SSH, The Secure Shell: The Definitive Guide, 2nd Edition

Summary

In this chapter, we discussed SSH port forwarding and X forwarding. Port forwarding is a general TCP proxying feature that tunnels TCP connections through an SSH session. This is useful for securing otherwise insecure protocols running on top of TCP or for tunneling TCP connections through firewalls that would otherwise forbid access. X forwarding is a special case of port forwarding for X Window System connections, for which SSH has extra support. This makes it easy to secure X connections with SSH, which is good because X, while popular and useful, is notoriously insecure. Access control on forwarded ports is normally coarse, but you can achieve finer control with the TCP-wrappers feature.