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Old 06-12-2003, 12:57 PM   #1
LQ Newbie
Registered: Jun 2003
Posts: 2

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Angry Port Forwarding not working for Internal requests

Hi there,

I am trying to configure port forwarding on my server running Redhat 7.3. The idea is to forward requests to xxxx:8080 to (xxxx is the static IP I have.) It works fine if I try to connect to xxxx:8080 from a computer outside of my LAN, but DOESN'T WORK inside my LAN. Of course, I can use to load the page, which means the problem is solely with port forwarding. I definitely need to get this working because I have IP addresses in my HTML and other files, and I can't do testing if xxxx:8080 doesn't work in my LAN.

Anyone else having the same problem with Port Forwarding of requests from $INTIF? Any advice? Did I do something wrong or is it possible to do? Thanks. My port forwarding section of is attached below....

//Accept connections to port 8080
iptables -A INPUT -i $EXTIF -p tcp -d $EXTIP --dport 8080 -j ACCEPT
iptables -A OUTPUT -o $EXTIF -p tcp -s $EXTIP --sport 8080 ! --syn -j ACCEPT

iptables -t nat -A PREROUTING -p tcp -i $EXTIF -d $EXTIP --dport 8080 -j DNAT --to
iptables -A FORWARD -p tcp -i $EXTIF -d --dport 80 -j ACCEPT
//This doesn't work.....
iptables -t nat -A PREROUTING -p tcp -i $INTIF -d $EXTIP --dport 8080 -j DNAT --to
iptables -A FORWARD -p tcp -i $INTIF -d --dport 80 -j ACCEPT
Old 06-28-2003, 11:36 AM   #2
Registered: Jan 2003
Location: Poland, Warsaw
Distribution: LFS, Gentoo
Posts: 591

Rep: Reputation: 32
1. You don't need use INPUT and OUTPUT chains. Is it not true you need them for accepting the packet sent to port 8080.
This is the big difference between ipchains & iptables. (with ipchains you need them).

2. Regarding your problem.
In more complex problem there is no way to present the solution seeing only few rules.... but I'll try to decribe the problem you probably met.
I assume you are using the box as router so somewhere you have a rule with SNAT or MASQUERADE target. It makes the packet leaving your system to have $EXTIP as the source address. It is done independently if it is sent to the "world" IP or $EXTIP.
As a result packets sent from internal boxes to $EXTIP port 8080 leaves your router with dest ip = but src ip = $EXTIP.
And there is no rule for forwarding packets which are coming back (they are sent to $EXTIP!)
Old 06-29-2003, 12:42 AM   #3
LQ Newbie
Registered: Jun 2003
Posts: 2

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Yep, MASQUERADE is the problem. I have solved it now. Thanks.


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