Imagine you want to connect via SSH to remote systems hostB and hostC on an intranet behind hostA. This could be achieved easily using port forwarding via hostA, just pick two arbitrary ports on the local machine and forward them to ports 22 of hostB and hostC. This works very well for a small amount of intranet hosts, but it get’s quite messy as the list of hosts grows. After some time you’ll have a rather huge amount of local ports to remember (or to lookup in your port forwarding script several times a day). It’d certainly be easier to just type ssh hostB and have the tunnel setup automatically.

Fortunately that is very well possible and quite easy to achieve using the ProxyCommand directive. Assuming hostA has nc installed, you can just add the following lines to your $HOME/.ssh/config.

Host hostB
HostKeyAlias hostB
ProxyCommand ssh hostA 'nc hostB 22'

Host hostC
HostKeyAlias hostC
ProxyCommand ssh hostA 'nc hostC 22'

Once done, you can easily connect to hostB via ssh hostB, or hostC via ssh hostC. No need to setup the tunnel first, it’ll be set up and teared down automagically as needed.