Configure openssh to tunnel through another server using SSH

[bogdi19]

Ever needed to SSH to a server that you can’t connect to directly and would have to bounce off another host?

Within your local ssh configuration file which is stored in $HOME/.ssh/config create a SSH host entry for the server you are going to tunnel through.

Host ssh-proxy-server
HostName ssh-proxy-server.example.com

The host ssh-proxy-server will need the netcat package installed which provides the nc binary.

Now create another SSH host entry for the final destination server. In order to SSH to this server, you have to SSH to the ssh-proxy-server, create a tunnel then SSH through that tunnel.

Host web-server
HostName web-server.example.com
ProxyCommand ssh -q -A ssh-proxy-server 'nc %h %p'

The important line in the final configuration item is the ProxyCommand. ProxyCommand specifies the command to use to connect to the server. %h is substituted for the hostname which is defined by the HostName directive. %p is substituted for the SSH destination port which defaults to 22.

Now SSH indirectly to web-server.example.com. Use -v to see SSH bounce off the ssh-proxy-server server.

$ ssh -v web-server.example.com

If the bounce host, in our tutorial being ‘ssh-proxy-server.example.com’, was a SOCKS5 host and not a server running SSH we would use a different ProxyCommand directive.

Host web-server
HostName web-server.example.com
ProxyCommand connect -R both -S ssh-proxy-server.example.com:1080 %h %p

The ProxyCommand above uses the ‘connect’ binary which creates a connection to a SOCKS4/5 proxy. Our SSH client will then tunnel over this SOCKS tunnel to the remote server.

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