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)
draft-ietf-uta-rfc6125bis-01
The information below is for an old version of the document.
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| Authors | Peter Saint-Andre , Jeff Hodges , Rich Salz | ||
| Last updated | 2021-07-08 | ||
| Replaces | draft-ietf-uta-use-san | ||
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draft-ietf-uta-rfc6125bis-01
Network Working Group P. Saint-Andre
Internet-Draft Mozilla
Obsoletes: 6125 (if approved) J. Hodges
Intended status: Standards Track Google
Expires: 9 January 2022 R. Salz
Akamai Technologies
8 July 2021
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)
draft-ietf-uta-rfc6125bis-01
Abstract
Many application technologies enable secure communication between two
entities by means of Transport Layer Security (TLS) with Internet
Public Key Infrastructure Using X.509 (PKIX) certificates. This
document specifies procedures for representing and verifying the
identity of application services in such interactions.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Using TLS in
Applications Working Group mailing list (uta@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/uta/.
Source for this draft and an issue tracker can be found at
https://github.com/richsalz/draft-ietf-uta-rfc6125bis.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on 9 January 2022.
Copyright Notice
Copyright (c) 2021 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 (https://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
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provided without warranty as described in the Simplified BSD License.
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. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.6.1. In Scope . . . . . . . . . . . . . . . . . . . . . . 6
1.6.2. Out of Scope . . . . . . . . . . . . . . . . . . . . 6
1.7. Terminology . . . . . . . . . . . . . . . . . . . . . . . 8
2. Naming of Application Services . . . . . . . . . . . . . . . 12
2.1. Naming Application Services . . . . . . . . . . . . . . . 12
2.2. DNS Domain Names . . . . . . . . . . . . . . . . . . . . 13
2.3. Subject Naming in PKIX Certificates . . . . . . . . . . . 14
3. Designing Application Protocols . . . . . . . . . . . . . . . 15
4. Representing Server Identity . . . . . . . . . . . . . . . . 16
4.1. Rules . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . 17
5. Requesting Server Certificates . . . . . . . . . . . . . . . 18
6. Verifying Service Identity . . . . . . . . . . . . . . . . . 19
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 19
6.2. Constructing a List of Reference Identifiers . . . . . . 19
6.2.1. Rules . . . . . . . . . . . . . . . . . . . . . . . . 19
6.2.2. Examples . . . . . . . . . . . . . . . . . . . . . . 22
6.3. Preparing to Seek a Match . . . . . . . . . . . . . . . . 22
6.4. Matching the DNS Domain Name Portion . . . . . . . . . . 24
6.4.1. Checking of Traditional Domain Names . . . . . . . . 24
6.4.2. Checking of Internationalized Domain Names . . . . . 24
6.4.3. Checking of Wildcard Certificates . . . . . . . . . . 24
6.5. Matching the Application Service Type Portion . . . . . . 25
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6.5.1. SRV-ID . . . . . . . . . . . . . . . . . . . . . . . 25
6.5.2. URI-ID . . . . . . . . . . . . . . . . . . . . . . . 26
6.6. Outcome . . . . . . . . . . . . . . . . . . . . . . . . . 26
6.6.1. Case #1: Match Found . . . . . . . . . . . . . . . . 26
6.6.2. Case #2: No Match Found, Pinned Certificate . . . . . 26
6.6.3. Case #3: No Match Found, No Pinned Certificate . . . 26
6.6.4. Fallback . . . . . . . . . . . . . . . . . . . . . . 26
7. Security Considerations . . . . . . . . . . . . . . . . . . . 27
7.1. Pinned Certificates . . . . . . . . . . . . . . . . . . . 27
7.2. Wildcard Certificates . . . . . . . . . . . . . . . . . . 27
7.3. Internationalized Domain Names . . . . . . . . . . . . . 28
7.4. Multiple Identifiers . . . . . . . . . . . . . . . . . . 28
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
8.1. Normative References . . . . . . . . . . . . . . . . . . 28
8.2. Informative References . . . . . . . . . . . . . . . . . 29
Appendix A. Sample Text . . . . . . . . . . . . . . . . . . . . 33
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
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.
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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.
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:
* Protocol designers might want to first read the checklist in
Section 3.
* Certification authorities might want to first read the
recommendations for representation of server identity in
Section 4.
