[Mailman-Users] Virtual hosts

Brad Knowles brad at shub-internet.org
Wed Apr 11 05:55:13 CEST 2007

At 9:27 PM -0500 4/10/07, Brad Knowles wrote:

>  The important sentence here is:
>  |              In the dot-atom form, this is interpreted as an Internet
>  |   domain name (either a host name or a mail exchanger name) as
>  |   described in [STD3, STD13, STD14].
>  In particular, note that it says "host name" or "mail exchanger name",
>  and specifically omits "CNAME alias", and refers you to STD3 (RFCs 1122
>  and 1123) and STD13 (RFC 1034) for definitions of these terms.

In contrast, here's a quote from the cf/README file for sendmail 8.13.8:

| confDONT_EXPAND_CNAMES  DontExpandCnames
|                                        [False] If set, $[ ... $] lookups that
|                                        do DNS based lookups do not expand
|                                        CNAME records.  This 
currently violates
|                                        the published standards, but the IETF
|                                        seems to be moving toward legalizing
|                                        this.  For example, if "FTP.Foo.ORG"
|                                        is a CNAME for "Cruft.Foo.ORG", then
|                                        with this option set a lookup of
|                                        "FTP" will return "FTP.Foo.ORG"; if
|                                        clear it returns 
"Cruft.FOO.ORG".  N.B.
|                                        you may not see any effect until your
|                                        downstream neighbors stop doing CNAME
|                                        lookups as well.

And the HISTORY file for postfix-2.4.1-RC1:

| 20021207
|        Performance: RFC 2821 blesses the use of CNAME domain names
|        in MAIL FROM and RCPT TO. Not having to expand CNAME domain
|        names speeds things up a bit.  File:  smtp/smtp_proto.c.

Now, I'm still not seeing the part that Wietse is referring to.  I'm 
still checking the document.

However, Eric's comment in the sendmail cf/README file makes it plain 
that this is a common behaviour that has been done for as long as 
Internet e-mail has existed, and therefore it's going to be a very, 
very long time before we can assume that most sites won't do this 
kind of thing -- even if the IETF does finally codify the practice.

So, we're still going to need a FAQ on this subject, regardless of 
whether the then-current RFCs require that CNAME aliases be resolved 
to their actual canonical names or not.

Brad Knowles <brad at shub-internet.org>, Consultant & Author
LinkedIn Profile: <http://tinyurl.com/y8kpxu>
Slides from Invited Talks: <http://tinyurl.com/tj6q4>

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