wiki:Openvpn23ManPage

Version 4 (modified by Samuli Seppänen, 7 years ago) (diff)

Updated man-page for openvpn-2.3_beta1

openvpn(8)                                                                                           openvpn(8)



NAME
       openvpn - secure IP tunnel daemon.

SYNOPSIS
       openvpn [ options ... ]

INTRODUCTION
       OpenVPN  is  an open source VPN daemon by James Yonan.  Because OpenVPN tries to be a universal VPN tool
       offering a great deal of 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 examples section where you will see how to construct simple
       VPNs on the command line without even needing a configuration file.

       Also note that there's more documentation and examples on the OpenVPN web site: http://openvpn.net/

       And if you would like to see a shorter version of this manual, see the openvpn usage message  which  can
       be obtained by running openvpn without any parameters.

DESCRIPTION
       OpenVPN  is a robust and highly flexible VPN daemon.  OpenVPN supports SSL/TLS security, ethernet bridg‐
       ing, TCP or UDP tunnel transport through proxies or NAT, support for  dynamic  IP  addresses  and  DHCP,
       scalability to hundreds or thousands of users, and portability to most major OS platforms.

       OpenVPN is tightly bound to the OpenSSL library, and derives much of its crypto capabilities from it.

       OpenVPN  supports  conventional encryption using a pre-shared secret key (Static Key mode) or public key
       security (SSL/TLS mode) using client & server certificates.  OpenVPN also supports non-encrypted TCP/UDP
       tunnels.

       OpenVPN is designed to work with the TUN/TAP virtual networking interface that exists on most platforms.

       Overall, OpenVPN aims to offer many of the key features of IPSec but with a relatively lightweight foot‐
       print.

OPTIONS
       OpenVPN allows any option to be placed either on the command line or in a  configuration  file.   Though
       all  command  line options are preceded by a double-leading-dash ("--"), this prefix can be removed when
       an option is placed in a configuration file.

       --help Show options.

       --config file
              Load additional config options from file where each line corresponds to one command line  option,
              but with the leading '--' removed.

              If  --config file is the only option to the openvpn command, the --config can be removed, and the
              command can be given as openvpn file

              Note that configuration files can be nested to a reasonable depth.

              Double quotation or single quotation characters ("", '') can be used to enclose single parameters
              containing  whitespace,  and "#" or ";" characters in the first column can be used to denote com‐
              ments.

              Note that OpenVPN 2.0 and higher performs backslash-based shell escaping for  characters  not  in
              single quotations, so the following mappings should be observed:

                  \\       Maps to a single backslash character (\).
                  \"       Pass a literal doublequote character ("), don't
                           interpret it as enclosing a parameter.
                  \[SPACE] Pass a literal space or tab character, don't
                           interpret it as a parameter delimiter.

              For example on Windows, use double backslashes to represent pathnames:

                  secret "c:\\OpenVPN\\secret.key"

              For examples of configuration files, see http://openvpn.net/examples.html

              Here is an example configuration file:

                  #
                  # Sample OpenVPN configuration file for
                  # using a pre-shared static key.
                  #
                  # '#' or ';' may be used to delimit comments.

                  # Use a dynamic tun device.
                  dev tun

                  # Our remote peer
                  remote mypeer.mydomain

                  # 10.1.0.1 is our local VPN endpoint
                  # 10.1.0.2 is our remote VPN endpoint
                  ifconfig 10.1.0.1 10.1.0.2

                  # Our pre-shared static key
                  secret static.key

   Tunnel Options:
       --mode m
              Set  OpenVPN  major  mode.  By default, OpenVPN runs in point-to-point mode ("p2p").  OpenVPN 2.0
              introduces a new mode ("server") which implements a multi-client server capability.

       --local host
              Local host name or IP address for bind.  If specified, OpenVPN will bind to  this  address  only.
              If unspecified, OpenVPN will bind to all interfaces.

       --remote host [port] [proto]
              Remote  host  name  or IP address.  On the client, multiple --remote options may be specified for
              redundancy, each referring to a different OpenVPN server.  Specifying multiple  --remote  options
              for this purpose is a special case of the more general connection-profile feature.  See the <con‐
              nection> documentation below.

              The OpenVPN client will try to connect to a server at host:port in the  order  specified  by  the
              list of --remote options.

              proto indicates the protocol to use when connecting with the remote, and may be "tcp" or "udp".

              The  client  will move on to the next host in the list, in the event of connection failure.  Note
              that at any given time, the OpenVPN client will at most be connected to one server.

              Note that since UDP is connectionless, connection failure is defined by the  --ping  and  --ping-
              restart options.

              Note  the following corner case:  If you use multiple --remote options, AND you are dropping root
              privileges on the client with --user and/or --group, AND the client is running a non-Windows  OS,
              if  the  client  needs  to  switch  to  a different server, and that server pushes back different
              TUN/TAP or route settings, the client may lack the necessary privileges to close and  reopen  the
              TUN/TAP interface.  This could cause the client to exit with a fatal error.

              If --remote is unspecified, OpenVPN will listen for packets from any IP address, but will not act
              on those packets unless they pass all authentication tests.  This requirement for  authentication
              is  binding on all potential peers, even those from known and supposedly trusted IP addresses (it
              is very easy to forge a source IP address on a UDP packet).

              When used in TCP mode, --remote will act as a filter, rejecting connections from any  host  which
              does not match host.

              If  host is a DNS name which resolves to multiple IP addresses, one will be randomly chosen, pro‐
              viding a sort of basic load-balancing and failover capability.

       --remote-random-hostname
              Add a random string (6 characters) to first DNS label of hostname to prevent  DNS  caching.   For
              example, "foo.bar.gov" would be modified to "<random-chars>.foo.bar.gov".

       <connection>
              Define  a  client  connection  profile.  Client connection profiles are groups of OpenVPN options
              that describe how to connect to a given OpenVPN server.  Client connection profiles are specified
              within an OpenVPN configuration file, and each profile is bracketed by <connection> and </connec‐
              tion>.

              An OpenVPN client will try each connection profile sequentially until it  achieves  a  successful
              connection.

              --remote-random can be used to initially "scramble" the connection list.

              Here is an example of connection profile usage:

                  client
                  dev tun

                  <connection>
                  remote 198.19.34.56 1194 udp
                  </connection>

                  <connection>
                  remote 198.19.34.56 443 tcp
                  </connection>

                  <connection>
                  remote 198.19.34.56 443 tcp
                  http-proxy 192.168.0.8 8080
                  http-proxy-retry
                  </connection>

                  <connection>
                  remote 198.19.36.99 443 tcp
                  http-proxy 192.168.0.8 8080
                  http-proxy-retry
                  </connection>

                  persist-key
                  persist-tun
                  pkcs12 client.p12
                  ns-cert-type server
                  verb 3

              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 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 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.

              The following OpenVPN options may be used inside of a <connection> block:

              bind, connect-retry, connect-retry-max, connect-timeout,  float,  http-proxy,  http-proxy-option,
              http-proxy-retry,  http-proxy-timeout,  local,  lport, nobind, port, proto, remote, rport, socks-
              proxy, and socks-proxy-retry.

              A defaulting mechanism exists for specifying options to apply to all <connection>  profiles.   If
              any  of the above options (with the exception of remote ) appear outside of a <connection> block,
              but in a configuration file which has one or more <connection> blocks, the option setting will be
              used as a default for <connection> blocks which follow it in the configuration file.

              For  example,  suppose the nobind option were placed in the sample configuration file above, near
              the top of the file, before the first <connection> block.  The effect would be as if nobind  were
              declared in all <connection> blocks below it.

       --proto-force p
              When   iterating   through   connection   profiles,  only  consider  profiles  using  protocol  p
              ('tcp'|'udp').

       --remote-random
              When multiple --remote address/ports are specified, or if connection  profiles  are  being  used,
              initially randomize the order of the list as a kind of basic load-balancing measure.

       --proto p
              Use protocol p for communicating with remote host.  p can be udp, tcp-client, or tcp-server.

              The default protocol is udp when --proto is not specified.

              For UDP operation, --proto udp should be specified on both peers.

              For  TCP  operation,  one  peer  must  use --proto tcp-server and the other must use --proto tcp-
              client.  A peer started with tcp-server will wait indefinitely for  an  incoming  connection.   A
              peer started with tcp-client will attempt to connect, and if that fails, will sleep for 5 seconds
              (adjustable via the --connect-retry option) and try again infinite or up to N retries (adjustable
              via  the --connect-retry-max option).  Both TCP client and server will simulate a SIGUSR1 restart
              signal if either side resets the connection.

              OpenVPN is designed to operate optimally over UDP, but TCP capability is provided for  situations
              where  UDP  cannot  be used.  In comparison with UDP, TCP will usually be somewhat less efficient
              and less robust when used over unreliable or congested networks.

              This article outlines some of problems with tunneling IP over TCP:

              http://sites.inka.de/sites/bigred/devel/tcp-tcp.html

              There are certain cases, however, where using TCP may be advantageous from a security and robust‐
              ness  perspective, such as tunneling non-IP or application-level UDP protocols, or tunneling pro‐
              tocols which don't possess a built-in reliability layer.

       --connect-retry n
              For --proto tcp-client, take n as the number  of  seconds  to  wait  between  connection  retries
              (default=5).

       --connect-timeout n
              For --proto tcp-client, set connection timeout to n seconds (default=10).

       --connect-retry-max n
              For --proto tcp-client, take n as the number of retries of connection attempt (default=infinite).

       --show-proxy-settings
              Show sensed HTTP or SOCKS proxy settings. Currently, only Windows clients support this option.

       --http-proxy server port [authfile|'auto'|'auto-nct'] [auth-method]
              Connect  to  remote  host  through an HTTP proxy at address server and port port.  If HTTP Proxy-
              Authenticate is required, authfile is a file containing a username and password on  2  lines,  or
              "stdin" to prompt from console.

              auth-method should be one of "none", "basic", or "ntlm".

              HTTP Digest authentication is supported as well, but only via the auto or auto-nct flags (below).

              The  auto  flag  causes OpenVPN to automatically determine the auth-method and query stdin or the
              management interface for username/password credentials, if required.  This flag exists on OpenVPN
              2.1 or higher.

              The auto-nct flag (no clear-text auth) instructs OpenVPN to automatically determine the authenti‐
              cation method, but to reject weak authentication protocols such as HTTP Basic Authentication.

       --http-proxy-retry
              Retry indefinitely on HTTP proxy errors.  If an HTTP  proxy  error  occurs,  simulate  a  SIGUSR1
              reset.

       --http-proxy-timeout n
              Set proxy timeout to n seconds, default=5.

       --http-proxy-option type [parm]
              Set extended HTTP proxy options.  Repeat to set multiple options.

              VERSION version -- Set HTTP version number to version (default=1.0).

              AGENT user-agent -- Set HTTP "User-Agent" string to user-agent.

       --socks-proxy server [port]
              Connect to remote host through a Socks5 proxy at address server and port port (default=1080).

       --socks-proxy-retry
              Retry  indefinitely  on  Socks  proxy  errors.  If a Socks proxy error occurs, simulate a SIGUSR1
              reset.

       --resolv-retry n
              If hostname resolve fails for --remote, retry resolve for n seconds before failing.

              Set n to "infinite" to retry indefinitely.

              By default, --resolv-retry infinite is enabled.  You can disable by setting n=0.

       --float
              Allow remote peer to change its IP address and/or port number, such as due to DHCP (this  is  the
              default if --remote is not used).  --float when specified with --remote allows an OpenVPN session
              to initially connect to a peer at a known address, however if packets arrive from a  new  address
              and  pass  all  authentication  tests, the new address will take control of the session.  This is
              useful when you are connecting to a peer which holds a dynamic address such as a dial-in user  or
              DHCP client.

              Essentially, --float tells OpenVPN to accept authenticated packets from any address, not only the
              address which was specified in the --remote option.

       --ipchange cmd
              Run command cmd when our remote ip-address is initially authenticated or changes.

              cmd consists of a path to script (or executable program), optionally followed by  arguments.  The
              path  and  arguments may be single- or double-quoted and/or escaped using a backslash, and should
              be separated by one or more spaces.

              When cmd is executed two arguments are appended after any arguments specified in cmd  ,  as  fol‐
              lows:

              cmd ip_address port_number

              Don't use --ipchange in --mode server mode.  Use a --client-connect script instead.

              See the "Environmental Variables" section below for additional parameters passed as environmental
              variables.

              If you are running in a dynamic IP address environment where the  IP  addresses  of  either  peer
              could  change  without  notice, you can use this script, for example, to edit the /etc/hosts file
              with the current address of the peer.  The script will be run every time the remote peer  changes
              its IP address.

              Similarly if our IP address changes due to DHCP, we should configure our IP address change script
              (see man page for dhcpcd(8) ) to deliver a SIGHUP or SIGUSR1 signal  to  OpenVPN.   OpenVPN  will
              then reestablish a connection with its most recently authenticated peer on its new IP address.

       --port port
              TCP/UDP  port number for both local and remote.  The current default of 1194 represents the offi‐
              cial IANA port number assignment for OpenVPN and has been used since version 2.0-beta17.   Previ‐
              ous versions used port 5000 as the default.

       --lport port
              TCP/UDP port number for bind.

       --rport port
              TCP/UDP port number for remote.

       --bind Bind  to  local  address and port. This is the default unless any of --proto tcp-client , --http-
              proxy or --socks-proxy are used.

       --nobind
              Do not bind to local address and port.  The IP stack will allocate a dynamic port  for  returning
              packets.   Since  the  value  of  the  dynamic port could not be known in advance by a peer, this
              option is only suitable for peers which will be initiating  connections  by  using  the  --remote
              option.

       --dev tunX | tapX | null
              TUN/TAP virtual network device ( X can be omitted for a dynamic device.)

              See examples section below for an example on setting up a TUN device.

              You  must use either tun devices on both ends of the connection or tap devices on both ends.  You
              cannot mix them, as they represent different underlying network layers.

              tun devices encapsulate IPv4 or IPv6 (OSI Layer 3) while tap devices encapsulate  Ethernet  802.3
              (OSI Layer 2).

       --dev-type device-type
              Which  device  type  are we using?  device-type should be tun (OSI Layer 3) or tap (OSI Layer 2).
              Use this option only if the TUN/TAP device used with --dev does not begin with tun or tap.

       --topology mode
              Configure virtual addressing topology when running in --dev tun  mode.   This  directive  has  no
              meaning in --dev tap mode, which always uses a subnet topology.

              If  you  set this directive on the server, the --server and --server-bridge directives will auto‐
              matically push your chosen topology setting to clients as well.  This directive can also be manu‐
              ally  pushed  to  clients.   Like  the  --dev directive, this directive must always be compatible
              between client and server.

              mode can be one of:

              net30 -- Use a point-to-point topology, by  allocating  one  /30  subnet  per  client.   This  is
              designed  to  allow  point-to-point semantics when some or all of the connecting clients might be
              Windows systems.  This is the default on OpenVPN 2.0.

              p2p -- Use a point-to-point topology where the remote endpoint  of  the  client's  tun  interface
              always  points to the local endpoint of the server's tun interface.  This mode allocates a single
              IP address per connecting client.  Only use when none of the connecting clients are Windows  sys‐
              tems.   This  mode  is  functionally  equivalent to the --ifconfig-pool-linear directive which is
              available in OpenVPN 2.0 and is now deprecated.

              subnet -- Use a subnet rather than a point-to-point topology by  configuring  the  tun  interface
              with  a  local IP address and subnet mask, similar to the topology used in --dev tap and ethernet
              bridging mode.  This mode allocates a single IP address per connecting client and works  on  Win‐
              dows as well.  Only available when server and clients are OpenVPN 2.1 or higher, or OpenVPN 2.0.x
              which has been manually patched with the  --topology  directive  code.   When  used  on  Windows,
              requires version 8.2 or higher of the TAP-Win32 driver.  When used on *nix, requires that the tun
              driver supports an ifconfig(8) command which sets a  subnet  instead  of  a  remote  endpoint  IP
              address.

              This option exists in OpenVPN 2.1 or higher.

       --tun-ipv6
              Build  a  tun  link capable of forwarding IPv6 traffic.  Should be used in conjunction with --dev
              tun or --dev tunX.  A warning will be displayed if no specific IPv6 TUN support for your  OS  has
              been compiled into OpenVPN.

              See below for further IPv6-related configuration options.

       --dev-node node
              Explicitly set the device node rather than using /dev/net/tun, /dev/tun, /dev/tap, etc.  If Open‐
              VPN cannot figure out whether node is a TUN or TAP device based on  the  name,  you  should  also
              specify --dev-type tun or --dev-type tap.

              On  Windows  systems, select the TAP-Win32 adapter which is named node in the Network Connections
              Control Panel or the raw GUID of the adapter enclosed  by  braces.   The  --show-adapters  option
              under  Windows  can also be used to enumerate all available TAP-Win32 adapters and will show both
              the network connections control panel name and the GUID for each TAP-Win32 adapter.

       --lladdr address
              Specify the link layer address, more commonly known as the MAC  address.   Only  applied  to  TAP
              devices.

       --iproute cmd
              Set  alternate  command  to execute instead of default iproute2 command.  May be used in order to
              execute OpenVPN in unprivileged environment.

       --ifconfig l rn
              Set TUN/TAP adapter parameters.  l is the IP address of the local VPN endpoint.  For TUN devices,
              rn  is  the IP address of the remote VPN endpoint.  For TAP devices, rn is the subnet mask of the
              virtual ethernet segment which is being created or connected to.

              For TUN devices, which facilitate virtual point-to-point IP  connections,  the  proper  usage  of
              --ifconfig is to use two private IP addresses which are not a member of any existing subnet which
              is in use.  The IP addresses may be consecutive and should  have  their  order  reversed  on  the
              remote peer.  After the VPN is established, by pinging rn, you will be pinging across the VPN.

              For  TAP  devices,  which  provide the ability to create virtual ethernet segments, --ifconfig is
              used to set an IP address and subnet mask just as a physical ethernet adapter would be  similarly
              configured.   If  you  are  attempting to connect to a remote ethernet bridge, the IP address and
              subnet should be set to values which would be valid on the the  bridged  ethernet  segment  (note
              also that DHCP can be used for the same purpose).

              This option, while primarily a proxy for the ifconfig(8) command, is designed to simplify TUN/TAP
              tunnel configuration by providing a standard interface to the different ifconfig  implementations
              on different platforms.

              --ifconfig  parameters  which  are IP addresses can also be specified as a DNS or /etc/hosts file
              resolvable name.

              For TAP devices, --ifconfig should not be used if the TAP interface will be getting an IP address
              lease from a DHCP server.

       --ifconfig-noexec
              Don't  actually  execute  ifconfig/netsh  commands, instead pass --ifconfig parameters to scripts
              using environmental variables.

       --ifconfig-nowarn
              Don't output an options consistency check warning if the --ifconfig option on this  side  of  the
              connection  doesn't  match  the  remote side.  This is useful when you want to retain the overall
              benefits of the options consistency check (also see --disable-occ option)  while  only  disabling
              the ifconfig component of the check.

              For example, if you have a configuration where the local host uses --ifconfig but the remote host
              does not, use --ifconfig-nowarn on the local host.

