DNS domain blocking using UDP payload

hi, , we are trying to filter some DNS quueries in our bigIP, but face some problems - running version is 10.1 - only LTM license that means we can not use DNS irules statements, so we though about using UDP payload features for that reason we tried the following

when CLIENT_ACCEPTED { set payload [UDP::payload] if {[matchclass $payload contains "google"]} { reject } } this is working and it is able to reject DNS queries to google, www.google.com, etc but if we write down $payload contains "www.google.com"]} it is not working, neither for google, nor for google.com we tried to check the payload itself (logging it) and it shows something like blablablawwwgooglecomblablabla, without the dot between google and com any idea? we are interested in filtering www.google.com and not google or google.com (this is just an example, URL is different in life system) thanks a lot in advance