There are many use cases for SSH tunnels as they allow accessing resources behind firewalls and other obstacles over a secure channel.
Since more and more services are containerized, it makes sense to use SSH tunnels also within the context of containers, especially for testing.
Using SSH tunnels within Docker containers would require installing an SSH client and mounting keys.
In many cases this is not possible without building a new Docker image which includes the client.
As this is a cumbersome approach, an easy but insecure solution exists, which is recommended in many tutorials or posts on StackOverflow.
This fix makes use of the
--net=host flag, which allows accessing all ports of the host - also open SSH tunnels.
But for obvious reasons, this is dangerous.
A better approach is to bind the SSH tunnel to the bridge network of the Docker service. This bridge is available to all the containers connected to the particular network and thus can also forward specific ports. This technique gives a much more fine granular control over which containers may access the tunnel.
You can list the bridges with
br-b273916af970: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500 inet 172.18.0.1 netmask 255.255.0.0 broadcast 172.18.255.255 ether dd:bb:aa:cc:bb txqueuelen 0 (Ethernet) RX packets 0 bytes 0 (0.0 B) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 205 bytes 22043 (22.0 KB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 br-c92ab5650a7a: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500 inet 172.19.0.1 netmask 255.255.0.0 broadcast 172.19.255.255 ether aa:bb:cc txqueuelen 0 (Ethernet) RX packets 0 bytes 0 (0.0 B) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 0 bytes 0 (0.0 B) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 docker0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500 inet 172.17.0.1 netmask 255.255.0.0 broadcast 172.17.255.255 ether bb:aa:cc:aa:bb: txqueuelen 0 (Ethernet) RX packets 3919 bytes 227485 (227.4 KB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 3205 bytes 8586636 (8.5 MB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
You can find out the bridge a container uses with docker inspect «container». The default bridge is called docker0. You then need to enable packet forwarding to this bridge for IP tables. Note that the change below is not persisted, you need to do it again after reboot or add it permanently.
sudo iptables -I INPUT 3 -i docker0 -j ACCEPT
After this step you open a SSH tunnel on the host and also use it inside your container. This way you do not have to install SSH clients, keys etc. The trick is to bind the SSH connection to the right interface. Below you can see an example command, which allows to connect to a MySQL database via a bastion host, a typical scenario for cloud services.
ssh -L 172.17.0.1:7002:mysqlcluster:3306 bastion.example.org
Then you can access the forwarded port within the docker container on the same IP / interface, e.g. 172.17.0.1:7200 This way you can for instance use the Percona PMM container for momitoring your cluster also on your local machine, without having to deploy it and expose it via the Web.