Version 5 (modified by Samuli Seppänen, 8 years ago) (diff)

Updated man-page for openvpn-2.3_rc1

openvpn(8)                                                          openvpn(8)

       openvpn - secure IP tunnel daemon.

       openvpn [ options ... ]

       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 Open‐
       VPN, 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:

       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.

       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  tun‐

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

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

       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  corre‐
              sponds  to  one  command  line option, but with the leading '--'

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

              Note that configuration files can  be  nested  to  a  reasonable

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

              Note that OpenVPN 2.0 and higher performs backslash-based  shell
              escaping for characters not in single quotations, so the follow‐
              ing 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

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

              For   examples   of   configuration   files,   see  http://open‐

              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

                  # is our local VPN endpoint
                  # is our remote VPN endpoint

                  # Our pre-shared static key
                  secret static.key

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

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

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

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

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

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

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

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

              If --remote is unspecified, OpenVPN will listen for packets from
              any IP address, but will not act on those  packets  unless  they
              pass all authentication tests.  This requirement for authentica‐
              tion 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-bal‐
              ancing and failover capability.

              Add  a  random string (6 characters) to first DNS label of host‐
              name to prevent DNS caching.  For example,  ""  would
              be modified to "<random-chars>".

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

              Here is an example of connection profile usage:

                  dev tun

                  remote 1194 udp

                  remote 443 tcp

                  remote 443 tcp
                  http-proxy 8080

                  remote 443 tcp
                  http-proxy 8080

                  pkcs12 client.p12
                  ns-cert-type server
                  verb 3

              First  we  try to connect to a server at using
              UDP.  If that fails, we then try to connect to
              using  TCP.   If that also fails, then try connecting through an
              HTTP proxy at to  using  TCP.
              Finally,  try  to  connect through the same proxy to a server at
     using TCP.

              The following OpenVPN options may be used inside of  a  <connec‐
              tion> 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 <con‐
              nection> blocks, the option setting will be used  as  a  default
              for  <connection>  blocks  which  follow it in the configuration

              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 pro‐
              files using protocol p ('tcp'|'udp').

              When multiple --remote address/ports are specified, or  if  con‐
              nection  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

              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  --con‐
              nect-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 capa‐
              bility  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


              There are certain cases, however, where using TCP may be  advan‐
              tageous from a security and robustness perspective, such as tun‐
              neling non-IP or application-level UDP protocols,  or  tunneling
              protocols which don't possess a built-in reliability layer.

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

       --connect-timeout n
              For --proto tcp-client, set  connection  timeout  to  n  seconds

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

              Show sensed HTTP or SOCKS proxy settings. Currently,  only  Win‐
              dows 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.

              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

              VERSION  version  --  Set  HTTP  version   number   to   version

              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).

              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 sec‐
              onds before failing.

              Set n to "infinite" to retry indefinitely.

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

              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,  how‐
              ever if packets arrive from a new address and pass all authenti‐
              cation tests, the new address will take control of the  session.
              This  is  useful when you are connecting to a peer which holds a
              dynamic address such as a dial-in user or DHCP client.

              Essentially, --float tells OpenVPN to accept authenticated pack‐
              ets  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  authen‐
              ticated 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  argu‐
              ments specified in cmd , as follows:

              cmd ip_address port_number

              Don't use --ipchange in --mode server mode.  Use a --client-con‐
              nect 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 con‐
              figure 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 assign‐
              ment  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.

              Do  not bind to local address and port.  The IP stack will allo‐
              cate 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  con‐
              nections by using the --remote option.

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

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

              You  must  use either tun devices on both ends of the connection
              or tap devices on both ends.  You cannot mix them, as they  rep‐
              resent 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  end‐
              point of the server's tun interface.  This mode allocates a sin‐
              gle IP address per connecting client.  Only use when none of the
              connecting  clients are Windows systems.  This mode is function‐
              ally 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 con‐
              necting  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 direc‐
              tive 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.

              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 con‐
              trol 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  unprivi‐
              leged environment.

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

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

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

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

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

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

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

              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  compo‐
              nent of the check.

