OpenVPN for Docker

OpenVPN server in a Docker container complete with an EasyRSA PKI CA, modified for TAP and bridge support

Disclaimer

• Primary credit: jpetazzo/dockvpn
• Secondary credit (of which this is a fork): kylemanna/docker-openvpn
• Tertiary credit (tap and bridge support principles): aktur/docker-openvpn

This image was modified for my own private use with my homelab. It was modified to support tap mode and network bridging out-of-the-box without the need of any additional or manual modifications. Unlike the image this is image is based on, it is not tested extensively with different network setups, host architectures or VPN configurations. It works the way I use it.

Upstream Links

• Docker Registry @ salvoxia/openvpn-tap
• GitHub @ salvoxia/docker-openvpn-tap

Environment Variables

Environment varible	Description
DEBUG	Set to 1 to enable debugging output
NFT_TABLES	Set to 1 to use iptables-nft command instead of legacy iptables command for setting up ACCEPT and FORWARD rules Use this if you intend to set up a bridge and your host uses the new iptables-nft.

Quick Start

• For TAP mode, see TAP and bridge support.

• Pick a name for the $OVPN_DATA data volume container. It's recommended to use the ovpn-data- prefix to operate seamlessly with the reference systemd service. Users are encouraged to replace example with a descriptive name of their choosing.

  OVPN_DATA="ovpn-data-example"

• Initialize the $OVPN_DATA container that will hold the configuration files and certificates. The container will prompt for a passphrase to protect the private key used by the newly generated certificate authority.

  docker volume create --name $OVPN_DATA
  docker run -v $OVPN_DATA:/etc/openvpn --rm salvoxia/openvpn-tap ovpn_genconfig -u udp://VPN.SERVERNAME.COM
  docker run -v $OVPN_DATA:/etc/openvpn --rm -it salvoxia/openvpn-tap ovpn_initpki

• Start OpenVPN server process

  docker run -v $OVPN_DATA:/etc/openvpn -d -p 1194:1194/udp --cap-add=NET_ADMIN salvoxia/openvpn-tap

• Generate a client certificate without a passphrase

  docker run -v $OVPN_DATA:/etc/openvpn --rm -it salvoxia/openvpn-tap easyrsa build-client-full CLIENTNAME nopass

• Retrieve the client configuration with embedded certificates

  docker run -v $OVPN_DATA:/etc/openvpn --rm salvoxia/openvpn-tap ovpn_getclient CLIENTNAME > CLIENTNAME.ovpn

TAP and bridge support

This image has been modified to work with tap mode and support network bridging out-of-the-box. You must start the container with host networking mode.

⚠️ Caution:

Bridge mode is not compatible with NetworkManager running on the host. The container creates a network bridge on the host, bridging the interface you specify and updating routing tables accordingly.
If the network interface you want to bridge is managed by NetworkManager, it will interfere with the routing tables and eventually render your host unreachable over the network!
NetworkManager provides various VPN plugins for direct VPN support.
If you still want to use this image, you must disable NetworkManager for the network device you want to bridge. Assuming that device is called eth0, you can set the device to "unmanaged" like this:

nmcli device set eth0 managed no

Configuring the container for TAP mode

• First, create an OpenVPN data volume or choose a local path on the host system to mount into the container.

  OVPN_DATA="ovpn-data-example"
  docker volume create --name $OVPN_DATA

• To set up a server using tap and bridging, use the -B argument when creating the configuration. You must supply all bridge-related arguments as well:

  docker run -v $OVPN_DATA:/etc/openvpn --rm salvoxia/openvpn-tap \
    ovpn_genconfig \
        -u udp://VPN.SERVERNAME.COM:PORT \
        -t \
        -B \
        --bridge-name 'br0' \
        --bridge-eth-if 'eth0' \
        --bridge-eth-ip '192.168.0.199' \
        --bridge-eth-subnet '255.255.255.0' \
        --bridge-eth-broadcast '192.168.0.255' \
        --bridge-eth-gateway '192.168.0.1' \
        --bridge-eth-mac 'b8:32:ac:8b:17:2e' \
        --bridge-dhcp-start '192.168.0.200' \
        --bridge-dhcp-end '192.168.0.220'

  ⚠️ Caution: Choosing the wrong bridge argument values may render your host machine unreachable over the network! Make sure to have direct access or choose wisely!

