Unix as a Second Language

Port Forwarding with Perl

Many people are thinking of ssh tunnels when they talk about port forwarding. It's easy to set up and you probably have everything you need (i.e., ssh) on most any system you want to use it on. Port forwarding does not have to be done through a tunnel, however, nor does it require ssh. Any script or program that rewrites packets in such a way as to accomplish the forwarding of packets from one port or system to another can be used for port forwarding. This doesn't mean that it's easy to build this kind of script or program, but it does mean thay tools like this are available.

One good example of a port forwarding script is Accordata's Port-Proxy. A Perl script which uses a number of modules such as IO::Socket and IO::Select, port-proxy.pl requires that you install your port forwarding specifications in a configuration file which it reads when invoked. The line "forward=1111, localhost:2222", for example, forwards connections sent to port 1111 to port 222 on the same system. If you put the following lines in your configuration file, port-proxy will listen on both ports 1111 and 3333 and will pass connections to the latter to fermion.particles.net.

Listen on 1111; forward to localhost:2222
Listen on 3333; forward to fermion.particles.net:4444

You can see that the specified ports are listening using netstat. One thing you might notice in this netstat output that is different than what we would see if we were forwarding the ports using ssh is that these LISTENs will accept connections from any system; unlike ssh, they do not listen only for connection requests generated locally. This means that you could easily set up systems to "daisy chain" port connections from one system to the next.

tcp4       0      0  *.1111          *.*             LISTEN
tcp4       0      0  *.3333          *.*             LISTEN

NOTE: The "*" in "*.1111" and "*.3333" means listen for requests from any system.

You can download port-proxy from Accordata GmbH using this URL:

http://www.accordata.de/downloads/port-proxy/index.html

The site is both in German and English. The notes online say that port-proxy has been tested with Linux and Windows, but I've used it on Solaris and Mac OS X as well. If you have Perl installed on your system and a reasonable set of modules, it will probably work for you as well.

If you test with two systems, you can run a listen script like that I wrote about last week on, say, port 4444 on the remote system.

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