SYNOPSIS
openvpn [ options ... ]
INTRODUCTION
OpenVPN is an open source VPN daemon by James Yonan. Because OpenVPN tries to be a uni‐
versal VPN tool offering a great deal of flexibility, there are a lot of options on this
manual page. If you're new to OpenVPN, you might want to skip ahead to the examples sec‐
tion 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:
Homepage
And if you would like to see a shorter version of this manual, see the openvpn usage mes‐
sage which can be obtained by running openvpn without any parameters.
DESCRIPTION
OpenVPN is a robust and highly flexible VPN daemon. OpenVPN supports SSL/TLS security,
ethernet bridging, TCP or UDP tunnel transport through proxies or NAT, support for
dynamic IP addresses and DHCP, scalability to hundreds or thousands of users, and porta‐
bility to most major OS platforms.
OpenVPN is tightly bound to the OpenSSL library, and derives much of its crypto capabili‐
ties from it.
OpenVPN supports conventional encryption using a pre-shared secret key (Static Key mode)
or public key security (SSL/TLS mode) using client & server certificates. OpenVPN also
supports non-encrypted TCP/UDP tunnels.
OpenVPN is designed to work with the TUN/TAP virtual networking interface that exists on
most platforms.
Overall, OpenVPN aims to offer many of the key features of IPSec but with a relatively
lightweight footprint.
OPTIONS
OpenVPN allows any option to be placed either on the command line or in a configuration
file. Though all command line options are preceded by a double-leading-dash ("--"), this
prefix can be removed when an option is placed in a configuration file.
--help Show options.
--config file
Load additional config options from file where each line corresponds to one com‐
mand line option, but with the leading '--' removed.
If --config file is the only option to the openvpn command, the --config can be
removed, and the command can be given as openvpn file
Note that configuration files can be nested to a reasonable depth.
Double quotation or single quotation characters ("", '') can be used to enclose
single parameters containing whitespace, and "#" or ";" characters in the first
column can be used to denote comments.
Note that OpenVPN 2.0 and higher performs backslash-based shell escaping for char‐
acters not in single quotations, so the following mappings should be observed:
\\ Maps to a single backslash character (\).
\" Pass a literal doublequote character ("), don't
interpret it as enclosing a parameter.
\[SPACE] Pass a literal space or tab character, don't
interpret it as a parameter delimiter.
For example on Windows, use double backslashes to represent pathnames:
secret "c:\\OpenVPN\\secret.key"
For examples of configuration files, see https://openvpn.net/examples.html
Here is an example configuration file:
#
# Sample OpenVPN configuration file for
# using a pre-shared static key.
#
# '#' or ';' may be used to delimit comments.
# Use a dynamic tun device.
dev tun
# Our remote peer
remote mypeer.mydomain
# 10.1.0.1 is our local VPN endpoint
# 10.1.0.2 is our remote VPN endpoint
ifconfig 10.1.0.1 10.1.0.2
# Our pre-shared static key
secret static.key
Tunnel Options:
--mode m
Set OpenVPN major mode. By default, OpenVPN runs in point-to-point mode ("p2p").
OpenVPN 2.0 introduces a new mode ("server") which implements a multi-client
server capability.
--local host
Local host name or IP address for bind. If specified, OpenVPN will bind to this
address only. If unspecified, OpenVPN will bind to all interfaces.
--remote host [port] [proto]
Remote host name or IP address. On the client, multiple --remote options may be
specified for redundancy, each referring to a different OpenVPN server. Specify‐
ing multiple --remote options for this purpose is a special case of the more gen‐
eral connection-profile feature. See the <connection> documentation below.
The OpenVPN client will try to connect to a server at host:port in the order spec‐
ified 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 con‐
nected to one server.
Note that since UDP is connectionless, connection failure is defined by the --ping
and --ping-restart options.
Note the following corner case: If you use multiple --remote options, AND you are
dropping root privileges on the client with --user and/or --group, AND the client
is running a non-Windows OS, if the client needs to switch to a different server,
and that server pushes back different TUN/TAP or route settings, the client may
lack the necessary privileges to close and reopen the TUN/TAP interface. This
could cause the client to exit with a fatal error.
If --remote is unspecified, OpenVPN will listen for packets from any IP address,
but will not act on those packets unless they pass all authentication tests. This
requirement for authentication is binding on all potential peers, even those from
known and supposedly trusted IP addresses (it is very easy to forge a source IP
address on a UDP packet).
When used in TCP mode, --remote will act as a filter, rejecting connections from
any host which does not match host.
If host is a DNS name which resolves to multiple IP addresses, one will be ran‐
domly chosen, providing a sort of basic load-balancing and failover capability.
--remote-random-hostname
Add a random string (6 characters) to first DNS label of hostname to prevent DNS
caching. For example, "foo.bar.gov" would be modified to "<random-
chars>.foo.bar.gov".
<connection>
Define a client connection profile. Client connection profiles are groups of
OpenVPN options that describe how to connect to a given OpenVPN server. Client
connection profiles are specified within an OpenVPN configuration file, and each
profile is bracketed by <connection> and </connection>.
An OpenVPN client will try each connection profile sequentially until it achieves
a successful connection.
--remote-random can be used to initially "scramble" the connection list.
Here is an example of connection profile usage:
client
dev tun
<connection>
remote 198.19.34.56 1194 udp
</connection>
<connection>
remote 198.19.34.56 443 tcp
</connection>
<connection>
remote 198.19.34.56 443 tcp
http-proxy 192.168.0.8 8080
http-proxy-retry
</connection>
<connection>
remote 198.19.36.99 443 tcp
http-proxy 192.168.0.8 8080
http-proxy-retry
</connection>
persist-key
persist-tun
pkcs12 client.p12
ns-cert-type server
verb 3
First we try to connect to a server at 198.19.34.56:1194 using UDP. If that
fails, we then try to connect to 198.19.34.56:443 using TCP. If that also fails,
then try connecting through an HTTP proxy at 192.168.0.8:8080 to 198.19.34.56:443
using TCP. Finally, try to connect through the same proxy to a server at
198.19.36.99:443 using TCP.
The following OpenVPN options may be used inside of a <connection> block:
bind, connect-retry, connect-retry-max, connect-timeout, float, http-proxy, http-
proxy-option, http-proxy-retry, http-proxy-timeout, local, lport, nobind, port,
proto, remote, rport, socks-proxy, and socks-proxy-retry.
A defaulting mechanism exists for specifying options to apply to all <connection>
profiles. If any of the above options (with the exception of remote ) appear out‐
side of a <connection> block, but in a configuration file which has one or more
<connection> blocks, the option setting will be used as a default for <connection>
blocks which follow it in the configuration file.
For example, suppose the nobind option were placed in the sample configuration
file above, near the top of the file, before the first <connection> block. The
effect would be as if nobind were declared in all <connection> blocks below it.
--proto-force p
When iterating through connection profiles, only consider profiles using protocol
p ('tcp'|'udp').
--remote-random
When multiple --remote address/ports are specified, or if connection profiles are
being used, initially randomize the order of the list as a kind of basic load-bal‐
ancing measure.
--proto p
Use protocol p for communicating with remote host. p can be udp, tcp-client, or
tcp-server.
The default protocol is udp when --proto is not specified.
For UDP operation, --proto udp should be specified on both peers.
For TCP operation, one peer must use --proto tcp-server and the other must use
--proto tcp-client. A peer started with tcp-server will wait indefinitely for an
incoming connection. A peer started with tcp-client will attempt to connect, and
if that fails, will sleep for 5 seconds (adjustable via the --connect-retry
option) and try again infinite or up to N retries (adjustable via the --connect-
retry-max option). Both TCP client and server will simulate a SIGUSR1 restart
signal if either side resets the connection.
OpenVPN is designed to operate optimally over UDP, but TCP capability is provided
for situations where UDP cannot be used. In comparison with UDP, TCP will usually
be somewhat less efficient and less robust when used over unreliable or congested
networks.
This article outlines some of problems with tunneling IP over TCP:
http://sites.inka.de/sites/bigred/devel/tcp-tcp.html
There are certain cases, however, where using TCP may be advantageous from a secu‐
rity and robustness perspective, such as tunneling 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 (default=10).
--connect-retry-max n
For --proto tcp-client, take n as the number of retries of connection attempt
(default=infinite).
--auto-proxy
Try to sense HTTP or SOCKS proxy settings automatically. If no settings are
present, a direct connection will be attempted. If both HTTP and SOCKS settings
are present, HTTP will be preferred. If the HTTP proxy server requires a pass‐
word, it will be queried from stdin or the management interface. If the underly‐
ing OS doesn't support an API for returning proxy settings, a direct connection
will be attempted. Currently, only Windows clients support this option via the
InternetQueryOption API. This option exists in OpenVPN 2.1 or higher.
--show-proxy-settings
Show sensed HTTP or SOCKS proxy settings. Currently, only Windows clients support
this option.
--http-proxy server port [authfile|'auto'|'auto-nct'] [auth-method]
Connect to remote host through an HTTP proxy at address server and port port. If
HTTP Proxy-Authenticate is required, authfile is a file containing a username and
password on 2 lines, or "stdin" to prompt from console.
auth-method should be one of "none", "basic", or "ntlm".
HTTP Digest authentication is supported as well, but only via the auto or auto-nct
flags (below).
The auto flag causes OpenVPN to automatically determine the auth-method and query
stdin or the management interface for username/password credentials, if required.
This flag exists on OpenVPN 2.1 or higher.
The auto-nct flag (no clear-text auth) instructs OpenVPN to automatically deter‐
mine the authentication method, but to reject weak authentication protocols such
as HTTP Basic Authentication.
--http-proxy-retry
Retry indefinitely on HTTP proxy errors. If an HTTP proxy error occurs, simulate
a SIGUSR1 reset.
--http-proxy-timeout n
Set proxy timeout to n seconds, default=5.
--http-proxy-option type [parm]
Set extended HTTP proxy options. Repeat to set multiple options.
VERSION version -- Set HTTP version number to version (default=1.0).
AGENT user-agent -- Set HTTP "User-Agent" string to user-agent.
--socks-proxy server [port]
Connect to remote host through a Socks5 proxy at address server and port port
(default=1080).
--socks-proxy-retry
Retry indefinitely on Socks proxy errors. If a Socks proxy error occurs, simulate
a SIGUSR1 reset.
--resolv-retry n
If hostname resolve fails for --remote, retry resolve for n seconds before fail‐
ing.
Set n to "infinite" to retry indefinitely.
By default, --resolv-retry infinite is enabled. You can disable by setting n=0.
--float
Allow remote peer to change its IP address and/or port number, such as due to DHCP
(this is the default if --remote is not used). --float when specified with
--remote allows an OpenVPN session to initially connect to a peer at a known
address, however if packets arrive from a new address and pass all authentication
tests, the new address will take control of the session. This is useful when you
are connecting to a peer which holds a dynamic address such as a dial-in user or
DHCP client.
Essentially, --float tells OpenVPN to accept authenticated packets from any
address, not only the address which was specified in the --remote option.
--ipchange cmd
Execute shell command cmd when our remote ip-address is initially authenticated or
changes.
Execute as:
cmd ip_address port_number
Don't use --ipchange in --mode server mode. Use a --client-connect script
instead.
See the "Environmental Variables" section below for additional parameters passed
as environmental variables.
Note that cmd can be a shell command with multiple arguments, in which case all
OpenVPN-generated arguments will be appended to cmd to build a command line which
will be passed to the script.
If you are running in a dynamic IP address environment where the IP addresses of
either peer could change without notice, you can use this script, for example, to
edit the /etc/hosts file with the current address of the peer. The script will be
run every time the remote peer changes its IP address.
Similarly if our IP address changes due to DHCP, we should configure our IP
address change script (see man page for dhcpcd(8) ) to deliver a SIGHUP or SIGUSR1
signal to OpenVPN. OpenVPN will then reestablish a connection with its most
recently authenticated peer on its new IP address.
--port port
TCP/UDP port number for both local and remote. The current default of 1194 repre‐
sents the official IANA port number assignment for OpenVPN and has been used since
version 2.0-beta17. Previous versions used port 5000 as the default.
--lport port
TCP/UDP port number for bind.
--rport port
TCP/UDP port number for remote.
--bind Bind to local address and port. This is the default unless any of --proto tcp-
client , --http-proxy or --socks-proxy are used.
--nobind
Do not bind to local address and port. The IP stack will allocate a dynamic port
for returning packets. Since the value of the dynamic port could not be known in
advance by a peer, this option is only suitable for peers which will be initiating
connections by using the --remote option.
--dev tunX | tapX | null
TUN/TAP virtual network device ( X can be omitted for a dynamic device.)
See examples section below for an example on setting up a TUN device.
You must use either tun devices on both ends of the connection or tap devices on
both ends. You cannot mix them, as they represent different underlying network
layers.
tun devices encapsulate IPv4 or IPv6 (OSI Layer 3) while tap devices encapsulate
Ethernet 802.3 (OSI Layer 2).
--dev-type device-type
Which device type are we using? device-type should be tun (OSI Layer 3) or tap
(OSI Layer 2). Use this option only if the TUN/TAP device used with --dev does
not begin with tun or tap.
--topology mode
Configure virtual addressing topology when running in --dev tun mode. This direc‐
tive 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 direc‐
tives 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 con‐
necting clients might be Windows systems. This is the default on OpenVPN 2.0.
p2p -- Use a point-to-point topology where the remote endpoint of the client's tun
interface always points to the local endpoint of the server's tun interface. This
mode allocates a single IP address per connecting client. Only use when none of
the connecting clients are Windows systems. This mode is functionally equivalent
to the --ifconfig-pool-linear directive which is available in OpenVPN 2.0 and is
now deprecated.
subnet -- Use a subnet rather than a point-to-point topology by configuring the
tun interface with a local IP address and subnet mask, similar to the topology
used in --dev tap and ethernet bridging mode. This mode allocates a single IP
address per connecting client and works on Windows as well. Only available when
server and clients are OpenVPN 2.1 or higher, or OpenVPN 2.0.x which has been man‐
ually patched with the --topology directive code. When used on Windows, requires
version 8.2 or higher of the TAP-Win32 driver. When used on *nix, requires that
the tun driver supports an ifconfig(8) command which sets a subnet instead of a
remote endpoint IP address.
