SYNOPSIS
openvpn [ --help ]
openvpn [ --config file ]
openvpn [ --genkey ] [ --secret file ]
openvpn [ --mktun ] [ --rmtun ] [ --dev tunX | tapX ]
[ --dev-type device-type ] [ --dev-node node ]
openvpn [ --test-crypto ] [ --secret file ] [ --auth alg ]
[ --cipher alg ] [ --engine ] [ --keysize n ] [ --no-replay ]
[ --no-iv ]
openvpn [ --askpass [file] ] [ --auth-nocache ] [ --auth-retry type ]
[ --auth-user-pass-verify script ] [ --auth-user-pass up ]
[ --auth alg ] [ --bcast-buffers n ] [ --ca file ]
[ --ccd-exclusive ] [ --cd dir ] [ --cert file ] [ --chroot dir ]
[ --cipher alg ] [ --client-cert-not-required ]
[ --client-config-dir dir ] [ --client-connect script ]
[ --client-disconnect ] [ --client-to-client ] [ --client ]
[ --comp-lzo ] [ --comp-noadapt ] [ --config file ]
[ --connect-freq n sec ] [ --connect-retry n ] [ --crl-verify crl ]
[ --cryptoapicert select-string ] [ --daemon [progname] ]
[ --dev-node node ] [ --dev-type device-type ]
[ --dev tunX | tapX | null ] [ --dev tunX | tapX ] [ --dh file ]
[ --dhcp-option type [parm] ] [ --dhcp-release ] [ --dhcp-renew ]
[ --disable-occ ] [ --disable ] [ --down-pre ] [ --down cmd ]
[ --duplicate-cn ] [ --echo [parms...] ] [ --engine [engine-name] ]
[ --explicit-exit-notify [n] ] [ --fast-io ] [ --float ]
[ --fragment max ] [ --genkey ] [ --group group ]
[ --hand-window n ] [ --hash-size r v ] [ --help ]
[ --http-proxy-option type [parm] ] [ --http-proxy-retry ]
[ --http-proxy-timeout n ]
[ --http-proxy server port [authfile] [auth-method] ]
[ --ifconfig-noexec ] [ --ifconfig-nowarn ]
[ --ifconfig-pool-linear ]
[ --ifconfig-pool-persist file [seconds] ]
[ --ifconfig-pool start-IP end-IP [netmask] ]
[ --ifconfig-push local remote-netmask ] [ --ifconfig l rn ]
[ --inactive n ] [ --inetd [wait|nowait] [progname] ]
[ --ip-win32 method ] [ --ipchange cmd ]
[ --iroute network [netmask] ] [ --keepalive n m ]
[ --key-method m ] [ --key file ] [ --keysize n ]
[ --learn-address cmd ] [ --link-mtu n ] [ --local host ]
[ --log-append file ] [ --log file ] [ --suppress-timestamps ]
[ --lport port ] [ --management-hold ] [ --management-log-cache n ]
[ --management-query-passwords ] [ --management IP port [pw-file] ]
[ --max-clients n ] [ --max-routes-per-client n ] [ --mktun ]
[ --mlock ] [ --mode m ] [ --mssfix max ] [ --mtu-disc type ]
[ --mtu-test ] [ --mute-replay-warnings ] [ --mute n ] [ --nice n ]
[ --no-iv ] [ --no-replay ] [ --nobind ]
[ --secret file [direction] ] [ --secret file ]
[ --server-bridge gateway netmask pool-start-IP pool-end-IP ]
[ --server network netmask ] [ --service exit-event [0|1] ]
[ --setenv name value ] [ --shaper n ] [ --show-adapters ]
[ --show-ciphers ] [ --show-digests ] [ --show-engines ]
[ --show-net-up ] [ --show-net ] [ --show-tls ]
[ --show-valid-subnets ] [ --single-session ] [ --sndbuf size ]
[ --socks-proxy-retry ] [ --socks-proxy server [port] ]
[ --status file [n] ] [ --status-version n ]
[ --syslog [progname] ] [ --tap-sleep n ] [ --tcp-queue-limit n ]
[ --test-crypto ] [ --tls-auth file [direction] ]
[ --tls-cipher l ] [ --tls-client ] [ --tls-exit ]
[ --tls-remote x509name ] [ --tls-server ] [ --tls-timeout n ]
[ --tls-verify cmd ] [ --tmp-dir dir ] [ --tran-window n ]
[ --tun-ipv6 ] [ --tun-mtu-extra n ] [ --tun-mtu n ]
[ --txqueuelen n ] [ --up-delay ] [ --up-restart ] [ --up cmd ]
[ --user user ] [ --username-as-common-name ] [ --verb n ]
[ --writepid file ]
INTRODUCTION
OpenVPN is an open source VPN daemon by James Yonan. Because OpenVPN
tries to be a universal VPN tool offering a great deal of flexibility,
there are a lot of options on this manual page. If you're new to Open-
VPN, you might want to skip ahead to the examples section where you
will see how to construct simple VPNs on the command line without even
needing a configuration file.
Also note that there's more documentation and examples on the OpenVPN
web site: http://openvpn.net/
And if you would like to see a shorter version of this manual, see the
openvpn usage message which can be obtained by running openvpn without
any parameters.
DESCRIPTION
OpenVPN is a robust and highly flexible VPN daemon. OpenVPN supports
SSL/TLS security, ethernet bridging, TCP or UDP tunnel transport
through proxies or NAT, support for dynamic IP addresses and DHCP,
scalability to hundreds or thousands of users, and portability to most
major OS platforms.
OpenVPN is tightly bound to the OpenSSL library, and derives much of
its crypto capabilities from it.
OpenVPN supports conventional encryption using a pre-shared secret key
(Static Key mode) or public key security (SSL/TLS mode) using client &
server certificates. OpenVPN also supports non-encrypted TCP/UDP tun-
nels.
OpenVPN is designed to work with the TUN/TAP virtual networking inter-
face that exists on most platforms.
moved.
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 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, so the following mappings should be observed:
\\ Maps to a single backslash character (\).
\" Pass a literal doublequote character ("), don't
interpret it as enclosing a parameter.
\[SPACE] Pass a literal space or tab character, don't
interpret it as a parameter delimiter.
For example on Windows, use double backslashes to represent pathnames:
secret "c:\\OpenVPN\\secret.key"
For examples of configuration files, see http://openvpn.net/exam-
ples.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
--remote host [port]
Remote host name or IP address. On the client, multiple --re-
mote options may be specified for redundancy, each referring to
a different OpenVPN server.
The OpenVPN client will try to connect to a server at host:port
in the order specified by the list of --remote options.
The client will move on to the next host in the list, in the
event of connection failure. Note that at any given time, the
OpenVPN client will at most be connected to one server.
Note that since UDP is connectionless, connection failure is de-
fined by the --ping and --ping-restart options.
Note the following corner case: If you use multiple --remote
options, AND you are dropping root privileges on the client with
--user and/or --group, AND the client is running a non-Windows
OS, if the client needs to switch to a different server, and
that server pushes back different TUN/TAP or route settings, the
client may lack the necessary privileges to close and reopen the
TUN/TAP interface. This could cause the client to exit with a
fatal error.
If --remote is unspecified, OpenVPN will listen for packets from
any IP address, but will not act on those packets unless they
pass all authentication tests. This requirement for authentica-
tion is binding on all potential peers, even those from known
and supposedly trusted IP addresses (it is very easy to forge a
source IP address on a UDP packet).
When used in TCP mode, --remote will act as a filter, rejecting
connections from any host which does not match host.
If host is a DNS name which resolves to multiple IP addresses,
one will be randomly chosen, providing a sort of basic load-bal-
ancing and failover capability.
--remote-random
When multiple --remote address/ports are specified, initially
randomize the order of the list as a kind of basic load-balanc-
ing 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.
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 advan-
tageous from a security and robustness perspective, such as tun-
neling non-IP or application-level UDP protocols, or tunneling
protocols which don't possess a built-in reliability layer.
--connect-retry n
For --proto tcp-client, take n as the number of seconds to wait
between connection retries (default=5).
--http-proxy server port [authfile] [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-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 op-
tions.
VERSION version -- Set HTTP version number to version (de-
fault=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 er-
ror occurs, simulate a SIGUSR1 reset.
--resolv-retry n
If hostname resolve fails for --remote, retry resolve for n sec-
onds before failing.
Set n to "infinite" to retry indefinitely.
ets 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 initial-
ly authenticated or changes.
Execute as:
cmd ip_address port_number
Don't use --ipchange in --mode server mode. Use a --client-con-
nect script instead.
