Книга: Iptables Tutorial 1.2.2



The FORWARD chain contains quite a few rules in this scenario. We have a single rule which sends all packets to the bad_tcp_packets chain, which was also used in the INPUT chain as described previously. The bad_tcp_packets chain is constructed in such a fashion that it can be used recycled in several calling chains, regardless of what packet traverses it.

After this first check for bad TCP packets, we have the main rules in the FORWARD chain. The first rule will allow all traffic from our $LAN_IFACE to any other interface to flow freely, without restrictions. This rule will in other words allow all traffic from our LAN to the Internet. The second rule will allow ESTABLISHED and RELATED traffic back through the firewall. This will in other words allow packets belonging to connections that were initiated from our internal network to flow freely back to our local network. These rules are required for our local network to be able to access the Internet, since the default policy of the FORWARD chain was previously set to DROP. This is quite clever, since it will allow hosts on our local network to connect to hosts on the Internet, but at the same time block hosts on the Internet from connecting to the hosts on our internal network.

Finally we also have a logging rule which will log packets that are not allowed in one or another way to pass through the FORWARD chain. This will most likely show one or another occurrence of a badly formed packet or other problem. One cause may be hacker attacks, and others may be malformed packets. This is exactly the same rule as the one used in the INPUT chain except for the logging prefix, "IPT FORWARD packet died: ". The logging prefix is mainly used to separate log entries, and may be used to distinguish log entries to find out where the packet was logged from and some header options.

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