Changes between Initial Version and Version 1 of Openvpn23ManPage


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Timestamp:
06/29/12 11:53:16 (12 years ago)
Author:
Samuli Seppänen
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  • Openvpn23ManPage

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     1{{{
     2openvpn(8)                                                                                                                  openvpn(8)
     3
     4
     5
     6NAME
     7       openvpn - secure IP tunnel daemon.
     8
     9SYNOPSIS
     10       openvpn [ options ... ]
     11
     12INTRODUCTION
     13       OpenVPN is an open source VPN daemon by James Yonan.  Because OpenVPN tries to be a universal VPN tool offering a great deal of
     14       flexibility, there are a lot of options on this manual page.  If you're new to OpenVPN, you might want to  skip  ahead  to  the
     15       examples section where you will see how to construct simple VPNs on the command line without even needing a configuration file.
     16
     17       Also note that there's more documentation and examples on the OpenVPN web site: http://openvpn.net/
     18
     19       And  if  you would like to see a shorter version of this manual, see the openvpn usage message which can be obtained by running
     20       openvpn without any parameters.
     21
     22DESCRIPTION
     23       OpenVPN is a robust and highly flexible VPN daemon.  OpenVPN supports SSL/TLS security, ethernet bridging, TCP  or  UDP  tunnel
     24       transport through proxies or NAT, support for dynamic IP addresses and DHCP, scalability to hundreds or thousands of users, and
     25       portability to most major OS platforms.
     26
     27       OpenVPN is tightly bound to the OpenSSL library, and derives much of its crypto capabilities from it.
     28
     29       OpenVPN supports conventional encryption using a pre-shared secret key (Static Key mode) or public key security (SSL/TLS  mode)
     30       using client & server certificates.  OpenVPN also supports non-encrypted TCP/UDP tunnels.
     31
     32       OpenVPN is designed to work with the TUN/TAP virtual networking interface that exists on most platforms.
     33
     34       Overall, OpenVPN aims to offer many of the key features of IPSec but with a relatively lightweight footprint.
     35
     36OPTIONS
     37       OpenVPN  allows any option to be placed either on the command line or in a configuration file.  Though all command line options
     38       are preceded by a double-leading-dash ("--"), this prefix can be removed when an option is placed in a configuration file.
     39
     40       --help Show options.
     41
     42       --config file
     43              Load additional config options from file where each line corresponds to one command line option, but  with  the  leading
     44              '--' removed.
     45
     46              If --config file is the only option to the openvpn command, the --config can be removed, and the command can be given as
     47              openvpn file
     48
     49              Note that configuration files can be nested to a reasonable depth.
     50
     51              Double quotation or single quotation characters ("", '') can be used to enclose single parameters containing whitespace,
     52              and "#" or ";" characters in the first column can be used to denote comments.
     53
     54              Note that OpenVPN 2.0 and higher performs backslash-based shell escaping for characters not in single quotations, so the
     55              following mappings should be observed:
     56
     57                  \\       Maps to a single backslash character (\).
     58                  \"       Pass a literal doublequote character ("), don't
     59                           interpret it as enclosing a parameter.
     60                  \[SPACE] Pass a literal space or tab character, don't
     61                           interpret it as a parameter delimiter.
     62
     63              For example on Windows, use double backslashes to represent pathnames:
     64
     65                  secret "c:\\OpenVPN\\secret.key"
     66
     67              For examples of configuration files, see http://openvpn.net/examples.html
     68
     69              Here is an example configuration file:
     70
     71                  #
     72                  # Sample OpenVPN configuration file for
     73                  # using a pre-shared static key.
     74                  #
     75                  # '#' or ';' may be used to delimit comments.
     76
     77                  # Use a dynamic tun device.
     78                  dev tun
     79
     80                  # Our remote peer
     81                  remote mypeer.mydomain
     82
     83                  # 10.1.0.1 is our local VPN endpoint
     84                  # 10.1.0.2 is our remote VPN endpoint
     85                  ifconfig 10.1.0.1 10.1.0.2
     86
     87                  # Our pre-shared static key
     88                  secret static.key
     89
     90   Tunnel Options:
     91       --mode m
     92              Set OpenVPN major mode.  By default, OpenVPN runs in point-to-point mode ("p2p").  OpenVPN 2.0  introduces  a  new  mode
     93              ("server") which implements a multi-client server capability.
     94
     95       --local host
     96              Local  host name or IP address for bind.  If specified, OpenVPN will bind to this address only.  If unspecified, OpenVPN
     97              will bind to all interfaces.
     98
     99       --remote host [port] [proto]
     100              Remote host name or IP address.  On the client, multiple --remote options may be specified for redundancy,  each  refer‐
     101              ring to a different OpenVPN server.  Specifying multiple --remote options for this purpose is a special case of the more
     102              general connection-profile feature.  See the <connection> documentation below.
     103
     104              The OpenVPN client will try to connect to a server at host:port in the order specified by the list of --remote options.
     105
     106              proto indicates the protocol to use when connecting with the remote, and may be "tcp" or "udp".
     107
     108              The client will move on to the next host in the list, in the event of connection failure.  Note that at any given  time,
     109              the OpenVPN client will at most be connected to one server.
     110
     111              Note that since UDP is connectionless, connection failure is defined by the --ping and --ping-restart options.
     112
     113              Note  the  following  corner  case:   If  you use multiple --remote options, AND you are dropping root privileges on the
     114              client with --user and/or --group, AND the client is running a non-Windows OS, if the client needs to switch to  a  dif‐
     115              ferent server, and that server pushes back different TUN/TAP or route settings, the client may lack the necessary privi‐
     116              leges to close and reopen the TUN/TAP interface.  This could cause the client to exit with a fatal error.
     117
     118              If --remote is unspecified, OpenVPN will listen for packets from any IP address, but  will  not  act  on  those  packets
     119              unless  they pass all authentication tests.  This requirement for authentication is binding on all potential peers, even
     120              those from known and supposedly trusted IP addresses (it is very easy to forge a source IP address on a UDP packet).
     121
     122              When used in TCP mode, --remote will act as a filter, rejecting connections from any host which does not match host.
     123
     124              If host is a DNS name which resolves to multiple IP addresses, one will be randomly chosen, providing a  sort  of  basic
     125              load-balancing and failover capability.
     126
     127       --remote-random-hostname
     128              Add  a  random  string (6 characters) to first DNS label of hostname to prevent DNS caching.  For example, "foo.bar.gov"
     129              would be modified to "<random-chars>.foo.bar.gov".
     130
     131       <connection>
     132              Define a client connection profile.  Client connection profiles are groups of OpenVPN options that describe how to  con‐
     133              nect to a given OpenVPN server.  Client connection profiles are specified within an OpenVPN configuration file, and each
     134              profile is bracketed by <connection> and </connection>.
     135
     136              An OpenVPN client will try each connection profile sequentially until it achieves a successful connection.
     137
     138              --remote-random can be used to initially "scramble" the connection list.
     139
     140              Here is an example of connection profile usage:
     141
     142                  client
     143                  dev tun
     144
     145                  <connection>
     146                  remote 198.19.34.56 1194 udp
     147                  </connection>
     148
     149                  <connection>
     150                  remote 198.19.34.56 443 tcp
     151                  </connection>
     152
     153                  <connection>
     154                  remote 198.19.34.56 443 tcp
     155                  http-proxy 192.168.0.8 8080
     156                  http-proxy-retry
     157                  </connection>
     158
     159                  <connection>
     160                  remote 198.19.36.99 443 tcp
     161                  http-proxy 192.168.0.8 8080
     162                  http-proxy-retry
     163                  </connection>
     164
     165                  persist-key
     166                  persist-tun
     167                  pkcs12 client.p12
     168                  ns-cert-type server
     169                  verb 3
     170
     171              First we try to connect to a server at 198.19.34.56:1194  using  UDP.   If  that  fails,  we  then  try  to  connect  to
     172              198.19.34.56:443  using  TCP.   If  that  also  fails,  then try connecting through an HTTP proxy at 192.168.0.8:8080 to
     173              198.19.34.56:443 using TCP.  Finally, try to connect through the same proxy to a server at 198.19.36.99:443 using TCP.
     174
     175              The following OpenVPN options may be used inside of a <connection> block:
     176
     177              bind, connect-retry, connect-retry-max, connect-timeout, float, http-proxy, http-proxy-option,  http-proxy-retry,  http-
     178              proxy-timeout, local, lport, nobind, port, proto, remote, rport, socks-proxy, and socks-proxy-retry.
     179
     180              A defaulting mechanism exists for specifying options to apply to all <connection> profiles.  If any of the above options
     181              (with the exception of remote ) appear outside of a <connection> block, but in a configuration file  which  has  one  or
     182              more  <connection>  blocks,  the option setting will be used as a default for <connection> blocks which follow it in the
     183              configuration file.
     184
     185              For example, suppose the nobind option were placed in the sample configuration file above, near the  top  of  the  file,
     186              before  the  first  <connection> block.  The effect would be as if nobind were declared in all <connection> blocks below
     187              it.
     188
     189       --proto-force p
     190              When iterating through connection profiles, only consider profiles using protocol p ('tcp'|'udp').
     191
     192       --remote-random
     193              When multiple --remote address/ports are specified, or if connection profiles are being used,  initially  randomize  the
     194              order of the list as a kind of basic load-balancing measure.
     195
     196       --proto p
     197              Use protocol p for communicating with remote host.  p can be udp, tcp-client, or tcp-server.
     198
     199              The default protocol is udp when --proto is not specified.
     200
     201              For UDP operation, --proto udp should be specified on both peers.
     202
     203              For  TCP operation, one peer must use --proto tcp-server and the other must use --proto tcp-client.  A peer started with
     204              tcp-server will wait indefinitely for an incoming connection.  A peer started with tcp-client will attempt  to  connect,
     205              and  if that fails, will sleep for 5 seconds (adjustable via the --connect-retry option) and try again infinite or up to
     206              N retries (adjustable via the --connect-retry-max option).  Both TCP client and server will simulate a  SIGUSR1  restart
     207              signal if either side resets the connection.
     208
     209              OpenVPN  is  designed  to  operate optimally over UDP, but TCP capability is provided for situations where UDP cannot be
     210              used.  In comparison with UDP, TCP will usually be somewhat less efficient and less robust when used over unreliable  or
     211              congested networks.
     212
     213              This article outlines some of problems with tunneling IP over TCP:
     214
     215              http://sites.inka.de/sites/bigred/devel/tcp-tcp.html
     216
     217              There  are  certain cases, however, where using TCP may be advantageous from a security and robustness perspective, such
     218              as tunneling non-IP or application-level UDP protocols, or tunneling protocols which don't possess a built-in  reliabil‐
     219              ity layer.
     220
     221       --connect-retry n
     222              For --proto tcp-client, take n as the number of seconds to wait between connection retries (default=5).
     223
     224       --connect-timeout n
     225              For --proto tcp-client, set connection timeout to n seconds (default=10).
     226
     227       --connect-retry-max n
     228              For --proto tcp-client, take n as the number of retries of connection attempt (default=infinite).
     229
     230       --show-proxy-settings
     231              Show sensed HTTP or SOCKS proxy settings. Currently, only Windows clients support this option.
     232
     233       --http-proxy server port [authfile|'auto'|'auto-nct'] [auth-method]
     234              Connect  to  remote host through an HTTP proxy at address server and port port.  If HTTP Proxy-Authenticate is required,
     235              authfile is a file containing a username and password on 2 lines, or "stdin" to prompt from console.
     236
     237              auth-method should be one of "none", "basic", or "ntlm".
     238
     239              HTTP Digest authentication is supported as well, but only via the auto or auto-nct flags (below).
     240
     241              The auto flag causes OpenVPN to automatically determine the auth-method and query stdin or the management interface  for
     242              username/password credentials, if required.  This flag exists on OpenVPN 2.1 or higher.
     243
     244              The  auto-nct  flag  (no clear-text auth) instructs OpenVPN to automatically determine the authentication method, but to
     245              reject weak authentication protocols such as HTTP Basic Authentication.
     246
     247       --http-proxy-retry
     248              Retry indefinitely on HTTP proxy errors.  If an HTTP proxy error occurs, simulate a SIGUSR1 reset.
     249
     250       --http-proxy-timeout n
     251              Set proxy timeout to n seconds, default=5.
     252
     253       --http-proxy-option type [parm]
     254              Set extended HTTP proxy options.  Repeat to set multiple options.
     255
     256              VERSION version -- Set HTTP version number to version (default=1.0).
     257
     258              AGENT user-agent -- Set HTTP "User-Agent" string to user-agent.
     259
     260       --socks-proxy server [port]
     261              Connect to remote host through a Socks5 proxy at address server and port port (default=1080).
     262
     263       --socks-proxy-retry
     264              Retry indefinitely on Socks proxy errors.  If a Socks proxy error occurs, simulate a SIGUSR1 reset.
     265
     266       --resolv-retry n
     267              If hostname resolve fails for --remote, retry resolve for n seconds before failing.
     268
     269              Set n to "infinite" to retry indefinitely.
     270
     271              By default, --resolv-retry infinite is enabled.  You can disable by setting n=0.
     272
     273       --float
     274              Allow remote peer to change its IP address and/or port number, such as due to DHCP (this is the default if  --remote  is
     275              not  used).   --float  when  specified with --remote allows an OpenVPN session to initially connect to a peer at a known
     276              address, however if packets arrive from a new address and pass all authentication tests, the new address will take  con‐
     277              trol  of  the session.  This is useful when you are connecting to a peer which holds a dynamic address such as a dial-in
     278              user or DHCP client.
     279
     280              Essentially, --float tells OpenVPN to accept authenticated packets from any address, not  only  the  address  which  was
     281              specified in the --remote option.
     282
     283       --ipchange cmd
     284              Run command cmd when our remote ip-address is initially authenticated or changes.
     285
     286              cmd  consists  of a path to script (or executable program), optionally followed by arguments. The path and arguments may
     287              be single- or double-quoted and/or escaped using a backslash, and should be separated by one or more spaces.
     288
     289              When cmd is executed two arguments are appended after any arguments specified in cmd , as follows:
     290
     291              cmd ip_address port_number
     292
     293              Don't use --ipchange in --mode server mode.  Use a --client-connect script instead.
     294
     295              See the "Environmental Variables" section below for additional parameters passed as environmental variables.
     296
     297              If you are running in a dynamic IP address environment where the IP  addresses  of  either  peer  could  change  without
     298              notice,  you  can  use  this script, for example, to edit the /etc/hosts file with the current address of the peer.  The
     299              script will be run every time the remote peer changes its IP address.
     300
     301              Similarly if our IP address changes due to DHCP, we should configure our IP address change  script  (see  man  page  for
     302              dhcpcd(8)  ) to deliver a SIGHUP or SIGUSR1 signal to OpenVPN.  OpenVPN will then reestablish a connection with its most
     303              recently authenticated peer on its new IP address.
     304
     305       --port port
     306              TCP/UDP port number for both local and remote.  The current default of 1194 represents the  official  IANA  port  number
     307              assignment for OpenVPN and has been used since version 2.0-beta17.  Previous versions used port 5000 as the default.
     308
     309       --lport port
     310              TCP/UDP port number for bind.
     311
     312       --rport port
     313              TCP/UDP port number for remote.
     314
     315       --bind Bind to local address and port. This is the default unless any of --proto tcp-client , --http-proxy or --socks-proxy are
     316              used.
     317
     318       --nobind
     319              Do not bind to local address and port.  The IP stack will allocate a dynamic port  for  returning  packets.   Since  the
     320              value  of the dynamic port could not be known in advance by a peer, this option is only suitable for peers which will be
     321              initiating connections by using the --remote option.
     322
     323       --dev tunX | tapX | null
     324              TUN/TAP virtual network device ( X can be omitted for a dynamic device.)
     325
     326              See examples section below for an example on setting up a TUN device.
     327
     328              You must use either tun devices on both ends of the connection or tap devices on both ends.  You  cannot  mix  them,  as
     329              they represent different underlying network layers.
     330
     331              tun devices encapsulate IPv4 or IPv6 (OSI Layer 3) while tap devices encapsulate Ethernet 802.3 (OSI Layer 2).
     332
     333       --dev-type device-type
     334              Which  device type are we using?  device-type should be tun (OSI Layer 3) or tap (OSI Layer 2).  Use this option only if
     335              the TUN/TAP device used with --dev does not begin with tun or tap.
     336
     337       --topology mode
     338              Configure virtual addressing topology when running in --dev tun mode.  This directive has no meaning in --dev tap  mode,
     339              which always uses a subnet topology.
     340
     341              If you set this directive on the server, the --server and --server-bridge directives will automatically push your chosen
     342              topology setting to clients as well.  This directive can also be manually pushed to clients.  Like the --dev  directive,
     343              this directive must always be compatible between client and server.
     344
     345              mode can be one of:
     346
     347              net30  --  Use  a point-to-point topology, by allocating one /30 subnet per client.  This is designed to allow point-to-
     348              point semantics when some or all of the connecting clients might be Windows systems.  This is  the  default  on  OpenVPN
     349              2.0.
     350
     351              p2p  -- Use a point-to-point topology where the remote endpoint of the client's tun interface always points to the local
     352              endpoint of the server's tun interface.  This mode allocates a single IP address per connecting client.  Only  use  when
     353              none  of the connecting clients are Windows systems.  This mode is functionally equivalent to the --ifconfig-pool-linear
     354              directive which is available in OpenVPN 2.0 and is now deprecated.
     355
     356              subnet -- Use a subnet rather than a point-to-point topology by configuring the tun interface with a  local  IP  address
     357              and  subnet mask, similar to the topology used in --dev tap and ethernet bridging mode.  This mode allocates a single IP
     358              address per connecting client and works on Windows as well.  Only available when server and clients are OpenVPN  2.1  or
     359              higher,  or  OpenVPN  2.0.x  which  has been manually patched with the --topology directive code.  When used on Windows,
     360              requires version 8.2 or higher of the TAP-Win32 driver.  When used on *nix, requires that the  tun  driver  supports  an
     361              ifconfig(8) command which sets a subnet instead of a remote endpoint IP address.
     362
     363              This option exists in OpenVPN 2.1 or higher.
     364
     365       --tun-ipv6
     366              Build  a  tun  link  capable of forwarding IPv6 traffic.  Should be used in conjunction with --dev tun or --dev tunX.  A
     367              warning will be displayed if no specific IPv6 TUN support for your OS has been compiled into OpenVPN.
     368
     369              See below for further IPv6-related configuration options.
     370
     371       --dev-node node
     372              Explicitly set the device node rather than using /dev/net/tun, /dev/tun, /dev/tap, etc.  If OpenVPN  cannot  figure  out
     373              whether node is a TUN or TAP device based on the name, you should also specify --dev-type tun or --dev-type tap.
     374
     375              On Windows systems, select the TAP-Win32 adapter which is named node in the Network Connections Control Panel or the raw
     376              GUID of the adapter enclosed by braces.  The --show-adapters option under Windows can also  be  used  to  enumerate  all
     377              available  TAP-Win32  adapters  and will show both the network connections control panel name and the GUID for each TAP-
     378              Win32 adapter.
     379
     380       --lladdr address
     381              Specify the link layer address, more commonly known as the MAC address.  Only applied to TAP devices.
     382
     383       --iproute cmd
     384              Set alternate command to execute instead of default iproute2 command.  May be  used  in  order  to  execute  OpenVPN  in
     385              unprivileged environment.
     386
     387       --ifconfig l rn
     388              Set  TUN/TAP  adapter parameters.  l is the IP address of the local VPN endpoint.  For TUN devices, rn is the IP address
     389              of the remote VPN endpoint.  For TAP devices, rn is the subnet mask of the virtual ethernet segment which is being  cre‐
     390              ated or connected to.
     391
     392              For  TUN  devices,  which facilitate virtual point-to-point IP connections, the proper usage of --ifconfig is to use two
     393              private IP addresses which are not a member of any existing subnet which is in use.  The IP addresses may be consecutive
     394              and should have their order reversed on the remote peer.  After the VPN is established, by pinging rn, you will be ping‐
     395              ing across the VPN.
     396
     397              For TAP devices, which provide the ability to create virtual ethernet segments, --ifconfig is used to set an IP  address
     398              and  subnet mask just as a physical ethernet adapter would be similarly configured.  If you are attempting to connect to
     399              a remote ethernet bridge, the IP address and subnet should be set to values which would be valid on the the bridged eth‐
     400              ernet segment (note also that DHCP can be used for the same purpose).
     401
     402              This  option,  while primarily a proxy for the ifconfig(8) command, is designed to simplify TUN/TAP tunnel configuration
     403              by providing a standard interface to the different ifconfig implementations on different platforms.
     404
     405              --ifconfig parameters which are IP addresses can also be specified as a DNS or /etc/hosts file resolvable name.
     406
     407              For TAP devices, --ifconfig should not be used if the TAP interface will be getting an IP  address  lease  from  a  DHCP
     408              server.
     409
     410       --ifconfig-noexec
     411              Don't  actually execute ifconfig/netsh commands, instead pass --ifconfig parameters to scripts using environmental vari‐
     412              ables.
     413
     414       --ifconfig-nowarn
     415              Don't output an options consistency check warning if the --ifconfig option on this side of the connection doesn't  match
     416              the remote side.  This is useful when you want to retain the overall benefits of the options consistency check (also see
     417              --disable-occ option) while only disabling the ifconfig component of the check.
