rfc6125
Internet Engineering Task Force (IETF) P. Saint-Andre
Request for Comments: 6125 Cisco
Category: Standards Track J. Hodges
ISSN: 2070-1721 PayPal
March 2011
Representation and Verification of Domain-Based Application Service
Identity within Internet Public Key Infrastructure Using X.509 (PKIX)
Certificates in the Context of Transport Layer Security (TLS)
Abstract
Many application technologies enable secure communication between two
entities by means of Internet Public Key Infrastructure Using X.509
(PKIX) certificates in the context of Transport Layer Security (TLS).
This document specifies procedures for representing and verifying the
identity of application services in such interactions.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6125.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Audience . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. How to Read This Document . . . . . . . . . . . . . . . . 4
1.4. Applicability . . . . . . . . . . . . . . . . . . . . . . 5
1.5. Overview of Recommendations . . . . . . . . . . . . . . . 5
1.6. Generalization from Current Technologies . . . . . . . . . 6
1.7. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.7.1. In Scope . . . . . . . . . . . . . . . . . . . . . . . 7
1.7.2. Out of Scope . . . . . . . . . . . . . . . . . . . . . 7
1.8. Terminology . . . . . . . . . . . . . . . . . . . . . . . 9
2. Naming of Application Services . . . . . . . . . . . . . . . . 13
2.1. Naming Application Services . . . . . . . . . . . . . . . 13
2.2. DNS Domain Names . . . . . . . . . . . . . . . . . . . . . 14
2.3. Subject Naming in PKIX Certificates . . . . . . . . . . . 15
2.3.1. Implementation Notes . . . . . . . . . . . . . . . . . 17
3. Designing Application Protocols . . . . . . . . . . . . . . . 18
4. Representing Server Identity . . . . . . . . . . . . . . . . . 18
4.1. Rules . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 20
5. Requesting Server Certificates . . . . . . . . . . . . . . . . 21
6. Verifying Service Identity . . . . . . . . . . . . . . . . . . 21
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.2. Constructing a List of Reference Identifiers . . . . . . . 22
6.2.1. Rules . . . . . . . . . . . . . . . . . . . . . . . . 22
6.2.2. Examples . . . . . . . . . . . . . . . . . . . . . . . 24
6.3. Preparing to Seek a Match . . . . . . . . . . . . . . . . 25
6.4. Matching the DNS Domain Name Portion . . . . . . . . . . . 26
6.4.1. Checking of Traditional Domain Names . . . . . . . . . 27
6.4.2. Checking of Internationalized Domain Names . . . . . . 27
6.4.3. Checking of Wildcard Certificates . . . . . . . . . . 27
6.4.4. Checking of Common Names . . . . . . . . . . . . . . . 28
6.5. Matching the Application Service Type Portion . . . . . . 28
6.5.1. SRV-ID . . . . . . . . . . . . . . . . . . . . . . . . 29
6.5.2. URI-ID . . . . . . . . . . . . . . . . . . . . . . . . 29
6.6. Outcome . . . . . . . . . . . . . . . . . . . . . . . . . 29
6.6.1. Case #1: Match Found . . . . . . . . . . . . . . . . . 29
6.6.2. Case #2: No Match Found, Pinned Certificate . . . . . 29
6.6.3. Case #3: No Match Found, No Pinned Certificate . . . . 30
6.6.4. Fallback . . . . . . . . . . . . . . . . . . . . . . . 30
7. Security Considerations . . . . . . . . . . . . . . . . . . . 30
7.1. Pinned Certificates . . . . . . . . . . . . . . . . . . . 30
7.2. Wildcard Certificates . . . . . . . . . . . . . . . . . . 31
7.3. Internationalized Domain Names . . . . . . . . . . . . . . 32
7.4. Multiple Identifiers . . . . . . . . . . . . . . . . . . . 32
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 33
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9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 33
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
10.1. Normative References . . . . . . . . . . . . . . . . . . . 34
10.2. Informative References . . . . . . . . . . . . . . . . . . 34
Appendix A. Sample Text . . . . . . . . . . . . . . . . . . . . . 40
Appendix B. Prior Art . . . . . . . . . . . . . . . . . . . . . . 42
B.1. IMAP, POP3, and ACAP (1999) . . . . . . . . . . . . . . . 42
B.2. HTTP (2000) . . . . . . . . . . . . . . . . . . . . . . . 43
B.3. LDAP (2000/2006) . . . . . . . . . . . . . . . . . . . . . 44
B.4. SMTP (2002/2007) . . . . . . . . . . . . . . . . . . . . . 47
B.5. XMPP (2004) . . . . . . . . . . . . . . . . . . . . . . . 49
B.6. NNTP (2006) . . . . . . . . . . . . . . . . . . . . . . . 50
B.7. NETCONF (2006/2009) . . . . . . . . . . . . . . . . . . . 51
B.8. Syslog (2009) . . . . . . . . . . . . . . . . . . . . . . 52
B.9. SIP (2010) . . . . . . . . . . . . . . . . . . . . . . . . 54
B.10. SNMP (2010) . . . . . . . . . . . . . . . . . . . . . . . 55
B.11. GIST (2010) . . . . . . . . . . . . . . . . . . . . . . . 55
1. Introduction
1.1. Motivation
The visible face of the Internet largely consists of services that
employ a client-server architecture in which an interactive or
automated client communicates with an application service in order to
retrieve or upload information, communicate with other entities, or
access a broader network of services. When a client communicates
with an application service using Transport Layer Security [TLS] or
Datagram Transport Layer Security [DTLS], it references some notion
of the server's identity (e.g., "the website at example.com") while
attempting to establish secure communication. Likewise, during TLS
negotiation, the server presents its notion of the service's identity
in the form of a public-key certificate that was issued by a
certification authority (CA) in the context of the Internet Public
Key Infrastructure using X.509 [PKIX]. Informally, we can think of
these identities as the client's "reference identity" and the
server's "presented identity" (these rough ideas are defined more
precisely later in this document through the concept of particular
identifiers). In general, a client needs to verify that the server's
presented identity matches its reference identity so it can
authenticate the communication.
Many application technologies adhere to the pattern just outlined.
Such protocols have traditionally specified their own rules for
representing and verifying application service identity.
Unfortunately, this divergence of approaches has caused some
confusion among certification authorities, application developers,
and protocol designers.
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Therefore, to codify secure procedures for the implementation and
deployment of PKIX-based authentication, this document specifies
recommended procedures for representing and verifying application
service identity in certificates intended for use in application
protocols employing TLS.
1.2. Audience
The primary audience for this document consists of application
protocol designers, who can reference this document instead of
defining their own rules for the representation and verification of
application service identity. Secondarily, the audience consists of
certification authorities, service providers, and client developers
from technology communities that might reuse the recommendations in
this document when defining certificate issuance policies, generating
certificate signing requests, or writing software algorithms for
identity matching.
1.3. How to Read This Document
This document is longer than the authors would have liked because it
was necessary to carefully define terminology, explain the underlying
concepts, define the scope, and specify recommended behavior for both
certification authorities and application software implementations.
The following sections are of special interest to various audiences:
o Protocol designers might want to first read the checklist in
Section 3.
o Certification authorities might want to first read the
recommendations for representation of server identity in
Section 4.
o Service providers might want to first read the recommendations for
requesting of server certificates in Section 5.
o Software implementers might want to first read the recommendations
for verification of server identity in Section 6.
The sections on terminology (Section 1.8), naming of application
services (Section 2), document scope (Section 1.7), and the like
provide useful background information regarding the recommendations
and guidelines that are contained in the above-referenced sections,
but are not absolutely necessary for a first reading of this
document.
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1.4. Applicability
This document does not supersede the rules for certificate issuance
or validation provided in [PKIX]. Therefore, [PKIX] is authoritative
on any point that might also be discussed in this document.
Furthermore, [PKIX] also governs any certificate-related topic on
which this document is silent, including but not limited to
certificate syntax, certificate extensions such as name constraints
and extended key usage, and handling of certification paths.
This document addresses only name forms in the leaf "end entity"
server certificate, not any name forms in the chain of certificates
used to validate the server certificate. Therefore, in order to
ensure proper authentication, application clients need to verify the
entire certification path per [PKIX].
This document also does not supersede the rules for verifying service
identity provided in specifications for existing application
protocols published prior to this document, such as those excerpted
under Appendix B. However, the procedures described here can be
referenced by future specifications, including updates to
specifications for existing application protocols if the relevant
technology communities agree to do so.
1.5. Overview of Recommendations
To orient the reader, this section provides an informational overview
of the recommendations contained in this document.
For the primary audience of application protocol designers, this
document provides recommended procedures for the representation and
verification of application service identity within PKIX certificates
used in the context of TLS.
For the secondary audiences, in essence this document encourages
certification authorities, application service providers, and
application client developers to coalesce on the following practices:
o Move away from including and checking strings that look like
domain names in the subject's Common Name.
o Move toward including and checking DNS domain names via the
subjectAlternativeName extension designed for that purpose:
dNSName.
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o Move toward including and checking even more specific
subjectAlternativeName extensions where appropriate for using the
protocol (e.g., uniformResourceIdentifier and the otherName form
SRVName).
o Move away from the issuance of so-called wildcard certificates
(e.g., a certificate containing an identifier for
"*.example.com").
These suggestions are not entirely consistent with all practices that
are currently followed by certification authorities, client
developers, and service providers. However, they reflect the best
aspects of current practices and are expected to become more widely
adopted in the coming years.
1.6. Generalization from Current Technologies
This document attempts to generalize best practices from the many
application technologies that currently use PKIX certificates with
TLS. Such technologies include, but are not limited to:
o The Internet Message Access Protocol [IMAP] and the Post Office
Protocol [POP3]; see also [USINGTLS]
o The Hypertext Transfer Protocol [HTTP]; see also [HTTP-TLS]
o The Lightweight Directory Access Protocol [LDAP]; see also
[LDAP-AUTH] and its predecessor [LDAP-TLS]
o The Simple Mail Transfer Protocol [SMTP]; see also [SMTP-AUTH] and
[SMTP-TLS]
o The Extensible Messaging and Presence Protocol [XMPP]; see also
[XMPP-OLD]
o The Network News Transfer Protocol [NNTP]; see also [NNTP-TLS]
o The NETCONF Configuration Protocol [NETCONF]; see also
[NETCONF-SSH] and [NETCONF-TLS]
o The Syslog Protocol [SYSLOG]; see also [SYSLOG-TLS] and
[SYSLOG-DTLS]
o The Session Initiation Protocol [SIP]; see also [SIP-CERTS]
o The Simple Network Management Protocol [SNMP]; see also [SNMP-TLS]
o The General Internet Signalling Transport [GIST]
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However, as noted, this document does not supersede the rules for
verifying service identity provided in specifications for those
application protocols.
1.7. Scope
1.7.1. In Scope
This document applies only to service identities associated with
fully qualified DNS domain names, only to TLS and DTLS (or the older
Secure Sockets Layer (SSL) technology), and only to PKIX-based
systems. As a result, the scenarios described in the following
section are out of scope for this specification (although they might
be addressed by future specifications).
1.7.2. Out of Scope
The following topics are out of scope for this specification:
o Client or end-user identities.
Certificates representing client or end-user identities (e.g., the
rfc822Name identifier) can be used for mutual authentication
between a client and server or between two clients, thus enabling
stronger client-server security or end-to-end security. However,
certification authorities, application developers, and service
operators have less experience with client certificates than with
server certificates, thus giving us fewer models from which to
generalize and a less solid basis for defining best practices.
o Identifiers other than fully qualified DNS domain names.
