wiki:Openvpn23ManPage

Version 1 (modified by Samuli Seppänen, 8 years ago) (diff)

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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 bridging, 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 footprint.

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 comments.

              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  refer‐
              ring 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 <connection> 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  dif‐
              ferent server, and that server pushes back different TUN/TAP or route settings, the client may lack the necessary privi‐
              leges 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, providing 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  con‐
              nect to a given OpenVPN server.  Client connection profiles are specified within an OpenVPN configuration file, and each
              profile is bracketed by <connection> and </connection>.

              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 robustness perspective, such
              as tunneling non-IP or application-level UDP protocols, or tunneling protocols which don't possess a built-in  reliabil‐
              ity 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 authentication 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  con‐
              trol  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 follows:

              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  official  IANA  port  number
              assignment for OpenVPN and has been used since version 2.0-beta17.  Previous 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 automatically push your chosen
              topology setting to clients as well.  This directive can also be manually 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 systems.  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 Windows 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 OpenVPN  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  cre‐
              ated 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 ping‐
              ing 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 eth‐
              ernet 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 vari‐
              ables.

       --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 --ifcon‐
              fig-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 automati‐
              cally 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 --ifcon‐
              fig 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 negotiation 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  imme‐
              diately  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 environmental 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,  how‐
              ever 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 --ifconfig, --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 create 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-gateway or the second param‐
              eter 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 gateway.

              bypass-dhcp  --  Add a direct route to the DHCP server (if it is non-local) which bypasses the tunnel (Available on Win‐
              dows 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 accom‐
              plished 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  parame‐
              ter 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 particu‐
              lar 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 configuration.  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 encapsula‐
              tion 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 dis‐
              covery 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 encapsula‐
              tion 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 fragmenta‐
              tion in the first place, and if big packets come through anyhow (from protocols other than TCP), --fragment will  inter‐
              nally 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 trigger 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  pack‐
              ets).  When used in one of OpenVPN's secure modes (where --secret, --tls-server, or --tls-client is specified), the ping
              packet will be cryptographically secure.

              This option has two intended uses:

              (1) Compatibility with stateful firewalls.  The periodic ping will ensure that a stateful  firewall  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 gener‐
              ated 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  --per‐
              sist-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.

              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 virtual memory paging oper‐
              ations 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 private 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 initialization 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' --persist-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 establishment with peer.

              In --proto udp mode, this option normally requires the use of --ping to allow connection initiation 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 establish‐
              ment, 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 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.

              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  privi‐
              lege.

       --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 prob‐
              lems 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 "OPENVPN_" 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  compatibility  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 inconsis‐
              tency 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  privi‐
              leged 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  stand‐
              point.

              Since the chroot operation is delayed until after initialization, most OpenVPN options that reference 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 sys‐
              tem call. You can of course combine all three, but please note that since setcon requires access to /proc you will  have
              to provide 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 sys‐
              tem 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 redirected.  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 logger  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  majority  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  initial‐
              ized and entered the packet forwarding 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 prog‐
              name 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 clas‐
              sic 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 particular, 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 inter‐
              faces, 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 logging 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 "adap‐
              tive" (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 periodically 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  (pass‐
              word  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 management functions.

              The management interface can also listen on a unix domain socket, for those platforms that support 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 connected 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 management 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 tel‐
              net 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 client to IP:port specified by --management  rather  than  listen  as  a  TCP
              server.

       --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 console.

       --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.

       --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 module initialization func‐
              tion.  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 connections.  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 sufficiently 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 directive will set up an Open‐
              VPN  server  which will allocate addresses to clients out of the given network/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 OpenVPN's server mode in  eth‐
              ernet bridging configurations.

              If  --server-bridge  is  used without any parameters, it will enable a DHCP-proxy mode, where connecting 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 effec‐
              tively allow a compromised server to execute arbitrary code on the client.  Other options such as TLS or MTU  parameters
              cannot 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 specified 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 --persist-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 com‐
              mon 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 --ifconfig-push

       --ifconfig-pool-linear
              Modifies  the  --ifconfig-pool directive to allocate individual TUN interface addresses for clients rather than /30 sub‐
              nets.  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 specified 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 interface.

              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 environmental 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 con‐
              fig 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-con‐
              nect 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 configu‐
              ration 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_control_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 pack‐
              ets 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 ultimately 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 com‐
              mon 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  cur‐
              rent  directory  if unspecified.  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 authentication capabilities.

              To  protect  against a client passing a maliciously formed username or password string, the username 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 underbar ('_').

              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 OpenVPN source distribution.