              This option will also silence warnings about potential address conflicts which occasionally annoy
              more experienced users by triggering "false positive" warnings.

       --route network/IP [netmask] [gateway] [metric]
              Add  route  to  routing table after connection is established.  Multiple routes can be specified.
              Routes will be automatically torn down in reverse order prior to TUN/TAP device close.

              This option is intended as a convenience proxy for the route(8) shell command, while at the  same
              time providing portable semantics across OpenVPN's platform space.

              netmask default -- 255.255.255.255

              gateway  default  --  taken from --route-gateway or the second parameter to --ifconfig when --dev
              tun is specified.

              metric default -- taken from --route-metric otherwise 0.

              The default can be specified by leaving an option blank or setting it to "default".

              The network and gateway parameters can also be specified as a DNS or /etc/hosts  file  resolvable
              name, or as one of three special keywords:

              vpn_gateway -- The remote VPN endpoint address (derived either from --route-gateway or the second
              parameter to --ifconfig when --dev tun is specified).

              net_gateway -- The pre-existing IP default gateway, read from the routing table (not supported on
              all OSes).

              remote_host  --  The --remote address if OpenVPN is being run in client mode, and is undefined in
              server mode.

       --max-routes n
              Allow a maximum number of n --route options to be specified, either in  the  local  configuration
              file, or pulled from an OpenVPN server.  By default, n=100.

       --route-gateway gw|'dhcp'
              Specify a default gateway gw for use with --route.

              If dhcp is specified as the parameter, the gateway address will be extracted from a DHCP negotia‐
              tion with the OpenVPN server-side LAN.

       --route-metric m
              Specify a default metric m for use with --route.

       --route-delay [n] [w]
              Delay n seconds (default=0) after connection establishment, before adding  routes.  If  n  is  0,
              routes  will  be  added  immediately upon connection establishment.  If --route-delay is omitted,
              routes will be added immediately after TUN/TAP device open and --up script execution, before  any
              --user or --group privilege downgrade (or --chroot execution.)

              This  option  is  designed  to  be  useful  in  scenarios  where  DHCP is used to set tap adapter
              addresses.  The delay will give the DHCP handshake time to complete before routes are added.

              On Windows, --route-delay tries to be more intelligent by waiting w seconds (w=30 by default) for
              the TAP-Win32 adapter to come up before adding routes.

       --route-up cmd
              Run command cmd after routes are added, subject to --route-delay.

              cmd  consists  of a path to script (or executable program), optionally followed by arguments. The
              path and arguments may be single- or double-quoted and/or escaped using a backslash,  and  should
              be separated by one or more spaces.

              See the "Environmental Variables" section below for additional parameters passed as environmental
              variables.

       --route-pre-down cmd
              Run command cmd before routes are removed upon disconnection.

              cmd consists of a path to script (or executable program), optionally followed by  arguments.  The
              path  and  arguments may be single- or double-quoted and/or escaped using a backslash, and should
              be separated by one or more spaces.

              See the "Environmental Variables" section below for additional parameters passed as environmental
              variables.

       --route-noexec
              Don't  add  or remove routes automatically.  Instead pass routes to --route-up script using envi‐
              ronmental variables.

       --route-nopull
              When used with --client or --pull, accept options pushed by server EXCEPT  for  routes  and  dhcp
              options like DNS servers.

              When  used  on  the  client,  this  option  effectively bars the server from adding routes to the
              client's routing table, however note that this option still allows the server to set  the  TCP/IP
              properties of the client's TUN/TAP interface.

       --allow-pull-fqdn
              Allow client to pull DNS names from server (rather than being limited to IP address) for --ifcon‐
              fig, --route, and --route-gateway.

       --client-nat snat|dnat network netmask alias
              This pushable client option sets up a stateless one-to-one NAT  rule  on  packet  addresses  (not
              ports),  and is useful in cases where routes or ifconfig settings pushed to the client would cre‐
              ate an IP numbering conflict.

              network/netmask (for example 192.168.0.0/255.255.0.0) defines the local view of a  resource  from
              the  client  perspective,  while  alias/netmask  (for  example 10.64.0.0/255.255.0.0) defines the
              remote view from the server perspective.

              Use snat (source NAT) for resources owned by the client and dnat  (destination  NAT)  for  remote
              resources.

              Set --verb 6 for debugging info showing the transformation of src/dest addresses in packets.

       --redirect-gateway flags...
              Automatically execute routing commands to cause all outgoing IP traffic to be redirected over the
              VPN.  This is a client-side option.

              This option performs three steps:

              (1) Create a static route for the --remote address which forwards  to  the  pre-existing  default
              gateway.  This is done so that (3) will not create a routing loop.

              (2) Delete the default gateway route.

              (3) Set the new default gateway to be the VPN endpoint address (derived either from --route-gate‐
              way or the second parameter to --ifconfig when --dev tun is specified).

              When the tunnel is torn down, all of the above steps are reversed so that  the  original  default
              route is restored.

              Option flags:

              local  --  Add the local flag if both OpenVPN servers are directly connected via a common subnet,
              such as with wireless.  The local flag will cause step 1 above to be omitted.

              autolocal -- Try to automatically determine whether to enable local flag above.

              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 has the benefit of overriding but not wiping out the original default gate‐
              way.

              bypass-dhcp -- Add a direct route to the DHCP server (if it is non-local) which bypasses the tun‐
              nel (Available on Windows clients, may not be available on non-Windows clients).

              bypass-dns  -- Add a direct route to the DNS server(s) (if they are non-local) which bypasses the
              tunnel (Available on Windows clients, may not be available on non-Windows clients).

              block-local -- Block access to local LAN when the tunnel is active, except for  the  LAN  gateway
              itself.   This is accomplished by routing the local LAN (except for the LAN gateway address) into
              the tunnel.

       --link-mtu n
              Sets an upper bound on the size of UDP packets which are sent between OpenVPN peers.   It's  best
              not to set this parameter unless you know what you're doing.

       --redirect-private [flags]
              Like  --redirect-gateway,  but  omit  actually changing the default gateway.  Useful when pushing
              private subnets.

       --tun-mtu n
              Take the TUN device MTU to be n and derive the link MTU from it (default=1500).  In  most  cases,
              you will probably want to leave this parameter set to its default value.

              The  MTU  (Maximum  Transmission  Units)  is  the maximum datagram size in bytes that can be sent
              unfragmented over a particular network path.  OpenVPN requires that packets  on  the  control  or
              data channels be sent unfragmented.

              MTU problems often manifest themselves as connections which hang during periods of active usage.

              It's best to use the --fragment and/or --mssfix options to deal with MTU sizing issues.

       --tun-mtu-extra n
              Assume  that  the  TUN/TAP device might return as many as n bytes more than the --tun-mtu size on
              read.  This parameter defaults to 0, which is sufficient for most TUN devices.  TAP  devices  may
              introduce  additional overhead in excess of the MTU size, and a setting of 32 is the default when
              TAP devices are used.  This parameter only controls internal OpenVPN buffer sizing, so  there  is
              no transmission overhead associated with using a larger value.

       --mtu-disc type
              Should  we  do  Path MTU discovery on TCP/UDP channel?  Only supported on OSes such as Linux that
              supports the necessary system call to set.

              'no' -- Never send DF (Don't Fragment) frames
              'maybe' -- Use per-route hints
              'yes' -- Always DF (Don't Fragment)

       --mtu-test
              To empirically measure MTU on connection startup, add the --mtu-test option  to  your  configura‐
              tion.  OpenVPN will send ping packets of various sizes to the remote peer and measure the largest
              packets which were successfully received.  The --mtu-test process normally takes about 3  minutes
              to complete.

       --fragment max
              Enable  internal  datagram  fragmentation so that no UDP datagrams are sent which are larger than
              max bytes.

              The max parameter is interpreted in the same way as the --link-mtu parameter, i.e. the UDP packet
              size after encapsulation overhead has been added in, but not including the UDP header itself.

              The --fragment option only makes sense when you are using the UDP protocol ( --proto udp ).

              --fragment adds 4 bytes of overhead per datagram.

              See the --mssfix option below for an important related option to --fragment.

              It  should  also  be  noted  that this option is not meant to replace UDP fragmentation at the IP
              stack level.  It is only meant as a last resort when path MTU discovery is  broken.   Using  this
              option  is  less  efficient  than  fixing path MTU discovery for your IP link and using native IP
              fragmentation instead.

              Having said that, there are circumstances where using OpenVPN's internal fragmentation capability
              may be your only option, such as tunneling a UDP multicast stream which requires fragmentation.

       --mssfix max
              Announce  to  TCP sessions running over the tunnel that they should limit their send packet sizes
              such that after OpenVPN has encapsulated them, the resulting UDP packet size that  OpenVPN  sends
              to its peer will not exceed max bytes. The default value is 1450.

              The max parameter is interpreted in the same way as the --link-mtu parameter, i.e. the UDP packet
              size after encapsulation overhead has been added in, but not including the UDP header itself.

              The --mssfix option only makes sense when you are using the UDP protocol for OpenVPN peer-to-peer
              communication, i.e.  --proto udp.

              --mssfix  and  --fragment  can be ideally used together, where --mssfix will try to keep TCP from
              needing packet fragmentation in the first place, and if big packets  come  through  anyhow  (from
              protocols other than TCP), --fragment will internally fragment them.

              Both --fragment and --mssfix are designed to work around cases where Path MTU discovery is broken
              on the network path between OpenVPN peers.

              The usual symptom of such a breakdown is an OpenVPN connection  which  successfully  starts,  but
              then stalls during active usage.

              If  --fragment  and --mssfix are used together, --mssfix will take its default max parameter from
              the --fragment max option.

              Therefore, one could lower the maximum UDP packet size to 1300 (a good first try for solving MTU-
              related connection problems) with the following options:

              --tun-mtu 1500 --fragment 1300 --mssfix

       --sndbuf size
              Set the TCP/UDP socket send buffer size.  Currently defaults to 65536 bytes.

       --rcvbuf size
              Set the TCP/UDP socket receive buffer size.  Currently defaults to 65536 bytes.

       --mark value
              Mark encrypted packets being sent with value. The mark value can be matched in policy routing and
              packetfilter rules. This option is only supported in Linux and does nothing  on  other  operating
              systems.

       --socket-flags flags...
              Apply the given flags to the OpenVPN transport socket.  Currently, only TCP_NODELAY is supported.

              The  TCP_NODELAY  socket flag is useful in TCP mode, and causes the kernel to send tunnel packets
              immediately over the TCP connection without trying to group several smaller packets into a larger
              packet.  This can result in a considerably improvement in latency.

              This  option  is pushable from server to client, and should be used on both client and server for
              maximum effect.

       --txqueuelen n
              (Linux only) Set the TX queue length on the TUN/TAP interface.  Currently defaults to 100.

       --shaper n
              Limit bandwidth of outgoing tunnel data to n bytes per second on the TCP/UDP port.  If  you  want
              to limit the bandwidth in both directions, use this option on both peers.

              OpenVPN uses the following algorithm to implement traffic shaping: Given a shaper rate of n bytes
              per second, after a datagram write of b bytes is queued on the TCP/UDP port, wait a minimum of (b
              / n) seconds before queuing the next write.

              It  should  be  noted that OpenVPN supports multiple tunnels between the same two peers, allowing
              you to construct full-speed and reduced bandwidth tunnels at the same time, routing  low-priority
              data  such  as  off-site backups over the reduced bandwidth tunnel, and other data over the full-
              speed tunnel.

              Also note that for low bandwidth tunnels (under 1000 bytes per second), you should  probably  use
              lower MTU values as well (see above), otherwise the packet latency will grow so large as to trig‐
              ger timeouts in the TLS layer and TCP connections running over the tunnel.

              OpenVPN allows n to be between 100 bytes/sec and 100 Mbytes/sec.

       --inactive n [bytes]
              Causes OpenVPN to exit after n seconds of inactivity on the TUN/TAP device. The  time  length  of
              inactivity is measured since the last incoming or outgoing tunnel packet.  The default value is 0
              seconds, which disables this feature.

              If the optional bytes parameter is included, exit if less than bytes of combined  in/out  traffic
              are produced on the tun/tap device in n seconds.

              In  any case, OpenVPN's internal ping packets (which are just keepalives) and TLS control packets
              are not considered "activity", nor are they counted as traffic, as they are  used  internally  by
              OpenVPN and are not an indication of actual user activity.

       --ping n
              Ping  remote over the TCP/UDP control channel if no packets have been sent for at least n seconds
              (specify --ping on both peers to cause ping packets to be sent in both directions  since  OpenVPN
              ping  packets  are  not echoed like IP ping packets).  When used in one of OpenVPN's secure modes
              (where --secret, --tls-server, or --tls-client is specified), the ping  packet  will  be  crypto‐
              graphically secure.

              This option has two intended uses:

              (1)  Compatibility  with stateful firewalls.  The periodic ping will ensure that a stateful fire‐
              wall rule which allows OpenVPN UDP packets to pass will not time out.

              (2) To provide a basis for the remote to test the existence of its  peer  using  the  --ping-exit
              option.

       --ping-exit n
              Causes  OpenVPN  to  exit  after  n seconds pass without reception of a ping or other packet from
              remote.  This option can be combined with --inactive, --ping, and --ping-exit to  create  a  two-
              tiered inactivity disconnect.

              For example,

              openvpn [options...] --inactive 3600 --ping 10 --ping-exit 60

              when used on both peers will cause OpenVPN to exit within 60 seconds if its peer disconnects, but
              will exit after one hour if no actual tunnel data is exchanged.

       --ping-restart n
              Similar to --ping-exit, but trigger a SIGUSR1 restart after n seconds pass without reception of a
              ping or other packet from remote.

              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 the IP address using a service such as http://dyndns.org/ + a  dynamic  DNS
              client such as ddclient.

              If  the  peer  cannot  be  reached,  a  restart will be triggered, causing the hostname used with
              --remote to be re-resolved (if --resolv-retry is also specified).

              In server mode, --ping-restart, --inactive, or any other type of internally generated signal will
              always be applied to individual client instance objects, never to whole server itself.  Note also
              in server mode that any internally generated signal which would normally cause  a  restart,  will
              cause the deletion of the client instance object instead.

              In client mode, the --ping-restart parameter is set to 120 seconds by default.  This default will
              hold until the client pulls a replacement value from the server, based on the --keepalive setting
              in  the  server  configuration.   To  disable the 120 second default, set --ping-restart 0 on the
              client.

              See the signals section below for more information on SIGUSR1.

              Note that the behavior of SIGUSR1 can be modified by the --persist-tun, --persist-key, --persist-
              local-ip, and --persist-remote-ip options.

              Also note that --ping-exit and --ping-restart are mutually exclusive and cannot be used together.

       --keepalive n m
              A  helper  directive  designed  to simplify the expression of --ping and --ping-restart in server
              mode configurations.

              The server timeout is set twice the value of the second argument.  This ensures that a timeout is
              dectected on client side before the server side drops the connection.

              For example, --keepalive 10 60 expands as follows:

                   if mode server:
                     ping 10
                     ping-restart 120
                     push "ping 10"
                     push "ping-restart 60"
                   else
                     ping 10
                     ping-restart 60

       --ping-timer-rem
              Run  the --ping-exit / --ping-restart timer only if we have a remote address.  Use this option if
              you are starting the daemon in listen mode (i.e. without an  explicit  --remote  peer),  and  you
              don't want to start clocking timeouts until a remote peer connects.

       --persist-tun
              Don't  close  and  reopen  TUN/TAP device or run up/down scripts across SIGUSR1 or --ping-restart
              restarts.

              SIGUSR1 is a restart signal similar to SIGHUP, but which offers finer-grained control over  reset
              options.

       --persist-key
              Don't re-read key files across SIGUSR1 or --ping-restart.

              This option can be combined with --user nobody to allow restarts triggered by the SIGUSR1 signal.
              Normally if you drop root privileges in OpenVPN, the daemon cannot be restarted since it will now
              be unable to re-read protected key files.

              This option solves the problem by persisting keys across SIGUSR1 resets, so they don't need to be
              re-read.

       --persist-local-ip
              Preserve initially resolved local IP address and port number  across  SIGUSR1  or  --ping-restart
              restarts.

       --persist-remote-ip
              Preserve  most recently authenticated remote IP address and port number across SIGUSR1 or --ping-
              restart restarts.

       --mlock
              Disable paging by calling the POSIX mlockall function.  Requires that OpenVPN be initially run as
              root (though OpenVPN can subsequently downgrade its UID using the --user option).

              Using this option ensures that key material and tunnel data are never written to disk due to vir‐
              tual memory paging operations which occur under most modern operating systems.  It  ensures  that
              even  if  an attacker was able to crack the box running OpenVPN, he would not be able to scan the
              system swap file to recover previously used ephemeral keys, which are used for a period  of  time
              governed by the --reneg options (see below), then are discarded.

              The  downside  of using --mlock is that it will reduce the amount of physical memory available to
              other applications.

       --up cmd
              Run command cmd after successful TUN/TAP device open (pre --user UID change).

              cmd consists of a path to script (or executable program), optionally followed by  arguments.  The
              path  and  arguments may be single- or double-quoted and/or escaped using a backslash, and should
              be separated by one or more spaces.

              The up command is useful for specifying route commands which route IP traffic destined  for  pri‐
              vate subnets which exist at the other end of the VPN connection into the tunnel.

              For --dev tun execute as:

              cmd tun_dev tun_mtu link_mtu ifconfig_local_ip ifconfig_remote_ip [ init | restart ]

              For --dev tap execute as:

              cmd tap_dev tap_mtu link_mtu ifconfig_local_ip ifconfig_netmask [ init | restart ]

              See the "Environmental Variables" section below for additional parameters passed as environmental
              variables.

              Note that if cmd includes arguments, all OpenVPN-generated arguments will be appended to them  to
              build an argument list with which the executable will be called.

              Typically, cmd will run a script to add routes to the tunnel.

              Normally  the  up script is called after the TUN/TAP device is opened.  In this context, the last
              command line parameter passed to the script will be init.  If the  --up-restart  option  is  also
              used, the up script will be called for restarts as well.  A restart is considered to be a partial
              reinitialization of OpenVPN where the TUN/TAP instance is  preserved  (the  --persist-tun  option
              will enable such preservation).  A restart can be generated by a SIGUSR1 signal, a --ping-restart
              timeout, or a connection reset when the TCP protocol is enabled with the --proto  option.   If  a
              restart occurs, and --up-restart has been specified, the up script will be called with restart as
              the last parameter.

              The following standalone example shows how the --up script can be called in both  an  initializa‐
              tion  and  restart  context.  (NOTE: for security reasons, don't run the following example unless
              UDP port 9999 is blocked by your firewall.  Also, the  example  will  run  indefinitely,  so  you
              should abort with control-c).

              openvpn --dev tun --port 9999 --verb 4 --ping-restart 10 --up 'echo up' --down 'echo down' --per‐
              sist-tun --up-restart

              Note that OpenVPN also provides the --ifconfig option to automatically ifconfig the  TUN  device,
              eliminating  the  need  to define an --up script, unless you also want to configure routes in the
              --up script.

