SSH is an extremely useful tool in that it allows you to do many things in a secure fashion that you might not otherwise be able to do. One of the things SSH allows you to do is to set up a reverse encrypted tunnel for data transfer. Typically, when you initiate an SSH tunnel, you forward a port on the local machine to a remote machine which can allow you to connect to an insecure service in a secure way, such as POP3 or IMAP. However, you can also do the reverse. You can forward a port on the remote machine to the local machine while still initiating the tunnel from the local machine.
This is useful if you have a service on the remote end that you want to have connected to something on the local machine, but you don't wish to open up your firewall or have SSH private keys stored on the remote machine. By using a reverse tunnel, you maintain all of the control on the local machine. An example usage for this would be for logging messages; by setting up a reverse SSH tunnel, you can have a logger on the remote system send logs to the local system (i.e., syslog-ng).
To set up the reverse tunnel, use:
$ ssh -nNT -R 1100:local.mydomain.com:1100 remote.mydomain.com
What this does is initiate a connection to remote.mydomain.com and forwards TCP port 1100 on remote.mydomain.com to TCP port 1100 on local.mydomain.com. The "-n" option tells ssh to associate standard input with /dev/null, "-N" tells ssh to just set up the tunnel and not to prepare a command stream, and "-T" tells ssh not to allocate a pseudo-tty on the remote system. These options are useful because all that is desired is the tunnel and no actual commands will be sent through the tunnel, unlike a normal SSH login session. The "-R" option tells ssh to set up the tunnel as a reverse tunnel.
Now, if anything connects to port 1100 on the remote system, it will be transparently forwarded to port 1100 on the local system.
Delivered each Tuesday, TechRepublic's free Linux NetNote provides tips, articles, and other resources to help you hone your Linux skills. Automatically sign up today!
Vincent Danen works on the Red Hat Security Response Team and lives in Canada. He has been writing about and developing on Linux for over 10 years and is a veteran Mac user.