How to connect SSH client via proxy

A proxy act as an intermediary host where you could tunnel your connection through the proxy to access another host. Tunneling your SSH connection via a proxy, among other things, could allow you to access hosts in a private network or under a NAT. As such, a proxy avoids the need to set up a more complex infrastructure such as a VPN.

OpenSSH's SSH client supports connecting through both SOCKS and HTTPS proxy. It is achieved with the ProxyCommand option alongside third-party programs such as nc or netcat.

1. Create SOCKS or HTTPS proxy if you dont already have one.

   Related: How to create an SSH SOCKS proxy
   Related: How to create SOCKS proxy on Windows

2. Test if the SOCKS or HTTPS proxy is reachable from the SSH client's host (optional).

   $ nc -zv 127.0.0.1 2222
   Connection to 127.0.0.1 2222 port [tcp/*] succeeded!

   -v      Produce more verbose output.
   -z      Only scan for listening daemons, without sending any data to
           them.  Cannot be used together with -l.

3. Use ProxyCommand as option for SSH client.

   $ ssh -o ProxyCommand='nc -X4 -x 127.0.0.1:2222 %h %p' remoteuser@remotehost

   -X proxy_protocol
           Use proxy_protocol when talking to the proxy server.  Supported
           protocols are 4 (SOCKS v.4), 5 (SOCKS v.5) and connect (HTTPS
           proxy).  If the protocol is not specified, SOCKS version 5 is
           used.
   
   -x proxy_address[:port]
           Connect to destination using a proxy at proxy_address and port.
           If port is not specified, the well-known port for the proxy pro‐
           tocol is used (1080 for SOCKS, 3128 for HTTPS).  An IPv6 address
           can be specified unambiguously by enclosing proxy_address in
           square brackets.  A proxy cannot be used with any of the options
           -lsuU.

4. Add ProxyCommand to SSH client configuration file for persistence.

   $ cat .ssh/config
   Host remotehost
       hostname 192.168.1.10
       user remoteuser
       ProxyCommand nc -X4 -x 127.0.0.1:2222 %h %p

5. Connect again using SSH client with just the Host name as parameter.

   $ ssh remotehost