     418
     419              For example, if you have a configuration where the local host uses --ifconfig but the remote host does not, use --ifcon‐
     420              fig-nowarn on the local host.
     421
     422              This option will also silence warnings about potential address conflicts which occasionally annoy more experienced users
     423              by triggering "false positive" warnings.
     424
     425       --route network/IP [netmask] [gateway] [metric]
     426              Add route to routing table after connection is established.  Multiple routes can be specified.  Routes will be automati‐
     427              cally torn down in reverse order prior to TUN/TAP device close.
     428
     429              This option is intended as a convenience proxy for the route(8) shell command, while at the same time providing portable
     430              semantics across OpenVPN's platform space.
     431
     432              netmask default -- 255.255.255.255
     433
     434              gateway default -- taken from --route-gateway or the second parameter to --ifconfig when --dev tun is specified.
     435
     436              metric default -- taken from --route-metric otherwise 0.
     437
     438              The default can be specified by leaving an option blank or setting it to "default".
     439
     440              The network and gateway parameters can also be specified as a DNS or /etc/hosts file resolvable name, or as one of three
     441              special keywords:
     442
     443              vpn_gateway  -- The remote VPN endpoint address (derived either from --route-gateway or the second parameter to --ifcon‐
     444              fig when --dev tun is specified).
     445
     446              net_gateway -- The pre-existing IP default gateway, read from the routing table (not supported on all OSes).
     447
     448              remote_host -- The --remote address if OpenVPN is being run in client mode, and is undefined in server mode.
     449
     450       --max-routes n
     451              Allow a maximum number of n --route options to be specified, either in the local configuration file, or pulled  from  an
     452              OpenVPN server.  By default, n=100.
     453
     454       --route-gateway gw|'dhcp'
     455              Specify a default gateway gw for use with --route.
     456
     457              If  dhcp  is  specified as the parameter, the gateway address will be extracted from a DHCP negotiation with the OpenVPN
     458              server-side LAN.
     459
     460       --route-metric m
     461              Specify a default metric m for use with --route.
     462
     463       --route-delay [n] [w]
     464              Delay n seconds (default=0) after connection establishment, before adding routes. If n is 0, routes will be added  imme‐
     465              diately  upon  connection  establishment.   If  --route-delay is omitted, routes will be added immediately after TUN/TAP
     466              device open and --up script execution, before any --user or --group privilege downgrade (or --chroot execution.)
     467
     468              This option is designed to be useful in scenarios where DHCP is used to set tap adapter addresses.  The delay will  give
     469              the DHCP handshake time to complete before routes are added.
     470
     471              On  Windows, --route-delay tries to be more intelligent by waiting w seconds (w=30 by default) for the TAP-Win32 adapter
     472              to come up before adding routes.
     473
     474       --route-up cmd
     475              Run command cmd after routes are added, subject to --route-delay.
     476
     477              cmd consists of a path to script (or executable program), optionally followed by arguments. The path and  arguments  may
     478              be single- or double-quoted and/or escaped using a backslash, and should be separated by one or more spaces.
     479
     480              See the "Environmental Variables" section below for additional parameters passed as environmental variables.
     481
     482       --route-pre-down cmd
     483              Run command cmd before routes are removed upon disconnection.
     484
     485              cmd  consists  of a path to script (or executable program), optionally followed by arguments. The path and arguments may
     486              be single- or double-quoted and/or escaped using a backslash, and should be separated by one or more spaces.
     487
     488              See the "Environmental Variables" section below for additional parameters passed as environmental variables.
     489
     490       --route-noexec
     491              Don't add or remove routes automatically.  Instead pass routes to --route-up script using environmental variables.
     492
     493       --route-nopull
     494              When used with --client or --pull, accept options pushed by server EXCEPT for routes and dhcp options like DNS servers.
     495
     496              When used on the client, this option effectively bars the server from adding routes to the client's routing table,  how‐
     497              ever note that this option still allows the server to set the TCP/IP properties of the client's TUN/TAP interface.
     498
     499       --allow-pull-fqdn
     500              Allow  client  to  pull  DNS  names  from  server (rather than being limited to IP address) for --ifconfig, --route, and
     501              --route-gateway.
     502
     503       --client-nat snat|dnat network netmask alias
     504              This pushable client option sets up a stateless one-to-one NAT rule on packet addresses (not ports), and  is  useful  in
     505              cases where routes or ifconfig settings pushed to the client would create an IP numbering conflict.
     506
     507              network/netmask  (for example 192.168.0.0/255.255.0.0) defines the local view of a resource from the client perspective,
     508              while alias/netmask (for example 10.64.0.0/255.255.0.0) defines the remote view from the server perspective.
     509
     510              Use snat (source NAT) for resources owned by the client and dnat (destination NAT) for remote resources.
     511
     512              Set --verb 6 for debugging info showing the transformation of src/dest addresses in packets.
     513
     514       --redirect-gateway flags...
     515              Automatically execute routing commands to cause all outgoing IP traffic to be  redirected  over  the  VPN.   This  is  a
     516              client-side option.
     517
     518              This option performs three steps:
     519
     520              (1)  Create a static route for the --remote address which forwards to the pre-existing default gateway.  This is done so
     521              that (3) will not create a routing loop.
     522
     523              (2) Delete the default gateway route.
     524
     525              (3) Set the new default gateway to be the VPN endpoint address (derived either from --route-gateway or the second param‐
     526              eter to --ifconfig when --dev tun is specified).
     527
     528              When the tunnel is torn down, all of the above steps are reversed so that the original default route is restored.
     529
     530              Option flags:
     531
     532              local  --  Add the local flag if both OpenVPN servers are directly connected via a common subnet, such as with wireless.
     533              The local flag will cause step 1 above to be omitted.
     534
     535              autolocal -- Try to automatically determine whether to enable local flag above.
     536
     537              def1 -- Use this flag to override the default gateway by using 0.0.0.0/1 and 128.0.0.0/1 rather  than  0.0.0.0/0.   This
     538              has the benefit of overriding but not wiping out the original default gateway.
     539
     540              bypass-dhcp  --  Add a direct route to the DHCP server (if it is non-local) which bypasses the tunnel (Available on Win‐
     541              dows clients, may not be available on non-Windows clients).
     542
     543              bypass-dns -- Add a direct route to the DNS server(s) (if they are non-local) which bypasses the  tunnel  (Available  on
     544              Windows clients, may not be available on non-Windows clients).
     545
     546              block-local  --  Block access to local LAN when the tunnel is active, except for the LAN gateway itself.  This is accom‐
     547              plished by routing the local LAN (except for the LAN gateway address) into the tunnel.
     548
     549       --link-mtu n
     550              Sets an upper bound on the size of UDP packets which are sent between OpenVPN peers.  It's best not to set this  parame‐
     551              ter unless you know what you're doing.
     552
     553       --redirect-private [flags]
     554              Like --redirect-gateway, but omit actually changing the default gateway.  Useful when pushing private subnets.
     555
     556       --tun-mtu n
     557              Take  the  TUN device MTU to be n and derive the link MTU from it (default=1500).  In most cases, you will probably want
     558              to leave this parameter set to its default value.
     559
     560              The MTU (Maximum Transmission Units) is the maximum datagram size in bytes that can be sent unfragmented over a particu‐
     561              lar network path.  OpenVPN requires that packets on the control or data channels be sent unfragmented.
     562
     563              MTU problems often manifest themselves as connections which hang during periods of active usage.
     564
     565              It's best to use the --fragment and/or --mssfix options to deal with MTU sizing issues.
     566
     567       --tun-mtu-extra n
     568              Assume  that  the  TUN/TAP  device might return as many as n bytes more than the --tun-mtu size on read.  This parameter
     569              defaults to 0, which is sufficient for most TUN devices.  TAP devices may introduce additional overhead in excess of the
     570              MTU  size,  and a setting of 32 is the default when TAP devices are used.  This parameter only controls internal OpenVPN
     571              buffer sizing, so there is no transmission overhead associated with using a larger value.
     572
     573       --mtu-disc type
     574              Should we do Path MTU discovery on TCP/UDP channel?  Only supported on OSes such as Linux that  supports  the  necessary
     575              system call to set.
     576
     577              'no' -- Never send DF (Don't Fragment) frames
     578              'maybe' -- Use per-route hints
     579              'yes' -- Always DF (Don't Fragment)
     580
     581       --mtu-test
     582              To  empirically  measure  MTU on connection startup, add the --mtu-test option to your configuration.  OpenVPN will send
     583              ping packets of various sizes to the remote peer and measure the largest packets which were successfully received.   The
     584              --mtu-test process normally takes about 3 minutes to complete.
     585
     586       --fragment max
     587              Enable internal datagram fragmentation so that no UDP datagrams are sent which are larger than max bytes.
     588
     589              The  max parameter is interpreted in the same way as the --link-mtu parameter, i.e. the UDP packet size after encapsula‐
     590              tion overhead has been added in, but not including the UDP header itself.
     591
     592              The --fragment option only makes sense when you are using the UDP protocol ( --proto udp ).
     593
     594              --fragment adds 4 bytes of overhead per datagram.
     595
     596              See the --mssfix option below for an important related option to --fragment.
     597
     598              It should also be noted that this option is not meant to replace UDP fragmentation at the IP stack level.   It  is  only
     599              meant as a last resort when path MTU discovery is broken.  Using this option is less efficient than fixing path MTU dis‐
     600              covery for your IP link and using native IP fragmentation instead.
     601
     602              Having said that, there are circumstances where using OpenVPN's internal  fragmentation  capability  may  be  your  only
     603              option, such as tunneling a UDP multicast stream which requires fragmentation.
     604
     605       --mssfix max
     606              Announce  to TCP sessions running over the tunnel that they should limit their send packet sizes such that after OpenVPN
     607              has encapsulated them, the resulting UDP packet size that OpenVPN sends to its peer  will  not  exceed  max  bytes.  The
     608              default value is 1450.
     609
     610              The  max parameter is interpreted in the same way as the --link-mtu parameter, i.e. the UDP packet size after encapsula‐
     611              tion overhead has been added in, but not including the UDP header itself.
     612
     613              The --mssfix option only makes sense when you are using the UDP protocol for OpenVPN  peer-to-peer  communication,  i.e.
     614              --proto udp.
     615
     616              --mssfix and --fragment can be ideally used together, where --mssfix will try to keep TCP from needing packet fragmenta‐
     617              tion in the first place, and if big packets come through anyhow (from protocols other than TCP), --fragment will  inter‐
     618              nally fragment them.
     619
     620              Both  --fragment  and  --mssfix are designed to work around cases where Path MTU discovery is broken on the network path
     621              between OpenVPN peers.
     622
     623              The usual symptom of such a breakdown is an OpenVPN connection which successfully starts, but then stalls during  active
     624              usage.
     625
     626              If  --fragment  and  --mssfix  are  used  together, --mssfix will take its default max parameter from the --fragment max
     627              option.
     628
     629              Therefore, one could lower the maximum UDP packet size to 1300 (a good first  try  for  solving  MTU-related  connection
     630              problems) with the following options:
     631
     632              --tun-mtu 1500 --fragment 1300 --mssfix
     633
     634       --sndbuf size
     635              Set the TCP/UDP socket send buffer size.  Currently defaults to 65536 bytes.
     636
     637       --rcvbuf size
     638              Set the TCP/UDP socket receive buffer size.  Currently defaults to 65536 bytes.
     639
     640       --mark value
     641              Mark  encrypted  packets  being sent with value. The mark value can be matched in policy routing and packetfilter rules.
     642              This option is only supported in Linux and does nothing on other operating systems.
     643
     644       --socket-flags flags...
     645              Apply the given flags to the OpenVPN transport socket.  Currently, only TCP_NODELAY is supported.
     646
     647              The TCP_NODELAY socket flag is useful in TCP mode, and causes the kernel to send tunnel packets immediately over the TCP
     648              connection  without  trying  to  group  several smaller packets into a larger packet.  This can result in a considerably
     649              improvement in latency.
     650
     651              This option is pushable from server to client, and should be used on both client and server for maximum effect.
     652
     653       --txqueuelen n
     654              (Linux only) Set the TX queue length on the TUN/TAP interface.  Currently defaults to 100.
     655
     656       --shaper n
     657              Limit bandwidth of outgoing tunnel data to n bytes per second on the TCP/UDP port.  If you want to limit  the  bandwidth
     658              in both directions, use this option on both peers.
     659
     660              OpenVPN  uses  the  following algorithm to implement traffic shaping: Given a shaper rate of n bytes per second, after a
     661              datagram write of b bytes is queued on the TCP/UDP port, wait a minimum of (b /  n)  seconds  before  queuing  the  next
     662              write.
     663
     664              It  should  be  noted that OpenVPN supports multiple tunnels between the same two peers, allowing you to construct full-
     665              speed and reduced bandwidth tunnels at the same time, routing low-priority  data  such  as  off-site  backups  over  the
     666              reduced bandwidth tunnel, and other data over the full-speed tunnel.
     667
     668              Also note that for low bandwidth tunnels (under 1000 bytes per second), you should probably use lower MTU values as well
     669              (see above), otherwise the packet latency will grow so large as to trigger timeouts in the TLS layer and TCP connections
     670              running over the tunnel.
     671
     672              OpenVPN allows n to be between 100 bytes/sec and 100 Mbytes/sec.
     673
     674       --inactive n [bytes]
     675              Causes  OpenVPN  to  exit after n seconds of inactivity on the TUN/TAP device. The time length of inactivity is measured
     676              since the last incoming or outgoing tunnel packet.  The default value is 0 seconds, which disables this feature.
     677
     678              If the optional bytes parameter is included, exit if less than bytes of combined in/out  traffic  are  produced  on  the
     679              tun/tap device in n seconds.
     680
     681              In  any  case,  OpenVPN's  internal  ping packets (which are just keepalives) and TLS control packets are not considered
     682              "activity", nor are they counted as traffic, as they are used internally by OpenVPN and are not an indication of  actual
     683              user activity.
     684
     685       --ping n
     686              Ping remote over the TCP/UDP control channel if no packets have been sent for at least n seconds (specify --ping on both
     687              peers to cause ping packets to be sent in both directions since OpenVPN ping packets are not echoed like IP  ping  pack‐
     688              ets).  When used in one of OpenVPN's secure modes (where --secret, --tls-server, or --tls-client is specified), the ping
     689              packet will be cryptographically secure.
     690
     691              This option has two intended uses:
     692
     693              (1) Compatibility with stateful firewalls.  The periodic ping will ensure that a stateful  firewall  rule  which  allows
     694              OpenVPN UDP packets to pass will not time out.
     695
     696              (2) To provide a basis for the remote to test the existence of its peer using the --ping-exit option.
     697
     698       --ping-exit n
     699              Causes OpenVPN to exit after n seconds pass without reception of a ping or other packet from remote.  This option can be
     700              combined with --inactive, --ping, and --ping-exit to create a two-tiered inactivity disconnect.
     701
     702              For example,
     703
     704              openvpn [options...] --inactive 3600 --ping 10 --ping-exit 60
     705
     706              when used on both peers will cause OpenVPN to exit within 60 seconds if its peer disconnects, but will  exit  after  one
     707              hour if no actual tunnel data is exchanged.
     708
     709       --ping-restart n
     710              Similar  to  --ping-exit, but trigger a SIGUSR1 restart after n seconds pass without reception of a ping or other packet
     711              from remote.
     712
     713              This option is useful in cases where the remote peer has a dynamic IP address and a low-TTL DNS name is  used  to  track
     714              the IP address using a service such as http://dyndns.org/ + a dynamic DNS client such as ddclient.
     715
     716              If  the  peer  cannot be reached, a restart will be triggered, causing the hostname used with --remote to be re-resolved
     717              (if --resolv-retry is also specified).
     718
     719              In server mode, --ping-restart, --inactive, or any other type of internally generated signal will always be  applied  to
     720              individual  client  instance objects, never to whole server itself.  Note also in server mode that any internally gener‐
     721              ated signal which would normally cause a restart, will cause the deletion of the client instance object instead.
     722
     723              In client mode, the --ping-restart parameter is set to 120 seconds by default.  This default will hold until the  client
     724              pulls a replacement value from the server, based on the --keepalive setting in the server configuration.  To disable the
     725              120 second default, set --ping-restart 0 on the client.
     726
     727              See the signals section below for more information on SIGUSR1.
     728
     729              Note that the behavior of SIGUSR1 can be modified by the --persist-tun, --persist-key,  --persist-local-ip,  and  --per‐
     730              sist-remote-ip options.
     731
     732              Also note that --ping-exit and --ping-restart are mutually exclusive and cannot be used together.
     733
     734       --keepalive n m
     735              A helper directive designed to simplify the expression of --ping and --ping-restart in server mode configurations.
     736
     737              For example, --keepalive 10 60 expands as follows:
     738
     739                   if mode server:
     740                     ping 10
     741                     ping-restart 120
     742                     push "ping 10"
     743                     push "ping-restart 60"
     744                   else
     745                     ping 10
     746                     ping-restart 60
     747
     748       --ping-timer-rem
     749              Run  the  --ping-exit  / --ping-restart timer only if we have a remote address.  Use this option if you are starting the
     750              daemon in listen mode (i.e. without an explicit --remote peer), and you don't want to start clocking  timeouts  until  a
     751              remote peer connects.
     752
     753       --persist-tun
     754              Don't close and reopen TUN/TAP device or run up/down scripts across SIGUSR1 or --ping-restart restarts.
     755
     756              SIGUSR1 is a restart signal similar to SIGHUP, but which offers finer-grained control over reset options.
     757
     758       --persist-key
     759              Don't re-read key files across SIGUSR1 or --ping-restart.
     760
     761              This  option can be combined with --user nobody to allow restarts triggered by the SIGUSR1 signal.  Normally if you drop
     762              root privileges in OpenVPN, the daemon cannot be restarted since it will now be unable to re-read protected key files.
     763
     764              This option solves the problem by persisting keys across SIGUSR1 resets, so they don't need to be re-read.
     765
     766       --persist-local-ip
     767              Preserve initially resolved local IP address and port number across SIGUSR1 or --ping-restart restarts.
     768
     769       --persist-remote-ip
     770              Preserve most recently authenticated remote IP address and port number across SIGUSR1 or --ping-restart restarts.
     771
     772       --mlock
     773              Disable paging by calling the POSIX mlockall function.  Requires that OpenVPN be initially run as root  (though  OpenVPN
     774              can subsequently downgrade its UID using the --user option).
     775
     776              Using this option ensures that key material and tunnel data are never written to disk due to virtual memory paging oper‐
     777              ations which occur under most modern operating systems.  It ensures that even if an attacker was able to crack  the  box
     778              running  OpenVPN, he would not be able to scan the system swap file to recover previously used ephemeral keys, which are
     779              used for a period of time governed by the --reneg options (see below), then are discarded.
     780
     781              The downside of using --mlock is that it will reduce the amount of physical memory available to other applications.
     782
     783       --up cmd
     784              Run command cmd after successful TUN/TAP device open (pre --user UID change).
     785
     786              cmd consists of a path to script (or executable program), optionally followed by arguments. The path and  arguments  may
     787              be single- or double-quoted and/or escaped using a backslash, and should be separated by one or more spaces.
     788
     789              The  up  command is useful for specifying route commands which route IP traffic destined for private subnets which exist
     790              at the other end of the VPN connection into the tunnel.
     791
     792              For --dev tun execute as:
     793
     794              cmd tun_dev tun_mtu link_mtu ifconfig_local_ip ifconfig_remote_ip [ init | restart ]
     795
     796              For --dev tap execute as:
     797
     798              cmd tap_dev tap_mtu link_mtu ifconfig_local_ip ifconfig_netmask [ init | restart ]
     799
     800              See the "Environmental Variables" section below for additional parameters passed as environmental variables.
     801
     802              Note that if cmd includes arguments, all OpenVPN-generated arguments will be appended to them to build an argument  list
     803              with which the executable will be called.
     804
     805              Typically, cmd will run a script to add routes to the tunnel.
     806
     807              Normally  the  up script is called after the TUN/TAP device is opened.  In this context, the last command line parameter
     808              passed to the script will be init.  If the --up-restart option is also used, the up script will be called  for  restarts
     809              as  well.   A  restart is considered to be a partial reinitialization of OpenVPN where the TUN/TAP instance is preserved
     810              (the --persist-tun option will enable such preservation).  A restart can be generated by a  SIGUSR1  signal,  a  --ping-
     811              restart  timeout,  or a connection reset when the TCP protocol is enabled with the --proto option.  If a restart occurs,
     812              and --up-restart has been specified, the up script will be called with restart as the last parameter.
     813
     814              The following standalone example shows how the --up script can be called in both an initialization and restart  context.
     815              (NOTE:  for  security  reasons, don't run the following example unless UDP port 9999 is blocked by your firewall.  Also,
     816              the example will run indefinitely, so you should abort with control-c).
     817
     818              openvpn --dev tun --port 9999 --verb 4 --ping-restart 10 --up 'echo up' --down 'echo down' --persist-tun --up-restart
     819
     820              Note that OpenVPN also provides the --ifconfig option to automatically ifconfig the TUN device, eliminating the need  to
     821              define an --up script, unless you also want to configure routes in the --up script.
