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commit b05ec6cbdacdf40d6c75326394461e22b7f8ab20
parent 9ef349af5c83f54b4d62fdcc90e9603aa55c149d
Author: Jonathan de Boyne Pollard <>
Date:   Fri, 12 Jul 2019 23:34:52 -0600

Apply Jonathan de Boyne Pollard's any-to-cname patch.

modifies the behaviour of qmail-remote to remove the workaround
that Dan Bernstein added on 1996-10-03 to work around a bug in
BIND versions earlier than version 4.9.4.

Applying this patch incurs a risk, but yields a benefit. It is
published in order to allow others to experiment with removing
the workaround.

The risk is twofold:

 * qmail-remote will not be able to relay any mail if one's own
   proxy DNS server is such a version of BIND. This is trivially
   overcome by replacing such an old version of BIND either with a
   new version of BIND that doesn't have the problem or with some
   other proxy DNS server software entirely (such as dnscache).

 * qmail-remote will not be able to relay mail to domains whose
   content DNS servers use such versions of BIND, because the
   "CNAME" resource record lookup will fail. To gauge the level of
   this risk, notice that Dan's own 2002-12-17 survey of content DNS
   servers reports a mere 2% of the "*.com." content DNS servers as
   employing BIND version 4 (but doesn't report how many of that 2%
   employ BIND 4 versions earlier than 4.9.4).

The benefit of this patch is that it reduces DNS query traffic
and proxy DNS server cache load.

 * Without it, qmail-remote issues "ANY" queries. Some proxy DNS
   server softwares (albeit not dnscache) pass such queries through
   directly to the back end, meaning that every query issued by
   qmail-remote will result in a back-end query to a content DNS
   server, no matter if the necessary information is already cached.
   Moreover: The results of such a query, which are often a large
   collection of resource record sets of various types, are cached
   in the proxy DNS server's cache, even though almost none of them
   will be used. A caching proxy DNS server dedicated to serving
   qmail will end up with all sorts of cruft in its cache that isn't
   actually relevant to mail transportation, taking up space that
   could be better put to use caching those resource record sets
   that are relevant.

 * With it, qmail-remote issues "CNAME" queries. All of the mainstream
   proxy DNS server softwares in popular use (apart from dnscache,
   because it has problems in this regard) don't pass such queries
   directly through, and will answer them from their caches without
   issuing a back-end query at all if the data are already there and
   still current. Moreover: A caching proxy DNS server dedicated to
   serving qmail will not have its cache cluttered with irrelevant

Mdns.c | 2+-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/dns.c b/dns.c @@ -197,7 +197,7 @@ stralloc *sa; if (!sa->len) return loop; if (sa->s[sa->len - 1] == ']') return loop; if (sa->s[sa->len - 1] == '.') { --sa->len; continue; } - switch(resolve(sa,T_ANY)) + switch(resolve(sa,T_CNAME)) { case DNS_MEM: return DNS_MEM; case DNS_SOFT: return DNS_SOFT;