MS DNS CNAMEs fail to resolve when Conditional Forwarding involved

Question

_{Wednesday, April 3, 2019 9:12 PM}

I have MS DNS servers set up to do recursive resolution. I have a number of domains set up to do conditional forwarding. When trying to look up a record in one of those domains that is a CNAME whose value is in another conditionally forwarded domain, the CNAME doesn't resolve properly.

The records involved look something like this:

alias.example.com CNAME host.extra.sub.example.net
sub.example.net NS ns.awsdns.com
host.extra.sub.example.net A 10.11.12.13

Both example.com and example.net are "conditionally forwarded" domains, forwarded to the same set of nameservers. sub.example.net is delegated to another set of nameservers at AWS.

If neither record is in the cache, the response looks correct, returning the CNAME and the resultant A record. If the CNAME record is in the cache but the A record is not in the cache, the MS DNS server either responds with only the CNAME record, or sometimes a SERVFAIL. (Which symptom seems to be dependent on the specific server I was requesting from; there may be different versions of MS DNS involved.) Requesting host.extra.sub.example.net directly always works and also inserts the record into the cache, causing the CNAME record request to start returning correct data.

It's possible for the one record to be in the cache and not the other because of differing TTL values for the records, or even if the TTL records are identical if the A record is directly requested before the CNAME record.

Removing the conditional forwarding for both resolves the problem. (But returns a logically incorrect result, as we were forwarding to "internal" nameservers with internal records.)

I feel like this is a bug in the MS DNS server, as removing the conditional forwarding results in a change in what MS DNS returns.

All replies (2)

_{Thursday, April 4, 2019 8:58 AM}

Hi,

I do not quite understand what you mean.

If you configure a zone and a conditional forwarder with the same name, the forwarder will not work.

Best regards,

Travis

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_{Monday, April 8, 2019 8:48 AM}

Hi,

Just checking in to see if the information provided was helpful.

Please let us know if you would like further assistance.

Best Regards,

Travis

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