This option exists in OpenVPN 2.1 or higher.
--tun-ipv6
Build a tun link capable of forwarding IPv6 traffic. Should be used in conjunc‐
tion 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.
--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 Net‐
work 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 avail‐
able TAP-Win32 adapters and will show both the network connections control panel
name and the GUID for each TAP-Win32 adapter.
--lladdr address
Specify the link layer address, more commonly known as the MAC address. Only
applied to TAP devices.
--iproute cmd
Set alternate command to execute instead of default iproute2 command. May be used
in order to execute OpenVPN in unprivileged environment.
--ifconfig l rn
Set TUN/TAP adapter parameters. l is the IP address of the local VPN endpoint.
For TUN devices, rn is the IP address of the remote VPN endpoint. For TAP
devices, rn is the subnet mask of the virtual ethernet segment which is being cre‐
ated or connected to.
For TUN devices, which facilitate virtual point-to-point IP connections, the
proper usage of --ifconfig is to use two private IP addresses which are not a mem‐
ber 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 estab‐
lished, 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 ether‐
net adapter would be similarly configured. If you are attempting to connect to a
remote ethernet bridge, the IP address and subnet should be set to values which
would be valid on the the bridged ethernet segment (note also that DHCP can be
used for the same purpose).
This option, while primarily a proxy for the ifconfig(8) command, is designed to
simplify TUN/TAP tunnel configuration by providing a standard interface to the
different ifconfig implementations on different platforms.
--ifconfig parameters which are IP addresses can also be specified as a DNS or
/etc/hosts file resolvable name.
For TAP devices, --ifconfig should not be used if the TAP interface will be get‐
ting an IP address lease from a DHCP server.
--ifconfig-noexec
Don't actually execute ifconfig/netsh commands, instead pass --ifconfig parameters
to scripts using environmental variables.
--ifconfig-nowarn
Don't output an options consistency check warning if the --ifconfig option on this
side of the connection doesn't match the remote side. This is useful when you
want to retain the overall benefits of the options consistency check (also see
--disable-occ option) while only disabling the ifconfig component of the check.
For example, if you have a configuration where the local host uses --ifconfig but
the remote host does not, use --ifconfig-nowarn on the local host.
This option will also silence warnings about potential address conflicts which
occasionally annoy more experienced users by triggering "false positive" warnings.
--route network/IP [netmask] [gateway] [metric]
Add route to routing table after connection is established. Multiple routes can
be specified. Routes will be automatically torn down in reverse order prior to
TUN/TAP device close.
This option is intended as a convenience proxy for the route(8) shell command,
while at the same time providing portable semantics across OpenVPN's platform
space.
netmask default -- 255.255.255.255
gateway default -- taken from --route-gateway or the second parameter to --ifcon‐
fig when --dev tun is specified.
metric default -- taken from --route-metric otherwise 0.
The default can be specified by leaving an option blank or setting it to
"default".
The network and gateway parameters can also be specified as a DNS or /etc/hosts
file resolvable name, or as one of three special keywords:
vpn_gateway -- The remote VPN endpoint address (derived either from --route-gate‐
way or the second parameter to --ifconfig when --dev tun is specified).
net_gateway -- The pre-existing IP default gateway, read from the routing table
(not supported on all OSes).
remote_host -- The --remote address if OpenVPN is being run in client mode, and is
undefined in server mode.
--max-routes n
Allow a maximum number of n --route options to be specified, either in the local
configuration file, or pulled from an OpenVPN server. By default, n=100.
--route-gateway gw|'dhcp'
Specify a default gateway gw for use with --route.
If dhcp is specified as the parameter, the gateway address will be extracted from
a DHCP 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 immediately upon connection establishment. If
--route-delay is omitted, routes will be added immediately after TUN/TAP device
open and --up script execution, before any --user or --group privilege downgrade
(or --chroot execution.)
This option is designed to be useful in scenarios where DHCP is used to set tap
adapter addresses. The delay will give the DHCP handshake time to complete before
routes are added.
On Windows, --route-delay tries to be more intelligent by waiting w seconds (w=30
by default) for the TAP-Win32 adapter to come up before adding routes.
--route-up cmd
Execute shell command cmd after routes are added, subject to --route-delay.
See the "Environmental Variables" section below for additional parameters passed
as environmental variables.
Note that cmd can be a shell command with multiple arguments.
--route-noexec
Don't add or remove routes automatically. Instead pass routes to --route-up
script using environmental variables.
--route-nopull
When used with --client or --pull, accept options pushed by server EXCEPT for
routes.
When used on the client, this option effectively bars the server from adding
routes to the client's routing table, however note that this option still allows
the server to set the TCP/IP properties of the client's TUN/TAP interface.
--allow-pull-fqdn
Allow client to pull DNS names from server (rather than being limited to IP
address) for --ifconfig, --route, and --route-gateway.
--redirect-gateway flags...
(Experimental) Automatically execute routing commands to cause all outgoing IP
traffic to be redirected over the VPN.
This option performs three steps:
(1) Create a static route for the --remote address which forwards to the pre-
existing default gateway. This is done so that (3) will not create a routing
loop.
(2) Delete the default gateway route.
(3) Set the new default gateway to be the VPN endpoint address (derived either
from --route-gateway or the second parameter to --ifconfig when --dev tun is spec‐
ified).
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.
def1 -- Use this flag to override the default gateway by using 0.0.0.0/1 and
128.0.0.0/1 rather than 0.0.0.0/0. This has the benefit of overriding but not
wiping out the original default gateway.
bypass-dhcp -- Add a direct route to the DHCP server (if it is non-local) which
bypasses the tunnel (Available on 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).
Using the def1 flag is highly recommended.
--redirect-private [flags]
Like --redirect-gateway, but omit actually changing the default gateway. Useful
when pushing private subnets.
--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.
--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 transmis‐
sion overhead associated with using a larger value.
--mtu-disc type
Should we do Path MTU discovery on TCP/UDP channel? Only supported on OSes such
as Linux that supports the necessary system call to set.
'no' -- Never send DF (Don't Fragment) frames
'maybe' -- Use per-route hints
'yes' -- Always DF (Don't Fragment)
--mtu-test
To empirically measure MTU on connection startup, add the --mtu-test option to
your configuration. OpenVPN will send ping packets of various sizes to the remote
peer and measure the largest packets which were successfully received. The --mtu-
test process normally takes about 3 minutes to complete.
--fragment max
Enable internal datagram fragmentation so that no UDP datagrams are sent which are
larger than max bytes.
The max parameter is interpreted in the same way as the --link-mtu parameter, i.e.
the UDP packet size after encapsulation overhead has been added in, but not
including the UDP header itself.
The --fragment option only makes sense when you are using the UDP protocol (
--proto udp ).
--fragment adds 4 bytes of overhead per datagram.
See the --mssfix option below for an important related option to --fragment.
It should also be noted that this option is not meant to replace UDP fragmentation
at the IP stack level. It is only meant as a last resort when path MTU discovery
is broken. Using this option is less efficient than fixing path MTU discovery for
your IP link and using native IP fragmentation instead.
Having said that, there are circumstances where using OpenVPN's internal fragmen‐
tation capability may be your only option, such as tunneling a UDP multicast
stream which requires fragmentation.
--mssfix max
Announce to TCP sessions running over the tunnel that they should limit their send
packet sizes such that after OpenVPN has encapsulated them, the resulting UDP
packet size that OpenVPN sends to its peer will not exceed max bytes. The default
value is 1450.
The max parameter is interpreted in the same way as the --link-mtu parameter, i.e.
the UDP packet size after encapsulation overhead has been added in, but not
including the UDP header itself.
The --mssfix option only makes sense when you are using the UDP protocol for Open‐
VPN peer-to-peer communication, i.e. --proto udp.
--mssfix and --fragment can be ideally used together, where --mssfix will try to
keep TCP from needing packet fragmentation in the first place, and if big packets
come through anyhow (from protocols other than TCP), --fragment will internally
fragment them.
Both --fragment and --mssfix are designed to work around cases where Path MTU dis‐
covery 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.
--socket-flags flags...
Apply the given flags to the OpenVPN transport socket. Currently, only TCP_NODE‐
LAY is supported.
The TCP_NODELAY socket flag is useful in TCP mode, and causes the kernel to send
tunnel packets immediately over the TCP connection without trying to group several
smaller packets into a larger packet. This can result in a considerably improve‐
ment in latency.
This option is pushable from server to client, and should be used on both client
and server for maximum effect.
--txqueuelen n
(Linux only) Set the TX queue length on the TUN/TAP interface. Currently defaults
to 100.
--shaper n
Limit bandwidth of outgoing tunnel data to n bytes per second on the TCP/UDP port.
If you want to limit the bandwidth in both directions, use this option on both
peers.
OpenVPN uses the following algorithm to implement traffic shaping: Given a shaper
rate of n bytes per second, after a datagram write of b bytes is queued on the
TCP/UDP port, wait a minimum of (b / n) seconds before queuing the next write.
It should be noted that OpenVPN supports multiple tunnels between the same two
peers, allowing you to construct full-speed and reduced bandwidth tunnels at the
same time, routing low-priority data such as off-site backups over the reduced
bandwidth tunnel, and other data over the full-speed tunnel.
Also note that for low bandwidth tunnels (under 1000 bytes per second), you should
probably use lower MTU values as well (see above), otherwise the packet latency
will grow so large as to trigger timeouts in the TLS layer and TCP connections
running over the tunnel.
OpenVPN allows n to be between 100 bytes/sec and 100 Mbytes/sec.
--inactive n [bytes]
Causes OpenVPN to exit after n seconds of inactivity on the TUN/TAP device. The
time length of inactivity is measured since the last incoming or outgoing tunnel
packet. The default value is 0 seconds, which disables this feature.
If the optional bytes parameter is included, exit if less than bytes of combined
in/out traffic are produced on the tun/tap device in n seconds.
In any case, OpenVPN's internal ping packets (which are just keepalives) and TLS
control packets are not considered "activity", nor are they counted as traffic, as
they are used internally by OpenVPN and are not an indication of actual user
activity.
--ping n
Ping remote over the TCP/UDP control channel if no packets have been sent for at
least n seconds (specify --ping on both peers to cause ping packets to be sent in
both directions since OpenVPN ping packets are not echoed like IP ping 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 remote.
This option is useful in cases where the remote peer has a dynamic IP address and
a low-TTL DNS name is used to track the IP address using a service such as
http://dyndns.org/ + a dynamic DNS client such as ddclient.
If the peer cannot be reached, a restart will be triggered, causing the hostname
used with --remote to be re-resolved (if --resolv-retry is also specified).
In server mode, --ping-restart, --inactive, or any other type of internally gener‐
ated signal will always be applied to individual client instance objects, never to
whole server itself. Note also in server mode that any internally generated sig‐
nal which would normally cause a restart, will cause the deletion of the client
instance object instead.
In client mode, the --ping-restart parameter is set to 120 seconds by default.
This default will hold until the client pulls a replacement value from the server,
based on the --keepalive setting in the server configuration. To disable the 120
second default, set --ping-restart 0 on the client.
See the signals section below for more information on SIGUSR1.
Note that the behavior of SIGUSR1 can be modified by the --persist-tun, --persist-
key, --persist-local-ip, and --persist-remote-ip options.
Also note that --ping-exit and --ping-restart are mutually exclusive and cannot be
used together.
--keepalive n m
A helper directive designed to simplify the expression of --ping and --ping-
restart in server mode configurations.
For example, --keepalive 10 60 expands as follows:
if mode server:
ping 10
ping-restart 120
push "ping 10"
push "ping-restart 60"
else
ping 10
ping-restart 60
--ping-timer-rem
Run the --ping-exit / --ping-restart timer only if we have a remote address. Use
this option if you are starting the daemon in listen mode (i.e. without an
explicit --remote peer), and you don't want to start clocking timeouts until a
remote peer connects.
--persist-tun
Don't close and reopen TUN/TAP device or run up/down scripts across SIGUSR1 or
--ping-restart restarts.
SIGUSR1 is a restart signal similar to SIGHUP, but which offers finer-grained con‐
trol over reset options.
--persist-key
Don't re-read key files across SIGUSR1 or --ping-restart.
This option can be combined with --user nobody to allow restarts triggered by the
SIGUSR1 signal. Normally if you drop root privileges in OpenVPN, the daemon can‐
not be restarted since it will now be unable to re-read protected key files.
This option solves the problem by persisting keys across SIGUSR1 resets, so they
don't need to be re-read.
--persist-local-ip
Preserve initially resolved local IP address and port number across SIGUSR1 or
--ping-restart restarts.
--persist-remote-ip
Preserve most recently authenticated remote IP address and port number across
SIGUSR1 or --ping-restart restarts.
--mlock
Disable paging by calling the POSIX mlockall function. Requires that OpenVPN be
initially run as root (though OpenVPN can subsequently downgrade its UID using the
--user option).
Using this option ensures that key material and tunnel data are never written to
disk due to virtual memory paging operations which occur under most modern operat‐
ing systems. It ensures that even if an attacker was able to crack the box run‐
ning OpenVPN, he would not be able to scan the system swap file to recover previ‐
ously 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
Shell command to run after successful TUN/TAP device open (pre --user UID change).
The up script is useful for specifying route commands which route IP traffic des‐
tined for private subnets which exist at the other end of the VPN connection into
the tunnel.
For --dev tun execute as:
cmd tun_dev tun_mtu link_mtu ifconfig_local_ip ifconfig_remote_ip [ init | restart
]
For --dev tap execute as:
cmd tap_dev tap_mtu link_mtu ifconfig_local_ip ifconfig_netmask [ init | restart ]
See the "Environmental Variables" section below for additional parameters passed
as environmental variables.
Note that cmd can be a shell command with multiple arguments, in which case all
OpenVPN-generated arguments will be appended to cmd to build a command line which
will be passed to the shell.
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 con‐
text, the last command line parameter passed to the script will be init. If the
--up-restart option is also used, the up script will be called for restarts as
well. A restart is considered to be a partial reinitialization of OpenVPN where
the TUN/TAP instance is preserved (the --persist-tun option will enable such
preservation). A restart can be generated by a SIGUSR1 signal, a --ping-restart
timeout, or a connection reset when the TCP protocol is enabled with the --proto
option. If a restart occurs, and --up-restart has been specified, the up script
will be called with restart as the last parameter.
