Thread: Linux: Explain NAT/Firewall to me shortly and clearly with examples

Hello people. I've been looking to learn about NAT but all the guides ive found are very long and go in depth; not so easy to read. Im sure its not that complicated. Can some nice person please explain it to me "in a nutshell"? Lets say I have a small LAN of about 3 pcs and a NAT/firewall pc connecting them to the internet. What commands would I need to run to set up network address translation so that those pcs could communicate with the outside world?

NAT :-
Network Address Translation generally involves "re-writing the source and/or destination addresses of IP packets as they pass through a router or firewall"

Refer this page for more info about nat.

TO share internet to other pc you need to add MASQUERADE rule in NAT, go through this page for the same

For more info:

Code:

man iptables

thanks i read those links and they are very useful.

I have a little LAN now im trying to do a rule to forward all dns traffic from outside world to my DNS server: I think this:

Code:

iptables -t nat -A PREROUTING -p udp --dport 53 -s ! 10.35.13.0/8 -j DNAT --to-destination 10.35.13.4:53
iptables -t nat -A POSTROUTING -j MASQUERADE

should work, but the first command is for some reason not working. If I remove the exclamation mark it works but Im pretty sure its the right syntax because this syntax I found on many guides and websites. I get error: Bad argument `10.35.13.0/8'. once again, without the "!" the command runs perfectly

May be you are using old version of iptables that's why it is not accepting "!" .


What you want to achieve in dns ? can you explain more.