Ubuntu Security :: Filtering Connection Strings With Iptables?

I have several CS servers running on ubuntu server, and sometimes someone is trying to brute server's RCON password with the program called HLBrute. I've found the following rules to prevent such hack attacks, but they don't work What can be wrong in these rules?

Quote:

iptables -A INPUT -p udp -m multiport --dport 26000:30000 -m string --algo kmp --string "HLBrute" -m limit --limit 1/hour --limit-burst 5 -j LOG --log-prefix " HLBrute_Ataka "
iptables -A INPUT -p udp -m multiport --dport 26000:30000 -m string --algo kmp --string "HLBrute" -j DROP