Some certification authorities issue server certificates based on
IP addresses, but preliminary evidence indicates that such
certificates are a very small percentage (less than 1%) of issued
certificates. Furthermore, IP addresses are not necessarily
reliable identifiers for application services because of the
existence of private internets [PRIVATE], host mobility, multiple
interfaces on a given host, Network Address Translators (NATs)
resulting in different addresses for a host from different
locations on the network, the practice of grouping many hosts
together behind a single IP address, etc. Most fundamentally,
most users find DNS domain names much easier to work with than IP
addresses, which is why the domain name system was designed in the
first place. We prefer to define best practices for the much more
common use case and not to complicate the rules in this
specification.
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Furthermore, we focus here on application service identities, not
specific resources located at such services. Therefore this
document discusses Uniform Resource Identifiers [URI] only as a
way to communicate a DNS domain name (via the URI "host" component
or its equivalent), not as a way to communicate other aspects of a
service such as a specific resource (via the URI "path" component)
or parameters (via the URI "query" component).
We also do not discuss attributes unrelated to DNS domain names,
such as those defined in [X.520] and other such specifications
(e.g., organizational attributes, geographical attributes, company
logos, and the like).
o Security protocols other than [TLS], [DTLS], or the older Secure
Sockets Layer (SSL) technology.
Although other secure, lower-layer protocols exist and even employ
PKIX certificates at times (e.g., IPsec [IPSEC]), their use cases
can differ from those of TLS-based and DTLS-based application
technologies. Furthermore, application technologies have less
experience with IPsec than with TLS, thus making it more difficult
to gather feedback on proposed best practices.
o Keys or certificates employed outside the context of PKIX-based
systems.
Some deployed application technologies use a web of trust model
based on or similar to OpenPGP [OPENPGP], or use self-signed
certificates, or are deployed on networks that are not directly
connected to the public Internet and therefore cannot depend on
Certificate Revocation Lists (CRLs) or the Online Certificate
Status Protocol [OCSP] to check CA-issued certificates. However,
the method for binding a public key to an identifier in OpenPGP
differs essentially from the method in X.509, the data in self-
signed certificates has not been certified by a third party in any
way, and checking of CA-issued certificates via CRLs or OCSP is
critically important to maintaining the security of PKIX-based
systems. Attempting to define best practices for such
technologies would unduly complicate the rules defined in this
specification.
o Certification authority policies, such as:
* What types or "classes" of certificates to issue and whether to
apply different policies for them (e.g., allow the wildcard
character in certificates issued to individuals who have
provided proof of identity but do not allow the wildcard
character in "Extended Validation" certificates [EV-CERTS]).
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* Whether to issue certificates based on IP addresses (or some
other form, such as relative domain names) in addition to fully
qualified DNS domain names.
* Which identifiers to include (e.g., whether to include SRV-IDs
or URI-IDs as defined in the body of this specification).
* How to certify or validate fully qualified DNS domain names and
application service types.
* How to certify or validate other kinds of information that
might be included in a certificate (e.g., organization name).
o Resolution of DNS domain names.
Although the process whereby a client resolves the DNS domain name
of an application service can involve several steps (e.g., this is
true of resolutions that depend on DNS SRV resource records,
Naming Authority Pointer (NAPTR) DNS resource records [NAPTR], and
related technologies such as [S-NAPTR]), for our purposes we care
only about the fact that the client needs to verify the identity
of the entity with which it communicates as a result of the
resolution process. Thus the resolution process itself is out of
scope for this specification.
o User interface issues.
In general, such issues are properly the responsibility of client
software developers and standards development organizations
dedicated to particular application technologies (see, for
example, [WSC-UI]).
1.8. Terminology
Because many concepts related to "identity" are often too vague to be
actionable in application protocols, we define a set of more concrete
terms for use in this specification.
application service: A service on the Internet that enables
interactive and automated clients to connect for the purpose of
retrieving or uploading information, communicating with other
entities, or connecting to a broader network of services.
application service provider: An organization or individual that
hosts or deploys an application service.
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application service type: A formal identifier for the application
protocol used to provide a particular kind of application service
at a domain; the application service type typically takes the form
of a Uniform Resource Identifier scheme [URI] or a DNS SRV Service
[DNS-SRV].
attribute-type-and-value pair: A colloquial name for the ASN.1-based
construction comprising a Relative Distinguished Name (RDN), which
itself is a building-block component of Distinguished Names. See
Section 2 of [LDAP-DN].
automated client: A software agent or device that is not directly
controlled by a human user.
delegated domain: A domain name or host name that is explicitly
configured for communicating with the source domain, by either (a)
the human user controlling an interactive client or (b) a trusted
administrator. In case (a), one example of delegation is an
account setup that specifies the domain name of a particular host
to be used for retrieving information or connecting to a network,
which might be different from the server portion of the user's
account name (e.g., a server at mailhost.example.com for
connecting to an IMAP server hosting an email address of
juliet@example.com). In case (b), one example of delegation is an
admin-configured host-to-address/address-to-host lookup table.
derived domain: A domain name or host name that a client has derived
from the source domain in an automated fashion (e.g., by means of
a [DNS-SRV] lookup).
identifier: A particular instance of an identifier type that is
either presented by a server in a certificate or referenced by a
client for matching purposes.
identifier type: A formally defined category of identifier that can
be included in a certificate and therefore that can also be used
for matching purposes. For conciseness and convenience, we define
the following identifier types of interest, which are based on
those found in the PKIX specification [PKIX] and various PKIX
extensions.
* CN-ID = a Relative Distinguished Name (RDN) in the certificate
subject field that contains one and only one attribute-type-
and-value pair of type Common Name (CN), where the value
matches the overall form of a domain name (informally, dot-
separated letter-digit-hyphen labels); see [PKIX] and also
[LDAP-SCHEMA]
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* DNS-ID = a subjectAltName entry of type dNSName; see [PKIX]
* SRV-ID = a subjectAltName entry of type otherName whose name
form is SRVName; see [SRVNAME]
* URI-ID = a subjectAltName entry of type
uniformResourceIdentifier whose value includes both (i) a
"scheme" and (ii) a "host" component (or its equivalent) that
matches the "reg-name" rule (where the quoted terms represent
the associated [ABNF] productions from [URI]); see [PKIX] and
[URI]
interactive client: A software agent or device that is directly
controlled by a human user. (Other specifications related to
security and application protocols, such as [WSC-UI], often refer
to this entity as a "user agent".)
pinning: The act of establishing a cached name association between
the application service's certificate and one of the client's
reference identifiers, despite the fact that none of the presented
identifiers matches the given reference identifier. Pinning is
accomplished by allowing a human user to positively accept the
mismatch during an attempt to communicate with the application
service. Once a cached name association is established, the
certificate is said to be pinned to the reference identifier and
in future communication attempts the client simply verifies that
the service's presented certificate matches the pinned
certificate, as described under Section 6.6.2. (A similar
definition of "pinning" is provided in [WSC-UI].)
PKIX: PKIX is a short name for the Internet Public Key
Infrastructure using X.509 defined in RFC 5280 [PKIX], which
comprises a profile of the X.509v3 certificate specifications and
X.509v2 certificate revocation list (CRL) specifications for use
in the Internet.
PKIX-based system: A software implementation or deployed service
that makes use of X.509v3 certificates and X.509v2 certificate
revocation lists (CRLs).
PKIX certificate: An X.509v3 certificate generated and employed in
the context of PKIX.
presented identifier: An identifier that is presented by a server to
a client within a PKIX certificate when the client attempts to
establish secure communication with the server; the certificate
can include one or more presented identifiers of different types,
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and if the server hosts more than one domain then the certificate
might present distinct identifiers for each domain.
reference identifier: An identifier, constructed from a source
domain and optionally an application service type, used by the
client for matching purposes when examining presented identifiers.
source domain: The fully qualified DNS domain name that a client
expects an application service to present in the certificate
(e.g., "www.example.com"), typically input by a human user,
configured into a client, or provided by reference such as in a
hyperlink. The combination of a source domain and, optionally, an
application service type enables a client to construct one or more
reference identifiers.
subjectAltName entry: An identifier placed in a subjectAltName
extension.
subjectAltName extension: A standard PKIX certificate extension
[PKIX] enabling identifiers of various types to be bound to the
certificate subject -- in addition to, or in place of, identifiers
that may be embedded within or provided as a certificate's subject
field.
subject field: The subject field of a PKIX certificate identifies
the entity associated with the public key stored in the subject
public key field (see Section 4.1.2.6 of [PKIX]).
subject name: In an overall sense, a subject's name(s) can be
represented by or in the subject field, the subjectAltName
extension, or both (see [PKIX] for details). More specifically,
the term often refers to the name of a PKIX certificate's subject,
encoded as the X.501 type Name and conveyed in a certificate's
subject field (see Section 4.1.2.6 of [PKIX]).
TLS client: An entity that assumes the role of a client in a
Transport Layer Security [TLS] negotiation. In this specification
we generally assume that the TLS client is an (interactive or
automated) application client; however, in application protocols
that enable server-to-server communication, the TLS client could
be a peer application service.
TLS server: An entity that assumes the role of a server in a
Transport Layer Security [TLS] negotiation; in this specification
we assume that the TLS server is an application service.
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Most security-related terms in this document are to be understood in
the sense defined in [SECTERMS]; such terms include, but are not
limited to, "attack", "authentication", "authorization",
"certification authority", "certification path", "certificate",
"credential", "identity", "self-signed certificate", "trust", "trust
anchor", "trust chain", "validate", and "verify".
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [KEYWORDS].
2. Naming of Application Services
This section discusses naming of application services on the
Internet, followed by a brief tutorial about subject naming in PKIX.
2.1. Naming Application Services
This specification assumes that the name of an application service is
based on a DNS domain name (e.g., "example.com") -- supplemented in
some circumstances by an application service type (e.g., "the IMAP
server at example.com").
From the perspective of the application client or user, some names
are direct because they are provided directly by a human user (e.g.,
via runtime input, prior configuration, or explicit acceptance of a
client communication attempt), whereas other names are indirect
because they are automatically resolved by the client based on user
input (e.g., a target name resolved from a source name using DNS SRV
or NAPTR records). This dimension matters most for certificate
consumption, specifically verification as discussed in this document.
From the perspective of the application service, some names are
unrestricted because they can be used in any type of service (e.g., a
certificate might be reused for both the HTTP service and the IMAP
service at example.com), whereas other names are restricted because
they can be used in only one type of service (e.g., a special-purpose
certificate that can be used only for an IMAP service). This
dimension matters most for certificate issuance.
Therefore, we can categorize the identifier types of interest as
follows:
o A CN-ID is direct and unrestricted.
o A DNS-ID is direct and unrestricted.
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o An SRV-ID can be either direct or (more typically) indirect, and
is restricted.
o A URI-ID is direct and restricted.
We summarize this taxonomy in the following table.
+-----------+-----------+---------------+
| | Direct | Restricted |
+-----------+-----------+---------------+
| CN-ID | Yes | No |
+-----------+-----------+---------------+
| DNS-ID | Yes | No |
+-----------+-----------+---------------+
| SRV-ID | Either | Yes |
+-----------+-----------+---------------+
| URI-ID | Yes | Yes |
+-----------+-----------+---------------+
When implementing software, deploying services, and issuing
certificates for secure PKIX-based authentication, it is important to
keep these distinctions in mind. In particular, best practices
differ somewhat for application server implementations, application
client implementations, application service providers, and
certification authorities. Ideally, protocol specifications that
reference this document will specify which identifiers are mandatory-
to-implement by servers and clients, which identifiers ought to be
supported by certificate issuers, and which identifiers ought to be
requested by application service providers. Because these
requirements differ across applications, it is impossible to
categorically stipulate universal rules (e.g., that all software
implementations, service providers, and certification authorities for
all application protocols need to use or support DNS-IDs as a
baseline for the purpose of interoperability).
