What is a SSH tunnel?

SSH tunneling is a method of transporting arbitrary networking data over an encrypted SSH connection. It can be used to add encryption to legacy applications. It can also be used to implement VPNs (Virtual Private Networks) and access intranet services across firewalls.

SSH is a standard for secure remote logins and file transfers over untrusted networks. It also provides a way to secure the data traffic of any given application using port forwarding, basically tunneling any TCP/IP port over SSH. This means that the application data traffic is directed to flow inside an encrypted SSH connection so that it cannot be eavesdropped or intercepted while it is in transit. SSH tunneling enables adding network security to legacy applications that do not natively support encryption.

The SSH connection is used by the application to connect to the application server. With tunneling enabled, the application contacts to a port on the local host that the SSH client listens on. The SSH client then forwards the application over its encrypted tunnel to the server. The server then connects to the actual application server - usually on the same machine or in the same data center as the SSH server. The application communication is thus secured, without having to modify the application or end user workflows.