Re: [dane] Review of DANE SMTP draft

mrex@sap.com (Martin Rex) Mon, 17 March 2014 22:34 UTC

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From: mrex@sap.com (Martin Rex)
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Subject: Re: [dane] Review of DANE SMTP draft
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Viktor Dukhovni wrote:
> On Fri, Mar 14, 2014 at 09:01:48PM -0400, James Cloos wrote:
> 
>>> The folks at Postini have a wildcard cert for "*.psmtp.com" and
>>> clients publish MX records of the form:
>>>
>>>   verisign.com.           IN      MX      100 verisign.com.s6a1.psmtp.com.
>>>   verisign.com.           IN      MX      200 verisign.com.s6a2.psmtp.com.
>>>   verisign.com.           IN      MX      300 verisign.com.s6b1.psmtp.com.
>>>   verisign.com.           IN      MX      400 verisign.com.s6b2.psmtp.com.
>> 
>> For some historical context, mozilla's original wildcarded ssl implement-
>> ation also allowed an *. to match any number of labels.
>> 
>> Several sites were broken by the change to limit a wildcard to a single label.
> 
> I take it you're suggesting to not perpetuate Postini's abuse of
> wildcard certs?  Implementations might choose to be more liberal,
> but servers can't expect multi-label wildcard support.  Right?

See:

 http://tools.ietf.org/html/rfc6125#section-6.4.3

   6.4.3.  Checking of Wildcard Certificates

   A client employing this specification's rules MAY match the reference
   identifier against a presented identifier whose DNS domain name
   portion contains the wildcard character '*' as part or all of a label
   (following the description of labels and domain names in
   [DNS-CONCEPTS]).

   For information regarding the security characteristics of wildcard
   certificates, see Section 7.2.

   If a client matches the reference identifier against a presented
   identifier whose DNS domain name portion contains the wildcard
   character '*', the following rules apply:

   1.  The client SHOULD NOT attempt to match a presented identifier in
       which the wildcard character comprises a label other than the
       left-most label (e.g., do not match bar.*.example.net).

   2.  If the wildcard character is the only character of the left-most
       label in the presented identifier, the client SHOULD NOT compare
       against anything but the left-most label of the reference
       identifier (e.g., *.example.com would match foo.example.com but
       not bar.foo.example.com or example.com).

   3.  The client MAY match a presented identifier in which the wildcard
       character is not the only character of the label (e.g.,
       baz*.example.net and *baz.example.net and b*z.example.net would
       be taken to match baz1.example.net and foobaz.example.net and
       buzz.example.net, respectively).  However, the client SHOULD NOT
       attempt to match a presented identifier where the wildcard
       character is embedded within an A-label or U-label [IDNA-DEFS] of
       an internationalized domain name [IDNA-PROTO].


-Martin