* Service providers might want to first read the recommendations for
requesting of server certificates in Section 5.
* Software implementers might want to first read the recommendations
for verification of server identity in Section 6.
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The sections on terminology (Section 1.7), naming of application
services (Section 2), document scope (Section 1.6), 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.
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. 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.
The previous version of this specification, [VERIFY], surveyed the
current practice from many IETF standards and tried to generalize
best practices. This document takes the lessons learned in the past
decade and codifies them as best practices.
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:
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* Stop including and checking strings that look like domain names in
the subject's Common Name.
* Check DNS domain names via the subjectAlternativeName extension
designed for that purpose: dNSName.
* Move toward including and checking even more specific
subjectAlternativeName extensions where appropriate for using the
protocol (e.g., uniformResourceIdentifier and the otherName form
SRVName).
* Constrain and simplify the validation of wildcard certificates
(e.g., a certificate containing an identifier for
"*.example.com").
1.6. Scope
1.6.1. 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).
1.6.2. Out of Scope
The following topics are out of scope for this specification:
* 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.
* 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
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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.
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).
* 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.
* 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
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systems. Attempting to define best practices for such
technologies would unduly complicate the rules defined in this
specification.
* Certification authority policies, such as:
- What types or "classes" of certificates to issue and whether to
apply different policies for them.
- 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).
* 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.
* 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]).
1.7. 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
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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.
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.
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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
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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,
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
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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:
* A DNS-ID is direct and unrestricted.
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* An SRV-ID can be either direct or (more typically) indirect, and
is restricted.
* A URI-ID is direct and restricted.
We summarize this taxonomy in the following table.
+-----------+-----------+---------------+
| | Direct | Restricted |
+-----------+-----------+---------------+
| 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 one or more of the following identifier types
within subjectAltName entries:
* DNS-ID
* SRV-ID
* URI-ID
The Common Name RDN should not be used to identify a service.
Reasons for this include:
* It is not strongly typed and therefore suffers from ambiguities in
interpretation.
* It can appear more than once in the Subject Name.
Likewise, other RDN's within the Subject Name SHOULD NOT be used to
identify a service.
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.
* 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].)
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* 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.)
* 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
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
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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. The certificate MAY contain more than one DNS-ID, SRV-ID, or URI-
ID as further explained under Section 7.4.
6. 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".
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".
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".
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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]).
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]).
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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):
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.
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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
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).
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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:
* 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).
* 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.
* 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.
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.
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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.
6.2.2. Examples
A web browser that is connecting via HTTPS to the website at
"www.example.com" would have a single reference identifier: a DNS-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 three
reference identifiers: an SRV-ID of "_imaps.example.net" (see
[EMAIL-SRV]), and DNS-IDs of "example.net" and "mail.example.net".
(A 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.
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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:
* 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
DNS-ID of "www.example.com" would result in a DNS domain name
portion of "www.example.com".
* 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]).
* 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.
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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
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".
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]), provided the requirements listed below are met.
For information regarding the security characteristics of wildcard
certificates, see Section 7.2.
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A client MUST NOT use the wildcard identifier if the reference
identifier does not follow the following rules:
1. There is more than one wildcard character.
2. The wildcard appears other than in the left-most label (e.g., do
not match "bar.*.example.net").
3. The wildcard is not the first character (e.g., do not match
"w*.example.com")
4. The wildcard character is embedded in an A-label or U-label
[IDNA-DEFS] of an internationalized domain name [IDNA-PROTO].
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 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.
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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 1.7), and the presented
certificate matches the pinned certificate (including the context as
described under Section 7.1), then the service identity check has
succeeded.
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.
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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 1.7, 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
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 SHOULD be checked by
application clients if the requirements specified in Section 6.4.3
are met.
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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).
Protection against a wildcard that identifies a so-called "public
suffix" (e.g., "*.co.uk" or "*.com") is beyond the scope of this
document.
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.
8. References
8.1. Normative References
[DNS-CONCEPTS]
Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/rfc/rfc1034>.
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[DNS-SRV] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/rfc/rfc2782>.
[IDNA-DEFS]
Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<https://www.rfc-editor.org/rfc/rfc5890>.
[IDNA-PROTO]
Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891,
DOI 10.17487/RFC5891, August 2010,
<https://www.rfc-editor.org/rfc/rfc5891>.
