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Monday, 27 June 2016
OpenVPN for Docker
Recipe to build an OpenVPN image for Docker.
CID=$(docker run -d --privileged -p 1194:1194/udp -p 443:443/tcp jpetazzo/dockvpn)
docker run -t -i -p 8080:8080 --volumes-from $CID jpetazzo/dockvpn serveconfig
Now download the file located at the indicated URL. You will get a certificate warning, since the connection is done over SSL, but we are using a self-signed certificate. After downloading the configuration, stop the serveconfig container. You can restart it later if you need to re-download the configuration, or to download it to multiple devices.
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!
Note: there is a bug in the Android Download Manager which prevents downloading files from untrusted SSL servers; and in that case, our self-signed certificate means that our server is untrusted. If you try to download with the default browser on your Android device, it will show the download as "in progress" but it will remain stuck. You can download it with Firefox; or you can transfer it with another way: Dropbox, USB, micro-SD card...
If you reboot the server (or stop the container) and 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/dockvpn image is started, it generates:
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/dockvpncomes with a script called serveconfig, which starts a pseudo HTTPS server on 8080/tcp. The pseudo server does not even check the HTTP request; it just sends the HTTP status line, headers, and body right away.
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 (184.108.40.206 and 220.127.116.11) or OpenDNS (18.104.22.168 and 22.214.171.124).
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.