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

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

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

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

              netmask default --

              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  set‐
              ting 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  spe‐
              cial keywords:

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

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

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

       --max-routes n
              Allow  a  maximum  number  of n --route options to be specified,
              either in the local configuration file, or pulled from an  Open‐
              VPN 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  immedi‐
              ately  upon connection establishment.  If --route-delay is omit‐
              ted, routes will be added immediately after TUN/TAP device  open
              and  --up  script execution, before any --user or --group privi‐
              lege 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  wait‐
              ing  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-

              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.

              Don't  add  or remove routes automatically.  Instead pass routes
              to --route-up script using environmental variables.

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

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

              Allow client to pull DNS names from server  (rather  than  being
              limited  to  IP  address)  for --ifconfig, --route, and --route-

       --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 defines
              the local view of a resource from the client perspective,  while
              alias/netmask  (for  example 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

              This option performs three steps:

              (1) Create a static route for the --remote  address  which  for‐
              wards 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 parameter to
              --ifconfig when --dev tun is specified).

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

              Option flags:

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

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

              def1 -- Use this flag to override the default gateway  by  using
      and rather than  This has the
              benefit of overriding but not wiping out  the  original  default

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

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

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

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

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

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

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

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

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

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

       --mtu-disc type
              Should  we  do Path MTU discovery on TCP/UDP channel?  Only sup‐
              ported 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)

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

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

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

              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

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

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

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

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

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

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

              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

       --socket-flags flags...
              Apply the given flags to the  OpenVPN  transport  socket.   Cur‐
              rently, 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  con‐
              nection  without  trying to group several smaller packets into a
              larger packet.  This can result in a considerably improvement in

              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 shap‐
              ing: 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-pri‐
              ority  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  "activ‐
              ity",  nor  are they counted as traffic, as they are used inter‐
              nally by OpenVPN and are not an indication of actual user activ‐

       --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 Open‐
              VPN ping packets are not echoed like  IP  ping  packets).   When
              used  in  one  of OpenVPN's secure modes (where --secret, --tls-
              server, or --tls-client is specified), the ping packet  will  be
              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

              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 + a
              dynamic DNS client such as ddclient.

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

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

              In  client mode, the --ping-restart parameter is set to 120 sec‐
              onds 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 --per‐
              sist-tun,  --persist-key,  --persist-local-ip,  and   --persist-
              remote-ip options.

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

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

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

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

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

              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  con‐

              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.

              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.

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

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

              Disable paging by calling the POSIX mlockall function.  Requires
              that OpenVPN be initially run as root (though OpenVPN can subse‐
              quently 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  operations
              which  occur  under  most  modern operating systems.  It ensures
              that even if an attacker was able to crack the box running Open‐
              VPN,  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   ifcon‐
              fig_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 argu‐
              ments  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  Open‐
              VPN  where  the TUN/TAP instance is preserved (the --persist-tun
              option will enable such preservation).  A restart can be  gener‐
              ated by a SIGUSR1 signal, a --ping-restart timeout, or a connec‐
              tion reset when the TCP protocol is  enabled  with  the  --proto
              option.   If  a restart occurs, and --up-restart has been speci‐
              fied, the up script will be called  with  restart  as  the  last

              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  auto‐
              matically  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 netmask gw $5

              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"  proto‐

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

       --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  spa‐

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

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

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

              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

       --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

              This option should be used with caution, as there are good secu‐
              rity reasons for having OpenVPN fail if it detects problems in a
              config  file.   Having  said  that,  there are valid reasons for
              wanting new software features to gracefully degrade when encoun‐
              tered 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
              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

              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
              3 -- Allow passwords to be passed to scripts  via  environmental
              variables (potentially unsafe).

              OpenVPN  releases before v2.3 also supported a method flag which
              indicated how OpenVPN should call external commands and scripts.
              This could be either execve or system.  As of OpenVPN v2.3, this
              flag is no longer  accepted.   In  most  *nix  environments  the
              execve() approach has been used without any issues.