     822
     823              If  --ifconfig  is  also specified, OpenVPN will pass the ifconfig local and remote endpoints on the command line to the
     824              --up script so that they can be used to configure routes such as:
     825
     826              route add -net 10.0.0.0 netmask 255.255.255.0 gw $5
     827
     828       --up-delay
     829              Delay TUN/TAP open and possible --up script execution until after TCP/UDP connection establishment with peer.
     830
     831              In --proto udp mode, this option normally requires the use of --ping to allow connection initiation to be sensed in  the
     832              absence of tunnel data, since UDP is a "connectionless" protocol.
     833
     834              On  Windows,  this  option will delay the TAP-Win32 media state transitioning to "connected" until connection establish‐
     835              ment, i.e. the receipt of the first authenticated packet from the peer.
     836
     837       --down cmd
     838              Run command cmd after TUN/TAP device close (post --user UID change and/or --chroot ).  cmd consists of a path to  script
     839              (or executable program), optionally followed by arguments. The path and arguments may be single- or double-quoted and/or
     840              escaped using a backslash, and should be separated by one or more spaces.
     841
     842              Called with the same parameters and environmental variables as the --up option above.
     843
     844              Note that if you reduce privileges by using --user and/or --group, your --down script will also run  at  reduced  privi‐
     845              lege.
     846
     847       --down-pre
     848              Call --down cmd/script before, rather than after, TUN/TAP close.
     849
     850       --up-restart
     851              Enable the --up and --down scripts to be called for restarts as well as initial program start.  This option is described
     852              more fully above in the --up option documentation.
     853
     854       --setenv name value
     855              Set a custom environmental variable name=value to pass to script.
     856
     857       --setenv FORWARD_COMPATIBLE 1
     858              Relax config file syntax checking so that unknown directives will trigger a warning  but  not  a  fatal  error,  on  the
     859              assumption that a given unknown directive might be valid in future OpenVPN versions.
     860
     861              This  option should be used with caution, as there are good security reasons for having OpenVPN fail if it detects prob‐
     862              lems in a config file.  Having said that, there are valid reasons  for  wanting  new  software  features  to  gracefully
     863              degrade when encountered by older software versions.
     864
     865       --setenv-safe name value
     866              Set a custom environmental variable OPENVPN_name=value to pass to script.
     867
     868              This  directive is designed to be pushed by the server to clients, and the prepending of "OPENVPN_" to the environmental
     869              variable is a safety precaution to prevent a LD_PRELOAD style attack from a malicious or compromised server.
     870
     871       --script-security level [method]
     872              This directive offers policy-level control over OpenVPN's usage of external programs and scripts.   Lower  level  values
     873              are more restrictive, higher values are more permissive.  Settings for level:
     874
     875              0 -- Strictly no calling of external programs.
     876              1 -- (Default) Only call built-in executables such as ifconfig, ip, route, or netsh.
     877              2 -- Allow calling of built-in executables and user-defined scripts.
     878              3 -- Allow passwords to be passed to scripts via environmental variables (potentially unsafe).
     879
     880              The method parameter indicates how OpenVPN should call external commands and scripts.  Settings for method:
     881
     882              execve -- (default) Use execve() function on Unix family OSes and CreateProcess() on Windows.
     883              system  --  Use  system() function (deprecated and less safe since the external program command line is subject to shell
     884              expansion).
     885
     886              The --script-security option was introduced in OpenVPN 2.1_rc9.  For  configuration  file  compatibility  with  previous
     887              OpenVPN versions, use: --script-security 3 system
     888
     889       --disable-occ
     890              Don't  output a warning message if option inconsistencies are detected between peers.  An example of an option inconsis‐
     891              tency would be where one peer uses --dev tun while the other peer uses --dev tap.
     892
     893              Use of this option is discouraged, but is provided as a temporary fix in situations where a recent  version  of  OpenVPN
     894              must connect to an old version.
     895
     896       --user user
     897              Change the user ID of the OpenVPN process to user after initialization, dropping privileges in the process.  This option
     898              is useful to protect the system in the event that some hostile party was able to gain control  of  an  OpenVPN  session.
     899              Though OpenVPN's security features make this unlikely, it is provided as a second line of defense.
     900
     901              By  setting  user  to  nobody or somebody similarly unprivileged, the hostile party would be limited in what damage they
     902              could cause.  Of course once you take away privileges, you cannot return them to an OpenVPN session.   This  means,  for
     903              example,  that  if  you want to reset an OpenVPN daemon with a SIGUSR1 signal (for example in response to a DHCP reset),
     904              you should make use of one or more of the --persist options to ensure that OpenVPN doesn't need to  execute  any  privi‐
     905              leged operations in order to restart (such as re-reading key files or running ifconfig on the TUN device).
     906
     907       --group group
     908              Similar to the --user option, this option changes the group ID of the OpenVPN process to group after initialization.
     909
     910       --cd dir
     911              Change  directory to dir prior to reading any files such as configuration files, key files, scripts, etc.  dir should be
     912              an absolute path, with a leading "/", and without any references to the current directory such as "." or "..".
     913
     914              This option is useful when you are running OpenVPN in --daemon mode, and you want to consolidate  all  of  your  OpenVPN
     915              control files in one location.
     916
     917       --chroot dir
     918              Chroot  to  dir  after  initialization.   --chroot  essentially redefines dir as being the top level directory tree (/).
     919              OpenVPN will therefore be unable to access any files outside this tree.  This can be desirable from  a  security  stand‐
     920              point.
     921
     922              Since the chroot operation is delayed until after initialization, most OpenVPN options that reference files will operate
     923              in a pre-chroot context.
     924
     925              In many cases, the dir parameter can point to an empty directory, however  complications  can  result  when  scripts  or
     926              restarts are executed after the chroot operation.
     927
     928       --setcon context
     929              Apply  SELinux  context after initialization. This essentially provides the ability to restrict OpenVPN's rights to only
     930              network I/O operations, thanks to SELinux. This goes further than --user and --chroot in that  those  two,  while  being
     931              great  security features, unfortunately do not protect against privilege escalation by exploitation of a vulnerable sys‐
     932              tem call. You can of course combine all three, but please note that since setcon requires access to /proc you will  have
     933              to provide it inside the chroot directory (e.g. with mount --bind).
     934
     935              Since the setcon operation is delayed until after initialization, OpenVPN can be restricted to just network-related sys‐
     936              tem calls, whereas by applying the context before startup (such as the OpenVPN one provided  in  the  SELinux  Reference
     937              Policies) you will have to allow many things required only during initialization.
     938
     939              Like  with  chroot,  complications can result when scripts or restarts are executed after the setcon operation, which is
     940              why you should really consider using the --persist-key and --persist-tun options.
     941
     942       --daemon [progname]
     943              Become a daemon after all initialization functions are completed.  This option will cause all message and  error  output
     944              to be sent to the syslog file (such as /var/log/messages), except for the output of scripts and ifconfig commands, which
     945              will go to /dev/null unless otherwise redirected.  The syslog redirection occurs immediately at the point that  --daemon
     946              is parsed on the command line even though the daemonization point occurs later.  If one of the --log options is present,
     947              it will supercede syslog redirection.
     948
     949              The optional progname parameter will cause OpenVPN to report its program name to the system logger  as  progname.   This
     950              can be useful in linking OpenVPN messages in the syslog file with specific tunnels.  When unspecified, progname defaults
     951              to "openvpn".
     952
     953              When OpenVPN is run with the --daemon option, it will try to delay daemonization until the  majority  of  initialization
     954              functions  which  are  capable of generating fatal errors are complete.  This means that initialization scripts can test
     955              the return status of the openvpn command for a fairly reliable indication of whether the command has correctly  initial‐
     956              ized and entered the packet forwarding event loop.
     957
     958              In OpenVPN, the vast majority of errors which occur after initialization are non-fatal.
     959
     960       --syslog [progname]
     961              Direct  log  output to system logger, but do not become a daemon.  See --daemon directive above for description of prog‐
     962              name parameter.
     963
     964       --errors-to-stderr
     965              Output errors to stderr instead of stdout unless log output is redirected by one of the --log options.
     966
     967       --passtos
     968              Set the TOS field of the tunnel packet to what the payload's TOS is.
     969
     970       --inetd [wait|nowait] [progname]
     971              Use this option when OpenVPN is being run from the inetd or xinetd(8) server.
     972
     973              The wait/nowait option must match what is specified in the inetd/xinetd config file.  The nowait mode can only  be  used
     974              with --proto tcp-server.  The default is wait.  The nowait mode can be used to instantiate the OpenVPN daemon as a clas‐
     975              sic TCP server, where client connection requests are serviced on a single port number.  For  additional  information  on
     976              this kind of configuration, see the OpenVPN FAQ: http://openvpn.net/faq.html#oneport
     977
     978              This  option precludes the use of --daemon, --local, or --remote.  Note that this option causes message and error output
     979              to be handled in the same way as the --daemon option.  The optional progname parameter is also  handled  exactly  as  in
     980              --daemon.
     981
     982              Also  note that in wait mode, each OpenVPN tunnel requires a separate TCP/UDP port and a separate inetd or xinetd entry.
     983              See the OpenVPN 1.x HOWTO for an example on using OpenVPN with xinetd: http://openvpn.net/1xhowto.html
     984
     985       --log file
     986              Output logging messages to file, including output to stdout/stderr which  is  generated  by  called  scripts.   If  file
     987              already  exists  it  will  be truncated.  This option takes effect immediately when it is parsed in the command line and
     988              will supercede syslog output if --daemon or --inetd is also specified.  This option is persistent over the entire course
     989              of an OpenVPN instantiation and will not be reset by SIGHUP, SIGUSR1, or --ping-restart.
     990
     991              Note  that  on Windows, when OpenVPN is started as a service, logging occurs by default without the need to specify this
     992              option.
     993
     994       --log-append file
     995              Append logging messages to file.  If file does not exist, it will be created.  This option behaves  exactly  like  --log
     996              except that it appends to rather than truncating the log file.
     997
     998       --suppress-timestamps
     999              Avoid  writing  timestamps  to log messages, even when they otherwise would be prepended. In particular, this applies to
     1000              log messages sent to stdout.
     1001
     1002       --writepid file
     1003              Write OpenVPN's main process ID to file.
     1004
     1005       --nice n
     1006              Change process priority after initialization ( n greater than 0 is lower priority, n less than zero is higher priority).
     1007
     1008       --fast-io
     1009              (Experimental) Optimize TUN/TAP/UDP I/O writes by avoiding a call to poll/epoll/select prior  to  the  write  operation.
     1010              The  purpose  of  such  a call would normally be to block until the device or socket is ready to accept the write.  Such
     1011              blocking is unnecessary on some platforms which don't support write blocking on UDP sockets or TUN/TAP devices.  In such
     1012              cases, one can optimize the event loop by avoiding the poll/epoll/select call, improving CPU efficiency by 5% to 10%.
     1013
     1014              This option can only be used on non-Windows systems, when --proto udp is specified, and when --shaper is NOT specified.
     1015
     1016       --multihome
     1017              Configure  a  multi-homed  UDP server.  This option can be used when OpenVPN has been configured to listen on all inter‐
     1018              faces, and will attempt to bind client sessions to the interface on which packets are being received, so  that  outgoing
     1019              packets will be sent out of the same interface.  Note that this option is only relevant for UDP servers and currently is
     1020              only implemented on Linux.
     1021
     1022              Note: clients connecting to a --multihome server should always use the --nobind option.
     1023
     1024       --echo [parms...]
     1025              Echo parms to log output.
     1026
     1027              Designed to be used to send messages to a controlling application which is receiving the OpenVPN log output.
     1028
     1029       --remap-usr1 signal
     1030              Control whether internally or externally generated SIGUSR1 signals are remapped to SIGHUP  (restart  without  persisting
     1031              state) or SIGTERM (exit).
     1032
     1033              signal can be set to "SIGHUP" or "SIGTERM".  By default, no remapping occurs.
     1034
     1035       --verb n
     1036              Set  output  verbosity to n (default=1).  Each level shows all info from the previous levels.  Level 3 is recommended if
     1037              you want a good summary of what's happening without being swamped by output.
     1038
     1039              0 -- No output except fatal errors.
     1040              1 to 4 -- Normal usage range.
     1041              5 -- Output R and W characters to the console for each packet read and write, uppercase is used for TCP/UDP packets  and
     1042              lowercase is used for TUN/TAP packets.
     1043              6 to 11 -- Debug info range (see errlevel.h for additional information on debug levels).
     1044
     1045       --status file [n]
     1046              Write operational status to file every n seconds.
     1047
     1048              Status can also be written to the syslog by sending a SIGUSR2 signal.
     1049
     1050       --status-version [n]
     1051              Choose the status file format version number.  Currently n can be 1, 2, or 3 and defaults to 1.
     1052
     1053       --mute n
     1054              Log  at most n consecutive messages in the same category.  This is useful to limit repetitive logging of similar message
     1055              types.
     1056
     1057       --comp-lzo [mode]
     1058              Use fast LZO compression -- may add up to 1 byte per packet for incompressible data.  mode may be "yes", "no", or "adap‐
     1059              tive" (default).
     1060
     1061              In a server mode setup, it is possible to selectively turn compression on or off for individual clients.
     1062
     1063              First,  make sure the client-side config file enables selective compression by having at least one --comp-lzo directive,
     1064              such as --comp-lzo no.  This will turn off compression by default, but allow a future directive push from the server  to
     1065              dynamically change the on/off/adaptive setting.
     1066
     1067              Next in a --client-config-dir file, specify the compression setting for the client, for example:
     1068
     1069                  comp-lzo yes
     1070                  push "comp-lzo yes"
     1071
     1072              The first line sets the comp-lzo setting for the server side of the link, the second sets the client side.
     1073
     1074       --comp-noadapt
     1075              When  used in conjunction with --comp-lzo, this option will disable OpenVPN's adaptive compression algorithm.  Normally,
     1076              adaptive compression is enabled with --comp-lzo.
     1077
     1078              Adaptive compression tries to optimize the case where you have compression enabled, but you  are  sending  predominantly
     1079              uncompressible  (or  pre-compressed)  packets  over  the tunnel, such as an FTP or rsync transfer of a large, compressed
     1080              file.  With adaptive compression, OpenVPN will periodically sample the compression process to  measure  its  efficiency.
     1081              If  the  data  being sent over the tunnel is already compressed, the compression efficiency will be very low, triggering
     1082              openvpn to disable compression for a period of time until the next re-sample test.
     1083
     1084       --management IP port [pw-file]
     1085              Enable a TCP server on IP:port to handle daemon management functions.  pw-file, if specified, is a password file  (pass‐
     1086              word  on  first  line)  or "stdin" to prompt from standard input.  The password provided will set the password which TCP
     1087              clients will need to provide in order to access management functions.
     1088
     1089              The management interface can also listen on a unix domain socket, for those platforms that support it.  To  use  a  unix
     1090              domain socket, specify the unix socket pathname in place of IP and set port to 'unix'.  While the default behavior is to
     1091              create a unix domain socket that may be connected to by any  process,  the  --management-client-user  and  --management-
     1092              client-group directives can be used to restrict access.
     1093
     1094              The  management  interface provides a special mode where the TCP management link can operate over the tunnel itself.  To
     1095              enable this mode, set IP = "tunnel".  Tunnel mode will cause the management interface to listen for a TCP connection  on
     1096              the local VPN address of the TUN/TAP interface.
     1097
     1098              While  the management port is designed for programmatic control of OpenVPN by other applications, it is possible to tel‐
     1099              net to the port, using a telnet client in "raw" mode.  Once connected, type "help" for a list of commands.
     1100
     1101              For detailed documentation on the management interface, see the management-notes.txt file in the  management  folder  of
     1102              the OpenVPN source distribution.
     1103
     1104              It is strongly recommended that IP be set to 127.0.0.1 (localhost) to restrict accessibility of the management server to
     1105              local clients.
     1106
     1107       --management-client
     1108              Management interface will connect as a TCP client to IP:port specified by --management  rather  than  listen  as  a  TCP
     1109              server.
     1110
     1111       --management-query-passwords
     1112              Query  management  channel  for  private key password and --auth-user-pass username/password.  Only query the management
     1113              channel for inputs which ordinarily would have been queried from the console.
     1114
     1115       --management-query-remote
     1116              Allow management interface to override --remote directives (client-only).
     1117
     1118       --management-forget-disconnect
     1119              Make OpenVPN forget passwords when management session disconnects.
     1120
     1121              This directive does not affect the --http-proxy username/password.  It is always cached.
     1122
     1123       --management-hold
     1124              Start OpenVPN in a hibernating state, until a client of the management interface explicitly  starts  it  with  the  hold
     1125              release command.
     1126
     1127       --management-signal
     1128              Send SIGUSR1 signal to OpenVPN if management session disconnects.  This is useful when you wish to disconnect an OpenVPN
     1129              session on user logoff.
     1130
     1131       --management-log-cache n
     1132              Cache the most recent n lines of log file history for usage by the management channel.
     1133
     1134       --management-up-down
     1135              Report tunnel up/down events to management interface.
     1136
     1137       --management-client-auth
     1138              Gives management interface client the responsibility to authenticate clients after their  client  certificate  has  been
     1139              verified.  See management-notes.txt in OpenVPN distribution for detailed notes.
     1140
     1141       --management-client-pf
     1142              Management  interface clients must specify a packet filter file for each connecting client.  See management-notes.txt in
     1143              OpenVPN distribution for detailed notes.
     1144
     1145       --management-client-user u
     1146              When the management interface is listening on a unix domain socket, only allow connections from user u.
     1147
     1148       --management-client-group g
     1149              When the management interface is listening on a unix domain socket, only allow connections from group g.
     1150
     1151       --plugin module-pathname [init-string]
     1152              Load plug-in module from the file module-pathname, passing init-string as an argument to the module initialization func‐
     1153              tion.  Multiple plugin modules may be loaded into one OpenVPN process.
     1154
     1155              For  more  information and examples on how to build OpenVPN plug-in modules, see the README file in the plugin folder of
     1156              the OpenVPN source distribution.
     1157
     1158              If you are using an RPM install of OpenVPN, see /usr/share/openvpn/plugin.  The documentation is in doc and  the  actual
     1159              plugin modules are in lib.
     1160
     1161              Multiple  plugin modules can be cascaded, and modules can be used in tandem with scripts.  The modules will be called by
     1162              OpenVPN in the order that they are declared in the config file.  If both a plugin and script are configured for the same
     1163              callback,  the  script will be called last.  If the return code of the module/script controls an authentication function
     1164              (such as tls-verify, auth-user-pass-verify, or client-connect), then every module and script must return success (0)  in
     1165              order for the connection to be authenticated.
     1166
     1167   Server Mode
     1168       Starting  with  OpenVPN 2.0, a multi-client TCP/UDP server mode is supported, and can be enabled with the --mode server option.
     1169       In server mode, OpenVPN will listen on a single port for incoming client connections.  All client connections  will  be  routed
     1170       through  a  single  tun or tap interface.  This mode is designed for scalability and should be able to support hundreds or even
     1171       thousands of clients on sufficiently fast hardware.  SSL/TLS authentication must be used in this mode.
     1172
     1173       --server network netmask
     1174              A helper directive designed to simplify the configuration of OpenVPN's server mode.  This directive will set up an Open‐
     1175              VPN  server  which will allocate addresses to clients out of the given network/netmask.  The server itself will take the
     1176              ".1" address of the given network for use as the server-side endpoint of the local TUN/TAP interface.
     1177
     1178              For example, --server 10.8.0.0 255.255.255.0 expands as follows:
     1179
     1180                   mode server
     1181                   tls-server
     1182                   push "topology [topology]"
     1183
     1184                   if dev tun AND (topology == net30 OR topology == p2p):
     1185                     ifconfig 10.8.0.1 10.8.0.2
     1186                     if !nopool:
     1187                       ifconfig-pool 10.8.0.4 10.8.0.251
     1188                     route 10.8.0.0 255.255.255.0
     1189                     if client-to-client:
     1190                       push "route 10.8.0.0 255.255.255.0"
     1191                     else if topology == net30:
     1192                       push "route 10.8.0.1"
     1193
     1194                   if dev tap OR (dev tun AND topology == subnet):
     1195                     ifconfig 10.8.0.1 255.255.255.0
     1196                     if !nopool:
     1197                       ifconfig-pool 10.8.0.2 10.8.0.254 255.255.255.0
     1198                     push "route-gateway 10.8.0.1"
     1199
     1200              Don't use --server if you are ethernet bridging.  Use --server-bridge instead.
     1201
     1202       --server-bridge gateway netmask pool-start-IP pool-end-IP
     1203
     1204       --server-bridge ['nogw']
     1205
     1206              A helper directive similar to --server which is designed to simplify the configuration of OpenVPN's server mode in  eth‐
     1207              ernet bridging configurations.
     1208
     1209              If  --server-bridge  is  used without any parameters, it will enable a DHCP-proxy mode, where connecting OpenVPN clients
     1210              will receive an IP address for their TAP adapter from the DHCP server running on the OpenVPN server-side LAN.  Note that
     1211              only  clients  that  support  the binding of a DHCP client with the TAP adapter (such as Windows) can support this mode.
     1212              The optional nogw flag (advanced) indicates that gateway information should not be pushed to the client.