• Create certificates for generating clients:

  docker run -v $OVPN_DATA:/etc/openvpn --rm -it salvoxia/openvpn-tap ovpn_initpki

• Then start the server with host networking mode:

  docker run -v $OVPN_DATA:/etc/openvpn -d --network host --cap-add=NET_ADMIN salvoxia/openvpn-tap

  Make sure to choose the correct iptables command, otherwise routing might not work (refer to Environment Variables. To use iptables-nft, start the container like this:

  docker run -v $OVPN_DATA:/etc/openvpn -d --network host --cap-add=NET_ADMIN -e NFT_TABLES=1 salvoxia/openvpn-tap

Next Steps

More Reading

Miscellaneous write-ups for advanced configurations are available in the docs folder.

Systemd Init Scripts

A systemd init script is available to manage the OpenVPN container. It will start the container on system boot, restart the container if it exits unexpectedly, and pull updates from Docker Hub to keep itself up to date.

Please refer to the systemd documentation to learn more.

Docker Compose

If you prefer to use docker-compose please refer to the documentation.

Debugging Tips

• Create an environment variable with the name DEBUG and value of 1 to enable debug output (using "docker -e").

  docker run -v $OVPN_DATA:/etc/openvpn -p 1194:1194/udp --cap-add=NET_ADMIN -e DEBUG=1 salvoxia/openvpn-tap

• Test using a client that has openvpn installed correctly

  $ openvpn --config CLIENTNAME.ovpn

• Run through a barrage of debugging checks on the client if things don't just work

  ping 8.8.8.8    # checks connectivity without touching name resolution
  dig google.com  # won't use the search directives in resolv.conf
  nslookup google.com # will use search

• Consider setting up a systemd service for automatic start-up at boot time and restart in the event the OpenVPN daemon or Docker crashes.

How Does It Work?

Initialize the volume container using the salvoxia/openvpn-tap 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 -v argument, 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, salvoxia/openvpn-tap uses EasyRSA via the easyrsa command in the container's path. The EASYRSA_* environmental variables place the PKI CA under /etc/openvpn/pki.

Conveniently, salvoxia/openvpn-tap 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.

To enable Two Factor Authentication for clients (a.k.a. OTP) see this document.

OpenVPN Details

You can choose between tun and tap mode when generating the OpenVPN server configuration by (not) setting the -t flag when running ovpn_genconfig.

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 and the script ovpn_copy_server_files 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 be 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.

Benefits of Running Inside a Docker Container

The Entire Daemon and Dependencies are in the Docker Image

This means that it will function correctly (after Docker itself is setup) on all distributions Linux distributions such as: Ubuntu, Arch, Debian, Fedora, etc. Furthermore, an old stable server can run a bleeding edge OpenVPN server without having to install/muck with library dependencies (i.e. run latest OpenVPN with latest OpenSSL on Ubuntu 12.04 LTS).

It Doesn't Stomp All Over the Server's Filesystem

Everything for the Docker container is contained in two images: the ephemeral run time image (salvoxia/openvpn-tap) and the $OVPN_DATA data volume. To remove it, remove the corresponding containers, $OVPN_DATA data volume and Docker image and it's completely removed. This also makes it easier to run multiple servers since each lives in the bubble of the container (of course multiple IPs or separate ports are needed to communicate with the world).

Some (arguable) Security Benefits

At the simplest level compromising the container may prevent additional compromise of the server. There are many arguments surrounding this, but the take away is that it certainly makes it more difficult to break out of the container. People are actively working on Linux containers to make this more of a guarantee in the future.

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
• Addition of tls-auth for HMAC security

Originally Tested On

• Docker hosts:
  • RaspberryPI 3B+ running Raspbian arm64 with kernel 5.15.61
• Clients
  • Windows OpenVPN Client 2.5.7 (64bit)
  • Android App VPN Client Pro (supports TAP with rooting the device)

Cheat Sheet

Build

docker build -t salvoxia/openvpn-tap .

Build Multi-Arch

docker buildx create --name multi-platform-builder --platform linux/arm/v7,linux/arm64/v8,linux/amd64
docker buildx build --builder multi-platform-builder -t salvoxia/openvpn-tap .

Original License