OpenVPN

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  • add IPv6 example

OpenVPN (Open Virtual Private Network) is software that enables the creation of secure point-to-point or site-to-site connections. This document describes the most common setup configuring a secured, routed VPN connection, using the linux tun interface and the net-vpn/openvpn version 2.6.x ebuild.

Installation

Kernel

KERNEL Enable CONFIG_TUN in the kernel
Device Drivers  --->
    [*] Network device support  --->
        [*] Network core driver support
        <*>   Universal TUN/TAP device driver support

USE flags

USE flags for net-vpn/openvpn Robust and highly flexible tunneling application compatible with many OSes

+lz4 Enable support for lz4 compression (as implemented in app-arch/lz4)
+lzo Enable support for lzo compression
+openssl Use OpenSSL as the backend crypto library
+plugins Enable the OpenVPN plugin system
dco Enable support for kernel data channel offload
down-root Enable the down-root plugin
examples Install examples, usually source code
inotify Enable inotify filesystem monitoring support
iproute2 Enabled iproute2 support instead of net-tools
mbedtls Use mbed TLS as the backend crypto library
pam Add support for PAM (Pluggable Authentication Modules) - DANGEROUS to arbitrarily flip
pkcs11 Enable PKCS#11 smartcard support
selinux !!internal use only!! Security Enhanced Linux support, this must be set by the selinux profile or breakage will occur
systemd Enable use of systemd-specific libraries and features like socket activation or session tracking
test Enable dependencies and/or preparations necessary to run tests (usually controlled by FEATURES=test but can be toggled independently)
Data provided by the Gentoo Package Database · Last update: 2025-04-03 15:25 More information about USE flags

Emerge

Install the OpenVPN package:

root #emerge --ask net-vpn/openvpn

Additional software

3 examples DNS server that could be used:

Choose one DNS server product from all availble in portage:

user $eix -Sc | grep DNS

Configuration

Tip
For a simple setup that does not require PKI, follow the peer-fingerprint authentication setup.

A secure and full setup of OpenVPN explained here, requires placing a large amount of files on different nodes, correctly. For a overview of all required files and their placement, lookup the openvpn.net community article about creation a PKI infrastructure - Key Files, verify with the table displayed at the end linked article.

The setup of DNS server is optional and not explained below. Please follow dnsmasq or BIND articles to setup a working DNS server.

Environment variables

Configuration:

  • dev - tun0 or tap0 device
  • proto - Define protocol udp, tcp, udp6 or tcp6 for communicating with peer.
  • port - TCP/UDP port for for local and remote.
  • topology - Set IP topology used for tun device: subnet or p2p

IP address table:

Hostname Interface IP address Gateway Network description
server1 eth0 192.168.100.1/24 Private
eth1 203.0.113.1/24 Public - internet
tun1 192.168.255.1/24 VPN
client1 eth0 192.0.2.10/24 192.0.2.1 Private or Public
tun1 192.168.255.2/24 VPN
dns1 eth0 192.168.100.100/24 192.168.100.1 Private
Note
This is a example IP scenario used in this document. For real world usage, change the according IP networking entries.

Files

Set the openvpn deamon running options to use local syslog. Add the --syslog running option to enable syslog logging:

FILE /etc/conf.d/openvpnSet logging to local syslog
[...]
# Additional arguments to pass to openvpn.
command_args="--syslog"

Apply this setting to all nodes, here in document to the server and to the client.

The file /etc/openvpn/*/ca.crt is the Certifcate Autority's (CA's) public certificate. That file is identical on all VPN domain participating nodes. It is used in different paths in this document:

  • /etc/openvpn/server/ca.crt
  • /etc/openvpn/client/ca.crt

Server

Files

  • /etc/conf.d/openvpn - Daemon (system wide) configuration file.
  • /etc/openvpn/openvpn.conf - Global (system wide) configuration file.
  • /etc/openvpn/server/ca.crt - CA (VPN domain wide) public certificate.
  • /etc/openvpn/server/server1.key - OpenVPN server private key
  • /etc/openvpn/server/server1.crt - OpenVPN server's CA signed certificate.
  • /etc/openvpn/server/dh2048.pem - Diffie-Helman (DH) parameter file.

