For clarity, I'll provide two examples where I think it is motivated to
obtain the FQDN reference identifier in an automated fashion and not through
direct user input or configuration (contrary to section 5.1).
1) Obtaining EU Trusted Lists
EU has standardized on XML based lists over national Trust Service Providers
and their certificates. Each country in the EU publish their own list.
The EU commission provides a central list with all the URLs to each national
The first step is for the client to establish a secure connection with the
EU commission and download the EU list. This is done by configuration.
The second step is to automatically obtain reference identifiers for all
national lists from the EU list.
2) Redirects from a trusted service.
If I connect to a trusted service and then get redirected to another host,
it can be reasonable to obtain the reference identifier from the rediricet.
Typical application I can think of is a redirect to a SAML IdP or a SAML
On 10-09-08 4:21 PM, "Stefan Santesson" <stefan(_at_)aaa-sec(_dot_)com> wrote:
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
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
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?
On 10-09-08 3:40 PM, "Stefan Santesson" <stefan(_at_)aaa-sec(_dot_)com> 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" <bernard_aboba(_at_)hotmail(_dot_)com>
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:
The DNS domain name of the domain where the specified service
a process would be consistent with a match between user inputs
(the source domain and service type) and the presented identifier
Since this is not the definition of SRVName, this type of matching does not
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
[TP] what I thought was about to be raised here was a contradiction that
is all about information gotten from a DNS retrieval whereas the wording of
in this I-D
domain name and service type ... MUST NOT be derived from
the user inputs in an automated fashion (e.g., ... discovered through
resolution ... "
would appear to exclude DNS resolution. If DNS resolution is off limits,
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
The client now has assurance from the CA that this host is in fact authorized
to provide this service.
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