[ietf-dkim] Are subdomains like parent domains?

[Charles Lindsey]

On Tue, 29 Apr 2008 01:25:26 +0100, John Levine <johnl at iecc.com> wrote:

>>> But I have to say, without any sort of domain blanket/coverage
>>> option, it seems like something is really missing here.
>
> I'm seeing an implicit assumption that if someone has an opinion about
> mail from foo.com, they will have a similar opinion of mail from
> subdomains a.foo.com or a.b.foo.com, or a.b.c.foo.com.  I've been
> thinking about the mail I actually see, and I am having great
> difficulty finding even a small set of real life scenarios where that
> is true.
>
> For corporate or ISP domains, they either use one domain for all the
> mail, e.g. aol.com, or a handful of regional or divisional subdomains,
> e.g. twcny.rr.com or watson.ibm.com.  The other 99% of the names under
> the main domain are random hosts whose names are unlikely to appear as
> mail addresses* in real mail.

Not necessarily so. The machine random.example.com, which sits on some  
secretary's desk somewhere, is likely to have an MX record pointing to  
mail.example.com. So all outgoing mail will go through mail.example.com  
(it will be the machine appearing in the EHLO). And the postmaster at  
example.com will ensure that all such necessary MX records are set up.

But the problem is what goes in the From header of mails coming from  
random.example.com? Those will be set by whatever MUA is running on that  
secretary's machine which, if that machine was configured by the Secratary  
herself, will most likely be left by default at random.example.com.

Now a smart postmaster who is an expert in hacking sendmail.cf may well  
arrange to have that From header fixed before it goes out, but don't bank  
on it. And what happens if the secretary wamts to set the From header to  
something completely different, e.g. From: boss at somewhere.example.com with  
Sender: secretary.example.com? Don't tell me you have never seen that  
particular scenario before :-).

-- 
Charles H. Lindsey ---------At Home, doing my own thing------------------------
Tel: +44 161 436 6131                       
   Web: http://www.cs.man.ac.uk/~chl
Email: chl at clerew.man.ac.uk      Snail: 5 Clerewood Ave, CHEADLE, SK8 3JU, U.K.
PGP: 2C15F1A9      Fingerprint: 73 6D C2 51 93 A0 01 E7 65 E8 64 7E 14 A4 AB A5