If this is the first time setting up an openvpn server, we will need to create a PKI (Public Key Infrastructure) from scratch.

In this section place all generated keys and certificates into the /etc/openvpn/server/ directory.

The document explains a simple setup for one OpenVPN server only. For multiple OpenVPN instances see the 'Gentoo specifics' section.

Write a server-side openvpn configuration.

FILE /etc/openvpn/openvpn.confSetup a example UDP OpenVPN server
# openvpn 2.6.x version 
# set interface
dev tun0

# set protocol
proto udp

# set port
port 1194

# certificates and keys paths
ca server/ca.crt
cert server/server1.crt
key server/server1.key
dh server/dh2048.pem

# set the topology
topology subnet

# vpn client IP subnet assigning pool
server 192.168.255.0 255.255.255.0

# persistent device and key settings
persist-key
persist-tun

# update vpn clients IP routing table
push "route 192.168.100.0 255.255.255.0"

# update vpn clients DNS entry
push "dhcp-option DNS 192.168.100.100"

# connection
keepalive 60 300

# notify clients on deamon restart to reconnect quickly
explicit-exit-notify 1

# run the server as system user nobody
user nobody
group nobody

Before starting the configured openvpn daemon on the server, display the current state of:

  • Active system interfaces
  • IP routing table

Use the ip link show up command to list the currently active system interfaces. The command line output shows 2 active interfaces:

  • eth0
  • eth1

when leaving out the lo - loopback interface.

user $ip link show up
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
    link/ether 0c:64:f8:f1:00:00 brd ff:ff:ff:ff:ff:ff
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
    link/ether 0c:64:f8:f1:00:01 brd ff:ff:ff:ff:ff:ff

Use the ip route command to display the IP routing table. The routing table displays 2 entries:

  • private IP network 192.168.100.0/24 conntected to the eth0 interface
  • public IP network 203.0.113.0/24 conntected to the eth1 interface
user $ip route
default via 203.0.113.1 dev eth0 metric 202 
192.168.100.0/24 dev eth1 proto kernel scope link src 192.168.100.1 
203.0.113.0/24 dev eth0 proto kernel scope link src 203.0.113.2

The openvpn server is the IP gateway for both connected networks at eth0 and the eth1 interfaces.

Start the daemon and add to the default startup:

root #rc-service openvpn start
root #rc-update add openvpn default

After the openvpn daemon has been started following system entries have changed.

Use the ip link show up command to list the currently active system interfaces. This output displays now 3 active interfaces on the server. Notice the new tun0 inteface:

user $ip link show up
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
    link/ether 0c:64:f8:f1:00:00 brd ff:ff:ff:ff:ff:ff
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
    link/ether 0c:64:f8:f1:00:01 brd ff:ff:ff:ff:ff:ff
8: tun0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN mode DEFAULT group default qlen 500
    link/none

Use the ip route command to display the servers IP routing table. This shows the server is now connected to a additional IP network 192.168.255.0/24 using the IP address 192.168.255.1 on the tun0 interface, this is the VPN client network:

user $ip route
default via 203.0.113.1 dev eth0 metric 202 
192.168.100.0/24 dev eth1 proto kernel scope link src 192.168.100.1
192.168.255.0/24 dev tun0 proto kernel scope link src 192.168.255.1
203.0.113.0/24 dev eth0 proto kernel scope link src 203.0.113.2

Verify the new tun0 interface by simply using the ping command to its configured IP address:

user $ping 192.168.255.1
64 bytes from 192.168.255.1: seq=0 ttl=42 time=0.067 ms
[...]
--- 192.168.255.1 ping statistics ---
1 packets transmitted, 5 packets received, 0% packet loss
round-trip min/avg/max = 0.067/0.067/0.067 ms

The interface should respond with usual ICMP echo reply message to the send ping command.

