Be warned, the MIRROR is dangerous and was only developed as an example code of the new conntrack and NAT code. It can cause dangerous things to happen, and very serious DDoS/DoS will be possible if used improperly. Avoif using it at all costs! It was removed from 2.5 and 2.6 kernels due to it's bad security implications!
The MIRROR target is an experimental and demonstration target only, and you are warned against using it, since it may result in really bad loops hence, among other things, resulting in serious Denial of Service. The MIRROR target is used to invert the source and destination fields in the IP header, and then to retransmit the packet. This can cause some really funny effects, and I'll bet that, thanks to this target, not just one red faced cracker has cracked his own box by now. The effect of using this target is stark, to say the least. Let's say we set up a MIRROR target for port 80 at computer A. If host B were to come from yahoo.com, and try to access the HTTP server at host A, the MIRROR target would return the yahoo host's own web page (since this is where the request came from).
Note that the MIRROR target is only valid within the INPUT, FORWARD and PREROUTING chains, and any user-defined chains which are called from those chains. Also note that outgoing packets resulting from the MIRROR target are not seen by any of the normal chains in the filter, nat or mangle tables, which could give rise to loops and other problems. This could make the target the cause of unforeseen headaches. For example, a host might send a spoofed packet to another host that uses the MIRROR command with a TTL of 255, at the same time spoofing its own packet, so as to seem as if it comes from a third host that uses the MIRROR command. The packet will then bounce back and forth incessantly, for the number of hops there are to be completed. If there is only 1 hop, the packet will jump back and forth 240-255 times. Not bad for a cracker, in other words, to send 1500 bytes of data and eat up 380 kbyte of your connection. Note that this is a best case scenario for the cracker or script kiddie, whatever we want to call them.
Works under Linux kernel 2.3 and 2.4. It was removed from 2.5 and 2.6 kernels due to it's inherent insecurity. Do not use this target!