              If --ifconfig is also specified, OpenVPN will pass the ifconfig local and remote endpoints on the
              command line to the --up script so that they can be used to configure routes such as:

              route add -net 10.0.0.0 netmask 255.255.255.0 gw $5

       --up-delay
              Delay  TUN/TAP  open and possible --up script execution until after TCP/UDP connection establish‐
              ment with peer.

              In --proto udp mode, this option normally requires the use of --ping to allow connection  initia‐
              tion to be sensed in the absence of tunnel data, since UDP is a "connectionless" protocol.

              On  Windows,  this option will delay the TAP-Win32 media state transitioning to "connected" until
              connection establishment, i.e. the receipt of the first authenticated packet from the peer.

       --down cmd
              Run command cmd after TUN/TAP device close (post --user UID change and/or --chroot ).   cmd  con‐
              sists of a path to script (or executable program), optionally followed by arguments. The path and
              arguments may be single- or double-quoted and/or escaped using a backslash, and should  be  sepa‐
              rated by one or more spaces.

              Called with the same parameters and environmental variables as the --up option above.

              Note  that  if you reduce privileges by using --user and/or --group, your --down script will also
              run at reduced privilege.

       --down-pre
              Call --down cmd/script before, rather than after, TUN/TAP close.

       --up-restart
              Enable the --up and --down scripts to be called for restarts as well as  initial  program  start.
              This option is described more fully above in the --up option documentation.

       --setenv name value
              Set a custom environmental variable name=value to pass to script.

       --setenv FORWARD_COMPATIBLE 1
              Relax  config  file  syntax  checking so that unknown directives will trigger a warning but not a
              fatal error, on the assumption that a given unknown directive might be valid  in  future  OpenVPN
              versions.

              This  option  should  be used with caution, as there are good security reasons for having OpenVPN
              fail if it detects problems in a config file.  Having said that,  there  are  valid  reasons  for
              wanting new software features to gracefully degrade when encountered by older software versions.

       --setenv-safe name value
              Set a custom environmental variable OPENVPN_name=value to pass to script.

              This  directive  is  designed to be pushed by the server to clients, and the prepending of "OPEN‐
              VPN_" to the environmental variable is a safety precaution to prevent a LD_PRELOAD  style  attack
              from a malicious or compromised server.

       --script-security level [method]
              This directive offers policy-level control over OpenVPN's usage of external programs and scripts.
              Lower level values are more restrictive, higher values are more permissive.  Settings for level:

              0 -- Strictly no calling of external programs.
              1 -- (Default) Only call built-in executables such as ifconfig, ip, route, or netsh.
              2 -- Allow calling of built-in executables and user-defined scripts.
              3 -- Allow passwords to be passed to scripts via environmental variables (potentially unsafe).

              The method parameter indicates how OpenVPN should call external commands and  scripts.   Settings
              for method:

              execve -- (default) Use execve() function on Unix family OSes and CreateProcess() on Windows.
              system -- Use system() function (deprecated and less safe since the external program command line
              is subject to shell expansion).

              The --script-security option was introduced in OpenVPN 2.1_rc9.  For configuration file  compati‐
              bility with previous OpenVPN versions, use: --script-security 3 system

       --disable-occ
              Don't  output a warning message if option inconsistencies are detected between peers.  An example
              of an option inconsistency would be where one peer uses --dev tun while the other peer uses --dev
              tap.

              Use  of  this  option  is  discouraged,  but is provided as a temporary fix in situations where a
              recent version of OpenVPN must connect to an old version.

       --user user
              Change the user ID of the OpenVPN process to user after initialization,  dropping  privileges  in
              the  process.   This  option is useful to protect the system in the event that some hostile party
              was able to gain control of an OpenVPN session.  Though OpenVPN's  security  features  make  this
              unlikely, it is provided as a second line of defense.

              By  setting user to nobody or somebody similarly unprivileged, the hostile party would be limited
              in what damage they could cause.  Of course once you take away privileges, you cannot return them
              to an OpenVPN session.  This means, for example, that if you want to reset an OpenVPN daemon with
              a SIGUSR1 signal (for example in response to a DHCP reset), you should make use of one or more of
              the --persist options to ensure that OpenVPN doesn't need to execute any privileged operations in
              order to restart (such as re-reading key files or running ifconfig on the TUN device).

       --group group
              Similar to the --user option, this option changes the group ID of the OpenVPN  process  to  group
              after initialization.

       --cd dir
              Change  directory  to  dir  prior  to  reading  any files such as configuration files, key files,
              scripts, etc.  dir should be an absolute path, with a leading "/", and without any references  to
              the current directory such as "." or "..".

              This  option is useful when you are running OpenVPN in --daemon mode, and you want to consolidate
              all of your OpenVPN control files in one location.

       --chroot dir
              Chroot to dir after initialization.  --chroot essentially redefines dir as being  the  top  level
              directory  tree  (/).   OpenVPN  will  therefore be unable to access any files outside this tree.
              This can be desirable from a security standpoint.

              Since the chroot operation is delayed until after initialization, most OpenVPN options that  ref‐
              erence files will operate in a pre-chroot context.

              In  many  cases,  the  dir  parameter  can point to an empty directory, however complications can
              result when scripts or restarts are executed after the chroot operation.

       --setcon context
              Apply SELinux context after initialization. This essentially provides  the  ability  to  restrict
              OpenVPN's rights to only network I/O operations, thanks to SELinux. This goes further than --user
              and --chroot in that those two, while being great security features, unfortunately do not protect
              against  privilege escalation by exploitation of a vulnerable system call. You can of course com‐
              bine all three, but please note that since setcon requires access to /proc you will have to  pro‐
              vide it inside the chroot directory (e.g. with mount --bind).

              Since  the  setcon  operation is delayed until after initialization, OpenVPN can be restricted to
              just network-related system calls, whereas by applying the context before startup  (such  as  the
              OpenVPN  one  provided  in  the  SELinux  Reference  Policies) you will have to allow many things
              required only during initialization.

              Like with chroot, complications can result when scripts or restarts are executed after the setcon
              operation,  which  is  why  you  should really consider using the --persist-key and --persist-tun
              options.

       --daemon [progname]
              Become a daemon after all initialization functions are completed.  This  option  will  cause  all
              message  and  error  output to be sent to the syslog file (such as /var/log/messages), except for
              the output of scripts and ifconfig commands, which will go to /dev/null  unless  otherwise  redi‐
              rected.   The  syslog  redirection occurs immediately at the point that --daemon is parsed on the
              command line even though the daemonization point occurs later.  If one of the  --log  options  is
              present, it will supercede syslog redirection.

              The  optional progname parameter will cause OpenVPN to report its program name to the system log‐
              ger as progname.  This can be useful in linking OpenVPN messages in the syslog file with specific
              tunnels.  When unspecified, progname defaults to "openvpn".

              When OpenVPN is run with the --daemon option, it will try to delay daemonization until the major‐
              ity of initialization functions which are capable of generating fatal errors are complete.   This
              means  that initialization scripts can test the return status of the openvpn command for a fairly
              reliable indication of whether the command has correctly initialized and entered the packet  for‐
              warding event loop.

              In OpenVPN, the vast majority of errors which occur after initialization are non-fatal.

       --syslog [progname]
              Direct log output to system logger, but do not become a daemon.  See --daemon directive above for
              description of progname parameter.

       --errors-to-stderr
              Output errors to stderr instead of stdout unless log output is redirected by  one  of  the  --log
              options.

       --passtos
              Set the TOS field of the tunnel packet to what the payload's TOS is.

       --inetd [wait|nowait] [progname]
              Use this option when OpenVPN is being run from the inetd or xinetd(8) server.

              The  wait/nowait option must match what is specified in the inetd/xinetd config file.  The nowait
              mode can only be used with --proto tcp-server.  The default is wait.  The nowait mode can be used
              to  instantiate  the OpenVPN daemon as a classic TCP server, where client connection requests are
              serviced on a single port number.  For additional information on this kind of configuration,  see
              the OpenVPN FAQ: http://openvpn.net/faq.html#oneport

              This  option  precludes  the use of --daemon, --local, or --remote.  Note that this option causes
              message and error output to be handled in the same way as  the  --daemon  option.   The  optional
              progname parameter is also handled exactly as in --daemon.

              Also  note that in wait mode, each OpenVPN tunnel requires a separate TCP/UDP port and a separate
              inetd or xinetd entry.  See the OpenVPN 1.x HOWTO for an example on using  OpenVPN  with  xinetd:
              http://openvpn.net/1xhowto.html

       --log file
              Output  logging  messages to file, including output to stdout/stderr which is generated by called
              scripts.  If file already exists it will be truncated.  This option takes effect immediately when
              it  is parsed in the command line and will supercede syslog output if --daemon or --inetd is also
              specified.  This option is persistent over the entire course of an OpenVPN instantiation and will
              not be reset by SIGHUP, SIGUSR1, or --ping-restart.

              Note that on Windows, when OpenVPN is started as a service, logging occurs by default without the
              need to specify this option.

       --log-append file
              Append logging messages to file.  If file does not  exist,  it  will  be  created.   This  option
              behaves exactly like --log except that it appends to rather than truncating the log file.

       --suppress-timestamps
              Avoid writing timestamps to log messages, even when they otherwise would be prepended. In partic‐
              ular, this applies to log messages sent to stdout.

       --writepid file
              Write OpenVPN's main process ID to file.

       --nice n
              Change process priority after initialization ( n greater than 0 is lower priority,  n  less  than
              zero is higher priority).

       --fast-io
              (Experimental)  Optimize  TUN/TAP/UDP I/O writes by avoiding a call to poll/epoll/select prior to
              the write operation.  The purpose of such a call would normally be to block until the  device  or
              socket  is ready to accept the write.  Such blocking is unnecessary on some platforms which don't
              support write blocking on UDP sockets or TUN/TAP devices.  In such cases, one  can  optimize  the
              event loop by avoiding the poll/epoll/select call, improving CPU efficiency by 5% to 10%.

              This  option  can  only  be  used on non-Windows systems, when --proto udp is specified, and when
              --shaper is NOT specified.

       --multihome
              Configure a multi-homed UDP server.  This option can be used when OpenVPN has been configured  to
              listen  on  all  interfaces,  and  will attempt to bind client sessions to the interface on which
              packets are being received, so that outgoing packets will be sent  out  of  the  same  interface.
              Note  that  this  option  is  only  relevant for UDP servers and currently is only implemented on
              Linux.

              Note: clients connecting to a --multihome server should always use the --nobind option.

       --echo [parms...]
              Echo parms to log output.

              Designed to be used to send messages to a controlling application which is receiving the  OpenVPN
              log output.

       --remap-usr1 signal
              Control  whether  internally  or  externally  generated  SIGUSR1  signals  are remapped to SIGHUP
              (restart without persisting state) or SIGTERM (exit).

              signal can be set to "SIGHUP" or "SIGTERM".  By default, no remapping occurs.

       --verb n
              Set output verbosity to n (default=1).  Each level shows  all  info  from  the  previous  levels.
              Level  3  is  recommended if you want a good summary of what's happening without being swamped by
              output.

              0 -- No output except fatal errors.
              1 to 4 -- Normal usage range.
              5 -- Output R and W characters to the console for each packet read and write, uppercase  is  used
              for TCP/UDP packets and lowercase is used for TUN/TAP packets.
              6 to 11 -- Debug info range (see errlevel.h for additional information on debug levels).

       --status file [n]
              Write operational status to file every n seconds.

              Status can also be written to the syslog by sending a SIGUSR2 signal.

       --status-version [n]
              Choose the status file format version number.  Currently n can be 1, 2, or 3 and defaults to 1.

       --mute n
              Log at most n consecutive messages in the same category.  This is useful to limit repetitive log‐
              ging of similar message types.

       --comp-lzo [mode]
              Use fast LZO compression -- may add up to 1 byte per packet for incompressible data.  mode may be
              "yes", "no", or "adaptive" (default).

              In  a  server mode setup, it is possible to selectively turn compression on or off for individual
              clients.

              First, make sure the client-side config file enables selective compression by having at least one
              --comp-lzo  directive,  such  as  --comp-lzo  no.  This will turn off compression by default, but
              allow a future directive push from the server to dynamically change the on/off/adaptive setting.

              Next in a --client-config-dir file, specify the compression setting for the client, for example:

                  comp-lzo yes
                  push "comp-lzo yes"

              The first line sets the comp-lzo setting for the server side of the link,  the  second  sets  the
              client side.

       --comp-noadapt
              When used in conjunction with --comp-lzo, this option will disable OpenVPN's adaptive compression
              algorithm.  Normally, adaptive compression is enabled with --comp-lzo.

              Adaptive compression tries to optimize the case where you have compression enabled, but  you  are
              sending  predominantly uncompressible (or pre-compressed) packets over the tunnel, such as an FTP
              or rsync transfer of a large, compressed file.  With adaptive compression, OpenVPN will  periodi‐
              cally  sample the compression process to measure its efficiency.  If the data being sent over the
              tunnel is already compressed, the compression efficiency will be very low, triggering openvpn  to
              disable compression for a period of time until the next re-sample test.

       --management IP port [pw-file]
              Enable  a TCP server on IP:port to handle daemon management functions.  pw-file, if specified, is
              a password file (password on first line) or "stdin" to prompt from standard input.  The  password
              provided  will set the password which TCP clients will need to provide in order to access manage‐
              ment functions.

              The management interface can also listen on a unix domain socket, for those platforms  that  sup‐
              port  it.   To  use a unix domain socket, specify the unix socket pathname in place of IP and set
              port to 'unix'.  While the default behavior is to create a unix domain socket that  may  be  con‐
              nected  to  by any process, the --management-client-user and --management-client-group directives
              can be used to restrict access.

              The management interface provides a special mode where the TCP management link can  operate  over
              the  tunnel  itself.  To enable this mode, set IP = "tunnel".  Tunnel mode will cause the manage‐
              ment interface to listen for a TCP connection on the local VPN address of the TUN/TAP interface.

              While the management port is designed for programmatic control of OpenVPN by other  applications,
              it  is possible to telnet to the port, using a telnet client in "raw" mode.  Once connected, type
              "help" for a list of commands.

              For detailed documentation on the management interface, see the management-notes.txt file in  the
              management folder of the OpenVPN source distribution.

              It  is  strongly recommended that IP be set to 127.0.0.1 (localhost) to restrict accessibility of
              the management server to local clients.

       --management-client
              Management interface will connect as a TCP/unix domain client to IP:port specified  by  --manage‐
              ment rather than listen as a TCP server or on a unix domain socket.

              If  the client connection fails to connect or is disconnected, a SIGTERM signal will be generated
              causing OpenVPN to quit.

       --management-query-passwords
              Query management channel for private key password and --auth-user-pass  username/password.   Only
              query  the  management  channel for inputs which ordinarily would have been queried from the con‐
              sole.

       --management-query-proxy
              Query management channel for proxy server information for a specific --remote (client-only).

       --management-query-remote
              Allow management interface to override --remote directives (client-only).

       --management-forget-disconnect
              Make OpenVPN forget passwords when management session disconnects.

              This directive does not affect the --http-proxy username/password.  It is always cached.

       --management-hold
              Start OpenVPN in a hibernating state, until a  client  of  the  management  interface  explicitly
              starts it with the hold release command.

       --management-signal
              Send  SIGUSR1  signal to OpenVPN if management session disconnects.  This is useful when you wish
              to disconnect an OpenVPN session on user logoff.  For  --management-client  this  option  is  not
              needed since a disconnect will always generate a SIGTERM.

       --management-log-cache n
              Cache the most recent n lines of log file history for usage by the management channel.

       --management-up-down
              Report tunnel up/down events to management interface.

       --management-client-auth
              Gives  management  interface client the responsibility to authenticate clients after their client
              certificate has been verified.  See management-notes.txt in  OpenVPN  distribution  for  detailed
              notes.

       --management-client-pf
              Management  interface  clients must specify a packet filter file for each connecting client.  See
              management-notes.txt in OpenVPN distribution for detailed notes.

       --management-client-user u
              When the management interface is listening on a unix domain socket, only allow  connections  from
              user u.

       --management-client-group g
              When  the  management interface is listening on a unix domain socket, only allow connections from
              group g.

       --plugin module-pathname [init-string]
              Load plug-in module from the file module-pathname, passing init-string as an argument to the mod‐
              ule initialization function.  Multiple plugin modules may be loaded into one OpenVPN process.

              For more information and examples on how to build OpenVPN plug-in modules, see the README file in
              the plugin folder of the OpenVPN source distribution.

              If you are using an RPM install of OpenVPN, see /usr/share/openvpn/plugin.  The documentation  is
              in doc and the actual plugin modules are in lib.

              Multiple  plugin  modules  can  be cascaded, and modules can be used in tandem with scripts.  The
              modules will be called by OpenVPN in the order that they are declared in  the  config  file.   If
              both  a  plugin  and script are configured for the same callback, the script will be called last.
              If the return code of the module/script controls an authentication function (such as  tls-verify,
              auth-user-pass-verify,  or  client-connect), then every module and script must return success (0)
              in order for the connection to be authenticated.

   Server Mode
       Starting with OpenVPN 2.0, a multi-client TCP/UDP server mode is supported, and can be enabled with  the
       --mode  server option.  In server mode, OpenVPN will listen on a single port for incoming client connec‐
       tions.  All client connections will be routed through a single tun  or  tap  interface.   This  mode  is
       designed  for  scalability and should be able to support hundreds or even thousands of clients on suffi‐
       ciently fast hardware.  SSL/TLS authentication must be used in this mode.

       --server network netmask
              A helper directive designed to simplify the configuration of OpenVPN's server mode.  This  direc‐
              tive will set up an OpenVPN server which will allocate addresses to clients out of the given net‐
              work/netmask.  The server itself will take the ".1" address of the given network for use  as  the
              server-side endpoint of the local TUN/TAP interface.

              For example, --server 10.8.0.0 255.255.255.0 expands as follows:

                   mode server
                   tls-server
                   push "topology [topology]"

                   if dev tun AND (topology == net30 OR topology == p2p):
                     ifconfig 10.8.0.1 10.8.0.2
                     if !nopool:
                       ifconfig-pool 10.8.0.4 10.8.0.251
                     route 10.8.0.0 255.255.255.0
                     if client-to-client:
                       push "route 10.8.0.0 255.255.255.0"
                     else if topology == net30:
                       push "route 10.8.0.1"

                   if dev tap OR (dev tun AND topology == subnet):
                     ifconfig 10.8.0.1 255.255.255.0
                     if !nopool:
                       ifconfig-pool 10.8.0.2 10.8.0.254 255.255.255.0
                     push "route-gateway 10.8.0.1"

              Don't use --server if you are ethernet bridging.  Use --server-bridge instead.

       --server-bridge gateway netmask pool-start-IP pool-end-IP

       --server-bridge ['nogw']

              A  helper  directive similar to --server which is designed to simplify the configuration of Open‐
              VPN's server mode in ethernet bridging configurations.

              If --server-bridge is used without any parameters, it will enable a DHCP-proxy mode,  where  con‐
              necting  OpenVPN  clients  will  receive an IP address for their TAP adapter from the DHCP server
              running on the OpenVPN server-side LAN.  Note that only clients that support  the  binding  of  a
              DHCP client with the TAP adapter (such as Windows) can support this mode.  The optional nogw flag
              (advanced) indicates that gateway information should not be pushed to the client.

