OpenVPN for Docker
OpenVPN server in a Docker container complete with an EasyRSA PKI CA.
Extensively tested on Digital Ocean $5/mo node and has
a corresponding Digital Ocean Community Tutorial.
Upstream Links
Quick Start
-
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="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 kylemanna/openvpn ovpn_genconfig -u udp://VPN.SERVERNAME.COM
docker run -v $OVPN_DATA:/etc/openvpn --rm -it kylemanna/openvpn ovpn_initpki
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Start OpenVPN server process
docker run -v $OVPN_DATA:/etc/openvpn -d -p 1194:1194/udp --cap-add=NET_ADMIN kylemanna/openvpn
-
Generate a client certificate without a passphrase
docker run -v $OVPN_DATA:/etc/openvpn --rm -it kylemanna/openvpn easyrsa build-client-full CLIENTNAME nopass
-
Retrieve the client configuration with embedded certificates
docker run -v $OVPN_DATA:/etc/openvpn --rm kylemanna/openvpn ovpn_getclient CLIENTNAME > CLIENTNAME.ovpn
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 kylemanna/openvpn
-
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 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.
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:
- server a Digital 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
License
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