[CentOS] Can't get forwarding to work

Neil Aggarwal neil at JAMMConsulting.com
Mon Oct 5 00:54:24 UTC 2009

Hello all:

I am having trouble getting port forwarding
with iptables to work.

I have this configuration:

Internet --- Linux machine --- Windows machine

The windows machine is set up to listen on 
IP address

I am able to use lynx to connect to on 
the linux machine so there is no problem going from
the linux machine to the windows machine.

I have tied a public IP address a.b.c.d to the
linux machine and entered these rules in its

/sbin/ifconfig eth0:1 $WIN_EXTERNAL_IP netmask $ETH0_NETMASK broadcast
/sbin/iptables -t nat -A PREROUTING -p tcp -i eth0 -d $WIN_EXTERNAL_IP
--dport 80 -j DNAT --to $WIN_INTERNAL_IP:80
/sbin/iptables -A FORWARD -p tcp -i eth0 -d $WIN_INTERNAL_IP --dport 80 -j

I have this log entry at the bottom of the firewall rules:

/sbin/iptables -A FORWARD -j LOG --log-prefix "FORWARD "

When I try to go to the external IP address http://a.b.c.d from
another network, I get an error.

Any ideas?


Neil Aggarwal, (281)846-8957, www.JAMMConsulting.com
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