See the "Environmental Variables" section below for additional
parameters passed as environmental variables.
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 con-
figure our IP address change script (see man page for dhcpcd(8)
) to deliver a SIGHUP or SIGUSR1 signal to OpenVPN. OpenVPN
will then reestablish a connection with its most recently au-
thenticated peer on its new IP address.
--port port
TCP/UDP port number for both local and remote. The current de-
fault of 1194 represents the official IANA port number assign-
ment for OpenVPN and has been used since version 2.0-beta17.
Previous versions used port 5000 as the default.
--lport port
TCP/UDP port number for local.
--rport port
TCP/UDP port number for remote.
--nobind
Do not bind to local address and port. The IP stack will allo-
cate a dynamic port for returning packets. Since the value of
the dynamic port could not be known in advance by a peer, this
option is only suitable for peers which will be initiating con-
nections by using the --remote option.
--dev-type device-type
Which device type are we using? device-type should be tun or
tap. Use this option only if the TUN/TAP device used with --dev
does not begin with tun or tap.
--tun-ipv6
Build a tun link capable of forwarding IPv6 traffic. Should be
used in conjunction with --dev tun or --dev tunX. A warning
will be displayed if no specific IPv6 TUN support for your OS
has been compiled into OpenVPN.
--dev-node node
Explicitly set the device node rather than using /dev/net/tun,
/dev/tun, /dev/tap, etc. If OpenVPN cannot figure out whether
node is a TUN or TAP device based on the name, you should also
specify --dev-type tun or --dev-type tap.
On Windows systems, select the TAP-Win32 adapter which is named
node in the Network Connections Control Panel or the raw GUID of
the adapter enclosed by braces. The --show-adapters option un-
der Windows can also be used to enumerate all available TAP-
Win32 adapters and will show both the network connections con-
trol panel name and the GUID for each TAP-Win32 adapter.
--ifconfig l rn
Set TUN/TAP adapter parameters. l is the IP address of the lo-
cal VPN endpoint. For TUN devices, rn is the IP address of the
remote VPN endpoint. For TAP devices, rn is the subnet mask of
the virtual ethernet segment which is being created or connected
to.
For TUN devices, which facilitate virtual point-to-point IP con-
nections, the proper usage of --ifconfig is to use two private
IP addresses which are not a member of any existing subnet which
is in use. The IP addresses may be consecutive and should have
their order reversed on the remote peer. After the VPN is es-
tablished, by pinging rn, you will be pinging across the VPN.
For TAP devices, which provide the ability to create virtual
ethernet segments, --ifconfig is used to set an IP address and
subnet mask just as a physical ethernet adapter would be simi-
larly configured. If you are attempting to connect to a remote
ethernet bridge, the IP address and subnet should be set to val-
ues which would be valid on the the bridged ethernet segment
(note also that DHCP can be used for the same purpose).
This option, while primarily a proxy for the ifconfig(8) com-
mand, is designed to simplify TUN/TAP tunnel configuration by
providing a standard interface to the different ifconfig imple-
mentations on different platforms.
--ifconfig parameters which are IP addresses can also be speci-
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 seman-
tics across OpenVPN's platform space.
netmask default -- 255.255.255.255
gateway default -- taken from --route-gateway or the second pa-
rameter to --ifconfig when --dev tun is specified.
The default can be specified by leaving an option blank or set-
ting it to "default".
The network and gateway parameters can also be specified as a
DNS or /etc/hosts file resolvable name, or as one of three spe-
cial keywords:
vpn_gateway -- The remote VPN endpoint address (derived either
from --route-gateway or the second parameter to --ifconfig when
--dev tun is specified).
net_gateway -- The pre-existing IP default gateway, read from
the routing table (not supported on all OSes).
remote_host -- The --remote address if OpenVPN is being run in
client mode, and is undefined in server mode.
--route-gateway gw
Specify a default gateway gw for use with --route.
--route-delay [n] [w]
Delay n seconds (default=0) after connection establishment, be-
fore 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.)
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.
--redirect-gateway ["local"] ["def1"]
(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 for-
wards to the pre-existing default gateway. This is done so that
(3) will not create a routing loop.
(2) Delete the default gateway route.
(3) Set the new default gateway to be the VPN endpoint address
(derived either from --route-gateway or the second parameter to
--ifconfig when --dev tun is specified).
When the tunnel is torn down, all of the above steps are re-
versed so that the original default route is restored.
Add the local flag if both OpenVPN servers are directly connect-
ed via a common subnet, such as with wireless. The local flag
will cause step 1 above to be omitted.
Add the def1 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.
Using the def1 flag is highly recommended, and is currently
planned to become the default by OpenVPN 2.1.
--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 un-
less 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
parameter only controls internal OpenVPN buffer sizing, so there
is no transmission overhead associated with using a larger val-
ue.
--mtu-disc type
Should we do Path MTU discovery on TCP/UDP channel? Only sup-
ported on OSes such as Linux that supports the necessary system
call to set.
'no' -- Never send DF (Don't Fragment) frames
'maybe' -- Use per-route hints
'yes' -- Always DF (Don't Fragment)
--mtu-test
To empirically measure MTU on connection startup, add the --mtu-
test option to your configuration. OpenVPN will send ping pack-
ets of various sizes to the remote peer and measure the largest
packets which were successfully received. The --mtu-test
process normally takes about 3 minutes to complete.
--fragment max
Enable internal datagram fragmentation so that no UDP datagrams
are sent which are larger than max bytes.
The max parameter is interpreted in the same way as the --link-
mtu parameter, i.e. the UDP packet size after encapsulation
overhead has been added in, but not including the UDP header it-
self.
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 op-
tion is less efficient than fixing path MTU discovery for your
IP link and using native IP fragmentation instead.
Having said that, there are circumstances where using OpenVPN's
internal fragmentation capability may be your only option, such
as tunneling a UDP multicast stream which requires fragmenta-
tion.
--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
protocols other than TCP), --fragment will internally fragment
them.
Both --fragment and --mssfix are designed to work around cases
where Path MTU discovery is broken on the network path between
OpenVPN peers.
The usual symptom of such a breakdown is an OpenVPN connection
which successfully starts, but then stalls during active usage.
If --fragment and --mssfix are used together, --mssfix will take
its default max parameter from the --fragment max option.
Therefore, one could lower the maximum UDP packet size to 1300
(a good first try for solving MTU-related connection problems)
with the following options:
--tun-mtu 1500 --fragment 1300 --mssfix
--sndbuf size
Set the TCP/UDP socket send buffer size. Currently defaults to
65536 bytes.
--rcvbuf size
Set the TCP/UDP socket receive buffer size. Currently defaults
to 65536 bytes.
--txqueuelen n
(Linux only) Set the TX queue length on the TUN/TAP interface.
Currently defaults to 100.
--shaper n
Limit bandwidth of outgoing tunnel data to n bytes per second on
the TCP/UDP port. If you want to limit the bandwidth in both
directions, use this option on both peers.
OpenVPN uses the following algorithm to implement traffic shap-
ing: Given a shaper rate of n bytes per second, after a datagram
write of b bytes is queued on the TCP/UDP port, wait a minimum
of (b / n) seconds before queuing the next write.
It should be noted that OpenVPN supports multiple tunnels be-
tween the same two peers, allowing you to construct full-speed
and reduced bandwidth tunnels at the same time, routing low-pri-
ority data such as off-site backups over the reduced bandwidth
tunnel, and other data over the full-speed tunnel.
Also note that for low bandwidth tunnels (under 1000 bytes per
second), you should probably use lower MTU values as well (see
above), otherwise the packet latency will grow so large as to
trigger timeouts in the TLS layer and TCP connections running
over the tunnel.
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 dy-
namic IP address and a low-TTL DNS name is used to track the IP
address using a service such as http://dyndns.org/ + a dynamic
DNS client such as ddclient.
If the peer cannot be reached, a restart will be triggered,
causing the hostname used with --remote to be re-resolved (if
--resolv-retry is also specified).
In server mode, --ping-restart, --inactive, or any other type of
internally generated signal will always be applied to individual
client instance objects, never to whole server itself. Note al-
so in server mode that any internally generated signal which
would normally cause a restart, will cause the deletion of the
client instance object instead.
In client mode, the --ping-restart parameter is set to 120 sec-
onds by default. This default will hold until the client pulls
a replacement value from the server, based on the --keepalive
setting in the server configuration. To disable the 120 second
default, set --ping-restart 0 on the client.
