draft-ietf-dane-srv-14.txt   rfc7673.txt 
DNS-Based Authentication of Named Entities (DANE) T. Finch Internet Engineering Task Force (IETF) T. Finch
Internet-Draft University of Cambridge Request for Comments: 7673 University of Cambridge
Intended status: Standards Track M. Miller Category: Standards Track M. Miller
Expires: October 25, 2015 Cisco Systems, Inc. ISSN: 2070-1721 Cisco Systems, Inc.
P. Saint-Andre P. Saint-Andre
&yet &yet
April 23, 2015 October 2015
Using DNS-Based Authentication of Named Entities (DANE) TLSA Records Using DNS-Based Authentication of Named Entities (DANE)
with SRV Records TLSA Records with SRV Records
draft-ietf-dane-srv-14
Abstract Abstract
The DANE specification (RFC 6698) describes how to use TLSA resource The DNS-Based Authentication of Named Entities (DANE) specification
records secured by DNSSEC (RFC 4033) to associate a server's (RFC 6698) describes how to use TLSA resource records secured by
connection endpoint with its TLS certificate (thus enabling DNSSEC (RFC 4033) to associate a server's connection endpoint with
its Transport Layer Security (TLS) certificate (thus enabling
administrators of domain names to specify the keys used in that administrators of domain names to specify the keys used in that
domain's TLS servers). However, application protocols that use SRV domain's TLS servers). However, application protocols that use SRV
records (RFC 2782) to indirectly name the target server connection records (RFC 2782) to indirectly name the target server connection
endpoints for a service domain cannot apply the rules from RFC 6698. endpoints for a service domain name cannot apply the rules from RFC
Therefore this document provides guidelines that enable such 6698. Therefore, this document provides guidelines that enable such
protocols to locate and use TLSA records. protocols to locate and use TLSA records.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
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 http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
This Internet-Draft will expire on October 25, 2015. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7673.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction ....................................................3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology .....................................................4
3. DNS Checks . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. DNS Checks ......................................................4
3.1. SRV Query . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. SRV Query ..................................................4
3.2. Address Queries . . . . . . . . . . . . . . . . . . . . . 5 3.2. Address Queries ............................................5
3.3. TLSA Queries . . . . . . . . . . . . . . . . . . . . . . 5 3.3. TLSA Queries ...............................................6
3.4. Impact on TLS Usage . . . . . . . . . . . . . . . . . . . 6 3.4. Impact on TLS Usage ........................................6
4. TLS Checks . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. TLS Checks ......................................................7
4.1. SRV Records Only . . . . . . . . . . . . . . . . . . . . 6 4.1. SRV Records Only ...........................................7
4.2. TLSA Records . . . . . . . . . . . . . . . . . . . . . . 7 4.2. TLSA Records ...............................................8
5. Guidance for Protocol Authors . . . . . . . . . . . . . . . . 7 5. Guidance for Protocol Authors ...................................8
6. Guidance for Server Operators . . . . . . . . . . . . . . . . 8 6. Guidance for Server Operators ...................................8
7. Guidance for Application Developers . . . . . . . . . . . . . 9 7. Guidance for Application Developers .............................9
8. Internationalization Considerations . . . . . . . . . . . . . 9 8. Internationalization Considerations .............................9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 9. Security Considerations ........................................10
10. Security Considerations . . . . . . . . . . . . . . . . . . . 9 9.1. Mixed Security Status .....................................10
10.1. Mixed Security Status . . . . . . . . . . . . . . . . . 9 9.2. Certificate Subject Name Matching .........................10
10.2. Certificate Subject Name Matching . . . . . . . . . . . 9 10. References ....................................................11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 10.1. Normative References .....................................11
11.1. Normative References . . . . . . . . . . . . . . . . . . 10 10.2. Informative References ...................................12
11.2. Informative References . . . . . . . . . . . . . . . . . 11 Appendix A. Examples ..............................................13
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 11 A.1. IMAP .......................................................13
A.1. IMAP . . . . . . . . . . . . . . . . . . . . . . . . . . 12 A.2. XMPP .......................................................13
A.2. XMPP . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Appendix B. Rationale .............................................14
Appendix B. Rationale . . . . . . . . . . . . . . . . . . . . . 13 Acknowledgements ..................................................15
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 14 Authors' Addresses ................................................16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction 1. Introduction
The base DANE specification [RFC6698] describes how to use TLSA The base DNS-Based Authentication of Named Entities (DANE)
resource records secured by DNSSEC [RFC4033] to associate a target specification [RFC6698] describes how to use TLSA resource records
server's connection endpoint with its TLS certificate (thus enabling secured by DNSSEC [RFC4033] to associate a target server's connection
administrators of domain names to specify the keys used in that endpoint with its Transport Layer Security (TLS) certificate (thus
domain's TLS servers). Some application protocols locate connection enabling administrators of domain names to specify the keys used in
endpoints indirectly via SRV records [RFC2782]. As a result of this that domain's TLS servers). Some application protocols locate
indirection, the rules specified in [RFC6698] cannot be directly connection endpoints indirectly via SRV records [RFC2782]. As a
applied to such application protocols. (Rules for SMTP [RFC5321], result of this indirection, the rules specified in [RFC6698] cannot
which uses MX resource records instead of SRV records, are described be directly applied to such application protocols. (Rules for SMTP
in [I-D.ietf-dane-smtp-with-dane].) [RFC5321], which uses MX resource records instead of SRV records, are
described in [RFC7672].)