Client

Files

  • /etc/conf.d/openvpn - Daemon (system wide) configuration file.
  • /etc/openvpn/openvpn.conf - Global (system wide) configuration file.
  • /etc/openvpn/client/ca.crt - CA (VPN domain wide) public certificate.
  • /etc/openvpn/client/client1.key - OpenVPN client private key.
  • /etc/openvpn/client/client1.crt - OpenVPN client CA signed certificate.

Write a client-side openvpn configuration file:

FILE /etc/openvpn/openvpn.confclient-side udp openvpn configuration
# openvpn 2.6.x version
# specify client-side
client

# tun/tap device
dev tun0

# protocol, according to server
proto udp

# target public IP server address
remote 203.0.113.2 1194

# persistent device and keys
persist-key
persist-tun

# keys settings
ca client/ca.crt
cert client/client1.crt
key client/client1.key

Usage

Before starting the configured openvpn on the client, display the current state of:

  • Active system interfaces
  • IP routing table
  • DNS setting, only the VPN client

Client

Use the ip link show up command to list the currently active system interfaces. The command line output shows 2 active interfaces:

  • lo
  • eth1
user $ip link show up
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
    link/ether 0c:f2:7d:16:00:00 brd ff:ff:ff:ff:ff:ff

Use the ip route command to display the IP routing table on the client:

  • IP network 192.0.2.0/24connected to the eth0 interface
user $ip route
default via 192.0.2.1 dev eth0 metric 202 
192.0.2.0/24 dev eth0 proto kernel scope link src 192.0.2.10
Note
It is not important for the setup here, on the client, to distinguish between private and public IP networks. The setup will work in both networks the same way.

Display the current setting for DNS on the opevpn client the system:

user $more /etc/resolv.conf
nameserver 192.0.2.1

The output shows the openvpn clients DNS server is set to a IP in the local area network 192.0.2.1. In most cases the local router IP address, the default gateway IP address is also the DNS server.

OpenRC

Now start the openvpn application:

root #rc-service openvpn start

systemd

root #systemctl start openvpn-client

The routine explained below is a set of commands to verify the proper working for this setup.

Client

link

Use the ip link show up command to list the currently active system interfaces. The output shows the tun0 interface has shown up among the active interfaces:

user $ip link show up
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
    link/ether 0c:f2:7d:16:00:00 brd ff:ff:ff:ff:ff:ff
3: tun0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN mode DEFAULT group default qlen 500
    link/none

IP routing

The clients routing table shows 2 new entries:

  • VPN client IP network 192.168.255.0/24 available on the tun0 interface
  • VPN client IP network 192.168.100.0/24 available via the tun0 interface
user $ip route
default via 192.0.2.1 dev eth0 metric 202 
192.0.2.0/24 dev eth0 proto kernel scope link src 192.0.2.10
192.168.100.0/24 via 192.168.255.1 dev tun0 
192.168.255.0/24 dev tun0 proto kernel scope link src 192.168.255.2

DNS

Display the current setting for DNS on the opevpn client the system:

user $more /etc/resolv.conf
# Generated by openvpn for interface tun0
domain example.net
nameserver 192.168.100.100

The output shows the openvpn clients DNS server entry has changed to the DNS server configured for the VPN network in the server configuration part. It is set to a IP address used in the VPN private IP network, (check the IP table overview) it is the 192.168.100.100.

The IP address set by the openvpn software, should be reachable and respond to send ping commands. Verify its reachability using ping:

user $ping 192.168.100.100
PING 192.168.100.100 (192.168.100.100): 56 data bytes
[...]
64 bytes from 192.168.100.100: seq=4 ttl=42 time=3.525 ms

--- 192.168.100.100 ping statistics ---
5 packets transmitted, 5 packets received, 0% packet loss
round-trip min/avg/max = 2.648/3.451/4.731 ms

Now as last check, try out if the name resolution works. Here pinging the dns1 server:

user $ping dns1
PING dns1 (192.168.100.100): 56 data bytes
[...]
64 bytes from 192.168.100.100: seq=4 ttl=42 time=3.525 ms

--- dns1 ping statistics ---
5 packets transmitted, 5 packets received, 0% packet loss
round-trip min/avg/max = 2.691/2.857/3.003 ms

The setup of a secure connection is now completed. Everything should work as expected at this point. If still running into issues read the troubleshooting section below.