The following standalone example shows how the --up script can be called in both
an initialization and restart context. (NOTE: for security reasons, don't run the
following example unless UDP port 9999 is blocked by your firewall. Also, the
example will run indefinitely, so you should abort with control-c).
openvpn --dev tun --port 9999 --verb 4 --ping-restart 10 --up 'echo up' --down
'echo down' --persist-tun --up-restart
Note that OpenVPN also provides the --ifconfig option to automatically ifconfig
the TUN device, eliminating the need to define an --up script, unless you also
want to configure routes in the --up script.
If --ifconfig is also specified, OpenVPN will pass the ifconfig local and remote
endpoints on the command line to the --up script so that they can be used to con‐
figure routes such as:
route add -net 10.0.0.0 netmask 255.255.255.0 gw $5
--up-delay
Delay TUN/TAP open and possible --up script execution until after TCP/UDP connec‐
tion establishment with peer.
In --proto udp mode, this option normally requires the use of --ping to allow con‐
nection initiation to be sensed in the absence of tunnel data, since UDP is a
"connectionless" protocol.
On Windows, this option will delay the TAP-Win32 media state transitioning to
"connected" until connection establishment, i.e. the receipt of the first authen‐
ticated packet from the peer.
--down cmd
Shell command to run after TUN/TAP device close (post --user UID change and/or
--chroot ). Called with the same parameters and environmental variables as the
--up option above.
Note that if you reduce privileges by using --user and/or --group, your --down
script will also run at reduced privilege.
--down-pre
Call --down cmd/script before, rather than after, TUN/TAP close.
--up-restart
Enable the --up and --down scripts to be called for restarts as well as initial
program start. This option is described more fully above in the --up option docu‐
mentation.
--setenv name value
Set a custom environmental variable name=value to pass to script.
--setenv FORWARD_COMPATIBLE 1
Relax config file syntax checking so that unknown directives will trigger a warn‐
ing but not a fatal error, on the assumption that a given unknown directive might
be valid in future OpenVPN versions.
This option should be used with caution, as there are good security reasons for
having OpenVPN fail if it detects problems in a config file. Having said that,
there are valid reasons for wanting new software features to gracefully degrade
when encountered by older software versions.
--setenv-safe name value
Set a custom environmental variable OPENVPN_name=value to pass to script.
This directive is designed to be pushed by the server to clients, and the prepend‐
ing of "OPENVPN_" to the environmental variable is a safety precaution to prevent
a LD_PRELOAD style attack from a malicious or compromised server.
--script-security level [method]
This directive offers policy-level control over OpenVPN's usage of external pro‐
grams and scripts. Lower level values are more restrictive, higher values are
more permissive. Settings for level:
0 -- Strictly no calling of external programs.
1 -- (Default) Only call built-in executables such as ifconfig, ip, route, or
netsh.
2 -- Allow calling of built-in executables and user-defined scripts.
3 -- Allow passwords to be passed to scripts via environmental variables (poten‐
tially unsafe).
The method parameter indicates how OpenVPN should call external commands and
scripts. Settings for method:
execve -- (default) Use execve() function on Unix family OSes and CreateProcess()
on Windows.
system -- Use system() function (deprecated and less safe since the external pro‐
gram command line is subject to shell expansion).
The --script-security option was introduced in OpenVPN 2.1_rc9. For configuration
file compatibility with previous OpenVPN versions, use: --script-security 3 system
--disable-occ
Don't output a warning message if option inconsistencies are detected between
peers. An example of an option inconsistency would be where one peer uses --dev
tun while the other peer uses --dev tap.
Use of this option is discouraged, but is provided as a temporary fix in situa‐
tions where a recent version of OpenVPN must connect to an old version.
--user user
Change the user ID of the OpenVPN process to user after initialization, dropping
privileges in the process. This option is useful to protect the system in the
event that some hostile party was able to gain control of an OpenVPN session.
Though OpenVPN's security features make this unlikely, it is provided as a second
line of defense.
By setting user to nobody or somebody similarly unprivileged, the hostile party
would be limited in what damage they could cause. Of course once you take away
privileges, you cannot return them to an OpenVPN session. This means, for exam‐
ple, that if you want to reset an OpenVPN daemon with a SIGUSR1 signal (for exam‐
ple in response to a DHCP reset), you should make use of one or more of the --per‐
sist options to ensure that OpenVPN doesn't need to execute any privileged opera‐
tions in order to restart (such as re-reading key files or running ifconfig on the
TUN device).
--group group
Similar to the --user option, this option changes the group ID of the OpenVPN
process to group after initialization.
--cd dir
Change directory to dir prior to reading any files such as configuration files,
key files, scripts, etc. dir should be an absolute path, with a leading "/", and
without any references to the current directory such as "." or "..".
This option is useful when you are running OpenVPN in --daemon mode, and you want
to consolidate all of your OpenVPN control files in one location.
--chroot dir
Chroot to dir after initialization. --chroot essentially redefines dir as being
the top level directory tree (/). OpenVPN will therefore be unable to access any
files outside this tree. This can be desirable from a security standpoint.
Since the chroot operation is delayed until after initialization, most OpenVPN
options that reference files will operate in a pre-chroot context.
In many cases, the dir parameter can point to an empty directory, however compli‐
cations can result when scripts or restarts are executed after the chroot opera‐
tion.
--setcon context
Apply SELinux context after initialization. This essentially provides the ability
to restrict OpenVPN's rights to only network I/O operations, thanks to SELinux.
This goes further than --user and --chroot in that those two, while being great
security features, unfortunately do not protect against privilege escalation by
exploitation of a vulnerable system call. You can of course combine all three, but
please note that since setcon requires access to /proc you will have to provide it
inside the chroot directory (e.g. with mount --bind).
Since the setcon operation is delayed until after initialization, OpenVPN can be
restricted to just network-related system calls, whereas by applying the context
before startup (such as the OpenVPN one provided in the SELinux Reference Poli‐
cies) you will have to allow many things required only during initialization.
Like with chroot, complications can result when scripts or restarts are executed
after the setcon operation, which is why you should really consider using the
--persist-key and --persist-tun options.
--daemon [progname]
Become a daemon after all initialization functions are completed. This option
will cause all message and error output to be sent to the syslog file (such as
/var/log/messages), except for the output of shell scripts and ifconfig commands,
which will go to /dev/null unless otherwise redirected. The syslog redirection
occurs immediately at the point that --daemon is parsed on the command line even
though the daemonization point occurs later. If one of the --log options is
present, it will supercede syslog redirection.
The optional progname parameter will cause OpenVPN to report its program name to
the system logger as progname. This can be useful in linking OpenVPN messages in
the syslog file with specific tunnels. When unspecified, progname defaults to
"openvpn".
When OpenVPN is run with the --daemon option, it will try to delay daemonization
until the majority of initialization functions which are capable of generating
fatal errors are complete. This means that initialization scripts can test the
return status of the openvpn command for a fairly reliable indication of whether
the command has correctly initialized and entered the packet forwarding event
loop.
In OpenVPN, the vast majority of errors which occur after initialization are non-
fatal.
--syslog [progname]
Direct log output to system logger, but do not become a daemon. See --daemon
directive above for description of progname parameter.
--passtos
Set the TOS field of the tunnel packet to what the payload's TOS is.
--inetd [wait|nowait] [progname]
Use this option when OpenVPN is being run from the inetd or xinetd(8) server.
The wait/nowait option must match what is specified in the inetd/xinetd config
file. The nowait mode can only be used with --proto tcp-server. The default is
wait. The nowait mode can be used to instantiate the OpenVPN daemon as a classic
TCP server, where client connection requests are serviced on a single port number.
For additional information on this kind of configuration, see the OpenVPN FAQ:
https://openvpn.net/faq.html#oneport
This option precludes the use of --daemon, --local, or --remote. Note that this
option causes message and error output to be handled in the same way as the --dae‐
mon option. The optional progname parameter is also handled exactly as in --dae‐
mon.
Also note that in wait mode, each OpenVPN tunnel requires a separate TCP/UDP port
and a separate inetd or xinetd entry. See the OpenVPN 1.x HOWTO for an example on
using OpenVPN with xinetd: https://openvpn.net/1xhowto.html
--log file
Output logging messages to file, including output to stdout/stderr which is gener‐
ated by called scripts. If file already exists it will be truncated. This option
takes effect immediately when it is parsed in the command line and will supercede
syslog output if --daemon or --inetd is also specified. This option is persistent
over the entire course of an OpenVPN instantiation and will not be reset by
SIGHUP, SIGUSR1, or --ping-restart.
Note that on Windows, when OpenVPN is started as a service, logging occurs by
default without the need to specify this option.
--log-append file
Append logging messages to file. If file does not exist, it will be created.
This option behaves exactly like --log except that it appends to rather than trun‐
cating the log file.
--suppress-timestamps
Avoid writing timestamps to log messages, even when they otherwise would be
prepended. In particular, this applies to log messages sent to stdout.
--writepid file
Write OpenVPN's main process ID to file.
--nice n
Change process priority after initialization ( n greater than 0 is lower priority,
n less than zero is higher priority).
--fast-io
(Experimental) Optimize TUN/TAP/UDP I/O writes by avoiding a call to
poll/epoll/select prior to the write operation. The purpose of such a call would
normally be to block until the device or socket is ready to accept the write.
Such blocking is unnecessary on some platforms which don't support write blocking
on UDP sockets or TUN/TAP devices. In such cases, one can optimize the event loop
by avoiding the poll/epoll/select call, improving CPU efficiency by 5% to 10%.
This option can only be used on non-Windows systems, when --proto udp is speci‐
fied, and when --shaper is NOT specified.
--multihome
Configure a multi-homed UDP server. This option can be used when OpenVPN has been
configured to listen on all interfaces, and will attempt to bind client sessions
to the interface on which packets are being received, so that outgoing packets
will be sent out of the same interface. Note that this option is only relevant
for UDP servers and currently is only implemented on Linux.
Note: clients connecting to a --multihome server should always use the --nobind
option.
--echo [parms...]
Echo parms to log output.
Designed to be used to send messages to a controlling application which is receiv‐
ing the OpenVPN log output.
--remap-usr1 signal
Control whether internally or externally generated SIGUSR1 signals are remapped to
SIGHUP (restart without persisting state) or SIGTERM (exit).
signal can be set to "SIGHUP" or "SIGTERM". By default, no remapping occurs.
--verb n
Set output verbosity to n (default=1). Each level shows all info from the previ‐
ous levels. Level 3 is recommended if you want a good summary of what's happening
without being swamped by output.
0 -- No output except fatal errors.
1 to 4 -- Normal usage range.
5 -- Output R and W characters to the console for each packet read and write,
uppercase is used for TCP/UDP packets and lowercase is used for TUN/TAP packets.
6 to 11 -- Debug info range (see errlevel.h for additional information on debug
levels).
--status file [n]
Write operational status to file every n seconds.
Status can also be written to the syslog by sending a SIGUSR2 signal.
--status-version [n]
Choose the status file format version number. Currently n can be 1, 2, or 3 and
defaults to 1.
--mute n
Log at most n consecutive messages in the same category. This is useful to limit
repetitive logging of similar message types.
--comp-lzo [mode]
Use fast LZO compression -- may add up to 1 byte per packet for incompressible
data. mode may be "yes", "no", or "adaptive" (default).
In a server mode setup, it is possible to selectively turn compression on or off
for individual clients.
First, make sure the client-side config file enables selective compression by hav‐
ing at least one --comp-lzo directive, such as --comp-lzo no. This will turn off
compression by default, but allow a future directive push from the server to
dynamically change the on/off/adaptive setting.
Next in a --client-config-dir file, specify the compression setting for the
client, for example:
comp-lzo yes
push "comp-lzo yes"
The first line sets the comp-lzo setting for the server side of the link, the sec‐
ond sets the client side.
--comp-noadapt
When used in conjunction with --comp-lzo, this option will disable OpenVPN's adap‐
tive compression algorithm. Normally, adaptive compression is enabled with
--comp-lzo.
Adaptive compression tries to optimize the case where you have compression
enabled, but you are sending predominantly uncompressible (or pre-compressed)
packets over the tunnel, such as an FTP or rsync transfer of a large, compressed
file. With adaptive compression, OpenVPN will periodically sample the compression
process to measure its efficiency. If the data being sent over the tunnel is
already compressed, the compression efficiency will be very low, triggering open‐
vpn to disable compression for a period of time until the next re-sample test.
--management IP port [pw-file]
Enable a TCP server on IP:port to handle daemon management functions. pw-file, if
specified, is a password file (password on first line) or "stdin" to prompt from
standard input. The password provided will set the password which TCP clients
will need to provide in order to access management functions.
The management interface can also listen on a unix domain socket, for those plat‐
forms that support it. To use a unix domain socket, specify the unix socket path‐
name in place of IP and set port to 'unix'. While the default behavior is to cre‐
ate a unix domain socket that may be connected to by any process, the --manage‐
ment-client-user and --management-client-group directives can be used to restrict
access.
The management interface provides a special mode where the TCP management link can
operate over the tunnel itself. To enable this mode, set IP = "tunnel". Tunnel
mode will cause the management interface to listen for a TCP connection on the
local VPN address of the TUN/TAP interface.
While the management port is designed for programmatic control of OpenVPN by other
applications, it is possible to telnet to the port, using a telnet client in "raw"
mode. Once connected, type "help" for a list of commands.
For detailed documentation on the management interface, see the management-
notes.txt file in the management folder of the OpenVPN source distribution.
It is strongly recommended that IP be set to 127.0.0.1 (localhost) to restrict
accessibility of the management server to local clients.
--management-client
Management interface will connect as a TCP client to IP:port specified by --man‐
agement rather than listen as a TCP server.
--management-query-passwords
Query management channel for private key password and --auth-user-pass user‐
name/password. Only query the management channel for inputs which ordinarily
would have been queried from the console.
--management-forget-disconnect
Make OpenVPN forget passwords when management session disconnects.
This directive does not affect the --http-proxy username/password. It is always
cached.
--management-hold
Start OpenVPN in a hibernating state, until a client of the management interface
explicitly starts it with the hold release command.