              To configure ethernet bridging, you must first use your OS's bridging capability  to  bridge  the
              TAP interface with the ethernet NIC interface.  For example, on Linux this is done with the brctl
              tool, and with Windows XP it is done in the Network Connections Panel by selecting  the  ethernet
              and TAP adapters and right-clicking on "Bridge Connections".

              Next  you  you must manually set the IP/netmask on the bridge interface.  The gateway and netmask
              parameters to --server-bridge can be set to either the IP/netmask of the bridge interface, or the
              IP/netmask of the default gateway/router on the bridged subnet.

              Finally,  set  aside  a IP range in the bridged subnet, denoted by pool-start-IP and pool-end-IP,
              for OpenVPN to allocate to connecting clients.

              For example, server-bridge 10.8.0.4 255.255.255.0 10.8.0.128 10.8.0.254 expands as follows:

                  mode server
                  tls-server

                  ifconfig-pool 10.8.0.128 10.8.0.254 255.255.255.0
                  push "route-gateway 10.8.0.4"

              In another example, --server-bridge (without parameters) expands as follows:

                  mode server
                  tls-server

                  push "route-gateway dhcp"

              Or --server-bridge nogw expands as follows:

                  mode server
                  tls-server

       --push option
              Push a config file option back to the client for remote execution.   Note  that  option  must  be
              enclosed  in  double quotes ("").  The client must specify --pull in its config file.  The set of
              options which can be pushed is limited by both feasibility and security.  Some  options  such  as
              those  which  would  execute scripts are banned, since they would effectively allow a compromised
              server to execute arbitrary code on the client.  Other options such as TLS or MTU parameters can‐
              not  be  pushed  because the client needs to know them before the connection to the server can be
              initiated.

              This is a partial list of options  which  can  currently  be  pushed:  --route,  --route-gateway,
              --route-delay,  --redirect-gateway,  --ip-win32,  --dhcp-option, --inactive, --ping, --ping-exit,
              --ping-restart,  --setenv,  --persist-key,  --persist-tun,  --echo,  --comp-lzo,  --socket-flags,
              --sndbuf, --rcvbuf

       --push-reset
              Don't  inherit  the  global  push  list for a specific client instance.  Specify this option in a
              client-specific context such as with a --client-config-dir configuration file.  This option  will
              ignore --push options at the global config file level.

       --push-peer-info
              Push  additional  information about the client to server.  The additional information consists of
              the following data:

              IV_VER=<version> -- the client OpenVPN version

              IV_PLAT=[linux|solaris|openbsd|mac|netbsd|freebsd|win] -- the client OS platform

              IV_HWADDR=<mac address> -- the MAC address of clients default gateway

              IV_LZO_STUB=1 -- if client was built with LZO stub capability

              UV_<name>=<value> -- client environment variables whose names start with "UV_"

       --disable
              Disable a particular client (based on the common name) from connecting.  Don't use this option to
              disable  a  client  due  to  key or password compromise.  Use a CRL (certificate revocation list)
              instead (see the --crl-verify option).

              This option must be associated with a specific client instance, which means that it must be spec‐
              ified  either in a client instance config file using --client-config-dir or dynamically generated
              using a --client-connect script.

       --ifconfig-pool start-IP end-IP [netmask]
              Set aside a pool of subnets to be dynamically allocated to connecting clients, similar to a  DHCP
              server.  For tun-style tunnels, each client will be given a /30 subnet (for interoperability with
              Windows clients).  For tap-style  tunnels,  individual  addresses  will  be  allocated,  and  the
              optional netmask parameter will also be pushed to clients.


       --ifconfig-pool-persist file [seconds]
              Persist/unpersist  ifconfig-pool  data to file, at seconds intervals (default=600), as well as on
              program startup and shutdown.

              The goal of this option is to provide a long-term association between clients (denoted  by  their
              common  name)  and the virtual IP address assigned to them from the ifconfig-pool.  Maintaining a
              long-term association is good for clients because it allows them to effectively  use  the  --per‐
              sist-tun option.

              file is a comma-delimited ASCII file, formatted as <Common-Name>,<IP-address>.

              If  seconds  =  0,  file will be treated as read-only.  This is useful if you would like to treat
              file as a configuration file.

              Note that the entries in this file are treated by OpenVPN as  suggestions  only,  based  on  past
              associations  between  a common name and IP address.  They do not guarantee that the given common
              name will always receive the given IP address.  If you want guaranteed assignment,  use  --ifcon‐
              fig-push

       --ifconfig-pool-linear
              Modifies the --ifconfig-pool directive to allocate individual TUN interface addresses for clients
              rather than /30 subnets.  NOTE:  This option is incompatible with Windows clients.

              This option is deprecated, and should be replaced  with  --topology  p2p  which  is  functionally
              equivalent.

       --ifconfig-push local remote-netmask [alias]
              Push virtual IP endpoints for client tunnel, overriding the --ifconfig-pool dynamic allocation.

              The  parameters  local and remote-netmask are set according to the --ifconfig directive which you
              want to execute on the client machine to configure the remote end of the tunnel.  Note  that  the
              parameters local and remote-netmask are from the perspective of the client, not the server.  They
              may be DNS names rather than IP addresses, in which case they will be resolved on the  server  at
              the time of client connection.

              The  optional  alias parameter may be used in cases where NAT causes the client view of its local
              endpoint to differ from the server view.  In this case local/remote-netmask  will  refer  to  the
              server view while alias/remote-netmask will refer to the client view.

              This option must be associated with a specific client instance, which means that it must be spec‐
              ified either in a client instance config file using --client-config-dir or dynamically  generated
              using a --client-connect script.

              Remember  also  to  include  a  --route  directive in the main OpenVPN config file which encloses
              local, so that the kernel will know to route it to the server's TUN/TAP interface.

              OpenVPN's internal client IP address selection algorithm works as follows:

              1 -- Use --client-connect script generated file for static IP (first choice).
              2 -- Use --client-config-dir file for static IP (next choice).
              3 -- Use --ifconfig-pool allocation for dynamic IP (last choice).

       --iroute network [netmask]
              Generate an internal route to a specific client. The netmask parameter, if omitted,  defaults  to
              255.255.255.255.

              This  directive  can  be  used  to  route  a fixed subnet from the server to a particular client,
              regardless of where the client is connecting from.  Remember that you must also add the route  to
              the  system  routing  table as well (such as by using the --route directive).  The reason why two
              routes are needed is that the --route directive routes the packet from  the  kernel  to  OpenVPN.
              Once in OpenVPN, the --iroute directive routes to the specific client.

              This  option  must be specified either in a client instance config file using --client-config-dir
              or dynamically generated using a --client-connect script.

              The --iroute directive also has an important  interaction  with  --push  "route  ...".   --iroute
              essentially  defines a subnet which is owned by a particular client (we will call this client A).
              If you would like other clients to be able to reach A's subnet, you can use  --push  "route  ..."
              together  with  --client-to-client  to  effect this.  In order for all clients to see A's subnet,
              OpenVPN must push this route to all clients EXCEPT for A, since the subnet is already owned by A.
              OpenVPN  accomplishes  this  by  not  not  pushing  a  route to a client if it matches one of the
              client's iroutes.

       --client-to-client
              Because the OpenVPN server mode handles multiple clients through a single tun or  tap  interface,
              it  is  effectively  a  router.   The  --client-to-client  flag tells OpenVPN to internally route
              client-to-client traffic rather than pushing all client-originating traffic to the TUN/TAP inter‐
              face.

              When this option is used, each client will "see" the other clients which are currently connected.
              Otherwise, each client will only see the server.  Don't use this option if you want  to  firewall
              tunnel traffic using custom, per-client rules.

       --duplicate-cn
              Allow multiple clients with the same common name to concurrently connect.  In the absence of this
              option, OpenVPN will disconnect a client instance upon connection of a new client having the same
              common name.

       --client-connect cmd
              Run command cmd on client connection.

              cmd  consists  of a path to script (or executable program), optionally followed by arguments. The
              path and arguments may be single- or double-quoted and/or escaped using a backslash,  and  should
              be separated by one or more spaces.

              The command is passed the common name and IP address of the just-authenticated client as environ‐
              mental variables (see environmental variable section below).  The  command  is  also  passed  the
              pathname  of a freshly created temporary file as the last argument (after any arguments specified
              in cmd ), to be used by the command to pass dynamically generated config file directives back  to
              OpenVPN.

              If the script wants to generate a dynamic config file to be applied on the server when the client
              connects, it should write it to the file named by the last argument.

              See the --client-config-dir option below for options which can be legally used in  a  dynamically
              generated config file.

              Note  that the return value of script is significant.  If script returns a non-zero error status,
              it will cause the client to be disconnected.

       --client-disconnect cmd
              Like --client-connect but called on client instance shutdown.  Will  not  be  called  unless  the
              --client-connect  script  and  plugins  (if defined) were previously called on this instance with
              successful (0) status returns.

              The exception to this rule is if the --client-disconnect command or plugins are cascaded, and  at
              least  one  client-connect  function  succeeded,  then ALL of the client-disconnect functions for
              scripts and plugins will be called on client instance object deletion, even in cases  where  some
              of the related client-connect functions returned an error status.

              The --client-disconnect command is passed the same pathname as the corresponding --client-connect
              command as its last argument. (after any arguments specified in cmd ).

       --client-config-dir dir
              Specify a directory dir for custom client config files.   After  a  connecting  client  has  been
              authenticated,  OpenVPN  will  look  in  this  directory  for  a file having the same name as the
              client's X509 common name.  If a matching file exists, it will be opened and parsed  for  client-
              specific  configuration  options.  If no matching file is found, OpenVPN will instead try to open
              and parse a default file called "DEFAULT", which may be provided but is not required.  Note  that
              the  configuration  files  must be readable by the OpenVPN process after it has dropped it's root
              privileges.

              This file can specify a fixed IP address for a given client using  --ifconfig-push,  as  well  as
              fixed subnets owned by the client using --iroute.

              One  of  the  useful properties of this option is that it allows client configuration files to be
              conveniently created, edited, or removed while the server is live, without needing to restart the
              server.

              The  following  options  are  legal in a client-specific context: --push, --push-reset, --iroute,
              --ifconfig-push, and --config.

       --ccd-exclusive
              Require, as a condition of authentication, that a connecting  client  has  a  --client-config-dir
              file.

       --tmp-dir dir
              Specify  a  directory  dir for temporary files.  This directory will be used by openvpn processes
              and script to communicate temporary data with openvpn main process. Note that the directory  must
              be writable by the OpenVPN process after it has dropped it's root privileges.

              This directory will be used by in the following cases:

              * --client-connect scripts to dynamically generate client-specific configuration files.

              *  OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY  plugin  hook  to  return  success/failure  via auth_con‐
              trol_file when using deferred auth method

              * OPENVPN_PLUGIN_ENABLE_PF plugin hook to pass filtering rules via pf_file

       --hash-size r v
              Set the size of the real address hash table to r and the virtual address table to v.  By default,
              both tables are sized at 256 buckets.

       --bcast-buffers n
              Allocate n buffers for broadcast datagrams (default=256).

       --tcp-queue-limit n
              Maximum number of output packets queued before TCP (default=64).

              When OpenVPN is tunneling data from a TUN/TAP device to a remote client over a TCP connection, it
              is possible that the TUN/TAP device might produce data at a faster rate than the  TCP  connection
              can  support.   When the number of output packets queued before sending to the TCP socket reaches
              this limit for a given client connection, OpenVPN will start to drop outgoing packets directed at
              this client.

       --tcp-nodelay
              This  macro  sets  the  TCP_NODELAY  socket flag on the server as well as pushes it to connecting
              clients.  The TCP_NODELAY flag disables the Nagle algorithm on TCP sockets causing packets to  be
              transmitted  immediately with low latency, rather than waiting a short period of time in order to
              aggregate several packets into a  larger  containing  packet.   In  VPN  applications  over  TCP,
              TCP_NODELAY is generally a good latency optimization.

              The macro expands as follows:

                   if mode server:
                     socket-flags TCP_NODELAY
                     push "socket-flags TCP_NODELAY"

       --max-clients n
              Limit server to a maximum of n concurrent clients.

       --max-routes-per-client n
              Allow  a maximum of n internal routes per client (default=256).  This is designed to help contain
              DoS attacks where an authenticated client floods the server with packets appearing to  come  from
              many  unique  MAC addresses, forcing the server to deplete virtual memory as its internal routing
              table expands.  This directive can be used in a --client-config-dir file or auto-generated  by  a
              --client-connect script to override the global value for a particular client.

              Note that this directive affects OpenVPN's internal routing table, not the kernel routing table.

       --stale-routes-check n [t]
              Remove routes haven't had activity for n seconds (i.e. the ageing time).

              This check is ran every t seconds (i.e. check interval).

              If t is not present it defaults to n

              This option helps to keep the dynamic routing table small.  See also --max-routes-per-client

       --connect-freq n sec
              Allow  a  maximum of n new connections per sec seconds from clients.  This is designed to contain
              DoS attacks which flood the server with connection requests using certificates which  will  ulti‐
              mately fail to authenticate.

              This  is  an  imperfect  solution however, because in a real DoS scenario, legitimate connections
              might also be refused.

              For the best protection against DoS attacks in server mode, use --proto udp and --tls-auth.

       --learn-address cmd
              Run command cmd to validate client virtual addresses or routes.

              cmd consists of a path to script (or executable program), optionally followed by  arguments.  The
              path  and  arguments may be single- or double-quoted and/or escaped using a backslash, and should
              be separated by one or more spaces.

              Three arguments will be appended to any arguments in cmd as follows:

              [1] operation -- "add", "update", or "delete" based on whether or not the address is being  added
              to, modified, or deleted from OpenVPN's internal routing table.
              [2]  address  --  The  address  being  learned or unlearned.  This can be an IPv4 address such as
              "198.162.10.14", an IPv4 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".
              [3]  common  name -- The common name on the certificate associated with the client linked to this
              address.  Only present for "add" or "update" operations, not "delete".

              On "add" or "update" methods, if the script returns  a  failure  code  (non-zero),  OpenVPN  will
              reject the address and will not modify its internal routing table.

              Normally,  the  cmd  script  will  use the information provided above to set appropriate firewall
              entries on the VPN TUN/TAP interface.  Since OpenVPN provides the association between virtual  IP
              or  MAC  address  and  the client's authenticated common name, it allows a user-defined script to
              configure firewall access policies with regard to the client's  high-level  common  name,  rather
              than the low level client virtual addresses.

       --auth-user-pass-verify cmd method
              Require  the client to provide a username/password (possibly in addition to a client certificate)
              for authentication.

              OpenVPN will run command cmd to validate the username/password provided by the client.

              cmd consists of a path to script (or executable program), optionally followed by  arguments.  The
              path  and  arguments may be single- or double-quoted and/or escaped using a backslash, and should
              be separated by one or more spaces.

              If method is set to "via-env", OpenVPN will call script with the environmental variables username
              and  password  set  to  the username/password strings provided by the client.  Be aware that this
              method is insecure on some platforms which make the environment of a process publicly visible  to
              other unprivileged processes.

              If  method  is  set  to "via-file", OpenVPN will write the username and password to the first two
              lines of a temporary file.  The filename will be passed as an argument to script,  and  the  file
              will be automatically deleted by OpenVPN after the script returns.  The location of the temporary
              file is controlled by the --tmp-dir option, and will default to the current directory if unspeci‐
              fied.  For security, consider setting --tmp-dir to a volatile storage medium such as /dev/shm (if
              available) to prevent the username/password file from touching the hard drive.

              The script should examine the username and password, returning a success exit  code  (0)  if  the
              client's authentication request is to be accepted, or a failure code (1) to reject the client.

              This directive is designed to enable a plugin-style interface for extending OpenVPN's authentica‐
              tion capabilities.

              To protect against a client passing a maliciously formed username or password string,  the  user‐
              name  string must consist only of these characters: alphanumeric, underbar ('_'), dash ('-'), dot
              ('.'), or at ('@').  The password string can consist of any printable characters except for CR or
              LF.  Any illegal characters in either the username or password string will be converted to under‐
              bar ('_').

              Care must be taken by any user-defined scripts to avoid creating a security vulnerability in  the
              way  that  these  strings  are handled.  Never use these strings in such a way that they might be
              escaped or evaluated by a shell interpreter.

              For a sample script that performs PAM authentication, see sample-scripts/auth-pam.pl in the Open‐
              VPN source distribution.

       --opt-verify
              Clients  that connect with options that are incompatible with those of the server will be discon‐
              nected.

              Options that will be compared for compatibility include dev-type, link-mtu, tun-mtu, proto,  tun-
              ipv6, ifconfig, comp-lzo, fragment, keydir, cipher, auth, keysize, secret, no-replay, no-iv, tls-
              auth, key-method, tls-server, and tls-client.

              This option requires that --disable-occ NOT be used.

       --auth-user-pass-optional
              Allow connections by clients that do not specify a  username/password.   Normally,  when  --auth-
              user-pass-verify  or  --management-client-auth is specified (or an authentication plugin module),
              the OpenVPN server daemon will require connecting clients to specify  a  username  and  password.
              This  option makes the submission of a username/password by clients optional, passing the respon‐
              sibility to the user-defined authentication module/script to accept or deny the client  based  on
              other  factors (such as the setting of X509 certificate fields).  When this option is used, and a
              connecting client does not submit  a  username/password,  the  user-defined  authentication  mod‐
              ule/script  will see the username and password as being set to empty strings ("").  The authenti‐
              cation module/script MUST have logic to detect this condition and respond accordingly.

       --client-cert-not-required
              Don't require client certificate, client will  authenticate  using  username/password  only.   Be
              aware that using this directive is less secure than requiring certificates from all clients.

              If  you use this directive, the entire responsibility of authentication will rest on your --auth-
              user-pass-verify script, so keep in mind that bugs in your script  could  potentially  compromise
              the security of your VPN.

              If  you  don't  use  this directive, but you also specify an --auth-user-pass-verify script, then
              OpenVPN will perform double authentication.  The client certificate verification AND the  --auth-
              user-pass-verify  script  will  need  to  succeed  in  order for a client to be authenticated and
              accepted onto the VPN.

       --username-as-common-name
              For --auth-user-pass-verify authentication, use the authenticated username as  the  common  name,
              rather than the common name from the client cert.

       --compat-names [no-remapping]
              Until OpenVPN v2.3 the format of the X.509 Subject fields was formatted like this:

              /C=US/L=Somewhere/CN=John Doe/emailAddress=john@example.com

              In  addition  the  old  behavivour was to remap any character other than alphanumeric, underscore
              ('_'), dash ('-'), dot ('.'), and slash ('/') to underscore ('_').  The X.509 Subject  string  as
              returned  by  the  tls_id environmental variable, could additionally contain colon (':') or equal
              ('=').

              When using the --compat-names option, this old formatting and remapping will be re-enabled again.
              This  is  purely implemented for compatibility reasons when using older plug-ins or scripts which
              does not handle the new formatting or UTF-8 characters.

