Question

IPTABLES: NAT and FORWARD

Asked by: telman

Hi,


I am trying to setup a spam filter gateway. We have iptables firewall running on Linux box. Our firewall is connected to the public internet and ip addresses behind firewall are public too.

So I want to redirect email traffic to the spam filter gateway using iptables.
Currently I am using forward rules to forward rules to forward my traffic from eth0 to eth1

   e.g.
    iptables -P INPUT DROP
    iptables -P FORWARD DROP
    iptables -P OUTPUT ACCEPT
    iptables -A FORWARD -i eth0 -o eth1 -m state --state ESTABLISHED,RELATED -j ACCEPT
    iptables -A FORWARD -p tcp -s 0/0 -d 0/0 --dport 80  -j ACCEPT
    iptables -A FORWARD -p tcp -s 0/0 -d 0/0--dport 443 -j ACCEPT

However I assumed I will have to use NAT to redirect emails to the spam gateway.
 
  iptables -P INPUT DROP
  iptables -P FORWARD DROP
  iptables -P OUTPUT ACCEPT
  iptables -t nat -P PREROUTING DROP
  iptables -t nat -P POSTROUTING DROP

  iptables -A FORWARD -i eth0 -o eth1 -m state --state ESTABLISHED,RELATED -j ACCEPT
  iptables -A FORWARD -p tcp -s 0/0 -d 0/0 --dport 80  -j ACCEPT
  iptables -A FORWARD -p tcp -s 0/0 -d 0/0--dport 443 -j ACCEPT

  iptables -A PREROUTING -i eth0 -t nat -p tcp -s 0/0 -d <Public_ip_1> --dport 25 -j DNAT --to-destination <Public_ip_2>:25

1. Can I use forward chain rules and NAT the same time?
2. Do I need to load NAT modules in  /etc/sysconfig/iptables-config?
3. Can I redirect public ip to public ip address or I will have to map public to non-public?
4. What will happen if I do not change default policy for NAT?
5. How do list rules for NAT?
6. Do I need to assign ip address to eth0 (external interface) in order use nat?


I enabled NAT today and firewall went down.


Thanks,

Telman

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Asked On
2004-07-26 at 14:29:58ID21071935
Tags

iptables

,

nat

,

forward

Topic

Linux Network Security

Participating Experts
1
Points
500
Comments
5

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Answers

 

by: telmanPosted on 2004-07-26 at 14:36:08ID: 11641885

iptables -A PREROUTING -i eth0 -t nat -p tcp -s 0/0 -d <Public_ip_1> --dport 25 -j DNAT --to-destination <Public_ip_2>:25


Public_ip_1 & Public_ip_2 are located behind firewall but they are public ip address.


Telman

 

by: ahoffmannPosted on 2004-07-27 at 08:47:07ID: 11647646

1. yes
2. yes, BUT I'd recommend to not use kernel modules for iptables, build all iptables modules static into your kernel
3. you can redirect whatever to whatever (as long as there'e no IP-conflict), SNAT and DNAT are your friends here
4. (don't know what you mean here)
5. iptables -L -n -t nat && iptables -L -n -t mangle
6. hmm, not shure here, simply test it
   AFAIK the kernel at least will not catch the packets destinied for forwarding (e.g. all those you want to NAT)

why would you not simply use MASQUERADING instead of [SD]NAT? makes things simpler ...

 

by: telmanPosted on 2004-07-27 at 10:24:23ID: 11648702

Hi  Ahoffmann,

I think I was confusing myself. Should not I assign public ip address to my external interface in order to use nat? Becouse <Public_ip_1> and <Public_ip_2> are behing firewall already. Basicly I was trying to  redirect traffic sent to <Public_ip_1> (which forwarded from eth0 to eht1)  to <Public_ip_2> (which forwarded from eth0 to eht1. I guess I was wrong right? or It is still posible? :)

How do I do this?
-----------------
2. yes, BUT I'd recommend to not use kernel modules for iptables, build all iptables modules static into your kernel
-----------------


Thanks,
Telman

 

by: ahoffmannPosted on 2004-07-27 at 23:29:50ID: 11653862

> Should not I assign public ip address to my external interface in order to use nat?
yes, you should assign an external IP (or det it via DHCP from your ISP)

> Basicly I was trying to  redirect traffic sent to <Public_ip_1> (which forwarded from eth0 to eht1)  to <Public_ip_2>
this is no problem, something like this (assuming eth0 with <Public_ip_1> and eth1 in the net segment of <Public_ip_2>):
iptables -t nat -A PREROUTING -i eth0 -p tcp  -j DNAT   --to <Public_ip_2>
iptables        -A INPUT      -i eth0 -p tcp  -j ACCEPT  -d  <Public_ip_2>
iptables        -A INPUT      -i eth1 -j ACCEPT
iptables        -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

2. ..  build all iptables modules static into your kernel
> How do I do this?
you need to rebuild your kernel.
go to the /usr/src/linux dir, start
  make menuconfig
and select Y instead of M for netfilter/iptables (and all its parts)

 

by: telmanPosted on 2004-07-28 at 09:16:59ID: 11658194

Hi Ahoffmann,

Thanks!!! This make sence.

Telman

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