--management-signal
Send SIGUSR1 signal to OpenVPN if management session disconnects. This is useful
when you wish to disconnect an OpenVPN session on user logoff.
--management-log-cache n
Cache the most recent n lines of log file history for usage by the management
channel.
--management-client-auth
Gives management interface client the responsibility to authenticate clients after
their client certificate has been verified. See management-notes.txt in OpenVPN
distribution for detailed notes.
--management-client-pf
Management interface clients must specify a packet filter file for each connecting
client. See management-notes.txt in OpenVPN distribution for detailed notes.
--management-client-user u
When the management interface is listening on a unix domain socket, only allow
connections from user u.
--management-client-group g
When the management interface is listening on a unix domain socket, only allow
connections from group g.
--plugin module-pathname [init-string]
Load plug-in module from the file module-pathname, passing init-string as an argu‐
ment 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/openvpn/plugin. The
documentation is in doc and the actual plugin modules are in lib.
Multiple plugin modules can be cascaded, and modules can be used in tandem with
scripts. The modules will be called by OpenVPN in the order that they are
declared in the config file. If both a plugin and script are configured for the
same callback, the script will be called last. If the return code of the mod‐
ule/script controls an authentication function (such as tls-verify, auth-user-
pass-verify, or client-connect), then every module and script must return success
(0) in order for the connection to be authenticated.
Server Mode
Starting with OpenVPN 2.0, a multi-client TCP/UDP server mode is supported, and can be
enabled with the --mode server option. In server mode, OpenVPN will listen on a single
port for incoming client connections. All client connections will be routed through a
single tun or tap interface. This mode is designed for scalability and should be able to
support hundreds or even thousands of clients on sufficiently fast hardware. SSL/TLS
authentication must be used in this mode.
--server network netmask
A helper directive designed to simplify the configuration of OpenVPN's server
mode. This directive will set up an 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 10.8.0.0 255.255.255.0 expands as follows:
mode server
tls-server
push "topology [topology]"
if dev tun AND (topology == net30 OR topology == p2p):
ifconfig 10.8.0.1 10.8.0.2
if !nopool:
ifconfig-pool 10.8.0.4 10.8.0.251
route 10.8.0.0 255.255.255.0
if client-to-client:
push "route 10.8.0.0 255.255.255.0"
else if topology == net30:
push "route 10.8.0.1"
if dev tap OR (dev tun AND topology == subnet):
ifconfig 10.8.0.1 255.255.255.0
if !nopool:
ifconfig-pool 10.8.0.2 10.8.0.254 255.255.255.0
push "route-gateway 10.8.0.1"
Don't use --server if you are ethernet bridging. Use --server-bridge instead.
--server-bridge gateway netmask pool-start-IP pool-end-IP
--server-bridge ['nogw']
A helper directive similar to --server which is designed to simplify the configu‐
ration 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 ethernet NIC interface. For example, on
Linux this is done with the brctl tool, and with Windows XP it is done in the Net‐
work Connections Panel by selecting the ethernet and TAP adapters and right-click‐
ing on "Bridge Connections".
Next you you must manually set the IP/netmask on the bridge interface. The gate‐
way and netmask parameters to --server-bridge can be set to either the IP/netmask
of the bridge interface, or the IP/netmask of the default gateway/router on the
bridged subnet.
Finally, set aside a IP range in the bridged subnet, denoted by pool-start-IP and
pool-end-IP, for OpenVPN to allocate to connecting clients.
For example, server-bridge 10.8.0.4 255.255.255.0 10.8.0.128 10.8.0.254 expands as
follows:
mode server
tls-server
ifconfig-pool 10.8.0.128 10.8.0.254 255.255.255.0
push "route-gateway 10.8.0.4"
In another example, --server-bridge (without parameters) expands as follows:
mode server
tls-server
push "route-gateway dhcp"
Or --server-bridge nogw expands as follows:
mode server
tls-server
--push option
Push a config file option back to the client for remote execution. Note that
option must be enclosed in double quotes (""). The client must specify --pull in
its config file. The set of options which can be pushed is limited by both feasi‐
bility and security. Some options such as those which would execute scripts are
banned, since they would effectively allow a compromised server to execute arbi‐
trary code on the client. Other options such as TLS or MTU parameters cannot be
pushed because the client needs to know them before the connection to the server
can be initiated.
This is a partial list of options which can currently be pushed: --route, --route-
gateway, --route-delay, --redirect-gateway, --ip-win32, --dhcp-option, --inactive,
--ping, --ping-exit, --ping-restart, --setenv, --persist-key, --persist-tun,
--echo, --comp-lzo, --socket-flags, --sndbuf, --rcvbuf
--push-reset
Don't inherit the global push list for a specific client instance. Specify this
option in a client-specific context such as with a --client-config-dir configura‐
tion file. This option will ignore --push options at the global config file
level.
--disable
Disable a particular client (based on the common name) from connecting. Don't use
this option to disable a client due to key or password compromise. Use a CRL
(certificate revocation list) instead (see the --crl-verify option).
This option must be associated with a specific client instance, which means that
it must be specified either in a client instance config file using --client-con‐
fig-dir or dynamically generated using a --client-connect script.
--ifconfig-pool start-IP end-IP [netmask]
Set aside a pool of subnets to be dynamically allocated to connecting clients,
similar to a DHCP server. For tun-style tunnels, each client will be given a /30
subnet (for interoperability with Windows clients). For tap-style tunnels, indi‐
vidual addresses will be allocated, and the optional netmask parameter will also
be pushed to clients.
--ifconfig-pool-persist file [seconds]
Persist/unpersist ifconfig-pool data to file, at seconds intervals (default=600),
as well as on program startup and shutdown.
The goal of this option is to provide a long-term association between clients
(denoted by their common name) and the virtual IP address assigned to them from
the ifconfig-pool. Maintaining a long-term association is good for clients
because it allows them to effectively use the --persist-tun option.
file is a comma-delimited ASCII file, formatted as <Common-Name>,<IP-address>.
If seconds = 0, file will be treated as read-only. This is useful if you would
like to treat file as a configuration file.
Note that the entries in this file are treated by OpenVPN as suggestions only,
based on past associations between a common name and IP address. They do not
guarantee that the given common name will always receive the given IP address. If
you want guaranteed assignment, use --ifconfig-push
--ifconfig-pool-linear
Modifies the --ifconfig-pool directive to allocate individual TUN interface
addresses for clients rather than /30 subnets. NOTE: This option is incompatible
with Windows clients.
This option is deprecated, and should be replaced with --topology p2p which is
functionally equivalent.
--ifconfig-push local remote-netmask
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 direc‐
tive 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.
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-con‐
fig-dir or dynamically generated using a --client-connect script.
Remember also to include a --route directive in the main OpenVPN config file which
encloses local, so that the kernel will know to route it to the server's TUN/TAP
interface.
OpenVPN's internal client IP address selection algorithm works as follows:
1 -- Use --client-connect script generated file for static IP (first choice).
2 -- Use --client-config-dir file for static IP (next choice).
3 -- Use --ifconfig-pool allocation for dynamic IP (last choice).
--iroute network [netmask]
Generate an internal route to a specific client. The netmask parameter, if omit‐
ted, defaults to 255.255.255.255.
This directive can be used to route a fixed subnet from the server to a particular
client, regardless of where the client is connecting from. Remember that you must
also add the route to the system routing table as well (such as by using the
--route directive). The reason why two routes are needed is that the --route
directive routes the packet from the kernel to OpenVPN. Once in OpenVPN, the
--iroute directive routes to the specific client.
This option must be specified either in a client instance config file using
--client-config-dir or dynamically generated using a --client-connect script.
The --iroute directive also has an important interaction with --push "route ...".
--iroute essentially defines a subnet which is owned by a particular client (we
will call this client A). If you would like other clients to be able to reach A's
subnet, you can use --push "route ..." together with --client-to-client to effect
this. In order for all clients to see A's subnet, OpenVPN must push this route to
all clients EXCEPT for A, since the subnet is already owned by A. OpenVPN accom‐
plishes this by not not pushing a route to a client if it matches one of the
client's iroutes.
--client-to-client
Because the OpenVPN server mode handles multiple clients through a single tun or
tap interface, it is effectively a router. The --client-to-client flag tells
OpenVPN to internally route client-to-client traffic rather than pushing all
client-originating traffic to the TUN/TAP interface.
When this option is used, each client will "see" the other clients which are cur‐
rently connected. Otherwise, each client will only see the server. Don't use
this option if you want to firewall tunnel traffic using custom, per-client rules.
--duplicate-cn
Allow multiple clients with the same common name to concurrently connect. In the
absence of this option, OpenVPN will disconnect a client instance upon connection
of a new client having the same common name.
--client-connect script
Run script on client connection. The script is passed the common name and IP
address of the just-authenticated client as environmental variables (see environ‐
mental variable section below). The script is also passed the pathname of a
freshly created temporary file as $1 (i.e. the first command line argument), to be
used by the script 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 $1.
See the --client-config-dir option below for options which can be legally used in
a dynamically generated config file.
Note that the return value of script is significant. If script returns a non-zero
error status, it will cause the client to be disconnected.
--client-disconnect
Like --client-connect but called on client instance shutdown. Will not be called
unless the --client-connect script and plugins (if defined) were previously called
on this instance with successful (0) status returns.
The exception to this rule is if the --client-disconnect script or plugins are
cascaded, and at least one client-connect function succeeded, then ALL of the
client-disconnect functions for scripts and plugins will be called on client
instance object deletion, even in cases where some of the related client-connect
functions returned an error status.
--client-config-dir dir
Specify a directory dir for custom client config files. After a connecting client
has been authenticated, OpenVPN will look in this directory for a file having the
same name as the client's X509 common name. If a matching file exists, it will be
opened and parsed for client-specific configuration options. If no matching file
is found, OpenVPN will instead try to open and parse a default file called
"DEFAULT", which may be provided but is not required. Note that the configuration
files must be readable by the OpenVPN process after it has dropped it's root priv‐
ileges.
This file can specify a fixed IP address for a given client using --ifconfig-push,
as well as fixed subnets owned by the client using --iroute.
One of the useful properties of this option is that it allows client configuration
files to be conveniently created, edited, or removed while the server is live,
without needing to restart the server.
The following options are legal in a client-specific context: --push, --push-
reset, --iroute, --ifconfig-push, and --config.
--ccd-exclusive
Require, as a condition of authentication, that a connecting client has a
--client-config-dir file.
--tmp-dir dir
Specify a directory dir for temporary files. This directory will be used by open‐
vpn processes and script to communicate temporary data with openvpn main process.
Note that the directory must be writable by the OpenVPN process after it has
dropped it's root privileges.
This directory will be used by in the following cases:
* --client-connect scripts to dynamically generate client-specific configuration
files.
* OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY plugin hook to return success/failure via
auth_control_file when using deferred auth method
* OPENVPN_PLUGIN_ENABLE_PF plugin hook to pass filtering rules via pf_file
--hash-size r v
Set the size of the real address hash table to r and the virtual address table to
v. By default, both tables are sized at 256 buckets.
--bcast-buffers n
Allocate n buffers for broadcast datagrams (default=256).
--tcp-queue-limit n
Maximum number of output packets queued before TCP (default=64).
When OpenVPN is tunneling data from a TUN/TAP device to a remote client over a TCP
connection, it is possible that the TUN/TAP device might produce data at a faster
rate than the TCP connection can support. When the number of output packets
queued before sending to the TCP socket reaches this limit for a given client con‐
nection, OpenVPN will start to drop outgoing packets directed at this client.
--tcp-nodelay
This macro sets the TCP_NODELAY socket flag on the server as well as pushes it to
connecting clients. The TCP_NODELAY flag disables the Nagle algorithm on TCP
sockets causing packets to be transmitted immediately with low latency, rather
than waiting a short period of time in order to aggregate several packets into a
larger containing packet. In VPN applications over TCP, TCP_NODELAY is generally
a good latency optimization.
The macro expands as follows:
if mode server:
socket-flags TCP_NODELAY
push "socket-flags TCP_NODELAY"
--max-clients n
Limit server to a maximum of n concurrent clients.
--max-routes-per-client n
Allow a maximum of n internal routes per client (default=256). This is designed
to help contain DoS attacks where an authenticated client floods the server with
packets appearing to come from many unique MAC addresses, forcing the server to
deplete virtual memory as its internal routing table expands. This directive can
be used in a --client-config-dir file or auto-generated by a --client-connect
script to override the global value for a particular client.
Note that this directive affects OpenVPN's internal routing table, not the kernel
routing table.
--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 script or shell command cmd to validate client virtual addresses or routes.
cmd will be executed with 3 parameters:
[1] operation -- "add", "update", or "delete" based on whether or not the address
is being added to, modified, or deleted from OpenVPN's internal routing table.
[2] address -- The address being learned or unlearned. This can be an IPv4
address such as "198.162.10.14", an IPv4 subnet such as "198.162.10.0/24", or an
ethernet MAC address (when --dev tap is being used) such as "00:FF:01:02:03:04".
[3] common name -- The common name on the certificate associated with the client
linked to this address. Only present for "add" or "update" operations, not
"delete".
On "add" or "update" methods, if the script returns a failure code (non-zero),
OpenVPN will reject the address and will not modify its internal routing table.
Normally, the cmd script will use the information provided above to set appropri‐
ate firewall entries on the VPN TUN/TAP interface. Since OpenVPN provides the
association between virtual IP or MAC address and the client's authenticated com‐
mon name, it allows a user-defined script to configure firewall access policies
with regard to the client's high-level common name, rather than the low level
client virtual addresses.
--auth-user-pass-verify script method
Require the client to provide a username/password (possibly in addition to a
client certificate) for authentication.
OpenVPN will execute script as a shell command to validate the username/password
provided by the client.
If method is set to "via-env", OpenVPN will call script with the environmental
variables username and password set to the username/password strings provided by
the client. Be aware that this method is insecure on some platforms which make
the environment of a process publicly visible to other unprivileged processes.
If method is set to "via-file", OpenVPN will write the username and password to
the first two lines of a temporary file. The filename will be passed as an argu‐
ment 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 prevent the username/password file from touching the hard drive.
The script should examine the username and password, returning a success exit code
(0) if the client's authentication request is to be accepted, or a failure code
(1) to reject the client.
This directive is designed to enable a plugin-style interface for extending Open‐
VPN's authentication capabilities.