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 re-
mote address. Use this option if you are starting the daemon in
listen mode (i.e. without an explicit --remote peer), and you
don't want to start clocking timeouts until a remote peer con-
nects.
--persist-tun
Don't close and reopen TUN/TAP device or run up/down scripts
across SIGUSR1 or --ping-restart restarts.
SIGUSR1 is a restart signal similar to SIGHUP, but which offers
finer-grained control over reset options.
--persist-key
Don't re-read key files across SIGUSR1 or --ping-restart.
This option can be combined with --user nobody to allow restarts
triggered by the SIGUSR1 signal. Normally if you drop root
privileges in OpenVPN, the daemon cannot be restarted since it
will now be unable to re-read protected key files.
This option solves the problem by persisting keys across SIGUSR1
resets, so they don't need to be re-read.
--persist-local-ip
Preserve initially resolved local IP address and port number
across SIGUSR1 or --ping-restart restarts.
--persist-remote-ip
Preserve most recently authenticated remote IP address and port
number across SIGUSR1 or --ping-restart restarts.
--mlock
Disable paging by calling the POSIX mlockall function. Requires
that OpenVPN be initially run as root (though OpenVPN can subse-
quently downgrade its UID using the --user option).
Using this option ensures that key material and tunnel data are
route commands which route IP traffic destined for private sub-
nets 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_re-
mote_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 context, the last command line parameter passed
to the script will be init. If the --up-restart option is also
used, the up script will be called for restarts as well. A
restart is considered to be a partial reinitialization of Open-
VPN where the TUN/TAP instance is preserved (the --persist-tun
option will enable such preservation). A restart can be gener-
ated by a SIGUSR1 signal, a --ping-restart timeout, or a connec-
tion reset when the TCP protocol is enabled with the --proto op-
tion. 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 auto-
matically ifconfig the TUN device, eliminating the need to de-
fine an --up script, unless you also want to configure routes in
the --up script.
If --ifconfig is also specified, OpenVPN will pass the ifconfig
local and remote endpoints on the command line to the --up
script so that they can be used to configure routes such as:
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 documentation.
--setenv name value
Set a custom environmental variable name=value to pass to
script.
--disable-occ
Don't output a warning message if option inconsistencies are de-
tected between peers. An example of an option inconsistency
would be where one peer uses --dev tun while the other peer uses
--dev tap.
Use of this option is discouraged, but is provided as a tempo-
rary fix in situations where a recent version of OpenVPN must
connect to an old version.
--user user
Change the user ID of the OpenVPN process to user after initial-
ization, dropping privileges in the process. This option is
useful to protect the system in the event that some hostile par-
ty was able to gain control of an OpenVPN session. Though Open-
VPN'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 re-
turn them to an OpenVPN session. This means, for example, that
if you want to reset an OpenVPN daemon with a SIGUSR1 signal
(for example in response to a DHCP reset), you should make use
of one or more of the --persist options to ensure that OpenVPN
doesn't need to execute any privileged operations in order to
restart (such as re-reading key files or running ifconfig on the
TUN device).
--group group
Chroot to dir after initialization. --chroot essentially rede-
fines dir as being the top level directory tree (/). OpenVPN
will therefore be unable to access any files outside this tree.
This can be desirable from a security standpoint.
Since the chroot operation is delayed until after initializa-
tion, most OpenVPN options that reference files will operate in
a pre-chroot context.
In many cases, the dir parameter can point to an empty directo-
ry, however complications can result when scripts or restarts
are executed after the chroot operation.
--daemon [progname]
Become a daemon after all initialization functions are complet-
ed. 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 redirect-
ion occurs immediately at the point that --daemon is parsed on
the command line even though the daemonization point occurs lat-
er. If one of the --log options is present, it will supercede
syslog redirection.
The optional progname parameter will cause OpenVPN to report its
program name to the system logger as progname. This can be use-
ful in linking OpenVPN messages in the syslog file with specific
tunnels. When unspecified, progname defaults to "openvpn".
When OpenVPN is run with the --daemon option, it will try to de-
lay 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 ini-
tialization are non-fatal.
--syslog [progname]
Direct log output to system logger, but do not become a daemon.
See --daemon directive above for description of progname parame-
ter.
--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.
Also note that in wait mode, each OpenVPN tunnel requires a sep-
arate TCP/UDP port and a separate inetd or xinetd entry. See
the OpenVPN 1.x HOWTO for an example on using OpenVPN with
xinetd: http://openvpn.net/1xhowto.html
--log file
Output logging messages to file, including output to std-
out/stderr which is generated by called scripts. If file al-
ready exists it will be truncated. This option takes effect im-
mediately when it is parsed in the command line and will su-
percede 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, log-
ging occurs by default without the need to specify this option.
--log-append file
Append logging messages to file. If file does not exist, it
will be created. This option behaves exactly like --log except
that it appends to rather than truncating the log file.
--suppress-timestamps
Avoid writing timestamps to log messages, even when they other-
wise would be prepended. In particular, this applies to log mes-
sages sent to stdout.
--writepid file
Write OpenVPN's main process ID to file.
--nice n
Change process priority after initialization ( n greater than 0
is lower priority, n less than zero is higher priority).
--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 de-
vice or socket is ready to accept the write. Such blocking is
unnecessary on some platforms which don't support write blocking
on UDP sockets or TUN/TAP devices. In such cases, one can opti-
mize the event loop by avoiding the poll/epoll/select call, im-
proving CPU efficiency by 5% to 10%.
This option can only be used on non-Windows systems, when --pro-
to udp is specified, and when --shaper is NOT specified.
--echo [parms...]
Echo parms to log output.
a good summary of what's happening without being swamped by out-
put.
0 -- No output except fatal errors.
1 to 4 -- Normal usage range.
5 -- Output R and W characters to the console for each packet
read and write, uppercase is used for TCP/UDP packets and lower-
case is used for TUN/TAP packets.
6 to 11 -- Debug info range (see errlevel.h for additional in-
formation 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 or 2 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
Use fast LZO compression -- may add up to 1 byte per packet for
incompressible data.
--comp-noadapt
When used in conjunction with --comp-lzo, this option will dis-
able OpenVPN's adaptive compression algorithm. Normally, adap-
tive compression is enabled with --comp-lzo.
Adaptive compression tries to optimize the case where you have
compression enabled, but you are sending predominantly uncom-
pressible (or pre-compressed) packets over the tunnel, such as
an FTP or rsync transfer of a large, compressed file. With
adaptive compression, OpenVPN will periodically sample the com-
pression process to measure its efficiency. If the data being
sent over the tunnel is already compressed, the compression ef-
ficiency will be very low, triggering openvpn to disable com-
pression for a period of time until the next re-sample test.
--management IP port [pw-file]
Enable a TCP server on IP:port to handle daemon management func-
tions. pw-file, if specified, is a password file (password on
first line) or "stdin" to prompt from standard input. The pass-
word provided will set the password which TCP clients will need
to provide in order to access management functions.
The management interface provides a special mode where the TCP
It is strongly recommended that IP be set to 127.0.0.1 (local-
host) to restrict accessibility of the management server to lo-
cal clients.
--management-query-passwords
Query management channel for private key password and --auth-us-
er-pass username/password. Only query the management channel
for inputs which ordinarily would have been queried from the
console.
--management-hold
Start OpenVPN in a hibernating state, until a client of the man-
agement interface explicitly starts it with the hold release
command.
--management-log-cache n
Cache the most recent n lines of log file history for usage by
the management channel.
--plugin module-pathname [init-string]
Load plug-in module from the file module-pathname, passing init-
string as an argument to the module initialization function.
Multiple plugin modules may be loaded into one OpenVPN process.
For more information and examples on how to build OpenVPN plug-
in modules, see the README file in the plugin folder of the
OpenVPN source distribution.
If you are using an RPM install of OpenVPN, see /usr/share/open-
vpn/plugin. The documentation is in doc and the actual plugin
modules are in lib.
Multiple plugin modules can be cascaded, and modules can be used
in tandem with scripts. The modules will be called by OpenVPN
in the order that they are declared in the config file. If both
a plugin and script are configured for the same callback, the
script will be called last. If the return code of the mod-
ule/script controls an authentication function (such as tls-ver-
ify, auth-user-pass-verify, or client-connect), then every mod-
ule and script must return success (0) in order for the connec-
tion to be authenticated.
Server Mode
Starting with OpenVPN 2.0, a multi-client TCP/UDP server mode is sup-
ported, and can be enabled with the --mode server option. In server
mode, OpenVPN will listen on a single port for incoming client connec-
tions. All client connections will be routed through a single tun or
tap interface. This mode is designed for scalability and should be
able to support hundreds or even thousands of clients on sufficiently
fast hardware. SSL/TLS authentication must be used in this mode.