              In OpenVPN v2.3 the formatting of these fields changed into a more standardised format.   It  now
              looks like:

              C=US, L=Somewhere, CN=John Doe, emailAddress=john@example.com

              The  new  default  format in OpenVPN v2.3 also does not do the character remapping which happened
              earlier.  This new format enables proper support for UTF-8 characters  in  the  usernames,  X.509
              Subject  fields and Common Name variables and it complies to the RFC 2253, UTF-8 String Represen‐
              tation of Distinguished Names.

              As a backwards compatibility for the removed --no-name-remapping feature in  older  OpenVPN  ver‐
              sions,  the  no-remapping  mode  flag can be used with the --compat-names option.  When this mode
              flag is used, the Common Name, Subject, and username strings are allowed to include any printable
              character  including  space, but excluding control characters such as tab, newline, and carriage-
              return. It ensures compatibility with the --no-name-remapping option of OpenVPN  versions  before
              v2.3.

              Please  note: This option will not be around for a long time.  It is only implemented to make the
              transition to the new formatting less intrusive.  It will be removed either in  OpenVPN  v2.4  or
              v2.5.   So please make sure you start the process to support the new formatting as soon as possi‐
              ble.

       --port-share host port [dir]
              When run in TCP server mode, share the OpenVPN port with another application, such  as  an  HTTPS
              server.   If  OpenVPN  senses  a connection to its port which is using a non-OpenVPN protocol, it
              will proxy the connection to the server at host:port.   Currently  only  designed  to  work  with
              HTTP/HTTPS, though it would be theoretically possible to extend to other protocols such as ssh.

              dir  specifies an optional directory where a temporary file with name N containing content C will
              be dynamically generated for each proxy connection, where N is the source IP:port of  the  client
              connection  and  C is the source IP:port of the connection to the proxy receiver.  This directory
              can be used as a dictionary by the proxy receiver to determine  the  origin  of  the  connection.
              Each generated file will be automatically deleted when the proxied connection is torn down.

              Not implemented on Windows.

   Client Mode
       Use  client  mode  when  connecting  to an OpenVPN server which has --server, --server-bridge, or --mode
       server in it's configuration.

       --client
              A helper directive designed to simplify the configuration of OpenVPN's client mode.  This  direc‐
              tive is equivalent to:

                   pull
                   tls-client

       --pull This  option must be used on a client which is connecting to a multi-client server.  It indicates
              to OpenVPN that it should accept options pushed by the server, provided  they  are  part  of  the
              legal set of pushable options (note that the --pull option is implied by --client ).

              In  particular,  --pull  allows  the  server  to push routes to the client, so you should not use
              --pull or --client in situations where you don't trust  the  server  to  have  control  over  the
              client's routing table.

       --auth-user-pass [up]
              Authenticate with server using username/password.  up is a file containing username/password on 2
              lines (Note: OpenVPN will only read passwords from a file if it has been built with the --enable-
              password-save  configure  option,  or  on  Windows  by  defining ENABLE_PASSWORD_SAVE in win/set‐
              tings.in).

              If up is omitted, username/password will be prompted from the console.

              The server configuration must specify an  --auth-user-pass-verify  script  to  verify  the  user‐
              name/password provided by the client.

       --auth-retry type
              Controls  how  OpenVPN  responds to username/password verification errors such as the client-side
              response to an AUTH_FAILED message from the server or verification failure  of  the  private  key
              password.

              Normally  used  to  prevent  auth errors from being fatal on the client side, and to permit user‐
              name/password requeries in case of error.

              An AUTH_FAILED message is generated by the server if the client fails --auth-user-pass  authenti‐
              cation,  or  if  the  server-side --client-connect script returns an error status when the client
              tries to connect.

              type can be one of:

              none -- Client will exit with a fatal error (this is the default).
              nointeract -- Client will retry the connection without requerying for an  --auth-user-pass  user‐
              name/password.  Use this option for unattended clients.
              interact  --  Client  will  requery  for an --auth-user-pass username/password and/or private key
              password before attempting a reconnection.

              Note that while this option cannot be pushed, it can be controlled from the management interface.

       --static-challenge t e
              Enable static challenge/response protocol using challenge text t,  with  echo  flag  given  by  e
              (0|1).

              The echo flag indicates whether or not the user's response to the challenge should be echoed.

              See  management-notes.txt  in  the  OpenVPN  distribution  for a description of the OpenVPN chal‐
              lenge/response protocol.

       --server-poll-timeout n
              when polling possible remote servers to connect to in a round-robin fashion, spend no more than n
              seconds waiting for a response before trying the next server.

       --explicit-exit-notify [n]
              In  UDP  client  mode  or point-to-point mode, send server/peer an exit notification if tunnel is
              restarted or OpenVPN process is exited.  In client mode, on exit/restart, this option  will  tell
              the  server  to  immediately  close its client instance object rather than waiting for a timeout.
              The n parameter (default=1) controls the maximum number of attempts that the client will  try  to
              resend  the  exit notification message.  OpenVPN will not send any exit notifications unless this
              option is enabled.

   Data Channel Encryption Options:
       These options are meaningful for both Static & TLS-negotiated key  modes  (must  be  compatible  between
       peers).

       --secret file [direction]
              Enable Static Key encryption mode (non-TLS).  Use pre-shared secret file which was generated with
              --genkey.

              The optional direction parameter enables the use of 4 distinct keys  (HMAC-send,  cipher-encrypt,
              HMAC-receive,  cipher-decrypt),  so that each data flow direction has a different set of HMAC and
              cipher keys.  This has a number of desirable security properties  including  eliminating  certain
              kinds of DoS and message replay attacks.

              When  the  direction  parameter is omitted, 2 keys are used bidirectionally, one for HMAC and the
              other for encryption/decryption.

              The direction parameter should always be complementary on either side of the connection, i.e. one
              side should use "0" and the other should use "1", or both sides should omit it altogether.

              The  direction  parameter  requires that file contains a 2048 bit key.  While pre-1.5 versions of
              OpenVPN generate 1024 bit key files, any version of OpenVPN which supports the direction  parame‐
              ter, will also support 2048 bit key file generation using the --genkey option.

              Static key encryption mode has certain advantages, the primary being ease of configuration.

              There  are  no  certificates or certificate authorities or complicated negotiation handshakes and
              protocols.  The only requirement is that you have a pre-existing secure channel  with  your  peer
              (such  as  ssh  ) to initially copy the key.  This requirement, along with the fact that your key
              never changes unless you manually generate a new one, makes it somewhat less secure than TLS mode
              (see  below).   If an attacker manages to steal your key, everything that was ever encrypted with
              it is compromised.  Contrast that to the perfect forward secrecy  features  of  TLS  mode  (using
              Diffie  Hellman  key  exchange), where even if an attacker was able to steal your private key, he
              would gain no information to help him decrypt past sessions.

              Another advantageous aspect of Static Key encryption mode is that it is a handshake-free protocol
              without any distinguishing signature or feature (such as a header or protocol handshake sequence)
              that would mark the ciphertext packets as being generated by OpenVPN.   Anyone  eavesdropping  on
              the wire would see nothing but random-looking data.

       --key-direction
              Alternative  way  of  specifying the optional direction parameter for the --tls-auth and --secret
              options. Useful when using inline files (See section on inline files).

       --auth alg
              Authenticate packets with HMAC using message digest algorithm alg.  (The default is SHA1 ).  HMAC
              is  a commonly used message authentication algorithm (MAC) that uses a data string, a secure hash
              algorithm, and a key, to produce a digital signature.

              OpenVPN's usage of HMAC is to first encrypt a packet, then HMAC the resulting ciphertext.

              In static-key encryption mode, the HMAC key is included in the key file  generated  by  --genkey.
              In  TLS  mode, the HMAC key is dynamically generated and shared between peers via the TLS control
              channel.  If OpenVPN receives a packet with a bad HMAC it will drop  the  packet.   HMAC  usually
              adds 16 or 20 bytes per packet.  Set alg=none to disable authentication.

              For more information on HMAC see http://www.cs.ucsd.edu/users/mihir/papers/hmac.html

       --cipher alg
              Encrypt  packets  with cipher algorithm alg.  The default is BF-CBC, an abbreviation for Blowfish
              in Cipher Block Chaining mode.  Blowfish has the advantages  of  being  fast,  very  secure,  and
              allowing  key  sizes of up to 448 bits.  Blowfish is designed to be used in situations where keys
              are changed infrequently.

              For more information on blowfish, see http://www.counterpane.com/blowfish.html

              To see other ciphers that are available with OpenVPN, use the --show-ciphers option.

              OpenVPN supports the CBC, CFB, and OFB cipher modes, however CBC is recommended and CFB  and  OFB
              should be considered advanced modes.

              Set alg=none to disable encryption.

       --keysize n
              Size of cipher key in bits (optional).  If unspecified, defaults to cipher-specific default.  The
              --show-ciphers option (see below) shows all available OpenSSL ciphers, their default  key  sizes,
              and whether the key size can be changed.  Use care in changing a cipher's default key size.  Many
              ciphers have not been extensively cryptanalyzed with non-standard key lengths, and a  larger  key
              may offer no real guarantee of greater security, or may even reduce security.

       --prng alg [nsl]
              (Advanced)  For  PRNG  (Pseudo-random number generator), use digest algorithm alg (default=sha1),
              and set nsl (default=16) to the size in bytes of the nonce secret length (between 16 and 64).

              Set alg=none to disable the PRNG and use the OpenSSL RAND_bytes function instead for all of Open‐
              VPN's pseudo-random number needs.

       --engine [engine-name]
              Enable OpenSSL hardware-based crypto engine functionality.

              If  engine-name  is  specified,  use a specific crypto engine.  Use the --show-engines standalone
              option to list the crypto engines which are supported by OpenSSL.

       --no-replay
              (Advanced) Disable OpenVPN's protection against replay attacks.  Don't use this option unless you
              are prepared to make a tradeoff of greater efficiency in exchange for less security.

              OpenVPN provides datagram replay protection by default.

              Replay  protection  is  accomplished by tagging each outgoing datagram with an identifier that is
              guaranteed to be unique for the key being used.  The peer that receives the datagram  will  check
              for the uniqueness of the identifier.  If the identifier was already received in a previous data‐
              gram, OpenVPN will drop the packet.  Replay protection is important to defeat attacks such  as  a
              SYN  flood attack, where the attacker listens in the wire, intercepts a TCP SYN packet (identify‐
              ing it by the context in which it occurs in relation to other packets), then floods the receiving
              peer with copies of this packet.

              OpenVPN's  replay protection is implemented in slightly different ways, depending on the key man‐
              agement mode you have selected.

              In Static Key mode or when using an CFB or OFB mode cipher, OpenVPN uses a 64 bit unique  identi‐
              fier that combines a time stamp with an incrementing sequence number.

              When  using  TLS mode for key exchange and a CBC cipher mode, OpenVPN uses only a 32 bit sequence
              number without a time stamp, since OpenVPN can guarantee the uniqueness of this  value  for  each
              key.  As in IPSec, if the sequence number is close to wrapping back to zero, OpenVPN will trigger
              a new key exchange.

              To check for replays, OpenVPN uses the sliding window algorithm used by IPSec.

       --replay-window n [t]
              Use a replay protection sliding-window of size n and a time window of t seconds.

              By default n is 64 (the IPSec default) and t is 15 seconds.

              This option is only relevant in UDP mode, i.e.  when  either  --proto  udp  is  specifed,  or  no
              --proto option is specified.

              When  OpenVPN tunnels IP packets over UDP, there is the possibility that packets might be dropped
              or delivered out of order.  Because OpenVPN, like IPSec, is emulating the physical network layer,
              it  will  accept an out-of-order packet sequence, and will deliver such packets in the same order
              they were received to the TCP/IP protocol stack, provided they satisfy several constraints.

              (a) The packet cannot be a replay (unless --no-replay is specified, which disables replay protec‐
              tion altogether).

              (b)  If  a  packet  arrives  out of order, it will only be accepted if the difference between its
              sequence number and the highest sequence number received so far is less than n.

              (c) If a packet arrives out of order, it will only be accepted if it arrives no later than t sec‐
              onds after any packet containing a higher sequence number.

              If  you are using a network link with a large pipeline (meaning that the product of bandwidth and
              latency is high), you may want to use a larger value for n.  Satellite links in particular  often
              require this.

              If  you  run OpenVPN at --verb 4, you will see the message "Replay-window backtrack occurred [x]"
              every time the maximum sequence number backtrack seen thus far increases.  This can  be  used  to
              calibrate n.

              There is some controversy on the appropriate method of handling packet reordering at the security
              layer.

              Namely, to what extent should the security layer protect the encapsulated protocol  from  attacks
              which masquerade as the kinds of normal packet loss and reordering that occur over IP networks?

              The IPSec and OpenVPN approach is to allow packet reordering within a certain fixed sequence num‐
              ber window.

              OpenVPN adds to the IPSec model by limiting the window size in time as well as sequence space.

              OpenVPN also adds TCP transport as an option (not offered by IPSec) in  which  case  OpenVPN  can
              adopt a very strict attitude towards message deletion and reordering:  Don't allow it.  Since TCP
              guarantees reliability, any packet loss or reordering event can be assumed to be an attack.

              In this sense, it could be argued that TCP tunnel transport is preferred when tunneling non-IP or
              UDP  application  protocols  which might be vulnerable to a message deletion or reordering attack
              which falls within the normal operational parameters of IP networks.

              So I would make the statement that one should never tunnel a non-IP protocol or  UDP  application
              protocol over UDP, if the protocol might be vulnerable to a message deletion or reordering attack
              that falls within the normal operating parameters of what is to be expected from the physical  IP
              layer.  The problem is easily fixed by simply using TCP as the VPN transport layer.

       --mute-replay-warnings
              Silence  the  output  of  replay warnings, which are a common false alarm on WiFi networks.  This
              option preserves the security of the replay protection code without the verbosity associated with
              warnings about duplicate packets.

       --replay-persist file
              Persist replay-protection state across sessions using file to save and reload the state.

              This  option  will  strengthen  protection  against replay attacks, especially when you are using
              OpenVPN in a dynamic context (such as with --inetd) when OpenVPN sessions are frequently  started
              and stopped.

              This  option  will  keep a disk copy of the current replay protection state (i.e. the most recent
              packet timestamp and sequence number received from the remote peer), so that if an  OpenVPN  ses‐
              sion  is stopped and restarted, it will reject any replays of packets which were already received
              by the prior session.

              This option only makes sense when replay protection is enabled (the default) and  you  are  using
              either --secret (shared-secret key mode) or TLS mode with --tls-auth.

       --no-iv
              (Advanced)  Disable  OpenVPN's  use  of IV (cipher initialization vector).  Don't use this option
              unless you are prepared to make a tradeoff of greater efficiency in exchange for less security.

              OpenVPN uses an IV by default, and requires it for CFB and OFB cipher modes  (which  are  totally
              insecure  without  it).   Using  an IV is important for security when multiple messages are being
              encrypted/decrypted with the same key.

              IV is implemented differently depending on the cipher mode used.

              In CBC mode, OpenVPN uses a pseudo-random IV for each packet.

              In CFB/OFB mode, OpenVPN uses a unique sequence number and time stamp as the  IV.   In  fact,  in
              CFB/OFB  mode,  OpenVPN uses a datagram space-saving optimization that uses the unique identifier
              for datagram replay protection as the IV.

       --use-prediction-resistance
              Enable prediction resistance on PolarSSL's RNG.

              Enabling prediction resistance causes the RNG to reseed in each call for random.  Reseeding  this
              often can quickly deplete the kernel entropy pool.

              If you need this option, please consider running a daemon that adds entropy to the kernel pool.

              Note that this option only works with PolarSSL versions greater than 1.1.

       --test-crypto
              Do  a  self-test  of OpenVPN's crypto options by encrypting and decrypting test packets using the
              data channel encryption options specified above.  This option does not require a  peer  to  func‐
              tion, and therefore can be specified without --dev or --remote.

              The typical usage of --test-crypto would be something like this:

              openvpn --test-crypto --secret key

              or

              openvpn --test-crypto --secret key --verb 9

              This option is very useful to test OpenVPN after it has been ported to a new platform, or to iso‐
              late problems in the compiler, OpenSSL crypto library, or OpenVPN's crypto code.  Since it  is  a
              self-test mode, problems with encryption and authentication can be debugged independently of net‐
              work and tunnel issues.

   TLS Mode Options:
       TLS mode is the most powerful crypto mode of OpenVPN in both security and flexibility.  TLS  mode  works
       by  establishing  control  and  data channels which are multiplexed over a single TCP/UDP port.  OpenVPN
       initiates a TLS session over the control channel and uses it to exchange cipher and HMAC keys to protect
       the  data  channel.   TLS  mode  uses a robust reliability layer over the UDP connection for all control
       channel communication, while the data channel, over which encrypted tunnel  data  passes,  is  forwarded
       without  any  mediation.   The result is the best of both worlds: a fast data channel that forwards over
       UDP with only the overhead of encrypt, decrypt, and HMAC functions, and a control channel that  provides
       all  of the security features of TLS, including certificate-based authentication and Diffie Hellman for‐
       ward secrecy.

       To use TLS mode, each peer that runs OpenVPN should have its own local certificate/key pair ( --cert and
       --key ), signed by the root certificate which is specified in --ca.

       When  two  OpenVPN peers connect, each presents its local certificate to the other.  Each peer will then
       check that its partner peer presented a certificate which was signed by the master root  certificate  as
       specified in --ca.

       If  that  check  on  both peers succeeds, then the TLS negotiation will succeed, both OpenVPN peers will
       exchange temporary session keys, and the tunnel will begin passing data.

       The OpenVPN distribution contains a set of scripts for managing RSA certificates & keys, located in  the
       easy-rsa subdirectory.

       The easy-rsa package is also rendered in web form here: http://openvpn.net/easyrsa.html

       --tls-server
              Enable TLS and assume server role during TLS handshake.  Note that OpenVPN is designed as a peer-
              to-peer application.  The designation of client or server is only for the purpose of  negotiating
              the TLS control channel.

       --tls-client
              Enable TLS and assume client role during TLS handshake.

       --ca file
              Certificate  authority  (CA) file in .pem format, also referred to as the root certificate.  This
              file can have multiple certificates in .pem format, concatenated  together.   You  can  construct
              your own certificate authority certificate and private key by using a command such as:

              openssl req -nodes -new -x509 -keyout ca.key -out ca.crt

              Then  edit your openssl.cnf file and edit the certificate variable to point to your new root cer‐
              tificate ca.crt.

              For testing purposes only, the OpenVPN distribution includes a sample  CA  certificate  (ca.crt).
              Of  course you should never use the test certificates and test keys distributed with OpenVPN in a
              production environment, since by virtue of the fact that they are distributed with OpenVPN,  they
              are totally insecure.

       --capath dir
              Directory  containing  trusted  certificates  (CAs  and CRLs).  Available with OpenSSL version >=
              0.9.7 dev.  Not available with PolarSSL.

       --dh file
              File containing Diffie Hellman parameters in .pem format (required for --tls-server only). Use

              openssl dhparam -out dh1024.pem 1024

              to generate your own, or use the existing dh1024.pem file included with the OpenVPN distribution.
              Diffie Hellman parameters may be considered public.