To protect against a client passing a maliciously formed username or password
string, the username string must consist only of these characters: alphanumeric,
underbar ('_'), dash ('-'), dot ('.'), or at ('@'). The password string can con‐
sist of any printable characters except for CR or LF. Any illegal characters in
either the username or password string will be converted to underbar ('_').
Care must be taken by any user-defined scripts to avoid creating a security vul‐
nerability in the way that these strings are handled. Never use these strings in
such a way that they might be escaped or evaluated by a shell interpreter.
For a sample script that performs PAM authentication, see sample-scripts/auth-
pam.pl in the OpenVPN source distribution.
--opt-verify
Clients that connect with options that are incompatible with those of the server
will be disconnected.
Options that will be compared for compatibility include dev-type, link-mtu, tun-
mtu, proto, tun-ipv6, ifconfig, comp-lzo, fragment, keydir, cipher, auth, keysize,
secret, no-replay, no-iv, tls-auth, key-method, tls-server, and tls-client.
This option requires that --disable-occ NOT be used.
--auth-user-pass-optional
Allow connections by clients that do not specify a username/password. Normally,
when --auth-user-pass-verify or --management-client-auth is specified (or an
authentication plugin module), the OpenVPN server daemon will require connecting
clients to specify a username and password. This option makes the submission of a
username/password by clients optional, passing the responsibility to the user-
defined authentication module/script to accept or deny the client based on other
factors (such as the setting of X509 certificate fields). When this option is
used, and a connecting client does not submit a username/password, the user-
defined authentication module/script will see the username and password as being
set to empty strings (""). The authentication module/script MUST have logic to
detect this condition and respond accordingly.
--client-cert-not-required
Don't require client certificate, client will authenticate using username/password
only. Be aware that using this directive is less secure than requiring certifi‐
cates from all clients.
If you use this directive, the entire responsibility of authentication will rest
on your --auth-user-pass-verify script, so keep in mind that bugs in your script
could potentially compromise the security of your VPN.
If you don't use this directive, but you also specify an --auth-user-pass-verify
script, then OpenVPN will perform double authentication. The client certificate
verification AND the --auth-user-pass-verify script will need to succeed in order
for a client to be authenticated and accepted onto the VPN.
--username-as-common-name
For --auth-user-pass-verify authentication, use the authenticated username as the
common name, rather than the common name from the client cert.
--no-name-remapping
Allow Common Name, X509 Subject, and username strings to include any printable
character including space, but excluding control characters such as tab, newline,
and carriage-return.
By default, OpenVPN will remap any character other than alphanumeric, underbar
('_'), dash ('-'), dot ('.'), and slash ('/') to underbar ('_'). The X509 Subject
string as returned by the tls_id environmental variable, can additionally contain
colon (':') or equal ('=').
While name remapping is performed for security reasons to reduce the possibility
of introducing string expansion security vulnerabilities in user-defined authenti‐
cation scripts, this option is provided for those cases where it is desirable to
disable the remapping feature. Don't use this option unless you know what you are
doing!
--port-share host port
When run in TCP server mode, share the OpenVPN port with another application, such
as an HTTPS server. If OpenVPN senses a connection to its port which is using a
non-OpenVPN protocol, it will proxy the connection to the server at host:port.
Currently only designed to work with HTTP/HTTPS, though it would be theoretically
possible to extend to other protocols such as ssh.
Not implemented on Windows.
Client Mode
Use client mode when connecting to an OpenVPN server which has --server, --server-bridge,
or --mode server in it's configuration.
--client
A helper directive designed to simplify the configuration of OpenVPN's client
mode. This directive is equivalent to:
pull
tls-client
--pull This option must be used on a client which is connecting to a multi-client server.
It indicates to OpenVPN that it should accept options pushed by the server, pro‐
vided 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 user‐
name/password on 2 lines (Note: OpenVPN will only read passwords from a file if it
has been built with the --enable-password-save configure option, or on Windows by
defining ENABLE_PASSWORD_SAVE in win/settings.in).
If up is omitted, username/password will be prompted from the console.
The server configuration must specify an --auth-user-pass-verify script to verify
the username/password provided by the client.
--auth-retry type
Controls how OpenVPN responds to username/password verification errors such as the
client-side response to an AUTH_FAILED message from the server or verification
failure of the private key password.
Normally used to prevent auth errors from being fatal on the client side, and to
permit username/password requeries in case of error.
An AUTH_FAILED message is generated by the server if the client fails --auth-user-
pass authentication, or if the server-side --client-connect script returns an
error status when the client tries to connect.
type can be one of:
none -- Client will exit with a fatal error (this is the default).
nointeract -- Client will retry the connection without requerying for an --auth-
user-pass username/password. Use this option for unattended clients.
interact -- Client will requery for an --auth-user-pass username/password and/or
private key password before attempting a reconnection.
Note that while this option cannot be pushed, it can be controlled from the man‐
agement interface.
--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 compati‐
ble 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 secu‐
rity properties including eliminating certain kinds of DoS and message replay
attacks.
When the direction parameter is omitted, 2 keys are used bidirectionally, one for
HMAC and the other for encryption/decryption.
The direction parameter should always be complementary on either side of the con‐
nection, 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 sup‐
ports 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 con‐
figuration.
There are no certificates or certificate authorities or complicated negotiation
handshakes and protocols. The only requirement is that you have a pre-existing
secure channel with your peer (such as ssh ) to initially copy the key. This
requirement, along with the fact that your key never changes unless you manually
generate a new one, makes it somewhat less secure than TLS mode (see below). If
an attacker manages to steal your key, everything that was ever encrypted with it
is compromised. Contrast that to the perfect forward secrecy features of TLS mode
(using Diffie Hellman key exchange), where even if an attacker was able to steal
your private key, he would gain no information to help him decrypt past sessions.
Another advantageous aspect of Static Key encryption mode is that it is a hand‐
shake-free protocol without any distinguishing signature or feature (such as a
header or protocol handshake sequence) that would mark the ciphertext packets as
being generated by OpenVPN. Anyone eavesdropping on the wire would see nothing
but random-looking data.
--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 sig‐
nature.
OpenVPN's usage of HMAC is to first encrypt a packet, then HMAC the resulting
ciphertext.
In static-key encryption mode, the HMAC key is included in the key file generated
by --genkey. In TLS mode, the HMAC key is dynamically generated and shared
between peers via the TLS control channel. If OpenVPN receives a packet with a
bad HMAC it will drop the packet. HMAC usually adds 16 or 20 bytes per packet.
Set alg=none to disable authentication.
For more information on HMAC see
http://www.cs.ucsd.edu/users/mihir/papers/hmac.html
--cipher alg
Encrypt packets with cipher algorithm alg. The default is BF-CBC, an abbreviation
for Blowfish in Cipher Block Chaining mode. Blowfish has the advantages of being
fast, very secure, and allowing key sizes of up to 448 bits. Blowfish is designed
to be used in situations where keys are changed infrequently.
For more information on blowfish, see http://www.counterpane.com/blowfish.html
To see other ciphers that are available with OpenVPN, use the --show-ciphers
option.
OpenVPN supports the CBC, CFB, and OFB cipher modes, however CBC is recommended
and CFB and OFB should be considered advanced modes.
Set alg=none to disable encryption.
--keysize n
Size of cipher key in bits (optional). If unspecified, defaults to cipher-spe‐
cific 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 exten‐
sively cryptanalyzed with non-standard key lengths, and a larger key may offer no
real guarantee of greater security, or may even reduce security.
--prng alg [nsl]
(Advanced) For PRNG (Pseudo-random number generator), use digest algorithm alg
(default=sha1), and set nsl (default=16) to the size in bytes of the nonce secret
length (between 16 and 64).
Set alg=none to disable the PRNG and use the OpenSSL RAND_bytes function instead
for all of OpenVPN's pseudo-random number needs.
--engine [engine-name]
Enable OpenSSL hardware-based crypto engine functionality.
If engine-name is specified, use a specific crypto engine. Use the --show-engines
standalone option to list the crypto engines which are supported by OpenSSL.
--no-replay
(Advanced) Disable OpenVPN's protection against replay attacks. Don't use this
option unless you are prepared to make a tradeoff of greater efficiency in
exchange for less security.
OpenVPN provides datagram replay protection by default.
Replay protection is accomplished by tagging each outgoing datagram with an iden‐
tifier 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 (iden‐
tifying 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 unique‐
ness of this value for each key. As in IPSec, if the sequence number is close to
wrapping back to zero, OpenVPN will trigger a new key exchange.
To check for replays, OpenVPN uses the sliding window algorithm used by IPSec.
--replay-window n [t]
Use a replay protection sliding-window of size n and a time window of t seconds.
By default n is 64 (the IPSec default) and t is 15 seconds.
This option is only relevant in UDP mode, i.e. when either --proto udp is
specifed, or no --proto option is specified.
When OpenVPN tunnels IP packets over UDP, there is the possibility that packets
might be dropped or delivered out of order. Because OpenVPN, like IPSec, is emu‐
lating the physical network layer, it will accept an out-of-order packet sequence,
and will deliver such packets in the same order they were received to the TCP/IP
protocol stack, provided they satisfy several constraints.
(a) The packet cannot be a replay (unless --no-replay is specified, which disables
replay protection altogether).
(b) If a packet arrives out of order, it will only be accepted if the difference
between its sequence number and the highest sequence number received so far is
less than n.
(c) If a packet arrives out of order, it will only be accepted if it arrives no
later than t seconds after any packet containing a higher sequence number.
If you are using a network link with a large pipeline (meaning that the product of
bandwidth and latency is high), you may want to use a larger value for n. Satel‐
lite links in particular often require this.
If you run OpenVPN at --verb 4, you will see the message "Replay-window backtrack
occurred [x]" every time the maximum sequence number backtrack seen thus far
increases. This can be used to calibrate n.
There is some controversy on the appropriate method of handling packet reordering
at the security layer.
Namely, to what extent should the security layer protect the encapsulated protocol
from attacks which masquerade as the kinds of normal packet loss and reordering
that occur over IP networks?
The IPSec and OpenVPN approach is to allow packet reordering within a certain
fixed sequence number window.
OpenVPN adds to the IPSec model by limiting the window size in time as well as
sequence space.
OpenVPN also adds TCP transport as an option (not offered by IPSec) in which case
OpenVPN can adopt a very strict 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 tun‐
neling 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 mes‐
sage deletion or reordering attack that falls within the normal operating parame‐
ters of what is to be expected from the physical IP layer. The problem is easily
fixed by simply using TCP as the VPN transport layer.
--mute-replay-warnings
Silence the output of replay warnings, which are a common false alarm on WiFi net‐
works. 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 ses‐
sions are frequently started and stopped.
This option will keep a disk copy of the current replay protection state (i.e. the
most recent packet timestamp and sequence number received from the remote peer),
so that if an OpenVPN session is stopped and restarted, it will reject any replays
of packets which were already received by the prior session.
This option only makes sense when replay protection is enabled (the default) and
you are using either --secret (shared-secret key mode) or TLS mode with --tls-
auth.
--no-iv
(Advanced) Disable OpenVPN's use of IV (cipher initialization vector). Don't use
this option unless you are prepared to make a tradeoff of greater efficiency in
exchange for less security.
OpenVPN uses an IV by default, and requires it for CFB and OFB cipher modes (which
are totally insecure without it). Using an IV is important for security when mul‐
tiple messages are being encrypted/decrypted with the same key.
IV is implemented differently depending on the cipher mode used.
In CBC mode, OpenVPN uses a pseudo-random IV for each packet.
In CFB/OFB mode, OpenVPN uses a unique sequence number and time stamp as the IV.
In fact, in CFB/OFB mode, OpenVPN uses a datagram space-saving optimization that
uses the unique identifier for datagram replay protection as the IV.
--test-crypto
Do a self-test of OpenVPN's crypto options by encrypting and decrypting test pack‐
ets using the data channel encryption options specified above. This option does
not require a peer to function, and therefore can be specified without --dev or
--remote.
The typical usage of --test-crypto would be something like this:
openvpn --test-crypto --secret key
or
openvpn --test-crypto --secret key --verb 9
This option is very useful to test OpenVPN after it has been ported to a new plat‐
form, or to isolate problems in the compiler, OpenSSL crypto library, or OpenVPN's
crypto code. Since it is a self-test mode, problems with encryption and authenti‐
cation can be debugged independently of network and tunnel issues.
TLS Mode Options:
TLS mode is the most powerful crypto mode of OpenVPN in both security and flexibility.
TLS mode works by establishing control and data channels which are multiplexed over a
single TCP/UDP port. OpenVPN initiates a TLS session over the control channel and uses
it to exchange cipher and HMAC keys to protect the data channel. TLS mode uses a robust
reliability layer over the UDP connection for all control channel communication, while
the data channel, over which encrypted tunnel data passes, is forwarded without any medi‐
ation. The result is the best of both worlds: a fast data channel that forwards over UDP
with only the overhead of encrypt, decrypt, and HMAC functions, and a control channel
that provides all of the security features of TLS, including certificate-based authenti‐
cation and Diffie Hellman forward secrecy.
To use TLS mode, each peer that runs OpenVPN should have its own local certificate/key
pair ( --cert and --key ), signed by the root certificate which is specified in --ca.
When two OpenVPN peers connect, each presents its local certificate to the other. Each
peer will then check that its partner peer presented a certificate which was signed by
the master root certificate as specified in --ca.
If that check on both peers succeeds, then the TLS negotiation will succeed, both OpenVPN
peers will exchange temporary session keys, and the tunnel will begin passing data.
The OpenVPN distribution contains a set of scripts for managing RSA certificates & keys,
located in the easy-rsa subdirectory.
The easy-rsa package is also rendered in web form here: https://openvpn.net/easyrsa.html
--tls-server
Enable TLS and assume server role during TLS handshake. Note that OpenVPN is
designed as a peer-to-peer application. The designation of client or server is
only for the purpose of negotiating the TLS control channel.
--tls-client
Enable TLS and assume client role during TLS handshake.
--ca file
Certificate authority (CA) file in .pem format, also referred to as the root cer‐
tificate. This file can have multiple certificates in .pem format, concatenated
together. You can construct your own certificate authority certificate and pri‐
vate key by using a command such as:
openssl req -nodes -new -x509 -keyout ca.key -out ca.crt
Then edit your openssl.cnf file and edit the certificate variable to point to your
new root certificate ca.crt.