              To run scripts in Windows in earlier OpenVPN versions you needed
              to either add a full path to the script  interpreter  which  can
              parse  the  script  or use the system flag to run these scripts.
              As of OpenVPN v2.3 it is now a strict requirement to  have  full
              path  to  the  script  interpreter  when running non-executables
              files.  This is not needed for executable files, such  as  .exe,
              .com,  .bat  or  .cmd  files.  For example, if you have a Visual
              Basic script, you must use this syntax now:

                  --up 'C:\\Windows\\System32\\wscript.exe C:\\Program\ Files\\OpenVPN\\config\\my-up-script.vbs'

              Please note the single quote marks and the escaping of the back‐
              slashes (\) and the space character.

              The reason the support for the system flag was removed is due to
              the security implications with shell expansions  when  executing
              scripts via the system() call.

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

              Use of this option is discouraged, but is provided as  a  tempo‐
              rary  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 initial‐
              ization,  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 sig‐
              nal (for example in response to a DHCP reset), you  should  make
              use of one or more of the --persist options to ensure that Open‐
              VPN doesn't need to execute any privileged operations  in  order
              to  restart (such as re-reading key files or running ifconfig on
              the TUN device).

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

       --cd dir
              Change  directory to dir prior to reading any files such as con‐
              figuration 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  rede‐
              fines  dir  as  being the top level directory tree (/).  OpenVPN
              will therefore be unable to access any files outside this  tree.
              This can be desirable from a security standpoint.

              Since  the  chroot  operation is delayed until after initializa‐
              tion, most OpenVPN options that reference files will operate  in
              a pre-chroot context.

              In  many  cases,  the dir parameter can point to an empty direc‐
              tory, 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  net‐
              work  I/O  operations, thanks to SELinux. This goes further than
              --user and --chroot in that those two, while being  great  secu‐
              rity  features,  unfortunately  do not protect against privilege
              escalation by exploitation of a vulnerable system 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 initializa‐
              tion, OpenVPN can be restricted to just  network-related  system
              calls,  whereas  by applying the context before startup (such as
              the OpenVPN one provided in the SELinux Reference Policies)  you
              will  have to allow many things required only during initializa‐

              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  --per‐
              sist-tun options.

       --daemon [progname]
              Become  a  daemon  after  all  initialization functions are com‐
              pleted.  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 redirect‐
              ion 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 use‐
              ful 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 func‐
              tions which are capable of generating fatal errors are complete.
              This  means that initialization scripts can test the return sta‐
              tus of the openvpn command for a fairly reliable  indication  of
              whether  the  command  has correctly initialized and entered the
              packet forwarding event loop.

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

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

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

              Set the TOS field of the tunnel packet to what the payload's TOS

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

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

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

              Also note that in wait mode, each OpenVPN tunnel requires a sep‐
              arate  TCP/UDP  port  and a separate inetd or xinetd entry.  See
              the OpenVPN 1.x HOWTO for  an  example  on  using  OpenVPN  with

       --log file
              Output  logging  messages  to  file,  including  output  to std‐
              out/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 speci‐
              fied.  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, log‐
              ging 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.

              Avoid  writing timestamps to log messages, even when they other‐
              wise would be prepended. In particular, this applies to log mes‐
              sages 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).

              (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 opti‐
              mize 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.

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

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

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

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

       --remap-usr1 signal
              Control whether internally or externally generated SIGUSR1  sig‐
              nals  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

              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 lower‐
              case 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

       --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 "adaptive"

              In a server mode setup, it is possible to selectively turn  com‐
              pression 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 set‐
              ting 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.

              When  used in conjunction with --comp-lzo, this option will dis‐
              able OpenVPN's adaptive compression algorithm.  Normally,  adap‐
              tive compression is enabled with --comp-lzo.

              Adaptive  compression  tries to optimize the case where you have
              compression enabled, but you are  sending  predominantly  uncom‐
              pressible  (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 com‐
              pression 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  com‐
              pression 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 func‐
              tions.  pw-file, if specified, is a password file  (password  on
              first line) or "stdin" to prompt from standard input.  The pass‐
              word 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 man‐
              agement interface to listen for a TCP connection  on  the  local
              VPN address of the TUN/TAP interface.

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

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

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

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

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

              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

              Query management channel for proxy server information for a spe‐
              cific --remote (client-only).

              Allow  management  interface  to  override  --remote  directives
              (client-only).   --management-external-key  Allows   usage   for
              external private key file instead of --key option (client-only).

