Sendmail/DNS

Alexander Stade alex at wego.com
Thu Mar 8 03:04:26 UTC 2001


Sendmail does accept mail without an MX record if it believes that it is 
the host that is called. Say "jim at example.com" goes to 1.2.3.4 which 
resolves as an A record:

example.com.    IN A    1.2.3.4

Sendmail accepts that. But as this may not be the optimal way of doing it, 
MX records are provided as a convenience and they should typically be used. 
Lyle's suggestion is therefore correct. NEVER point MX records to a CNAME. 
Sendmail looks for an MX record and then canonicalizes that. This means it 
doesn't query the DNS for an MX record which then turns into a CNAME which 
then turns in to an A record. It will stop at the MX record. If the MX 
record doesn't resolve into an A record (read IP address), the mail goes to 
the bit-bucket.

Something that has already been said, is that you should look for MX 
records, not A records. Therefore, set your querytype to "mx".

Alex

At 04:40 PM 3/14/2001 -0600, you wrote:

>Also unless you use NSLOOKUP to grab the MX records or do AXFR, how do you
>know if there is an MX record being returned by DNS? Just querying the
>domain without specifying the MX records returns the A record which is not
>what Sendmail needs.
>
>Lyle
>
>-----Original Message-----
>From: Joseph S D Yao [mailto:jsdy at cospo.osis.gov]
>Sent: Wednesday, March 14, 2001 4:16 PM
>To: Ziad Ghafour
>Cc: Fred Garlock; 'bind-users at isc.org'
>Subject: Re: Sendmail/DNS
>
>
>
>On Wed, Mar 14, 2001 at 01:54:49AM -0500, Ziad Ghafour wrote:
> > I am using a mail relay on a server, and when I try to send any mail out,
>it
> > gives me the following error:
> >
> > Deferred: Name server: X.mydomain.com: host name lookup failure
> >
> > but when I try to do nslookup on the X.mydomain.com, it resolve it ok
>
>You probably have one of the several systems that have
>/etc/nsswitch.conf or some other file within which you have to turn on
>DNS.  And it is not turned on.  One of 'nslookup's features is that it
>ignores such files and uses DNS even if it is not enabled on your
>machine.
>
>--
>Joe Yao                         jsdy at cospo.osis.gov - Joseph S. D. Yao
>COSPO/OSIS Computer Support                                     EMT-B
>-----------------------------------------------------------------------
>This message is not an official statement of COSPO policies.



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