OpenVPN Update: Fixed!

I finally had time to really do some more troubleshooting on the OpenVPN problem I recently posted about. After trying some of the same old things to get the server to be able to access the Internet and have the VPN service running at the same time, I decided it was time to purge the installation and start fresh:

sudo apt-get purge openvpn

No fun. It’s always depressing when a configuration problem gets to the point where the only thing left you can think to try is to start over from scratch. I did have the good sense to back up all the server’s keys and configuration files, which in the long run did end up being useful.

As I was once again going through the OpenVPN tutorial provided by Ubuntu and scanning through the help comments in /etc/openvpn/server.conf, when I found the following:

Use “dev tap0” if you are ethernet bridging and have precreated a tap0 virtual interface and bridged it with your ethernet interface.

For a while, I mistakenly thought that this might be the answer I was looking for, since it seemed like that was what I was doing, but only because I didn’t understand everything that was going on under the hood as well as I do now. You see, when the tutorial has you add the line:

up "/etc/openvpn/ br0 eth1"

this isn’t exactly what is called to bring up the bridged interfaces. Taking another look at the script, there are three network interface devices referenced, even though only two are explicitly given in the configuration line above. OpenVPN adds the necessary tap0 device to the system, and then adds its interface name as a parameter to the end of the string you specify.

A minimum of three interfaces are needed to create a bridge of the sort that OpenVPN needs: 1) the physical interface with access to the “real world” through which the VPN traffic will initially come in, 2) the destination interface which is actually handled by OpenVPN, and 3) the bridging interface that connects the first two at the kernel level.

As I was figuring out all this, I thought to check the man page of brctl since I knew that the script called by OpenVPN as it starts up uses brctl to do the kernel-level changes to bridge eth1 and tap0. Once I really started to understand how the various options worked, I tried refreshing the bridged interface by calling brctl dellbr br0 and then brctl addbr br0, followed by restarting the openvpn service.

Doing this had positive results, but none of them persisted through a reboot of the machine. Finally, after trying several different things, I came up with hard-coding a refresh of the bridged interface in the script. This, plus changing the network configuration of eth0 from static to dhcp seems to have done the trick. The machine has access to the Internet, and clients can connect just as they could before. I even copied over the old configuration, certificate, and key files, which prevented me from having to re-issue all the client keys and re-distributing the server’s certificate.

The new contents of my script are below:



# Reset the bridge device first
/sbin/ip link set "$BR" down
/sbin/brctl delbr $BR
/sbin/brctl addbr $BR
/sbin/brctl addif $BR $ETHDEV

/sbin/ip link set "$TAPDEV" up
/sbin/ip link set "$ETHDEV" promisc on
/sbin/brctl addif $BR $TAPDEV