Routing Select Docker Containers through Wireguard VPN
Wouldn’t be a docker post without an image of a ship
Scenario: You have a host running many Docker containers. Several sets of these containers need to route traffic through different VPNs. Below I’ll describe my solution that doesn’t resort to VMs and doesn’t require modification to any docker images.
This post assumes that one has already set up working wireguard servers, and will focus only on client side. For a quick wireguard intro: see WireGuard VPN Walkthrough.
If you’re familiar with the openvpn client trick then this will look familiar. We’re going to create a Wireguard container and link all desired containers to this Wireguard container.
First we’re going to create a Wireguard Dockerfile:
Some notes about this Dockerfile:
- One needs to supply
wgnet0.conf(described in previously linked post), as it’ll contain VPN configuration
EXPOSEcontains all the applications in the VPN that will need their port exposed. Feel free to omit
EXPOSEas it’s completely optional
startup.shscript connects to the VPN, verifies that the container’s IP address is the same VPN server’s, and if not to stop the container. Implementation below.
- Every minute we check to see what our IP address is from dyndns.com. This is the same endpoint that the dynamic dns client, ddclient, uses.
- A more efficient kill switch would integrate with ip tables to ensure that all traffic is routed via the VPN. But don’t use the example provided in
wg-quickas that will block traffic from the host (192.168.1.x) to the containers. I have a cool trick later on with port forwarding.
- When the script exits, it brings down the VPN
The best way to see this in action is through a docker compose file. We’ll have grafana traffic routed through the VPN.
- Just like the OpenVPN solution, we need the
- Since Wireguard is a kernel module we need the
- See the
sysctlsconfiguration? This affects only the container’s networking.
- All ports exposed for the
wireguardcontainer are the ones that would normally be exposed in other services. For instance, I have wireguard exposing the grafana port 3000.
network_mode: "service:wireguard"is the magic that has grafana use the wireguard vpn
When dependant services bind to wireguard’s network they are binding to that container’s id. If you rebuild the wireguard container, you’ll need to rebuild all dependant containers. This is somewhat annoying. Ideally, they would bind to whatever container’s network that had the name of wireguard.
Quick quiz, which of these addresses will resolve to our grafana instance (taken from the host machine)?
If I hadn’t ran the experiment, I would have gotten this wrong!
If we log onto the VPN server, we see that only curling only our client IP address will return Grafana. This is good news, it means we are not accidentally exposing services on our VPN’s external IP address. It also allows a cool trick to see the services locally through the host machine without being on the VPN. Normally one would would put in
http://host-ip.com:3000 to see Grafana, but as we just discovered, that no longer routes to Grafana because it lives on the VPN. We can, however, ssh into the host machine and port forward localhost:3000 to great success!
The end result gives me a good feeling about the security of the implementation. There is no way someone can access the services routed through the VPN unless they are also on the VPN, they are on the host machine, or port forward to the host machine. We have a rudimentary kill switch as well for some more comfort.
Our second solution will involve installing Wireguard on the host machine. This requires gcc and other build tools, which is annoying as the whole point of docker is to keep hosts disposable, but we’ll see how this solution shakes out as it has some nice properties too.
Initial plans were to follow Wireguard’s official Routing & Network Namespace Integration, as it explicitly mentions docker as a use case, but it’s light on docker instructions. It mentions only using the pid of a docker process. This doesn’t seem like the “docker” approach, as it’s cumbersome. If you are a linux networking guru, I may be missing something obvious, and this may be your most viable solution. For mere mortals like myself, I’ll show a similar approach, but more docker friendly.
wg-quick has been my crutch, as it abstracts away some of the routing configuration. Running
wg-quick up wgnet0 to have all traffic routed through the Wireguard interface is a desirable property, and it was a struggle to figure out how to route only select traffic.
For those coming from
wg-quick we’re going to be doing things manually, so to avoid confusion, I’m going to be creating another interface called
wg1. Our beloved
Address configurations found in
wgnet0.conf have to be commented out as they are
wg-quick specific. These settings are explicitly written in manual invocation:
At this point if your VPN is hosted externally you can test that the Wireguard interface is working by comparing these two outputs:
For my future self, I’m going to break down what just happened by annotating the commands.
Now for the docker fun. We’re going to create a new docker network for our VPN docker containers:
Now to route traffic for
docker-vpn0 through our new
My layman understanding is that we mark traffic from our docker subnet as “200”, kinda like fwmark. We then set the default route for the docker subnet to our
wg1 interface. The default route allows the docker subnet to query unknown IPs and hosts (ie. everything that is not a docker container in the
10.193.0.0/16 space). By having the route be more specific, as it mentions a table, data is routed through
wg1 instead of
You can test it out with:
docker network remove docker-vpn0, we slim down our docker compose file.
Now when we bring up grafana, it will automatically be connected through the VPN thanks to the subnet routing.
If we want to bring the VPN up on boot, we need to create
/etc/network/interfaces.d/wg1 with encoded commands:
auto wg1 is what starts the interface automatically on boot, else we’d have to rely on
ifdown. Everything else should look familiar.
The last thing that needs mentioning is the kill switch. We’ve seen calling curl inside our networked docker container. We could use this to periodically check the IP address is as expected. But I know we can do better, but I can’t quite yet formulate a complete solution, so I’ll include my work in progress.
We can deny all traffic from our subnet with the following:
But how to run this command when the VPN disintegrates? I’ve thought about putting it in a
post-down step for
wg1, but I don’t think it’s surefire approach. The scary part is if
wg1 goes down, the docker ip table rule is no longer effective, so instead of dropping packets for an interface that no longer exists, they are sent to the next applicable rule which is
eth0! We have to be smarter. For reference, the kill switch used as an example in
The end result should look something like this. It will be more complete than any
curl kill switch. I didn’t want to bumble through to a halfway decent solution, so I called it a night! If I come across the solution or someone shouts it at me, I’ll update the post.
I think both solutions should be in one’s toolkit. At this stage I’m not sure if there is a clear winner. I ran the first solution for about a week. It can feel like a bit of a hack, but knowing that everything is isolated in the container and managed by docker can be a relief.
I’ve been running the second solution for about a day or so, as I’ve only just figured out how all the pieces fit together. The solution feels more flexible. The apps can be more easily deployed anywhere, as there is nothing encoded about a VPN in the compose file. The only thing that will stand out as different is the networking section. I also find pros and cons to having the VPN managed by the host machine. On one hand, having all the linux tools and
wg show readily available to monitor the tunnel is nice. Like collectd, it’s easiest to report stats on top level interfaces. But on the other hand, installing build tools is annoying and managing routing tables makes me anxious if I think too much about it. In testing these solutions, I’ve locked myself out of a VM more than once – forcing a reboot, and I don’t take rebooting actual servers lightly.
Only time will tell which solution is best, but I thought I should document both.