Port Forwarding http://www.monmouth.demon.co.uk/ipsubs/portforwarding.html 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. 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 10.0.0.2 (an internal machine). The incoming web connection would be labelled: Source: 163.158.1.2/7890 Dest: 194.160.1.1/80 This would be forwarded on to the internal host as: Source: 163.158.1.2/7890 Dest: 10.0.0.2/80 Replies would be labelled: Source: 10.0.0.2/80 Dest: 163.158.1.2/7890 and would be rewritten by the gateway to: Source 194.160.1.1 Dest: 163.158.1.2/7890 This has a number of advantages over using tools like nc and plug-gw to do the forwarding: As there is less copying of packets in memory, it's much faster. Internal hosts see the original connection source so so logs are meaningful. It's easy to load split between the internal hosts. The choice of host can also depend on live feedback such as which machine has the lower load average. This decision making is done entirely in userspace so is easy to implement. As redirection is done at a packet level, you don't need different gatewaying tools for different services. It's very stable. Our gateway machine has been crashed many times (it's not difficult to crash a machine with only 4Mb of memory) but it's carried on forwarding/masquerading although someone notices that they can't log in and reboots it. 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. 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. Lastly... Last updated on 24th January, 2000 www.monmouth.demon.co.uk -------------------------------------------------------------- Patch the kernel, configure the kernel (remember to enable the CONFIG_IP_MASQUERADE_IPPORTFW option) and recompile (replace x with appropriate version): cd /usr/src/linux zcat subs-patch-1.3x.gz | patch -p1 Compile the configuration utility: gcc ipportfw.c -o ipportfw chmod a+x ipportfw Before attempting to setup Port forwarding, set up and use IP Masquerading. Until it's enabled with ipportfw, Port forwarding will not affect IP Masquerading in any way. To configure redirection of a local port (local refers to the machine running the masquerading), you need to use ipportfw. It's options can be described as: ipportfw -h ipportfw Usage: ipportfw -A -[t|u] l.l.l.l/lport -R a.a.a.a/rport add entry ipportfw -D -[t|u] l.l.l.l/lport delete entry ipportfw -C clear table ipportfw -L list table l.l.l.l is the local interface receiving packets to be forwarded. a.a.a.a is the remote address. lport is the port being redirected. rport is the port being redirected to. You need an example? ok. /sbin/ipfwadm -I -f /sbin/ipfwadm -O -f /sbin/ipfwadm -F -f /sbin/ipfwadm -F -p deny /sbin/ipfwadm -F -a masquerade -S 10.0.0.0/8 -D 0.0.0.0/0 /usr/local/sbin/ipportfw -C /usr/local/sbin/ipportfw -A -t194.160.1.1/80 -R 10.0.0.2/80 /usr/local/sbin/ipportfw -A -u194.160.1.1/525 -R 10.0.0.2/525 /usr/local/sbin/ipportfw -L Prot Local Addr/Port > Remote Addr/Port TCP 194.160.1.1/80 > 10.0.0.2/80 UDP 194.160.1.1/525 > 10.0.0.2/525