Forum Discussion

Leilow214's avatar
Icon for Altostratus rankAltostratus
Aug 09, 2022

F5 DNS iRule for disabling DNS Express for a CNAME query on a particular domain

Hey Folks!

So we're using an F5 DNS as a resolver for some of our linux servers and at the same time it is the authoritative DNS for that particular domain

let's say we host on the F5 DNS.

When we create a CNAME record on CNAME

and hosts 3 A records.

What happens is DNS Express wont let us use the recursion since its built that way.

What we want is to disable DNSExpress when a CNAME query for specifically. 

Is there an irule to do that?

Any help is greatly appreciated!

Thank you!




7 Replies

  • Leilow214 - this has been hanging out for a while - have you figured out a path forward?
    If not...let me know if anything has changed since the 9th and I'll see if I can find someone to lend a hand.

    • Leilow214's avatar
      Icon for Altostratus rankAltostratus

      Hey Lief,

      Good day!

      Thanks for checking out on my question, I havent tested any solutions yet. But here's what Irule I did, Im not sure if this would work.

      Name: CNAME_DNSX_iRule

      when DNS_REQUEST {

      if { ([string toupper [DNS::question type]] eq "CNAME") }
      DNS::disable dnsx


      Thank you.

      • JRahm's avatar
        Icon for Admin rankAdmin

        yes, that should work fine, DNS iRules processing is handled up front (as shown in this lightboard lesson). You can further refine by extending your if condition to the targeted domain in the query received if necessary.

        I'm pretty sure DNS::question type returns all caps already, but you could test to make sure and then eliminate the extra string command for the comparison.