     1213
     1214              To configure ethernet bridging, you must first use your OS's bridging capability to bridge the TAP  interface  with  the
     1215              ethernet  NIC  interface.  For example, on Linux this is done with the brctl tool, and with Windows XP it is done in the
     1216              Network Connections Panel by selecting the ethernet and TAP adapters and right-clicking on "Bridge Connections".
     1217
     1218              Next you you must manually set the IP/netmask on the bridge interface.  The gateway and netmask parameters to  --server-
     1219              bridge  can  be set to either the IP/netmask of the bridge interface, or the IP/netmask of the default gateway/router on
     1220              the bridged subnet.
     1221
     1222              Finally, set aside a IP range in the bridged subnet, denoted by pool-start-IP and pool-end-IP, for OpenVPN  to  allocate
     1223              to connecting clients.
     1224
     1225              For example, server-bridge 10.8.0.4 255.255.255.0 10.8.0.128 10.8.0.254 expands as follows:
     1226
     1227                  mode server
     1228                  tls-server
     1229
     1230                  ifconfig-pool 10.8.0.128 10.8.0.254 255.255.255.0
     1231                  push "route-gateway 10.8.0.4"
     1232
     1233              In another example, --server-bridge (without parameters) expands as follows:
     1234
     1235                  mode server
     1236                  tls-server
     1237
     1238                  push "route-gateway dhcp"
     1239
     1240              Or --server-bridge nogw expands as follows:
     1241
     1242                  mode server
     1243                  tls-server
     1244
     1245       --push option
     1246              Push  a  config file option back to the client for remote execution.  Note that option must be enclosed in double quotes
     1247              ("").  The client must specify --pull in its config file.  The set of options which can be pushed  is  limited  by  both
     1248              feasibility  and  security.   Some options such as those which would execute scripts are banned, since they would effec‐
     1249              tively allow a compromised server to execute arbitrary code on the client.  Other options such as TLS or MTU  parameters
     1250              cannot be pushed because the client needs to know them before the connection to the server can be initiated.
     1251
     1252              This  is  a  partial list of options which can currently be pushed: --route, --route-gateway, --route-delay, --redirect-
     1253              gateway, --ip-win32, --dhcp-option, --inactive, --ping, --ping-exit, --ping-restart, --setenv, --persist-key, --persist-
     1254              tun, --echo, --comp-lzo, --socket-flags, --sndbuf, --rcvbuf
     1255
     1256       --push-reset
     1257              Don't  inherit  the  global  push list for a specific client instance.  Specify this option in a client-specific context
     1258              such as with a --client-config-dir configuration file.  This option will ignore --push options at the global config file
     1259              level.
     1260
     1261       --push-peer-info
     1262              Push additional information about the client to server.  The additional information consists of the following data:
     1263
     1264              IV_VER=<version> -- the client OpenVPN version
     1265
     1266              IV_PLAT=[linux|solaris|openbsd|mac|netbsd|freebsd|win] -- the client OS platform
     1267
     1268              IV_HWADDR=<mac address> -- the MAC address of clients default gateway
     1269
     1270              IV_LZO_STUB=1 -- if client was built with LZO stub capability
     1271
     1272              UV_<name>=<value> -- client environment variables whose names start with "UV_"
     1273
     1274       --disable
     1275              Disable  a  particular client (based on the common name) from connecting.  Don't use this option to disable a client due
     1276              to key or password compromise.  Use a CRL (certificate revocation list) instead (see the --crl-verify option).
     1277
     1278              This option must be associated with a specific client instance, which means that it must be specified either in a client
     1279              instance config file using --client-config-dir or dynamically generated using a --client-connect script.
     1280
     1281       --ifconfig-pool start-IP end-IP [netmask]
     1282              Set  aside a pool of subnets to be dynamically allocated to connecting clients, similar to a DHCP server.  For tun-style
     1283              tunnels, each client will be given a /30 subnet (for interoperability with Windows  clients).   For  tap-style  tunnels,
     1284              individual addresses will be allocated, and the optional netmask parameter will also be pushed to clients.
     1285
     1286
     1287       --ifconfig-pool-persist file [seconds]
     1288              Persist/unpersist  ifconfig-pool  data  to  file,  at seconds intervals (default=600), as well as on program startup and
     1289              shutdown.
     1290
     1291              The goal of this option is to provide a long-term association between clients (denoted by their  common  name)  and  the
     1292              virtual  IP  address  assigned  to them from the ifconfig-pool.  Maintaining a long-term association is good for clients
     1293              because it allows them to effectively use the --persist-tun option.
     1294
     1295              file is a comma-delimited ASCII file, formatted as <Common-Name>,<IP-address>.
     1296
     1297              If seconds = 0, file will be treated as read-only.  This is useful if you would like to treat file  as  a  configuration
     1298              file.
     1299
     1300              Note that the entries in this file are treated by OpenVPN as suggestions only, based on past associations between a com‐
     1301              mon name and IP address.  They do not guarantee that the given common name will always receive the given IP address.  If
     1302              you want guaranteed assignment, use --ifconfig-push
     1303
     1304       --ifconfig-pool-linear
     1305              Modifies  the  --ifconfig-pool directive to allocate individual TUN interface addresses for clients rather than /30 sub‐
     1306              nets.  NOTE:  This option is incompatible with Windows clients.
     1307
     1308              This option is deprecated, and should be replaced with --topology p2p which is functionally equivalent.
     1309
     1310       --ifconfig-push local remote-netmask [alias]
     1311              Push virtual IP endpoints for client tunnel, overriding the --ifconfig-pool dynamic allocation.
     1312
     1313              The parameters local and remote-netmask are set according to the --ifconfig directive which you want to execute  on  the
     1314              client  machine  to  configure the remote end of the tunnel.  Note that the parameters local and remote-netmask are from
     1315              the perspective of the client, not the server.  They may be DNS names rather than IP addresses, in which case they  will
     1316              be resolved on the server at the time of client connection.
     1317
     1318              The  optional alias parameter may be used in cases where NAT causes the client view of its local endpoint to differ from
     1319              the server view.  In this case local/remote-netmask will refer to the server view while alias/remote-netmask will  refer
     1320              to the client view.
     1321
     1322              This option must be associated with a specific client instance, which means that it must be specified either in a client
     1323              instance config file using --client-config-dir or dynamically generated using a --client-connect script.
     1324
     1325              Remember also to include a --route directive in the main OpenVPN config file which encloses local, so  that  the  kernel
     1326              will know to route it to the server's TUN/TAP interface.
     1327
     1328              OpenVPN's internal client IP address selection algorithm works as follows:
     1329
     1330              1 -- Use --client-connect script generated file for static IP (first choice).
     1331              2 -- Use --client-config-dir file for static IP (next choice).
     1332              3 -- Use --ifconfig-pool allocation for dynamic IP (last choice).
     1333
     1334       --iroute network [netmask]
     1335              Generate an internal route to a specific client. The netmask parameter, if omitted, defaults to 255.255.255.255.
     1336
     1337              This  directive  can  be  used  to  route a fixed subnet from the server to a particular client, regardless of where the
     1338              client is connecting from.  Remember that you must also add the route to the system routing table as well  (such  as  by
     1339              using the --route directive).  The reason why two routes are needed is that the --route directive routes the packet from
     1340              the kernel to OpenVPN.  Once in OpenVPN, the --iroute directive routes to the specific client.
     1341
     1342              This option must be specified either in a client instance config file using --client-config-dir or dynamically generated
     1343              using a --client-connect script.
     1344
     1345              The --iroute directive also has an important interaction with --push "route ...".  --iroute essentially defines a subnet
     1346              which is owned by a particular client (we will call this client A).  If you would like other clients to be able to reach
     1347              A's  subnet,  you can use --push "route ..."  together with --client-to-client to effect this.  In order for all clients
     1348              to see A's subnet, OpenVPN must push this route to all clients EXCEPT for A, since the subnet is  already  owned  by  A.
     1349              OpenVPN accomplishes this by not not pushing a route to a client if it matches one of the client's iroutes.
     1350
     1351       --client-to-client
     1352              Because  the  OpenVPN  server  mode  handles multiple clients through a single tun or tap interface, it is effectively a
     1353              router.  The --client-to-client flag tells OpenVPN to internally route client-to-client traffic rather than pushing  all
     1354              client-originating traffic to the TUN/TAP interface.
     1355
     1356              When  this  option  is  used,  each  client will "see" the other clients which are currently connected.  Otherwise, each
     1357              client will only see the server.  Don't use this option if you want to firewall tunnel traffic using custom,  per-client
     1358              rules.
     1359
     1360       --duplicate-cn
     1361              Allow  multiple  clients with the same common name to concurrently connect.  In the absence of this option, OpenVPN will
     1362              disconnect a client instance upon connection of a new client having the same common name.
     1363
     1364       --client-connect cmd
     1365              Run command cmd on client connection.
     1366
     1367              cmd consists of a path to script (or executable program), optionally followed by arguments. The path and  arguments  may
     1368              be single- or double-quoted and/or escaped using a backslash, and should be separated by one or more spaces.
     1369
     1370              The  command  is  passed the common name and IP address of the just-authenticated client as environmental variables (see
     1371              environmental variable section below).  The command is also passed the pathname of a freshly created temporary  file  as
     1372              the  last argument (after any arguments specified in cmd ), to be used by the command to pass dynamically generated con‐
     1373              fig file directives back to OpenVPN.
     1374
     1375              If the script wants to generate a dynamic config file to be applied on the server when the client  connects,  it  should
     1376              write it to the file named by the last argument.
     1377
     1378              See the --client-config-dir option below for options which can be legally used in a dynamically generated config file.
     1379
     1380              Note  that  the  return  value  of  script is significant.  If script returns a non-zero error status, it will cause the
     1381              client to be disconnected.
     1382
     1383       --client-disconnect cmd
     1384              Like --client-connect but called on client instance shutdown.  Will not be called unless the --client-connect script and
     1385              plugins (if defined) were previously called on this instance with successful (0) status returns.
     1386
     1387              The  exception  to this rule is if the --client-disconnect command or plugins are cascaded, and at least one client-con‐
     1388              nect function succeeded, then ALL of the client-disconnect functions for scripts and plugins will be  called  on  client
     1389              instance object deletion, even in cases where some of the related client-connect functions returned an error status.
     1390
     1391              The  --client-disconnect  command  is passed the same pathname as the corresponding --client-connect command as its last
     1392              argument. (after any arguments specified in cmd ).
     1393
     1394       --client-config-dir dir
     1395              Specify a directory dir for custom client config files.  After a connecting client has been authenticated, OpenVPN  will
     1396              look  in this directory for a file having the same name as the client's X509 common name.  If a matching file exists, it
     1397              will be opened and parsed for client-specific configuration options.  If no matching file is found, OpenVPN will instead
     1398              try to open and parse a default file called "DEFAULT", which may be provided but is not required. Note that the configu‐
     1399              ration files must be readable by the OpenVPN process after it has dropped it's root privileges.
     1400
     1401              This file can specify a fixed IP address for a given client using --ifconfig-push, as well as fixed subnets owned by the
     1402              client using --iroute.
     1403
     1404              One  of  the  useful  properties of this option is that it allows client configuration files to be conveniently created,
     1405              edited, or removed while the server is live, without needing to restart the server.
     1406
     1407              The following options are legal in a client-specific  context:  --push,  --push-reset,  --iroute,  --ifconfig-push,  and
     1408              --config.
     1409
     1410       --ccd-exclusive
     1411              Require, as a condition of authentication, that a connecting client has a --client-config-dir file.
     1412
     1413       --tmp-dir dir
     1414              Specify a directory dir for temporary files.  This directory will be used by openvpn processes and script to communicate
     1415              temporary data with openvpn main process. Note that the directory must be writable by the OpenVPN process after  it  has
     1416              dropped it's root privileges.
     1417
     1418              This directory will be used by in the following cases:
     1419
     1420              * --client-connect scripts to dynamically generate client-specific configuration files.
     1421
     1422              *  OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY  plugin hook to return success/failure via auth_control_file when using deferred
     1423              auth method
     1424
     1425              * OPENVPN_PLUGIN_ENABLE_PF plugin hook to pass filtering rules via pf_file
     1426
     1427       --hash-size r v
     1428              Set the size of the real address hash table to r and the virtual address table to v.  By default, both tables are  sized
     1429              at 256 buckets.
     1430
     1431       --bcast-buffers n
     1432              Allocate n buffers for broadcast datagrams (default=256).
     1433
     1434       --tcp-queue-limit n
     1435              Maximum number of output packets queued before TCP (default=64).
     1436
     1437              When  OpenVPN  is tunneling data from a TUN/TAP device to a remote client over a TCP connection, it is possible that the
     1438              TUN/TAP device might produce data at a faster rate than the TCP connection can support.  When the number of output pack‐
     1439              ets queued before sending to the TCP socket reaches this limit for a given client connection, OpenVPN will start to drop
     1440              outgoing packets directed at this client.
     1441
     1442       --tcp-nodelay
     1443              This macro sets the TCP_NODELAY socket flag on the server as well as pushes it to connecting clients.   The  TCP_NODELAY
     1444              flag  disables the Nagle algorithm on TCP sockets causing packets to be transmitted immediately with low latency, rather
     1445              than waiting a short period of time in order to aggregate several packets into  a  larger  containing  packet.   In  VPN
     1446              applications over TCP, TCP_NODELAY is generally a good latency optimization.
     1447
     1448              The macro expands as follows:
     1449
     1450                   if mode server:
     1451                     socket-flags TCP_NODELAY
     1452                     push "socket-flags TCP_NODELAY"
     1453
     1454       --max-clients n
     1455              Limit server to a maximum of n concurrent clients.
     1456
     1457       --max-routes-per-client n
     1458              Allow  a  maximum  of n internal routes per client (default=256).  This is designed to help contain DoS attacks where an
     1459              authenticated client floods the server with packets appearing to come from many unique MAC addresses, forcing the server
     1460              to  deplete  virtual  memory as its internal routing table expands.  This directive can be used in a --client-config-dir
     1461              file or auto-generated by a --client-connect script to override the global value for a particular client.
     1462
     1463              Note that this directive affects OpenVPN's internal routing table, not the kernel routing table.
     1464
     1465       --stale-routes-check n [t]
     1466              Remove routes haven't had activity for n seconds (i.e. the ageing time).
     1467
     1468              This check is ran every t seconds (i.e. check interval).
     1469
     1470              If t is not present it defaults to n
     1471
     1472              This option helps to keep the dynamic routing table small.  See also --max-routes-per-client
     1473
     1474       --connect-freq n sec
     1475              Allow a maximum of n new connections per sec seconds from clients.  This is designed to contain DoS attacks which  flood
     1476              the server with connection requests using certificates which will ultimately fail to authenticate.
     1477
     1478              This is an imperfect solution however, because in a real DoS scenario, legitimate connections might also be refused.
     1479
     1480              For the best protection against DoS attacks in server mode, use --proto udp and --tls-auth.
     1481
     1482       --learn-address cmd
     1483              Run command cmd to validate client virtual addresses or routes.
     1484
     1485              cmd  consists  of a path to script (or executable program), optionally followed by arguments. The path and arguments may
     1486              be single- or double-quoted and/or escaped using a backslash, and should be separated by one or more spaces.
     1487
     1488              Three arguments will be appended to any arguments in cmd as follows:
     1489
     1490              [1] operation -- "add", "update", or "delete" based on whether or not the  address  is  being  added  to,  modified,  or
     1491              deleted from OpenVPN's internal routing table.
     1492              [2]  address  --  The  address being learned or unlearned.  This can be an IPv4 address such as "198.162.10.14", an IPv4
     1493              subnet such as "198.162.10.0/24", or an ethernet MAC address (when --dev tap is being used) such as "00:FF:01:02:03:04".
     1494              [3] common name -- The common name on the certificate associated with the client linked to this address.   Only  present
     1495              for "add" or "update" operations, not "delete".
     1496
     1497              On  "add" or "update" methods, if the script returns a failure code (non-zero), OpenVPN will reject the address and will
     1498              not modify its internal routing table.
     1499
     1500              Normally, the cmd script will use the information provided above to set appropriate firewall entries on the VPN  TUN/TAP
     1501              interface.  Since OpenVPN provides the association between virtual IP or MAC address and the client's authenticated com‐
     1502              mon name, it allows a user-defined script to configure firewall access policies with regard to the  client's  high-level
     1503              common name, rather than the low level client virtual addresses.
     1504
     1505       --auth-user-pass-verify cmd method
     1506              Require the client to provide a username/password (possibly in addition to a client certificate) for authentication.
     1507
     1508              OpenVPN will run command cmd to validate the username/password provided by the client.
     1509
     1510              cmd  consists  of a path to script (or executable program), optionally followed by arguments. The path and arguments may
     1511              be single- or double-quoted and/or escaped using a backslash, and should be separated by one or more spaces.
     1512
     1513              If method is set to "via-env", OpenVPN will call script with the environmental variables username and  password  set  to
     1514              the  username/password  strings  provided  by the client.  Be aware that this method is insecure on some platforms which
     1515              make the environment of a process publicly visible to other unprivileged processes.
     1516
     1517              If method is set to "via-file", OpenVPN will write the username and password to the first two lines of a temporary file.
     1518              The  filename  will  be passed as an argument to script, and the file will be automatically deleted by OpenVPN after the
     1519              script returns.  The location of the temporary file is controlled by the --tmp-dir option, and will default to the  cur‐
     1520              rent  directory  if unspecified.  For security, consider setting --tmp-dir to a volatile storage medium such as /dev/shm
     1521              (if available) to prevent the username/password file from touching the hard drive.
     1522
     1523              The script should examine the username and password, returning a success exit code (0) if  the  client's  authentication
     1524              request is to be accepted, or a failure code (1) to reject the client.
     1525
     1526              This directive is designed to enable a plugin-style interface for extending OpenVPN's authentication capabilities.
     1527
     1528              To  protect  against a client passing a maliciously formed username or password string, the username string must consist
     1529              only of these characters: alphanumeric, underbar ('_'), dash ('-'), dot ('.'), or at ('@').   The  password  string  can
     1530              consist  of  any  printable  characters  except for CR or LF.  Any illegal characters in either the username or password
     1531              string will be converted to underbar ('_').
     1532
     1533              Care must be taken by any user-defined scripts to avoid creating a security vulnerability in the way that these  strings
     1534              are handled.  Never use these strings in such a way that they might be escaped or evaluated by a shell interpreter.
     1535
     1536              For a sample script that performs PAM authentication, see sample-scripts/auth-pam.pl in the OpenVPN source distribution.
     1537
     1538       --opt-verify
     1539              Clients that connect with options that are incompatible with those of the server will be disconnected.
     1540
     1541              Options  that  will  be compared for compatibility include dev-type, link-mtu, tun-mtu, proto, tun-ipv6, ifconfig, comp-
     1542              lzo, fragment, keydir, cipher, auth, keysize, secret, no-replay,  no-iv,  tls-auth,  key-method,  tls-server,  and  tls-
     1543              client.
     1544
     1545              This option requires that --disable-occ NOT be used.
     1546
     1547       --auth-user-pass-optional
     1548              Allow  connections by clients that do not specify a username/password.  Normally, when --auth-user-pass-verify or --man‐
     1549              agement-client-auth is specified (or an authentication plugin module), the OpenVPN server daemon will require connecting
     1550              clients  to  specify  a  username  and  password.   This  option  makes the submission of a username/password by clients
     1551              optional, passing the responsibility to the user-defined authentication module/script to accept or deny the client based
     1552              on  other  factors  (such as the setting of X509 certificate fields).  When this option is used, and a connecting client
     1553              does not submit a username/password, the user-defined authentication module/script will see the username and password as
     1554              being  set to empty strings ("").  The authentication module/script MUST have logic to detect this condition and respond
     1555              accordingly.
     1556
     1557       --client-cert-not-required
     1558              Don't require client certificate, client will authenticate using username/password  only.   Be  aware  that  using  this
     1559              directive is less secure than requiring certificates from all clients.
     1560
     1561              If you use this directive, the entire responsibility of authentication will rest on your --auth-user-pass-verify script,
     1562              so keep in mind that bugs in your script could potentially compromise the security of your VPN.
     1563
     1564              If you don't use this directive, but you also specify an --auth-user-pass-verify script, then OpenVPN will perform  dou‐
     1565              ble  authentication.  The client certificate verification AND the --auth-user-pass-verify script will need to succeed in
     1566              order for a client to be authenticated and accepted onto the VPN.
     1567
     1568       --username-as-common-name
     1569              For --auth-user-pass-verify authentication, use the authenticated username as the common name, rather  than  the  common
     1570              name from the client cert.
     1571
     1572       --port-share host port [dir]
     1573              When  run  in  TCP  server  mode,  share the OpenVPN port with another application, such as an HTTPS server.  If OpenVPN
     1574              senses a connection to its port which is using a non-OpenVPN protocol, it will proxy the connection  to  the  server  at
     1575              host:port.   Currently  only  designed  to  work with HTTP/HTTPS, though it would be theoretically possible to extend to
     1576              other protocols such as ssh.
     1577
     1578              dir specifies an optional directory where a temporary file with name N containing content C will be  dynamically  gener‐
     1579              ated  for  each  proxy connection, where N is the source IP:port of the client connection and C is the source IP:port of
     1580              the connection to the proxy receiver.  This directory can be used as a dictionary by the proxy receiver to determine the
     1581              origin of the connection.  Each generated file will be automatically deleted when the proxied connection is torn down.
