From: Frank Gruellich <frank@der-frank.org>
To: netfilter@lists.netfilter.org
Subject: Re: bad tcp packets
Date: Tue, 23 Nov 2004 14:59:10 +0100 [thread overview]
Message-ID: <20041123135910.GT21819@der-frank.org> (raw)
In-Reply-To: <web-23112413@infinito.it>
* hamals@infinito.it <hamals@infinito.it> 23. Nov 04:
> Hello to everyone
Hi,
> I'm reading "Iptables Tutorial 1.1.19" by Oskar
> Andreasoon, and I cant understand these bad packets rules
Well choosen. Please, don't wrap commands. I'll fix in quoting.
> $IPTABLES -N bad_tcp_packets
> #
> #
> # bad_tcp_packets chain
> #
> $IPTABLES -A bad_tcp_packets -p tcp --tcp-flags SYN,ACK SYN,ACK \
> -m state --state NEW -j REJECT --reject-with tcp-reset
Conntrack treats a packet as state NEW, if it hits netfilter the first
time. The first packet of a TCP stream will never have set both of SYN
and ACK. This is the correct answer after a connection request (SYN)
from your network, but then it wouldn't be the first packet in stream.
> $IPTABLES -A bad_tcp_packets -p tcp ! --syn -m state --state NEW -j LOG \
> --log-prefix "New not syn:"
Same goes here: NEW in conntrack, but synflag not set should never
occure. So LOG...
> $IPTABLES -A bad_tcp_packets -p tcp ! --syn -m state --state NEW -j DROP
... and DROP it.
> someone could explain me why that are bad tcp packets?
HTH,
regards, Frank.
--
Sigmentation fault
next prev parent reply other threads:[~2004-11-23 13:59 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-11-23 12:18 bad tcp packets hamals
2004-11-23 12:42 ` Miguel Angel Amador L
2004-11-23 13:49 ` Jason Opperisano
2004-11-23 13:59 ` Frank Gruellich [this message]
-- strict thread matches above, loose matches on Subject: below --
2004-11-23 13:39 Scott Knake
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