              Make  OpenVPN  forget  passwords when management session discon‐

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

              Start OpenVPN in a hibernating state, until a client of the man‐
              agement interface explicitly starts it  with  the  hold  release

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

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

              Report tunnel up/down events to management interface.

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

              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 function.
              Multiple plugin modules may be loaded into one OpenVPN process.

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

              If you are using an RPM install of OpenVPN, see /usr/share/open‐
              vpn/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  mod‐
              ule/script controls an authentication function (such as tls-ver‐
              ify, auth-user-pass-verify, or client-connect), then every  mod‐
              ule  and script must return success (0) in order for the connec‐
              tion to be authenticated.

   Server Mode
       Starting with OpenVPN 2.0, a multi-client TCP/UDP server mode  is  sup‐
       ported,  and  can  be enabled with the --mode server option.  In server
       mode, OpenVPN will listen on a single port for incoming client  connec‐
       tions.   All  client connections will be routed through a single tun or
       tap interface.  This mode is designed for  scalability  and  should  be
       able  to  support hundreds or even thousands of clients on 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 OpenVPN
              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 expands as follows:

                   mode server
                   push "topology [topology]"

                   if dev tun AND (topology == net30 OR topology == p2p):
                     if !nopool:
                     if client-to-client:
                       push "route"
                     else if topology == net30:
                       push "route"

                   if dev tap OR (dev tun AND topology == subnet):
                     if !nopool:
                     push "route-gateway"

              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 sim‐
              plify  the  configuration  of  OpenVPN's server mode in ethernet
              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  ether‐
              net  NIC interface.  For example, on Linux this is done with the
              brctl tool, and with Windows XP it is done in the  Network  Con‐
              nections  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  inter‐
              face,  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 con‐
              necting clients.

              For example,  server-bridge
     expands as follows:

                  mode server

                  push "route-gateway"

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

                  mode server

                  push "route-gateway dhcp"

              Or --server-bridge nogw expands as follows:

                  mode server

       --push option
              Push a config file option back to the client for  remote  execu‐
              tion.   Note that option must be enclosed in double quotes ("").
              The client must specify --pull in its config file.  The  set  of
              options  which  can be pushed is limited by both feasibility and
              security.  Some  options  such  as  those  which  would  execute
              scripts are banned, since they would effectively allow a compro‐
              mised 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

              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 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

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

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

              Disable a particular client (based on the common name) from con‐
              necting.   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  con‐
              necting  clients,  similar to a DHCP server.  For tun-style tun‐
              nels, each client will be given a /30 subnet (for interoperabil‐
              ity  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 inter‐
              vals (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-

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

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

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

              This option is deprecated, and should be replaced with  --topol‐
              ogy 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

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

              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

              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.

              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-origi‐
              nating 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.

              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 envi‐
              ronmental variable section below).  The command is  also  passed
              the  pathname  of  a  freshly created temporary file as the last
              argument (after any arguments specified in cmd ), to be used  by
              the command to pass dynamically generated config file directives
              back to OpenVPN.

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

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

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

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

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

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

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

              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.

              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 tem‐
              porary  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-spe‐
              cific configuration files.

              * OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY  plugin  hook  to  return
              success/failure  via  auth_control_file when using deferred auth

              * 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

       --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 connec‐
              tion can support.  When the  number  of  output  packets  queued
              before  sending to the TCP socket reaches this limit for a given
              client connection, OpenVPN will start to drop  outgoing  packets
              directed at this client.

              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  authenti‐
              cated  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-gen‐
              erated by a --client-connect script to override the global value
              for a particular client.

