Re: Name Service error but resolver is working

[Viktor Dukhovni]

> On Nov 7, 2018, at 3:27 PM, James B. Byrne <byrn...@harte-lyne.ca> wrote:
> 
> Neither dns02 nor dns04 are listed in the /etc/resolv.conf file on the
> affected services.
> 
> With respect to Viktor's answer.
> 
> My understanding is that: in the absence of a specified MX record then
> the A RR is supposed to be used. In this case MX31 is one of the MX
> for the entire domain. Why is the failure to lookup an MX record
> fatal? Why is not the A record value used in its absence?

Because absence != lookup failure.  Absence means "NXDOMAIN" or "NODATA"
not SERVFAIL.  Anything else would be disastrously fragile.

DNS MX lookups can result in:

  1.  RCODE:NoError, ANCOUNT>0  -- Hooray, MX RRset, use it
  2.  RCODE:NoError, ANCOUNT:0  -- (a.k.a. NoData), try A record instead.
  3.  RCODE:NXDomain,           -- Hardfail, the domain does not exist
  4.  RCODE:SerVFfail, ...      -- Lookup failure, try again later.

-- 
        Viktor.