Maybe try adding -t filter.
https://www.centos.org/forums/viewtopic.php?t=8995Code:iptables -t filter -A INPUT -p udp -m string --algo bm --string "BANNED_CDKEY" --sport 20700 --dport 28961 -j DROP iptables -t filter -A INPUT -p udp -m string --algo bm --string "INVALID_CDKEY" --sport 20700 --dport 28961 -j DROP