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

       --stale-routes-check n [t]
              Remove  routes haven't had activity for n seconds (i.e. the age‐
              ing 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 fol‐

              [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 "", an IPv4 subnet such
              as "", 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  inter‐
              face.  Since OpenVPN provides the association between virtual IP
              or MAC address and the client's authenticated  common  name,  it
              allows  a user-defined script to configure firewall access poli‐
              cies 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 user‐
              name/password strings provided by the  client.   Be  aware  that
              this  method  is insecure on some platforms which make the envi‐
              ronment of a process publicly visible to other unprivileged pro‐

              If  method is set to "via-file", OpenVPN will write the username
              and password to the first two lines of a  temporary  file.   The
              filename  will  be passed as an argument to script, and the file
              will be  automatically  deleted  by  OpenVPN  after  the  script
              returns.   The  location  of the temporary file is controlled by
              the --tmp-dir option, and will default to the current  directory
              if  unspecified.   For security, consider setting --tmp-dir to a
              volatile storage medium such as /dev/shm (if available) to  pre‐
              vent 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  user‐
              name  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  charac‐
              ters 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 han‐
              dled.  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 sam‐
              ple-scripts/ in the OpenVPN source distribution.

              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.

              Allow connections  by  clients  that  do  not  specify  a  user‐
              name/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, pass‐
              ing the responsibility to the user-defined  authentication  mod‐
              ule/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 user‐
              name/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.

              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 authen‐
              tication will rest on your  --auth-user-pass-verify  script,  so
              keep  in mind that bugs in your script could potentially compro‐
              mise the security of your VPN.

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

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

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

              /C=US/L=Somewhere/CN=John Doe/

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

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

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

              C=US, L=Somewhere, CN=John Doe,

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

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

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

       --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  con‐
              nection  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 theo‐
              retically possible to extend to other protocols such as ssh.

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

              Not implemented on Windows.

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

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


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

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

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

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

              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 mes‐
              sage from the server or verification failure of the private  key

              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
              nointeract  -- Client will retry the connection without requery‐
              ing for an --auth-user-pass username/password.  Use this  option
              for unattended clients.
              interact  --  Client  will requery for an --auth-user-pass user‐
              name/password and/or private key password  before  attempting  a

              Note  that  while  this  option cannot be pushed, it can be con‐
              trolled 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

              When  the  direction parameter is omitted, 2 keys are used bidi‐
              rectionally, one for HMAC and the other  for  encryption/decryp‐

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

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

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

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

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

              In static-key encryption mode, the HMAC key is included  in  the
              key  file  generated  by --genkey.  In TLS mode, the HMAC key is
              dynamically generated and shared between peers via the TLS  con‐
              trol  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

       --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.counter‐

              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

              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  secu‐
              rity, 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  number

       --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.

              (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 data‐
              gram 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 (identifying 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 wrap‐
              ping 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 win‐
              dow 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 possibil‐
              ity  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  con‐

              (a) The packet cannot be a replay (unless --no-replay is  speci‐
              fied, 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  contain‐
              ing 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  net‐

              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  attitude
              towards message deletion and reordering:  Don't allow it.  Since
              TCP guarantees reliability, any packet loss or reordering  event
              can be assumed to be an attack.

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

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

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

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

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

              This  option will keep a disk copy of the current replay protec‐
              tion 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.

              (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 data‐
              gram space-saving optimization that uses the  unique  identifier
              for datagram replay protection as the IV.

              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.

              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

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

              openvpn --test-crypto --secret key


              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  com‐
              piler,  OpenSSL crypto library, or OpenVPN's crypto code.  Since
              it is a self-test mode, problems with encryption and authentica‐
              tion 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 chan‐
       nels  which are multiplexed over a single TCP/UDP port.  OpenVPN initi‐
       ates 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  chan‐
       nel  communication, while the data channel, over which encrypted tunnel
       data passes, is forwarded without any mediation.   The  result  is  the
       best  of  both  worlds: a fast data channel that forwards over UDP with
       only the overhead of encrypt, decrypt, and HMAC functions, and  a  con‐
       trol channel that provides all of the security features of TLS, includ‐
       ing  certificate-based  authentication  and  Diffie   Hellman   forward

       To  use TLS mode, each peer that runs OpenVPN should have its own local
       certificate/key pair ( --cert and --key ), signed by the root  certifi‐
       cate 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 speci‐
       fied 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://open‐

              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 nego‐
              tiating the TLS control channel.

              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  certifi‐
              cates  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 vari‐
              able 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

       --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  parame‐
              ters may be considered public.

