Modifying SSH port forwardings mid-session

I frequently use SSH port forwarding to access services on computers I'm connected to (e.g. VNC, web servers, Zeya). For example,

ssh -L 8001:localhost:8080 foobar

connects port 8001 on my local machine to whatever service is running on foobar port 8080.

Sometimes I'll discover mid-session that I wish to connect to a new service I've just started up remotely, or that I forgot to add the -L flag for some service I wanted. I could always just disconnect, add the appropriate port forwardings, and reconnect.

However, I just learned that SSH also supports some escape sequences, one of which lets you break out to a command line, where you can change port forwardings mid-session without disconnecting.

With the default settings, type ~C at the beginning of your session or after a newline. You'll see a command prompt:

ssh>

At this prompt, you can add additional forwardings using the same syntax that ssh accepts:

• Local forwarding to remote service: -L local_port:hostname:hostport
• Remote forwarding to local service: -R remote_port:hostname:hostport
• Dynamic forwarding, e.g. for SOCKS: -D port

Further reading:

• More about escapes from the ssh man page. (Escapes provide access to some nifty other features, too.)
• Reverse SSH (Tricks for making port forwarding even more useful.)