Configuring IP Filter for firewall usage.

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Step 1 - Block out "bad" IP packets.

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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.

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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.

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For each service that uses TCP, create a rule as follows:

pass in on <int-a> proto tcp from <int-net> to any port <ext-service> 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.