RE: Review of draft-saintandre-tls-server-id-check

["Bernard Aboba" <[e-mail]>]

So the statement that "RFC 4985 appears to require matching of the source
domain/service type to the SRV-ID in the certificate" is correct, right? 

If the "reference identifier" is  _Service.Name then the match is being done
on the *input* to the SRV lookup process, not the output, and prohibition on
DNS lookups would not apply (or even make any sense). 

-----Original Message-----
From: Stefan Santesson [mailto:[e-mail]] 
Sent: Wednesday, September 08, 2010 7:21 AM
To: Bernard Aboba; [e-mail]; [e-mail]; [e-mail]
Subject: Re: Review of draft-saintandre-tls-server-id-check

My apology,

I just realized that the document defines "source domain" as what I thought
would be the "target domain"

   source domain:  The fully-qualified DNS domain name that a client
      expects an application service to present in the certificate.

Which makes my comments below a bit wrong.

I think it would be better to discuss this in terms of "reference
identifier" and "presented Identifier".

   presented identifier:  An identifier that is presented by a server to
      a client within the server's PKIX certificate when the client
      attempts to establish a secure connection with the server; the
      certificate can include one or more presented identifiers of
      different types.

   reference identifier:  An identifier that is used by the client for
      matching purposes when checking the presented identifiers; the
      client can attempt to match multiple reference identifiers of
      different types.

I see no problem in obtaining the reference identifier from a DNS lookup an
the comparing it with a presented identifier in the certificate.

Why would you require the reference identity to be provided by a human user?

/Stefan



On 10-09-08 3:40 PM, "Stefan Santesson" <[e-mail]> wrote:

> Being the author of RFC 4985 I agree with most of you say here.
> 
> Comments in line;
> 
> On 10-09-06 8:48 PM, "Bernard Aboba" <[e-mail]> wrote:
> 
>> That was in fact my original question.
>> 
>> Section 5.1 states that the source domain and service type MUST be 
>> provided by a human user, and can't be derived.  Yet in an SRV or 
>> DDDS lookup, it is not the source domain that is derived, it is the 
>> target domain.  Given that, it's not clear to me what types of DNS 
>> resolutions are to be discouraged.
>> 
> 
> This puzzled me as well. The domain of interest is the domain where 
> the requested service is located = target domain.
> 
>> As noted elsewhere, RFC 4985 appears to require matching of the 
>> source domain/service type to the SRV-ID in the certificate.
> 
> It is not. RFC 4985 says the following in section 2:
> 
>       _Service.Name
> 
> <snip>
> 
>       Name
>          The DNS domain name of the domain where the specified service
>          is located.
> 
> 
>>  Such
>> a process would be consistent with a match between user inputs (the 
>> source domain and service type) and the presented identifier (the 
>> SRV-ID).
>> 
> 
> Since this is not the definition of SRVName, this type of matching 
> does not apply.
> 
>> 
>>> Yet, Section 5.1 states:
>>> 
>>> When the connecting application is an interactive client, the source
>>>    domain name and service type MUST be provided by a human user (e.g.
>>>    when specifying the server portion of the user's account name on the
>>>    server or when explicitly configuring the client to connect to a
>>>    particular host or URI as in [SIP-LOC]) and MUST NOT be derived from
>>>    the user inputs in an automated fashion (e.g., a host name or domain
>>>    name discovered through DNS resolution of the source domain).  This
>>>    rule is important because only a match between the user inputs (in
>>>    the form of a reference identifier) and a presented identifier
>>>    enables the client to be sure that the certificate can legitimately
>>>    be used to secure the connection.
>>> 
>>>    However, an interactive client MAY provide a configuration setting
>>>    that enables a human user to explicitly specify a particular host
>>>    name or domain name (called a "target domain") to be checked for
>>>    connection purposes.
>>> 
>>> [TP] what I thought was about to be raised here was a contradiction 
>>> that
>>> RFC4985
>>> is all about information gotten from a DNS retrieval whereas the 
>>> wording of
>>> s5.1
>>> in this I-D
>>> 
>>> "the source
>>>    domain name and service type  ...  MUST NOT be derived from
>>>    the user inputs in an automated fashion (e.g., ... discovered 
>>> through DNS resolution ... "
>>> 
>>> would appear to exclude DNS resolution.  If DNS resolution is off 
>>> limits, then
>>> RFC4985 would appear not to apply.
>>> 
> 
> RFC 4985 provides the client with a way to authenticate a host that it 
> believes is authorized to provide a specific service in the target domain.
> 
> It does not matter from where the client has obtained that 
> authorization information or whether that information is trustworthy.
> 
> A client may very well do an insecure DNS lookup to discover what host 
> is providing the requested service. The client would then contact that 
> host and obtained it's certificate. If the certificate is trusted and 
> it's SRVName matches the information provided from the DNS server, then
everything is fine.
> 
> The client now has assurance from the CA that this host is in fact 
> authorized to provide this service.
> 
> 
> /Stefan
>