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

              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 --man‐
              agement-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 responsibility 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 module/script will see the username and password as
              being  set to empty strings ("").  The authentication 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  dou‐
              ble  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.

       --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  gener‐
              ated  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 configura‐
       tion.

       --client
              A helper directive designed to simplify the configuration of OpenVPN's client mode.  This directive 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 situa‐
              tions 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  Win‐
              dows by defining ENABLE_PASSWORD_SAVE in win/settings.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 username/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 username/password requeries in
              case of error.

              An AUTH_FAILED message is generated by the server if the client fails --auth-user-pass authentication, 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 username/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 challenge/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  encryp‐
              tion/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 parameter, 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 some‐
              what  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 Hell‐
              man  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  distin‐
              guishing  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.

       --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 OpenVPN's pseudo-random num‐
              ber 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 datagram, 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 (identi‐
              fying 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 management mode you have
              selected.

              In Static Key mode or when using an CFB or OFB mode cipher, OpenVPN uses a 64 bit unique identifier 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 protection 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 seconds 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 number 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 atti‐
              tude 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 proto‐
              cols 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 secu‐
              rity 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 con‐
              text (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 session 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 function, 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 isolate 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 network 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  for‐
       warded  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 forward 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 certificate 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 param‐
              eters 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  addi‐
              tion,  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
              commercial service such as thawte.com (in which case you will be helping to finance the world's second 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  pub‐
              lic  medium).   Note that the openssl ca command reads the location of the certificate authority key from its configura‐
              tion 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 certificate 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 cer‐
              tificate, 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 following:

              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 certificate, 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 cer‐
              tificate 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 connection.

              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 certificates 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 security 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 seconds (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 exponential 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 renegotia‐
              tion 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  reautho‐
              rize 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 connection 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 initiation 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  sec‐
              onds).   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 fin‐
              ished.

       --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 packets 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 signature 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,  including  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 private 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 dur‐
              ing 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  certi‐
              fication (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 follows:

              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 certificate  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  uppercased  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  connections  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 associated 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 designated 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 designated 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 designated 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 designated server.

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

              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  Authenti‐
              cation"

              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  com‐
              puter  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 containing 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  per‐
       sistent 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  envi‐
              ronment  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  gener‐
              ated  by  the kernel.  This mode is probably the "cleanest" solution for setting 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 masquerade as the IP address at broad‐
              cast 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 prevents 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" command.  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 practice.  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 pack‐
              ages 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  Neon‐
              Surge@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 envi‐
              ronment 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 connection 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 service 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 control‐
              ling 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 out‐
              put 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  sys‐
              tems, 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 necessary 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 (net‐
              mask 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 OpenVPN's internal routing ta‐
              ble.

       --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 per‐
       mitted 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 parame‐
       ters 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 ('=').  Alphanu‐
       meric 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 ('@') character 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 program 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 execu‐
              tion.

       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  sys‐
              tem, 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 version 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  execu‐
              tion.

       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 ifcon‐
              fig) 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  execu‐
              tion.

       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 normally 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 ifconfig) 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 specified,  or  otherwise  from
              the  ifconfig  pool  (controlled  by  the --ifconfig-pool config file directive).  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 execution 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 execution.

       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 execution.

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

              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 documentation 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  (controlled  by  --inactive
              option),  ping-exit  (controlled by --ping-exit option), ping-restart (controlled 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 execution of the --client-dis‐
              connect script.

       time_unix
              Client  connection  timestamp, formatted as a unix integer date/time value.  Set prior to execution of the --client-con‐
              nect 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 connec‐
              tions.  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
              contrib/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 execution 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 execution 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  con‐
              necting  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 endpoints (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 execution 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  connec‐
              tions.  Set prior to execution of --tls-verify script.  This variable is similar to tls_id_{n} except the component X509
              subject fields are broken out, and no string remapping occurs on these field values (except  for  remapping  of  control
              characters  to  "_").   For  example, the following variables would be set on the OpenVPN server using the sample client
              certificate in sample-keys (client.crt).  Note that the verification level is 0 for the client certificate 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

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 authenticated remote IP address/port based on --per‐
              sist-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 stdout 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 connectivity 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 directions.   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 constructing a VPN over the inter‐
       net,  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 meaning 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) indepen‐
       dently of the VPN, by choosing whether to use (a) the VPN endpoint 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 subnet 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 certificates  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 negotiation.

       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 seconds.  If n is less than the stateful firewall connection timeout, you
       can maintain an OpenVPN connection 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://open‐
       vpn.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/opensource/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)