However, it is preferable that each application protocol will at
least define a baseline that applies to the community of software
developers, application service providers, and CAs actively using or
supporting that technology (one such community, the CA/Browser Forum,
has codified such a baseline for "Extended Validation Certificates"
in [EV-CERTS]).
2.2. DNS Domain Names
For the purposes of this specification, the name of an application
service is (or is based on) a DNS domain name that conforms to one of
the following forms:
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1. A "traditional domain name", i.e., a fully qualified DNS domain
name or "FQDN" (see [DNS-CONCEPTS]) all of whose labels are "LDH
labels" as described in [IDNA-DEFS]. Informally, such labels are
constrained to [US-ASCII] letters, digits, and the hyphen, with
the hyphen prohibited in the first character position.
Additional qualifications apply (please refer to the above-
referenced specifications for details), but they are not relevant
to this specification.
2. An "internationalized domain name", i.e., a DNS domain name that
conforms to the overall form of a domain name (informally, dot-
separated letter-digit-hyphen labels) but includes at least one
label containing appropriately encoded Unicode code points
outside the traditional US-ASCII range. That is, it contains at
least one U-label or A-label, but otherwise may contain any
mixture of NR-LDH labels, A-labels, or U-labels, as described in
[IDNA-DEFS] and the associated documents.
2.3. Subject Naming in PKIX Certificates
In theory, the Internet Public Key Infrastructure using X.509 [PKIX]
employs the global directory service model defined in [X.500] and
[X.501]. Under that model, information is held in a directory
information base (DIB) and entries in the DIB are organized in a
hierarchy called the directory information tree (DIT). An object or
alias entry in that hierarchy consists of a set of attributes (each
of which has a defined type and one or more values) and is uniquely
identified by a Distinguished Name (DN). The DN of an entry is
constructed by combining the Relative Distinguished Names of its
superior entries in the tree (all the way down to the root of the
DIT) with one or more specially nominated attributes of the entry
itself (which together comprise the Relative Distinguished Name (RDN)
of the entry, so-called because it is relative to the Distinguished
Names of the superior entries in the tree). The entry closest to the
root is sometimes referred to as the "most significant" entry, and
the entry farthest from the root is sometimes referred to as the
"least significant" entry. An RDN is a set (i.e., an unordered
group) of attribute-type-and-value pairs (see also [LDAP-DN]), each
of which asserts some attribute about the entry.
In practice, the certificates used in [X.509] and [PKIX] borrow key
concepts from X.500 and X.501 (e.g., DNs and RDNs) to identify
entities, but such certificates are not necessarily part of a global
directory information base. Specifically, the subject field of a
PKIX certificate is an X.501 type Name that "identifies the entity
associated with the public key stored in the subject public key
field" (see Section 4.1.2.6 of [PKIX]). However, it is perfectly
acceptable for the subject field to be empty, as long as the
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certificate contains a subject alternative name ("subjectAltName")
extension that includes at least one subjectAltName entry, because
the subjectAltName extension allows various identities to be bound to
the subject (see Section 4.2.1.6 of [PKIX]). The subjectAltName
extension itself is a sequence of typed entries, where each type is a
distinct kind of identifier.
For our purposes, an application service can be identified by a name
or names carried in the subject field (i.e., a CN-ID) and/or in one
of the following identifier types within subjectAltName entries:
o DNS-ID
o SRV-ID
o URI-ID
Existing certificates often use a CN-ID in the subject field to
represent a fully qualified DNS domain name; for example, consider
the following three subject names, where the attribute of type Common
Name contains a string whose form matches that of a fully qualified
DNS domain name ("im.example.org", "mail.example.net", and
"www.example.com", respectively):
CN=im.example.org,O=Example Org,C=GB
C=CA,O=Example Internetworking,CN=mail.example.net
O=Examples-R-Us,CN=www.example.com,C=US
However, the Common Name is not strongly typed because a Common Name
might contain a human-friendly string for the service, rather than a
string whose form matches that of a fully qualified DNS domain name
(a certificate with such a single Common Name will typically have at
least one subjectAltName entry containing the fully qualified DNS
domain name):
CN=A Free Chat Service,O=Example Org,C=GB
Or, a certificate's subject might contain both a CN-ID as well as
another common name attribute containing a human-friendly string:
CN=A Free Chat Service,CN=im.example.org,O=Example Org,C=GB
In general, this specification recommends and prefers use of
subjectAltName entries (DNS-ID, SRV-ID, URI-ID, etc.) over use of the
subject field (CN-ID) where possible, as more completely described in
the following sections. However, specifications that reuse this one
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can legitimately encourage continued support for the CN-ID identifier
type if they have good reasons to do so, such as backward
compatibility with deployed infrastructure (see, for example,
[EV-CERTS]).
2.3.1. Implementation Notes
Confusion sometimes arises from different renderings or encodings of
the hierarchical information contained in a certificate.
Certificates are binary objects and are encoded using the
Distinguished Encoding Rules (DER) specified in [X.690]. However,
some implementations generate displayable (a.k.a. printable)
renderings of the certificate issuer, subject field, and
subjectAltName extension, and these renderings convert the DER-
encoded sequences into a "string representation" before being
displayed. Because a certificate subject field (of type Name
[X.509], the same as for a Distinguished Name (DN) [X.501]) is an
ordered sequence, order is typically preserved in subject string
representations, although the two most prevalent subject (and DN)
string representations differ in employing left-to-right vs. right-
to-left ordering. However, because a Relative Distinguished Name
(RDN) is an unordered group of attribute-type-and-value pairs, the
string representation of an RDN can differ from the canonical DER
encoding (and the order of attribute-type-and-value pairs can differ
in the RDN string representations or display orders provided by
various implementations). Furthermore, various specifications refer
to the order of RDNs in DNs or certificate subject fields using
terminology that is implicitly related to an information hierarchy
(which may or may not actually exist), such as "most specific" vs.
"least specific", "left-most" vs. "right-most", "first" vs. "last",
or "most significant" vs. "least significant" (see, for example,
[LDAP-DN]).
To reduce confusion, in this specification we avoid such terms and
instead use the terms provided under Section 1.8; in particular, we
do not use the term "(most specific) Common Name field in the subject
field" from [HTTP-TLS] and instead state that a CN-ID is a Relative
Distinguished Name (RDN) in the certificate subject containing one
and only one attribute-type-and-value pair of type Common Name (thus
removing the possibility that an RDN might contain multiple AVAs
(Attribute Value Assertions) of type CN, one of which could be
considered "most specific").
Finally, although theoretically some consider the order of RDNs
within a subject field to have meaning, in practice that rule is
often not observed. An AVA of type CN is considered to be valid at
any position within the subject field.
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3. Designing Application Protocols
This section provides guidelines for designers of application
protocols, in the form of a checklist to follow when reusing the
recommendations provided in this document.
o Does your technology use DNS SRV records to resolve the DNS domain
names of application services? If so, consider recommending or
requiring support for the SRV-ID identifier type in PKIX
certificates issued and used in your technology community. (Note
that many existing application technologies use DNS SRV records to
resolve the DNS domain names of application services, but do not
rely on representations of those records in PKIX certificates by
means of SRV-IDs as defined in [SRVNAME].)
o Does your technology use URIs to identify application services?
If so, consider recommending or requiring support for the URI-ID
identifier type. (Note that many existing application
technologies use URIs to identify application services, but do not
rely on representation of those URIs in PKIX certificates by means
of URI-IDs.)
o Does your technology need to use DNS domain names in the Common
Name of certificates for the sake of backward compatibility? If
so, consider recommending support for the CN-ID identifier type as
a fallback.
o Does your technology need to allow the wildcard character in DNS
domain names? If so, consider recommending support for wildcard
certificates, and specify exactly where the wildcard character is
allowed to occur (e.g., only the complete left-most label of a DNS
domain name).
Sample text is provided under Appendix A.
4. Representing Server Identity
This section provides rules and guidelines for issuers of
certificates.
4.1. Rules
When a certification authority issues a certificate based on the
fully qualified DNS domain name at which the application service
provider will provide the relevant application, the following rules
apply to the representation of application service identities. The
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reader needs to be aware that some of these rules are cumulative and
can interact in important ways that are illustrated later in this
document.
1. The certificate SHOULD include a "DNS-ID" if possible as a
baseline for interoperability.
2. If the service using the certificate deploys a technology for
which the relevant specification stipulates that certificates
ought to include identifiers of type SRV-ID (e.g., this is true
of [XMPP]), then the certificate SHOULD include an SRV-ID.
3. If the service using the certificate deploys a technology for
which the relevant specification stipulates that certificates
ought to include identifiers of type URI-ID (e.g., this is true
of [SIP] as specified by [SIP-CERTS], but not true of [HTTP]
since [HTTP-TLS] does not describe usage of a URI-ID for HTTP
services), then the certificate SHOULD include a URI-ID. The
scheme SHALL be that of the protocol associated with the
application service type and the "host" component (or its
equivalent) SHALL be the fully qualified DNS domain name of the
service. A specification that reuses this one MUST specify which
URI schemes are to be considered acceptable in URI-IDs contained
in PKIX certificates used for the application protocol (e.g.,
"sip" but not "sips" or "tel" for SIP as described in [SIP-SIPS],
or perhaps http and https for HTTP as might be described in a
future specification).
4. The certificate MAY include other application-specific
identifiers for types that were defined before publication of
[SRVNAME] (e.g., XmppAddr for [XMPP]) or for which service names
or URI schemes do not exist; however, such application-specific
identifiers are not applicable to all application technologies
and therefore are out of scope for this specification.
5. Even though many deployed clients still check for the CN-ID
within the certificate subject field, certification authorities
are encouraged to migrate away from issuing certificates that
represent the server's fully qualified DNS domain name in a
CN-ID. Therefore, the certificate SHOULD NOT include a CN-ID
unless the certification authority issues the certificate in
accordance with a specification that reuses this one and that
explicitly encourages continued support for the CN-ID identifier
type in the context of a given application technology.
6. The certificate MAY contain more than one DNS-ID, SRV-ID, or
URI-ID but SHOULD NOT contain more than one CN-ID, as further
explained under Section 7.4.
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7. Unless a specification that reuses this one allows continued
support for the wildcard character '*', the DNS domain name
portion of a presented identifier SHOULD NOT contain the wildcard
character, whether as the complete left-most label within the
identifier (following the description of labels and domain names
in [DNS-CONCEPTS], e.g., "*.example.com") or as a fragment
thereof (e.g., *oo.example.com, f*o.example.com, or
fo*.example.com). A more detailed discussion of so-called
"wildcard certificates" is provided under Section 7.2.
4.2. Examples
Consider a simple website at "www.example.com", which is not
discoverable via DNS SRV lookups. Because HTTP does not specify the
use of URIs in server certificates, a certificate for this service
might include only a DNS-ID of "www.example.com". It might also
include a CN-ID of "www.example.com" for backward compatibility with
deployed infrastructure.
Consider an IMAP-accessible email server at the host
"mail.example.net" servicing email addresses of the form
"user@example.net" and discoverable via DNS SRV lookups on the
application service name of "example.net". A certificate for this
service might include SRV-IDs of "_imap.example.net" and
"_imaps.example.net" (see [EMAIL-SRV]) along with DNS-IDs of
"example.net" and "mail.example.net". It might also include CN-IDs
of "example.net" and "mail.example.net" for backward compatibility
with deployed infrastructure.
Consider a SIP-accessible voice-over-IP (VoIP) server at the host
"voice.example.edu" servicing SIP addresses of the form
"user@voice.example.edu" and identified by a URI of <sip:
voice.example.edu>. A certificate for this service would include a
URI-ID of "sip:voice.example.edu" (see [SIP-CERTS]) along with a
DNS-ID of "voice.example.edu". It might also include a CN-ID of
"voice.example.edu" for backward compatibility with deployed
infrastructure.