     1582
     1583              Not implemented on Windows.
     1584
     1585   Client Mode
     1586       Use  client  mode when connecting to an OpenVPN server which has --server, --server-bridge, or --mode server in it's configura‐
     1587       tion.
     1588
     1589       --client
     1590              A helper directive designed to simplify the configuration of OpenVPN's client mode.  This directive is equivalent to:
     1591
     1592                   pull
     1593                   tls-client
     1594
     1595       --pull This option must be used on a client which is connecting to a multi-client server.  It  indicates  to  OpenVPN  that  it
     1596              should  accept  options pushed by the server, provided they are part of the legal set of pushable options (note that the
     1597              --pull option is implied by --client ).
     1598
     1599              In particular, --pull allows the server to push routes to the client, so you should not use --pull or --client in situa‐
     1600              tions where you don't trust the server to have control over the client's routing table.
     1601
     1602       --auth-user-pass [up]
     1603              Authenticate  with  server using username/password.  up is a file containing username/password on 2 lines (Note: OpenVPN
     1604              will only read passwords from a file if it has been built with the --enable-password-save configure option, or  on  Win‐
     1605              dows by defining ENABLE_PASSWORD_SAVE in win/settings.in).
     1606
     1607              If up is omitted, username/password will be prompted from the console.
     1608
     1609              The  server configuration must specify an --auth-user-pass-verify script to verify the username/password provided by the
     1610              client.
     1611
     1612       --auth-retry type
     1613              Controls how OpenVPN responds  to  username/password  verification  errors  such  as  the  client-side  response  to  an
     1614              AUTH_FAILED message from the server or verification failure of the private key password.
     1615
     1616              Normally  used  to prevent auth errors from being fatal on the client side, and to permit username/password requeries in
     1617              case of error.
     1618
     1619              An AUTH_FAILED message is generated by the server if the client fails --auth-user-pass authentication, or if the server-
     1620              side --client-connect script returns an error status when the client tries to connect.
     1621
     1622              type can be one of:
     1623
     1624              none -- Client will exit with a fatal error (this is the default).
     1625              nointeract  --  Client will retry the connection without requerying for an --auth-user-pass username/password.  Use this
     1626              option for unattended clients.
     1627              interact -- Client will requery for an --auth-user-pass username/password and/or private key password before  attempting
     1628              a reconnection.
     1629
     1630              Note that while this option cannot be pushed, it can be controlled from the management interface.
     1631
     1632       --static-challenge t e
     1633              Enable static challenge/response protocol using challenge text t, with echo flag given by e (0|1).
     1634
     1635              The echo flag indicates whether or not the user's response to the challenge should be echoed.
     1636
     1637              See management-notes.txt in the OpenVPN distribution for a description of the OpenVPN challenge/response protocol.
     1638
     1639       --server-poll-timeout n
     1640              when  polling possible remote servers to connect to in a round-robin fashion, spend no more than n seconds waiting for a
     1641              response before trying the next server.
     1642
     1643       --explicit-exit-notify [n]
     1644              In UDP client mode or point-to-point mode, send server/peer an exit notification  if  tunnel  is  restarted  or  OpenVPN
     1645              process  is  exited.   In client mode, on exit/restart, this option will tell the server to immediately close its client
     1646              instance object rather than waiting for a timeout.  The n parameter (default=1) controls the maximum number of  attempts
     1647              that  the  client will try to resend the exit notification message.  OpenVPN will not send any exit notifications unless
     1648              this option is enabled.
     1649
     1650   Data Channel Encryption Options:
     1651       These options are meaningful for both Static & TLS-negotiated key modes (must be compatible between peers).
     1652
     1653       --secret file [direction]
     1654              Enable Static Key encryption mode (non-TLS).  Use pre-shared secret file which was generated with --genkey.
     1655
     1656              The optional direction parameter enables the use of 4 distinct keys (HMAC-send,  cipher-encrypt,  HMAC-receive,  cipher-
     1657              decrypt),  so that each data flow direction has a different set of HMAC and cipher keys.  This has a number of desirable
     1658              security properties including eliminating certain kinds of DoS and message replay attacks.
     1659
     1660              When the direction parameter is omitted, 2 keys are used bidirectionally,  one  for  HMAC  and  the  other  for  encryp‐
     1661              tion/decryption.
     1662
     1663              The  direction  parameter  should always be complementary on either side of the connection, i.e. one side should use "0"
     1664              and the other should use "1", or both sides should omit it altogether.
     1665
     1666              The direction parameter requires that file contains a 2048 bit key.  While pre-1.5 versions of OpenVPN generate 1024 bit
     1667              key files, any version of OpenVPN which supports the direction parameter, will also support 2048 bit key file generation
     1668              using the --genkey option.
     1669
     1670              Static key encryption mode has certain advantages, the primary being ease of configuration.
     1671
     1672              There are no certificates or certificate authorities or complicated negotiation  handshakes  and  protocols.   The  only
     1673              requirement  is  that  you  have  a pre-existing secure channel with your peer (such as ssh ) to initially copy the key.
     1674              This requirement, along with the fact that your key never changes unless you manually generate a new one, makes it some‐
     1675              what  less  secure  than  TLS  mode  (see  below).   If  an attacker manages to steal your key, everything that was ever
     1676              encrypted with it is compromised.  Contrast that to the perfect forward secrecy features of TLS mode (using Diffie Hell‐
     1677              man  key  exchange),  where even if an attacker was able to steal your private key, he would gain no information to help
     1678              him decrypt past sessions.
     1679
     1680              Another advantageous aspect of Static Key encryption mode is that it is a handshake-free protocol  without  any  distin‐
     1681              guishing  signature  or feature (such as a header or protocol handshake sequence) that would mark the ciphertext packets
     1682              as being generated by OpenVPN.  Anyone eavesdropping on the wire would see nothing but random-looking data.
     1683
     1684       --auth alg
     1685              Authenticate packets with HMAC using message digest algorithm alg.  (The default is SHA1 ).  HMAC  is  a  commonly  used
     1686              message authentication algorithm (MAC) that uses a data string, a secure hash algorithm, and a key, to produce a digital
     1687              signature.
     1688
     1689              OpenVPN's usage of HMAC is to first encrypt a packet, then HMAC the resulting ciphertext.
     1690
     1691              In static-key encryption mode, the HMAC key is included in the key file generated by --genkey.  In TLS  mode,  the  HMAC
     1692              key  is dynamically generated and shared between peers via the TLS control channel.  If OpenVPN receives a packet with a
     1693              bad HMAC it will drop the packet.  HMAC usually adds 16 or 20 bytes per packet.  Set alg=none to disable authentication.
     1694
     1695              For more information on HMAC see http://www.cs.ucsd.edu/users/mihir/papers/hmac.html
     1696
     1697       --cipher alg
     1698              Encrypt packets with cipher algorithm alg.  The default is BF-CBC, an abbreviation for Blowfish in Cipher Block Chaining
     1699              mode.   Blowfish  has  the advantages of being fast, very secure, and allowing key sizes of up to 448 bits.  Blowfish is
     1700              designed to be used in situations where keys are changed infrequently.
     1701
     1702              For more information on blowfish, see http://www.counterpane.com/blowfish.html
     1703
     1704              To see other ciphers that are available with OpenVPN, use the --show-ciphers option.
     1705
     1706              OpenVPN supports the CBC, CFB, and OFB cipher modes, however CBC is recommended and CFB and  OFB  should  be  considered
     1707              advanced modes.
     1708
     1709              Set alg=none to disable encryption.
     1710
     1711       --keysize n
     1712              Size  of cipher key in bits (optional).  If unspecified, defaults to cipher-specific default.  The --show-ciphers option
     1713              (see below) shows all available OpenSSL ciphers, their default key sizes, and whether the key size can be changed.   Use
     1714              care  in  changing  a cipher's default key size.  Many ciphers have not been extensively cryptanalyzed with non-standard
     1715              key lengths, and a larger key may offer no real guarantee of greater security, or may even reduce security.
     1716
     1717       --prng alg [nsl]
     1718              (Advanced) For PRNG (Pseudo-random number generator), use digest algorithm alg (default=sha1), and set nsl  (default=16)
     1719              to the size in bytes of the nonce secret length (between 16 and 64).
     1720
     1721              Set alg=none to disable the PRNG and use the OpenSSL RAND_bytes function instead for all of OpenVPN's pseudo-random num‐
     1722              ber needs.
     1723
     1724       --engine [engine-name]
     1725              Enable OpenSSL hardware-based crypto engine functionality.
     1726
     1727              If engine-name is specified, use a specific crypto engine.  Use the --show-engines standalone option to list the  crypto
     1728              engines which are supported by OpenSSL.
     1729
     1730       --no-replay
     1731              (Advanced) Disable OpenVPN's protection against replay attacks.  Don't use this option unless you are prepared to make a
     1732              tradeoff of greater efficiency in exchange for less security.
     1733
     1734              OpenVPN provides datagram replay protection by default.
     1735
     1736              Replay protection is accomplished by tagging each outgoing datagram with an identifier that is guaranteed to  be  unique
     1737              for  the  key  being used.  The peer that receives the datagram will check for the uniqueness of the identifier.  If the
     1738              identifier was already received in a previous datagram, OpenVPN will drop the packet.  Replay protection is important to
     1739              defeat  attacks such as a SYN flood attack, where the attacker listens in the wire, intercepts a TCP SYN packet (identi‐
     1740              fying it by the context in which it occurs in relation to other packets), then floods the receiving peer with copies  of
     1741              this packet.
     1742
     1743              OpenVPN's  replay  protection  is  implemented in slightly different ways, depending on the key management mode you have
     1744              selected.
     1745
     1746              In Static Key mode or when using an CFB or OFB mode cipher, OpenVPN uses a 64 bit unique identifier that combines a time
     1747              stamp with an incrementing sequence number.
     1748
     1749              When  using  TLS  mode for key exchange and a CBC cipher mode, OpenVPN uses only a 32 bit sequence number without a time
     1750              stamp, since OpenVPN can guarantee the uniqueness of this value for each key.  As in IPSec, if the  sequence  number  is
     1751              close to wrapping back to zero, OpenVPN will trigger a new key exchange.
     1752
     1753              To check for replays, OpenVPN uses the sliding window algorithm used by IPSec.
     1754
     1755       --replay-window n [t]
     1756              Use a replay protection sliding-window of size n and a time window of t seconds.
     1757
     1758              By default n is 64 (the IPSec default) and t is 15 seconds.
     1759
     1760              This option is only relevant in UDP mode, i.e.  when either --proto udp is specifed, or no --proto option is specified.
     1761
     1762              When  OpenVPN  tunnels  IP  packets over UDP, there is the possibility that packets might be dropped or delivered out of
     1763              order.  Because OpenVPN, like IPSec, is emulating the physical network layer, it  will  accept  an  out-of-order  packet
     1764              sequence, and will deliver such packets in the same order they were received to the TCP/IP protocol stack, provided they
     1765              satisfy several constraints.
     1766
     1767              (a) The packet cannot be a replay (unless --no-replay is specified, which disables replay protection altogether).
     1768
     1769              (b) If a packet arrives out of order, it will only be accepted if the difference between its  sequence  number  and  the
     1770              highest sequence number received so far is less than n.
     1771
     1772              (c)  If  a  packet arrives out of order, it will only be accepted if it arrives no later than t seconds after any packet
     1773              containing a higher sequence number.
     1774
     1775              If you are using a network link with a large pipeline (meaning that the product of bandwidth and latency is  high),  you
     1776              may want to use a larger value for n.  Satellite links in particular often require this.
     1777
     1778              If  you  run OpenVPN at --verb 4, you will see the message "Replay-window backtrack occurred [x]" every time the maximum
     1779              sequence number backtrack seen thus far increases.  This can be used to calibrate n.
     1780
     1781              There is some controversy on the appropriate method of handling packet reordering at the security layer.
     1782
     1783              Namely, to what extent should the security layer protect the encapsulated protocol from attacks which masquerade as  the
     1784              kinds of normal packet loss and reordering that occur over IP networks?
     1785
     1786              The IPSec and OpenVPN approach is to allow packet reordering within a certain fixed sequence number window.
     1787
     1788              OpenVPN adds to the IPSec model by limiting the window size in time as well as sequence space.
     1789
     1790              OpenVPN  also adds TCP transport as an option (not offered by IPSec) in which case OpenVPN can adopt a very strict atti‐
     1791              tude towards message deletion and reordering:  Don't allow it.  Since TCP guarantees reliability,  any  packet  loss  or
     1792              reordering event can be assumed to be an attack.
     1793
     1794              In this sense, it could be argued that TCP tunnel transport is preferred when tunneling non-IP or UDP application proto‐
     1795              cols which might be vulnerable to a message deletion or reordering attack which  falls  within  the  normal  operational
     1796              parameters of IP networks.
     1797
     1798              So  I  would  make the statement that one should never tunnel a non-IP protocol or UDP application protocol over UDP, if
     1799              the protocol might be vulnerable to a message deletion or reordering attack  that  falls  within  the  normal  operating
     1800              parameters of what is to be expected from the physical IP layer.  The problem is easily fixed by simply using TCP as the
     1801              VPN transport layer.
     1802
     1803       --mute-replay-warnings
     1804              Silence the output of replay warnings, which are a common false alarm on WiFi networks.  This option preserves the secu‐
     1805              rity of the replay protection code without the verbosity associated with warnings about duplicate packets.
     1806
     1807       --replay-persist file
     1808              Persist replay-protection state across sessions using file to save and reload the state.
     1809
     1810              This  option  will strengthen protection against replay attacks, especially when you are using OpenVPN in a dynamic con‐
     1811              text (such as with --inetd) when OpenVPN sessions are frequently started and stopped.
     1812
     1813              This option will keep a disk copy of the current replay protection state (i.e. the  most  recent  packet  timestamp  and
     1814              sequence  number  received from the remote peer), so that if an OpenVPN session is stopped and restarted, it will reject
     1815              any replays of packets which were already received by the prior session.
     1816
     1817              This option only makes sense when replay protection is enabled (the default) and you are using either --secret  (shared-
     1818              secret key mode) or TLS mode with --tls-auth.
     1819
     1820       --no-iv
     1821              (Advanced) Disable OpenVPN's use of IV (cipher initialization vector).  Don't use this option unless you are prepared to
     1822              make a tradeoff of greater efficiency in exchange for less security.
     1823
     1824              OpenVPN uses an IV by default, and requires it for CFB and OFB cipher modes (which are  totally  insecure  without  it).
     1825              Using an IV is important for security when multiple messages are being encrypted/decrypted with the same key.
     1826
     1827              IV is implemented differently depending on the cipher mode used.
     1828
     1829              In CBC mode, OpenVPN uses a pseudo-random IV for each packet.
     1830
     1831              In CFB/OFB mode, OpenVPN uses a unique sequence number and time stamp as the IV.  In fact, in CFB/OFB mode, OpenVPN uses
     1832              a datagram space-saving optimization that uses the unique identifier for datagram replay protection as the IV.
     1833
     1834       --use-prediction-resistance
     1835              Enable prediction resistance on PolarSSL's RNG.
     1836
     1837              Enabling prediction resistance causes the RNG to reseed in each call  for  random.  Reseeding  this  often  can  quickly
     1838              deplete the kernel entropy pool.
     1839
     1840              If you need this option, please consider running a daemon that adds entropy to the kernel pool.
     1841
     1842              Note that this option only works with PolarSSL versions greater than 1.1.
     1843
     1844       --test-crypto
     1845              Do  a  self-test of OpenVPN's crypto options by encrypting and decrypting test packets using the data channel encryption
     1846              options specified above.  This option does not require a peer to function, and therefore can be specified without  --dev
     1847              or --remote.
     1848
     1849              The typical usage of --test-crypto would be something like this:
     1850
     1851              openvpn --test-crypto --secret key
     1852
     1853              or
     1854
     1855              openvpn --test-crypto --secret key --verb 9
     1856
     1857              This  option  is  very  useful to test OpenVPN after it has been ported to a new platform, or to isolate problems in the
     1858              compiler, OpenSSL crypto library, or OpenVPN's crypto code.  Since it is a self-test mode, problems with encryption  and
     1859              authentication can be debugged independently of network and tunnel issues.
     1860
     1861   TLS Mode Options:
     1862       TLS  mode is the most powerful crypto mode of OpenVPN in both security and flexibility.  TLS mode works by establishing control
     1863       and data channels which are multiplexed over a single TCP/UDP port.  OpenVPN initiates a TLS session over the  control  channel
     1864       and  uses  it  to exchange cipher and HMAC keys to protect the data channel.  TLS mode uses a robust reliability layer over the
     1865       UDP connection for all control channel communication, while the data channel, over which encrypted tunnel data passes, is  for‐
     1866       warded  without any mediation.  The result is the best of both worlds: a fast data channel that forwards over UDP with only the
     1867       overhead of encrypt, decrypt, and HMAC functions, and a control channel that provides all of  the  security  features  of  TLS,
     1868       including certificate-based authentication and Diffie Hellman forward secrecy.
     1869
     1870       To use TLS mode, each peer that runs OpenVPN should have its own local certificate/key pair ( --cert and --key ), signed by the
     1871       root certificate which is specified in --ca.
     1872
     1873       When two OpenVPN peers connect, each presents its local certificate to the other.  Each peer will then check that  its  partner
     1874       peer presented a certificate which was signed by the master root certificate as specified in --ca.
     1875
     1876       If that check on both peers succeeds, then the TLS negotiation will succeed, both OpenVPN peers will exchange temporary session
     1877       keys, and the tunnel will begin passing data.
     1878
     1879       The OpenVPN distribution contains a set of scripts for managing RSA certificates & keys, located in the easy-rsa subdirectory.
     1880
     1881       The easy-rsa package is also rendered in web form here: http://openvpn.net/easyrsa.html
     1882
     1883       --tls-server
     1884              Enable TLS and assume server role during TLS handshake.  Note that OpenVPN is designed as  a  peer-to-peer  application.
     1885              The designation of client or server is only for the purpose of negotiating the TLS control channel.
     1886
     1887       --tls-client
     1888              Enable TLS and assume client role during TLS handshake.
     1889
     1890       --ca file
     1891              Certificate  authority  (CA) file in .pem format, also referred to as the root certificate.  This file can have multiple
     1892              certificates in .pem format, concatenated together.  You can construct your own certificate  authority  certificate  and
     1893              private key by using a command such as:
     1894
     1895              openssl req -nodes -new -x509 -keyout ca.key -out ca.crt
     1896
     1897              Then edit your openssl.cnf file and edit the certificate variable to point to your new root certificate ca.crt.
     1898
     1899              For  testing  purposes  only,  the OpenVPN distribution includes a sample CA certificate (ca.crt).  Of course you should
     1900              never use the test certificates and test keys distributed with OpenVPN in a production environment, since by  virtue  of
     1901              the fact that they are distributed with OpenVPN, they are totally insecure.
     1902
     1903       --capath dir
     1904              Directory  containing  trusted certificates (CAs and CRLs).  Available with OpenSSL version >= 0.9.7 dev.  Not available
     1905              with PolarSSL.
     1906
     1907       --dh file
     1908              File containing Diffie Hellman parameters in .pem format (required for --tls-server only). Use
     1909
     1910              openssl dhparam -out dh1024.pem 1024
     1911
     1912              to generate your own, or use the existing dh1024.pem file included with the OpenVPN distribution.  Diffie Hellman param‐
     1913              eters may be considered public.
     1914
     1915       --cert file
     1916              Local peer's signed certificate in .pem format -- must be signed by a certificate authority whose certificate is in --ca
     1917              file.  Each peer in an OpenVPN link running in TLS mode should have its own certificate and private key file.  In  addi‐
     1918              tion,  each  certificate  should  have been signed by the key of a certificate authority whose public key resides in the
     1919              --ca certificate authority file.  You can easily make your own certificate authority (see above) or pay money to  use  a
     1920              commercial service such as thawte.com (in which case you will be helping to finance the world's second space tourist :).
     1921              To generate a certificate, you can use a command such as:
     1922
     1923              openssl req -nodes -new -keyout mycert.key -out mycert.csr
     1924
     1925              If your certificate authority private key lives on another machine, copy the certificate signing request (mycert.csr) to
     1926              this  other  machine (this can be done over an insecure channel such as email).  Now sign the certificate with a command
     1927              such as:
     1928
     1929              openssl ca -out mycert.crt -in mycert.csr
     1930
     1931              Now copy the certificate (mycert.crt) back to the peer which initially generated the .csr file (this can be over a  pub‐
     1932              lic  medium).   Note that the openssl ca command reads the location of the certificate authority key from its configura‐
     1933              tion file such as /usr/share/ssl/openssl.cnf -- note also that for certificate authority functions, you must set up  the
     1934              files index.txt (may be empty) and serial (initialize to 01 ).
     1935
     1936       --extra-certs file
     1937              Specify a file containing one or more PEM certs (concatenated together) that complete the local certificate chain.
     1938
     1939              This option is useful for "split" CAs, where the CA for server certs is different than the CA for client certs.  Putting
     1940              certs in this file allows them to be used to complete the local certificate chain without trusting them  to  verify  the
     1941              peer-submitted certificate, as would be the case if the certs were placed in the ca file.
     1942
     1943       --key file
     1944              Local peer's private key in .pem format.  Use the private key which was generated when you built your peer's certificate
     1945              (see -cert file above).
