I reviewed draft-saintandre-tls-server-id-check.
In a number of instances, this document is vague on the verification of an
SRV-ID, and in one instance, it appears to contradict RFC 4985, even though it
does not update that document.
Section 2.1 states:
o An SRV-ID can be either direct (provided by a user) or more
typically indirect (resolved by a client) and is restricted (can
be used for only a single application).
This is consistent with RFC 4985 Section 2.1 which states:
The SRVName, if present, MUST contain a service name and a domain
name in the following form:
_Service.Name
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.
[BA] As I understand RFC 4985, the SRV-ID provided in the target certificate
is to be
matched against components (service name and domain name) of the SRV RR
obtained
via lookup within the source domain. As a result, I don't believe that RFC 4985
is
consistent with this advice (e.g. the reference identifier is not matched
against the
SRV-ID).
Section 4.1 is not as clear as it could be on this point, given that it talks
about both
matching of the source domain and the target domain:
4. When checking a reference identifier against a presented
identifier, the client (a) MUST match the source domain (or, in
some cases, target domain) of the identifiers and (b) MAY also
match the service type of the identifiers.
Implementation Note: Naturally, in addition to checking
identifiers, a client might complete further checks to ensure that
the server is authorized to provide the requested service.
However, such checking is not a matter of verifying the
application service identity presented in a certificate, and
therefore methods for doing so (e.g., consulting local policy
information) are out of scope for this document.
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