~ IPTables : Limit Incoming UDP Packets With a Certain Length ~

Hello,

I am currently trying to limit incoming UDP length 20 packets on a per IP basis to 5 a second using IPTables on a Linux machine (CentOS 5.2).

Basically, if an IP is sending more than 5 length 20 UDP packet a second to the local machine, I would like the machine to drop the excess length 20 packets coming from that IP.

The modules that should work perfectly for this type of "rule set" are;

• Limiting module
• Length module

Both of which are installed / compiled with the kernel/IPTables correctly and functioning.

I have tried several rule sets, and they all seem to not fully work. Either they drop all UDP length 20 packets going to the local machine or allow all them through.

Below is one of the rule sets I use, and it is not working. Any ideas what the issue could be?

Code:
iptables -N CHECK1
iptables -A INPUT -p udp -m length --length 20 -j CHECK1
iptables -A CHECK1 -p udp -m length --length 20 -m limit --limit 5/second -j ACCEPT
iptables -A CHECK1 -j DROP

Any help would be appreciated. Thanks ahead of time!

I'm not sure, but the second "-m length --length 20" is redundant. You only get to that table if this condition is true. What I'm not clear about the length module is if it is the rule that gets limited or something else. If it's the rule, then this would drop any packet over 20 bytes after there have been 5 per second.

Do you want to limit it by IP address? Then I recommend you use the "recent" feature:

iptables -N CHECK1
iptables -A INPUT -p udp -m length --length 20 -j CHECK1

iptables -A CHECK1 -m recent --name longudp --rcheck 1 --hitcount 5 -j DROP
iptables -A CHECK1 -m recent --name longudp --set -j RETURN