       --cert file
              Local peer's signed certificate in .pem format -- must be signed by a certificate authority whose
              certificate is in --ca file.  Each peer in an OpenVPN link running in TLS mode  should  have  its
              own  certificate  and private key file.  In addition, each certificate should have been signed by
              the key of a certificate authority whose public key resides in  the  --ca  certificate  authority
              file.   You can easily make your own certificate authority (see above) or pay money to use a com‐
              mercial service such as thawte.com (in which case you will be helping to finance the world's sec‐
              ond space tourist :).  To generate a certificate, you can use a command such as:

              openssl req -nodes -new -keyout mycert.key -out mycert.csr

              If  your certificate authority private key lives on another machine, copy the certificate signing
              request (mycert.csr) to this other machine (this can be done over an  insecure  channel  such  as
              email).  Now sign the certificate with a command such as:

              openssl ca -out mycert.crt -in mycert.csr

              Now  copy  the  certificate (mycert.crt) back to the peer which initially generated the .csr file
              (this can be over a public medium).  Note that the openssl ca command reads the location  of  the
              certificate  authority key from its configuration file such as /usr/share/ssl/openssl.cnf -- note
              also that for certificate authority functions, you must set up the files index.txt (may be empty)
              and serial (initialize to 01 ).

       --extra-certs file
              Specify  a  file containing one or more PEM certs (concatenated together) that complete the local
              certificate chain.

              This option is useful for "split" CAs, where the CA for server certs is different than the CA for
              client  certs.   Putting certs in this file allows them to be used to complete the local certifi‐
              cate chain without trusting them to verify the peer-submitted certificate, as would be  the  case
              if the certs were placed in the ca file.

       --key file
              Local  peer's private key in .pem format.  Use the private key which was generated when you built
              your peer's certificate (see -cert file above).

       --pkcs12 file
              Specify a PKCS #12 file containing local private key, local certificate, and root CA certificate.
              This option can be used instead of --ca, --cert, and --key.  Not available with PolarSSL.

       --verify-hash hash
              Specify  SHA1  fingerprint  for  level-1 cert.  The level-1 cert is the CA (or intermediate cert)
              that signs the leaf certificate, and is one removed from the leaf certificate in the direction of
              the  root.  When accepting a connection from a peer, the level-1 cert fingerprint must match hash
              or  certificate  verification  will  fail.   Hash  is  specified  as  XX:XX:...    For   example:
              AD:B0:95:D8:09:C8:36:45:12:A9:89:C8:90:09:CB:13:72:A6:AD:16

       --pkcs11-cert-private [0|1]...
              Set  if access to certificate object should be performed after login.  Every provider has its own
              setting.

       --pkcs11-id name
              Specify the serialized certificate id to be used. The id can be gotten by the standalone  --show-
              pkcs11-ids option.

       --pkcs11-id-management
              Acquire  PKCS#11  id from management interface. In this case a NEED-STR 'pkcs11-id-request' real-
              time message will be triggered, application may use pkcs11-id-count command to retrieve available
              number  of  certificates,  and  pkcs11-id-get  command to retrieve certificate id and certificate
              body.

       --pkcs11-pin-cache seconds
              Specify how many seconds the PIN can be cached, the default is until the token is removed.

       --pkcs11-protected-authentication [0|1]...
              Use PKCS#11 protected authentication path, useful for  biometric  and  external  keypad  devices.
              Every provider has its own setting.

       --pkcs11-providers provider...
              Specify  a RSA Security Inc. PKCS #11 Cryptographic Token Interface (Cryptoki) providers to load.
              This option can be used instead of --cert, --key, and --pkcs12.

       --pkcs11-private-mode mode...
              Specify which method to use in order to perform private key operations.  A different mode can  be
              specified for each provider.  Mode is encoded as hex number, and can be a mask one of the follow‐
              ing:

              0 (default) -- Try to determind automatically.
              1 -- Use sign.
              2 -- Use sign recover.
              4 -- Use decrypt.
              8 -- Use unwrap.

       --cryptoapicert select-string
              Load the certificate and private key from the Windows Certificate System  Store  (Windows/OpenSSL
              Only).

              Use this option instead of --cert and --key.

              This makes it possible to use any smart card, supported by Windows, but also any kind of certifi‐
              cate, residing in the Cert Store, where you have access to the private key.  This option has been
              tested  with a couple of different smart cards (GemSAFE, Cryptoflex, and Swedish Post Office eID)
              on the client side, and also an imported PKCS12 software certificate on the server side.

              To select a certificate, based on a substring search in the certificate's subject:

              cryptoapicert "SUBJ:Peter Runestig"

              To select a certificate, based on certificate's thumbprint:

              cryptoapicert "THUMB:f6 49 24 41 01 b4 ..."

              The thumbprint hex string can easily be copy-and-pasted from the Windows Certificate Store GUI.


       --key-method m
              Use data channel key negotiation method m.  The key method must match on both sides of  the  con‐
              nection.

              After  OpenVPN negotiates a TLS session, a new set of keys for protecting the tunnel data channel
              is generated and exchanged over the TLS session.

              In method 1 (the default for OpenVPN 1.x), both sides generate random encrypt and HMAC-send  keys
              which are forwarded to the other host over the TLS channel.

              In  method  2,  (the default for OpenVPN 2.0) the client generates a random key.  Both client and
              server also generate some random seed material.  All key source material is  exchanged  over  the
              TLS channel. The actual keys are generated using the TLS PRF function, taking source entropy from
              both client and server.  Method 2 is designed to closely parallel the key generation process used
              by TLS 1.0.

              Note that in TLS mode, two separate levels of keying occur:

              (1)  The TLS connection is initially negotiated, with both sides of the connection producing cer‐
              tificates and verifying the certificate (or other authentication  info  provided)  of  the  other
              side.  The --key-method parameter has no effect on this process.

              (2)  After  the  TLS connection is established, the tunnel session keys are separately negotiated
              over the existing secure TLS channel.  Here, --key-method determines the derivation of the tunnel
              session keys.

       --tls-cipher l
              A  list  l  of  allowable TLS ciphers delimited by a colon (":").  If you require a high level of
              security, you may want to set this parameter manually, to prevent a version rollback attack where
              a  man-in-the-middle  attacker tries to force two peers to negotiate to the lowest level of secu‐
              rity they both support.  Use --show-tls to see a list of supported TLS ciphers.

       --tls-timeout n
              Packet retransmit timeout on TLS control channel if no acknowledgment from remote within  n  sec‐
              onds  (default=2).  When OpenVPN sends a control packet to its peer, it will expect to receive an
              acknowledgement within n seconds or it will retransmit the packet, subject to a TCP-like exponen‐
              tial  backoff  algorithm.   This parameter only applies to control channel packets.  Data channel
              packets (which carry encrypted tunnel data) are never acknowledged, sequenced,  or  retransmitted
              by  OpenVPN  because  the higher level network protocols running on top of the tunnel such as TCP
              expect this role to be left to them.

       --reneg-bytes n
              Renegotiate data channel key after n bytes sent  or  received  (disabled  by  default).   OpenVPN
              allows  the  lifetime of a key to be expressed as a number of bytes encrypted/decrypted, a number
              of packets, or a number of seconds.  A key renegotiation will be forced if  any  of  these  three
              criteria are met by either peer.

       --reneg-pkts n
              Renegotiate data channel key after n packets sent and received (disabled by default).

       --reneg-sec n
              Renegotiate data channel key after n seconds (default=3600).

              When  using dual-factor authentication, note that this default value may cause the end user to be
              challenged to reauthorize once per hour.

              Also, keep in mind that this option can be used on both the client and server, and whichever uses
              the  lower  value  will  be  the  one  to  trigger the renegotiation.  A common mistake is to set
              --reneg-sec to a higher value on either the client or server, while the other side of the connec‐
              tion  is still using the default value of 3600 seconds, meaning that the renegotiation will still
              occur once per 3600 seconds.  The solution is to increase --reneg-sec  on  both  the  client  and
              server,  or  set  it to 0 on one side of the connection (to disable), and to your chosen value on
              the other side.

       --hand-window n
              Handshake Window -- the TLS-based key exchange must finalize within n seconds of handshake initi‐
              ation  by  any  peer (default = 60 seconds).  If the handshake fails we will attempt to reset our
              connection with our peer and try again.  Even in the event of handshake failure we will still use
              our expiring key for up to --tran-window seconds to maintain continuity of transmission of tunnel
              data.

       --tran-window n
              Transition window -- our old key can live this many seconds  after  a  new  a  key  renegotiation
              begins  (default  = 3600 seconds).  This feature allows for a graceful transition from old to new
              key, and removes the key renegotiation sequence from the critical path of tunnel data forwarding.

       --single-session
              After initially connecting to a remote peer, disallow any new  connections.   Using  this  option
              means that a remote peer cannot connect, disconnect, and then reconnect.

              If the daemon is reset by a signal or --ping-restart, it will allow one new connection.

              --single-session  can  be  used with --ping-exit or --inactive to create a single dynamic session
              that will exit when finished.

       --tls-exit
              Exit on TLS negotiation failure.

       --tls-auth file [direction]
              Add an additional layer of HMAC authentication on top of  the  TLS  control  channel  to  protect
              against DoS attacks.

              In  a nutshell, --tls-auth enables a kind of "HMAC firewall" on OpenVPN's TCP/UDP port, where TLS
              control channel packets bearing an incorrect HMAC signature can be  dropped  immediately  without
              response.

              file (required) is a key file which can be in one of two formats:

              (1) An OpenVPN static key file generated by --genkey (required if direction parameter is used).

              (2)  A  freeform  passphrase  file.  In this case the HMAC key will be derived by taking a secure
              hash of this file, similar to the md5sum(1) or sha1sum(1) commands.

              OpenVPN will first try format (1), and if the file fails to parse as a static  key  file,  format
              (2) will be used.

              See the --secret option for more information on the optional direction parameter.

              --tls-auth  is recommended when you are running OpenVPN in a mode where it is listening for pack‐
              ets from any IP address, such as when --remote is not specified, or --remote  is  specified  with
              --float.

              The  rationale for this feature is as follows.  TLS requires a multi-packet exchange before it is
              able to authenticate a peer.  During this  time  before  authentication,  OpenVPN  is  allocating
              resources  (memory  and  CPU)  to  this potential peer.  The potential peer is also exposing many
              parts of OpenVPN and the OpenSSL library to the packets it is sending.  Most  successful  network
              attacks  today seek to either exploit bugs in programs (such as buffer overflow attacks) or force
              a program to consume so many resources that it becomes unusable.  Of course  the  first  line  of
              defense  is  always  to  produce  clean, well-audited code.  OpenVPN has been written with buffer
              overflow attack prevention as a top priority.  But as history has shown, many of the most  widely
              used network applications have, from time to time, fallen to buffer overflow attacks.

              So as a second line of defense, OpenVPN offers this special layer of authentication on top of the
              TLS control channel so that every packet on the control channel is authenticated by an HMAC  sig‐
              nature  and a unique ID for replay protection.  This signature will also help protect against DoS
              (Denial of Service) attacks.  An important rule of thumb in reducing vulnerability to DoS attacks
              is to minimize the amount of resources a potential, but as yet unauthenticated, client is able to
              consume.

              --tls-auth does this by signing every TLS control channel packet with an HMAC signature,  includ‐
              ing  packets  which are sent before the TLS level has had a chance to authenticate the peer.  The
              result is that packets without the correct signature can be dropped immediately  upon  reception,
              before  they  have  a  chance  to consume additional system resources such as by initiating a TLS
              handshake.  --tls-auth can be strengthened by adding the --replay-persist option which will  keep
              OpenVPN's replay protection state in a file so that it is not lost across restarts.

              It  should be emphasized that this feature is optional and that the passphrase/key file used with
              --tls-auth gives a peer nothing more than the power to initiate a TLS handshake.  It is not  used
              to encrypt or authenticate any tunnel data.

       --askpass [file]
              Get certificate password from console or file before we daemonize.

              For the extremely security conscious, it is possible to protect your private key with a password.
              Of course this means that every time the OpenVPN daemon is started you must be there to type  the
              password.  The --askpass option allows you to start OpenVPN from the command line.  It will query
              you for a password before it daemonizes.  To protect a private key with  a  password  you  should
              omit the -nodes option when you use the openssl command line tool to manage certificates and pri‐
              vate keys.

              If file is specified, read the password from the first line of file.  Keep in mind  that  storing
              your  password  in a file to a certain extent invalidates the extra security provided by using an
              encrypted key (Note: OpenVPN will only read passwords from a file if it has been built  with  the
              --enable-password-save  configure  option,  or  on  Windows  by  defining ENABLE_PASSWORD_SAVE in
              win/settings.in).

       --auth-nocache
              Don't cache --askpass or --auth-user-pass username/passwords in virtual memory.

              If specified, this directive will cause OpenVPN to immediately  forget  username/password  inputs
              after  they  are  used.   As a result, when OpenVPN needs a username/password, it will prompt for
              input from stdin, which may be multiple times during the duration of an OpenVPN session.

              This directive does not affect the --http-proxy username/password.  It is always cached.

       --tls-verify cmd
              Run command cmd to verify the X509 name of a pending TLS connection that has otherwise passed all
              other  tests  of  certification (except for revocation via --crl-verify directive; the revocation
              test occurs after the --tls-verify test).

              cmd should return 0 to allow the TLS handshake to proceed, or 1 to fail.

              cmd consists of a path to script (or executable program), optionally followed by  arguments.  The
              path  and  arguments may be single- or double-quoted and/or escaped using a backslash, and should
              be separated by one or more spaces.

              When cmd is executed two arguments are appended after any arguments specified in cmd  ,  as  fol‐
              lows:

              cmd certificate_depth subject

              These arguments are, respectively, the current certificate depth and the X509 common name (cn) of
              the peer.

              This feature is useful if the peer you want to trust has a certificate which was signed by a cer‐
              tificate  authority  who also signed many other certificates, where you don't necessarily want to
              trust all of them, but rather be selective about which peer certificate you  will  accept.   This
              feature  allows  you  to write a script which will test the X509 name on a certificate and decide
              whether or not it should be accepted.  For a simple perl script which will test the  common  name
              field on the certificate, see the file verify-cn in the OpenVPN distribution.

              See the "Environmental Variables" section below for additional parameters passed as environmental
              variables.

       --tls-export-cert directory
              Store the certificates the clients uses upon connection to this  directory.  This  will  be  done
              before  --tls-verify  is  called.  The certificates will use a temporary name and will be deleted
              when the tls-verify script returns.  The file name used for the certificate is available via  the
              peer_cert environment variable.

       --x509-username-field fieldname
              Field  in x509 certificate subject to be used as username (default=CN).  Fieldname will be upper‐
              cased before matching. When this option is used, the --tls-remote option will match  against  the
              chosen fieldname instead of the CN.

       --tls-remote name
              Accept connections only from a host with X509 name or common name equal to name.  The remote host
              must also pass all other tests of verification.

              NOTE: Because tls-remote may test against a common name prefix, only use this option when you are
              using  OpenVPN  with  a  custom CA certificate that is under your control.  Never use this option
              when your client certificates are signed by a third party, such as a commercial web CA.

              Name can also be a common name prefix, for example if you want a client to  only  accept  connec‐
              tions to "Server-1", "Server-2", etc., you can simply use --tls-remote Server

              Using  a  common  name  prefix  is a useful alternative to managing a CRL (Certificate Revocation
              List) on the client, since it allows the client to refuse all certificates except for those asso‐
              ciated with designated servers.

              --tls-remote  is  a  useful  replacement  for  the --tls-verify option to verify the remote host,
              because --tls-remote works in a --chroot environment too.

       --x509-track attribute
              Save peer X509 attribute value in environment  for  use  by  plugins  and  management  interface.
              Prepend  a  '+'  to  attribute  to  save  values from full cert chain.  Values will be encoded as
              X509_<depth>_<attribute>=<value>.  Multiple --x509-track options can be defined to track multiple
              attributes.  Not available with PolarSSL.

       --ns-cert-type client|server
              Require  that  peer certificate was signed with an explicit nsCertType designation of "client" or
              "server".

              This is a useful security option for clients, to ensure that the host they connect with is a des‐
              ignated server.

              See the easy-rsa/build-key-server script for an example of how to generate a certificate with the
              nsCertType field set to "server".

              If the server certificate's nsCertType field is set to "server", then the clients can verify this
              with --ns-cert-type server.

              This  is  an important security precaution to protect against a man-in-the-middle attack where an
              authorized client attempts to connect to another client by impersonating the server.  The  attack
              is  easily  prevented by having clients verify the server certificate using any one of --ns-cert-
              type, --tls-remote, or --tls-verify.

       --remote-cert-ku v...
              Require that peer certificate was signed with an explicit key usage.

              This is a useful security option for clients, to ensure that the host they connect to is a desig‐
              nated server.

              The key usage should be encoded in hex, more than one key usage can be specified.

       --remote-cert-eku oid
              Require that peer certificate was signed with an explicit extended key usage.

              This is a useful security option for clients, to ensure that the host they connect to is a desig‐
              nated server.

              The extended key usage should be encoded in oid notation, or OpenSSL symbolic representation.

       --remote-cert-tls client|server
              Require that peer certificate was signed with an explicit key usage and extended key usage  based
              on RFC3280 TLS rules.

              This is a useful security option for clients, to ensure that the host they connect to is a desig‐
              nated server.

              The --remote-cert-tls client option is equivalent to --remote-cert-ku 80 08 88  --remote-cert-eku
              "TLS Web Client Authentication"

              The key usage is digitalSignature and/or keyAgreement.

              The  --remote-cert-tls  server  option  is equivalent to --remote-cert-ku a0 88 --remote-cert-eku
              "TLS Web Server Authentication"

              The key usage is digitalSignature and ( keyEncipherment or keyAgreement ).

              This is an important security precaution to protect against a man-in-the-middle attack  where  an
              authorized  client attempts to connect to another client by impersonating the server.  The attack
              is easily prevented by having clients verify the server certificate using any  one  of  --remote-
              cert-tls, --tls-remote, or --tls-verify.

       --crl-verify crl ['dir']
              Check peer certificate against the file crl in PEM format.

              A  CRL  (certificate  revocation  list) is used when a particular key is compromised but when the
              overall PKI is still intact.

              Suppose you had a PKI consisting of a CA, root certificate, and a number of client  certificates.
              Suppose  a  laptop  computer  containing  a client key and certificate was stolen.  By adding the
              stolen certificate to the CRL file, you could reject any connection which  attempts  to  use  it,
              while preserving the overall integrity of the PKI.

              The  only  time when it would be necessary to rebuild the entire PKI from scratch would be if the
              root certificate key itself was compromised.

              If the optional dir flag is specified, enable a different mode where crl is a directory  contain‐
              ing  files named as revoked serial numbers (the files may be empty, the contents are never read).
              If a client requests a connection, where the client certificate serial number (decimal string) is
              the name of a file present in the directory, it will be rejected.

   SSL Library information:
       --show-ciphers
              (Standalone) Show all cipher algorithms to use with the --cipher option.

       --show-digests
              (Standalone) Show all message digest algorithms to use with the --auth option.