Consider an XMPP-compatible instant messaging (IM) server at the host
"im.example.org" servicing IM addresses of the form
"user@im.example.org" and discoverable via DNS SRV lookups on the
"im.example.org" domain. A certificate for this service might
include SRV-IDs of "_xmpp-client.im.example.org" and
"_xmpp-server.im.example.org" (see [XMPP]), a DNS-ID of
"im.example.org", and an XMPP-specific "XmppAddr" of "im.example.org"
(see [XMPP]). It might also include a CN-ID of "im.example.org" for
backward compatibility with deployed infrastructure.
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5. Requesting Server Certificates
This section provides rules and guidelines for service providers
regarding the information to include in certificate signing requests
(CSRs).
In general, service providers are encouraged to request certificates
that include all of the identifier types that are required or
recommended for the application service type that will be secured
using the certificate to be issued.
If the certificate might be used for any type of application service,
then the service provider is encouraged to request a certificate that
includes only a DNS-ID.
If the certificate will be used for only a single type of application
service, then the service provider is encouraged to request a
certificate that includes a DNS-ID and, if appropriate for the
application service type, an SRV-ID or URI-ID that limits the
deployment scope of the certificate to only the defined application
service type.
If a service provider offering multiple application service types
(e.g., a World Wide Web service, an email service, and an instant
messaging service) wishes to limit the applicability of certificates
using SRV-IDs or URI-IDs, then the service provider is encouraged to
request multiple certificates, i.e., one certificate per application
service type. Conversely, the service provider is discouraged from
requesting a single certificate containing multiple SRV-IDs or URI-
IDs identifying each different application service type. This
guideline does not apply to application service type "bundles" that
are used to identify manifold distinct access methods to the same
underlying application (e.g., an email application with access
methods denoted by the application service types of "imap", "imaps",
"pop3", "pop3s", and "submission" as described in [EMAIL-SRV]).
6. Verifying Service Identity
This section provides rules and guidelines for implementers of
application client software regarding algorithms for verification of
application service identity.
6.1. Overview
At a high level, the client verifies the application service's
identity by performing the actions listed below (which are defined in
the following subsections of this document):
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1. The client constructs a list of acceptable reference identifiers
based on the source domain and, optionally, the type of service
to which the client is connecting.
2. The server provides its identifiers in the form of a PKIX
certificate.
3. The client checks each of its reference identifiers against the
presented identifiers for the purpose of finding a match.
4. When checking a reference identifier against a presented
identifier, the client matches the source domain of the
identifiers and, optionally, their application service type.
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.
6.2. Constructing a List of Reference Identifiers
6.2.1. Rules
The client MUST construct a list of acceptable reference identifiers,
and MUST do so independently of the identifiers presented by the
service.
The inputs used by the client to construct its list of reference
identifiers might be a URI that a user has typed into an interface
(e.g., an HTTPS URL for a website), configured account information
(e.g., the domain name of a particular host or URI used for
retrieving information or connecting to a network, which might be
different from the DNS domain name portion of a username), a
hyperlink in a web page that triggers a browser to retrieve a media
object or script, or some other combination of information that can
yield a source domain and an application service type.
The client might need to extract the source domain and application
service type from the input(s) it has received. The extracted data
MUST include only information that can be securely parsed out of the
inputs (e.g., parsing the fully qualified DNS domain name out of the
"host" component (or its equivalent) of a URI or deriving the
application service type from the scheme of a URI) or information
that is derived in a manner not subject to subversion by network
attackers (e.g., pulling the data from a delegated domain that is
explicitly established via client or system configuration, resolving
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the data via [DNSSEC], or obtaining the data from a third-party
domain mapping service in which a human user has explicitly placed
trust and with which the client communicates over a connection or
association that provides both mutual authentication and integrity
checking). These considerations apply only to extraction of the
source domain from the inputs; naturally, if the inputs themselves
are invalid or corrupt (e.g., a user has clicked a link provided by a
malicious entity in a phishing attack), then the client might end up
communicating with an unexpected application service.
Example: Given an input URI of <sips:alice@example.net>, a client
would derive the application service type "sip" from the "scheme"
and parse the domain name "example.net" from the "host" component
(or its equivalent).
Each reference identifier in the list SHOULD be based on the source
domain and SHOULD NOT be based on a derived domain (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 and a presented identifier enables the client to be sure
that the certificate can legitimately be used to secure the client's
communication with the server. There is only one scenario in which
it is acceptable for an interactive client to override the
recommendation in this rule and therefore communicate with a domain
name other than the source domain: because a human user has "pinned"
the application service's certificate to the alternative domain name
as further discussed under Section 6.6.4 and Section 7.1. In this
case, the inputs used by the client to construct its list of
reference identifiers might include more than one fully qualified DNS
domain name, i.e., both (a) the source domain and (b) the alternative
domain contained in the pinned certificate.
Using the combination of fully qualified DNS domain name(s) and
application service type, the client constructs a list of reference
identifiers in accordance with the following rules:
o The list SHOULD include a DNS-ID. A reference identifier of type
DNS-ID can be directly constructed from a fully qualified DNS
domain name that is (a) contained in or securely derived from the
inputs (i.e., the source domain), or (b) explicitly associated
with the source domain by means of user configuration (i.e., a
derived domain).
o If a server for the application service type is typically
discovered by means of DNS SRV records, then the list SHOULD
include an SRV-ID.
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o If a server for the application service type is typically
associated with a URI for security purposes (i.e., a formal
protocol document specifies the use of URIs in server
certificates), then the list SHOULD include a URI-ID.
o The list MAY include a CN-ID, mainly for the sake of backward
compatibility with deployed infrastructure.
Which identifier types a client includes in its list of reference
identifiers is a matter of local policy. For example, in certain
deployment environments, a client that is built to connect only to a
particular kind of service (e.g., only IM services) might be
configured to accept as valid only certificates that include an
SRV-ID for that application service type; in this case, the client
would include only SRV-IDs matching the application service type in
its list of reference identifiers (not, for example, DNS-IDs). By
contrast, a more lenient client (even one built to connect only to a
particular kind of service) might include both SRV-IDs and DNS-IDs in
its list of reference identifiers.
Implementation Note: It is highly likely that implementers of
client software will need to support CN-IDs for the foreseeable
future, because certificates containing CN-IDs are so widely
deployed. Implementers are advised to monitor the state of the
art with regard to certificate issuance policies and migrate away
from support CN-IDs in the future if possible.
Implementation Note: The client does not need to construct the
foregoing identifiers in the actual formats found in a certificate
(e.g., as ASN.1 types); it only needs to construct the functional
equivalent of such identifiers for matching purposes.
Security Warning: A client MUST NOT construct a reference
identifier corresponding to Relative Distinguished Names (RDNs)
other than those of type Common Name and MUST NOT check for RDNs
other than those of type Common Name in the presented identifiers.
6.2.2. Examples
A web browser that is connecting via HTTPS to the website at
"www.example.com" might have two reference identifiers: a DNS-ID of
"www.example.com" and, as a fallback, a CN-ID of "www.example.com".
A mail user agent that is connecting via IMAPS to the email service
at "example.net" (resolved as "mail.example.net") might have five
reference identifiers: an SRV-ID of "_imaps.example.net" (see
[EMAIL-SRV]), DNS-IDs of "example.net" and "mail.example.net", and,
as a fallback, CN-IDs of "example.net" and "mail.example.net". (A
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legacy email user agent would not support [EMAIL-SRV] and therefore
would probably be explicitly configured to connect to
"mail.example.net", whereas an SRV-aware user agent would derive
"example.net" from an email address of the form "user@example.net"
but might also accept "mail.example.net" as the DNS domain name
portion of reference identifiers for the service.)
A voice-over-IP (VoIP) user agent that is connecting via SIP to the
voice service at "voice.example.edu" might have only one reference
identifier: a URI-ID of "sip:voice.example.edu" (see [SIP-CERTS]).
An instant messaging (IM) client that is connecting via XMPP to the
IM service at "im.example.org" might have three reference
identifiers: an SRV-ID of "_xmpp-client.im.example.org" (see [XMPP]),
a DNS-ID of "im.example.org", and an XMPP-specific "XmppAddr" of
"im.example.org" (see [XMPP]).
6.3. Preparing to Seek a Match
Once the client has constructed its list of reference identifiers and
has received the server's presented identifiers in the form of a PKIX
certificate, the client checks its reference identifiers against the
presented identifiers for the purpose of finding a match. The search
fails if the client exhausts its list of reference identifiers
without finding a match. The search succeeds if any presented
identifier matches one of the reference identifiers, at which point
the client SHOULD stop the search.
Implementation Note: A client might be configured to perform
multiple searches, i.e., to match more than one reference
identifier. Although such behavior is not forbidden by this
specification, rules for matching multiple reference identifiers
are a matter for implementation or future specification.
Security Warning: A client MUST NOT seek a match for a reference
identifier of CN-ID if the presented identifiers include a DNS-ID,
SRV-ID, URI-ID, or any application-specific identifier types
supported by the client.
Before applying the comparison rules provided in the following
sections, the client might need to split the reference identifier
into its DNS domain name portion and its application service type
portion, as follows:
o A reference identifier of type DNS-ID does not include an
application service type portion and thus can be used directly as
the DNS domain name for comparison purposes. As an example, a
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DNS-ID of "www.example.com" would result in a DNS domain name
portion of "www.example.com".
o A reference identifier of type CN-ID also does not include an
application service type portion and thus can be used directly as
the DNS domain name for comparison purposes. As previously
mentioned, this document specifies that a CN-ID always contains a
string whose form matches that of a DNS domain name (thus
differentiating a CN-ID from a Common Name containing a human-
friendly name).
o For a reference identifier of type SRV-ID, the DNS domain name
portion is the Name and the application service type portion is
the Service. As an example, an SRV-ID of "_imaps.example.net"
would be split into a DNS domain name portion of "example.net" and
an application service type portion of "imaps" (mapping to an
application protocol of IMAP as explained in [EMAIL-SRV]).
o For a reference identifier of type URI-ID, the DNS domain name
portion is the "reg-name" part of the "host" component (or its
equivalent) and the application service type portion is the
application service type associated with the scheme name matching
the [ABNF] "scheme" rule from [URI] (not including the ':'
separator). As previously mentioned, this document specifies that
a URI-ID always contains a "host" component (or its equivalent)
containing a "reg-name". (Matching only the "reg-name" rule from
[URI] limits verification to DNS domain names, thereby
differentiating a URI-ID from a uniformResourceIdentifier entry
that contains an IP address or a mere host name, or that does not
contain a "host" component at all.) Furthermore, note that
extraction of the "reg-name" might necessitate normalization of
the URI (as explained in [URI]). As an example, a URI-ID of "sip:
voice.example.edu" would be split into a DNS domain name portion
of "voice.example.edu" and an application service type of "sip"
(associated with an application protocol of SIP as explained in
[SIP-CERTS]).
Detailed comparison rules for matching the DNS domain name portion
and application service type portion of the reference identifier are
provided in the following sections.
6.4. Matching the DNS Domain Name Portion
The client MUST match the DNS domain name portion of a reference
identifier according to the following rules (and SHOULD also check
the application service type as described under Section 6.5). The
rules differ depending on whether the domain to be checked is a
"traditional domain name" or an "internationalized domain name" (as
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defined under Section 2.2). Furthermore, to meet the needs of
clients that support presented identifiers containing the wildcard
character '*', we define a supplemental rule for so-called "wildcard
certificates". Finally, we also specify the circumstances under
which it is acceptable to check the "CN-ID" identifier type.