--server network netmask
ifconfig 10.8.0.1 10.8.0.2
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
push "route 10.8.0.1"
if dev tap:
ifconfig 10.8.0.1 255.255.255.0
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
A helper directive similar to --server which is designed to sim-
plify the configuration of OpenVPN's server mode in ethernet
bridging configurations.
To configure ethernet bridging, you must first use your OS's
bridging capability to bridge the TAP interface with the ether-
net NIC interface. For example, on Linux this is done with the
brctl tool, and with Windows XP it is done in the Network Con-
nections Panel by selecting the ethernet and TAP adapters and
right-clicking on "Bridge Connections".
Next you you must manually set the IP/netmask on the bridge in-
terface. The gateway and netmask parameters to --server-bridge
can be set to either the IP/netmask of the bridge interface, or
the IP/netmask of the default gateway/router on the bridged sub-
net.
Finally, set aside a IP range in the bridged subnet, denoted by
pool-start-IP and pool-end-IP, for OpenVPN to allocate to con-
necting clients.
For example, server-bridge 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"
--push option
--ping-restart, --setenv, --persist-key, --persist-tun, --echo
--push-reset
Don't inherit the global push list for a specific client in-
stance. Specify this option in a client-specific context such
as with a --client-config-dir configuration file. This option
will ignore --push options at the global config file level.
--disable
Disable a particular client (based on the common name) from con-
necting. Don't use this option to disable a client due to key
or password compromise. Use a CRL (certificate revocation list)
instead (see the --crl-verify option).
This option must be associated with a specific client instance,
which means that it must be specified either in a client in-
stance config file using --client-config-dir or dynamically gen-
erated using a --client-connect script.
--ifconfig-pool start-IP end-IP [netmask]
Set aside a pool of subnets to be dynamically allocated to con-
necting clients, similar to a DHCP server. For tun-style tun-
nels, each client will be given a /30 subnet (for interoperabil-
ity with Windows clients). For tap-style tunnels, individual
addresses will be allocated, and the optional netmask parameter
will also be pushed to clients.
--ifconfig-pool-persist file [seconds]
Persist/unpersist ifconfig-pool data to file, at seconds inter-
vals (default=600), as well as on program startup and shutdown.
The goal of this option is to provide a long-term association
between clients (denoted by their common name) and the virtual
IP address assigned to them from the ifconfig-pool. Maintaining
a long-term association is good for clients because it allows
them to effectively use the --persist-tun option.
file is a comma-delimited ASCII file, formatted as <Common-
Name>,<IP-address>.
If seconds = 0, file will be treated as read-only. This is use-
ful if you would like to treat file as a configuration file.
Note that the entries in this file are treated by OpenVPN as
suggestions only, based on past associations between a common
name and IP address. They do not guarantee that the given com-
mon name will always receive the given IP address. If you want
guaranteed assignment, use --ifconfig-push
--ifconfig-pool-linear
Modifies the --ifconfig-pool directive to allocate individual
This option must be associated with a specific client instance,
which means that it must be specified either in a client in-
stance config file using --client-config-dir or dynamically gen-
erated 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 pa-
rameter, if omitted, defaults to 255.255.255.255.
This directive can be used to route a fixed subnet from the
server to a particular client, regardless of where the client is
connecting from. Remember that you must also add the route to
the system routing table as well (such as by using the --route
directive). The reason why two routes are needed is that the
--route directive routes the packet from the kernel to OpenVPN.
Once in OpenVPN, the --iroute directive routes to the specific
client.
This option must be specified either in a client instance config
file using --client-config-dir or dynamically generated using a
--client-connect script.
The --iroute directive also has an important interaction with
--push "route ...". --iroute essentially defines a subnet which
is owned by a particular client (we will call this client A).
If you would like other clients to be able to reach A's subnet,
you can use --push "route ..." together with --client-to-client
to effect this. In order for all clients to see A's subnet,
OpenVPN must push this route to all clients EXCEPT for A, since
the subnet is already owned by A. OpenVPN accomplishes this by
not not pushing a route to a client if it matches one of the
client's iroutes.
--client-to-client
Because the OpenVPN server mode handles multiple clients through
a single tun or tap interface, it is effectively a router. The
--client-to-client flag tells OpenVPN to internally route
client-to-client traffic rather than pushing all client-origi-
Run script on client connection. The script is passed the com-
mon name and IP address of the just-authenticated client as en-
vironmental variables (see environmental variable section be-
low). The script is also passed the pathname of a not-yet-cre-
ated temporary file as $1 (i.e. the first command line argu-
ment), 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 ap-
plied 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 plug-
ins (if defined) were previously called on this instance with
successful (0) status returns.
The exception to this rule is if the --client-disconnect script
or plugins are cascaded, and at least one client-connect func-
tion succeeded, then ALL of the client-disconnect functions for
scripts and plugins will be called on client instance object
deletion, even in cases where some of the related client-connect
functions returned an error status.
--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.
This file can specify a fixed IP address for a given client us-
ing --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:
buckets.
--bcast-buffers n
Allocate n buffers for broadcast datagrams (default=256).
--tcp-queue-limit n
Maximum number of queued TCP output packets (default=64).
When OpenVPN is tunneling data from a TUN/TAP device to a remote
client over a TCP connection, it is possible that the TUN/TAP
device might produce data at a faster rate than the TCP connec-
tion can support. When the number of queued TCP output packets
reaches this limit for a given client connection, OpenVPN will
start to drop outgoing packets directed at this client.
--max-clients n
Limit server to a maximum of n concurrent clients.
--max-routes-per-client n
Allow a maximum of n internal routes per client (default=256).
This is designed to help contain DoS attacks where an authenti-
cated client floods the server with packets appearing to come
from many unique MAC addresses, forcing the server to deplete
virtual memory as its internal routing table expands. This di-
rective can be used in a --client-config-dir file or auto-gener-
ated by a --client-connect script to override the global value
for a particular client.
Note that this directive affects OpenVPN's internal routing ta-
ble, not the kernel routing table.
--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 ad-
dresses 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.
face. Since OpenVPN provides the association between virtual IP
or MAC address and the client's authenticated common name, it
allows a user-defined script to configure firewall access poli-
cies with regard to the client's high-level common name, rather
than the low level client virtual addresses.
--auth-user-pass-verify 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 user-
name/password strings provided by the client. Be aware that
this method is insecure on some platforms which make the envi-
ronment of a process publicly visible to other unprivileged pro-
cesses.
If method is set to "via-file", OpenVPN will write the username
and password to the first two lines of a temporary file. The
filename will be passed as an argument to script, and the file
will be automatically deleted by OpenVPN after the script re-
turns. The location of the temporary file is controlled by the
--tmp-dir option, and will default to the current directory if
unspecified. For security, consider setting --tmp-dir to a
volatile storage medium such as /dev/shm (if available) to pre-
vent the username/password file from touching the hard drive.
The script should examine the username and password, returning a
success exit code (0) if the client's authentication request is
to be accepted, or a failure code (1) to reject the client.
This directive is designed to enable a plugin-style interface
for extending OpenVPN's authentication capabilities.
To protect against a client passing a maliciously formed user-
name or password string, the username string must consist only
of these characters: alphanumeric, underbar ('_'), dash ('-'),
dot ('.'), or at ('@'). The password string can consist of any
printable characters except for CR or LF. Any illegal charac-
ters in either the username or password string will be converted
to underbar ('_').
Care must be taken by any user-defined scripts to avoid creating
a security vulnerability in the way that these strings are han-
dled. Never use these strings in such a way that they might be
escaped or evaluated by a shell interpreter.
For a sample script that performs PAM authentication, see sam-
ple-scripts/auth-pam.pl in the OpenVPN source distribution.
--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 authenticat-
ed username as the common name, rather than the common name from
the client cert.
Client Mode
Use client mode when connecting to an OpenVPN server which has --serv-
er, --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 ac-
cept options pushed by the server, provided they are part of the
legal set of pushable options (note that the --pull option is
implied by --client ).
In particular, --pull allows the server to push routes to the
client, so you should not use --pull or --client in situations
where you don't trust the server to have control over the
client's routing table.
--auth-user-pass [up]
Authenticate with server using username/password. up is a file
containing username/password on 2 lines (Note: OpenVPN will only
read passwords from a file if it has been built with the --en-
able-password-save configure option, or on Windows by defining
ENABLE_PASSWORD_SAVE in config-win32.h).
