Plenty to find,
http://ubuntuforums.org/showthread.php?t=2141628
https://www.google.nl/#q=iptables%20string%20match%20not%20working
http://ubuntuforums.org/showthread.php?t=2141628
https://www.google.nl/#q=iptables%20string%20match%20not%20working