       --show-tls
              (Standalone)  Show all TLS ciphers (TLS used only as a control channel).  The TLS ciphers will be
              sorted from highest preference (most secure) to lowest.

       --show-engines
              (Standalone) Show currently available hardware-based crypto acceleration engines supported by the
              OpenSSL library.

   Generate a random key:
       Used only for non-TLS static key encryption mode.

       --genkey
              (Standalone)  Generate  a  random  key  to  be used as a shared secret, for use with the --secret
              option.  This file must be shared with the peer over a pre-existing secure channel such as scp(1)

       --secret file
              Write key to file.

   TUN/TAP persistent tunnel config mode:
       Available with linux 2.4.7+.  These options comprise a standalone mode of OpenVPN which can be  used  to
       create and delete persistent tunnels.

       --mktun
              (Standalone)  Create a persistent tunnel on platforms which support them such as Linux.  Normally
              TUN/TAP tunnels exist only for the period of time that an application has them open.  This option
              takes  advantage  of  the  TUN/TAP driver's ability to build persistent tunnels that live through
              multiple instantiations of OpenVPN and die only when they are deleted or the machine is rebooted.

              One of the advantages of persistent tunnels is that they eliminate the need for separate --up and
              --down  scripts  to run the appropriate ifconfig(8) and route(8) commands.  These commands can be
              placed in the the same shell script which starts or terminates an OpenVPN session.

              Another advantage is that open connections through the TUN/TAP-based tunnel will not be reset  if
              the  OpenVPN peer restarts.  This can be useful to provide uninterrupted connectivity through the
              tunnel in the event of a DHCP reset of the peer's public IP address (see  the  --ipchange  option
              above).

              One  disadvantage of persistent tunnels is that it is harder to automatically configure their MTU
              value (see --link-mtu and --tun-mtu above).

              On some platforms such as Windows, TAP-Win32 tunnels are persistent by default.

       --rmtun
              (Standalone) Remove a persistent tunnel.

       --dev tunX | tapX
              TUN/TAP device

       --user user
              Optional user to be owner of this tunnel.

       --group group
              Optional group to be owner of this tunnel.

   Windows-Specific Options:
       --win-sys path
              Set the Windows system directory pathname to use when looking  for  system  executables  such  as
              route.exe  and  netsh.exe.   By default, if this directive is not specified, OpenVPN will use the
              SystemRoot environment variable.

              This option have changed behaviour in OpenVPN 2.3.  Earlier you had to define  --win-sys  env  to
              use  the SystemRoot environment variable, otherwise it defaulted to C:\WINDOWS.  It is not needed
              to use the env keyword any more, and it will just be ignored. A warning is logged  when  this  is
              found in the configuration file.

       --ip-win32 method
              When  using --ifconfig on Windows, set the TAP-Win32 adapter IP address and netmask using method.
              Don't use this option unless you are also using --ifconfig.

              manual -- Don't set the IP address or netmask automatically.  Instead output  a  message  to  the
              console  telling  the  user to configure the adapter manually and indicating the IP/netmask which
              OpenVPN expects the adapter to be set to.

              dynamic [offset] [lease-time] -- Automatically set the IP address and netmask by replying to DHCP
              query  messages  generated by the kernel.  This mode is probably the "cleanest" solution for set‐
              ting the TCP/IP properties since it uses the well-known DHCP protocol.  There are,  however,  two
              prerequisites  for  using  this mode: (1) The TCP/IP properties for the TAP-Win32 adapter must be
              set to "Obtain an IP address automatically," and (2) OpenVPN needs to claim an IP address in  the
              subnet  for  use  as the virtual DHCP server address.  By default in --dev tap mode, OpenVPN will
              take the normally unused first address in the subnet.  For example, if your subnet is 192.168.4.0
              netmask  255.255.255.0,  then  OpenVPN will take the IP address 192.168.4.0 to use as the virtual
              DHCP server address.  In --dev tun mode, OpenVPN will cause the DHCP server to masquerade  as  if
              it  were coming from the remote endpoint.  The optional offset parameter is an integer which is >
              -256 and < 256 and which defaults to 0.  If offset is positive, the DHCP server  will  masquerade
              as  the IP address at network address + offset.  If offset is negative, the DHCP server will mas‐
              querade as the IP address at broadcast address + offset.  The Windows ipconfig /all  command  can
              be  used  to  show  what  Windows  thinks  the DHCP server address is.  OpenVPN will "claim" this
              address, so make sure to use a free address.  Having said that, different OpenVPN instantiations,
              including  different ends of the same connection, can share the same virtual DHCP server address.
              The lease-time parameter controls the lease time of the DHCP assignment given  to  the  TAP-Win32
              adapter, and is denoted in seconds.  Normally a very long lease time is preferred because it pre‐
              vents routes involving the TAP-Win32 adapter from being lost when the system goes to sleep.   The
              default lease time is one year.

              netsh -- Automatically set the IP address and netmask using the Windows command-line "netsh" com‐
              mand.  This method appears to work correctly on Windows XP but not Windows 2000.

              ipapi -- Automatically set the IP address and netmask using the  Windows  IP  Helper  API.   This
              approach  does not have ideal semantics, though testing has indicated that it works okay in prac‐
              tice.  If you use this option, it is best to  leave  the  TCP/IP  properties  for  the  TAP-Win32
              adapter in their default state, i.e. "Obtain an IP address automatically."

              adaptive -- (Default) Try dynamic method initially and fail over to netsh if the DHCP negotiation
              with the TAP-Win32 adapter does not succeed in 20 seconds.  Such  failures  have  been  known  to
              occur  when  certain third-party firewall packages installed on the client machine block the DHCP
              negotiation used by the TAP-Win32 adapter.  Note that if the netsh failover occurs, the TAP-Win32
              adapter  TCP/IP  properties will be reset from DHCP to static, and this will cause future OpenVPN
              startups using the adaptive mode to use netsh immediately, rather than trying dynamic first.   To
              "unstick" the adaptive mode from using netsh, run OpenVPN at least once using the dynamic mode to
              restore the TAP-Win32 adapter TCP/IP properties to a DHCP configuration.

       --route-method m
              Which method m to use for adding routes on Windows?

              adaptive (default) -- Try IP helper API first.  If that fails, fall back to the  route.exe  shell
              command.
              ipapi -- Use IP helper API.
              exe -- Call the route.exe shell command.

       --dhcp-option type [parm]
              Set  extended  TAP-Win32  TCP/IP  properties,  must be used with --ip-win32 dynamic or --ip-win32
              adaptive.  This option can be used to set additional TCP/IP properties on the TAP-Win32  adapter,
              and  is particularly useful for configuring an OpenVPN client to access a Samba server across the
              VPN.

              DOMAIN name -- Set Connection-specific DNS Suffix.

              DNS addr -- Set primary domain name server address.  Repeat this  option  to  set  secondary  DNS
              server addresses.

              WINS  addr  --  Set  primary  WINS server address (NetBIOS over TCP/IP Name Server).  Repeat this
              option to set secondary WINS server addresses.

              NBDD addr -- Set primary NBDD server address (NetBIOS over TCP/IP Datagram  Distribution  Server)
              Repeat this option to set secondary NBDD server addresses.

              NTP  addr  --  Set primary NTP server address (Network Time Protocol).  Repeat this option to set
              secondary NTP server addresses.

              NBT type -- Set NetBIOS over TCP/IP Node type.  Possible options: 1 = b-node (broadcasts), 2 = p-
              node  (point-to-point  name  queries  to  a  WINS  server), 4 = m-node (broadcast then query name
              server), and 8 = h-node (query name server, then broadcast).

              NBS scope-id -- Set NetBIOS over TCP/IP Scope. A NetBIOS Scope ID  provides  an  extended  naming
              service for the NetBIOS over TCP/IP (Known as NBT) module. The primary purpose of a NetBIOS scope
              ID is to isolate NetBIOS traffic on a single network to only those nodes with  the  same  NetBIOS
              scope  ID.   The NetBIOS scope ID is a character string that is appended to the NetBIOS name. The
              NetBIOS scope ID on two hosts must match, or the two hosts will not be able to  communicate.  The
              NetBIOS  Scope  ID  also  allows  computers to use the same computer name, as they have different
              scope IDs. The Scope ID becomes a part of the  NetBIOS  name,  making  the  name  unique.   (This
              description of NetBIOS scopes courtesy of NeonSurge@abyss.com)

              DISABLE-NBT -- Disable Netbios-over-TCP/IP.

              Note that if --dhcp-option is pushed via --push to a non-windows client, the option will be saved
              in the client's environment before the up script is called, under the name "foreign_option_{n}".

       --tap-sleep n
              Cause OpenVPN to sleep for n seconds immediately after the TAP-Win32  adapter  state  is  set  to
              "connected".

              This  option  is  intended to be used to troubleshoot problems with the --ifconfig and --ip-win32
              options, and is used to give the TAP-Win32 adapter time to come up before Windows IP  Helper  API
              operations are applied to it.

       --show-net-up
              Output  OpenVPN's  view of the system routing table and network adapter list to the syslog or log
              file after the TUN/TAP adapter has been brought up and any routes have been added.

       --dhcp-renew
              Ask Windows to renew the TAP adapter lease on startup.  This option is normally  unnecessary,  as
              Windows  automatically triggers a DHCP renegotiation on the TAP adapter when it comes up, however
              if you set the TAP-Win32 adapter Media Status property to "Always Connected", you may  need  this
              flag.

       --dhcp-release
              Ask  Windows  to  release the TAP adapter lease on shutdown.  This option has the same caveats as
              --dhcp-renew above.

       --register-dns
              Run net stop dnscache, net start dnscache, ipconfig /flushdns and ipconfig /registerdns  on  con‐
              nection initiation.  This is known to kick Windows into recognizing pushed DNS servers.

       --pause-exit
              Put  up a "press any key to continue" message on the console prior to OpenVPN program exit.  This
              option is automatically used by the Windows explorer when OpenVPN is run on a configuration  file
              using the right-click explorer menu.

       --service exit-event [0|1]
              Should  be used when OpenVPN is being automatically executed by another program in such a context
              that no interaction with the user via display or keyboard is  possible.   In  general,  end-users
              should never need to explicitly use this option, as it is automatically added by the OpenVPN ser‐
              vice wrapper when a given OpenVPN configuration is being run as a service.

              exit-event is the name of a Windows global event object, and OpenVPN  will  continuously  monitor
              the state of this event object and exit when it becomes signaled.

              The second parameter indicates the initial state of exit-event and normally defaults to 0.

              Multiple OpenVPN processes can be simultaneously executed with the same exit-event parameter.  In
              any case, the controlling process can signal exit-event, causing all such  OpenVPN  processes  to
              exit.

              When executing an OpenVPN process using the --service directive, OpenVPN will probably not have a
              console window to output status/error messages, therefore it is useful to  use  --log  or  --log-
              append to write these messages to a file.

       --show-adapters
              (Standalone) Show available TAP-Win32 adapters which can be selected using the --dev-node option.
              On non-Windows systems, the ifconfig(8) command provides similar functionality.

       --allow-nonadmin [TAP-adapter]
              (Standalone) Set TAP-adapter to allow access from non-administrative accounts.  If TAP-adapter is
              omitted,  all  TAP adapters on the system will be configured to allow non-admin access.  The non-
              admin access setting will only persist for the length of time that the  TAP-Win32  device  object
              and  driver  remain  loaded,  and  will need to be re-enabled after a reboot, or if the driver is
              unloaded and reloaded.  This directive can only be used by an administrator.

       --show-valid-subnets
              (Standalone) Show valid subnets for --dev tun emulation.  Since the TAP-Win32 driver  exports  an
              ethernet  interface  to Windows, and since TUN devices are point-to-point in nature, it is neces‐
              sary for the TAP-Win32 driver to impose certain constraints on TUN endpoint address selection.

              Namely, the point-to-point endpoints used  in  TUN  device  emulation  must  be  the  middle  two
              addresses of a /30 subnet (netmask 255.255.255.252).

       --show-net
              (Standalone) Show OpenVPN's view of the system routing table and network adapter list.

   PKCS#11 Standalone Options:
       --show-pkcs11-ids provider [cert_private]
              (Standalone) Show PKCS#11 token object list. Specify cert_private as 1 if certificates are stored
              as private objects.

              --verb option can be used BEFORE this option to produce debugging information.

   IPv6 Related Options
       The following options exist to support IPv6 tunneling in peer-to-peer and  client-server  mode.   As  of
       now,  this is just very basic documentation of the IPv6-related options. More documentation can be found
       on http://www.greenie.net/ipv6/openvpn.html.

       --ifconfig-ipv6 ipv6addr/bits ipv6remote
              configure IPv6 address ipv6addr/bits on the ``tun'' device.  The  second  parameter  is  used  as
              route target for --route-ipv6 if no gateway is specified.

       --route-ipv6 ipv6addr/bits [gateway] [metric]
              setup IPv6 routing in the system to send the specified IPv6 network into OpenVPN's ``tun'' device

       --server-ipv6 ipv6addr/bits
              convenience-function  to enable a number of IPv6 related options at once, namely --ifconfig-ipv6,
              --ifconfig-ipv6-pool, --tun-ipv6 and --push tun-ipv6 Is only accepted  if  ``--mode  server''  or
              ``--server'' is set.

       --ifconfig-ipv6-pool ipv6addr/bits
              Specify  an IPv6 address pool for dynamic assignment to clients.  The pool starts at ipv6addr and
              increments by +1 for every new client (linear mode).  The /bits setting controls the size of  the
              pool.

       --ifconfig-ipv6-push ipv6addr/bits ipv6remote
              for  ccd/ per-client static IPv6 interface configuration, see --client-config-dir and --ifconfig-
              push for more details.

       --iroute-ipv6 ipv6addr/bits
              for ccd/ per-client static IPv6 route configuration, see --iroute for more details how  to  setup
              and use this, and how --iroute and --route interact.


SCRIPTING AND ENVIRONMENTAL VARIABLES
       OpenVPN exports a series of environmental variables for use by user-defined scripts.

   Script Order of Execution
       --up   Executed after TCP/UDP socket bind and TUN/TAP open.

       --tls-verify
              Executed when we have a still untrusted remote peer.

       --ipchange
              Executed after connection authentication, or remote IP address change.

       --client-connect
              Executed in --mode server mode immediately after client authentication.

       --route-up
              Executed  after  connection  authentication,  either immediately after, or some number of seconds
              after as defined by the --route-delay option.

       --route-pre-down
              Executed right before the routes are removed.

       --client-disconnect
              Executed in --mode server mode on client instance shutdown.

       --down Executed after TCP/UDP and TUN/TAP close.

       --learn-address
              Executed in --mode server mode whenever an IPv4 address/route or MAC address is  added  to  Open‐
              VPN's internal routing table.

       --auth-user-pass-verify
              Executed in --mode server mode on new client connections, when the client is still untrusted.

   String Types and Remapping
       In  certain cases, OpenVPN will perform remapping of characters in strings.  Essentially, any characters
       outside the set of permitted characters for each string type will be converted to underbar ('_').

       Q: Why is string remapping necessary?

       A: It's an important security feature to prevent the malicious coding of strings from untrusted  sources
       to  be passed as parameters to scripts, saved in the environment, used as a common name, translated to a
       filename, etc.

       Q: Can string remapping be disabled?

       A: Yes, by using the --no-name-remapping option, however this should be considered an advanced option.

       Here is a brief rundown of OpenVPN's current string types and the permitted  character  class  for  each
       string:

       X509 Names: Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), at ('@'), colon (':'), slash ('/'), and
       equal ('=').  Alphanumeric is defined as a character which will cause the C library  isalnum()  function
       to return true.

       Common Names: Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), and at ('@').

       --auth-user-pass  username:  Same  as  Common Name, with one exception: starting with OpenVPN 2.0.1, the
       username is passed to the OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY plugin in its raw  form,  without  string
       remapping.

       --auth-user-pass  password:  Any  "printable"  character  except CR or LF.  Printable is defined to be a
       character which will cause the C library isprint() function to return true.

       --client-config-dir filename as derived from common name or username: Alphanumeric, underbar ('_'), dash
       ('-'),  and dot ('.') except for "." or ".." as standalone strings.  As of 2.0.1-rc6, the at ('@') char‐
       acter has been added as well for compatibility with the common name character class.

       Environmental variable names: Alphanumeric or underbar ('_').

       Environmental variable values: Any printable character.

       For all cases, characters in a string which are not members of the legal character class for that string
       type will be remapped to underbar ('_').

