Clever Firewall Rules - Second Edition - Update

From: Rob 'Feztaa' Park (
Date: 09/19/01

Date: Tue, 18 Sep 2001 16:03:15 -0600 (MDT)
From: Rob 'Feztaa' Park <>
To: Bugtraq - Focus Linux <>
Subject: Clever Firewall Rules - Second Edition - Update
Message-ID: <>

Just a quick update, I don't want to send out the whole thing again.

I've managed to get my SYN limiting rules to filter the closed ports,
eliminating the problem of ports randomly flickering between closed and
filtered during a SYN scan:

iptables -A INPUT -i eth0 -p tcp --syn -m limit --limit 10/m \
     --limit-burst 10 -j syn_limit
iptables -A INPUT -i eth0 -p tcp --syn -j DROP
iptables -A syn_limit -p tcp --dport 80 -j ACCEPT
[Rules for opening any other ports you need]
iptables -A syn_limit -p tcp -j DROP

If I can, I'm going to make the limiting host-specific. As I mentioned
before, somebody who wanted to prevent access to my webserver could just
continually SYN scan me - the only way of fixing this is to limit SYN
packets on a per-host basis. That way people can SYN scan me until the
cows come home and find nothing, while people sending single, legitimate
SYN packets to my webserver can get in.

The rest of my rules have stayed pretty much the same.

Rob 'Feztaa' Park
ICQ#: 49781692

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