       --cert file
              Local peer's signed certificate in .pem format -- must be signed
              by a certificate authority whose certificate is  in  --ca  file.
              Each peer in an OpenVPN link running in TLS mode should have its
              own certificate and private key file.  In  addition,  each  cer‐
              tificate  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 (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

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

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

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

              This option is useful for "split" CAs, where the CA  for  server
              certs  is different than the CA for client certs.  Putting certs
              in this file allows them to be used to complete the  local  cer‐
              tificate  chain without trusting them to verify the peer-submit‐
              ted 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 cer‐
              tificate, and root CA certificate.   This  option  can  be  used
              instead   of  --ca,  --cert,  and  --key.   Not  available  with

       --verify-hash hash
              Specify SHA1 fingerprint for level-1 cert.  The level-1 cert  is
              the  CA  (or intermediate cert) that signs the leaf certificate,
              and is one removed from the leaf certificate in the direction of
              the  root.  When accepting a connection from a peer, the level-1
              cert fingerprint must match  hash  or  certificate  verification
              will  fail.   Hash  is  specified  as  XX:XX:...   For  example:

       --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.

              Acquire PKCS#11 id from management interface.  In  this  case  a
              NEED-STR  'pkcs11-id-request'  real-time  message  will be trig‐
              gered, 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  set‐

       --pkcs11-providers provider...
              Specify  a RSA Security Inc. PKCS #11 Cryptographic Token Inter‐
              face (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  Certifi‐
              cate System Store (Windows/OpenSSL Only).

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

              This  makes it possible to use any smart card, supported by Win‐
              dows, 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 certificate on the server side.

              To select a certificate, based on a substring search in the cer‐
              tificate'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  cer‐
              tificate  (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  acknowl‐
              edgment  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 renegotiation will be
              forced if any of these three criteria are met by either peer.

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

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

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

              Also, keep in mind that this option can  be  used  on  both  the
              client  and  server,  and whichever uses the lower value will be
              the one to trigger the renegotiation.  A common  mistake  is  to
              set  --reneg-sec  to  a  higher  value  on  either the client or
              server, while the other side of the connection  is  still  using
              the  default  value of 3600 seconds, meaning that the renegotia‐
              tion 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 seconds).
              This  feature  allows  for a graceful transition from old to new
              key, and removes the key renegotiation sequence from the  criti‐
              cal path of tunnel data forwarding.

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

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

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

              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 bear‐
              ing an incorrect HMAC signature can be dropped immediately with‐
              out response.

              file  (required)  is  a key file which can be in one of two for‐

              (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

              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 poten‐
              tial 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 sig‐
              nature  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 poten‐
              tial, 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 protec‐
              tion 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 noth‐
              ing 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 daemo‐

              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  com‐
              mand 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 con‐
              figure option, or on Windows by defining ENABLE_PASSWORD_SAVE in

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

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

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

       --tls-verify cmd
              Run command cmd to verify the X509 name of a pending TLS connec‐
              tion that has otherwise passed all other tests of  certification
              (except  for  revocation via --crl-verify directive; the revoca‐
              tion 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  argu‐
              ments 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  cer‐
              tificate  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  cer‐
              tificate 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  associ‐
              ated 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

       --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

              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

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

              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 Authentica‐

              The key usage is digitalSignature and/or keyAgreement.

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

              The key usage is digitalSignature and ( keyEncipherment or keyA‐
              greement ).

              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

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

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

              Suppose you had a PKI consisting of a CA, root certificate,  and
              a number of client certificates.  Suppose a laptop computer con‐
              taining a client key and certificate was stolen.  By adding  the
              stolen certificate to the CRL file, you could reject any connec‐
              tion 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

              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 cer‐
              tificate  serial  number  (decimal string) is the name of a file
              present in the directory, it will be rejected.

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

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

              (Standalone) Show all TLS ciphers (TLS used only  as  a  control
              channel).   The  TLS ciphers will be sorted from highest prefer‐
              ence (most secure) to lowest.

              (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.

              (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

       --secret file
              Write key to file.

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

              (Standalone)  Create a persistent tunnel on platforms which sup‐
              port 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

              One  of the advantages of persistent tunnels is that they elimi‐
              nate 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  ter‐
              minates 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 persis‐
              tent by default.

              (Standalone) Remove a persistent tunnel.

