Книга: Iptables Tutorial 1.2.2

Starting SNAT and the POSTROUTING chain

Starting SNAT and the POSTROUTING chain

So, our final mission would be to get the Network Address Translation up, correct? At least to me. First of all we add a rule to the nat table, in the POSTROUTING chain that will NAT all packets going out on our interface connected to the Internet. For me this would be eth0. However, there are specific variables added to all of the example scripts that may be used to automatically configure these settings. The -t option tells iptables which table to insert the rule in, in this case the nat table. The -A command tells us that we want to Append a new rule to an existing chain named POSTROUTING and -o $INET_IFACE tells us to match all outgoing packets on the INET_IFACE interface (or eth0, per default settings in this script) and finally we set the target to SNAT the packets. So all packets that match this rule will be SNAT'ed to look as if they came from your Internet interface. Do note that you must set which IP address to give outgoing packets with the --to-source option sent to the SNAT target.

In this script we have chosen to use the SNAT target instead of MASQUERADE for a couple of reasons. The first one is that this script was supposed to run on a firewall that has a static IP address. A follow up reason to the first one, would hence be that it is faster and more efficient to use the SNAT target if possible. Of course, it was also used to show how it would work and how it would be used in a real live example. If you do not have a static IP address, you should definitely give thought to use the MASQUERADE target instead which provides a simple and easy facility that will also do NAT for you, but that will automatically grab the IP address that it should use. This takes a little bit extra computing power, but it may most definitely be worth it if you use DHCP for instance. If you would like to have a closer look at how the MASQUERADE target may look, you should look at the rc.DHCP.firewall.txt script.

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