For testing purposes only, the OpenVPN distribution includes a sample CA certifi‐
cate (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.
--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 Open‐
VPN distribution. Diffie Hellman parameters may be considered public.
--cert file
Local peer's signed certificate in .pem format -- must be signed by a certificate
authority whose certificate is in --ca file. Each peer in an OpenVPN link running
in TLS mode should have its own certificate and private key file. In addition,
each certificate should have been signed by the key of a certificate authority
whose public key resides in the --ca certificate authority file. You can easily
make your own certificate authority (see above) or pay money to use a commercial
service such as thawte.com (in which case you will be helping to finance the
world's second space tourist :). To generate a certificate, you can use a command
such as:
openssl req -nodes -new -keyout mycert.key -out mycert.csr
If your certificate authority private key lives on another machine, copy the cer‐
tificate signing request (mycert.csr) to this other machine (this can be done over
an insecure channel such as email). Now sign the certificate with a command such
as:
openssl ca -out mycert.crt -in mycert.csr
Now copy the certificate (mycert.crt) back to the peer which initially generated
the .csr file (this can be over a public medium). Note that the openssl ca com‐
mand reads the location of the certificate authority key from its configuration
file such as /usr/share/ssl/openssl.cnf -- note also that for certificate author‐
ity functions, you must set up the files index.txt (may be empty) and serial (ini‐
tialize to 01 ).
--key file
Local peer's private key in .pem format. Use the private key which was generated
when you built your peer's certificate (see -cert file above).
--pkcs12 file
Specify a PKCS #12 file containing local private key, local certificate, and root
CA certificate. This option can be used instead of --ca, --cert, and --key.
--pkcs11-cert-private [0|1]...
Set if access to certificate object should be performed after login. Every
provider has its own setting.
--pkcs11-id name
Specify the serialized certificate id to be used. The id can be gotten by the
standalone --show-pkcs11-ids option.
--pkcs11-id-management
Acquire PKCS#11 id from management interface. In this case a NEED-STR 'pkcs11-id-
request' real-time message will be triggered, application may use pkcs11-id-count
command to retrieve available number of certificates, and pkcs11-id-get command to
retrieve certificate id and certificate body.
--pkcs11-pin-cache seconds
Specify how many seconds the PIN can be cached, the default is until the token is
removed.
--pkcs11-protected-authentication [0|1]...
Use PKCS#11 protected authentication path, useful for biometric and external key‐
pad devices. Every provider has its own setting.
--pkcs11-providers provider...
Specify a RSA Security Inc. PKCS #11 Cryptographic Token Interface (Cryptoki)
providers to load. This option can be used instead of --cert, --key, and
--pkcs12.
--pkcs11-private-mode mode...
Specify which method to use in order to perform private key operations. A differ‐
ent mode can be specified for each provider. Mode is encoded as hex number, and
can be a mask one of the following:
0 (default) -- Try to determind automatically.
1 -- Use sign.
2 -- Use sign recover.
4 -- Use decrypt.
8 -- Use unwrap.
--cryptoapicert select-string
Load the certificate and private key from the Windows Certificate System Store
(Windows Only).
Use this option instead of --cert and --key.
This makes it possible to use any smart card, supported by Windows, but also any
kind of certificate, residing in the Cert Store, where you have access to the pri‐
vate key. This option has been tested with a couple of different smart cards
(GemSAFE, Cryptoflex, and Swedish Post Office eID) on the client side, and also an
imported PKCS12 software certificate on the server side.
To select a certificate, based on a substring search in the certificate's subject:
cryptoapicert "SUBJ:Peter Runestig"
To select a certificate, based on certificate's thumbprint:
cryptoapicert "THUMB:f6 49 24 41 01 b4 ..."
The thumbprint hex string can easily be copy-and-pasted from the Windows Certifi‐
cate 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 tun‐
nel data channel is generated and exchanged over the TLS session.
In method 1 (the default for OpenVPN 1.x), both sides generate random encrypt and
HMAC-send keys which are forwarded to the other host over the TLS channel.
In method 2, (the default for OpenVPN 2.0) the client generates a random key.
Both client and server also generate some random seed material. All key source
material is exchanged over the TLS channel. The actual keys are generated using
the TLS PRF function, taking source entropy from both client and server. Method 2
is designed to closely parallel the key generation process used by TLS 1.0.
Note that in TLS mode, two separate levels of keying occur:
(1) The TLS connection is initially negotiated, with both sides of the connection
producing certificates and verifying the certificate (or other authentication info
provided) of the other side. The --key-method parameter has no effect on this
process.
(2) After the TLS connection is established, the tunnel session keys are sepa‐
rately negotiated over the existing secure TLS channel. Here, --key-method deter‐
mines the derivation of the tunnel session keys.
--tls-cipher l
A list l of allowable TLS ciphers delimited by a colon (":"). If you require a
high level of security, you may want to set this parameter manually, to prevent a
version rollback attack where a man-in-the-middle attacker tries to force two
peers to negotiate to the lowest level of security they both support. Use --show-
tls to see a list of supported TLS ciphers.
--tls-timeout n
Packet retransmit timeout on TLS control channel if no acknowledgment from remote
within n seconds (default=2). When OpenVPN sends a control packet to its peer, it
will expect to receive an acknowledgement within n seconds or it will retransmit
the packet, subject to a TCP-like exponential backoff algorithm. This parameter
only applies to control channel packets. Data channel packets (which carry
encrypted tunnel data) are never acknowledged, sequenced, or retransmitted by
OpenVPN because the higher level network protocols running on top of the tunnel
such as TCP expect this role to be left to them.
--reneg-bytes n
Renegotiate data channel key after n bytes sent or received (disabled by default).
OpenVPN allows the lifetime of a key to be expressed as a number of bytes
encrypted/decrypted, a number of packets, or a number of seconds. A key renegoti‐
ation 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 renegotiation will still occur once per 3600 sec‐
onds. 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 critical path of tunnel data forwarding.
--single-session
After initially connecting to a remote peer, disallow any new connections. Using
this option means that a remote peer cannot connect, disconnect, and then recon‐
nect.
If the daemon is reset by a signal or --ping-restart, it will allow one new con‐
nection.
--single-session can be used with --ping-exit or --inactive to create a single
dynamic session that will exit when finished.
--tls-exit
Exit on TLS negotiation failure.
--tls-auth file [direction]
Add an additional layer of HMAC authentication on top of the TLS control channel
to protect against DoS attacks.
In a nutshell, --tls-auth enables a kind of "HMAC firewall" on OpenVPN's TCP/UDP
port, where TLS control channel packets bearing an incorrect HMAC signature can be
dropped immediately without response.
file (required) is a key file which can be in one of two formats:
(1) An OpenVPN static key file generated by --genkey (required if direction param‐
eter is used).
(2) A freeform passphrase file. In this case the HMAC key will be derived by tak‐
ing 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 lis‐
tening for packets from any IP address, such as when --remote is not specified, or
--remote is specified with --float.
The rationale for this feature is as follows. TLS requires a multi-packet
exchange before it is able to authenticate a peer. During this time before
authentication, OpenVPN is allocating resources (memory and CPU) to this potential
peer. The potential peer is also exposing many parts of OpenVPN and the OpenSSL
library to the packets it is sending. Most successful network attacks today seek
to either exploit bugs in programs (such as buffer overflow attacks) or force a
program to consume so many resources that it becomes unusable. Of course the
first line of defense is always to produce clean, well-audited code. OpenVPN has
been written with buffer overflow attack prevention as a top priority. But as
history has shown, many of the most widely used network applications have, from
time to time, fallen to buffer overflow attacks.
So as a second line of defense, OpenVPN offers this special layer of authentica‐
tion on top of the TLS control channel so that every packet on the control channel
is authenticated by an HMAC signature and a unique ID for replay protection. This
signature will also help protect against DoS (Denial of Service) attacks. An
important rule of thumb in reducing vulnerability to DoS attacks is to minimize
the amount of resources a potential, but as yet unauthenticated, client is able to
consume.
--tls-auth does this by signing every TLS control channel packet with an HMAC sig‐
nature, including packets which are sent before the TLS level has had a chance to
authenticate the peer. The result is that packets without the correct signature
can be dropped immediately upon reception, before they have a chance to consume
additional system resources such as by initiating a TLS handshake. --tls-auth can
be strengthened by adding the --replay-persist option which will keep OpenVPN's
replay protection state in a file so that it is not lost across restarts.
It should be emphasized that this feature is optional and that the passphrase/key
file used with --tls-auth gives a peer nothing more than the power to initiate a
TLS handshake. It is not used to encrypt or authenticate any tunnel data.
--askpass [file]
Get certificate password from console or file before we daemonize.
For the extremely security conscious, it is possible to protect your private key
with a password. Of course this means that every time the OpenVPN daemon is
started you must be there to type the password. The --askpass option allows you
to start OpenVPN from the command line. It will query you for a password before
it daemonizes. To protect a private key with a password you should omit the
-nodes option when you use the openssl command line tool to manage certificates
and private keys.
If file is specified, read the password from the first line of file. Keep in mind
that storing your password in a file to a certain extent invalidates the extra
security provided by using an encrypted key (Note: OpenVPN will only read pass‐
words from a file if it has been built with the --enable-password-save configure
option, or on Windows by defining ENABLE_PASSWORD_SAVE in win/settings.in).
--auth-nocache
Don't cache --askpass or --auth-user-pass username/passwords in virtual memory.
If specified, this directive will cause OpenVPN to immediately forget user‐
name/password inputs after they are used. As a result, when OpenVPN needs a user‐
name/password, it will prompt for input from stdin, which may be multiple times
during the duration of an OpenVPN session.
This directive does not affect the --http-proxy username/password. It is always
cached.
--tls-verify cmd
Execute shell command cmd to verify the X509 name of a pending TLS connection that
has otherwise passed all other tests of certification (except for revocation via
--crl-verify directive; the revocation test occurs after the --tls-verify test).
cmd should return 0 to allow the TLS handshake to proceed, or 1 to fail.
Note that cmd is a command line and as such may (if enclosed in quotes) contain
whitespace separated arguments. The first word of cmd is the shell command to
execute and the remaining words are its arguments. When cmd is executed two argu‐
ments are appended, as follows:
cmd certificate_depth X509_NAME_oneline
These arguments are, respectively, the current certificate depth and the X509 com‐
mon name (cn) of the peer.
This feature is useful if the peer you want to trust has a certificate which was
signed by a certificate authority who also signed many other certificates, where
you don't necessarily want to trust all of them, but rather be selective about
which peer certificate you will accept. This feature allows you to write a script
which will test the X509 name on a certificate and decide whether or not it should
be accepted. For a simple perl script which will test the common name field on
the certificate, see the file verify-cn in the OpenVPN distribution.
See the "Environmental Variables" section below for additional parameters passed
as environmental variables.
--tls-export-cert directory
Store the certificates the clients uses upon connection to this directory. This
will be done before --tls-verify is called. The certificates will use a temporary
name and will be deleted when the tls-verify script returns. The file name used
for the certificate is available via the peer_cert environment variable.
--x509-username-field fieldname
Field in x509 certificate subject to be used as username (default=CN). Fieldname
will be uppercased before matching. When this option is used, the --tls-remote
option will match against the chosen fieldname instead of the CN.
--tls-remote name
Accept connections only from a host with X509 name or common name equal to name.
The remote host must also pass all other tests of verification.
NOTE: Because tls-remote may test against a common name prefix, only use this
option when you are using OpenVPN with a custom CA certificate that is under your
control. Never use this option when your client certificates are signed by a
third party, such as a commercial web CA.
Name can also be a common name prefix, for example if you want a client to only
accept connections to "Server-1", "Server-2", etc., you can simply use --tls-
remote Server
Using a common name prefix is a useful alternative to managing a CRL (Certificate
Revocation List) on the client, since it allows the client to refuse all certifi‐
cates except for those associated with designated servers.
--tls-remote is a useful replacement for the --tls-verify option to verify the
remote host, because --tls-remote works in a --chroot environment too.
--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 cer‐
tificate with the nsCertType field set to "server".
If the server certificate's nsCertType field is set to "server", then the clients
can verify this with --ns-cert-type server.
This is an important security precaution to protect against a man-in-the-middle
attack where an authorized client attempts to connect to another client by imper‐
sonating the server. The attack is easily prevented by having clients verify the
server certificate using any one of --ns-cert-type, --tls-remote, or --tls-verify.
--remote-cert-ku v...
Require that peer certificate was signed with an explicit key usage.
This is a useful security option for clients, to ensure that the host they connect
to is a designated server.
The key usage should be encoded in hex, more than one key usage can be specified.
--remote-cert-eku oid
Require that peer certificate was signed with an explicit extended key usage.
This is a useful security option for clients, to ensure that the host they connect
to is a designated server.
The extended key usage should be encoded in oid notation, or OpenSSL symbolic rep‐
resentation.
--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 Authentication"
The key usage is digitalSignature and/or keyAgreement.
The --remote-cert-tls server option is equivalent to --remote-cert-ku a0 88
--remote-cert-eku "TLS Web Server Authentication"
The key usage is digitalSignature and ( keyEncipherment or keyAgreement ).
This is an important security precaution to protect against a man-in-the-middle
attack where an authorized client attempts to connect to another client by imper‐
sonating the server. The attack is easily prevented by having clients verify the
server certificate using any one of --remote-cert-tls, --tls-remote, or --tls-ver‐
ify.
--crl-verify crl
Check peer certificate against the file crl in PEM format.
A CRL (certificate revocation list) is used when a particular key is compromised
but when the overall PKI is still intact.
Suppose you had a PKI consisting of a CA, root certificate, and a number of client
certificates. Suppose a laptop computer containing a client key and certificate
was stolen. By adding the stolen certificate to the CRL file, you could reject
any connection which attempts to use it, while preserving the overall integrity of
the PKI.
The only time when it would be necessary to rebuild the entire PKI from scratch
would be if the root certificate key itself was compromised.
SSL Library information:
--show-ciphers
(Standalone) Show all cipher algorithms to use with the --cipher option.
--show-digests
(Standalone) Show all message digest algorithms to use with the --auth option.
