Iptables - forward email ports?

Question

im trying to open some local ports (LAN) and then re-direct them to another server (WAN) using iptables.

Here is my config:

#WAN
allow-hotplug eth1
auto eth1
iface eth1 inet static
#Tarjeta red WAN
address 192.168.2.2
gateway 192.168.2.1 
netmask 255.255.255.0

#LAN
allow-hotplug eth0 
auto eth0
iface eth0 inet static
address 192.168.16.6
netmask 255.255.255.0
network 192.168.16.0 
broadcast 192.168.16.255

I try this:

iptables -A PREROUTING -t nat -i eth0 -p tcp --dport 110 -j DNAT --to 200.40.30.218:110
iptables -A FORWARD -p tcp -i eth0 -o eth1 -d 200.40.30.218 --dport 110 -j ACCEPT 

iptables -A PREROUTING -t nat -i eth0 -p tcp --dport 25 -j DNAT --to 200.40.30.218:25
iptables -A FORWARD -p tcp -i eth0 -o ethq -d 200.40.30.218 --dport 25 -j ACCEPT

but, it did not work. i also try changeing eth0 to eth1 (and eth1 to eth0) but nothing happened.

Starting Nmap 5.00 ( http://nmap.org ) at 2011-10-03 14:44 UYST
Interesting ports on 192.168.16.6:
Not shown: 997 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
111/tcp  open  rpcbind
8080/tcp open  http-proxy

Im running debian. Can u guys help me to check what is happening?

edit: IPTABLES-SAVE

# Generated by iptables-save v1.4.8 on Mon Oct  3 15:43:14 2011
*mangle
:PREROUTING ACCEPT [139993:77867651]
:INPUT ACCEPT [139385:77761761]
:FORWARD ACCEPT [186:12071]
:OUTPUT ACCEPT [173556:74341650]
:POSTROUTING ACCEPT [173734:74352988]
COMMIT
# Completed on Mon Oct  3 15:43:14 2011
# Generated by iptables-save v1.4.8 on Mon Oct  3 15:43:14 2011
*nat
:PREROUTING ACCEPT [1649:190626]
:POSTROUTING ACCEPT [6729:339646]
:OUTPUT ACCEPT [6697:337660]
-A PREROUTING -i eth0 -p tcp -m tcp --dport 110 -j DNAT --to-destination 200.40.30.218:110 
-A PREROUTING -i eth0 -p tcp -m tcp --dport 25 -j DNAT --to-destination 200.40.30.218:25 
COMMIT
# Completed on Mon Oct  3 15:43:14 2011
# Generated by iptables-save v1.4.8 on Mon Oct  3 15:43:14 2011
*filter
:INPUT ACCEPT [138307:77066136]
:FORWARD ACCEPT [168:11207]
:OUTPUT ACCEPT [172288:73655708]
-A FORWARD -d 200.40.30.218/32 -i eth0 -o eth1 -p tcp -m tcp --dport 110 -j ACCEPT 
-A FORWARD -d 200.40.30.218/32 -i eth0 -o ethq -p tcp -m tcp --dport 25 -j ACCEPT 
COMMIT
# Completed on Mon Oct  3 15:43:14 2011

Regards

Answer 1

it looks like MASQUERADING is not setup. Add the following before the first PREROUTING STATEMENT

-A POSTROUTING -o eth0 -j MASQUERADE

So it would be something like:

-A POSTROUTING -o eth0 -j MASQUERADE
-A PREROUTING -i eth0 -p tcp -m tcp --dport 110 -j DNAT --to-destination 200.40.30.218:110 
-A PREROUTING -i eth0 -p tcp -m tcp --dport 25 -j DNAT --to-destination 200.40.30.218:25 

Also, make sure that IP forwarding is enabled. ou can set it dynamically with either:

sysctl -w net.ipv4.conf.all.forwarding =1

Or:

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

To change it permanently (which is what you want), add or change the net.ipv4.conf.all.forwarding value in /etc/sysctl.conf to:

net.ipv4.ip_forward = 1 

The last one is I think how you would do it on Red Hat, so if you don't have that file, you can add the first command in /etc/rc.local or look up the Ubuntu way of making the change.

Also, you last FORWARD statement appears to be error. It looks like it is supposed to be:

-A FORWARD -d 200.40.30.218/32 -i eth1 -o eth0 -p tcp -m tcp --dport 110 -j ACCEPT 
-A FORWARD -d 200.40.30.218/32 -i eth1 -o eth0 -p tcp -m tcp --dport 25 -j ACCEPT

I think that is how you get it working. I re-read the post and you want to forward inbound traffic to servers on the WAN, so I made the above changes to account for that (it a little confusing, because you usually go from WAN -> LAN with NAT, not the other way around).

But the important thing is that you need to ensure that IP forwarding is enabled as well as MASQUERADING.