Valid MX response includes A record for mailexchanger ?

[Mark_Andrews at isc.org]

> Thanks for your message, Kevin.
> What I conclude from your reply is this:
> It's optional for nameservers to return A records for mailexchangers, and it
> is mandatory for mailservers to do additional lookups.
> Do all mailserver-implementations do additional lookups ?

	Yes.
 
> Best regards
> Peter Juul Sørensen
> 
> "Kevin Darcy" <kcd at daimlerchrysler.com> wrote in message
> news:amare9$1pho$1 at isrv4.isc.org...
> >
> > "Peter Juul S=F8rensen" wrote:
> >
> > > My ISP has configured DNS-records for our mailserver. The mailserver is
> > > located at another ISP. Some mailservers on the Internet can mail us -
> > > others cannot. My guess is that the DNS-records have been configured wr=
> > ongly
> > > and some mailservers therefore can't mail us.
> > > When I do a MX-lookup on the ISP nameserver, that is authoritative for =
> > my
> > > domain, I get this response
> > > -----------------------------------------------------------------------=
> > -----
> > > -----------------------------------
> > > C:\T>nslookup      -type=3DMX     mail.mydomain.com.   ns.isp1.net
> > > Server:   ns.isp1.net
> > > Address: 200.200.200.201
> > >
> > > mail.mydomain.com              MX preference =3D 10, mail exchanger =3D
> > > mailhotel.isp2.com
> > > mydomain.com                      nameserver =3D ns.isp1.net
> > > ns.isp1.net                             internet address =3D 200.200.20=
> > 0.201
> > > -----------------------------------------------------------------------=
> > -----
> > > -----------------------------------
> > > When I repeat this lookup a number of times, no additional records show=
> >  up,
> > > so the ISP nameserver apparently doesn't forward requests.
> > >
> > > Now, this response is not sufficient information for a mailserver to de=
> > liver
> > > mail to my mailserver.  The ip-address of my mailserver is missing, so =
> > a
> > > mailserver will need to do a second nameserver lookup for the ip-addres=
> > s.
> > > Some mailservers might not do this.
> > >
> > > My question is, must valid nameserver responses include an A-record for=
> >  the
> > > mailexchanger ?
> > >
> > > mailhotel.isp2.com                internet address =3D 100.100.100.101
> > > <=3D=3D=3D=3D  MUST be included ?
> >
> > Responses to MX queries usually include A records for the MX targets in t=
> > heir
> > Additional Section, but even if they are missing, the sending server shou=
> > ld be
> > issuing separate queries to resolve them. Any mail server that doesn't do=
> >  this
> > is broken. It's not always *possible* to include all of those A records i=
> > n the
> > response (think truncation), so it is *required* for mail servers to fall=
> >  back
> > to issuing separate queries in at least some cases.
> >
> >
> > - Kevin
> >
> >
> >
> 
> 
> 
--
Mark Andrews, Internet Software Consortium
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark.Andrews at isc.org