Re: [9fans] $smtp dns failure

[erik quanstrom <quanstro@coraid.com>]

§5 of rfc 2821 spells out how to do the lookup.  smtpd should do that.
if there are any huristics to apply while running the lookup, it
would make sense to me to do as sendmail/postifix/qmail do.
smtp is an interface to the world.  being different isn't a virtue
in this case.

- erik

On Tue Jan 16 13:46:09 EST 2007, rsc@swtch.com wrote:
> > there is an interesting case  with mxlookup1() in mxdial.c.
> > if a dns /failure/ is reported for the mx lookup, then mxdial doesn't
> > try a/cname records.  i've had trouble in the past with mx failures when the
> > a record looks up just fine.  i'm not sure if the standard says one must quit
> > on dns failure.
> 
> if there is a dns failure, the logic there is that since the
> mx could not be determined due to some transient dns error,
> we are not going to blindly try the underlying host name,
> because that is likely to be wrong.  i am sure that i put this
> in because of actual problems.
> 
> the real problem is that smtp is doing a dns lookup on 192.168.0.22,
> which ndb/dns interprets as a request for a reverse lookup
> (that is, it looks up 22.0.168.192.in-addr.arpa), which fails.
> mxlookup should probably check whether ds->host is a
> valid ip address all by itself, and then return 0 if so, rather
> than ask dns anything at all.  unfortunately i don't think there
> is a library routine that will tell us whether a string is a valid
> ip address.  parseip doesn't.
> 
> russ