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 mes-
sage from the server or verification failure of the private key
password.
Normally used to prevent auth errors from being fatal on the
for unattended clients.
interact -- Client will requery for an --auth-user-pass user-
name/password and/or private key password before attempting a
reconnection.
Note that while this option cannot be pushed, it can be con-
trolled from the management interface.
--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 retries that the client will at-
tempt to resend the exit notification message.
Data Channel Encryption Options:
These options are meaningful for both Static & TLS-negotiated key modes
(must be compatible between peers).
--secret file [direction]
Enable Static Key encryption mode (non-TLS). Use pre-shared se-
cret file which was generated with --genkey.
The optional direction parameter enables the use of 4 distinct
keys (HMAC-send, cipher-encrypt, HMAC-receive, cipher-decrypt),
so that each data flow direction has a different set of HMAC and
cipher keys. This has a number of desirable security properties
including eliminating certain kinds of DoS and message replay
attacks.
When the direction parameter is omitted, 2 keys are used bidi-
rectionally, one for HMAC and the other for encryption/decryp-
tion.
The direction parameter should always be complementary on either
side of the connection, i.e. one side should use "0" and the
other should use "1", or both sides should omit it altogether.
The direction parameter requires that file contains a 2048 bit
key. While pre-1.5 versions of OpenVPN generate 1024 bit key
files, any version of OpenVPN which supports the direction pa-
rameter, will also support 2048 bit key file generation using
the --genkey option.
Static key encryption mode has certain advantages, the primary
being ease of configuration.
There are no certificates or certificate authorities or compli-
cated negotiation handshakes and protocols. The only require-
ment is that you have a pre-existing secure channel with your
ed by OpenVPN. Anyone eavesdropping on the wire would see noth-
ing but random-looking data.
--auth alg
Authenticate packets with HMAC using message digest algorithm
alg. (The default is SHA1 ). HMAC is a commonly used message
authentication algorithm (MAC) that uses a data string, a secure
hash algorithm, and a key, to produce a digital signature.
OpenVPN's usage of HMAC is to first encrypt a packet, then HMAC
the resulting ciphertext.
In static-key encryption mode, the HMAC key is included in the
key file generated by --genkey. In TLS mode, the HMAC key is
dynamically generated and shared between peers via the TLS con-
trol channel. If OpenVPN receives a packet with a bad HMAC it
will drop the packet. HMAC usually adds 16 or 20 bytes per
packet. Set alg=none to disable authentication.
For more information on HMAC see
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 al-
lowing key sizes of up to 448 bits. Blowfish is designed to be
used in situations where keys are changed infrequently.
For more information on blowfish, see http://www.counter-
pane.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.
Set alg=none to disable encryption.
--keysize n
Size of cipher key in bits (optional). If unspecified, defaults
to cipher-specific default. The --show-ciphers option (see be-
low) shows all available OpenSSL ciphers, their default key
sizes, and whether the key size can be changed. Use care in
changing a cipher's default key size. Many ciphers have not
been extensively cryptanalyzed with non-standard key lengths,
and a larger key may offer no real guarantee of greater securi-
ty, or may even reduce security.
--engine [engine-name]
Enable OpenSSL hardware-based crypto engine functionality.
for the uniqueness of the identifier. If the identifier was al-
ready received in a previous datagram, OpenVPN will drop the
packet. Replay protection is important to defeat attacks such
as a SYN flood attack, where the attacker listens in the wire,
intercepts a TCP SYN packet (identifying it by the context in
which it occurs in relation to other packets), then floods the
receiving peer with copies of this packet.
OpenVPN's replay protection is implemented in slightly different
ways, depending on the key management mode you have selected.
In Static Key mode or when using an CFB or OFB mode cipher,
OpenVPN uses a 64 bit unique identifier that combines a time
stamp with an incrementing sequence number.
When using TLS mode for key exchange and a CBC cipher mode,
OpenVPN uses only a 32 bit sequence number without a time stamp,
since OpenVPN can guarantee the uniqueness of this value for
each key. As in IPSec, if the sequence number is close to wrap-
ping back to zero, OpenVPN will trigger a new key exchange.
To check for replays, OpenVPN uses the sliding window algorithm
used by IPSec.
--replay-window n [t]
Use a replay protection sliding-window of size n and a time win-
dow of t seconds.
By default n is 64 (the IPSec default) and t is 15 seconds.
This option is only relevant in UDP mode, i.e. when either
--proto udp is specifed, or no --proto option is specified.
When OpenVPN tunnels IP packets over UDP, there is the possibil-
ity that packets might be dropped or delivered out of order.
Because OpenVPN, like IPSec, is emulating the physical network
layer, it will accept an out-of-order packet sequence, and will
deliver such packets in the same order they were received to the
TCP/IP protocol stack, provided they satisfy several con-
straints.
(a) The packet cannot be a replay (unless --no-replay is speci-
fied, which disables replay protection altogether).
(b) If a packet arrives out of order, it will only be accepted
if the difference between its sequence number and the highest
sequence number received so far is less than n.
(c) If a packet arrives out of order, it will only be accepted
if it arrives no later than t seconds after any packet contain-
ing a higher sequence number.
capsulated protocol from attacks which masquerade as the kinds
of normal packet loss and reordering that occur over IP net-
works?
The IPSec and OpenVPN approach is to allow packet reordering
within a certain fixed sequence number window.
OpenVPN adds to the IPSec model by limiting the window size in
time as well as sequence space.
OpenVPN also adds TCP transport as an option (not offered by
IPSec) in which case OpenVPN can adopt a very strict attitude
towards message deletion and reordering: Don't allow it. Since
TCP guarantees reliability, any packet loss or reordering event
can be assumed to be an attack.
In this sense, it could be argued that TCP tunnel transport is
preferred when tunneling non-IP or UDP application protocols
which might be vulnerable to a message deletion or reordering
attack which falls within the normal operational parameters of
IP networks.
So I would make the statement that one should never tunnel a
non-IP protocol or UDP application protocol over UDP, if the
protocol might be vulnerable to a message deletion or reordering
attack that falls within the normal operating parameters of what
is to be expected from the physical IP layer. The problem is
easily fixed by simply using TCP as the VPN transport layer.
--mute-replay-warnings
Silence the output of replay warnings, which are a common false
alarm on WiFi networks. This option preserves the security of
the replay protection code without the verbosity associated with
warnings about duplicate packets.
--replay-persist file
Persist replay-protection state across sessions using file to
save and reload the state.
This option will strengthen protection against replay attacks,
especially when you are using OpenVPN in a dynamic context (such
as with --inetd) when OpenVPN sessions are frequently started
and stopped.
This option will keep a disk copy of the current replay protec-
tion state (i.e. the most recent packet timestamp and sequence
number received from the remote peer), so that if an OpenVPN
session is stopped and restarted, it will reject any replays of
packets which were already received by the prior session.
This option only makes sense when replay protection is enabled
(the default) and you are using either --secret (shared-secret
In CBC mode, OpenVPN uses a pseudo-random IV for each packet.
In CFB/OFB mode, OpenVPN uses a unique sequence number and time
stamp as the IV. In fact, in CFB/OFB mode, OpenVPN uses a data-
gram space-saving optimization that uses the unique identifier
for datagram replay protection as the IV.
--test-crypto
Do a self-test of OpenVPN's crypto options by encrypting and de-
crypting test packets using the data channel encryption options
specified above. This option does not require a peer to func-
tion, and therefore can be specified without --dev or --remote.
The typical usage of --test-crypto would be something like this:
openvpn --test-crypto --secret key
or
openvpn --test-crypto --secret key --verb 9
This option is very useful to test OpenVPN after it has been
ported to a new platform, or to isolate problems in the compil-
er, OpenSSL crypto library, or OpenVPN's crypto code. Since it
is a self-test mode, problems with encryption and authentication
can be debugged independently of network and tunnel issues.
TLS Mode Options:
TLS mode is the most powerful crypto mode of OpenVPN in both security
and flexibility. TLS mode works by establishing control and data chan-
nels which are multiplexed over a single TCP/UDP port. OpenVPN initi-
ates a TLS session over the control channel and uses it to exchange ci-
pher and HMAC keys to protect the data channel. TLS mode uses a robust
reliability layer over the UDP connection for all control channel com-
munication, while the data channel, over which encrypted tunnel data
passes, is forwarded without any mediation. The result is the best of
both worlds: a fast data channel that forwards over UDP with only the
overhead of encrypt, decrypt, and HMAC functions, and a control channel
that provides all of the security features of TLS, including certifi-
cate-based authentication and Diffie Hellman forward secrecy.