   Environmental Variables
       Once set, a variable is persisted indefinitely until it is reset by a new value or a restart,

       As of OpenVPN 2.0-beta12, in server mode, environmental variables set by OpenVPN are scoped according to
       the client objects they are associated with, so there should not  be  any  issues  with  scripts  having
       access to stale, previously set variables which refer to different client instances.

       bytes_received
              Total  number  of  bytes  received from client during VPN session.  Set prior to execution of the
              --client-disconnect script.

       bytes_sent
              Total number of bytes sent to client during VPN session.  Set prior to execution of the --client-
              disconnect script.

       common_name
              The  X509  common  name  of an authenticated client.  Set prior to execution of --client-connect,
              --client-disconnect, and --auth-user-pass-verify scripts.

       config Name of first --config file.  Set on program initiation and reset on SIGHUP.

       daemon Set to "1" if the --daemon directive is specified, or "0" otherwise.  Set on  program  initiation
              and reset on SIGHUP.

       daemon_log_redirect
              Set  to "1" if the --log or --log-append directives are specified, or "0" otherwise.  Set on pro‐
              gram initiation and reset on SIGHUP.

       dev    The actual name of the TUN/TAP device, including a unit number if it exists.  Set prior  to  --up
              or --down script execution.

       foreign_option_{n}
              An option pushed via --push to a client which does not natively support it, such as --dhcp-option
              on a non-Windows system, will be recorded to this environmental variable sequence prior  to  --up
              script execution.

       ifconfig_broadcast
              The  broadcast  address  for  the  virtual  ethernet segment which is derived from the --ifconfig
              option when --dev tap is used.  Set prior to OpenVPN calling the ifconfig or netsh (windows  ver‐
              sion of ifconfig) commands which normally occurs prior to --up script execution.

       ifconfig_ipv6_local
              The  local  VPN  endpoint IPv6 address specified in the --ifconfig-ipv6 option (first parameter).
              Set prior to OpenVPN calling the ifconfig or netsh (windows version of ifconfig)  commands  which
              normally occurs prior to --up script execution.

       ifconfig_ipv6_netbits
              The  prefix  length of the IPv6 network on the VPN interface.  Derived from the /nnn parameter of
              the IPv6 address in the --ifconfig-ipv6 option (first parameter).  Set prior to  OpenVPN  calling
              the  ifconfig or netsh (windows version of ifconfig) commands which normally occurs prior to --up
              script execution.

       ifconfig_ipv6_remote
              The remote VPN endpoint IPv6 address specified in the --ifconfig-ipv6 option (second  parameter).
              Set  prior  to OpenVPN calling the ifconfig or netsh (windows version of ifconfig) commands which
              normally occurs prior to --up script execution.

       ifconfig_local
              The local VPN endpoint IP address specified in the  --ifconfig  option  (first  parameter).   Set
              prior  to OpenVPN calling the ifconfig or netsh (windows version of ifconfig) commands which nor‐
              mally occurs prior to --up script execution.

       ifconfig_remote
              The remote VPN endpoint IP address specified in the --ifconfig  option  (second  parameter)  when
              --dev tun is used.  Set prior to OpenVPN calling the ifconfig or netsh (windows version of ifcon‐
              fig) commands which normally occurs prior to --up script execution.

       ifconfig_netmask
              The subnet mask of the virtual ethernet segment that is specified  as  the  second  parameter  to
              --ifconfig  when  --dev  tap  is  being used.  Set prior to OpenVPN calling the ifconfig or netsh
              (windows version of ifconfig) commands which normally occurs prior to --up script execution.

       ifconfig_pool_local_ip
              The local virtual IP address for the TUN/TAP tunnel taken from an  --ifconfig-push  directive  if
              specified,  or  otherwise  from  the ifconfig pool (controlled by the --ifconfig-pool config file
              directive).  Only set for --dev tun tunnels.  This option is set on the server prior to execution
              of the --client-connect and --client-disconnect scripts.

       ifconfig_pool_netmask
              The  virtual  IP netmask for the TUN/TAP tunnel taken from an --ifconfig-push directive if speci‐
              fied, or otherwise from the ifconfig pool (controlled by the --ifconfig-pool config  file  direc‐
              tive).   Only  set for --dev tap tunnels.  This option is set on the server prior to execution of
              the --client-connect and --client-disconnect scripts.

       ifconfig_pool_remote_ip
              The remote virtual IP address for the TUN/TAP tunnel taken from an --ifconfig-push  directive  if
              specified,  or  otherwise  from  the ifconfig pool (controlled by the --ifconfig-pool config file
              directive).  This option is set on the server prior to  execution  of  the  --client-connect  and
              --client-disconnect scripts.

       link_mtu
              The  maximum  packet  size  (not  including the IP header) of tunnel data in UDP tunnel transport
              mode.  Set prior to --up or --down script execution.

       local  The --local parameter.  Set on program initiation and reset on SIGHUP.

       local_port
              The local port number, specified by --port or --lport.  Set on program initiation  and  reset  on
              SIGHUP.

       password
              The password provided by a connecting client.  Set prior to --auth-user-pass-verify script execu‐
              tion only when the via-env modifier is specified, and deleted  from  the  environment  after  the
              script returns.

       proto  The --proto parameter.  Set on program initiation and reset on SIGHUP.

       remote_{n}
              The --remote parameter.  Set on program initiation and reset on SIGHUP.

       remote_port_{n}
              The  remote  port number, specified by --port or --rport.  Set on program initiation and reset on
              SIGHUP.

       route_net_gateway
              The pre-existing default IP gateway in the system routing table.  Set prior to --up script execu‐
              tion.

       route_vpn_gateway
              The default gateway used by --route options, as specified in either the --route-gateway option or
              the second parameter to --ifconfig when --dev tun is specified.  Set prior to --up script  execu‐
              tion.

       route_{parm}_{n}
              A  set  of variables which define each route to be added, and are set prior to --up script execu‐
              tion.

              parm will be one of "network", "netmask", "gateway", or "metric".

              n is the OpenVPN route number, starting from 1.

              If the network or gateway are resolvable  DNS  names,  their  IP  address  translations  will  be
              recorded rather than their names as denoted on the command line or configuration file.

       route_ipv6_{parm}_{n}
              A  set  of  variables  which define each IPv6 route to be added, and are set prior to --up script
              execution.

              parm  will  be  one  of  "network"  or  "gateway"  ("netmask"  is  contained  as  "/nnn"  in  the
              route_ipv6_network_{n}, unlike IPv4 where it is passed in a separate environment variable).

              n is the OpenVPN route number, starting from 1.

              If  the  network  or  gateway  are  resolvable  DNS  names, their IP address translations will be
              recorded rather than their names as denoted on the command line or configuration file.

       peer_cert
              Temporary file name containing the client certificate upon  connection.   Useful  in  conjunction
              with --tls-verify

       script_context
              Set to "init" or "restart" prior to up/down script execution.  For more information, see documen‐
              tation for --up.

       script_type
              Prior to execution of any script, this variable is set to the type of script being run.   It  can
              be one of the following: up, down, ipchange, route-up, tls-verify, auth-user-pass-verify, client-
              connect, client-disconnect, or learn-address.  Set prior to execution of any script.

       signal The reason for exit or restart.  Can be one of sigusr1, sighup, sigterm, sigint,  inactive  (con‐
              trolled  by  --inactive option), ping-exit (controlled by --ping-exit option), ping-restart (con‐
              trolled by --ping-restart option), connection-reset (triggered on TCP connection  reset),  error,
              or unknown (unknown signal).  This variable is set just prior to down script execution.

       time_ascii
              Client  connection  timestamp, formatted as a human-readable time string.  Set prior to execution
              of the --client-connect script.

       time_duration
              The duration (in seconds) of the client session which is now disconnecting.  Set prior to  execu‐
              tion of the --client-disconnect script.

       time_unix
              Client connection timestamp, formatted as a unix integer date/time value.  Set prior to execution
              of the --client-connect script.

       tls_id_{n}
              A series of certificate fields from the remote peer, where n is the verification level.  Only set
              for TLS connections.  Set prior to execution of --tls-verify script.

       tls_serial_{n}
              The  serial  number  of  the certificate from the remote peer, where n is the verification level.
              Only set for TLS connections.  Set prior to execution of --tls-verify script. This is in the form
              of  a  hex  string  like  "37AB46E0", which is suitable for doing serial-based OCSP queries (with
              OpenSSL, you have to prepend "0x" to the string). If something goes wrong while reading the value
              from  the  certificate  it will be an empty string, so your code should check that.  See the con‐
              trib/OCSP_check/OCSP_check.sh script for an example.

       tun_mtu
              The MTU of the TUN/TAP device.  Set prior to --up or --down script execution.

       trusted_ip (or trusted_ip6)
              Actual IP address of connecting client or peer which has been authenticated.  Set prior to execu‐
              tion  of  --ipchange, --client-connect, and --client-disconnect scripts.  If using ipv6 endpoints
              (udp6, tcp6), trusted_ip6 will be set instead.

       trusted_port
              Actual port number of connecting client or peer which has been authenticated.  Set prior to  exe‐
              cution of --ipchange, --client-connect, and --client-disconnect scripts.

       untrusted_ip (or untrusted_ip6)
              Actual  IP  address of connecting client or peer which has not been authenticated yet.  Sometimes
              used to nmap the connecting host in a --tls-verify script to ensure it  is  firewalled  properly.
              Set  prior  to execution of --tls-verify and --auth-user-pass-verify scripts.  If using ipv6 end‐
              points (udp6, tcp6), untrusted_ip6 will be set instead.

       untrusted_port
              Actual port number of connecting client or peer which has not been authenticated yet.  Set  prior
              to execution of --tls-verify and --auth-user-pass-verify scripts.

       username
              The username provided by a connecting client.  Set prior to --auth-user-pass-verify script execu‐
              tion only when the via-env modifier is specified.

       X509_{n}_{subject_field}
              An X509 subject field from the remote peer certificate, where n is the verification level.   Only
              set  for TLS connections.  Set prior to execution of --tls-verify script.  This variable is simi‐
              lar to tls_id_{n} except the component X509 subject fields are broken out, and no  string  remap‐
              ping occurs on these field values (except for remapping of control characters to "_").  For exam‐
              ple, the following variables would be set on the OpenVPN server using the sample client  certifi‐
              cate  in sample-keys (client.crt).  Note that the verification level is 0 for the client certifi‐
              cate and 1 for the CA certificate.

                  X509_0_emailAddress=me@myhost.mydomain
                  X509_0_CN=Test-Client
                  X509_0_O=OpenVPN-TEST
                  X509_0_ST=NA
                  X509_0_C=KG
                  X509_1_emailAddress=me@myhost.mydomain
                  X509_1_O=OpenVPN-TEST
                  X509_1_L=BISHKEK
                  X509_1_ST=NA
                  X509_1_C=KG

INLINE FILE SUPPORT
       OpenVPN allows including files in the main configuration for  the  --ca,  --cert,  --dh,  --extra-certs,
       --key, --pkcs12, --secret and --tls-auth options.

       Each inline file started by the line <option> and ended by the line </option>

       Here is an example of an inline file usage

           <cert>
           -----BEGIN CERTIFICATE-----
           [...]
           -----END CERTIFICATE-----
           </cert>

       When using the inline file feature with --pkcs12 the inline file has to be base64 encoded. Encoding of a
       .p12 file into base64 can be done for example with OpenSSL by running openssl base64 -in input.p12


SIGNALS
       SIGHUP Cause OpenVPN to close all TUN/TAP and network connections, restart,  re-read  the  configuration
              file (if any), and reopen TUN/TAP and network connections.

       SIGUSR1
              Like SIGHUP, except don't re-read configuration file, and possibly don't close and reopen TUN/TAP
              device, re-read key files, preserve local IP address/port, or preserve  most  recently  authenti‐
              cated  remote  IP  address/port  based  on  --persist-tun, --persist-key, --persist-local-ip, and
              --persist-remote-ip options respectively (see above).

              This signal may also be internally generated by a timeout  condition,  governed  by  the  --ping-
              restart option.

              This  signal,  when combined with --persist-remote-ip, may be sent when the underlying parameters
              of the host's network interface change such as when the host is a DHCP client and is  assigned  a
              new IP address.  See --ipchange above for more information.

       SIGUSR2
              Causes OpenVPN to display its current statistics (to the syslog file if --daemon is used, or std‐
              out otherwise).

       SIGINT, SIGTERM
              Causes OpenVPN to exit gracefully.

TUN/TAP DRIVER SETUP
       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 things you need to do:

       Make device: mknod /dev/net/tun c 10 200

       Load driver: modprobe tun

EXAMPLES
       Prior  to running these examples, you should have OpenVPN installed on two machines with network connec‐
       tivity between them.  If you have not yet installed OpenVPN, consult the INSTALL file  included  in  the
       OpenVPN distribution.

   TUN/TAP Setup:
       If you are using Linux 2.4 or higher, make the tun device node and load the tun module:

              mknod /dev/net/tun c 10 200

              modprobe tun

       If you installed from RPM, the mknod step may be omitted, because the RPM install does that for you.

       Only Linux 2.4 and newer are supported.

       For other platforms, consult the INSTALL file at http://openvpn.net/install.html for more information.

   Firewall Setup:
       If  firewalls exist between the two machines, they should be set to forward UDP port 1194 in both direc‐
       tions.  If you do not have control over the firewalls between the two machines, you may still be able to
       use  OpenVPN  by  adding --ping 15 to each of 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 15 seconds which  will  cause  many
       stateful firewalls to forward packets in both directions without an explicit firewall rule).

       If  you  are using a Linux iptables-based firewall, you may need to enter the following command to allow
       incoming packets on the TUN device:

              iptables -A INPUT -i tun+ -j ACCEPT

       See the firewalls section below for more information on configuring firewalls for use with OpenVPN.

   VPN Address Setup:
       For purposes of our example, our two machines will be called may.kg and june.kg.  If you are  construct‐
       ing  a  VPN  over the internet, then replace may.kg and june.kg with the internet hostname or IP address
       that each machine will use to contact the other over the internet.

       Now we will choose the tunnel endpoints.  Tunnel endpoints are private IP addresses that only have mean‐
       ing in the context of the VPN.  Each machine will use the tunnel endpoint of the other machine to access
       it over the VPN.  In our example, the tunnel endpoint for may.kg  will  be  10.4.0.1  and  for  june.kg,
       10.4.0.2.

       Once  the VPN is established, you have essentially created a secure alternate path between the two hosts
       which is addressed by using the tunnel endpoints.  You can control which network traffic passes  between
       the  hosts (a) over the VPN or (b) independently of the VPN, by choosing whether to use (a) the VPN end‐
       point address or (b) the public internet address, to access the 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 its own built-in
       security) you would use the command ssh june.kg.  However in the same scenario, you could also  use  the
       command  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 ssh.

       You can use any address you wish for the tunnel endpoints but make sure that they are private  addresses
       (such  as  those that 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 bridging.  If you use an address that is part of your local sub‐
       net for either of the tunnel endpoints, you will get a weird feedback loop.

   Example 1: A simple tunnel without security
       On may:

              openvpn --remote june.kg --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --verb 9

       On june:

              openvpn --remote may.kg --dev tun1 --ifconfig 10.4.0.2 10.4.0.1 --verb 9

       Now verify the tunnel is working by pinging across the tunnel.

       On may:

              ping 10.4.0.2

       On june:

              ping 10.4.0.1

       The  --verb  9 option will produce verbose output, similar to the tcpdump(8) program.  Omit the --verb 9
       option to have OpenVPN run quietly.

   Example 2: A tunnel with static-key security (i.e. using a pre-shared secret)
       First build a static key on may.

              openvpn --genkey --secret key

       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 using the scp(1) program.

       On may:

              openvpn --remote june.kg --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --verb 5 --secret key

       On june:

              openvpn --remote may.kg --dev tun1 --ifconfig 10.4.0.2 10.4.0.1 --verb 5 --secret key

       Now verify the tunnel is working by pinging across the tunnel.

       On may:

              ping 10.4.0.2

       On june:

              ping 10.4.0.1

   Example 3: A tunnel with full TLS-based security
       For  this test, we will designate may as the TLS client and june as the TLS server.  Note that client or
       server designation only has meaning for the TLS subsystem. It has no bearing on OpenVPN's  peer-to-peer,
       UDP-based communication model.

       First,  build a separate certificate/key pair for both may and june (see above where --cert is discussed
       for more info).  Then construct Diffie Hellman parameters (see above where --dh is  discussed  for  more
       info).   You  can  also  use  the included test files client.crt, client.key, server.crt, server.key and
       ca.crt.  The .crt files are certificates/public-keys, the .key files are private keys, and ca.crt  is  a
       certification  authority  who  has signed both client.crt and server.crt.  For Diffie Hellman parameters
       you can use the included file dh1024.pem.  Note that all client, server, and certificate authority  cer‐
       tificates  and  keys  included  in  the OpenVPN distribution are totally insecure and should be used for
       testing only.

       On may:

              openvpn --remote june.kg --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --tls-client --ca ca.crt  --cert
              client.crt --key client.key --reneg-sec 60 --verb 5

       On june:

              openvpn --remote may.kg --dev tun1 --ifconfig 10.4.0.2 10.4.0.1 --tls-server --dh dh1024.pem --ca
              ca.crt --cert server.crt --key server.key --reneg-sec 60 --verb 5

       Now verify the tunnel is working by pinging across the tunnel.

       On may:

              ping 10.4.0.2

       On june:

              ping 10.4.0.1

       Notice the --reneg-sec 60 option we used above.  That tells OpenVPN to renegotiate the data channel keys
       every  minute.   Since  we used --verb 5 above, you will see status information on each new key negotia‐
       tion.

       For production operations, a key renegotiation interval of 60 seconds is probably  too  frequent.   Omit
       the --reneg-sec 60 option to use OpenVPN's default key renegotiation interval of one hour.

   Routing:
       Assuming you can ping across the tunnel, the next step is to route a real subnet over the secure tunnel.
       Suppose that may and june have two network interfaces each, one connected to the internet, and the other
       to a private network.  Our goal is to 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.

       First, ensure that IP forwarding is enabled on both peers.  On Linux, enable routing:

              echo 1 > /proc/sys/net/ipv4/ip_forward

       and enable TUN packet forwarding through the firewall:

              iptables -A FORWARD -i tun+ -j ACCEPT

       On may:

              route add -net 10.0.1.0 netmask 255.255.255.0 gw 10.4.0.2

       On june:

              route add -net 10.0.0.0 netmask 255.255.255.0 gw 10.4.0.1

       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 versa).

       In  a  production  environment, you could put the route command(s) in a script and execute with the --up
       option.

FIREWALLS
       OpenVPN's usage of a single UDP port makes it fairly firewall-friendly.  You should add an entry to your
       firewall rules to allow incoming OpenVPN packets.  On Linux 2.4+:

              iptables -A INPUT -p udp -s 1.2.3.4 --dport 1194 -j ACCEPT

       This  will  allow incoming packets on UDP port 1194 (OpenVPN's default UDP port) from an OpenVPN peer at
       1.2.3.4.

       If you are using HMAC-based packet authentication (the default in any of OpenVPN's secure modes), having
       the  firewall filter on source address can be considered optional, since HMAC packet authentication is a
       much more secure method of verifying the authenticity of a packet source.  In that case:

              iptables -A INPUT -p udp --dport 1194 -j ACCEPT

       would be adequate and would not render the host inflexible with respect to its peer having a dynamic  IP
       address.

       OpenVPN  also works well on stateful firewalls.  In some cases, you may not need to add any static rules
       to the firewall list if you are using a stateful firewall that knows how to track UDP  connections.   If
       you specify --ping n, OpenVPN will be guaranteed to send a packet to its peer at least once every n sec‐
       onds.  If n is less than the stateful firewall connection timeout, you can maintain an  OpenVPN  connec‐
       tion indefinitely without explicit firewall rules.

       You should also add firewall rules to allow incoming IP traffic on TUN or TAP devices such as:

              iptables -A INPUT -i tun+ -j ACCEPT

       to allow input packets from tun devices,

              iptables -A FORWARD -i tun+ -j ACCEPT

       to allow input packets from tun devices to be forwarded to other hosts on the local network,

              iptables -A INPUT -i tap+ -j ACCEPT

       to allow input packets from tap devices, and

              iptables -A FORWARD -i tap+ -j ACCEPT

       to allow input packets from tap devices to be forwarded to other hosts on the local network.

       These  rules are secure if you use packet authentication, since no incoming packets will arrive on a TUN
       or TAP virtual device unless they first pass an HMAC authentication test.

FAQ
       http://openvpn.net/faq.html

HOWTO
       For a more comprehensive guide to setting up OpenVPN in a production setting, see the OpenVPN  HOWTO  at
       http://openvpn.net/howto.html

PROTOCOL
       For a description of OpenVPN's underlying protocol, see http://openvpn.net/security.html

WEB
       OpenVPN's web site is at http://openvpn.net/

       Go here to download the latest version of OpenVPN, subscribe to the mailing lists, read the mailing list
       archives, or browse the SVN repository.

BUGS
       Report all bugs to the OpenVPN team <info@openvpn.net>.

SEE ALSO
       dhcpcd(8), ifconfig(8), openssl(1), route(8), scp(1) ssh(1)

NOTES
       This product includes software developed by the OpenSSL Project ( http://www.openssl.org/ )

       For more information on the TLS protocol, see http://www.ietf.org/rfc/rfc2246.txt

       For more information  on  the  LZO  real-time  compression  library  see  http://www.oberhumer.com/open‐
       source/lzo/

COPYRIGHT
       Copyright  (C)  2002-2010 OpenVPN Technologies, Inc. This program is free software; you can redistribute
       it and/or modify it under the terms of the GNU General Public License version 2 as published by the Free
       Software Foundation.

AUTHORS
       James Yonan <jim@yonan.net>



                                                17 November 2008                                     openvpn(8)