Configuring IP Filter for firewall usage. ========================================= Step 1 - Block out "bad" IP packets. ------------------------------------ Run the perl script "mkfilters". This will generate a list of blocking rules which: a) blocks all packets which might belong to an IP Spoofing attack; b) blocks all packets with IP options; c) blocks all packets which have a length which is too short for any legal packet; Step 2 - Convert Network Security Policy to filter rules. --------------------------------------------------------- Draw up a list of which services you want to allow users to use on the Internet (e.g. WWW, ftp, etc). Draw up a separate list for what you want each host that is part of your firewall to be allowed to do, including communication with internal hosts. Step 3 - Create TCP "keep state" rules. --------------------------------------- For each service that uses TCP, create a rule as follows: pass in on proto tcp from to any port flags S/SA keep state where * "int-a" is the internal interface of the firewall. That is, it is the closest to your internal network in terms of network hops. * "int-net" is the internal network IP# subnet address range. This might be something like 10.1.0.0/16, or 128.33.1.0/24 * "ext-service" is the service to which you wish to connect or if it doesn't have a proper name, a number can be used. The translation of "ext-service" as a name to a number is controlled with the /etc/services file.