I am starting a new post as I still have not been able to accomplish my goal, and I really need to get this figured out. Any and all help would be greatly appreciated.

My problem. I need to set up a Linux machine containing one nic to accept connections on a port and forward them on to the same port on another computer. For example, when the Linux machine (10.1.1.2) receives a packet on port 25 I need for the machine running SMTP at 10.1.1.1 to answer. I think this is called port forwarding, but so far I have not been able to make it work.

I have deleted my /etc/sysconfig/iptables file and am starting with a clean slate after a reboot. How can I simply set up my Linux machine (10.1.1.2) to forward all packets that it receives on port 25 of eth0 (it's only nic) on to 10.1.1.1. No other security needed, just yet.

Once I get that working, I can then add the security portion piece by piece.

I think i figured it out, after a small amount of thinking...

I don't believe we actually setup the masq. part of your firewall

iptables stop
iptables save
iptables start

echo 1 > /proc/sys/net/ipv4/ip_forward

iptables -t nat -A PREROUTING -t tcp --dport 25 -j SNAT --to 10.1.1.1:25

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

That worked! Thank you, thank you, and thank you again!!!

ya i was being stupid at first...

dunno today has not even been close to being my day...

If you know anything about masq'ing.. it has to be done, because your computer isn't going to accept a tcp connection from the other machine..

ex:

Your on Machine A, and attempting to HTTP to Machine B,

But Machine B forwards to Machine C...

Well.. without masqing, Machine C is going to attempt to contact Machine A, about its request.. sorry for not noticing sooner... having 1 fucking sucking ass day at work.

make sure you edit the /etc/sysctl.conf file to having ip_forward = 1

so when it reboots you dont need to re-echo it