Server

To find out which IP address, from the configured IP pool, 192.168.255.0/24 has been assigned to the connected client VPN client, use the OpenVPN management interface or use the command:

root #grep 192.168.255 /var/log/messages
...
Jan 30 07:44:40 server1 daemon.notice openvpn[2121]: client1/192.0.2.10:52523 MULTI_sva: pool returned IPv4=192.168.255.2, IPv6=(Not enabled)

Verify the client connectivity by using the ping command. Ping the clients tun0IP address, after the connection from the client has been established.

user $ping 192.168.255.2
PING 192.168.255.2 (192.168.255.2): 56 data bytes
[...]
--- 192.168.255.2 ping statistics ---
5 packets transmitted, 5 packets received, 0% packet loss
round-trip min/avg/max = 1.784/1.939/2.094 ms

The remote target IP of the client, connected to the tun0 interface, should respond with ICMP echo reply message to the send ping command.

Gentoo specifics

The init script allows multiple tunnels. Decide on a name for the tunnel - eg EXAMPLE.

OpenRC

First you create a config file for the connection in /etc/openvpn, such as EXAMPLE.conf

root #nano -w /etc/openvpn/EXAMPLE.conf

Then you create a link to the existing OpenRC script, with the name of your connection in it:

root #ln -s /etc/init.d/openvpn /etc/init.d/openvpn.EXAMPLE

To tell the system to automatically start it at boot, you add it to the Default runlevel: {{RootCmd|rc-upda

Finally, if you want it to start right now:

root #rc-service openvpn.EXAMPLE start

systemd

Due to dependencies server and client operations are separated into two units.

Create the server config as /etc/openvpn/server/EXAMPLE.conf:

root #systemctl start openvpn-server@EXAMPLE

Create the client config as /etc/openvpn/client/EXAMPLE.conf:

root #systemctl start openvpn-client@EXAMPLE

More tunnels can be created by replacing EXAMPLE with more names. Each one has its own configuration and can be stopped and started individually. The default is simply to use openvpn.conf and not symlink the service. Both methods may of course be used.

Troubleshooting

Sanitize configuration

Verifying /etc/openvpn/openvpn.conf configuration file with --verbose. Change into the /etc/openvpn directory, the configuration has relative configuration paths included:

root #cd /etc/openvpn

The command line output will display the matching configuration line and config option used 18:cipher, if the syntax does not match. openvpn verbosity levels --verb are 0 - 11. The default verbosity level is 1. Example --verb 2 command output displaying a configuration error:

root #openvpn --config openvpn.conf --verb 2
Options error: Unrecognized option or missing or extra parameter(s) in openvpn.conf:18: cipher (2.6.8)
Use --help for more information.

If the config is sane, openvpn will start the in the terminal.

Management Interface

Add following lines to the /etc/openvpn/openvpn.conf. Choose any desirable, free TCP port available at localhost. In example 7301:

FILE /etc/openvpn/openvpn.confAdd right at the top, 1-st configuration entry
management localhost 7301
...

Restart the daemon:

root #rc-service openvpn restart

Connect to the openvpn management interface. The management session output running on the openvpn server side.

The output shows a successfull authentication procedure done with openvpn software from a client using configuration from this example.

