Just to build on dangel's comment.
ipchains doesn't do reversed port forwarding through network address translation.
iptables will do this as part of the new MASQ requirements for 2.4 Kernels.
For more info on what's needed for Linux 2.0 & 2.2 Kernels read this:
The correct syntax is with a lower case "-a"
Also install the module as Linux 2.x doesn't use a Monolithic Kernel.
ipmasqadm portfw -a -P tcp -L 188.8.131.52 80 -R 10.0.21.2 80
What does it all mean:
ipmasqadm = ipmasq admin
portfw = internal port forward option
-a = add
-P = Protocol type
-L = source address port number
-R redirect address port number