CNAME & MX record confusion.

[Chris Buxton]

On Mar 3, 2008, at 1:25 AM, Stephane Bortzmeyer wrote:
> On Sun, Mar 02, 2008 at 02:32:34PM +0000,
> Stroller <stroller at bigfoot.com> wrote
> a message of 49 lines which said:
>> This seems to result in mail delivery problems, and I don't
>> understand why.  The CNAME for the website surely shouldn't be
>> treated as an MX, should it?
>
> How could the DNS know that the CNAME was only for the Web site? A
> CNAME is just a way to set up an alias in the DNS, and this alias will
> be for all protocols (MX records, used only for email, are an
> exception).


Begging your pardon, Stephane, but what exactly do you mean by "MX  
records... are an exception"? I can think of two interpretations of  
what you said, and they're both wrong. If you meant something  
different, please explain.

1) The following is invalid:

domain.  MX     10  mail.server.
domain.  CNAME  otherdomain.

2) A CNAME record, as you said, creates an alias that then inherits  
all behaviors of the original. That is, given the following rrsets (in  
different zones):

alias.     CNAME  original.
original.  A  192.0.2.1
original.  MX 10  mail.server.

A request for the address of "alias" will yield the address of  
"original" - web requests will go there. But, in apparent  
contradiction to what you said, an email message sent to "user at alias"  
will arrive at the mail server of "original", "mail.server". To put it  
more directly, an alias does in fact inherit the mail routing of its  
original.

Chris Buxton
Professional Services
Men & Mice
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Phone:   +354 412 1500
Email:   cbuxton at menandmice.com
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