Resources
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Firewall FAQ. This is a comprehensive FAQ on all kinds of Internet firewalls.
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TrinityOS. Written by David Ranch, who also wrote the IP Masquerade How-to, TrinityOS has lots of tips on securing Linux. It is a large document that can take a long time to download.
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Freesco (Free Cisco).
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Armoring Linux. This is Lance Spitzner's guide to Linux security for beginners.
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Maximum Linux Security (published by SAMS).
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Linux Security Toolkit, by David A. Bandel (published by IDG Books).
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Linux Firewalls, by Robert Ziegler (published by New Riders).