     1946
     1947       --pkcs12 file
     1948              Specify a PKCS #12 file containing local private key, local certificate, and root CA certificate.  This  option  can  be
     1949              used instead of --ca, --cert, and --key.  Not available with PolarSSL.
     1950
     1951       --verify-hash hash
     1952              Specify  SHA1  fingerprint for level-1 cert.  The level-1 cert is the CA (or intermediate cert) that signs the leaf cer‐
     1953              tificate, and is one removed from the leaf certificate in the direction of the root.  When accepting a connection from a
     1954              peer,  the  level-1  cert  fingerprint  must  match  hash  or  certificate verification will fail.  Hash is specified as
     1955              XX:XX:...  For example: AD:B0:95:D8:09:C8:36:45:12:A9:89:C8:90:09:CB:13:72:A6:AD:16
     1956
     1957       --pkcs11-cert-private [0|1]...
     1958              Set if access to certificate object should be performed after login.  Every provider has its own setting.
     1959
     1960       --pkcs11-id name
     1961              Specify the serialized certificate id to be used. The id can be gotten by the standalone --show-pkcs11-ids option.
     1962
     1963       --pkcs11-id-management
     1964              Acquire PKCS#11 id from management interface. In this case a NEED-STR  'pkcs11-id-request'  real-time  message  will  be
     1965              triggered,  application  may use pkcs11-id-count command to retrieve available number of certificates, and pkcs11-id-get
     1966              command to retrieve certificate id and certificate body.
     1967
     1968       --pkcs11-pin-cache seconds
     1969              Specify how many seconds the PIN can be cached, the default is until the token is removed.
     1970
     1971       --pkcs11-protected-authentication [0|1]...
     1972              Use PKCS#11 protected authentication path, useful for biometric and external keypad devices.  Every provider has its own
     1973              setting.
     1974
     1975       --pkcs11-providers provider...
     1976              Specify  a  RSA  Security  Inc. PKCS #11 Cryptographic Token Interface (Cryptoki) providers to load.  This option can be
     1977              used instead of --cert, --key, and --pkcs12.
     1978
     1979       --pkcs11-private-mode mode...
     1980              Specify which method to use in order to perform private key operations.  A different mode  can  be  specified  for  each
     1981              provider.  Mode is encoded as hex number, and can be a mask one of the following:
     1982
     1983              0 (default) -- Try to determind automatically.
     1984              1 -- Use sign.
     1985              2 -- Use sign recover.
     1986              4 -- Use decrypt.
     1987              8 -- Use unwrap.
     1988
     1989       --cryptoapicert select-string
     1990              Load the certificate and private key from the Windows Certificate System Store (Windows/OpenSSL Only).
     1991
     1992              Use this option instead of --cert and --key.
     1993
     1994              This  makes  it  possible to use any smart card, supported by Windows, but also any kind of certificate, residing in the
     1995              Cert Store, where you have access to the private key.  This option has been tested with  a  couple  of  different  smart
     1996              cards  (GemSAFE,  Cryptoflex, and Swedish Post Office eID) on the client side, and also an imported PKCS12 software cer‐
     1997              tificate on the server side.
     1998
     1999              To select a certificate, based on a substring search in the certificate's subject:
     2000
     2001              cryptoapicert "SUBJ:Peter Runestig"
     2002
     2003              To select a certificate, based on certificate's thumbprint:
     2004
     2005              cryptoapicert "THUMB:f6 49 24 41 01 b4 ..."
     2006
     2007              The thumbprint hex string can easily be copy-and-pasted from the Windows Certificate Store GUI.
     2008
     2009
     2010       --key-method m
     2011              Use data channel key negotiation method m.  The key method must match on both sides of the connection.
     2012
     2013              After OpenVPN negotiates a TLS session, a new set of keys for protecting  the  tunnel  data  channel  is  generated  and
     2014              exchanged over the TLS session.
     2015
     2016              In  method 1 (the default for OpenVPN 1.x), both sides generate random encrypt and HMAC-send keys which are forwarded to
     2017              the other host over the TLS channel.
     2018
     2019              In method 2, (the default for OpenVPN 2.0) the client generates a random key.  Both client and server also generate some
     2020              random  seed  material.   All key source material is exchanged over the TLS channel. The actual keys are generated using
     2021              the TLS PRF function, taking source entropy from both client and server.  Method 2 is designed to closely  parallel  the
     2022              key generation process used by TLS 1.0.
     2023
     2024              Note that in TLS mode, two separate levels of keying occur:
     2025
     2026              (1)  The  TLS connection is initially negotiated, with both sides of the connection producing certificates and verifying
     2027              the certificate (or other authentication info provided) of the other side.  The --key-method parameter has no effect  on
     2028              this process.
     2029
     2030              (2)  After the TLS connection is established, the tunnel session keys are separately negotiated over the existing secure
     2031              TLS channel.  Here, --key-method determines the derivation of the tunnel session keys.
     2032
     2033       --tls-cipher l
     2034              A list l of allowable TLS ciphers delimited by a colon (":").  If you require a high level of security, you may want  to
     2035              set  this parameter manually, to prevent a version rollback attack where a man-in-the-middle attacker tries to force two
     2036              peers to negotiate to the lowest level of security they both support.  Use --show-tls to see a  list  of  supported  TLS
     2037              ciphers.
     2038
     2039       --tls-timeout n
     2040              Packet  retransmit  timeout  on TLS control channel if no acknowledgment from remote within n seconds (default=2).  When
     2041              OpenVPN sends a control packet to its peer, it will expect to receive an acknowledgement within n  seconds  or  it  will
     2042              retransmit  the  packet,  subject  to  a TCP-like exponential backoff algorithm.  This parameter only applies to control
     2043              channel packets.  Data channel packets (which carry  encrypted  tunnel  data)  are  never  acknowledged,  sequenced,  or
     2044              retransmitted by OpenVPN because the higher level network protocols running on top of the tunnel such as TCP expect this
     2045              role to be left to them.
     2046
     2047       --reneg-bytes n
     2048              Renegotiate data channel key after n bytes sent or received (disabled by default).  OpenVPN allows the lifetime of a key
     2049              to be expressed as a number of bytes encrypted/decrypted, a number of packets, or a number of seconds.  A key renegotia‐
     2050              tion will be forced if any of these three criteria are met by either peer.
     2051
     2052       --reneg-pkts n
     2053              Renegotiate data channel key after n packets sent and received (disabled by default).
     2054
     2055       --reneg-sec n
     2056              Renegotiate data channel key after n seconds (default=3600).
     2057
     2058              When using dual-factor authentication, note that this default value may cause the end user to be challenged to  reautho‐
     2059              rize once per hour.
     2060
     2061              Also,  keep  in mind that this option can be used on both the client and server, and whichever uses the lower value will
     2062              be the one to trigger the renegotiation.  A common mistake is to set --reneg-sec to a higher value on either the  client
     2063              or  server,  while  the  other side of the connection is still using the default value of 3600 seconds, meaning that the
     2064              renegotiation will still occur once per 3600 seconds.  The solution is to increase --reneg-sec on both  the  client  and
     2065              server, or set it to 0 on one side of the connection (to disable), and to your chosen value on the other side.
     2066
     2067       --hand-window n
     2068              Handshake  Window  --  the  TLS-based  key  exchange  must finalize within n seconds of handshake initiation by any peer
     2069              (default = 60 seconds).  If the handshake fails we will attempt to reset our connection with our  peer  and  try  again.
     2070              Even  in  the  event of handshake failure we will still use our expiring key for up to --tran-window seconds to maintain
     2071              continuity of transmission of tunnel data.
     2072
     2073       --tran-window n
     2074              Transition window -- our old key can live this many seconds after a new a key renegotiation begins (default = 3600  sec‐
     2075              onds).   This  feature  allows for a graceful transition from old to new key, and removes the key renegotiation sequence
     2076              from the critical path of tunnel data forwarding.
     2077
     2078       --single-session
     2079              After initially connecting to a remote peer, disallow any new connections.  Using this option means that a  remote  peer
     2080              cannot connect, disconnect, and then reconnect.
     2081
     2082              If the daemon is reset by a signal or --ping-restart, it will allow one new connection.
     2083
     2084              --single-session  can be used with --ping-exit or --inactive to create a single dynamic session that will exit when fin‐
     2085              ished.
     2086
     2087       --tls-exit
     2088              Exit on TLS negotiation failure.
     2089
     2090       --tls-auth file [direction]
     2091              Add an additional layer of HMAC authentication on top of the TLS control channel to protect against DoS attacks.
     2092
     2093              In a nutshell, --tls-auth enables a kind of "HMAC firewall" on OpenVPN's TCP/UDP port, where TLS control channel packets
     2094              bearing an incorrect HMAC signature can be dropped immediately without response.
     2095
     2096              file (required) is a key file which can be in one of two formats:
     2097
     2098              (1) An OpenVPN static key file generated by --genkey (required if direction parameter is used).
     2099
     2100              (2) A freeform passphrase file.  In this case the HMAC key will be derived by taking a secure hash of this file, similar
     2101              to the md5sum(1) or sha1sum(1) commands.
     2102
     2103              OpenVPN will first try format (1), and if the file fails to parse as a static key file, format (2) will be used.
     2104
     2105              See the --secret option for more information on the optional direction parameter.
     2106
     2107              --tls-auth is recommended when you are running OpenVPN in a mode where it is listening for packets from any IP  address,
     2108              such as when --remote is not specified, or --remote is specified with --float.
     2109
     2110              The  rationale for this feature is as follows.  TLS requires a multi-packet exchange before it is able to authenticate a
     2111              peer.  During this time before authentication, OpenVPN is allocating resources (memory and CPU) to this potential  peer.
     2112              The  potential  peer  is also exposing many parts of OpenVPN and the OpenSSL library to the packets it is sending.  Most
     2113              successful network attacks today seek to either exploit bugs in programs (such as buffer overflow attacks)  or  force  a
     2114              program to consume so many resources that it becomes unusable.  Of course the first line of defense is always to produce
     2115              clean, well-audited code.  OpenVPN has been written with buffer overflow attack prevention as a top  priority.   But  as
     2116              history  has shown, many of the most widely used network applications have, from time to time, fallen to buffer overflow
     2117              attacks.
     2118
     2119              So as a second line of defense, OpenVPN offers this special layer of authentication on top of the TLS control channel so
     2120              that  every  packet  on the control channel is authenticated by an HMAC signature and a unique ID for replay protection.
     2121              This signature will also help protect against DoS (Denial of Service) attacks.  An important rule of thumb  in  reducing
     2122              vulnerability  to  DoS attacks is to minimize the amount of resources a potential, but as yet unauthenticated, client is
     2123              able to consume.
     2124
     2125              --tls-auth does this by signing every TLS control channel packet with an HMAC signature,  including  packets  which  are
     2126              sent  before  the  TLS  level has had a chance to authenticate the peer.  The result is that packets without the correct
     2127              signature can be dropped immediately upon reception, before they have a chance to consume  additional  system  resources
     2128              such  as by initiating a TLS handshake.  --tls-auth can be strengthened by adding the --replay-persist option which will
     2129              keep OpenVPN's replay protection state in a file so that it is not lost across restarts.
     2130
     2131              It should be emphasized that this feature is optional and that the passphrase/key file used with --tls-auth gives a peer
     2132              nothing more than the power to initiate a TLS handshake.  It is not used to encrypt or authenticate any tunnel data.
     2133
     2134       --askpass [file]
     2135              Get certificate password from console or file before we daemonize.
     2136
     2137              For  the extremely security conscious, it is possible to protect your private key with a password.  Of course this means
     2138              that every time the OpenVPN daemon is started you must be there to type the password.  The --askpass option  allows  you
     2139              to  start  OpenVPN  from the command line.  It will query you for a password before it daemonizes.  To protect a private
     2140              key with a password you should omit the -nodes option when you use the openssl command line tool to manage  certificates
     2141              and private keys.
     2142
     2143              If  file is specified, read the password from the first line of file.  Keep in mind that storing your password in a file
     2144              to a certain extent invalidates the extra security provided by using an encrypted key  (Note:  OpenVPN  will  only  read
     2145              passwords  from  a file if it has been built with the --enable-password-save configure option, or on Windows by defining
     2146              ENABLE_PASSWORD_SAVE in win/settings.in).
     2147
     2148       --auth-nocache
     2149              Don't cache --askpass or --auth-user-pass username/passwords in virtual memory.
     2150
     2151              If specified, this directive will cause OpenVPN to immediately forget username/password inputs after they are used.   As
     2152              a  result, when OpenVPN needs a username/password, it will prompt for input from stdin, which may be multiple times dur‐
     2153              ing the duration of an OpenVPN session.
     2154
     2155              This directive does not affect the --http-proxy username/password.  It is always cached.
     2156
     2157       --tls-verify cmd
     2158              Run command cmd to verify the X509 name of a pending TLS connection that has otherwise passed all other tests of  certi‐
     2159              fication (except for revocation via --crl-verify directive; the revocation test occurs after the --tls-verify test).
     2160
     2161              cmd should return 0 to allow the TLS handshake to proceed, or 1 to fail.
     2162
     2163              cmd  consists  of a path to script (or executable program), optionally followed by arguments. The path and arguments may
     2164              be single- or double-quoted and/or escaped using a backslash, and should be separated by one or more spaces.
     2165
     2166              When cmd is executed two arguments are appended after any arguments specified in cmd , as follows:
     2167
     2168              cmd certificate_depth subject
     2169
     2170              These arguments are, respectively, the current certificate depth and the X509 common name (cn) of the peer.
     2171
     2172              This feature is useful if the peer you want to trust has a certificate which was signed by a certificate  authority  who
     2173              also  signed  many  other  certificates,  where you don't necessarily want to trust all of them, but rather be selective
     2174              about which peer certificate you will accept.  This feature allows you to write a script which will test the  X509  name
     2175              on  a  certificate and decide whether or not it should be accepted.  For a simple perl script which will test the common
     2176              name field on the certificate, see the file verify-cn in the OpenVPN distribution.
     2177
     2178              See the "Environmental Variables" section below for additional parameters passed as environmental variables.
     2179
     2180       --tls-export-cert directory
     2181              Store the certificates the clients uses upon connection to this directory. This will  be  done  before  --tls-verify  is
     2182              called.   The  certificates  will use a temporary name and will be deleted when the tls-verify script returns.  The file
     2183              name used for the certificate is available via the peer_cert environment variable.
     2184
     2185       --x509-username-field fieldname
     2186              Field in x509 certificate subject to be used as username (default=CN).  Fieldname will be  uppercased  before  matching.
     2187              When this option is used, the --tls-remote option will match against the chosen fieldname instead of the CN.
     2188
     2189       --tls-remote name
     2190              Accept  connections  only  from  a host with X509 name or common name equal to name.  The remote host must also pass all
     2191              other tests of verification.
     2192
     2193              NOTE: Because tls-remote may test against a common name prefix, only use this option when you are using OpenVPN  with  a
     2194              custom  CA  certificate that is under your control.  Never use this option when your client certificates are signed by a
     2195              third party, such as a commercial web CA.
     2196
     2197              Name can also be a common name prefix, for example if you want a  client  to  only  accept  connections  to  "Server-1",
     2198              "Server-2", etc., you can simply use --tls-remote Server
     2199
     2200              Using  a common name prefix is a useful alternative to managing a CRL (Certificate Revocation List) on the client, since
     2201              it allows the client to refuse all certificates except for those associated with designated servers.
     2202
     2203              --tls-remote is a useful replacement for the --tls-verify option to verify the remote host, because  --tls-remote  works
     2204              in a --chroot environment too.
     2205
     2206       --x509-track attribute
     2207              Save  peer  X509 attribute value in environment for use by plugins and management interface.  Prepend a '+' to attribute
     2208              to save values from full cert chain.  Values will be encoded as X509_<depth>_<attribute>=<value>.  Multiple --x509-track
     2209              options can be defined to track multiple attributes.  Not available with PolarSSL.
     2210
     2211       --ns-cert-type client|server
     2212              Require that peer certificate was signed with an explicit nsCertType designation of "client" or "server".
     2213
     2214              This is a useful security option for clients, to ensure that the host they connect with is a designated server.
     2215
     2216              See  the  easy-rsa/build-key-server script for an example of how to generate a certificate with the nsCertType field set
     2217              to "server".
     2218
     2219              If the server certificate's nsCertType field is set to "server", then the clients can verify  this  with  --ns-cert-type
     2220              server.
     2221
     2222              This  is  an  important  security  precaution  to  protect against a man-in-the-middle attack where an authorized client
     2223              attempts to connect to another client by impersonating the server.  The attack is easily  prevented  by  having  clients
     2224              verify the server certificate using any one of --ns-cert-type, --tls-remote, or --tls-verify.
     2225
     2226       --remote-cert-ku v...
     2227              Require that peer certificate was signed with an explicit key usage.
     2228
     2229              This is a useful security option for clients, to ensure that the host they connect to is a designated server.
     2230
     2231              The key usage should be encoded in hex, more than one key usage can be specified.
     2232
     2233       --remote-cert-eku oid
     2234              Require that peer certificate was signed with an explicit extended key usage.
     2235
     2236              This is a useful security option for clients, to ensure that the host they connect to is a designated server.
     2237
     2238              The extended key usage should be encoded in oid notation, or OpenSSL symbolic representation.
     2239
     2240       --remote-cert-tls client|server
     2241              Require that peer certificate was signed with an explicit key usage and extended key usage based on RFC3280 TLS rules.
     2242
     2243              This is a useful security option for clients, to ensure that the host they connect to is a designated server.
     2244
     2245              The --remote-cert-tls client option is equivalent to --remote-cert-ku 80 08 88 --remote-cert-eku "TLS Web Client Authen‐
     2246              tication"
     2247
     2248              The key usage is digitalSignature and/or keyAgreement.
     2249
     2250              The --remote-cert-tls server option is equivalent to --remote-cert-ku a0 88 --remote-cert-eku "TLS Web Server  Authenti‐
     2251              cation"
     2252
     2253              The key usage is digitalSignature and ( keyEncipherment or keyAgreement ).
     2254
     2255              This  is  an  important  security  precaution  to  protect against a man-in-the-middle attack where an authorized client
     2256              attempts to connect to another client by impersonating the server.  The attack is easily  prevented  by  having  clients
     2257              verify the server certificate using any one of --remote-cert-tls, --tls-remote, or --tls-verify.
     2258
     2259       --crl-verify crl ['dir']
     2260              Check peer certificate against the file crl in PEM format.
     2261
     2262              A  CRL  (certificate  revocation  list)  is  used when a particular key is compromised but when the overall PKI is still
     2263              intact.
     2264
     2265              Suppose you had a PKI consisting of a CA, root certificate, and a number of client certificates.  Suppose a laptop  com‐
     2266              puter  containing  a client key and certificate was stolen.  By adding the stolen certificate to the CRL file, you could
     2267              reject any connection which attempts to use it, while preserving the overall integrity of the PKI.
     2268
     2269              The only time when it would be necessary to rebuild the entire PKI from scratch would be if  the  root  certificate  key
     2270              itself was compromised.
     2271
     2272              If  the  optional  dir  flag  is  specified,  enable a different mode where crl is a directory containing files named as
     2273              revoked serial numbers (the files may be empty, the contents are never read).  If a client requests a connection,  where
     2274              the  client  certificate  serial  number  (decimal  string)  is  the name of a file present in the directory, it will be
     2275              rejected.
     2276
     2277   SSL Library information:
     2278       --show-ciphers
     2279              (Standalone) Show all cipher algorithms to use with the --cipher option.
     2280
     2281       --show-digests
     2282              (Standalone) Show all message digest algorithms to use with the --auth option.
     2283
     2284       --show-tls
     2285              (Standalone) Show all TLS ciphers (TLS used only as a control channel).  The TLS ciphers will  be  sorted  from  highest
     2286              preference (most secure) to lowest.
     2287
     2288       --show-engines
     2289              (Standalone) Show currently available hardware-based crypto acceleration engines supported by the OpenSSL library.
     2290
     2291   Generate a random key:
     2292       Used only for non-TLS static key encryption mode.
     2293
     2294       --genkey
     2295              (Standalone)  Generate  a random key to be used as a shared secret, for use with the --secret option.  This file must be
     2296              shared with the peer over a pre-existing secure channel such as scp(1)
     2297
     2298       --secret file
     2299              Write key to file.
     2300
     2301   TUN/TAP persistent tunnel config mode:
     2302       Available with linux 2.4.7+.  These options comprise a standalone mode of OpenVPN which can be used to create and  delete  per‐
     2303       sistent tunnels.
     2304
     2305       --mktun
     2306              (Standalone)  Create  a persistent tunnel on platforms which support them such as Linux.  Normally TUN/TAP tunnels exist
     2307              only for the period of time that an application has them open.  This option takes  advantage  of  the  TUN/TAP  driver's
     2308              ability  to  build  persistent  tunnels  that live through multiple instantiations of OpenVPN and die only when they are
     2309              deleted or the machine is rebooted.
     2310
     2311              One of the advantages of persistent tunnels is that they eliminate the need for separate --up and --down scripts to  run
     2312              the  appropriate  ifconfig(8)  and  route(8)  commands.  These commands can be placed in the the same shell script which
     2313              starts or terminates an OpenVPN session.
