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From: Wietse Venema (wietseporcupine.org)
Date: Fri May 29 2009 - 13:42:37 CDT
> Wietse Venema:
> > > Is it even neccessary to have a hostname at all since the days of having s
> > >-eperate machines for seperate daemons are behind us for most websites? Afte
> > >-r all, mail.example1.com, www.example1.com, pop3.example1.com,... are all th
> > >-e same machine these days.
> > A machine name is required for several email-related Internet standards.
> > Besides, having all DNS names resolve to the same box is not normal usage.
> I'll post some numbers today, based on domain names found in open
> mailing lists.
To test the hypothesis that mail.domain, www.domain, etc. all have
the same IP address, I did a quick survey based on email sender
domain names found in open mailing lists on postfix, freebsd, php,
and bugtraq, for the past 1.5 year. The list is available on request.
These mailing lists represents a technical audience. At least in
this population, the "domain in a host" approach is not dominant
For the analysis I queried each domain for its A records, for the
A records of "www.domain", and for the A records of the domain's
primary MX records. Queries were done in parallel to reduce
inconsistencies due to short TTLs. All queries followed CNAME
indirections. I deleted domains that produced no result at all.
A special case is when a domain has no MX records. In this case
RFC 5321 requires that mail is sent using domain's A records instead.
Thus, a domain without MX records is the same as a domain with an
MX record that points at the domain itself. Thus, the default DNS
rules correspond to a domain-in-a-box architecture.
The raw results are at the end of this message.
For this domain population, the "domain in a host" approach is not
dominant with email: 1116 or 3755 domains used the same IP addresses
for the domain itself and as for email (with or without explicit
MX records), and 917 of 3755 domains used the same IP addresses
for web services (http://www.domain) as for email (userdomain).
The "domain in a host" approach appears to be more common with web
services: 3346 of 3755 domains used the same IP addresses for the
domain itself as for web services (http://www.domain). I had
expected a larger number here.
Conclusion: SMTP is not HTTP; don't assume that what is valid for
web services is necessarily valid for email services.
Number of domains = 3755
with A records = 3147 (following CNAME)
with MX records = 3525 (following CNAME)
with www.domain A records = 3346 (following CNAME)
with A records == www.domain A records = 2589
with A records == primary MX A records = 886
with www.domain A records == primary MX A records = 917
with A records or www.domain A records == primary MX A records = 986
with A records == primary MX A records, or no MX records = 1116