This document describes how to use DANE TLSA records with SRV This document describes how to use DANE TLSA records with SRV
records. To summarize: records. To summarize:
o We rely on DNSSEC to secure SRV records that map the desired o We rely on DNSSEC to secure SRV records that map the desired
service, transport protocol, and service domain to the service, transport protocol, and service domain name to the
corresponding target server connection endpoints (i.e., the target corresponding target server connection endpoints (i.e., the target
server host names and port numbers returned in the SRV records for server hostnames and port numbers returned in the SRV records for
that service type). that service type).
o Although in accordance with [RFC2782] a service domain can o Although in accordance with [RFC2782] a service domain name can
advertise a number of SRV records (some of which might map to advertise a number of SRV records (some of which might map to
connection endpoints that do not support TLS), the intent of this connection endpoints that do not support TLS), the intent of this
specification is for a client to securely discover connection specification is for a client to securely discover connection
endpoints that support TLS. endpoints that support TLS.
o The TLSA records for each connection endpoint are located using o The TLSA records for each connection endpoint are located using
the transport protocol, port number, and host name for the target the transport protocol, port number, and hostname for the target
server (not the service domain). server (not the service domain name).
o When DNSSEC-validated TLSA records are published for a given o When DNSSEC-validated TLSA records are published for a given
connection endpoint, clients always use TLS when connecting (even connection endpoint, clients always use TLS when connecting (even
if the connection endpoint supports cleartext communication). if the connection endpoint supports cleartext communication).
o If there is at least one usable TLSA record for a given connection o If there is at least one usable TLSA record for a given connection
endpoint, the connection endpoint's TLS certificate or public key endpoint, the connection endpoint's TLS certificate or public key
needs to match at least one of those usable TLSA records. needs to match at least one of those usable TLSA records.
o If there are no usable TLSA records for a given connection o If there are no usable TLSA records for a given connection
endpoint, the target server host name is used as one of the endpoint, the target server hostname is used as one of the
acceptable reference identifiers, as described in [RFC6125]. acceptable reference identifiers, as described in [RFC6125].
Other reference identifiers might arise through CNAME expansion of Other reference identifiers might arise through CNAME expansion of
either the service domain or target server host name, as detailed either the service domain name or target server hostname, as
in [I-D.ietf-dane-ops]. detailed in [RFC7671].
o If there are no usable TLSA records for any connection endpoint o If there are no usable TLSA records for any connection endpoint
(and thus the client cannot securely discover a connection (and thus the client cannot securely discover a connection
endpoint that supports TLS), the client's behavior is a matter for endpoint that supports TLS), the client's behavior is a matter for
the application protocol or client implementation; this might the application protocol or client implementation; this might
involve a fallback to non-DANE behavior using the public key involve a fallback to non-DANE behavior using the public key
infrastructure [RFC5280]. infrastructure [RFC5280].
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this memo are to be interpreted as described in "OPTIONAL" in this memo are to be interpreted as described in
[RFC2119]. [RFC2119].
This draft uses the definitions for "secure", "insecure", "bogus", This document uses the definitions for "secure", "insecure", "bogus",
and "indeterminate" from Section 4.3 of [RFC4035]. This draft uses and "indeterminate" from Section 4.3 of [RFC4035]. This document
the acronyms from [RFC7218] for the values of TLSA fields where uses the acronyms from [RFC7218] for the values of TLSA fields where
appropriate. appropriate.
Additionally, this document uses the following terms: Additionally, this document uses the following terms:
connection endpoint: A tuple of a fully qualified DNS host name, connection endpoint: A tuple of a fully qualified DNS hostname,
transport protocol, and port number that a client uses to transport protocol, and port number that a client uses to
establish a connection to the target server. establish a connection to the target server.
service domain: The fully qualified DNS domain name that identifies service domain name: The fully qualified DNS domain name that
an application service; corresponds to the term "source domain" identifies an application service; corresponds to the term "source
from [RFC6125]. domain" from [RFC6125].
This document uses the term "target server host name" in place of the This document uses the term "target server hostname" in place of the
term "derived domain" from the CertID specification [RFC6125]. term "derived domain" from the so-called CertID specification
[RFC6125].
3. DNS Checks 3. DNS Checks
3.1. SRV Query 3.1. SRV Query
When the client makes an SRV query, a successful result will When the client makes an SRV query, a successful result will
typically be a list of one or more SRV records (or possibly a chain typically be a list of one or more SRV records (or possibly a chain
of CNAME / DNAME aliases leading to such a list). of CNAME/DNAME aliases leading to such a list).
