SSH remote port forwarding

Second kind of forwarding with ssh is so called remote port forwarding. This time the service available on ssh client can be forwarded to be available on ssh server. Suppose that we have machine with httpd (or any other TCP service) somewhere behind NAT and we want to make it available on our public standing ssh machine:

 ssh-client$ ssh -R 8080:localhost:80 root@ssh-server-ip-addr

What it does is forwarding ssh-client's httpd server (port 80) to port 8080 on ssh-server.

From now on, connecting to ssh-server-ip-addr:8080 will effectively connect us with ssh-client:80

Use cases for this functionality:
- remote system administering of machine behind NAT (see my serverfault's answer)
- encrypted forwarding service to another machine

SSH local port forwarding

Recently I advised on serverfault.com how to do local port forwarding with ssh:

On remote machine with sshd server, start a service that you would like to give access to:

 echo "hello" | nc -l -p 2222

On local machine initiate port forwarding:

 ssh -L 1234:localhost:2222 root@remoteserver.com

try it (from local machine):

 nc localhost 1234

If you will see "hello" that means port forwarding worked as expected.

 Use cases for this kind of functionality:
 - tunneling otherwise insecure TCP traffic
 - accessing services behind firewall when only sshd access is available