Multi-homed GTM, how to restrict internal/external DNS queries

Here's a slight twist to get multi-homed records.

1. Create an "internal" topology region - include all of the local/internal IP subnets.

2. Create separate internal and external pools for each WIP resource with a common naming extension (ex. int_foo.example.com_pool and ext_foo.example.com_pool).

3. Create a "drop" pool - no members, preferred LB Method: Fallback IP, Fallback IP: 1.1.1.1, Alternate and Fallback LB Methods set to none.

4. Assign the external pool to the WIP.

5. Apply this iRule to all multi-homed WIPs:

   when DNS_REQUEST {
       if { [matchregion [IP::client_addr] internal_network] } {
           if { [catch {
                try to send internal GTM pool
               set pool [findstr [LB::server pool] "ext_" 4]
               pool "int_$pool"
           } error] } {
                internal GTM pool doesn't exist - send nothing
               pool drop_pool
           }
       } 
   }
   

This is really nothing more than a variation on some of the examples above, and probably pretty close to Jason's comments, but can be done completely inside a GTM iRule and will allow you to serve up internal and external DNS entries for the same resources (if they exist).