DNS and qmail : CNAME failure

[Christopher K Davis]

Murray A Walker <maw at cstr.ed.ac.uk> writes:

> I have a problem getting mail delivered to my site cstr.ed.ac.uk from
> sites that use qmail - seems to be ok with mail from sendmail, etc
> sites.

Yes.  qmail (which is claimed to have "no limits other than memory" to
the number of MX records it can deal with) has a 512-byte limit on DNS
responses due to a small, fixed-size buffer.

> The cname lookups are performed on the MX values which, in turn, are
> liddell.cstr.ed.ac.uk and mailrelay.ed.ac.uk which are both actual
> addresses, not cname's, so the cname query obviously fails. So why
> does qmail screw up? Any ideas? After mailing the qmail discussion
> list I got a reply suggesting that our records were > 512 bytes which
> would caused qmail to fail. Is this an issue if there are no cname
> records for those hosts? If it is, is there a way to reduce the size
> of the record?

When qmail does a query to check for CNAME records, it actually does an
"ANY" query--on the domain name given--before looking for MXes.

It looks like you've compensated for this problem by reducing the size
of an ANY query response.  Of course, AOL can trigger the same problem
(and they haven't cut their ANY response size down :-); many qmail users
have simply used one of several patches available to fix the problem.

-- 
Christopher Davis * <ckd-sig at ckdhr.com> * <URL:http://www.ckdhr.com/ckd/>
Put location information in your DNS! <URL:http://www.ckdhr.com/dns-loc/>