From mboxrd@z Thu Jan  1 00:00:00 1970
From: Brent Clark <bclark@eccotours.co.za>
Subject: use of -m  limit for Syn Flood protection
Date: Sat, 14 Oct 2006 15:16:34 +0200
Message-ID: <4530E332.4090306@eccotours.co.za>
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Hey all

In my continuous quest to understand TCP and netfilter / iptables more, I have started experimenting with rate limiting and different TCP Flags.

On my Lan I have mail server and obviously ident requests are perform etc, so I currently im trying this

# we allow 4 TCP connects per second, no more
$IPT -N syn-flood
$IPT -A syn-flood -m limit --limit 1/s --limit-burst 4 -j LOG --log-level info --log-prefix '#### Syn Flood ####'
$IPT -A syn-flood -m limit --limit 1/s --limit-burst 4 -j RETURN
$IPT -A syn-flood -j DROP

$IPT -t filter -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
$IPT -A FORWARD -p tcp --syn -j syn-flood

so now I get this

Oct 14 14:51:46 gate kernel: #### Syn Flood ####IN=eth1 OUT=eth0 SRC=192.168.111.11 DST=218.15.249.32 LEN=60 TOS=0x00 PREC=0x00 TTL=63 ID=23007 DF PROTO=TCP SPT=40108 DPT=113 WINDOW=5840 RES=0x00 SYN 
URGP=0
Oct 14 14:51:49 gate kernel: #### Syn Flood ####IN=eth1 OUT=eth0 SRC=192.168.111.11 DST=218.15.249.32 LEN=60 TOS=0x00 PREC=0x00 TTL=63 ID=23008 DF PROTO=TCP SPT=40108 DPT=113 WINDOW=5840 RES=0x00 SYN 
URGP=0
Oct 14 14:51:55 gate kernel: #### Syn Flood ####IN=eth1 OUT=eth0 SRC=192.168.111.11 DST=218.15.249.32 LEN=60 TOS=0x00 PREC=0x00 TTL=63 ID=23009 DF PROTO=TCP SPT=40108 DPT=113 WINDOW=5840 RES=0x00 SYN 
URGP=0
Oct 14 14:52:07 gate kernel: #### Syn Flood ####IN=eth1 OUT=eth0 SRC=192.168.111.11 DST=218.15.249.32 LEN=60 TOS=0x00 PREC=0x00 TTL=63 ID=23010 DF PROTO=TCP SPT=40108 DPT=113 WINDOW=5840 RES=0x00 SYN 
URGP=0

So my question is, have I maybe been to aggressive on the limit.

I use the -j syn-flood for both FORWARD and INPUT