--show-tls
(Standalone) Show all TLS ciphers (TLS used only as a control channel). The TLS
ciphers will be sorted from highest preference (most secure) to lowest.
--show-engines
(Standalone) Show currently available hardware-based crypto acceleration engines
supported by the OpenSSL library.
Generate a random key:
Used only for non-TLS static key encryption mode.
--genkey
(Standalone) Generate a random key to be used as a shared secret, for use with the
--secret option. This file must be shared with the peer over a pre-existing
secure channel such as scp(1)
--secret file
Write key to file.
TUN/TAP persistent tunnel config mode:
Available with linux 2.4.7+. These options comprise a standalone mode of OpenVPN which
can be used to create and delete persistent tunnels.
--mktun
(Standalone) Create a persistent tunnel on platforms which support them such as
Linux. Normally TUN/TAP tunnels exist only for the period of time that an appli‐
cation has them open. This option takes advantage of the TUN/TAP driver's ability
to build persistent tunnels that live through multiple instantiations of OpenVPN
and die only when they are deleted or the machine is rebooted.
One of the advantages of persistent tunnels is that they eliminate the need for
separate --up and --down scripts to run the appropriate ifconfig(8) and route(8)
commands. These commands can be placed in the the same shell script which starts
or terminates an OpenVPN session.
Another advantage is that open connections through the TUN/TAP-based tunnel will
not be reset if the OpenVPN peer restarts. This can be useful to provide uninter‐
rupted 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 con‐
figure their MTU value (see --link-mtu and --tun-mtu above).
On some platforms such as Windows, TAP-Win32 tunnels are persistent by default.
--rmtun
(Standalone) Remove a persistent tunnel.
--dev tunX | tapX
TUN/TAP device
--user user
Optional user to be owner of this tunnel.
--group group
Optional group to be owner of this tunnel.
Windows-Specific Options:
--win-sys path|'env'
Set the Windows system directory pathname to use when looking for system executa‐
bles such as route.exe and netsh.exe. By default, if this directive is not speci‐
fied, the pathname will be set to "C:\WINDOWS"
The special string 'env' indicates that the pathname should be read from the Sys‐
temRoot environmental variable.
--ip-win32 method
When using --ifconfig on Windows, set the TAP-Win32 adapter IP address and netmask
using method. Don't use this option unless you are also using --ifconfig.
manual -- Don't set the IP address or netmask automatically. Instead output a
message to the console telling the user to configure the adapter manually and
indicating the IP/netmask which OpenVPN expects the adapter to be set to.
dynamic [offset] [lease-time] -- Automatically set the IP address and netmask by
replying to DHCP query messages generated by the kernel. This mode is probably
the "cleanest" solution for setting the TCP/IP properties since it uses the well-
known DHCP protocol. There are, however, two prerequisites for using this mode:
(1) The TCP/IP properties for the TAP-Win32 adapter must be set to "Obtain an IP
address automatically," and (2) OpenVPN needs to claim an IP address in the subnet
for use as the virtual DHCP server address. By default in --dev tap mode, OpenVPN
will take the normally unused first address in the subnet. For example, if your
subnet is 192.168.4.0 netmask 255.255.255.0, then OpenVPN will take the IP address
192.168.4.0 to use as the virtual DHCP server address. In --dev tun mode, OpenVPN
will cause the DHCP server to masquerade as if it were coming from the remote end‐
point. 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 Win‐
dows ipconfig /all command can be used to show what Windows thinks the DHCP server
address is. OpenVPN will "claim" this address, so make sure to use a free
address. Having said that, different OpenVPN instantiations, including different
ends of the same connection, can share the same virtual DHCP server address. The
lease-time parameter controls the lease time of the DHCP assignment given to the
TAP-Win32 adapter, and is denoted in seconds. Normally a very long lease time is
preferred because it prevents routes involving the TAP-Win32 adapter from being
lost when the system goes to sleep. The default lease time is one year.
netsh -- Automatically set the IP address and netmask using the Windows command-
line "netsh" command. This method appears to work correctly on Windows XP but not
Windows 2000.
ipapi -- Automatically set the IP address and netmask using the Windows IP Helper
API. This approach does not have ideal semantics, though testing has indicated
that it works okay in practice. If you use this option, it is best to leave the
TCP/IP properties for the TAP-Win32 adapter in their default state, i.e. "Obtain
an IP address automatically."
adaptive -- (Default) Try dynamic method initially and fail over to netsh if the
DHCP negotiation with the TAP-Win32 adapter does not succeed in 20 seconds. Such
failures have been known to occur when certain third-party firewall 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 proper‐
ties 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 Distri‐
bution Server) Repeat this option to set secondary NBDD server addresses.
NTP addr -- Set primary NTP server address (Network Time Protocol). Repeat this
option to set secondary NTP server addresses.
NBT type -- Set NetBIOS over TCP/IP Node type. Possible options: 1 = b-node
(broadcasts), 2 = p-node (point-to-point name queries to a WINS server), 4 = m-
node (broadcast then query name server), and 8 = h-node (query name server, then
broadcast).
NBS scope-id -- Set NetBIOS over TCP/IP Scope. A NetBIOS Scope ID provides an
extended naming service for the NetBIOS over TCP/IP (Known as NBT) module. The
primary purpose of a NetBIOS scope ID is to isolate NetBIOS traffic on a single
network to only those nodes with the same NetBIOS scope ID. The NetBIOS scope ID
is a character string that is appended to the NetBIOS name. The NetBIOS scope ID
on two hosts must match, or the two hosts will not be able to communicate. The
NetBIOS Scope ID also allows computers to use the same computer name, as they have
different scope IDs. The Scope ID becomes a part of the NetBIOS name, making the
name unique. (This description of NetBIOS scopes courtesy of NeonSurge@abyss.com)
DISABLE-NBT -- Disable Netbios-over-TCP/IP.
Note that if --dhcp-option is pushed via --push to a non-windows client, the
option will be saved in the client's environment before the up script is called,
under the name "foreign_option_{n}".
--tap-sleep n
Cause OpenVPN to sleep for n seconds immediately after the TAP-Win32 adapter state
is set to "connected".
This option is intended to be used to troubleshoot problems with the --ifconfig
and --ip-win32 options, and is used to give the TAP-Win32 adapter time to come up
before Windows IP Helper API operations are applied to it.
--show-net-up
Output OpenVPN's view of the system routing table and network adapter list to the
syslog or log file after the TUN/TAP adapter has been brought up and any routes
have been added.
--dhcp-renew
Ask Windows to renew the TAP adapter lease on startup. This option is normally
unnecessary, as Windows automatically triggers a DHCP renegotiation on the TAP
adapter when it comes up, however if you set the TAP-Win32 adapter Media Status
property to "Always Connected", you may need this flag.
--dhcp-release
Ask Windows to release the TAP adapter lease on shutdown. This option has the
same caveats as --dhcp-renew above.
--register-dns
Run net stop dnscache, net start dnscache, ipconfig /flushdns and ipconfig /regis‐
terdns on connection initiation. This is known to kick Windows into recognizing
pushed DNS servers.
--pause-exit
Put up a "press any key to continue" message on the console prior to OpenVPN pro‐
gram exit. This option is automatically used by the Windows explorer when OpenVPN
is run on a configuration file using the right-click explorer menu.
--service exit-event [0|1]
Should be used when OpenVPN is being automatically executed by another program in
such a context that no interaction with the user via display or keyboard is possi‐
ble. In general, end-users should never need to explicitly use this option, as it
is automatically added by the OpenVPN service wrapper when a given OpenVPN config‐
uration is being run as a service.
exit-event is the name of a Windows global event object, and OpenVPN will continu‐
ously monitor the state of this event object and exit when it becomes signaled.
The second parameter indicates the initial state of exit-event and normally
defaults to 0.
Multiple OpenVPN processes can be simultaneously executed with the same exit-event
parameter. In any case, the controlling process can signal exit-event, causing
all such OpenVPN processes to exit.
When executing an OpenVPN process using the --service directive, OpenVPN will
probably not have a console window to output status/error messages, therefore it
is useful to use --log or --log-append to write these messages to a file.
--show-adapters
(Standalone) Show available TAP-Win32 adapters which can be selected using the
--dev-node option. On non-Windows systems, the ifconfig(8) command provides simi‐
lar functionality.
--allow-nonadmin [TAP-adapter]
(Standalone) Set TAP-adapter to allow access from non-administrative accounts. If
TAP-adapter is omitted, all TAP adapters on the system will be configured to allow
non-admin access. The non-admin access setting will only persist for the length
of time that the TAP-Win32 device object and driver remain loaded, and will need
to be re-enabled after a reboot, or if the driver is unloaded and reloaded. This
directive can only be used by an administrator.
--show-valid-subnets
(Standalone) Show valid subnets for --dev tun emulation. Since the TAP-Win32
driver exports an ethernet interface to Windows, and since TUN devices are point-
to-point in nature, it is necessary for the TAP-Win32 driver to impose certain
constraints on TUN endpoint address selection.
Namely, the point-to-point endpoints used in TUN device emulation must be the mid‐
dle two addresses of a /30 subnet (netmask 255.255.255.252).
--show-net
(Standalone) Show OpenVPN's view of the system routing table and network adapter
list.
PKCS#11 Standalone Options:
--show-pkcs11-ids provider [cert_private]
(Standalone) Show PKCS#11 token object list. Specify cert_private as 1 if certifi‐
cates are stored as private objects.
--verb option can be used BEFORE this option to produce debugging information.
SCRIPTING AND ENVIRONMENTAL VARIABLES
OpenVPN exports a series of environmental variables for use by user-defined scripts.
Script Order of Execution
--up Executed after TCP/UDP socket bind and TUN/TAP open.
--tls-verify
Executed when we have a still untrusted remote peer.
--ipchange
Executed after connection authentication, or remote IP address change.
--client-connect
Executed in --mode server mode immediately after client authentication.
--route-up
Executed after connection authentication, either immediately after, or some number
of seconds after as defined by the --route-delay option.
--client-disconnect
Executed in --mode server mode on client instance shutdown.
--down Executed after TCP/UDP and TUN/TAP close.
--learn-address
Executed in --mode server mode whenever an IPv4 address/route or MAC address is
added to OpenVPN's internal routing table.
--auth-user-pass-verify
Executed in --mode server mode on new client connections, when the client is still
untrusted.
String Types and Remapping
In certain cases, OpenVPN will perform remapping of characters in strings. Essentially,
any characters outside the set of permitted characters for each string type will be con‐
verted to underbar ('_').
Q: Why is string remapping necessary?
A: It's an important security feature to prevent the malicious coding of strings from
untrusted sources to be passed as parameters to scripts, saved in the environment, used
as a common name, translated to a filename, etc.
Q: Can string remapping be disabled?
A: Yes, by using the --no-name-remapping option, however this should be considered an
advanced option.
Here is a brief rundown of OpenVPN's current string types and the permitted character
class for each string:
X509 Names: Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), at ('@'), colon (':'),
slash ('/'), and equal ('='). Alphanumeric is defined as a character which will cause
the C library isalnum() function to return true.
Common Names: Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), and at ('@').
--auth-user-pass username: Same as Common Name, with one exception: starting with OpenVPN
2.0.1, the username is passed to the OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY plugin in its
raw form, without string remapping.
--auth-user-pass password: Any "printable" character except CR or LF. Printable is
defined to be a character which will cause the C library isprint() function to return
true.
--client-config-dir filename as derived from common name or username: Alphanumeric,
underbar ('_'), dash ('-'), and dot ('.') except for "." or ".." as standalone strings.
As of 2.0.1-rc6, the at ('@') character has been added as well for compatibility with the
common name character class.
Environmental variable names: Alphanumeric or underbar ('_').
Environmental variable values: Any printable character.
For all cases, characters in a string which are not members of the legal character class
for that string type will be remapped to underbar ('_').
Environmental Variables
Once set, a variable is persisted indefinitely until it is reset by a new value or a
restart,
As of OpenVPN 2.0-beta12, in server mode, environmental variables set by OpenVPN are
scoped according to the client objects they are associated with, so there should not be
any issues with scripts having access to stale, previously set variables which refer to
different client instances.
bytes_received
Total number of bytes received from client during VPN session. Set prior to exe‐
cution of the --client-disconnect script.
bytes_sent
Total number of bytes sent to client during VPN session. Set prior to execution
of the --client-disconnect script.
common_name
The X509 common name of an authenticated client. Set prior to execution of
--client-connect, --client-disconnect, and --auth-user-pass-verify scripts.
config Name of first --config file. Set on program initiation and reset on SIGHUP.
daemon Set to "1" if the --daemon directive is specified, or "0" otherwise. Set on pro‐
gram initiation and reset on SIGHUP.
daemon_log_redirect
Set to "1" if the --log or --log-append directives are specified, or "0" other‐
wise. Set on program initiation and reset on SIGHUP.
dev The actual name of the TUN/TAP device, including a unit number if it exists. Set
prior to --up or --down script execution.
foreign_option_{n}
An option pushed via --push to a client which does not natively support it, such
as --dhcp-option on a non-Windows system, will be recorded to this environmental
variable sequence prior to --up script execution.
ifconfig_broadcast
The broadcast address for the virtual ethernet segment which is derived from the
--ifconfig option when --dev tap is used. Set prior to OpenVPN calling the ifcon‐
fig or netsh (windows version of ifconfig) commands which normally occurs prior to
--up script execution.
ifconfig_local
The local VPN endpoint IP address specified in the --ifconfig option (first param‐
eter). Set prior to OpenVPN calling the ifconfig or netsh (windows version of
ifconfig) commands which normally occurs prior to --up script execution.
ifconfig_remote
The remote VPN endpoint IP address specified in the --ifconfig option (second
parameter) when --dev tun is used. Set prior to OpenVPN calling the ifconfig or
netsh (windows version of ifconfig) commands which normally occurs prior to --up
script execution.
ifconfig_netmask
The subnet mask of the virtual ethernet segment that is specified as the second
parameter to --ifconfig when --dev tap is being used. Set prior to OpenVPN call‐
ing the ifconfig or netsh (windows version of ifconfig) commands which normally
occurs prior to --up script execution.
ifconfig_pool_local_ip
The local virtual IP address for the TUN/TAP tunnel taken from an --ifconfig-push
directive if specified, or otherwise from the ifconfig pool (controlled by the
--ifconfig-pool config file directive). Only set for --dev tun tunnels. This
option is set on the server prior to execution of the --client-connect and
--client-disconnect scripts.
ifconfig_pool_netmask
The virtual IP netmask for the TUN/TAP tunnel taken from an --ifconfig-push direc‐
tive if specified, or otherwise from the ifconfig pool (controlled by the --ifcon‐
fig-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-discon‐
nect scripts.
ifconfig_pool_remote_ip
The remote virtual IP address for the TUN/TAP tunnel taken from an --ifconfig-push
directive if specified, or otherwise from the ifconfig pool (controlled by the
--ifconfig-pool config file directive). This option is set on the server prior to
execution of the --client-connect and --client-disconnect scripts.
link_mtu
The maximum packet size (not including the IP header) of tunnel data in UDP tunnel
transport mode. Set prior to --up or --down script execution.
local The --local parameter. Set on program initiation and reset on SIGHUP.
local_port
The local port number, specified by --port or --lport. Set on program initiation
and reset on SIGHUP.
password
The password provided by a connecting client. Set prior to --auth-user-pass-ver‐
ify script execution only when the via-env modifier is specified, and deleted from
the environment after the script returns.
proto The --proto parameter. Set on program initiation and reset on SIGHUP.
remote_{n}
The --remote parameter. Set on program initiation and reset on SIGHUP.
remote_port_{n}
The remote port number, specified by --port or --rport. Set on program initiation
and reset on SIGHUP.
route_net_gateway
The pre-existing default IP gateway in the system routing table. Set prior to
--up script execution.
route_vpn_gateway
The default gateway used by --route options, as specified in either the --route-
gateway option or the second parameter to --ifconfig when --dev tun is specified.
