connectionless port forwarding

["Giacomo A. Catenazzi" <cate@debian.org>]

Hello.

I'm searching if I can do (or why not) a connectionless
port forwarding. Google didn't help me, and now I'm
using a std port forwarding using nat tables, but
a smaller solution is better IMHO.

I admin an "high" traffic web site. In last time there was
an huge increment of web-spam/blog-spam traffic, which I would
avoid.

I want to direct traffic from a blacklist into
an other port, so that a simple http server will
advise user (and offer a graphical challenge) to unblock.

Practically I want to mangle the port of blacklist-originated
packets, from 80 to 81, and the opposite for outgoing traffic.
Port 81 will be firewalled from extern, so I think there cannot
be problem with connection identification / collision.

Would it be possible?
Would it be lighter than std nat solution (and conncetion tracking)?
Are there already some netfilter module? (or
i should implement myself one?)

ciao
	cate

PS: please CC: me. It is easier to reply