To use TLS mode, each peer that runs OpenVPN should have its own local
certificate/key pair ( --cert and --key ), signed by the root certifi-
cate which is specified in --ca.
When two OpenVPN peers connect, each presents its local certificate to
the other. Each peer will then check that its partner peer presented a
certificate which was signed by the master root certificate as speci-
fied in --ca.
If that check on both peers succeeds, then the TLS negotiation will
succeed, both OpenVPN peers will exchange temporary session keys, and
--tls-client
Enable TLS and assume client role during TLS handshake.
--ca file
Certificate authority (CA) file in .pem format, also referred to
as the root certificate. This file can have multiple certifi-
cates in .pem format, concatenated together. You can construct
your own certificate authority certificate and private key by
using a command such as:
openssl req -nodes -new -x509 -keyout tmp-ca.key -out tmp-ca.crt
Then edit your openssl.cnf file and edit the certificate vari-
able to point to your new root certificate tmp-ca.crt.
For testing purposes only, the OpenVPN distribution includes a
sample CA certificate (tmp-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.
--dh file
File containing Diffie Hellman parameters in .pem format (re-
quired for --tls-server only). Use
openssl dhparam -out dh1024.pem 1024
to generate your own, or use the existing dh1024.pem file in-
cluded with the OpenVPN distribution. Diffie Hellman parameters
may be considered public.
--cert file
Local peer's signed certificate in .pem format -- must be signed
by a certificate authority whose certificate is in --ca file.
Each peer in an OpenVPN link running in TLS mode should have its
own certificate and private key file. In addition, each cer-
tificate should have been signed by the key of a certificate au-
thority whose public key resides in the --ca certificate author-
ity 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 ma-
chine, copy the certificate signing request (mycert.csr) to this
other machine (this can be done over an insecure channel such as
email). Now sign the certificate with a command such as:
openssl ca -out mycert.crt -in mycert.csr
--pkcs12 file
Specify a PKCS #12 file containing local private key, local cer-
tificate, and root CA certificate. This option can be used in-
stead of --ca, --cert, and --key.
--cryptoapicert select-string
Load the certificate and private key from the Windows Certifi-
cate System Store (Windows Only).
Use this option instead of --cert and --key.
This makes it possible to use any smart card, supported by Win-
dows, but also any kind of certificate, residing in the Cert
Store, where you have access to the private key. This option
has been tested with a couple of different smart cards (GemSAFE,
Cryptoflex, and Swedish Post Office eID) on the client side, and
also an imported PKCS12 software certificate on the server side.
To select a certificate, based on a substring search in the cer-
tificate's subject:
cryptoapicert "SUBJ:Peter Runestig"
To select a certificate, based on certificate's thumbprint:
cryptoapicert "THUMB:f6 49 24 41 01 b4 ..."
The thumbprint hex string can easily be copy-and-pasted from the
Windows Certificate Store GUI.
--key-method m
Use data channel key negotiation method m. The key method must
match on both sides of the connection.
After OpenVPN negotiates a TLS session, a new set of keys for
protecting the tunnel data channel is generated and exchanged
over the TLS session.
In method 1 (the default for OpenVPN 1.x), both sides generate
random encrypt and HMAC-send keys which are forwarded to the
other host over the TLS channel.
In method 2, (the default for OpenVPN 2.0) the client generates
a random key. Both client and server also generate some random
seed material. All key source material is exchanged over the
TLS channel. The actual keys are generated using the TLS PRF
function, taking source entropy from both client and server.
Method 2 is designed to closely parallel the key generation
process used by TLS 1.0.
Note that in TLS mode, two separate levels of keying occur:
this parameter manually, to prevent a version rollback attack
where a man-in-the-middle attacker tries to force two peers to
negotiate to the lowest level of security they both support.
Use --show-tls to see a list of supported TLS ciphers.
--tls-timeout n
Packet retransmit timeout on TLS control channel if no acknowl-
edgment from remote within n seconds (default=2). When OpenVPN
sends a control packet to its peer, it will expect to receive an
acknowledgement within n seconds or it will retransmit the pack-
et, 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 acknowl-
edged, sequenced, or retransmitted by OpenVPN because the higher
level network protocols running on top of the tunnel such as TCP
expect this role to be left to them.
--reneg-bytes n
Renegotiate data channel key after n bytes sent or received
(disabled by default). OpenVPN allows the lifetime of a key to
be expressed as a number of bytes encrypted/decrypted, a number
of packets, or a number of seconds. A key renegotiation will be
forced if any of these three criteria are met by either peer.
--reneg-pkts n
Renegotiate data channel key after n packets sent and received
(disabled by default).
--reneg-sec n
Renegotiate data channel key after n seconds (default=3600).
--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 af-
ter a new a key renegotiation begins (default = 3600 seconds).
This feature allows for a graceful transition from old to new
key, and removes the key renegotiation sequence from the criti-
cal path of tunnel data forwarding.
--single-session
After initially connecting to a remote peer, disallow any new
connections. Using this option means that a remote peer cannot
connect, disconnect, and then reconnect.
OpenVPN's TCP/UDP port, where TLS control channel packets bear-
ing an incorrect HMAC signature can be dropped immediately with-
out response.
file (required) is a key file which can be in one of two for-
mats:
(1) An OpenVPN static key file generated by --genkey (required
if direction parameter is used).
(2) A freeform passphrase file. In this case the HMAC key will
be derived by taking a secure hash of this file, similar to the
md5sum(1) or sha1sum(1) commands.
OpenVPN will first try format (1), and if the file fails to
parse as a static key file, format (2) will be used.
See the --secret option for more information on the optional di-
rection parameter.
--tls-auth is recommended when you are running OpenVPN in a mode
where it is listening for packets from any IP address, such as
when --remote is not specified, or --remote is specified with
--float.
The rationale for this feature is as follows. TLS requires a
multi-packet exchange before it is able to authenticate a peer.
During this time before authentication, OpenVPN is allocating
resources (memory and CPU) to this potential peer. The poten-
tial peer is also exposing many parts of OpenVPN and the OpenSSL
library to the packets it is sending. Most successful network
attacks today seek to either exploit bugs in programs (such as
buffer overflow attacks) or force a program to consume so many
resources that it becomes unusable. Of course the first line of
defense is always to produce clean, well-audited code. OpenVPN
has been written with buffer overflow attack prevention as a top
priority. But as history has shown, many of the most widely
used network applications have, from time to time, fallen to
buffer overflow attacks.
So as a second line of defense, OpenVPN offers this special lay-
er of authentication on top of the TLS control channel so that
every packet on the control channel is authenticated by an HMAC
signature and a unique ID for replay protection. This signature
will also help protect against DoS (Denial of Service) attacks.
An important rule of thumb in reducing vulnerability to DoS at-
tacks is to minimize the amount of resources a potential, but as
yet unauthenticated, client is able to consume.
--tls-auth does this by signing every TLS control channel packet
with an HMAC signature, including packets which are sent before
the TLS level has had a chance to authenticate the peer. The
nize.
For the extremely security conscious, it is possible to protect
your private key with a password. Of course this means that ev-
ery time the OpenVPN daemon is started you must be there to type
the password. The --askpass option allows you to start OpenVPN
from the command line. It will query you for a password before
it daemonizes. To protect a private key with a password you
should omit the -nodes option when you use the openssl command
line tool to manage certificates and private keys.
If file is specified, read the password from the first line of
file. Keep in mind that storing your password in a file to a
certain extent invalidates the extra security provided by using
an encrypted key (Note: OpenVPN will only read passwords from a
file if it has been built with the --enable-password-save con-
figure option, or on Windows by defining ENABLE_PASSWORD_SAVE in
config-win32.h).
--auth-nocache
Don't cache --askpass or --auth-user-pass username/passwords in
virtual memory.
If specified, this directive will cause OpenVPN to immediately
forget username/password inputs after they are used. As a re-
sult, when OpenVPN needs a username/password, it will prompt for
input from stdin, which may be multiple times during the dura-
tion of an OpenVPN session.
This directive does not affect the --http-proxy username/pass-
word. 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 cer-
tification (except for revocation via --crl-verify directive;
the revocation test occurs after the --tls-verify test).
cmd should return 0 to allow the TLS handshake to proceed, or 1
to fail. cmd is executed as
cmd certificate_depth X509_NAME_oneline
This feature is useful if the peer you want to trust has a cer-
tificate which was signed by a certificate authority who also
signed many other certificates, where you don't necessarily want
to trust all of them, but rather be selective about which peer
certificate you will accept. This feature allows you to write a
script which will test the X509 name on a certificate and decide
whether or not it should be accepted. For a simple perl script
which will test the common name field on the certificate, see
the file verify-cn in the OpenVPN distribution.
client to only accept connections to "Server-1", "Server-2",
etc., you can simply use --tls-remote Server
Using a common name prefix is a useful alternative to managing a
CRL (Certificate Revocation List) on the client, since it allows
the client to refuse all certificates except for those associat-
ed 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 --ch-
root 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 certificate with the nsCertType field set to
"server".