       --dev tunX | tapX
              TUN/TAP device

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

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

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

              This  option have changed behaviour in OpenVPN 2.3.  Earlier you
              had to define --win-sys env to use  the  SystemRoot  environment
              variable,  otherwise  it  defaulted  to  C:\WINDOWS.   It is not
              needed to use the env keyword any more,  and  it  will  just  be
              ignored.  A warning is logged when this is found in the configu‐
              ration 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 con‐
              figure the adapter manually and indicating the IP/netmask  which
              OpenVPN expects the adapter to be set to.

              dynamic  [offset]  [lease-time]  --  Automatically  set  the  IP
              address and netmask by replying to DHCP query messages generated
              by  the  kernel.   This mode is probably the "cleanest" solution
              for 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  sub‐
              net.   For  example,  if  your  subnet  is  netmask
    , then OpenVPN will take the IP address
              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 broadcast
              address + offset.  The Windows ipconfig /all command can be used
              to show what Windows thinks the DHCP server address is.  OpenVPN
              will "claim" this address, so make sure to use a  free  address.
              Having  said  that,  different OpenVPN instantiations, including
              different ends of the same connection, can share the  same  vir‐
              tual 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 seman‐
              tics, though testing has indicated that it works okay  in  prac‐
              tice.   If  you  use this option, it is best to leave the TCP/IP
              properties for the TAP-Win32 adapter  in  their  default  state,
              i.e. "Obtain an IP address automatically."

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

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

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

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

              DOMAIN name -- Set Connection-specific DNS Suffix.

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

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

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

              NTP addr -- Set primary NTP server address (Network Time  Proto‐
              col).  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  broad‐

              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

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

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

       --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.

              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.

              Ask  Windows  to  renew  the TAP adapter lease on startup.  This
              option is normally unnecessary, as Windows  automatically  trig‐
              gers  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.

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

              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.

              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 configura‐
              tion 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  auto‐
              matically  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  controlling
              process  can  signal  exit-event,  causing all such OpenVPN pro‐
              cesses to exit.

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

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

       --allow-nonadmin [TAP-adapter]
              (Standalone)  Set  TAP-adapter to allow access from non-adminis‐
              trative 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

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

              Namely, the point-to-point endpoints used in TUN  device  emula‐
              tion  must  be the middle two addresses of a /30 subnet (netmask

              (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_pri‐
              vate as 1 if certificates are stored as private objects.

              --verb option can be used BEFORE this option to  produce  debug‐
              ging 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  documenta‐
       tion  of  the  IPv6-related options. More documentation can be found on

       --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 net‐
              work 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.

       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.

              Executed when we have a still untrusted remote peer.

              Executed  after  connection authentication, or remote IP address

              Executed in --mode server mode immediately after client  authen‐

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

              Executed right before the routes are removed.

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

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

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

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

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

       Q: Why is string remapping necessary?

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

       Q: Can string remapping be disabled?

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

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

       X509 Names: Alphanumeric, underbar ('_'), dash  ('-'),  dot  ('.'),  at
       ('@'),  colon  (':'),  slash  ('/'),  and equal ('=').  Alphanumeric is
       defined as a character which will cause the C library  isalnum()  func‐
       tion 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 OPEN‐
       VPN_PLUGIN_AUTH_USER_PASS_VERIFY plugin in its raw form, without string

       --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

       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 associ‐
       ated with, so there should not be any issues with scripts having access
       to  stale,  previously  set  variables  which refer to different client

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

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

              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" other‐
              wise.  Set on program initiation and reset on SIGHUP.

              Set to "1" if the --log or --log-append  directives  are  speci‐
              fied,  or "0" otherwise.  Set on program initiation and reset on

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

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

              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

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

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

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

              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.

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

              The  subnet  mask of the virtual ethernet segment that is speci‐
              fied 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.

              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.

              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.

              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.

              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

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

              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

              The  --remote parameter.  Set on program initiation and reset on

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

              The pre-existing default IP gateway in the system routing table.
              Set prior to --up script execution.

              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.

              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  "met‐

              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.

              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  con‐
              tained  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.