6.4.1. Checking of Traditional Domain Names
If the DNS domain name portion of a reference identifier is a
"traditional domain name", then matching of the reference identifier
against the presented identifier is performed by comparing the set of
domain name labels using a case-insensitive ASCII comparison, as
clarified by [DNS-CASE] (e.g., "WWW.Example.Com" would be lower-cased
to "www.example.com" for comparison purposes). Each label MUST match
in order for the names to be considered to match, except as
supplemented by the rule about checking of wildcard labels
(Section 6.4.3).
6.4.2. Checking of Internationalized Domain Names
If the DNS domain name portion of a reference identifier is an
internationalized domain name, then an implementation MUST convert
any U-labels [IDNA-DEFS] in the domain name to A-labels before
checking the domain name. In accordance with [IDNA-PROTO], A-labels
MUST be compared as case-insensitive ASCII. Each label MUST match in
order for the domain names to be considered to match, except as
supplemented by the rule about checking of wildcard labels
(Section 6.4.3; but see also Section 7.2 regarding wildcards in
internationalized domain names).
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).
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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].
6.4.4. Checking of Common Names
As noted, a client MUST NOT seek a match for a reference identifier
of CN-ID if the presented identifiers include a DNS-ID, SRV-ID,
URI-ID, or any application-specific identifier types supported by the
client.
Therefore, if and only if the presented identifiers do not include a
DNS-ID, SRV-ID, URI-ID, or any application-specific identifier types
supported by the client, then the client MAY as a last resort check
for a string whose form matches that of a fully qualified DNS domain
name in a Common Name field of the subject field (i.e., a CN-ID). If
the client chooses to compare a reference identifier of type CN-ID
against that string, it MUST follow the comparison rules for the DNS
domain name portion of an identifier of type DNS-ID, SRV-ID, or
URI-ID, as described under Section 6.4.1, Section 6.4.2, and
Section 6.4.3.
6.5. Matching the Application Service Type Portion
When a client checks identifiers of type SRV-ID and URI-ID, it MUST
check not only the DNS domain name portion of the identifier but also
the application service type portion. The client does this by
splitting the identifier into the DNS domain name portion and the
application service type portion (as described under Section 6.3),
then checking both the DNS domain name portion (as described under
Section 6.4) and the application service type portion as described in
the following subsections.
Implementation Note: An identifier of type SRV-ID or URI-ID
provides an application service type portion to be checked, but
that portion is combined only with the DNS domain name portion of
the SRV-ID or URI-ID itself. For example, if a client's list of
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reference identifiers includes an SRV-ID of "_xmpp-
client.im.example.org" and a DNS-ID of "apps.example.net", the
client would check (a) the combination of an application service
type of "xmpp-client" and a DNS domain name of "im.example.org"
and (b) a DNS domain name of "apps.example.net". However, the
client would not check (c) the combination of an application
service type of "xmpp-client" and a DNS domain name of
"apps.example.net" because it does not have an SRV-ID of "_xmpp-
client.apps.example.net" in its list of reference identifiers.
6.5.1. SRV-ID
The application service name portion of an SRV-ID (e.g., "imaps")
MUST be matched in a case-insensitive manner, in accordance with
[DNS-SRV]. Note that the "_" character is prepended to the service
identifier in DNS SRV records and in SRV-IDs (per [SRVNAME]), and
thus does not need to be included in any comparison.
6.5.2. URI-ID
The scheme name portion of a URI-ID (e.g., "sip") MUST be matched in
a case-insensitive manner, in accordance with [URI]. Note that the
":" character is a separator between the scheme name and the rest of
the URI, and thus does not need to be included in any comparison.
6.6. Outcome
The outcome of the matching procedure is one of the following cases.
6.6.1. Case #1: Match Found
If the client has found a presented identifier that matches a
reference identifier, then the service identity check has succeeded.
In this case, the client MUST use the matched reference identifier as
the validated identity of the application service.
6.6.2. Case #2: No Match Found, Pinned Certificate
If the client does not find a presented identifier matching any of
the reference identifiers but the client has previously pinned the
application service's certificate to one of the reference identifiers
in the list it constructed for this communication attempt (as
"pinning" is explained under Section 1.8), and the presented
certificate matches the pinned certificate (including the context as
described under Section 7.1), then the service identity check has
succeeded.
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6.6.3. Case #3: No Match Found, No Pinned Certificate
If the client does not find a presented identifier matching any of
the reference identifiers and the client has not previously pinned
the certificate to one of the reference identifiers in the list it
constructed for this communication attempt, then the client MUST
proceed as described under Section 6.6.4.
6.6.4. Fallback
If the client is an interactive client that is directly controlled by
a human user, then it SHOULD inform the user of the identity mismatch
and automatically terminate the communication attempt with a bad
certificate error; this behavior is preferable because it prevents
users from inadvertently bypassing security protections in hostile
situations.
Security Warning: Some interactive clients give advanced users the
option of proceeding with acceptance despite the identity
mismatch, thereby "pinning" the certificate to one of the
reference identifiers in the list constructed by the client for
this communication attempt. Although this behavior can be
appropriate in certain specialized circumstances, in general it
ought to be exposed only to advanced users. Even then it needs to
be handled with extreme caution, for example by first encouraging
even an advanced user to terminate the communication attempt and,
if the advanced user chooses to proceed anyway, by forcing the
user to view the entire certification path and only then allowing
the user to pin the certificate (on a temporary or permanent
basis, at the user's option).
Otherwise, if the client is an automated application not directly
controlled by a human user, then it SHOULD terminate the
communication attempt with a bad certificate error and log the error
appropriately. An automated application MAY provide a configuration
setting that disables this behavior, but MUST enable the behavior by
default.
7. Security Considerations
7.1. Pinned Certificates
As defined under Section 1.8, a certificate is said to be "pinned" to
a DNS domain name when a user has explicitly chosen to associate a
service's certificate with that DNS domain name despite the fact that
the certificate contains some other DNS domain name (e.g., the user
has explicitly approved "apps.example.net" as a domain associated
with a source domain of "example.com"). The cached name association
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MUST take account of both the certificate presented and the context
in which it was accepted or configured (where the "context" includes
the chain of certificates from the presented certificate to the trust
anchor, the source domain, the application service type, the
service's derived domain and port number, and any other relevant
information provided by the user or associated by the client).
7.2. Wildcard Certificates
This document states that the wildcard character '*' SHOULD NOT be
included in presented identifiers but MAY be checked by application
clients (mainly for the sake of backward compatibility with deployed
infrastructure). As a result, the rules provided in this document
are more restrictive than the rules for many existing application
technologies (such as those excerpted under Appendix B). Several
security considerations justify tightening the rules:
o Wildcard certificates automatically vouch for any and all host
names within their domain. This can be convenient for
administrators but also poses the risk of vouching for rogue or
buggy hosts. See for example [Defeating-SSL] (beginning at slide
91) and [HTTPSbytes] (slides 38-40).
o Specifications for existing application technologies are not clear
or consistent about the allowable location of the wildcard
character, such as whether it can be:
* only the complete left-most label (e.g., *.example.com)
* some fragment of the left-most label (e.g., fo*.example.com,
f*o.example.com, or *oo.example.com)
* all or part of a label other than the left-most label (e.g.,
www.*.example.com or www.foo*.example.com)
* all or part of a label that identifies a so-called "public
suffix" (e.g., *.co.uk or *.com)
* included more than once in a given label (e.g.,
f*b*r.example.com
* included as all or part of more than one label (e.g.,
*.*.example.com)
These ambiguities might introduce exploitable differences in
identity checking behavior among client implementations and
necessitate overly complex and inefficient identity checking
algorithms.
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o There is no specification that defines how the wildcard character
may be embedded within the A-labels or U-labels [IDNA-DEFS] of an
internationalized domain name [IDNA-PROTO]; as a result,
implementations are strongly discouraged from including or
attempting to check for the wildcard character embedded within the
A-labels or U-labels of an internationalized domain name (e.g.,
"xn--kcry6tjko*.example.org"). Note, however, that a presented
domain name identifier MAY contain the wildcard character as long
as that character occupies the entire left-most label position,
where all of the remaining labels are valid NR-LDH labels,
A-labels, or U-labels (e.g., "*.xn--kcry6tjko.example.org").
Notwithstanding the foregoing security considerations, specifications
that reuse this one can legitimately encourage continued support for
the wildcard character if they have good reasons to do so, such as
backward compatibility with deployed infrastructure (see, for
example, [EV-CERTS]).
7.3. Internationalized Domain Names
Allowing internationalized domain names can lead to the inclusion of
visually similar (so-called "confusable") characters in certificates;
for discussion, see for example [IDNA-DEFS].
7.4. Multiple Identifiers
A given application service might be addressed by multiple DNS domain
names for a variety of reasons, and a given deployment might service
multiple domains (e.g., in so-called "virtual hosting" environments).
In the default TLS handshake exchange, the client is not able to
indicate the DNS domain name with which it wants to communicate, and
the TLS server returns only one certificate for itself. Absent an
extension to TLS, a typical workaround used to facilitate mapping an
application service to multiple DNS domain names is to embed all of
the domain names into a single certificate.
A more recent approach, formally specified in [TLS-EXT], is for the
client to use the TLS "Server Name Indication" (SNI) extension when
sending the client_hello message, stipulating the DNS domain name it
desires or expects of the service. The service can then return the
appropriate certificate in its Certificate message, and that
certificate can represent a single DNS domain name.
To accommodate the workaround that was needed before the development
of the SNI extension, this specification allows multiple DNS-IDs,
SRV-IDs, or URI-IDs in a certificate; however, it explicitly
discourages multiple CN-IDs. Although it would be preferable to
forbid multiple CN-IDs entirely, there are several reasons at this
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time why this specification states that they SHOULD NOT (instead of
MUST NOT) be included:
o At least one significant technology community of interest
explicitly allows multiple CN-IDs [EV-CERTS].
o At least one significant certification authority is known to issue
certificates containing multiple CN-IDs.
o Many service providers often deem inclusion of multiple CN-IDs
necessary in virtual hosting environments because at least one
widely deployed operating system does not yet support the SNI
extension.
It is hoped that the recommendation regarding multiple CN-IDs can be
further tightened in the future.
8. Contributors
The following individuals made important contributions to the text of
this document: Shumon Huque, RL 'Bob' Morgan, and Kurt Zeilenga.
9. Acknowledgements
The editors and contributors wish to thank the following individuals
for their feedback and suggestions: Bernard Aboba, Richard Barnes,
Uri Blumenthal, Nelson Bolyard, Kaspar Brand, Anthony Bryan, Scott
Cantor, Wan-Teh Chang, Bil Corry, Dave Cridland, Dave Crocker, Cyrus
Daboo, Charles Gardiner, Philip Guenther, Phillip Hallam-Baker, Bruno
Harbulot, Wes Hardaker, David Harrington, Paul Hoffman, Love
Hornquist Astrand, Henry Hotz, Russ Housley, Jeffrey Hutzelman,
Cullen Jennings, Simon Josefsson, Geoff Keating, John Klensin, Scott
Lawrence, Matt McCutchen, Alexey Melnikov, Subramanian Moonesamy,
Eddy Nigg, Ludwig Nussel, Joe Orton, Tom Petch, Yngve N. Pettersen,
Tim Polk, Robert Relyea, Eric Rescorla, Pete Resnick, Martin Rex, Joe
Salowey, Stefan Santesson, Jim Schaad, Rob Stradling, Michael
Stroeder, Andrew Sullivan, Peter Sylvester, Martin Thomson, Paul
Tiemann, Sean Turner, Nicolas Williams, Dan Wing, Dan Winship, and
Stefan Winter.