All important settings used in the configure section above in this document in the Server and the Client part, be easily spotted in the management interface output log:

user $telnet localhost 7301
Trying ::1...
Connected to localhost.
Escape character is '^]'.
>INFO:OpenVPN Management Interface Version 5 -- type 'help' for more info
>CLIENT:ESTABLISHED,0
>CLIENT:ENV,n_clients=1
>CLIENT:ENV,time_unix=1706538709
>CLIENT:ENV,time_ascii=2024-01-29 15:31:49
>CLIENT:ENV,ifconfig_pool_netmask=255.255.255.0
>CLIENT:ENV,ifconfig_pool_remote_ip=192.168.255.2
>CLIENT:ENV,trusted_port=33675
>CLIENT:ENV,trusted_ip=192.0.2.10
>CLIENT:ENV,common_name=VPNcl1
>CLIENT:ENV,IV_COMP_STUBv2=1
>CLIENT:ENV,IV_COMP_STUB=1
>CLIENT:ENV,IV_LZO_STUB=1
>CLIENT:ENV,IV_PROTO=990
>CLIENT:ENV,IV_CIPHERS=AES-256-GCM:AES-128-GCM:CHACHA20-POLY1305
>CLIENT:ENV,IV_NCP=2
>CLIENT:ENV,IV_MTU=1600
>CLIENT:ENV,IV_TCPNL=1
>CLIENT:ENV,IV_PLAT=linux
>CLIENT:ENV,IV_VER=2.6.8
>CLIENT:ENV,untrusted_port=33675
>CLIENT:ENV,untrusted_ip=192.0.2.10
>CLIENT:ENV,tls_serial_hex_0=3b:92:62:75:d9:25:8f:62:f2:c8:63:25:8c:a9:bc:e7
>CLIENT:ENV,tls_serial_0=79184524105695947877959046836863483111
>CLIENT:ENV,tls_digest_sha256_0=58:0e:a2:c2:29:b4:51:19:9e:33:d0:c2:18:5a:7f:82:12:12:33:cc:bb:80:b9:58:09:be:73:3e:9e:01:d4:7f
>CLIENT:ENV,tls_digest_0=2f:47:20:f4:ff:4b:66:f8:2b:42:45:f2:c0:8d:b0:0b:7d:e2:9d:28
>CLIENT:ENV,tls_id_0=CN=VPNcl1
>CLIENT:ENV,X509_0_CN=VPNcl1
>CLIENT:ENV,tls_serial_hex_1=07:d1:dd:c5:64:d2:20:52:69:c0:88:45:e9:f3:8a:31:e5:0b:f6:89
>CLIENT:ENV,tls_serial_1=44643110127090816830928405865790676542383978121
>CLIENT:ENV,tls_digest_sha256_1=e9:15:25:77:c6:33:58:69:a3:c7:ce:d0:ea:33:95:94:fd:a1:11:d1:fe:ac:78:e3:5c:3c:e6:c6:7e:c0:f0:46
>CLIENT:ENV,tls_digest_1=c7:07:fd:09:96:fd:0d:60:38:bd:cb:d1:9f:54:2a:ee:12:e5:4c:e6
>CLIENT:ENV,tls_id_1=CN=Easy-RSA CA
>CLIENT:ENV,X509_1_CN=Easy-RSA CA
>CLIENT:ENV,remote_port_1=1194
>CLIENT:ENV,local_port_1=1194
>CLIENT:ENV,proto_1=udp
>CLIENT:ENV,daemon_pid=2388
>CLIENT:ENV,daemon_start_time=1706538686
>CLIENT:ENV,daemon_log_redirect=0
>CLIENT:ENV,daemon=1
>CLIENT:ENV,verb=1
>CLIENT:ENV,config=/etc/openvpn/openvpn.conf
>CLIENT:ENV,PEER_DNS=yes
>CLIENT:ENV,RC_SVCNAME=openvpn
>CLIENT:ENV,ifconfig_local=192.168.255.1
>CLIENT:ENV,ifconfig_netmask=255.255.255.0
>CLIENT:ENV,script_context=init
>CLIENT:ENV,tun_mtu=1500
>CLIENT:ENV,dev=tun0
>CLIENT:ENV,dev_type=tun
>CLIENT:ENV,redirect_gateway=0
>CLIENT:ENV,END

The management interface output shows the Certificate Authority (CA) name was using Easy-RSA.

Leave the OpenVPN management interface by using the exit command:

user $quit

For further information read OpenVPN management interface usage manual.

See also

External resources

Retrieved from "https://wiki.gentoo.org/index.php?title=OpenVPN&oldid=1366052"