Temporary redirection with DNAT and SNAT

[Kirk <whereisgui@gmail.com>]

Hello,

I have to shutdown a proxy server for a few days and I need to
redirect its traffic to a server behind an iptables firewall. Here's
what I want to do:


Original request to $PUBLIC_IP:80 is redirected  to $PRIVATE_IP:2050
(machine behind firewall)

Packets from $PRIVATE_IP:2050 come out of the firewall as coming from
$PUBLIC_IP:80


I binded the proxy's public IP to the firewall's  external interface
(eth0) and added the following rules:

I think I got the first part right.
#test for ezproxy
-A FORWARD -i eth0 -o eth1 -p tcp --syn -d 192.168.0.3 --dport 2050 -j ACCEPT

But I'm having problems with the second part. The SNAT rule:
-I POSTROUTING -s 192.168.0.3 --sport 2050  -o eth0 -j SNAT --to 130.17.174.108

#This one seems OK too.
-A PREROUTING -i eth0 -p tcp -d $PUBLIC_IP --dport  80 -j DNAT --to
$PRIVATE_IP:2050


The SNAT rule generates the error:
Applying iptables firewall rules: iptables-restore v1.2.11: Unknown
arg `--sport'

One of the restrictions I have is that *only* the packets from
$PRIVATE_IP:2050 can go out as coming from $PUBLIC_IP:80.

Could someone provide help to solve this problem?

Thanks.
-K