OpenVPN for Docker

Quick instructions:

CID=$(docker run -d -privileged -p 1194:1194/udp -p 443:443/tcp jpetazzo/openvpn)
SERVERADDR=$(curl http://myip.enix.org/REMOTE_ADDR)
echo "Download your OpenVPN configuration file at http://$SERVERADDR:8080/"
docker run -p 8080:8080 -volumes-from $CID jpetazzo/openvpn serveconfig

Now download the file located at the indicated URL. The configuration server exits after 1 download, so if you need to download the configuration on multiple devices, repeat the last docker run.

The file can be used immediately as an OpenVPN profile. It embeds all the required configuration and credentials. It has been tested successfully on Linux, Windows, and Android clients. If you can test it on OS X and iPhone, let me know!

If you reboot the server (or stop the container), if you docker run again, you will create a new service (with a new configuration) and you will have to re-download the configuration file. However, you can use docker start to restart the service without touching the configuration.

How does it work?

When the jpetazzo/openvpn image is started, it generates:

• Diffie-Hellman parameters,
• a private key,
• a self-certificate matching the private key,
• two OpenVPN server configurations (for UDP and TCP),
• an OpenVPN client profile.

Then, it starts two OpenVPN server processes (one on 1194/udp, another on 443/tcp).

The configuration is located in /etc/openvpn, and the Dockerfile declares that directory as a volume. It means that you can start another container with the -volumes-from flag, and access the configuration. Conveniently, jpetazzo/openvpn comes with a script called serveconfig, which starts a pseudo HTTP server on 8080/tcp. The pseudo server will accept only one request, and send the content of the configuration file, then it will exit.

OpenVPN details

We use tun mode, because it works on the widest range of devices. tap mode, for instance, does not work on Android, except if the device is rooted.

The topology used is net30, because it works on the widest range of OS. p2p, for instance, does not work on Windows.

The TCP server uses 192.168.255.0/25 and the UDP server uses 192.168.255.128/25.

The client profile specifies redirect-gateway def1, meaning that after establishing the VPN connection, all traffic will go through the VPN. This might cause problems if you use local DNS recursors which are not directly reachable, since you will try to reach them through the VPN and they might not answer to you. If that happens, use public DNS resolvers like those of Google (8.8.4.4 and 8.8.8.8) or OpenDNS (208.67.222.222 and 208.67.220.220).

Security discussion

For simplicity, the client and the server use the same private key and certificate. This is certainly a terrible idea. If someone can get their hands on the configuration on one of your clients, they will be able to connect to your VPN, and you will have to generate new keys. Which is, by the way, extremely easy, since each time you docker run the OpenVPN image, a new key is created. If someone steals your configuration file (and key), they will also be able to impersonate the VPN server (if they can also somehow hijack your connection).

It would probably be a good idea to generate two sets of keys.

It would probably be even better to generate the server key when running the container for the first time (as it is done now), but generate a new client key each time the serveconfig command is called. The command could even take the client CN as argument, and another revoke command could be used to revoke previously issued keys.

Also, the configuration could be served over SSL. This should be fairly enough, since we use socat for the pseudo HTTP server, and socat can also do SSL.