OpenVPN for CTF

This is a OpenVPN server used in CTF offline match, this docker allow players in the external network accessing the game intranet without barriers.

Upstream Links

• GitHub @ kylemanna/docker-openvpn

Quick Start

• Build the docker in order to use it.

    git clone https://github.com/yuguorui/docker-offline-game-vpn
    cd docker-offline-game-vpn/
    docker build . -t ctf_vpn_docker
  

• 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 encourage to replace example with a descriptive name of their choosing.

    OVPN_DATA="vpn_data"
  

• Initialize the $OVPN_DATA container that will hold the configuration files and certificates.

    docker volume create --name $OVPN_DATA
    docker run -v $OVPN_DATA:/etc/openvpn --log-driver=none --rm ctf_vpn_docker ovpn_genconfig -u udp://YOUR_VPS_ADDRESS:4242
    docker run -v $OVPN_DATA:/etc/openvpn --log-driver=none --rm -it ctf_vpn_docker ovpn_initpki nopass
  

• Start OpenVPN server process

    docker run -v $OVPN_DATA:/etc/openvpn -d -p 4242:1194/udp --cap-add=NET_ADMIN --name ctf_vpn ctf_vpn_docker
  

• Add new iroute to access the intranet and restart the docker. (You have to figure out the CTF subnet range to make the VPN work.)

    docker run -v $OVPN_DATA:/etc/openvpn --log-driver=none --rm -it ctf_vpn_docker ovpn_addiroute NETWORK_ID MASK
    docker restart ctf_vpn
  

• Generate a router certificate without a passphrase (DO NOT CHANGE THE CERTIFICATE NAME "router")

    docker run -v $OVPN_DATA:/etc/openvpn --log-driver=none --rm -it ctf_vpn_docker easyrsa build-client-full router nopass
    docker run -v $OVPN_DATA:/etc/openvpn --log-driver=none --rm ctf_vpn_docker ovpn_getclient router > router.ovpn
  

• Generate a player certificate without a passphrase

    docker run -v $OVPN_DATA:/etc/openvpn --log-driver=none --rm -it ctf_vpn_docker easyrsa build-client-full player nopass
    docker run -v $OVPN_DATA:/etc/openvpn --log-driver=none --rm ctf_vpn_docker ovpn_getclient player > player.ovpn
  

• Configure the Server(router or PC) in the playing field (used the file router.ovpn generated in last step)

    # Allow forward packets in kernel
    sysctl -w net.ipv4.ip_forward=1
    
    # Install OpenVPN client
    apt update && apt install openvpn
    
    # Run VPN client
    nohup openvpn --config router.ovpn &
  
    # Configure the iptables
    tun_interface=tun0      # Attention: tun0 is your tun interface, you should modify the interface to fit your need.
    wan_interface=eth0      # eth0 is your WAN interface
    iptables -t filter -I FORWARD -i ${tun_interface} -o ${wan_interface} -j ACCEPT
    iptables -t filter -I FORWARD -i ${wan_interface} -o ${tun_interface} -j ACCEPT         # Dual direction
    iptables -t nat -I POSTROUTING -o ${wan_interface} -j MASQUERADE                        # setting SNAT
  

• Configure the client of offline player (used the file player.ovpn generated in last step, and you should distribute the player.ovpn file to your team members.)

    # Install OpenVPN client
    apt update && apt install openvpn
    # Run VPN client
    nohup openvpn --config player.ovpn &
    # It's OK!
  

• Clear the iroute to reconfigure

    docker run -v $OVPN_DATA:/etc/openvpn --log-driver=none --rm -it ctf_vpn_docker ovpn_cleariroute
    docker restart ctf_vpn
    # Now, you can re-add new iroute rules.
  

Next Steps

Network topology

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 --privileged -e DEBUG=1 ctf_vpn_docker
  

• 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 (run in the docker)

    $ ping 192.168.255.1 # checks if you can connect to the VPN server
    $ ping ${CTF_SUBNET} # checks if you can connect to the CTF subnet
  

• 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 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 -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, kylemanna/openvpn uses EasyRSA via the easyrsa command in the container's path. The EASYRSA_* environmental variables place the PKI CA under /etc/openvpn/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.

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

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 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 (kylemanna/openvpn) 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.

License