Need help for iRule

The iRule would require a data-group, i.e.:

ltm data-group internal dg_internal_external {
    records {
        10.131.131.125/32 {
            data 10.141.141.125
        }
        10.131.131.126/32 {
            data 10.141.141.126
        }
    }
    type ip
}

An LTM (!) iRule to translate a response would look as follows:

when DNS_RESPONSE {

    foreach item [DNS::answer] {
        if {[DNS::type $item] eq "A"} {
            log local0. "[DNS::rdata $item]"
            log local0. "[DNS::type $item]"
            log local0. "[class match -value [DNS::rdata $item] equals dg_internal_external]"
            DNS::rdata $item [class match -value [DNS::rdata $item] equals dg_internal_external]
        } else {

            log local0. "[DNS::type $item]"
        }

    }
}

The iRule will be associated with your GMT listener (as well in the LTM context):

modify ltm virtual vs_10_131_131_53_53_gtm rules { rule_modify_dns_response }

Now the translation should work as I just tested:

 host -t A test.lb-net.bit 10.131.131.53
Using domain server:
Name: 10.131.131.53
Address: 10.131.131.5353
Aliases:

test.lb-net.bit has address 10.141.141.126
 host -t A test.lb-net.bit 10.131.131.53
Using domain server:
Name: 10.131.131.53
Address: 10.131.131.5353
Aliases:

test.lb-net.bit has address 10.141.141.125

You may want to use the iRule editor to craft the data-group and to assign it to the listener. This would be required with a plain GTM license.

Make sure to build your iRule in the LTM context. It sounds a bit odd, but the GTM listener is an object in the LTM context.