nanog mailing list archives
Re: Is Hotmail in the habit of ignoring MX records?
From: David Miller <dmiller () tiggee com>
Date: Fri, 27 Jul 2012 21:40:18 -0400
On 7/27/2012 9:00 PM, Jimmy Hess wrote:
On 7/27/12, Tony Finch <dot () dotat at> wrote:That would be a seriously broken violation of the SMTP specification.I would definitely agree it would be quite broken behavior, but you know, I never said Hotmail's processing wasn't broken -- only that they seem to honor MX records in the common case. If you are doing something unusual like "mail MX bla bla" I would say you can't rule that out as a possible cause, just because some RFC suggests it should be OK. The spec does say that you're not allowed to chain MX records. But i'm not so sure that the specification actually prohibits a SMTP server from doing that, if someone does try to chain MX records. it may also be out of spec to have a "MX" record point to a dns label that a MX record exists for in the first place.
MX records don't "chain". If they did, then example.com. 1800 IN MX 10 example.com. would be an infinite loop. This isn't an infinite loop and is instead a perfectly valid configuration. If you made a DNS query for the MX records for example.com you would get back an answer that might include: ;; ANSWER SECTION: example.com. 1800 IN MX 10 example.com. ;; ADDITIONAL SECTION: example.com. 1800 IN A 10.10.10.10
From RFC 1035:
3.3.9. MX RDATA format +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | PREFERENCE | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / EXCHANGE / / / +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ where: PREFERENCE A 16 bit integer which specifies the preference given to this RR among others at the same owner. Lower values are preferred. EXCHANGE A <domain-name> which specifies a host willing to act as a mail exchange for the owner name. MX records cause type A additional section processing for the host specified by EXCHANGE. The use of MX RRs is explained in detail in [RFC-974]. -DMM
Current thread:
- Is Hotmail in the habit of ignoring MX records? Lou Katz (Jul 26)
- Re: Is Hotmail in the habit of ignoring MX records? Suresh Ramasubramanian (Jul 26)
- Re: Is Hotmail in the habit of ignoring MX records? Ryan Rawdon (Jul 26)
- Re: Is Hotmail in the habit of ignoring MX records? Tony Finch (Jul 27)
- Re: Is Hotmail in the habit of ignoring MX records? Jimmy Hess (Jul 27)
- Re: Is Hotmail in the habit of ignoring MX records? David Miller (Jul 27)
- Re: Is Hotmail in the habit of ignoring MX records? Michael J Wise (Jul 27)
- Re: Is Hotmail in the habit of ignoring MX records? Mark Andrews (Jul 27)
- Re: Is Hotmail in the habit of ignoring MX records? Michael J Wise (Jul 27)
- Re: Is Hotmail in the habit of ignoring MX records? Suresh Ramasubramanian (Jul 26)
- Re: Is Hotmail in the habit of ignoring MX records? Lou Katz (Jul 26)
- Re: Is Hotmail in the habit of ignoring MX records? Mark Foster (Jul 26)
- Re: Is Hotmail in the habit of ignoring MX records? Michael J Wise (Jul 26)
- Re: Is Hotmail in the habit of ignoring MX records? Mark Andrews (Jul 26)
- Re: Is Hotmail in the habit of ignoring MX records? Michael J Wise (Jul 26)
- Re: Is Hotmail in the habit of ignoring MX records? Mark Andrews (Jul 26)