OpenVPN for Docker

OpenVPN server in a Docker container complete with an EasyRSA PKI CA.

Quick Start

• Create the openvpn-data volume container

    docker run --name openvpn-data -v /etc/openvpn busybox
  

• Initalize the openvpn-data container that will hold the configuration files and certificates

    docker run --volumes-from openvpn-data kylemanna/openvpn ovpn_genconfig -u udp://VPN.SERVERNAME.COM:1194
    docker run --volumes-from openvpn-data -it kylemanna/openvpn ovpn_initpki
  

• Start OpenVPN server process

    docker run --volumes-from openvpn-data -d -p 1194:1194/udp --privileged kylemanna/openvpn
  

• Generate a client certificate without a passphrase

    docker run --volumes-from openvpn-data --rm -it kylemanna/openvpn easyrsa build-client-full CLIENTNAME nopass
  

• Retrieve the client configuration with embedded certificates

    docker run --volumes-from openvpn-data --rm kylemanna/openvpn ovpn_getclient CLIENTNAME > CLIENTNAME.ovpn
  

How Does It Work?

Initialize the volume container using the kylemanna/openvpn image with the included scripts to automatically generate:

• Diffie-Hellman parameters
• a private key
• a self-certificate matching the private key for the OpenVPN server
• an EasyRSA CA key and certificate
• a TLS auth key from HMAC security

The OpenVPN server is started with the default run cmd of ovpn_run

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. The volume also holds the PKI keys and certs so that it could be backed up.

To generate a client certificate, kylemanna/openvpn uses EasyRSA via the easyrsa command in the container's path. The EASYRSA_* environmental variables place the PKI CA under /etc/opevpn/pki.

Conveniently, kylemanna/openvpn comes with a script called ovpn_getclient, which dumps an inline OpenVPN client configuration file. This single file can then be given to a client for access to the VPN.

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 UDP server uses192.168.255.0/24 for dynamic clients by default.

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

The Docker container runs its own EasyRSA PKI Certificate Authority. This was chosen as a good way to compromise on security and convenience. The container runs under the assumption that the OpenVPN container is running on a secure host, that is to say that an adversary does not have access to the PKI files under /etc/openvpn/pki. This is a fairly reasonable compromise because if an adversary had access to these files, the adversary could manipulate the function of the OpenVPN server itself (sniff packets, create a new PKI CA, MITM packets, etc).

• The certificate authority key is kept in the container by default for simplicity. It's highly recommended to secure the CA key with some passphrase to protect against a filesystem compromise. A more secure system would put the EasyRSA PKI CA on an offline system (can use the same Docker image to accomplish this).
• It would be impossible for an adversary to sign bad or forged certificates without first cracking the key's passphase should the adversary have root access to the filesystem.
• The EasyRSA build-client-full command will generate and leave keys on the server, again possible to compromise and steal the keys. The keys generated need to signed by the CA which the user hopefully configured with a passphrase as described above.
• Assuming the rest of the Docker container's filesystem is secure, TLS + PKI security should prevent any malicious host from using the VPN.

Differences from jpetazzo/dockvpn

• No longer uses serveconfig to distribute the configuration via https
• Proper PKI support integrated into image
• OpenVPN config files, PKI keys and certs are stored on a storage volume for re-use across containers
• Only offer UDP support for now, I don't have a good use case for TCP
• Addition of tls-auth for HMAC security

Tested On

• Docker hosts:
  • server a Digitial Ocean Droplet with 512 MB RAM running Ubuntu 14.04
• Clients
  • Android App OpenVPN Connect 1.1.14 (built 56)
    • OpenVPN core 3.0 android armv7a thumb2 32-bit
  • OS X Mavericks with Tunnelblick 3.4beta26 (build 3828) using openvpn-2.3.4
  • ArchLinux OpenVPN pkg 2.3.4-1