Set prior to --up script execution.
route_{parm}_{n}
A set of variables which define each route to be added, and are set prior to --up
script execution.
parm will be one of "network", "netmask", "gateway", or "metric".
n is the OpenVPN route number, starting from 1.
If the network or gateway are resolvable DNS names, their IP address translations
will be recorded rather than their names as denoted on the command line or config‐
uration file.
peer_cert
Temporary file name containing the client certificate upon connection. Useful in
conjunction with --tls-verify
script_context
Set to "init" or "restart" prior to up/down script execution. For more informa‐
tion, see documentation for --up.
script_type
Prior to execution of any script, this variable is set to the type of script being
run. It can be one of the following: up, down, ipchange, route-up, tls-verify,
auth-user-pass-verify, client-connect, client-disconnect, or learn-address.
signal The reason for exit or restart. Can be one of sigusr1, sighup, sigterm, sigint,
inactive (controlled by --inactive option), ping-exit (controlled by --ping-exit
option), ping-restart (controlled by --ping-restart option), connection-reset
(triggered on TCP connection reset), error, or unknown (unknown signal). This
variable is set just prior to down script execution.
time_ascii
Client connection timestamp, formatted as a human-readable time string. Set prior
to execution of the --client-connect script.
time_duration
The duration (in seconds) of the client session which is now disconnecting. Set
prior to execution of the --client-disconnect script.
time_unix
Client connection timestamp, formatted as a unix integer date/time value. Set
prior to execution of the --client-connect script.
tls_id_{n}
A series of certificate fields from the remote peer, where n is the verification
level. Only set for TLS connections. Set prior to execution of --tls-verify
script.
tls_serial_{n}
The serial number of the certificate from the remote peer, where n is the verifi‐
cation level. Only set for TLS connections. Set prior to execution of --tls-ver‐
ify script. This is in the form of a hex string like "37AB46E0", which is suitable
for doing serial-based OCSP queries (with OpenSSL, you have to prepend "0x" to the
string). If something goes wrong while reading the value from the certificate it
will be an empty string, so your code should check that. See the con‐
trib/OCSP_check/OCSP_check.sh script for an example.
tun_mtu
The MTU of the TUN/TAP device. Set prior to --up or --down script execution.
trusted_ip
Actual IP address of connecting client or peer which has been authenticated. Set
prior to execution of --ipchange, --client-connect, and --client-disconnect
scripts.
trusted_port
Actual port number of connecting client or peer which has been authenticated. Set
prior to execution of --ipchange, --client-connect, and --client-disconnect
scripts.
untrusted_ip
Actual IP address of connecting client or peer which has not been authenticated
yet. Sometimes used to nmap the connecting host in a --tls-verify script to
ensure it is firewalled properly. Set prior to execution of --tls-verify and
--auth-user-pass-verify scripts.
untrusted_port
Actual port number of connecting client or peer which has not been authenticated
yet. Set prior to execution of --tls-verify and --auth-user-pass-verify scripts.
username
The username provided by a connecting client. Set prior to --auth-user-pass-ver‐
ify script execution only when the via-env modifier is specified.
X509_{n}_{subject_field}
An X509 subject field from the remote peer certificate, where n is the verifica‐
tion 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.
X509_0_emailAddress=me@myhost.mydomain
X509_0_CN=Test-Client
X509_0_O=OpenVPN-TEST
X509_0_ST=NA
X509_0_C=KG
X509_1_emailAddress=me@myhost.mydomain
X509_1_O=OpenVPN-TEST
X509_1_L=BISHKEK
X509_1_ST=NA
X509_1_C=KG
SIGNALS
SIGHUP Cause OpenVPN to close all TUN/TAP and network connections, restart, re-read the
configuration file (if any), and reopen TUN/TAP and network connections.
SIGUSR1
Like SIGHUP, except don't re-read configuration file, and possibly don't close and
reopen TUN/TAP device, re-read key files, preserve local IP address/port, or pre‐
serve 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 condition, governed by
the --ping-restart option.
This signal, when combined with --persist-remote-ip, may be sent when the underly‐
ing parameters of the host's network interface change such as when the host is a
DHCP client and is assigned a new IP address. See --ipchange above for more
information.
SIGUSR2
Causes OpenVPN to display its current statistics (to the syslog file if --daemon
is used, or stdout otherwise).
SIGINT, SIGTERM
Causes OpenVPN to exit gracefully.
TUN/TAP DRIVER SETUP
If you are running Linux 2.4.7 or higher, you probably have the TUN/TAP driver already
installed. If so, there are still a few things you need to do:
Make device: mknod /dev/net/tun c 10 200
Load driver: modprobe tun
EXAMPLES
Prior to running these examples, you should have OpenVPN installed on two machines with
network connectivity between them. If you have not yet installed OpenVPN, consult the
INSTALL file included in the OpenVPN distribution.
TUN/TAP Setup:
If you are using Linux 2.4 or higher, make the tun device node and load the tun module:
mknod /dev/net/tun c 10 200
modprobe tun
If you installed from RPM, the mknod step may be omitted, because the RPM install does
that for you.
Only Linux 2.4 and newer are supported.
For other platforms, consult the INSTALL file at https://openvpn.net/install.html for more
information.
Firewall Setup:
If firewalls exist between the two machines, they should be set to forward UDP port 1194
in both directions. If you do not have control over the firewalls between the two
machines, you may still be able to use OpenVPN by adding --ping 15 to each of the openvpn
commands used below in the examples (this will cause each peer to send out a UDP ping to
its remote peer once every 15 seconds which will cause many stateful firewalls to forward
packets in both directions without an explicit firewall rule).
If you are using a Linux iptables-based firewall, you may need to enter the following
command to allow incoming packets on the TUN device:
iptables -A INPUT -i tun+ -j ACCEPT
See the firewalls section below for more information on configuring firewalls for use
with OpenVPN.
VPN Address Setup:
For purposes of our example, our two machines will be called may.kg and june.kg. If you
are constructing a VPN over the internet, then replace may.kg and june.kg with the inter‐
net hostname or IP address that each machine will use to contact the other over the
internet.
Now we will choose the tunnel endpoints. Tunnel endpoints are private IP addresses that
only have meaning in the context of the VPN. Each machine will use the tunnel endpoint
of the other machine to access it over the VPN. In our example, the tunnel endpoint for
may.kg will be 10.4.0.1 and for june.kg, 10.4.0.2.
Once the VPN is established, you have essentially created a secure alternate path between
the two hosts which is addressed by using the tunnel endpoints. You can control which
network traffic passes between the hosts (a) over the VPN or (b) 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 may.kg and you wish to con‐
nect to june.kg via ssh without using the VPN (since ssh has its own built-in security)
you would use the command ssh june.kg. However in the same scenario, you could also use
the command telnet 10.4.0.2 to create a telnet session with june.kg over the VPN, that
would use the VPN to secure the session rather than ssh.
You can use any address you wish for the tunnel endpoints but make sure that they are
private addresses (such as those that begin with 10 or 192.168) and that they are not
part of any existing subnet on the networks of either peer, unless you are bridging. If
you use an address that is part of your local subnet for either of the tunnel endpoints,
you will get a weird feedback loop.
Example 1: A simple tunnel without security
On may:
openvpn --remote june.kg --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --verb 9
On june:
openvpn --remote may.kg --dev tun1 --ifconfig 10.4.0.2 10.4.0.1 --verb 9
Now verify the tunnel is working by pinging across the tunnel.
On may:
ping 10.4.0.2
On june:
ping 10.4.0.1
The --verb 9 option will produce verbose output, similar to the tcpdump(8) program. Omit
the --verb 9 option to have OpenVPN run quietly.
Example 2: A tunnel with static-key security (i.e. using a pre-shared secret)
First build a static key on may.
openvpn --genkey --secret key
This command will build a random key file called key (in ascii format). Now copy key to
june over a secure medium such as by using the scp(1) program.
On may:
openvpn --remote june.kg --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --verb 5 --secret
key
On june:
openvpn --remote may.kg --dev tun1 --ifconfig 10.4.0.2 10.4.0.1 --verb 5 --secret
key
Now verify the tunnel is working by pinging across the tunnel.
On may:
ping 10.4.0.2
On june:
ping 10.4.0.1
Example 3: A tunnel with full TLS-based security
For this test, we will designate may as the TLS client and june as the TLS server. Note
that client or server designation only has meaning for the TLS subsystem. It has no bear‐
ing 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 author‐
ity 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 author‐
ity certificates and keys included in the OpenVPN distribution are totally insecure and
should be used for testing only.
On may:
openvpn --remote june.kg --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --tls-client --ca
ca.crt --cert client.crt --key client.key --reneg-sec 60 --verb 5
On june:
openvpn --remote may.kg --dev tun1 --ifconfig 10.4.0.2 10.4.0.1 --tls-server --dh
dh1024.pem --ca ca.crt --cert server.crt --key server.key --reneg-sec 60 --verb 5
Now verify the tunnel is working by pinging across the tunnel.
On may:
ping 10.4.0.2
On june:
ping 10.4.0.1
Notice the --reneg-sec 60 option we used above. That tells OpenVPN to renegotiate the
data channel keys every minute. Since we used --verb 5 above, you will see status infor‐
mation on each new key negotiation.
For production operations, a key renegotiation interval of 60 seconds is probably too
frequent. Omit the --reneg-sec 60 option to use OpenVPN's default key renegotiation
interval of one hour.
Routing:
Assuming you can ping across the tunnel, the next step is to route a real subnet over the
secure tunnel. Suppose that may and june have two network interfaces each, one connected
to the internet, and the other to a private network. Our goal is to securely connect
both private networks. We will assume that may's private subnet is 10.0.0.0/24 and
june's is 10.0.1.0/24.
First, ensure that IP forwarding is enabled on both peers. On Linux, enable routing:
echo 1 > /proc/sys/net/ipv4/ip_forward
and enable TUN packet forwarding through the firewall:
iptables -A FORWARD -i tun+ -j ACCEPT
On may:
route add -net 10.0.1.0 netmask 255.255.255.0 gw 10.4.0.2
On june:
route add -net 10.0.0.0 netmask 255.255.255.0 gw 10.4.0.1
Now any machine on the 10.0.0.0/24 subnet can access any machine on the 10.0.1.0/24 sub‐
net over the secure tunnel (or vice versa).
In a production environment, you could put the route command(s) in a shell script and
execute with the --up option.
FIREWALLS
OpenVPN's usage of a single UDP port makes it fairly firewall-friendly. You should add
an entry to your firewall rules to allow incoming OpenVPN packets. On Linux 2.4+:
iptables -A INPUT -p udp -s 1.2.3.4 --dport 1194 -j ACCEPT
This will allow incoming packets on UDP port 1194 (OpenVPN's default UDP port) from an
OpenVPN peer at 1.2.3.4.
If you are using HMAC-based packet authentication (the default in any of OpenVPN's secure
modes), having the firewall filter on source address can be considered optional, since
HMAC packet authentication is a much more secure method of verifying the authenticity of
a packet source. In that case:
iptables -A INPUT -p udp --dport 1194 -j ACCEPT
would be adequate and would not render the host inflexible with respect to its peer hav‐
ing 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 fire‐
wall 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 net‐
work,
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 net‐
work.
These rules are secure if you use packet authentication, since no incoming packets will
arrive on a TUN or TAP virtual device unless they first pass an HMAC authentication test.
FAQ
https://openvpn.net/faq.html
HOWTO
For a more comprehensive guide to setting up OpenVPN in a production setting, see the
OpenVPN HOWTO at https://openvpn.net/howto.html
PROTOCOL
For a description of OpenVPN's underlying protocol, see https://openvpn.net/security.html
WEB
OpenVPN's web site is at https://openvpn.net/
Go here to download the latest version of OpenVPN, subscribe to the mailing lists, read
the mailing list archives, or browse the SVN repository.
BUGS
Report all bugs to the OpenVPN team <info@openvpn.net>.
SEE ALSO
dhcpcd(8), ifconfig(8), openssl(1), route(8), scp(1) ssh(1)
NOTES
This product includes software developed by the OpenSSL Project ( http://www.openssl.org/
)
For more information on the TLS protocol, see http://www.ietf.org/rfc/rfc2246.txt
For more information on the LZO real-time compression library see http://www.ober‐
humer.com/opensource/lzo/
COPYRIGHT
Copyright (C) 2002-2010 OpenVPN Technologies, Inc. This program is free software; you can
redistribute it and/or modify it under the terms of the GNU General Public License ver‐
sion 2 as published by the Free Software Foundation.
AUTHORS
James Yonan <jim@yonan.net>