DNS irule, do I need a GTM license?

This is an often confusing issue on two fronts: licensing AND functionality. iRules for GTM and iRules for LTM are not the same, even if they share similarities.

• GTM iRules are applied to a GTM wide IP, require a GTM license, and lack a lot of functionality in LTM iRules.
• LTM iRules are applied to an LTM virtual server, require either an LTM+GTM license or an LTM+DNS services license, and lack some functionality in GTM iRules, but not much. Mostly GTM pool/member state information.

The iRule you show above is already an LTM iRule. We can know this by using the context clues in the iRule by what commands are in use. If you review the DNS namespace for iRules on Clouddocs, you'll see that whereas DNS_REQUEST, DNS::question, and DNS::rr are available in both GTM and LTM, DNS::answer, DNS::additional, and DNS::return are available in LTM only.

So to answer your question...no, you do not need a GTM license to use this iRule if you already have a DNS Services license, and it should only need to be tailored to your needs as it is already an LTM iRule.