david wong

Hey! I'm David, a security consultant at Cryptography Services, the crypto team of NCC Group . This is my blog about cryptography and security and other related topics that I find interesting.

NAT with iptables : super fast tutorial

posted April 2014

So I know how to use iptables, I know what a NAT is, but I don't want to learn how to exactly do it. Misery... I have to learn how to do it because I have an exam that will probably ask me how to do it in a few days. So I've been looking for a super simple tutorial, a 1 minute tutorial, on how to setup a NAT configuration with iptables in 1 minute. Couldn't really find it so here it is, if this is somewhat useful for someone, you're welcome.

First Step

For NAT to work, you have to allow forwarding on your server. Easy peasy:

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

Also, before adding new iptables rules, be sure to check what rules you already have

$ iptables -L

you should allow some forwarding for it to work (if the policy is default to DROP). But this not a tutorial about iptables.

Static

I have a server with:

  • eth0 connected to the network

  • eth1 connected to internet

Let's modify the PREROUTING part. Traffic coming from internet on our public address (@pub) and trying to reach our machine:

$ iptables -t nat -A PREROUTING -d @pub -i eth0 -j DNAT --to-destination @priv

Let's modify the table nat, append a rule to the pretrouting section : something is trying to reach @pub ? Let's put it in our input interface eth0, jump to the Destination Nat protocol, which tells us to send the packet to @priv.

Now Let's modify the POSTROUTING part. Traffic coming from inside our network and trying to reach something, somewhere on internet:

$ iptables -t nat -A POSTROUTING -s @priv -o eth1 -j SNAT --to-source @pub

If the packet is coming from @priv, let's put it on our output interface eth1 and jump to the Source Nat Protocol that will modify the packet so it has the public address (@pub) as source.

Here! You did it. One private IP address mapped to one public IP address.

Dynamic

Same kind of configuration but now we have several private addresses and only one public address.

$ iptables -t nat -A POSTROUTING -s @priv/mask -j MASQUERADE

We can modify every packets coming from the subnetwork @priv to get masqueraded.

$ iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE

Or we can just tell all the network to get masqueraded.

And this is it. No PREROUTING Needed.

Again, you're welcome ;)

Well done! You've reached the end of my post. Now you can leave me a comment :)

Kat

Hi, David. Thanks for the tutorial. It is indeed a super fast one :)

Note, that these instructions assume an empty firewall table. Packets could get blocked due to existing FORWARD rules. Another thing, the computer has to be specifically configured to allow forwarding for NAT to work at all! (net.ipv4.ip_forward).

Cheers!

David

You're right Kat! Adding that right away :)