              Temporary file name containing the client certificate upon  con‐
              nection.  Useful in conjunction with --tls-verify

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

              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 (con‐
              trolled  by  --ping-restart option), connection-reset (triggered
              on TCP connection reset), error, or  unknown  (unknown  signal).
              This variable is set just prior to down script execution.

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

              The duration (in seconds) of the client  session  which  is  now
              disconnecting.   Set  prior to execution of the --client-discon‐
              nect script.

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

              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.

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

              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 end‐
              points (udp6, tcp6), trusted_ip6 will be set instead.

              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 connecting
              host in a --tls-verify script to ensure it is  firewalled  prop‐
              erly.   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.

              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.

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

              An  X509 subject field from the remote peer certificate, where n
              is the verification level.  Only set for TLS  connections.   Set
              prior  to  execution  of  --tls-verify script.  This variable is
              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.


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

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

       Here is an example of an inline file usage

           -----BEGIN CERTIFICATE-----
           -----END CERTIFICATE-----

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

       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.

              Like SIGHUP, except don't re-read configuration file, and possi‐
              bly 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  --persist-tun,
              --persist-key,   --persist-local-ip,   and   --persist-remote-ip
              options respectively (see above).

              This signal may also be internally generated by a timeout condi‐
              tion, 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.

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

              Causes OpenVPN to exit gracefully.

       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

       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

   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://open‐ for more information.

   Firewall Setup:
       If firewalls exist between the two machines, they should be set to for‐
       ward 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 and  If you are constructing a VPN over the internet, then replace and 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  will  be and for,

       Once  the  VPN  is  established,  you have essentially created a secure
       alternate path between the two hosts which is addressed  by  using  the
       tunnel endpoints.  You can control which network traffic passes between
       the hosts (a) over the VPN or (b) independently of the VPN, by choosing
       whether  to use (a) the VPN endpoint address or (b) the public internet
       address, to access the remote host. For example if you  are  on
       and you wish to connect to via ssh without using the VPN (since
       ssh has its own built-in  security)  you  would  use  the  command  ssh   However in the same scenario, you could also use the command
       telnet to create a telnet session with 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 net‐
       works 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 --dev tun1 --ifconfig
              --verb 9

       On june:

              openvpn  --remote --dev tun1 --ifconfig
              --verb 9

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

       On may:


       On june:


       The --verb 9 option will produce verbose output, similar  to  the  tcp‐
       dump(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
       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)

       On may:

              openvpn --remote --dev tun1 --ifconfig
              --verb 5 --secret key

       On june:

              openvpn  --remote --dev tun1 --ifconfig
              --verb 5 --secret key

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

       On may:


       On june:


   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  certifi‐
       cates/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 --dev tun1 --ifconfig
              --tls-client --ca  ca.crt  --cert  client.crt  --key  client.key
              --reneg-sec 60 --verb 5

       On june:

              openvpn  --remote --dev tun1 --ifconfig
              --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:


       On june:


       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  Open‐
       VPN's default key renegotiation interval of one hour.

       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 and
       june's is

       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 netmask gw

       On june:

              route add -net netmask gw

       Now any machine on the subnet can access any machine on the 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.

       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  Open‐
       VPN packets.  On Linux 2.4+:

              iptables -A INPUT -p udp -s --dport 1194 -j ACCEPT

       This  will  allow  incoming packets on UDP port 1194 (OpenVPN's default
       UDP port) from an OpenVPN peer at

       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.


       For a more comprehensive guide to setting up OpenVPN  in  a  production
       setting, see the OpenVPN HOWTO at

       For  a  description  of OpenVPN's underlying protocol, see http://open‐

       OpenVPN's web site is at

       Go here to download the latest version of  OpenVPN,  subscribe  to  the
       mailing lists, read the mailing list archives, or browse the SVN repos‐

       Report all bugs to the OpenVPN team <>.

       dhcpcd(8), ifconfig(8), openssl(1), route(8), scp(1) ssh(1)

       This product includes software  developed  by  the  OpenSSL  Project  ( )

       For     more     information     on     the     TLS    protocol,    see

       For more information on  the  LZO  real-time  compression  library  see

       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 Soft‐
       ware Foundation.

       James Yonan <>

                               17 November 2008                     openvpn(8)