Thanks also to Barry Leiba and Ben Campbell for their reviews on
behalf of the Security Directorate and the General Area Review Team,
respectively.
The responsible Area Director was Alexey Melnikov.
Saint-Andre & Hodges Standards Track [Page 33]
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10. References
10.1. Normative References
[DNS-CONCEPTS] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, November 1987.
[DNS-SRV] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR
for specifying the location of services (DNS SRV)",
RFC 2782, February 2000.
[IDNA-DEFS] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document
Framework", RFC 5890, August 2010.
[IDNA-PROTO] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891,
August 2010.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[LDAP-DN] Zeilenga, K., Ed., "Lightweight Directory Access
Protocol (LDAP): String Representation of
Distinguished Names", RFC 4514, June 2006.
[PKIX] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", RFC 5280, May 2008.
[SRVNAME] Santesson, S., "Internet X.509 Public Key
Infrastructure Subject Alternative Name for
Expression of Service Name", RFC 4985, August 2007.
[URI] Berners-Lee, T., Fielding, R., and L. Masinter,
"Uniform Resource Identifier (URI): Generic Syntax",
STD 66, RFC 3986, January 2005.
10.2. Informative References
[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
January 2008.
[DNS-CASE] Eastlake 3rd, D., "Domain Name System (DNS) Case
Insensitivity Clarification", RFC 4343,
January 2006.
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RFC 6125 Service Identity March 2011
[DNSSEC] Arends, R., Austein, R., Larson, M., Massey, D., and
S. Rose, "DNS Security Introduction and
Requirements", RFC 4033, March 2005.
[DTLS] Rescorla, E. and N. Modadugu, "Datagram Transport
Layer Security", RFC 4347, April 2006.
[Defeating-SSL] Marlinspike, M., "New Tricks for Defeating SSL in
Practice", BlackHat DC, February 2009,
<http://www.blackhat.com/presentations/
bh-dc-09/Marlinspike/ BlackHat-DC-09-Marlinspike-
Defeating-SSL.pdf>.
[EMAIL-SRV] Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186, March 2011.
[EV-CERTS] CA/Browser Forum, "Guidelines For The Issuance And
Management Of Extended Validation Certificates",
October 2009,
<http://www.cabforum.org/Guidelines_v1_2.pdf>.
[GIST] Schulzrinne, H. and R. Hancock, "GIST: General
Internet Signalling Transport", RFC 5971,
October 2010.
[HTTP] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
June 1999.
[HTTP-TLS] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[HTTPSbytes] Sokol, J. and R. Hansen, "HTTPS Can Byte Me",
BlackHat Abu Dhabi, November 2010,
<https://media.blackhat.com/bh-ad-10/Hansen/
Blackhat-AD-2010-Hansen-Sokol-HTTPS-Can-Byte-Me-
slides.pdf>.
[IDNA2003] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications
(IDNA)", RFC 3490, March 2003.
[IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
VERSION 4rev1", RFC 3501, March 2003.
[IP] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
Saint-Andre & Hodges Standards Track [Page 35]
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[IPSEC] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[IPv6] Deering, S. and R. Hinden, "Internet Protocol,
Version 6 (IPv6) Specification", RFC 2460,
December 1998.
[LDAP] Sermersheim, J., "Lightweight Directory Access
Protocol (LDAP): The Protocol", RFC 4511, June 2006.
[LDAP-AUTH] Harrison, R., "Lightweight Directory Access Protocol
(LDAP): Authentication Methods and Security
Mechanisms", RFC 4513, June 2006.
[LDAP-SCHEMA] Sciberras, A., Ed., "Lightweight Directory Access
Protocol (LDAP): Schema for User Applications",
RFC 4519, June 2006.
[LDAP-TLS] Hodges, J., Morgan, R., and M. Wahl, "Lightweight
Directory Access Protocol (v3): Extension for
Transport Layer Security", RFC 2830, May 2000.
[NAPTR] Mealling, M., "Dynamic Delegation Discovery System
(DDDS) Part Three: The Domain Name System (DNS)
Database", RFC 3403, October 2002.
[NETCONF] Enns, R., Ed., "NETCONF Configuration Protocol",
RFC 4741, December 2006.
[NETCONF-SSH] Wasserman, M. and T. Goddard, "Using the NETCONF
Configuration Protocol over Secure SHell (SSH)",
RFC 4742, December 2006.
[NETCONF-TLS] Badra, M., "NETCONF over Transport Layer Security
(TLS)", RFC 5539, May 2009.
[NNTP] Feather, C., "Network News Transfer Protocol
(NNTP)", RFC 3977, October 2006.
[NNTP-TLS] Murchison, K., Vinocur, J., and C. Newman, "Using
Transport Layer Security (TLS) with Network News
Transfer Protocol (NNTP)", RFC 4642, October 2006.
[OCSP] Myers, M., Ankney, R., Malpani, A., Galperin, S.,
and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol -
OCSP", RFC 2560, June 1999.
Saint-Andre & Hodges Standards Track [Page 36]
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[OPENPGP] Callas, J., Donnerhacke, L., Finney, H., Shaw, D.,
and R. Thayer, "OpenPGP Message Format", RFC 4880,
November 2007.
[PKIX-OLD] Housley, R., Ford, W., Polk, T., and D. Solo,
"Internet X.509 Public Key Infrastructure
Certificate and CRL Profile", RFC 2459,
January 1999.
[POP3] Myers, J. and M. Rose, "Post Office Protocol -
Version 3", STD 53, RFC 1939, May 1996.
[PRIVATE] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot,
G., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, February 1996.
[S-NAPTR] Daigle, L. and A. Newton, "Domain-Based Application
Service Location Using SRV RRs and the Dynamic
Delegation Discovery Service (DDDS)", RFC 3958,
January 2005.
[SECTERMS] Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949, August 2007.
[SIP] Rosenberg, J., Schulzrinne, H., Camarillo, G.,
Johnston, A., Peterson, J., Sparks, R., Handley, M.,
and E. Schooler, "SIP: Session Initiation Protocol",
RFC 3261, June 2002.
[SIP-CERTS] Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain
Certificates in the Session Initiation Protocol
(SIP)", RFC 5922, June 2010.
[SIP-SIPS] Audet, F., "The Use of the SIPS URI Scheme in the
Session Initiation Protocol (SIP)", RFC 5630,
October 2009.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol",
RFC 5321, October 2008.
[SMTP-AUTH] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP
Service Extension for Authentication", RFC 4954,
July 2007.
[SMTP-TLS] Hoffman, P., "SMTP Service Extension for Secure SMTP
over Transport Layer Security", RFC 3207,
February 2002.
Saint-Andre & Hodges Standards Track [Page 37]
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[SNMP] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network
Management Protocol (SNMP) Management Frameworks",
STD 62, RFC 3411, December 2002.
[SNMP-TLS] Hardaker, W., "Transport Layer Security (TLS)
Transport Model for the Simple Network Management
Protocol (SNMP)", RFC 5953, August 2010.
[SYSLOG] Gerhards, R., "The Syslog Protocol", RFC 5424,
March 2009.
[SYSLOG-DTLS] Salowey, J., Petch, T., Gerhards, R., and H. Feng,
"Datagram Transport Layer Security (DTLS) Transport
Mapping for Syslog", RFC 6012, October 2010.
[SYSLOG-TLS] Miao, F., Ed., Ma, Y., Ed., and J. Salowey, Ed.,
"Transport Layer Security (TLS) Transport Mapping
for Syslog", RFC 5425, March 2009.
[TLS] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.2", RFC 5246,
August 2008.
[TLS-EXT] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
January 2011.
[US-ASCII] American National Standards Institute, "Coded
Character Set - 7-bit American Standard Code for
Information Interchange", ANSI X3.4, 1986.
[USINGTLS] Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, June 1999.
[WSC-UI] Saldhana, A. and T. Roessler, "Web Security Context:
User Interface Guidelines", World Wide Web
Consortium LastCall WD-wsc-ui-20100309, March 2010,
<http://www.w3.org/TR/2010/WD-wsc-ui-20100309>.
[X.500] International Telecommunications Union, "Information
Technology - Open Systems Interconnection - The
Directory: Overview of concepts, models and
services", ITU-T Recommendation X.500, ISO Standard
9594-1, August 2005.
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[X.501] International Telecommunications Union, "Information
Technology - Open Systems Interconnection - The
Directory: Models", ITU-T Recommendation X.501,
ISO Standard 9594-2, August 2005.
[X.509] International Telecommunications Union, "Information
Technology - Open Systems Interconnection - The
Directory: Public-key and attribute certificate
frameworks", ITU-T Recommendation X.509,
ISO Standard 9594-8, August 2005.
[X.520] International Telecommunications Union, "Information
Technology - Open Systems Interconnection - The
Directory: Selected attribute types", ITU-
T Recommendation X.509, ISO Standard 9594-6,
August 2005.
[X.690] International Telecommunications Union, "Information
Technology - ASN.1 encoding rules: Specification of
Basic Encoding Rules (BER), Canonical Encoding Rules
(CER) and Distinguished Encoding Rules (DER)", ITU-
T Recommendation X.690, ISO Standard 8825-1,
August 2008.
[XMPP] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011.
[XMPP-OLD] Saint-Andre, P., Ed., "Extensible Messaging and
Presence Protocol (XMPP): Core", RFC 3920,
October 2004.
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Appendix A. Sample Text
At the time of this writing, two application technologies reuse the
recommendations in this specification: email [EMAIL-SRV] and XMPP
[XMPP]. Here we include the text from [XMPP] to illustrate the
thought process that might be followed by protocol designers for
other application technologies. Specifically, because XMPP uses DNS
SRV records for resolution of the DNS domain names for application
services, the XMPP specification recommends the use of SRV-IDs.
The text regarding certificate issuance is as follows:
######
In a PKIX certificate to be presented by an XMPP server (i.e., a
"server certificate"), the certificate MUST include one or more XMPP
addresses (i.e., domainparts) associated with XMPP services hosted at
the server. The rules and guidelines defined in [this specification]
apply to XMPP server certificates, with the following XMPP-specific
considerations:
o Support for the DNS-ID identifier type [PKIX] is REQUIRED in XMPP
client and server software implementations. Certification
authorities that issue XMPP-specific certificates MUST support the
DNS-ID identifier type. XMPP service providers SHOULD include the
DNS-ID identifier type in certificate requests.
o Support for the SRV-ID identifier type [SRVNAME] is REQUIRED for
XMPP client and server software implementations (for verification
purposes XMPP client implementations need to support only the
"_xmpp-client" application service type, whereas XMPP server
implementations need to support both the "_xmpp-client" and
"_xmpp-server" application service types). Certification
authorities that issue XMPP-specific certificates SHOULD support
the SRV-ID identifier type. XMPP service providers SHOULD include
the SRV-ID identifier type in certificate requests.
o Support for the XmppAddr identifier type is encouraged in XMPP
client and server software implementations for the sake of
backward-compatibility, but is no longer encouraged in
certificates issued by certification authorities or requested by
XMPP service providers.
o DNS domain names in server certificates MAY contain the wildcard
character '*' as the complete left-most label within the
identifier.
######
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The text regarding certificate verification is as follows:
######
For server certificates, the rules and guidelines defined in [this
specification] apply, with the proviso that the XmppAddr identifier
is allowed as a reference identifier.
The identities to be checked are set as follows:
o The initiating entity sets its reference identifier to the 'to'
address it communicates in the initial stream header; i.e., this
is the identity it expects the receiving entity to provide in a
PKIX certificate.
o The receiving entity sets its reference identifier to the 'from'
address communicated by the initiating entity in the initial
stream header; i.e., this is the identity that the initiating
entity is trying to assert.
######
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Appendix B. Prior Art
(This section is non-normative.)