     2314
     2315              Another advantage is that open connections through the TUN/TAP-based tunnel will  not  be  reset  if  the  OpenVPN  peer
     2316              restarts.   This  can be useful to provide uninterrupted connectivity through the tunnel in the event of a DHCP reset of
     2317              the peer's public IP address (see the --ipchange option above).
     2318
     2319              One disadvantage of persistent tunnels is that it is harder to automatically configure their MTU value  (see  --link-mtu
     2320              and --tun-mtu above).
     2321
     2322              On some platforms such as Windows, TAP-Win32 tunnels are persistent by default.
     2323
     2324       --rmtun
     2325              (Standalone) Remove a persistent tunnel.
     2326
     2327       --dev tunX | tapX
     2328              TUN/TAP device
     2329
     2330       --user user
     2331              Optional user to be owner of this tunnel.
     2332
     2333       --group group
     2334              Optional group to be owner of this tunnel.
     2335
     2336   Windows-Specific Options:
     2337       --win-sys path
     2338              Set  the  Windows  system directory pathname to use when looking for system executables such as route.exe and netsh.exe.
     2339              By default, if this directive is not specified, OpenVPN will use the SystemRoot environment variable.
     2340
     2341              This option have changed behaviour in OpenVPN 2.3.  Earlier you had to define --win-sys env to use the SystemRoot  envi‐
     2342              ronment  variable,  otherwise it defaulted to C:\WINDOWS.  It is not needed to use the env keyword any more, and it will
     2343              just be ignored. A warning is logged when this is found in the configuration file.
     2344
     2345       --ip-win32 method
     2346              When using --ifconfig on Windows, set the TAP-Win32 adapter IP address and netmask using method.  Don't use this  option
     2347              unless you are also using --ifconfig.
     2348
     2349              manual  --  Don't set the IP address or netmask automatically.  Instead output a message to the console telling the user
     2350              to configure the adapter manually and indicating the IP/netmask which OpenVPN expects the adapter to be set to.
     2351
     2352              dynamic [offset] [lease-time] -- Automatically set the IP address and netmask by replying to DHCP query messages  gener‐
     2353              ated  by  the kernel.  This mode is probably the "cleanest" solution for setting the TCP/IP properties since it uses the
     2354              well-known DHCP protocol.  There are, however, two prerequisites for using this mode: (1) The TCP/IP properties for  the
     2355              TAP-Win32  adapter  must be set to "Obtain an IP address automatically," and (2) OpenVPN needs to claim an IP address in
     2356              the subnet for use as the virtual DHCP server address.  By default in --dev tap mode, OpenVPN  will  take  the  normally
     2357              unused first address in the subnet.  For example, if your subnet is 192.168.4.0 netmask 255.255.255.0, then OpenVPN will
     2358              take the IP address 192.168.4.0 to use as the virtual DHCP server address.  In --dev tun mode, OpenVPN  will  cause  the
     2359              DHCP  server  to  masquerade as if it were coming from the remote endpoint.  The optional offset parameter is an integer
     2360              which is > -256 and < 256 and which defaults to 0.  If offset is positive, the DHCP server will  masquerade  as  the  IP
     2361              address at network address + offset.  If offset is negative, the DHCP server will masquerade as the IP address at broad‐
     2362              cast address + offset.  The Windows ipconfig /all command can be used to  show  what  Windows  thinks  the  DHCP  server
     2363              address is.  OpenVPN will "claim" this address, so make sure to use a free address.  Having said that, different OpenVPN
     2364              instantiations, including different ends of the same connection, can share the same virtual DHCP  server  address.   The
     2365              lease-time  parameter  controls  the lease time of the DHCP assignment given to the TAP-Win32 adapter, and is denoted in
     2366              seconds.  Normally a very long lease time is preferred because it prevents routes involving the TAP-Win32  adapter  from
     2367              being lost when the system goes to sleep.  The default lease time is one year.
     2368
     2369              netsh  --  Automatically  set  the  IP  address and netmask using the Windows command-line "netsh" command.  This method
     2370              appears to work correctly on Windows XP but not Windows 2000.
     2371
     2372              ipapi -- Automatically set the IP address and netmask using the Windows IP Helper API.   This  approach  does  not  have
     2373              ideal  semantics,  though  testing  has indicated that it works okay in practice.  If you use this option, it is best to
     2374              leave the TCP/IP properties for the TAP-Win32 adapter in their default state, i.e. "Obtain an IP address automatically."
     2375
     2376              adaptive -- (Default) Try dynamic method initially and fail over to netsh if the DHCP  negotiation  with  the  TAP-Win32
     2377              adapter  does not succeed in 20 seconds.  Such failures have been known to occur when certain third-party firewall pack‐
     2378              ages installed on the client machine block the DHCP negotiation used by the TAP-Win32 adapter.  Note that if  the  netsh
     2379              failover  occurs,  the TAP-Win32 adapter TCP/IP properties will be reset from DHCP to static, and this will cause future
     2380              OpenVPN startups using the adaptive mode to use netsh immediately, rather than trying dynamic first.  To  "unstick"  the
     2381              adaptive mode from using netsh, run OpenVPN at least once using the dynamic mode to restore the TAP-Win32 adapter TCP/IP
     2382              properties to a DHCP configuration.
     2383
     2384       --route-method m
     2385              Which method m to use for adding routes on Windows?
     2386
     2387              adaptive (default) -- Try IP helper API first.  If that fails, fall back to the route.exe shell command.
     2388              ipapi -- Use IP helper API.
     2389              exe -- Call the route.exe shell command.
     2390
     2391       --dhcp-option type [parm]
     2392              Set extended TAP-Win32 TCP/IP properties, must be used with --ip-win32 dynamic or --ip-win32 adaptive.  This option  can
     2393              be  used  to  set  additional  TCP/IP properties on the TAP-Win32 adapter, and is particularly useful for configuring an
     2394              OpenVPN client to access a Samba server across the VPN.
     2395
     2396              DOMAIN name -- Set Connection-specific DNS Suffix.
     2397
     2398              DNS addr -- Set primary domain name server address.  Repeat this option to set secondary DNS server addresses.
     2399
     2400              WINS addr -- Set primary WINS server address (NetBIOS over TCP/IP Name Server).  Repeat this  option  to  set  secondary
     2401              WINS server addresses.
     2402
     2403              NBDD  addr  --  Set primary NBDD server address (NetBIOS over TCP/IP Datagram Distribution Server) Repeat this option to
     2404              set secondary NBDD server addresses.
     2405
     2406              NTP addr -- Set primary NTP server address (Network Time Protocol).  Repeat this option  to  set  secondary  NTP  server
     2407              addresses.
     2408
     2409              NBT  type  --  Set NetBIOS over TCP/IP Node type.  Possible options: 1 = b-node (broadcasts), 2 = p-node (point-to-point
     2410              name queries to a WINS server), 4 = m-node (broadcast then query name server), and 8 = h-node (query name  server,  then
     2411              broadcast).
     2412
     2413              NBS  scope-id  --  Set NetBIOS over TCP/IP Scope. A NetBIOS Scope ID provides an extended naming service for the NetBIOS
     2414              over TCP/IP (Known as NBT) module. The primary purpose of a NetBIOS scope ID is to isolate NetBIOS traffic on  a  single
     2415              network to only those nodes with the same NetBIOS scope ID.  The NetBIOS scope ID is a character string that is appended
     2416              to the NetBIOS name. The NetBIOS scope ID on two hosts must match, or the two hosts will not be able to communicate. The
     2417              NetBIOS  Scope  ID  also  allows computers to use the same computer name, as they have different scope IDs. The Scope ID
     2418              becomes a part of the NetBIOS name, making the name unique.  (This description  of  NetBIOS  scopes  courtesy  of  Neon‐
     2419              Surge@abyss.com)
     2420
     2421              DISABLE-NBT -- Disable Netbios-over-TCP/IP.
     2422
     2423              Note  that if --dhcp-option is pushed via --push to a non-windows client, the option will be saved in the client's envi‐
     2424              ronment before the up script is called, under the name "foreign_option_{n}".
     2425
     2426       --tap-sleep n
     2427              Cause OpenVPN to sleep for n seconds immediately after the TAP-Win32 adapter state is set to "connected".
     2428
     2429              This option is intended to be used to troubleshoot problems with the --ifconfig and --ip-win32 options, and is  used  to
     2430              give the TAP-Win32 adapter time to come up before Windows IP Helper API operations are applied to it.
     2431
     2432       --show-net-up
     2433              Output  OpenVPN's  view of the system routing table and network adapter list to the syslog or log file after the TUN/TAP
     2434              adapter has been brought up and any routes have been added.
     2435
     2436       --dhcp-renew
     2437              Ask Windows to renew the TAP adapter lease on startup.  This option is normally unnecessary,  as  Windows  automatically
     2438              triggers a DHCP renegotiation on the TAP adapter when it comes up, however if you set the TAP-Win32 adapter Media Status
     2439              property to "Always Connected", you may need this flag.
     2440
     2441       --dhcp-release
     2442              Ask Windows to release the TAP adapter lease on shutdown.  This option has the same caveats as --dhcp-renew above.
     2443
     2444       --register-dns
     2445              Run net stop dnscache, net start dnscache, ipconfig /flushdns and ipconfig /registerdns on connection initiation.   This
     2446              is known to kick Windows into recognizing pushed DNS servers.
     2447
     2448       --pause-exit
     2449              Put up a "press any key to continue" message on the console prior to OpenVPN program exit.  This option is automatically
     2450              used by the Windows explorer when OpenVPN is run on a configuration file using the right-click explorer menu.
     2451
     2452       --service exit-event [0|1]
     2453              Should be used when OpenVPN is being automatically executed by another program in such a  context  that  no  interaction
     2454              with  the  user  via  display  or  keyboard is possible.  In general, end-users should never need to explicitly use this
     2455              option, as it is automatically added by the OpenVPN service wrapper when a given OpenVPN configuration is being run as a
     2456              service.
     2457
     2458              exit-event  is  the name of a Windows global event object, and OpenVPN will continuously monitor the state of this event
     2459              object and exit when it becomes signaled.
     2460
     2461              The second parameter indicates the initial state of exit-event and normally defaults to 0.
     2462
     2463              Multiple OpenVPN processes can be simultaneously executed with the same exit-event parameter.  In any case, the control‐
     2464              ling process can signal exit-event, causing all such OpenVPN processes to exit.
     2465
     2466              When executing an OpenVPN process using the --service directive, OpenVPN will probably not have a console window to out‐
     2467              put status/error messages, therefore it is useful to use --log or --log-append to write these messages to a file.
     2468
     2469       --show-adapters
     2470              (Standalone) Show available TAP-Win32 adapters which can be selected using the --dev-node option.  On  non-Windows  sys‐
     2471              tems, the ifconfig(8) command provides similar functionality.
     2472
     2473       --allow-nonadmin [TAP-adapter]
     2474              (Standalone)  Set  TAP-adapter  to  allow  access  from non-administrative accounts.  If TAP-adapter is omitted, all TAP
     2475              adapters on the system will be configured to allow non-admin access.  The non-admin access setting will only persist for
     2476              the  length  of  time  that the TAP-Win32 device object and driver remain loaded, and will need to be re-enabled after a
     2477              reboot, or if the driver is unloaded and reloaded.  This directive can only be used by an administrator.
     2478
     2479       --show-valid-subnets
     2480              (Standalone) Show valid subnets for --dev tun emulation.  Since the TAP-Win32 driver exports an  ethernet  interface  to
     2481              Windows,  and since TUN devices are point-to-point in nature, it is necessary for the TAP-Win32 driver to impose certain
     2482              constraints on TUN endpoint address selection.
     2483
     2484              Namely, the point-to-point endpoints used in TUN device emulation must be the middle two addresses of a /30 subnet (net‐
     2485              mask 255.255.255.252).
     2486
     2487       --show-net
     2488              (Standalone) Show OpenVPN's view of the system routing table and network adapter list.
     2489
     2490   PKCS#11 Standalone Options:
     2491       --show-pkcs11-ids provider [cert_private]
     2492              (Standalone) Show PKCS#11 token object list. Specify cert_private as 1 if certificates are stored as private objects.
     2493
     2494              --verb option can be used BEFORE this option to produce debugging information.
     2495
     2496   IPv6 Related Options
     2497       The  following  options  exist  to support IPv6 tunneling in peer-to-peer and client-server mode.  As of now, this is just very
     2498       basic documentation of the IPv6-related options. More documentation can be found on http://www.greenie.net/ipv6/openvpn.html.
     2499
     2500       --ifconfig-ipv6 ipv6addr/bits ipv6remote
     2501              configure IPv6 address ipv6addr/bits on the ``tun'' device.  The second parameter is used as route target  for  --route-
     2502              ipv6 if no gateway is specified.
     2503
     2504       --route-ipv6 ipv6addr/bits [gateway] [metric]
     2505              setup IPv6 routing in the system to send the specified IPv6 network into OpenVPN's ``tun'' device
     2506
     2507       --server-ipv6 ipv6addr/bits
     2508              convenience-function  to  enable a number of IPv6 related options at once, namely --ifconfig-ipv6, --ifconfig-ipv6-pool,
     2509              --tun-ipv6 and --push tun-ipv6 Is only accepted if ``--mode server'' or ``--server'' is set.
     2510
     2511       --ifconfig-ipv6-pool ipv6addr/bits
     2512              Specify an IPv6 address pool for dynamic assignment to clients.  The pool starts at ipv6addr and increments  by  +1  for
     2513              every new client (linear mode).  The /bits setting controls the size of the pool.
     2514
     2515       --ifconfig-ipv6-push ipv6addr/bits ipv6remote
     2516              for ccd/ per-client static IPv6 interface configuration, see --client-config-dir and --ifconfig-push for more details.
     2517
     2518       --iroute-ipv6 ipv6addr/bits
     2519              for  ccd/  per-client  static IPv6 route configuration, see --iroute for more details how to setup and use this, and how
     2520              --iroute and --route interact.
     2521
     2522
     2523SCRIPTING AND ENVIRONMENTAL VARIABLES
     2524       OpenVPN exports a series of environmental variables for use by user-defined scripts.
     2525
     2526   Script Order of Execution
     2527       --up   Executed after TCP/UDP socket bind and TUN/TAP open.
     2528
     2529       --tls-verify
     2530              Executed when we have a still untrusted remote peer.
     2531
     2532       --ipchange
     2533              Executed after connection authentication, or remote IP address change.
     2534
     2535       --client-connect
     2536              Executed in --mode server mode immediately after client authentication.
     2537
     2538       --route-up
     2539              Executed after connection authentication, either immediately after, or some number of seconds after as  defined  by  the
     2540              --route-delay option.
     2541
     2542       --route-pre-down
     2543              Executed right before the routes are removed.
     2544
     2545       --client-disconnect
     2546              Executed in --mode server mode on client instance shutdown.
     2547
     2548       --down Executed after TCP/UDP and TUN/TAP close.
     2549
     2550       --learn-address
     2551              Executed  in --mode server mode whenever an IPv4 address/route or MAC address is added to OpenVPN's internal routing ta‐
     2552              ble.
     2553
     2554       --auth-user-pass-verify
     2555              Executed in --mode server mode on new client connections, when the client is still untrusted.
     2556
     2557   String Types and Remapping
     2558       In certain cases, OpenVPN will perform remapping of characters in strings.  Essentially, any characters outside the set of per‐
     2559       mitted characters for each string type will be converted to underbar ('_').
     2560
     2561       Q: Why is string remapping necessary?
     2562
     2563       A: It's an important security feature to prevent the malicious coding of strings from untrusted sources to be passed as parame‐
     2564       ters to scripts, saved in the environment, used as a common name, translated to a filename, etc.
     2565
     2566       Q: Can string remapping be disabled?
     2567
     2568       A: Yes, by using the --no-name-remapping option, however this should be considered an advanced option.
     2569
     2570       Here is a brief rundown of OpenVPN's current string types and the permitted character class for each string:
     2571
     2572       X509 Names: Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), at ('@'), colon (':'), slash ('/'), and equal ('=').  Alphanu‐
     2573       meric is defined as a character which will cause the C library isalnum() function to return true.
     2574
     2575       Common Names: Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), and at ('@').
     2576
     2577       --auth-user-pass  username: Same as Common Name, with one exception: starting with OpenVPN 2.0.1, the username is passed to the
     2578       OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY plugin in its raw form, without string remapping.
     2579
     2580       --auth-user-pass password: Any "printable" character except CR or LF.  Printable is defined to be a character which will  cause
     2581       the C library isprint() function to return true.
     2582
     2583       --client-config-dir  filename  as derived from common name or username: Alphanumeric, underbar ('_'), dash ('-'), and dot ('.')
     2584       except for "." or ".." as standalone strings.  As of 2.0.1-rc6, the at ('@') character has been added as well for compatibility
     2585       with the common name character class.
     2586
     2587       Environmental variable names: Alphanumeric or underbar ('_').
     2588
     2589       Environmental variable values: Any printable character.
     2590
     2591       For  all cases, characters in a string which are not members of the legal character class for that string type will be remapped
     2592       to underbar ('_').
     2593
     2594   Environmental Variables
     2595       Once set, a variable is persisted indefinitely until it is reset by a new value or a restart,
     2596
     2597       As of OpenVPN 2.0-beta12, in server mode, environmental variables set by OpenVPN are scoped according  to  the  client  objects
     2598       they are associated with, so there should not be any issues with scripts having access to stale, previously set variables which
     2599       refer to different client instances.
     2600
     2601       bytes_received
     2602              Total number of bytes received from client during VPN session.   Set  prior  to  execution  of  the  --client-disconnect
     2603              script.
     2604
     2605       bytes_sent
     2606              Total number of bytes sent to client during VPN session.  Set prior to execution of the --client-disconnect script.
     2607
     2608       common_name
     2609              The  X509  common name of an authenticated client.  Set prior to execution of --client-connect, --client-disconnect, and
     2610              --auth-user-pass-verify scripts.
     2611
     2612       config Name of first --config file.  Set on program initiation and reset on SIGHUP.
     2613
     2614       daemon Set to "1" if the --daemon directive is specified, or "0" otherwise.  Set on program initiation and reset on SIGHUP.
     2615
     2616       daemon_log_redirect
     2617              Set to "1" if the --log or --log-append directives are specified, or "0" otherwise.  Set on program initiation and reset
     2618              on SIGHUP.
     2619
     2620       dev    The  actual name of the TUN/TAP device, including a unit number if it exists.  Set prior to --up or --down script execu‐
     2621              tion.
     2622
     2623       foreign_option_{n}
     2624              An option pushed via --push to a client which does not natively support it, such as --dhcp-option on a non-Windows  sys‐
     2625              tem, will be recorded to this environmental variable sequence prior to --up script execution.
     2626
     2627       ifconfig_broadcast
     2628              The  broadcast  address  for  the virtual ethernet segment which is derived from the --ifconfig option when --dev tap is
     2629              used.  Set prior to OpenVPN calling the ifconfig or netsh (windows version of ifconfig) commands which  normally  occurs
     2630              prior to --up script execution.
     2631
     2632       ifconfig_ipv6_local
     2633              The  local  VPN  endpoint  IPv6 address specified in the --ifconfig-ipv6 option (first parameter).  Set prior to OpenVPN
     2634              calling the ifconfig or netsh (windows version of ifconfig) commands which normally occurs prior to --up  script  execu‐
     2635              tion.
     2636
     2637       ifconfig_ipv6_netbits
     2638              The  prefix length of the IPv6 network on the VPN interface.  Derived from the /nnn parameter of the IPv6 address in the
     2639              --ifconfig-ipv6 option (first parameter).  Set prior to OpenVPN calling the ifconfig or netsh (windows version of ifcon‐
     2640              fig) commands which normally occurs prior to --up script execution.
     2641
     2642       ifconfig_ipv6_remote
     2643              The  remote  VPN endpoint IPv6 address specified in the --ifconfig-ipv6 option (second parameter).  Set prior to OpenVPN
     2644              calling the ifconfig or netsh (windows version of ifconfig) commands which normally occurs prior to --up  script  execu‐
     2645              tion.
     2646
     2647       ifconfig_local
     2648              The  local  VPN  endpoint IP address specified in the --ifconfig option (first parameter).  Set prior to OpenVPN calling
     2649              the ifconfig or netsh (windows version of ifconfig) commands which normally occurs prior to --up script execution.
     2650
     2651       ifconfig_remote
     2652              The remote VPN endpoint IP address specified in the --ifconfig option (second parameter) when --dev tun  is  used.   Set
     2653              prior  to  OpenVPN  calling  the ifconfig or netsh (windows version of ifconfig) commands which normally occurs prior to
     2654              --up script execution.
     2655
     2656       ifconfig_netmask
     2657              The subnet mask of the virtual ethernet segment that is specified as the second parameter to --ifconfig when  --dev  tap
     2658              is being used.  Set prior to OpenVPN calling the ifconfig or netsh (windows version of ifconfig) commands which normally
     2659              occurs prior to --up script execution.
     2660
     2661       ifconfig_pool_local_ip
     2662              The local virtual IP address for the TUN/TAP tunnel taken from an --ifconfig-push directive if specified,  or  otherwise
     2663              from the ifconfig pool (controlled by the --ifconfig-pool config file directive).  Only set for --dev tun tunnels.  This
     2664              option is set on the server prior to execution of the --client-connect and --client-disconnect scripts.