If the server certificate's nsCertType field is set to "server",
then the clients can verify this with --ns-cert-type server.
This is an important security precaution to protect against a
man-in-the-middle attack where an authorized client attempts to
connect to another client by impersonating the server. The at-
tack is easily prevented by having clients verify the server
certificate using any one of --ns-cert-type, --tls-remote, or
--tls-verify.
--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 con-
taining a client key and certificate was stolen. By adding the
stolen certificate to the CRL file, you could reject any connec-
tion which attempts to use it, while preserving the overall in-
tegrity 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
celeration 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 se-
cret, 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 sup-
port them such as Linux. Normally TUN/TAP tunnels exist only
for the period of time that an application has them open. This
option takes advantage of the TUN/TAP driver's ability to build
persistent tunnels that live through multiple instantiations of
OpenVPN and die only when they are deleted or the machine is re-
booted.
One of the advantages of persistent tunnels is that they elimi-
nate the need for separate --up and --down scripts to run the
appropriate ifconfig(8) and route(8) commands. These commands
can be placed in the the same shell script which starts or ter-
minates an OpenVPN session.
Another advantage is that open connections through the TUN/TAP-
based tunnel will not be reset if the OpenVPN peer restarts.
This can be useful to provide uninterrupted connectivity through
the tunnel in the event of a DHCP reset of the peer's public IP
address (see the --ipchange option above).
One disadvantage of persistent tunnels is that it is harder to
automatically configure their MTU value (see --link-mtu and
--tun-mtu above).
On some platforms such as Windows, TAP-Win32 tunnels are persis-
tent by default.
--rmtun
(Standalone) Remove a persistent tunnel.
--dev tunX | tapX
TUN/TAP device
tion for setting the TCP/IP properties since it uses the well-
known DHCP protocol. There are, however, two prerequisites for
using this mode: (1) The TCP/IP properties for the TAP-Win32
adapter must be set to "Obtain an IP address automatically," and
(2) OpenVPN needs to claim an IP address in the subnet for use
as the virtual DHCP server address. By default in --dev tap
mode, OpenVPN will take the normally unused first address in the
subnet. For example, if your subnet is 192.168.4.0 netmask
255.255.255.0, then OpenVPN will take the IP address 192.168.4.0
to use as the virtual DHCP server address. In --dev tun mode,
OpenVPN will cause the DHCP server to masquerade as if it were
coming from the remote endpoint. The optional offset parameter
is an integer which is > -256 and < 256 and which defaults to 0.
If offset is positive, the DHCP server will masquerade as the IP
address at network address + offset. If offset is negative, the
DHCP server will masquerade as the IP address at broadcast ad-
dress + offset. The Windows ipconfig /all command can be used
to show what Windows thinks the DHCP server address is. OpenVPN
will "claim" this address, so make sure to use a free address.
Having said that, different OpenVPN instantiations, including
different ends of the same connection, can share the same virtu-
al DHCP server address. The lease-time parameter controls the
lease time of the DHCP assignment given to the TAP-Win32
adapter, and is denoted in seconds. Normally a very long lease
time is preferred because it prevents routes involving the TAP-
Win32 adapter from being lost when the system goes to sleep.
The default lease time is one year.
netsh -- Automatically set the IP address and netmask using the
Windows command-line "netsh" command. This method appears to
work correctly on Windows XP but not Windows 2000.
ipapi -- Automatically set the IP address and netmask using the
Windows IP Helper API. This approach does not have ideal seman-
tics, though testing has indicated that it works okay in prac-
tice. If you use this option, it is best to leave the TCP/IP
properties for the TAP-Win32 adapter in their default state,
i.e. "Obtain an IP address automatically."
--route-method m
Which method m to use for adding routes on Windows?
ipapi (default) -- 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. 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 serv-
er across the VPN.
col). Repeat this option to set secondary NTP server addresses.
NBT type -- Set NetBIOS over TCP/IP Node type. Possible op-
tions: 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 be-
fore the up script is called, under the name "foreign_op-
tion_{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 op-
tion is normally unnecessary, as Windows automatically triggers
a DHCP renegotiation on the TAP adapter when it comes up, howev-
er 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.
exit-event is the name of a Windows global event object, and
OpenVPN will continuously monitor the state of this event object
and exit when it becomes signaled.
The second parameter indicates the initial state of exit-event
and normally defaults to 0.
Multiple OpenVPN processes can be simultaneously executed with
the same exit-event parameter. In any case, the controlling
process can signal exit-event, causing all such OpenVPN process-
es to exit.
When executing an OpenVPN process using the --service directive,
OpenVPN will probably not have a console window to output sta-
tus/error messages, therefore it is useful to use --log or
--log-append to write these messages to a file.
--show-adapters
(Standalone) Show available TAP-Win32 adapters which can be se-
lected using the --dev-node option. On non-Windows systems, the
ifconfig(8) command provides similar functionality.
--show-valid-subnets
(Standalone) Show valid subnets for --dev tun emulation. Since
the TAP-Win32 driver exports an ethernet interface to Windows,
and since TUN devices are point-to-point in nature, it is neces-
sary for the TAP-Win32 driver to impose certain constraints on
TUN endpoint address selection.
Namely, the point-to-point endpoints used in TUN device emula-
tion must be the middle two addresses of a /30 subnet (netmask
255.255.255.252).
--show-net
(Standalone) Show OpenVPN's view of the system routing table and
network adapter list.
SCRIPTING AND ENVIRONMENTAL VARIABLES
OpenVPN exports a series of environmental variables for use by user-de-
fined 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 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 converted to underbar ('_').
Q: Why is string remapping necessary?
A: It's an important security feature to prevent the malicious coding
of strings from untrusted sources to be passed as parameters to
scripts, saved in the environment, used as a common name, translated to
a filename, etc.
Here is a brief rundown of OpenVPN's current string types and the per-
mitted character class for each string:
X509 Names: Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), at
('@'), colon (':'), slash ('/'), and equal ('='). Alphanumeric is de-
fined 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 OPEN-
VPN_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 le-
gal character class for that string type will be remapped to underbar
('_').
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 re-
set on SIGHUP.
daemon Set to "1" if the --daemon directive is specified, or "0" other-
wise. Set on program initiation and reset on SIGHUP.
daemon_log_redirect
Set to "1" if the --log or --log-append directives are speci-
fied, or "0" otherwise. Set on program initiation and reset on
SIGHUP.
dev The actual name of the TUN/TAP device, including a unit number
if it exists. Set prior to --up or --down script execution.
foreign_option_{n}
An option pushed via --push to a client which does not natively
support it, such as --dhcp-option on a non-Windows system, will
be recorded to this environmental variable sequence prior to
--up script execution.
ifconfig_broadcast
The broadcast address for the virtual ethernet segment which is
derived from the --ifconfig option when --dev tap is used. Set
prior to OpenVPN calling the ifconfig or netsh (windows version
of ifconfig) commands which normally occurs prior to --up script
execution.
ifconfig_local
The local VPN endpoint IP address specified in the --ifconfig
option (first parameter). Set prior to OpenVPN calling the if-
config 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 ifcon-
fig) commands which normally occurs prior to --up script execu-
tion.
ifconfig_netmask
The subnet mask of the virtual ethernet segment that is speci-
The virtual IP netmask for the TUN/TAP tunnel taken from an
--ifconfig-push directive if specified, or otherwise from the
ifconfig pool (controlled by the --ifconfig-pool config file di-
rective). Only set for --dev tap tunnels. This option is set
on the server prior to execution of the --client-connect and
--client-disconnect scripts.
ifconfig_pool_remote_ip
The remote virtual IP address for the TUN/TAP tunnel taken from
an --ifconfig-push directive if specified, or otherwise from the
ifconfig pool (controlled by the --ifconfig-pool config file di-
rective). This option is set on the server prior to execution
of the --client-connect and --client-disconnect scripts.
link_mtu
The maximum packet size (not including the IP header) of tunnel
data in UDP tunnel transport mode. Set prior to --up or --down
script execution.
local The --local parameter. Set on program initiation and reset on
SIGHUP.
local_port
The local port number, specified by --port or --lport. Set on
program initiation and reset on SIGHUP.
password
The password provided by a connecting client. Set prior to
--auth-user-pass-verify script execution only when the via-env
modifier is specified, and deleted from the environment after
the script returns.
proto The --proto parameter. Set on program initiation and reset on
SIGHUP.
remote_{n}
The --remote parameter. Set on program initiation and reset on
SIGHUP.
remote_port_{n}
The remote port number, specified by --port or --rport. Set on
program initiation and reset on SIGHUP.
route_net_gateway
The pre-existing default IP gateway in the system routing table.