The recommendations in this document are an abstraction from
recommendations in specifications for a wide range of application
protocols. For the purpose of comparison and to delineate the
history of thinking about application service identity verification
within the IETF, this informative section gathers together prior art
by including the exact text from various RFCs (the only modifications
are changes to the names of several references to maintain coherence
with the main body of this document, and the elision of irrelevant
text as marked by the characters "[...]").
B.1. IMAP, POP3, and ACAP (1999)
In 1999, [USINGTLS] specified the following text regarding
application service identity verification in IMAP, POP3, and ACAP:
######
2.4. Server Identity Check
During the TLS negotiation, the client MUST check its understanding
of the server hostname against the server's identity as presented in
the server Certificate message, in order to prevent man-in-the-middle
attacks. Matching is performed according to these rules:
o The client MUST use the server hostname it used to open the
connection as the value to compare against the server name as
expressed in the server certificate. The client MUST NOT use any
form of the server hostname derived from an insecure remote source
(e.g., insecure DNS lookup). CNAME canonicalization is not done.
o If a subjectAltName extension of type dNSName is present in the
certificate, it SHOULD be used as the source of the server's
identity.
o Matching is case-insensitive.
o A "*" wildcard character MAY be used as the left-most name
component in the certificate. For example, *.example.com would
match a.example.com, foo.example.com, etc. but would not match
example.com.
o If the certificate contains multiple names (e.g. more than one
dNSName field), then a match with any one of the fields is
considered acceptable.
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If the match fails, the client SHOULD either ask for explicit user
confirmation, or terminate the connection and indicate the server's
identity is suspect.
######
B.2. HTTP (2000)
In 2000, [HTTP-TLS] specified the following text regarding
application service identity verification in HTTP:
######
3.1. Server Identity
In general, HTTP/TLS requests are generated by dereferencing a URI.
As a consequence, the hostname for the server is known to the client.
If the hostname is available, the client MUST check it against the
server's identity as presented in the server's Certificate message,
in order to prevent man-in-the-middle attacks.
If the client has external information as to the expected identity of
the server, the hostname check MAY be omitted. (For instance, a
client may be connecting to a machine whose address and hostname are
dynamic but the client knows the certificate that the server will
present.) In such cases, it is important to narrow the scope of
acceptable certificates as much as possible in order to prevent man
in the middle attacks. In special cases, it may be appropriate for
the client to simply ignore the server's identity, but it must be
understood that this leaves the connection open to active attack.
If a subjectAltName extension of type dNSName is present, that MUST
be used as the identity. Otherwise, the (most specific) Common Name
field in the Subject field of the certificate MUST be used. Although
the use of the Common Name is existing practice, it is deprecated and
Certification Authorities are encouraged to use the dNSName instead.
Matching is performed using the matching rules specified by
[PKIX-OLD]. If more than one identity of a given type is present in
the certificate (e.g., more than one dNSName name, a match in any one
of the set is considered acceptable.) Names may contain the wildcard
character * which is considered to match any single domain name
component or component fragment. E.g., *.a.com matches foo.a.com but
not bar.foo.a.com. f*.com matches foo.com but not bar.com.
In some cases, the URI is specified as an IP address rather than a
hostname. In this case, the iPAddress subjectAltName must be present
in the certificate and must exactly match the IP in the URI.
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If the hostname does not match the identity in the certificate, user
oriented clients MUST either notify the user (clients MAY give the
user the opportunity to continue with the connection in any case) or
terminate the connection with a bad certificate error. Automated
clients MUST log the error to an appropriate audit log (if available)
and SHOULD terminate the connection (with a bad certificate error).
Automated clients MAY provide a configuration setting that disables
this check, but MUST provide a setting which enables it.
Note that in many cases the URI itself comes from an untrusted
source. The above-described check provides no protection against
attacks where this source is compromised. For example, if the URI
was obtained by clicking on an HTML page which was itself obtained
without using HTTP/TLS, a man in the middle could have replaced the
URI. In order to prevent this form of attack, users should carefully
examine the certificate presented by the server to determine if it
meets their expectations.
######
B.3. LDAP (2000/2006)
In 2000, [LDAP-TLS] specified the following text regarding
application service identity verification in LDAP:
######
3.6. Server Identity Check
The client MUST check its understanding of the server's hostname
against the server's identity as presented in the server's
Certificate message, in order to prevent man-in-the-middle attacks.
Matching is performed according to these rules:
o The client MUST use the server hostname it used to open the LDAP
connection as the value to compare against the server name as
expressed in the server's certificate. The client MUST NOT use
the server's canonical DNS name or any other derived form of name.
o If a subjectAltName extension of type dNSName is present in the
certificate, it SHOULD be used as the source of the server's
identity.
o Matching is case-insensitive.
o The "*" wildcard character is allowed. If present, it applies
only to the left-most name component.
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E.g. *.bar.com would match a.bar.com, b.bar.com, etc. but not
bar.com. If more than one identity of a given type is present in the
certificate (e.g. more than one dNSName name), a match in any one of
the set is considered acceptable.
If the hostname does not match the dNSName-based identity in the
certificate per the above check, user-oriented clients SHOULD either
notify the user (clients MAY give the user the opportunity to
continue with the connection in any case) or terminate the connection
and indicate that the server's identity is suspect. Automated
clients SHOULD close the connection, returning and/or logging an
error indicating that the server's identity is suspect.
Beyond the server identity checks described in this section, clients
SHOULD be prepared to do further checking to ensure that the server
is authorized to provide the service it is observed to provide. The
client MAY need to make use of local policy information.
######
In 2006, [LDAP-AUTH] specified the following text regarding
application service identity verification in LDAP:
######
3.1.3. Server Identity Check
In order to prevent man-in-the-middle attacks, the client MUST verify
the server's identity (as presented in the server's Certificate
message). In this section, the client's understanding of the
server's identity (typically the identity used to establish the
transport connection) is called the "reference identity".
The client determines the type (e.g., DNS name or IP address) of the
reference identity and performs a comparison between the reference
identity and each subjectAltName value of the corresponding type
until a match is produced. Once a match is produced, the server's
identity has been verified, and the server identity check is
complete. Different subjectAltName types are matched in different
ways. Sections 3.1.3.1 - 3.1.3.3 explain how to compare values of
various subjectAltName types.
The client may map the reference identity to a different type prior
to performing a comparison. Mappings may be performed for all
available subjectAltName types to which the reference identity can be
mapped; however, the reference identity should only be mapped to
types for which the mapping is either inherently secure (e.g.,
extracting the DNS name from a URI to compare with a subjectAltName
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of type dNSName) or for which the mapping is performed in a secure
manner (e.g., using [DNSSEC], or using user- or admin-configured
host-to-address/address-to-host lookup tables).
The server's identity may also be verified by comparing the reference
identity to the Common Name (CN) [LDAP-SCHEMA] value in the last
Relative Distinguished Name (RDN) of the subject field of the
server's certificate (where "last" refers to the DER-encoded order,
not the order of presentation in a string representation of DER-
encoded data). This comparison is performed using the rules for
comparison of DNS names in Section 3.1.3.1, below, with the exception
that no wildcard matching is allowed. Although the use of the Common
Name value is existing practice, it is deprecated, and Certification
Authorities are encouraged to provide subjectAltName values instead.
Note that the TLS implementation may represent DNs in certificates
according to X.500 or other conventions. For example, some X.500
implementations order the RDNs in a DN using a left-to-right (most
significant to least significant) convention instead of LDAP's right-
to-left convention.
If the server identity check fails, user-oriented clients SHOULD
either notify the user (clients may give the user the opportunity to
continue with the LDAP session in this case) or close the transport
connection and indicate that the server's identity is suspect.
Automated clients SHOULD close the transport connection and then
return or log an error indicating that the server's identity is
suspect or both.
Beyond the server identity check described in this section, clients
should be prepared to do further checking to ensure that the server
is authorized to provide the service it is requested to provide. The
client may need to make use of local policy information in making
this determination.
3.1.3.1. Comparison of DNS Names
If the reference identity is an internationalized domain name,
conforming implementations MUST convert it to the ASCII Compatible
Encoding (ACE) format as specified in Section 4 of RFC 3490
[IDNA2003] before comparison with subjectAltName values of type
dNSName. Specifically, conforming implementations MUST perform the
conversion operation specified in Section 4 of RFC 3490 as follows:
o in step 1, the domain name SHALL be considered a "stored string";
o in step 3, set the flag called "UseSTD3ASCIIRules";
o in step 4, process each label with the "ToASCII" operation; and
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o in step 5, change all label separators to U+002E (full stop).
After performing the "to-ASCII" conversion, the DNS labels and names
MUST be compared for equality according to the rules specified in
Section 3 of RFC3490.
The '*' (ASCII 42) wildcard character is allowed in subjectAltName
values of type dNSName, and then only as the left-most (least
significant) DNS label in that value. This wildcard matches any
left-most DNS label in the server name. That is, the subject
*.example.com matches the server names a.example.com and
b.example.com, but does not match example.com or a.b.example.com.
3.1.3.2. Comparison of IP Addresses
When the reference identity is an IP address, the identity MUST be
converted to the "network byte order" octet string representation
[IP] [IPv6]. For IP Version 4, as specified in RFC 791, the octet
string will contain exactly four octets. For IP Version 6, as
specified in RFC 2460, the octet string will contain exactly sixteen
octets. This octet string is then compared against subjectAltName
values of type iPAddress. A match occurs if the reference identity
octet string and value octet strings are identical.
3.1.3.3. Comparison of Other subjectName Types
Client implementations MAY support matching against subjectAltName
values of other types as described in other documents.
######
B.4. SMTP (2002/2007)
In 2002, [SMTP-TLS] specified the following text regarding
application service identity verification in SMTP:
######
4.1 Processing After the STARTTLS Command
[...]
The decision of whether or not to believe the authenticity of the
other party in a TLS negotiation is a local matter. However, some
general rules for the decisions are:
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o A SMTP client would probably only want to authenticate an SMTP
server whose server certificate has a domain name that is the
domain name that the client thought it was connecting to.
[...]
######
In 2006, [SMTP-AUTH] specified the following text regarding
application service identity verification in SMTP:
######
14. Additional Requirements When Using SASL PLAIN over TLS
[...]
After a successful [TLS] negotiation, the client MUST check its
understanding of the server hostname against the server's identity as
presented in the server Certificate message, in order to prevent man-
in-the-middle attacks. If the match fails, the client MUST NOT
attempt to authenticate using the SASL PLAIN mechanism. Matching is
performed according to the following rules:
The client MUST use the server hostname it used to open the
connection as the value to compare against the server name as
expressed in the server certificate. The client MUST NOT use any
form of the server hostname derived from an insecure remote source
(e.g., insecure DNS lookup). CNAME canonicalization is not done.
If a subjectAltName extension of type dNSName is present in the
certificate, it SHOULD be used as the source of the server's
identity.
Matching is case-insensitive.
A "*" wildcard character MAY be used as the leftmost name
component in the certificate. For example, *.example.com would
match a.example.com, foo.example.com, etc., but would not match
example.com.
If the certificate contains multiple names (e.g., more than one
dNSName field), then a match with any one of the fields is
considered acceptable.
######
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B.5. XMPP (2004)
In 2004, [XMPP-OLD] specified the following text regarding
application service identity verification in XMPP:
######
14.2. Certificate Validation
When an XMPP peer communicates with another peer securely, it MUST
validate the peer's certificate. There are three possible cases:
Case #1: The peer contains an End Entity certificate which appears
to be certified by a certification path terminating in a trust
anchor (as described in Section 6.1 of [PKIX]).
Case #2: The peer certificate is certified by a Certificate
Authority not known to the validating peer.
