# OpenVPN for Docker OpenVPN server in a Docker container complete with an EasyRSA PKI CA. ## Quick Start * Initalize the `openvpn-data` container that will hold the configuration files and certificates docker run --name openvpn-data -it kylemanna/openvpn ovpn_init udp://VPN.SERVERNAME.COM:1194 * 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 `ovpn_init` 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 uses`192.168.255.0/24` 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