VPN via the TUN/TAP device

This article describes how to use VPN via the TUN/TAP device inside a container.

Kernel TUN/TAP support

OpenVZ supports VPN inside a container via kernel TUN/TAP module and device. To allow container #101 to use the TUN/TAP device the following should be done:

Make sure the tun module has been already loaded on the hardware node:

lsmod | grep tun

If it is not there, use the following command to load tun module:

modprobe tun

To make sure that tun module will be automatically loaded on every reboot you can also add it or into /etc/modules.conf (on RHEL see /etc/sysconfig/modules/ directory).

Granting container an access to TUN/TAP

Allow your container to use the tun/tap device by running the following commands on the host node:

CTID=101
vzctl set $CTID --devnodes net/tun:rw --capability net_admin:on --save

Configuring VPN inside container

After the configuration steps above are done it is possible to use VPN software working with TUN/TAP inside container just like on a usual standalone Linux box.

The following software can be used for VPN with TUN/TAP:

• Tinc (http://tinc-vpn.org)
• OpenVPN (http://openvpn.net)
• Virtual TUNnel (http://vtun.sourceforge.net)

Reaching hosts behind VPN container

In order to reach hosts behind VPN container you must configure it to use a VETH interface instead a VENET one, at least with an OpenVPN server.

With a VENET interface you will only reach the VPN container.

To use a VETH device follow Veth article.

If you insist on using a VENET interface and need to reach hosts behind the OpenVPN VE then you can use source NAT. You need to mangle source packets so that they appear to originate from the OpenVPN server VE.

Tinc problems

Using the default venet0:0 interface on the container, tinc seems to have problems as it complains the port 655 is already used on 0.0.0.0.

Netstat shows that the port 655 is available:

root@132 / [3]# netstat -l
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State      
tcp        0      0 localhost.localdom:8001 *:*                     LISTEN     
tcp        0      0 *:2223                  *:*                     LISTEN     
tcp6       0      0 [::]:2223               [::]:*                  LISTEN     
udp6       0      0 [::]:talk               [::]:*                             
udp6       0      0 [::]:ntalk              [::]:*                             
Active UNIX domain sockets (only servers)
Proto RefCnt Flags       Type       State         I-Node   Path
unix  2      [ ACC ]     STREAM     LISTENING     4831020  /var/run/uml-utilities/uml_switch.ctl

Starting the Tincd daemon where it complains that port 655 is not available:

root@132 / [4]# tincd -n myvpn
root@132 / [5]# tail -f /var/log/syslog
Jul 26 14:08:01 132 /USR/SBIN/CRON[15159]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)
Jul 26 14:37:42 132 -- MARK --
Jul 26 14:57:42 132 -- MARK --
Jul 26 15:08:01 132 /USR/SBIN/CRON[15178]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)
Jul 26 15:11:23 132 tinc.myvpn[15139]: Got TERM signal
Jul 26 15:11:23 132 tinc.myvpn[15139]: Terminating
Jul 26 15:11:37 132 tinc.myvpn[15191]: tincd 1.0.8 (Aug 14 2007 13:51:23) starting, debug level 0
Jul 26 15:11:37 132 tinc.myvpn[15191]: /dev/net/tun is a Linux tun/tap device (tun mode)
Jul 26 15:11:37 132 tinc.myvpn[15191]: Can't bind to 0.0.0.0 port 655/tcp: Address already in use
Jul 26 15:11:37 132 tinc.myvpn[15191]: Ready
^C
root@132 / [6]# 

An echo to Bindv6only (see discussion here) seems to resolve the problem:

root@132 / [12]# echo 1 > /proc/sys/net/ipv6/bindv6only

Or put in your /etc/sysctl.conf file:

net.ipv6.bindv6only = 1

Then apply the changes with:

root@132 / [14]# sysctl -p

The tunctl problem

Unfortunately, you are limited to non-persistent tunnels inside the VEs:

# tunctl
enabling TUNSETPERSIST: Operation not permitted

Get a patched tunctl here, and run it with the -n option. It will create a non-persistent tun device and sleep instead of terminating, to keep the device from deletion. To remove the tunnel, kill the tunctl process.

Troubleshooting

If NAT is needed within the VE, this error will occur on attempts to use NAT:

# iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -o venet0 -j MASQUERADE
iptables v1.4.3.2: can't initialize iptables table `nat': Table does not exist (do you need to insmod?)
Perhaps iptables or your kernel needs to be upgraded.

The solution is given here:

http://kb.sp.parallels.com/en/5228

Also see page 69-70 of:

http://download.openvz.org/doc/OpenVZ-Users-Guide.pdf

Note that the above steps do not solve the problem if a gentoo VE sits on a Centos HN; it's still an unsolved mystery.

External links

• Virtual TUNnel
• OpenVPN
• Tinc
• How to run OpenVPN Access Server in OpenVZ
• Odin KB #696: Is VPN via the TUN/TAP device supported inside a Container?