Case #3: The peer certificate is self-signed.
In Case #1, the validating peer MUST do one of two things:
1. Verify the peer certificate according to the rules of [PKIX].
The certificate SHOULD then be checked against the expected
identity of the peer following the rules described in [HTTP-TLS],
except that a subjectAltName extension of type "xmpp" MUST be
used as the identity if present. If one of these checks fails,
user-oriented clients MUST either notify the user (clients MAY
give the user the opportunity to continue with the connection in
any case) or terminate the connection with a bad certificate
error. Automated clients SHOULD terminate the connection (with a
bad certificate error) and log the error to an appropriate audit
log. Automated clients MAY provide a configuration setting that
disables this check, but MUST provide a setting that enables it.
2. The peer SHOULD show the certificate to a user for approval,
including the entire certification path. The peer MUST cache the
certificate (or some non-forgeable representation such as a
hash). In future connections, the peer MUST verify that the same
certificate was presented and MUST notify the user if it has
changed.
In Case #2 and Case #3, implementations SHOULD act as in (2) above.
######
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Although [XMPP-OLD] defined its own rules, [XMPP] reuses the rules in
this document regarding application service identity verification in
XMPP.
B.6. NNTP (2006)
In 2006, [NNTP-TLS] specified the following text regarding
application service identity verification in NNTP:
######
5. Security Considerations
[...]
During the TLS negotiation, the client MUST check its understanding
of the server hostname against the server's identity as presented in
the server Certificate message, in order to prevent man-in-the-middle
attacks. Matching is performed according to these rules:
o The client MUST use the server hostname it used to open the
connection (or the hostname specified in TLS "server_name"
extension [TLS]) as the value to compare against the server name
as expressed in the server certificate. The client MUST NOT use
any form of the server hostname derived from an insecure remote
source (e.g., insecure DNS lookup). CNAME canonicalization is not
done.
o If a subjectAltName extension of type dNSName is present in the
certificate, it SHOULD be used as the source of the server's
identity.
o Matching is case-insensitive.
o A "*" wildcard character MAY be used as the left-most name
component in the certificate. For example, *.example.com would
match a.example.com, foo.example.com, etc., but would not match
example.com.
o If the certificate contains multiple names (e.g., more than one
dNSName field), then a match with any one of the fields is
considered acceptable.
If the match fails, the client SHOULD either ask for explicit user
confirmation or terminate the connection with a QUIT command and
indicate the server's identity is suspect.
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Additionally, clients MUST verify the binding between the identity of
the servers to which they connect and the public keys presented by
those servers. Clients SHOULD implement the algorithm in Section 6
of [PKIX] for general certificate validation, but MAY supplement that
algorithm with other validation methods that achieve equivalent
levels of verification (such as comparing the server certificate
against a local store of already-verified certificates and identity
bindings).
######
B.7. NETCONF (2006/2009)
In 2006, [NETCONF-SSH] specified the following text regarding
application service identity verification in NETCONF:
######
6. Security Considerations
The identity of the server MUST be verified and authenticated by the
client according to local policy before password-based authentication
data or any configuration or state data is sent to or received from
the server. The identity of the client MUST also be verified and
authenticated by the server according to local policy to ensure that
the incoming client request is legitimate before any configuration or
state data is sent to or received from the client. Neither side
should establish a NETCONF over SSH connection with an unknown,
unexpected, or incorrect identity on the opposite side.
######
In 2009, [NETCONF-TLS] specified the following text regarding
application service identity verification in NETCONF:
######
3.1. Server Identity
During the TLS negotiation, the client MUST carefully examine the
certificate presented by the server to determine if it meets the
client's expectations. Particularly, the client MUST check its
understanding of the server hostname against the server's identity as
presented in the server Certificate message, in order to prevent man-
in-the-middle attacks.
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Matching is performed according to the rules below (following the
example of [NNTP-TLS]):
o The client MUST use the server hostname it used to open the
connection (or the hostname specified in the TLS "server_name"
extension [TLS]) as the value to compare against the server name
as expressed in the server certificate. The client MUST NOT use
any form of the server hostname derived from an insecure remote
source (e.g., insecure DNS lookup). CNAME canonicalization is not
done.
o If a subjectAltName extension of type dNSName is present in the
certificate, it MUST be used as the source of the server's
identity.
o Matching is case-insensitive.
o A "*" wildcard character MAY be used as the leftmost name
component in the certificate. For example, *.example.com would
match a.example.com, foo.example.com, etc., but would not match
example.com.
o If the certificate contains multiple names (e.g., more than one
dNSName field), then a match with any one of the fields is
considered acceptable.
If the match fails, the client MUST either ask for explicit user
confirmation or terminate the connection and indicate the server's
identity is suspect.
Additionally, clients MUST verify the binding between the identity of
the servers to which they connect and the public keys presented by
those servers. Clients SHOULD implement the algorithm in Section 6
of [PKIX] for general certificate validation, but MAY supplement that
algorithm with other validation methods that achieve equivalent
levels of verification (such as comparing the server certificate
against a local store of already-verified certificates and identity
bindings).
If the client has external information as to the expected identity of
the server, the hostname check MAY be omitted.
######
B.8. Syslog (2009)
In 2009, [SYSLOG-TLS] specified the following text regarding
application service identity verification in Syslog:
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RFC 6125 Service Identity March 2011
######
5.2. Subject Name Authorization
Implementations MUST support certification path validation [PKIX].
In addition, they MUST support specifying the authorized peers using
locally configured host names and matching the name against the
certificate as follows.
o Implementations MUST support matching the locally configured host
name against a dNSName in the subjectAltName extension field and
SHOULD support checking the name against the common name portion
of the subject distinguished name.
o The '*' (ASCII 42) wildcard character is allowed in the dNSName of
the subjectAltName extension (and in common name, if used to store
the host name), but only as the left-most (least significant) DNS
label in that value. This wildcard matches any left-most DNS
label in the server name. That is, the subject *.example.com
matches the server names a.example.com and b.example.com, but does
not match example.com or a.b.example.com. Implementations MUST
support wildcards in certificates as specified above, but MAY
provide a configuration option to disable them.
o Locally configured names MAY contain the wildcard character to
match a range of values. The types of wildcards supported MAY be
more flexible than those allowed in subject names, making it
possible to support various policies for different environments.
For example, a policy could allow for a trust-root-based
authorization where all credentials issued by a particular CA
trust root are authorized.
o If the locally configured name is an internationalized domain
name, conforming implementations MUST convert it to the ASCII
Compatible Encoding (ACE) format for performing comparisons, as
specified in Section 7 of [PKIX].
o Implementations MAY support matching a locally configured IP
address against an iPAddress stored in the subjectAltName
extension. In this case, the locally configured IP address is
converted to an octet string as specified in [PKIX], Section
4.2.1.6. A match occurs if this octet string is equal to the
value of iPAddress in the subjectAltName extension.
######
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B.9. SIP (2010)
In 2010, [SIP-CERTS] specified the following text regarding
application service identity verification in SIP:
######
7.2. Comparing SIP Identities
When an implementation (either client or server) compares two values
as SIP domain identities:
Implementations MUST compare only the DNS name component of each
SIP domain identifier; an implementation MUST NOT use any scheme
or parameters in the comparison.
Implementations MUST compare the values as DNS names, which means
that the comparison is case insensitive as specified by
[DNS-CASE]. Implementations MUST handle Internationalized Domain
Names (IDNs) in accordance with Section 7.2 of [PKIX].
Implementations MUST match the values in their entirety:
Implementations MUST NOT match suffixes. For example,
"foo.example.com" does not match "example.com".
Implementations MUST NOT match any form of wildcard, such as a
leading "." or "*." with any other DNS label or sequence of
labels. For example, "*.example.com" matches only
"*.example.com" but not "foo.example.com". Similarly,
".example.com" matches only ".example.com", and does not match
"foo.example.com."
[HTTP-TLS] allows the dNSName component to contain a
wildcard; e.g., "DNS:*.example.com". [PKIX], while not
disallowing this explicitly, leaves the interpretation of
wildcards to the individual specification. [SIP] does not
provide any guidelines on the presence of wildcards in
certificates. Through the rule above, this document
prohibits such wildcards in certificates for SIP domains.
######
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B.10. SNMP (2010)
In 2010, [SNMP-TLS] specified the following text regarding
application service identity verification in SNMP:
######
If the server's presented certificate has passed certification path
validation [PKIX] to a configured trust anchor, and an active row
exists with a zero-length snmpTlstmAddrServerFingerprint value, then
the snmpTlstmAddrServerIdentity column contains the expected host
name. This expected host name is then compared against the server's
certificate as follows:
o Implementations MUST support matching the expected host name
against a dNSName in the subjectAltName extension field and MAY
support checking the name against the CommonName portion of the
subject distinguished name.
o The '*' (ASCII 0x2a) wildcard character is allowed in the dNSName
of the subjectAltName extension (and in common name, if used to
store the host name), but only as the left-most (least
significant) DNS label in that value. This wildcard matches any
left-most DNS label in the server name. That is, the subject
*.example.com matches the server names a.example.com and
b.example.com, but does not match example.com or a.b.example.com.
Implementations MUST support wildcards in certificates as
specified above, but MAY provide a configuration option to disable
them.
o If the locally configured name is an internationalized domain
name, conforming implementations MUST convert it to the ASCII
Compatible Encoding (ACE) format for performing comparisons, as
specified in Section 7 of [PKIX].
If the expected host name fails these conditions then the connection
MUST be closed.
######
B.11. GIST (2010)
In 2010, [GIST] specified the following text regarding application
service identity verification in the General Internet Signalling
Transport:
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RFC 6125 Service Identity March 2011
######
5.7.3.1. Identity Checking in TLS
After TLS authentication, a node MUST check the identity presented by
the peer in order to avoid man-in-the-middle attacks, and verify that
the peer is authorised to take part in signalling at the GIST layer.
The authorisation check is carried out by comparing the presented
identity with each Authorised Peer Database (APD) entry in turn, as
discussed in Section 4.4.2. This section defines the identity
comparison algorithm for a single APD entry.
For TLS authentication with X.509 certificates, an identity from the
DNS namespace MUST be checked against each subjectAltName extension
of type dNSName present in the certificate. If no such extension is
present, then the identity MUST be compared to the (most specific)
Common Name in the Subject field of the certificate. When matching
DNS names against dNSName or Common Name fields, matching is case-
insensitive. Also, a "*" wildcard character MAY be used as the left-
most name component in the certificate or identity in the APD. For
example, *.example.com in the APD would match certificates for
a.example.com, foo.example.com, *.example.com, etc., but would not
match example.com. Similarly, a certificate for *.example.com would
be valid for APD identities of a.example.com, foo.example.com,
*.example.com, etc., but not example.com.
Additionally, a node MUST verify the binding between the identity of
the peer to which it connects and the public key presented by that
peer. Nodes SHOULD implement the algorithm in Section 6 of [PKIX]
for general certificate validation, but MAY supplement that algorithm
with other validation methods that achieve equivalent levels of
verification (such as comparing the server certificate against a
local store of already-verified certificates and identity bindings).
For TLS authentication with pre-shared keys, the identity in the
psk_identity_hint (for the server identity, i.e. the Responding node)
or psk_identity (for the client identity, i.e. the Querying node)
MUST be compared to the identities in the APD.
######
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Authors' Addresses
Peter Saint-Andre
Cisco
1899 Wyknoop Street, Suite 600
Denver, CO 80202
USA
Phone: +1-303-308-3282
EMail: psaintan@cisco.com
Jeff Hodges
PayPal
2211 North First Street
San Jose, California 95131
US
EMail: Jeff.Hodges@PayPal.com
Saint-Andre & Hodges Standards Track [Page 57]
ERRATA