[LDAP-DN] Zeilenga, K., Ed., "Lightweight Directory Access Protocol
(LDAP): String Representation of Distinguished Names",
RFC 4514, DOI 10.17487/RFC4514, June 2006,
<https://www.rfc-editor.org/rfc/rfc4514>.
[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, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/rfc/rfc5280>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[SRVNAME] Santesson, S., "Internet X.509 Public Key Infrastructure
Subject Alternative Name for Expression of Service Name",
RFC 4985, DOI 10.17487/RFC4985, August 2007,
<https://www.rfc-editor.org/rfc/rfc4985>.
[URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/rfc/rfc3986>.
8.2. Informative References
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[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/rfc/rfc5234>.
[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>.
[DNS-CASE] Eastlake 3rd, D., "Domain Name System (DNS) Case
Insensitivity Clarification", RFC 4343,
DOI 10.17487/RFC4343, January 2006,
<https://www.rfc-editor.org/rfc/rfc4343>.
[DNSSEC] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/rfc/rfc4033>.
[DTLS] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, DOI 10.17487/RFC4347, April 2006,
<https://www.rfc-editor.org/rfc/rfc4347>.
[EMAIL-SRV]
Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186,
DOI 10.17487/RFC6186, March 2011,
<https://www.rfc-editor.org/rfc/rfc6186>.
[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>.
[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,
DOI 10.17487/RFC2616, June 1999,
<https://www.rfc-editor.org/rfc/rfc2616>.
[HTTP-TLS] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/rfc/rfc2818>.
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[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>.
[IPSEC] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <https://www.rfc-editor.org/rfc/rfc4301>.
[NAPTR] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part Three: The Domain Name System (DNS) Database",
RFC 3403, DOI 10.17487/RFC3403, October 2002,
<https://www.rfc-editor.org/rfc/rfc3403>.
[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,
DOI 10.17487/RFC2560, June 1999,
<https://www.rfc-editor.org/rfc/rfc2560>.
[OPENPGP] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007,
<https://www.rfc-editor.org/rfc/rfc4880>.
[PRIVATE] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
J., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
February 1996, <https://www.rfc-editor.org/rfc/rfc1918>.
[S-NAPTR] Daigle, L. and A. Newton, "Domain-Based Application
Service Location Using SRV RRs and the Dynamic Delegation
Discovery Service (DDDS)", RFC 3958, DOI 10.17487/RFC3958,
January 2005, <https://www.rfc-editor.org/rfc/rfc3958>.
[SECTERMS] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/rfc/rfc4949>.
[SIP] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/rfc/rfc3261>.
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[SIP-CERTS]
Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain
Certificates in the Session Initiation Protocol (SIP)",
RFC 5922, DOI 10.17487/RFC5922, June 2010,
<https://www.rfc-editor.org/rfc/rfc5922>.
[SIP-SIPS] Audet, F., "The Use of the SIPS URI Scheme in the Session
Initiation Protocol (SIP)", RFC 5630,
DOI 10.17487/RFC5630, October 2009,
<https://www.rfc-editor.org/rfc/rfc5630>.
[TLS] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/rfc/rfc5246>.
[TLS-EXT] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/rfc/rfc6066>.
[US-ASCII] American National Standards Institute, "Coded Character
Set - 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[VERIFY] Saint-Andre, P. and J. Hodges, "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)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/rfc/rfc6125>.
[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.
[X.501] International Telecommunications Union, "Information
Technology - Open Systems Interconnection - The Directory:
Models", ITU-T Recommendation X.501, ISO Standard 9594-2,
August 2005.
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[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.
[XMPP] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <https://www.rfc-editor.org/rfc/rfc6120>.
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:
* 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.
* 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
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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.
* 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.
* DNS domain names in server certificates MAY contain the wildcard
character "*" as the complete left-most label within the
identifier.
######
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:
* 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.
* 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.
######
Acknowledgements
We gratefully acknowledge everyone who contributed to the previous
version of this document, [VERIFY].
Authors' Addresses
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Peter Saint-Andre
Mozilla
United States of America
Email: stpeter@mozilla.com
Jeff Hodges
Google
United States of America
Email: jdhodges@google.com
Rich Salz
Akamai Technologies
United States of America
Email: rsalz@akamai.com
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