Set prior to --up script execution.
route_vpn_gateway
The default gateway used by --route options, as specified in ei-
ther the --route-gateway option or the second parameter to --if-
config when --dev tun is specified. Set prior to --up script
execution.
script_context
Set to "init" or "restart" prior to up/down script execution.
For more information, see documentation for --up.
script_type
One of up, down, ipchange, route-up, tls-verify, auth-user-pass-
verify, client-connect, client-disconnect, or learn-address.
Set prior to execution of any script.
signal The reason for exit or restart. Can be one of sigusr1, sighup,
sigterm, sigint, inactive (controlled by --inactive option),
ping-exit (controlled by --ping-exit option), ping-restart (con-
trolled by --ping-restart option), connection-reset (triggered
on TCP connection reset), error, or unknown (unknown signal).
This variable is set just prior to down script execution.
tls_id_{n}
A series of certificate fields from the remote peer, where n is
the verification level. Only set for TLS connections. Set pri-
or to execution of --tls-verify script.
tls_serial_{n}
The serial number of the certificate from the remote peer, where
n is the verification level. Only set for TLS connections. Set
prior to execution of --tls-verify script.
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 proper-
ly. 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.
thenticated remote IP address/port based on --persist-tun,
--persist-key, --persist-local-ip, and --persist-remote-ip op-
tions respectively (see above).
This signal may also be internally generated by a timeout condi-
tion, governed by the --ping-restart option.
This signal, when combined with --persist-remote-ip, may be sent
when the underlying parameters of the host's network interface
change such as when the host is a DHCP client and is assigned a
new IP address. See --ipchange above for more information.
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
If you have Linux 2.2 or earlier, you should obtain version 1.1 of the
TUN/TAP driver from http://vtun.sourceforge.net/tun/ and follow the in-
stallation instructions.
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.
If you have Linux 2.2, you should obtain version 1.1 of the TUN/TAP
driver from http://vtun.sourceforge.net/tun/ and follow the installa-
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 internet hostname or IP address that each
machine will use to contact the other over the internet.
Now we will choose the tunnel endpoints. Tunnel endpoints are private
IP addresses that only have meaning in the context of the VPN. Each
machine will use the tunnel endpoint of the other machine to access it
over the VPN. In our example, the tunnel endpoint for may.kg will be
10.4.0.1 and for june.kg, 10.4.0.2.
Once the VPN is established, you have essentially created a secure al-
ternate path between the two hosts which is addressed by using the tun-
nel 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 connect to june.kg via ssh without using the VPN (since
ssh has its own built-in security) you would use the command ssh
june.kg. However in the same scenario, you could also use the command
telnet 10.4.0.2 to create a telnet session with june.kg over the VPN,
that would use the VPN to secure the session rather than ssh.
You can use any address you wish for the tunnel endpoints but make sure
that they are private addresses (such as those that begin with 10 or
192.168) and that they are not part of any existing subnet on the net-
works of either peer, unless you are bridging. If you use an address
that is part of your local subnet for either of the tunnel endpoints,
you will get a weird feedback loop.
Example 1: A simple tunnel without security
On may:
openvpn --remote 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.
First build a static key on may.
openvpn --genkey --secret key
This command will build a random key file called key (in ascii format).
Now copy key to june over a secure medium such as by using the scp(1)
program.
On may:
openvpn --remote june.kg --dev tun1 --ifconfig 10.4.0.1 10.4.0.2
--verb 5 --secret key
On june:
openvpn --remote may.kg --dev tun1 --ifconfig 10.4.0.2 10.4.0.1
--verb 5 --secret key
Now verify the tunnel is working by pinging across the tunnel.
On may:
ping 10.4.0.2
On june:
ping 10.4.0.1
Example 3: A tunnel with full TLS-based security
For this test, we will designate may as the TLS client and june as the
TLS server. Note that client or server designation only has meaning
for the TLS subsystem. It has no bearing on OpenVPN's peer-to-peer,
UDP-based communication model.
First, build a separate certificate/key pair for both may and june (see
above where --cert is discussed for more info). Then construct Diffie
Hellman parameters (see above where --dh is discussed for more info).
You can also use the included test files client.crt, client.key, serv-
er.crt, server.key and tmp-ca.crt. The .crt files are certifi-
cates/public-keys, the .key files are private keys, and tmp-ca.crt is a
certification authority who has signed both client.crt and server.crt.
For Diffie Hellman parameters you can use the included file dh1024.pem.
Note that all client, server, and certificate authority certificates
and keys included in the OpenVPN distribution are totally insecure and
should be used for testing only.
On may:
openvpn --remote june.kg --dev tun1 --ifconfig 10.4.0.1 10.4.0.2
--tls-client --ca tmp-ca.crt --cert client.crt --key client.key
--reneg-sec 60 --verb 5
ping 10.4.0.1
Notice the --reneg-sec 60 option we used above. That tells OpenVPN to
renegotiate the data channel keys every minute. Since we used --verb 5
above, you will see status information on each new key negotiation.
For production operations, a key renegotiation interval of 60 seconds
is probably too frequent. Omit the --reneg-sec 60 option to use Open-
VPN's default key renegotiation interval of one hour.
Routing:
Assuming you can ping across the tunnel, the next step is to route a
real subnet over the secure tunnel. Suppose that may and june have two
network interfaces each, one connected to the internet, and the other
to a private network. Our goal is to securely connect both private
networks. We will assume that may's private subnet is 10.0.0.0/24 and
june's is 10.0.1.0/24.
First, ensure that IP forwarding is enabled on both peers. On Linux,
enable routing:
echo 1 > /proc/sys/net/ipv4/ip_forward
and enable TUN packet forwarding through the firewall:
iptables -A FORWARD -i tun+ -j ACCEPT
On may:
route add -net 10.0.1.0 netmask 255.255.255.0 gw 10.4.0.2
On june:
route add -net 10.0.0.0 netmask 255.255.255.0 gw 10.4.0.1
Now any machine on the 10.0.0.0/24 subnet can access any machine on the
10.0.1.0/24 subnet over the secure tunnel (or vice versa).
In a production environment, you could put the route command(s) in a
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 Open-
VPN 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
peer at least once every n seconds. If n is less than the stateful
firewall connection timeout, you can maintain an OpenVPN connection in-
definitely without explicit firewall rules.
You should also add firewall rules to allow incoming IP traffic on TUN
or TAP devices such as:
iptables -A INPUT -i tun+ -j ACCEPT
to allow input packets from tun devices,
iptables -A FORWARD -i tun+ -j ACCEPT
to allow input packets from tun devices to be forwarded to other hosts
on the local network,
iptables -A INPUT -i tap+ -j ACCEPT
to allow input packets from tap devices, and
iptables -A FORWARD -i tap+ -j ACCEPT
to allow input packets from tap devices to be forwarded to other hosts
on the local network.
These rules are secure if you use packet authentication, since no in-
coming packets will arrive on a TUN or TAP virtual device unless they
first pass an HMAC authentication test.
FAQ
http://openvpn.net/faq.html
HOWTO
For a more comprehensive guide to setting up OpenVPN in a production
setting, see the OpenVPN HOWTO at http://openvpn.net/howto.html
PROTOCOL
For a description of OpenVPN's underlying protocol, see http://open-
vpn.net/security.html
WEB
OpenVPN's web site is at http://openvpn.net/
Go here to download the latest version of OpenVPN, subscribe to the
mailing lists, read the mailing list archives, or browse the CVS repos-
itory.
BUGS
Report all bugs to the OpenVPN users list <openvpn-users@lists.source-
forge.net>. To subscribe to the list or see the archives, go to
http://openvpn.net/mail.html
Copyright (C) 2002-2005 OpenVPN Solutions LLC. This program is free
software; you can redistribute it and/or modify it under the terms of
the GNU General Public License version 2 as published by the Free Soft-
ware Foundation.
AUTHORS
James Yonan <jim@yonan.net>
3 August 2005 openvpn(8)
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