--On Sunday, 27 November, 2005 08:23 +0100 Arnt Gulbrandsen
<arnt(_at_)gulbrandsen(_dot_)priv(_dot_)no> wrote:
Valdis Klētnieks writes:
I think Arnt's meaning was "If you get a SERVFAIL trying to
get the MX, then you *don't* know there's no MX, so dropping
back to an A is improper. The message should be queued and
retried later".
Exactly.
I'd say that SERVFAIL is best covered by RFC 2821 section 6.1,
which has this to say about error handling: «It [the MTA]
must take this responsibility seriously. It MUST NOT lose the
message for frivolous reasons, such as because the host later
crashes or because of a predictable resource shortage.»
Although the DNS is not mentioned, the passage doesn't seem to
condone speculatively falling back to A in case of an MX query
timeout, SERVFAIL, or other uncertain response.
Arnt
That was the intent. If it doesn't seem sufficient clear, text
would be welcome.
"There is no MX" means that one actually gets an authoritative
reply that says that there isn't one. Only then does one look
for the associated A RR and assume it is equivalent to a
best-priority MX.
john
john