Use VPS as a proxy port forwarder

Question

I have 2 internet connections at home. I surf on both of them: every other connection switches isp.

I already use my VPS as a regular proxy for surfing, ensuring me I always have the same IP address for certain sites.

However: I can not use a proxy for a certain Usenet program, so I would like to set up a port on my VPS as a very specific proxy. How do I do that?

I've tried this rule:

iptables -A FORWARD -p tcp -i eth1 -o eth1 -d 256.256.256.256 --dport 563 -m state --state NEW -j ACCEPT

ACCEPT     tcp  --  anywhere             usenet-server.com tcp dpt:nntps state NEW 
ACCEPT     udp  --  anywhere             usenet-server.com udp dpt:nntps state NEW 

But they do not work.

Answer 1

This did the trick:

YourIP=1.2.3.4
YourPort=80
TargetIP=2.3.4.5
TargetPort=22

iptables -t nat -A PREROUTING --dst $YourIP -p tcp --dport $YourPort -j DNAT \
--to-destination $TargetIP:$TargetPort
iptables -t nat -A POSTROUTING -p tcp --dst $TargetIP --dport $TargetPort -j SNAT \
--to-source $YourIP
iptables -t nat -A OUTPUT --dst $YourIP -p tcp --dport $YourPort -j DNAT \
--to-destination $TargetIP:$TargetPort

Lots of thanks to this article: http://www.debian-administration.org/articles/595