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