RFC 6125
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)
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
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.
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.
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):
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
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.
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
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
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
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).
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
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.
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
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.
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
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.
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.
[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.
[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.
[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.
[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.
[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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