Port Forwarding

This is my first attempt at documenting the code I wrote to expand on the functionality of the IP Masquerading code included in Linux 2.0.* kernels.


Using Port Forwarding with Linux 2.2.

Port Forwarding - Why?

Before I describe what port forwarding is, let me describe the situation that persuaded me to write this code. A local computer group had 3 computers and 1 IP address. They had implemented an IP masquerading solution to allow people to use all 3 computers with the configuration:

                              | (
                         linux box 1
                              | (
                +--------------------------+ internal ethernet
                | (               | (
           linux box 2                linux box 3
The first linux box was a 386SX20 with only 4Mb and couldn't have coped with any real users. We then redirected telnet, mail, web, etc. to one of the two internal machines with plug-gw or nc (netcat) from inetd.

This had a number of problems:

Port Forwarding - What is it?

The easiest way to imagine Port Forwarding is a combination of routing by port combined with packet rewriting. A convention router examines the packet header and dispatches the packet on one of it's other interfaces, depending on the packet's destination address. Port Forwarding examines the packet header and forwards it on to another host (after a little header rewriting) depending on the destination port.

In more detail: Port forwarding forwards all packets intended for one forwarding port on the gateway from the external networks to routed on a specified port on one of the internal machines (after a little rewriting of headers). This is (in some ways) a reverse of masquerading and uses many of the maquerading functions - particularly the packet header rewriting code.

Confused? Here's an example:

On the gateway, we setup the rule that all connections to port 80/tcp should be redirected to port 80 on (an internal machine). The incoming web connection would be labelled:

Source:  Dest:
This would be forwarded on to the internal host as:
Source:  Dest:
Replies would be labelled:
Source:       Dest:
and would be rewritten by the gateway to:
Source        Dest:
This has a number of advantages over using tools like nc and plug-gw to do the forwarding:

Port Forwarding - How does it work?

[This section is optional reading and is not necessary to use Port Forwarding]

Port forwarding uses the existing masquerading scheme to do all the rewriting of packets. The masquerading table (what you see when you type netstat -M or ipfwadm -M -l) is setup as if the connection started internally. When the existing masquerading code receives a packet from the external interface, it checks whether the destination port is in the range 61000-64999 and, if so, checks for any current entries in the masquerading table. If there is a matching entry, it rewrites the packet header and forwards it onto its new desintation. Port forwarding performs an additional check on the destination port if it isn't in the masquerading range to see whether it's a forwarding port. If it is, we let the existing code check for an entry in the masquerading table. If a corresponding entry exists in the masquerading table, the existing masquerading code rewrites the header and sends the packet out. If the destination port is a forwarding port but doesn't have an entry in the masquerading table, we create a suitable entry in the table before rewriting the packet and sending it out.

Port Forwarding - How do I use it?

The code was original written for Linux 2.0.27-29 and worked with many other 2.0.* kernel. Changes were made to the kerne's masquerading code in 2.0.30 and this provoked me to improve and upgrade my patches while converting them for 2.0.30. The new patches use 17k less memory when in use and are smaller, neater and better integrated into the kernel distribution and with a Makefile option.

Using the Linux 2.0.27-29 patches.
Using the Linux 2.0.30-38 patches.
Using Port Forwarding with Linux 2.2.

Port Forwarding - Anything else?

When I wrote this code, I didn't imagine anyone else ever using this code other than the computer group I mentioned. I would appreciate all comments on the idea of port forwarding and its implementation, whether it's "didn't you know there's a much easier way of doing it" or "I liked the idea but I extended it by doing ...". If there are any mistakes or areas that aren't clear in the above documentation, please also let me know. My email address is: steven@monmouth.demon.co.uk

First FAQ: I now know what port forwarding is but what's IP Substitution or IPSubs?

Answer: IP Substitution was the original name I gave to the code I wrote. I subsequently decided I didn't like the name and changed it to Port Forwarding instead.


Last updated on 22nd October, 2000