     2665
     2666       ifconfig_pool_netmask
     2667              The virtual IP netmask for the TUN/TAP tunnel taken from an --ifconfig-push directive if specified,  or  otherwise  from
     2668              the  ifconfig  pool  (controlled  by  the --ifconfig-pool config file directive).  Only set for --dev tap tunnels.  This
     2669              option is set on the server prior to execution of the --client-connect and --client-disconnect scripts.
     2670
     2671       ifconfig_pool_remote_ip
     2672              The remote virtual IP address for the TUN/TAP tunnel taken from an --ifconfig-push directive if specified, or  otherwise
     2673              from  the  ifconfig  pool  (controlled  by the --ifconfig-pool config file directive).  This option is set on the server
     2674              prior to execution of the --client-connect and --client-disconnect scripts.
     2675
     2676       link_mtu
     2677              The maximum packet size (not including the IP header) of tunnel data in UDP tunnel transport mode.  Set prior to --up or
     2678              --down script execution.
     2679
     2680       local  The --local parameter.  Set on program initiation and reset on SIGHUP.
     2681
     2682       local_port
     2683              The local port number, specified by --port or --lport.  Set on program initiation and reset on SIGHUP.
     2684
     2685       password
     2686              The  password provided by a connecting client.  Set prior to --auth-user-pass-verify script execution only when the via-
     2687              env modifier is specified, and deleted from the environment after the script returns.
     2688
     2689       proto  The --proto parameter.  Set on program initiation and reset on SIGHUP.
     2690
     2691       remote_{n}
     2692              The --remote parameter.  Set on program initiation and reset on SIGHUP.
     2693
     2694       remote_port_{n}
     2695              The remote port number, specified by --port or --rport.  Set on program initiation and reset on SIGHUP.
     2696
     2697       route_net_gateway
     2698              The pre-existing default IP gateway in the system routing table.  Set prior to --up script execution.
     2699
     2700       route_vpn_gateway
     2701              The default gateway used by --route options, as specified in either the --route-gateway option or the  second  parameter
     2702              to --ifconfig when --dev tun is specified.  Set prior to --up script execution.
     2703
     2704       route_{parm}_{n}
     2705              A set of variables which define each route to be added, and are set prior to --up script execution.
     2706
     2707              parm will be one of "network", "netmask", "gateway", or "metric".
     2708
     2709              n is the OpenVPN route number, starting from 1.
     2710
     2711              If  the  network  or  gateway are resolvable DNS names, their IP address translations will be recorded rather than their
     2712              names as denoted on the command line or configuration file.
     2713
     2714       route_ipv6_{parm}_{n}
     2715              A set of variables which define each IPv6 route to be added, and are set prior to --up script execution.
     2716
     2717              parm will be one of "network" or "gateway" ("netmask" is contained as "/nnn" in the route_ipv6_network_{n}, unlike  IPv4
     2718              where it is passed in a separate environment variable).
     2719
     2720              n is the OpenVPN route number, starting from 1.
     2721
     2722              If  the  network  or  gateway are resolvable DNS names, their IP address translations will be recorded rather than their
     2723              names as denoted on the command line or configuration file.
     2724
     2725       peer_cert
     2726              Temporary file name containing the client certificate upon connection.  Useful in conjunction with --tls-verify
     2727
     2728       script_context
     2729              Set to "init" or "restart" prior to up/down script execution.  For more information, see documentation for --up.
     2730
     2731       script_type
     2732              Prior to execution of any script, this variable is set to the type of script being run.  It can be one of the following:
     2733              up,  down,  ipchange,  route-up, tls-verify, auth-user-pass-verify, client-connect, client-disconnect, or learn-address.
     2734              Set prior to execution of any script.
     2735
     2736       signal The reason for exit or restart.  Can be one of sigusr1, sighup, sigterm,  sigint,  inactive  (controlled  by  --inactive
     2737              option),  ping-exit  (controlled by --ping-exit option), ping-restart (controlled by --ping-restart option), connection-
     2738              reset (triggered on TCP connection reset), error, or unknown (unknown signal).  This variable is set just prior to  down
     2739              script execution.
     2740
     2741       time_ascii
     2742              Client  connection timestamp, formatted as a human-readable time string.  Set prior to execution of the --client-connect
     2743              script.
     2744
     2745       time_duration
     2746              The duration (in seconds) of the client session which is now disconnecting.  Set prior to execution of the --client-dis‐
     2747              connect script.
     2748
     2749       time_unix
     2750              Client  connection  timestamp, formatted as a unix integer date/time value.  Set prior to execution of the --client-con‐
     2751              nect script.
     2752
     2753       tls_id_{n}
     2754              A series of certificate fields from the remote peer, where n is the verification level.  Only set for  TLS  connections.
     2755              Set prior to execution of --tls-verify script.
     2756
     2757       tls_serial_{n}
     2758              The  serial number of the certificate from the remote peer, where n is the verification level.  Only set for TLS connec‐
     2759              tions.  Set prior to execution of --tls-verify script. This is in the form of a hex string  like  "37AB46E0",  which  is
     2760              suitable  for  doing serial-based OCSP queries (with OpenSSL, you have to prepend "0x" to the string). If something goes
     2761              wrong while reading the value from the certificate it will be an empty string, so your code should check that.  See  the
     2762              contrib/OCSP_check/OCSP_check.sh script for an example.
     2763
     2764       tun_mtu
     2765              The MTU of the TUN/TAP device.  Set prior to --up or --down script execution.
     2766
     2767       trusted_ip (or trusted_ip6)
     2768              Actual  IP  address  of  connecting  client or peer which has been authenticated.  Set prior to execution of --ipchange,
     2769              --client-connect, and --client-disconnect scripts.  If using ipv6  endpoints  (udp6,  tcp6),  trusted_ip6  will  be  set
     2770              instead.
     2771
     2772       trusted_port
     2773              Actual  port  number  of  connecting client or peer which has been authenticated.  Set prior to execution of --ipchange,
     2774              --client-connect, and --client-disconnect scripts.
     2775
     2776       untrusted_ip (or untrusted_ip6)
     2777              Actual IP address of connecting client or peer which has not been authenticated yet.  Sometimes used to  nmap  the  con‐
     2778              necting  host  in a --tls-verify script to ensure it is firewalled properly.  Set prior to execution of --tls-verify and
     2779              --auth-user-pass-verify scripts.  If using ipv6 endpoints (udp6, tcp6), untrusted_ip6 will be set instead.
     2780
     2781       untrusted_port
     2782              Actual port number of connecting client or peer which has not been authenticated yet.  Set prior to execution of  --tls-
     2783              verify and --auth-user-pass-verify scripts.
     2784
     2785       username
     2786              The  username provided by a connecting client.  Set prior to --auth-user-pass-verify script execution only when the via-
     2787              env modifier is specified.
     2788
     2789       X509_{n}_{subject_field}
     2790              An X509 subject field from the remote peer certificate, where n is the verification level.  Only  set  for  TLS  connec‐
     2791              tions.  Set prior to execution of --tls-verify script.  This variable is similar to tls_id_{n} except the component X509
     2792              subject fields are broken out, and no string remapping occurs on these field values (except  for  remapping  of  control
     2793              characters  to  "_").   For  example, the following variables would be set on the OpenVPN server using the sample client
     2794              certificate in sample-keys (client.crt).  Note that the verification level is 0 for the client certificate and 1 for the
     2795              CA certificate.
     2796
     2797                  X509_0_emailAddress=me@myhost.mydomain
     2798                  X509_0_CN=Test-Client
     2799                  X509_0_O=OpenVPN-TEST
     2800                  X509_0_ST=NA
     2801                  X509_0_C=KG
     2802                  X509_1_emailAddress=me@myhost.mydomain
     2803                  X509_1_O=OpenVPN-TEST
     2804                  X509_1_L=BISHKEK
     2805                  X509_1_ST=NA
     2806                  X509_1_C=KG
     2807
     2808SIGNALS
     2809       SIGHUP Cause OpenVPN to close all TUN/TAP and network connections, restart, re-read the configuration file (if any), and reopen
     2810              TUN/TAP and network connections.
     2811
     2812       SIGUSR1
     2813              Like SIGHUP, except don't re-read configuration file, and possibly don't close and reopen TUN/TAP  device,  re-read  key
     2814              files,  preserve  local  IP address/port, or preserve most recently authenticated remote IP address/port based on --per‐
     2815              sist-tun, --persist-key, --persist-local-ip, and --persist-remote-ip options respectively (see above).
     2816
     2817              This signal may also be internally generated by a timeout condition, governed by the --ping-restart option.
     2818
     2819              This signal, when combined with --persist-remote-ip, may be sent when the underlying parameters of  the  host's  network
     2820              interface change such as when the host is a DHCP client and is assigned a new IP address.  See --ipchange above for more
     2821              information.
     2822
     2823       SIGUSR2
     2824              Causes OpenVPN to display its current statistics (to the syslog file if --daemon is used, or stdout otherwise).
     2825
     2826       SIGINT, SIGTERM
     2827              Causes OpenVPN to exit gracefully.
     2828
     2829TUN/TAP DRIVER SETUP
     2830       If you are running Linux 2.4.7 or higher, you probably have the TUN/TAP driver already installed.  If so, there are still a few
     2831       things you need to do:
     2832
     2833       Make device: mknod /dev/net/tun c 10 200
     2834
     2835       Load driver: modprobe tun
     2836
     2837EXAMPLES
     2838       Prior  to running these examples, you should have OpenVPN installed on two machines with network connectivity between them.  If
     2839       you have not yet installed OpenVPN, consult the INSTALL file included in the OpenVPN distribution.
     2840
     2841   TUN/TAP Setup:
     2842       If you are using Linux 2.4 or higher, make the tun device node and load the tun module:
     2843
     2844              mknod /dev/net/tun c 10 200
     2845
     2846              modprobe tun
     2847
     2848       If you installed from RPM, the mknod step may be omitted, because the RPM install does that for you.
     2849
     2850       Only Linux 2.4 and newer are supported.
     2851
     2852       For other platforms, consult the INSTALL file at http://openvpn.net/install.html for more information.
     2853
     2854   Firewall Setup:
     2855       If firewalls exist between the two machines, they should be set to forward UDP port 1194 in both directions.   If  you  do  not
     2856       have  control  over the firewalls between the two machines, you may still be able to use OpenVPN by adding --ping 15 to each of
     2857       the openvpn commands used below in the examples (this will cause each peer to send out a UDP ping to its remote peer once every
     2858       15 seconds which will cause many stateful firewalls to forward packets in both directions without an explicit firewall rule).
     2859
     2860       If  you are using a Linux iptables-based firewall, you may need to enter the following command to allow incoming packets on the
     2861       TUN device:
     2862
     2863              iptables -A INPUT -i tun+ -j ACCEPT
     2864
     2865       See the firewalls section below for more information on configuring firewalls for use with OpenVPN.
     2866
     2867   VPN Address Setup:
     2868       For purposes of our example, our two machines will be called may.kg and june.kg.  If you are constructing a VPN over the inter‐
     2869       net,  then  replace may.kg and june.kg with the internet hostname or IP address that each machine will use to contact the other
     2870       over the internet.
     2871
     2872       Now we will choose the tunnel endpoints.  Tunnel endpoints are private IP addresses that only have meaning in  the  context  of
     2873       the VPN.  Each machine will use the tunnel endpoint of the other machine to access it over the VPN.  In our example, the tunnel
     2874       endpoint for may.kg will be 10.4.0.1 and for june.kg, 10.4.0.2.
     2875
     2876       Once the VPN is established, you have essentially created a secure alternate path between the two hosts which is  addressed  by
     2877       using  the  tunnel  endpoints.  You can control which network traffic passes between the hosts (a) over the VPN or (b) indepen‐
     2878       dently of the VPN, by choosing whether to use (a) the VPN endpoint address or (b) the public internet address,  to  access  the
     2879       remote  host.  For example if you are on may.kg and you wish to connect to june.kg via ssh without using the VPN (since ssh has
     2880       its own built-in security) you would use the command ssh june.kg.  However in the same scenario, you could also use the command
     2881       telnet  10.4.0.2 to create a telnet session with june.kg over the VPN, that would use the VPN to secure the session rather than
     2882       ssh.
     2883
     2884       You can use any address you wish for the tunnel endpoints but make sure that they are private addresses  (such  as  those  that
     2885       begin  with  10  or  192.168)  and that they are not part of any existing subnet on the networks of either peer, unless you are
     2886       bridging.  If you use an address that is part of your local subnet for either of the tunnel endpoints, you  will  get  a  weird
     2887       feedback loop.
     2888
     2889   Example 1: A simple tunnel without security
     2890       On may:
     2891
     2892              openvpn --remote june.kg --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --verb 9
     2893
     2894       On june:
     2895
     2896              openvpn --remote may.kg --dev tun1 --ifconfig 10.4.0.2 10.4.0.1 --verb 9
     2897
     2898       Now verify the tunnel is working by pinging across the tunnel.
     2899
     2900       On may:
     2901
     2902              ping 10.4.0.2
     2903
     2904       On june:
     2905
     2906              ping 10.4.0.1
     2907
     2908       The  --verb  9 option will produce verbose output, similar to the tcpdump(8) program.  Omit the --verb 9 option to have OpenVPN
     2909       run quietly.
     2910
     2911   Example 2: A tunnel with static-key security (i.e. using a pre-shared secret)
     2912       First build a static key on may.
     2913
     2914              openvpn --genkey --secret key
     2915
     2916       This command will build a random key file called key (in ascii format).  Now copy key to june over a secure medium such  as  by
     2917       using the scp(1) program.
     2918
     2919       On may:
     2920
     2921              openvpn --remote june.kg --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --verb 5 --secret key
     2922
     2923       On june:
     2924
     2925              openvpn --remote may.kg --dev tun1 --ifconfig 10.4.0.2 10.4.0.1 --verb 5 --secret key
     2926
     2927       Now verify the tunnel is working by pinging across the tunnel.
     2928
     2929       On may:
     2930
     2931              ping 10.4.0.2
     2932
     2933       On june:
     2934
     2935              ping 10.4.0.1
     2936
     2937   Example 3: A tunnel with full TLS-based security
     2938       For this test, we will designate may as the TLS client and june as the TLS server.  Note that client or server designation only
     2939       has meaning for the TLS subsystem. It has no bearing on OpenVPN's peer-to-peer, UDP-based communication model.
     2940
     2941       First, build a separate certificate/key pair for both may and june (see above where --cert is discussed for more  info).   Then
     2942       construct  Diffie  Hellman  parameters  (see  above where --dh is discussed for more info).  You can also use the included test
     2943       files client.crt, client.key, server.crt, server.key and ca.crt.  The .crt files are certificates/public-keys, the  .key  files
     2944       are  private  keys,  and ca.crt is a certification authority who has signed both client.crt and server.crt.  For Diffie Hellman
     2945       parameters you can use the included file dh1024.pem.  Note that all client, server, and certificate authority certificates  and
     2946       keys included in the OpenVPN distribution are totally insecure and should be used for testing only.
     2947
     2948       On may:
     2949
     2950              openvpn  --remote  june.kg  --dev  tun1  --ifconfig  10.4.0.1  10.4.0.2 --tls-client --ca ca.crt --cert client.crt --key
     2951              client.key --reneg-sec 60 --verb 5
     2952
     2953       On june:
     2954
     2955              openvpn --remote may.kg --dev tun1  --ifconfig  10.4.0.2  10.4.0.1  --tls-server  --dh  dh1024.pem  --ca  ca.crt  --cert
     2956              server.crt --key server.key --reneg-sec 60 --verb 5
     2957
     2958       Now verify the tunnel is working by pinging across the tunnel.
     2959
     2960       On may:
     2961
     2962              ping 10.4.0.2
     2963
     2964       On june:
     2965
     2966              ping 10.4.0.1
     2967
     2968       Notice  the  --reneg-sec 60 option we used above.  That tells OpenVPN to renegotiate the data channel keys every minute.  Since
     2969       we used --verb 5 above, you will see status information on each new key negotiation.
     2970
     2971       For production operations, a key renegotiation interval of 60 seconds is probably too frequent.  Omit the --reneg-sec 60 option
     2972       to use OpenVPN's default key renegotiation interval of one hour.
     2973
     2974   Routing:
     2975       Assuming  you can ping across the tunnel, the next step is to route a real subnet over the secure tunnel.  Suppose that may and
     2976       june have two network interfaces each, one connected to the internet, and the other to a  private  network.   Our  goal  is  to
     2977       securely connect both private networks.  We will assume that may's private subnet is 10.0.0.0/24 and june's is 10.0.1.0/24.
     2978
     2979       First, ensure that IP forwarding is enabled on both peers.  On Linux, enable routing:
     2980
     2981              echo 1 > /proc/sys/net/ipv4/ip_forward
     2982
     2983       and enable TUN packet forwarding through the firewall:
     2984
     2985              iptables -A FORWARD -i tun+ -j ACCEPT
     2986
     2987       On may:
     2988
     2989              route add -net 10.0.1.0 netmask 255.255.255.0 gw 10.4.0.2
     2990
     2991       On june:
     2992
     2993              route add -net 10.0.0.0 netmask 255.255.255.0 gw 10.4.0.1
     2994
     2995       Now  any  machine  on  the  10.0.0.0/24 subnet can access any machine on the 10.0.1.0/24 subnet over the secure tunnel (or vice
     2996       versa).
     2997
     2998       In a production environment, you could put the route command(s) in a script and execute with the --up option.
     2999
     3000FIREWALLS
     3001       OpenVPN's usage of a single UDP port makes it fairly firewall-friendly.  You should add an entry  to  your  firewall  rules  to
     3002       allow incoming OpenVPN packets.  On Linux 2.4+:
     3003
     3004              iptables -A INPUT -p udp -s 1.2.3.4 --dport 1194 -j ACCEPT
     3005
     3006       This will allow incoming packets on UDP port 1194 (OpenVPN's default UDP port) from an OpenVPN peer at 1.2.3.4.
     3007
     3008       If you are using HMAC-based packet authentication (the default in any of OpenVPN's secure modes), having the firewall filter on
     3009       source address can be considered optional, since HMAC packet authentication is a much  more  secure  method  of  verifying  the
     3010       authenticity of a packet source.  In that case:
     3011
     3012              iptables -A INPUT -p udp --dport 1194 -j ACCEPT
     3013
     3014       would be adequate and would not render the host inflexible with respect to its peer having a dynamic IP address.
     3015
     3016       OpenVPN also works well on stateful firewalls.  In some cases, you may not need to add any static rules to the firewall list if
     3017       you are using a stateful firewall that knows how to track UDP connections.  If you specify --ping n, OpenVPN will be guaranteed
     3018       to  send  a  packet to its peer at least once every n seconds.  If n is less than the stateful firewall connection timeout, you
     3019       can maintain an OpenVPN connection indefinitely without explicit firewall rules.
     3020
     3021       You should also add firewall rules to allow incoming IP traffic on TUN or TAP devices such as:
     3022
     3023              iptables -A INPUT -i tun+ -j ACCEPT
     3024
     3025       to allow input packets from tun devices,
     3026
     3027              iptables -A FORWARD -i tun+ -j ACCEPT
     3028
     3029       to allow input packets from tun devices to be forwarded to other hosts on the local network,
     3030
     3031              iptables -A INPUT -i tap+ -j ACCEPT
     3032
     3033       to allow input packets from tap devices, and
     3034
     3035              iptables -A FORWARD -i tap+ -j ACCEPT
     3036
     3037       to allow input packets from tap devices to be forwarded to other hosts on the local network.
     3038
     3039       These rules are secure if you use packet authentication, since no incoming packets will arrive on a TUN or TAP  virtual  device
     3040       unless they first pass an HMAC authentication test.
     3041
     3042FAQ
     3043       http://openvpn.net/faq.html
     3044
     3045HOWTO
     3046       For  a  more  comprehensive  guide  to  setting  up  OpenVPN  in  a  production  setting, see the OpenVPN HOWTO at http://open‐
     3047       vpn.net/howto.html
     3048
     3049PROTOCOL
     3050       For a description of OpenVPN's underlying protocol, see http://openvpn.net/security.html
     3051
     3052WEB
     3053       OpenVPN's web site is at http://openvpn.net/
     3054
     3055       Go here to download the latest version of OpenVPN, subscribe to the mailing lists, read the mailing list  archives,  or  browse
     3056       the SVN repository.
     3057
     3058BUGS
     3059       Report all bugs to the OpenVPN team <info@openvpn.net>.
     3060
     3061SEE ALSO
     3062       dhcpcd(8), ifconfig(8), openssl(1), route(8), scp(1) ssh(1)
     3063
     3064NOTES
     3065       This product includes software developed by the OpenSSL Project ( http://www.openssl.org/ )
     3066
     3067       For more information on the TLS protocol, see http://www.ietf.org/rfc/rfc2246.txt
     3068
     3069       For more information on the LZO real-time compression library see http://www.oberhumer.com/opensource/lzo/
     3070
     3071COPYRIGHT
     3072       Copyright  (C)  2002-2010  OpenVPN  Technologies,  Inc. This program is free software; you can redistribute it and/or modify it
     3073       under the terms of the GNU General Public License version 2 as published by the Free Software Foundation.
     3074
     3075AUTHORS
     3076       James Yonan <jim@yonan.net>
     3077
     3078
     3079
     3080                                                           17 November 2008                                                 openvpn(8)
     3081}}}