NOTE: Implementers need to be aware that unsuccessful results can NOTE: Implementers need to be aware that unsuccessful results can
occur because of various DNS-related errors; guidance on avoiding occur because of various DNS-related errors; guidance on avoiding
downgrade attacks can be found in Section 2.1 of downgrade attacks can be found in Section 2.1 of [RFC7672].
[I-D.ietf-dane-smtp-with-dane].
For this specification to apply, the entire chain of DNS RRset(s) For this specification to apply, the entire chain of DNS RRset(s)
returned MUST be "secure" according to DNSSEC validation (Section 5 returned MUST be "secure" according to DNSSEC validation (Section 5
of [RFC4035]). In the case where the answer is obtained via a chain of [RFC4035]). In the case where the answer is obtained via a chain
of CNAME and/or DNAME aliases, the whole chain of CNAME and DNAME of CNAME and/or DNAME aliases, the whole chain of CNAME and DNAME
RRsets MUST also be secure. RRsets MUST also be secure.
If the SRV lookup fails because the RRset is "bogus" (or the lookup If the SRV lookup fails because the RRset is "bogus" (or the lookup
fails for reasons other than no records), the client MUST abort its fails for reasons other than no records), the client MUST abort its
attempt to connect to the desired service. If the lookup result is attempt to connect to the desired service. If the lookup result is
skipping to change at page 5, line 15 skipping to change at page 5, line 26
possibly non-SRV) behavior. possibly non-SRV) behavior.
When the lookup returns a "secure" RRset (possibly via a chain of When the lookup returns a "secure" RRset (possibly via a chain of
"secure" CNAME/DNAME records), the client now has an authentic list "secure" CNAME/DNAME records), the client now has an authentic list
of target server connection endpoints with weight and priority of target server connection endpoints with weight and priority
values. It performs server ordering and selection using the weight values. It performs server ordering and selection using the weight
and priority values without regard to the presence or absence of and priority values without regard to the presence or absence of
DNSSEC or TLSA records. It also takes note of the DNSSEC validation DNSSEC or TLSA records. It also takes note of the DNSSEC validation
status of the SRV response for use when checking certificate names status of the SRV response for use when checking certificate names
(see Section 4). The client can then proceed to making address (see Section 4). The client can then proceed to making address
queries on the target server host names as described in the following queries on the target server hostnames as described in the following
section. section.
3.2. Address Queries 3.2. Address Queries
For each SRV target server connnection endpoint, the client makes A For each SRV target server connection endpoint, the client makes
and/or AAAA queries, performs DNSSEC validation on the address (A or A and/or AAAA queries, performs DNSSEC validation on the address
AAAA) response, and continues as follows based on the results: (A or AAAA) response, and continues as follows, based on the results:
o If a returned RRSet is "secure", the client MUST perform a TLSA o If a returned RRSet is "secure", the client MUST perform a TLSA
query for that target server connection endpoint as described in query for that target server connection endpoint, as described in
the next section. the next section.
o If no returned RRsets are "secure", the client MUST NOT perform a o If no returned RRsets are "secure", the client MUST NOT perform a
TLSA query for that target server connection endpoint; the TLSA TLSA query for that target server connection endpoint; the TLSA
query will most likely fail or produce spurious results. query will most likely fail or produce spurious results.
o If the address record lookup fails (this a validation status of o If the address record lookup fails (a validation status of either
either "bogus" or "indeterminate"), the client MUST NOT connect to "bogus" or "indeterminate"), the client MUST NOT connect to this
this connection endpoint; instead it uses the next most connection endpoint; instead, it uses the next most appropriate
appropriate SRV target. This mitigates against downgrade attacks. SRV target. This helps prevent downgrade attacks.
3.3. TLSA Queries 3.3. TLSA Queries
The client SHALL construct the TLSA query name as described in The client SHALL construct the TLSA query name as described in
Section 3 of [RFC6698], based on the fields from the SRV record: the Section 3 of [RFC6698], based on the fields from the SRV record: the
port number from the SRV RDATA, the transport protocol from the SRV port number from the SRV RDATA, the transport protocol from the SRV
query name, and the TLSA base domain from the SRV target server host query name, and the TLSA base domain from the SRV target server
name. hostname.
For example, the following SRV record for IMAP (see [RFC6186]): For example, the following SRV record for IMAP (see [RFC6186])
_imap._tcp.example.com. 86400 IN SRV 10 0 9143 imap.example.net. _imap._tcp.example.com. 86400 IN SRV 10 0 9143 imap.example.net.
leads to the TLSA query shown below: leads to the TLSA query shown below:
_9143._tcp.imap.example.net. IN TLSA ? _9143._tcp.imap.example.net. IN TLSA ?
3.4. Impact on TLS Usage 3.4. Impact on TLS Usage
The client SHALL determine if the TLSA records returned in the The client SHALL determine if the TLSA records returned in the
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o If the TLSA response is "insecure" (or no TLSA records exist), o If the TLSA response is "insecure" (or no TLSA records exist),
then the client SHALL proceed as if the target server had no TLSA then the client SHALL proceed as if the target server had no TLSA
records. It MAY connect to the target server with or without TLS, records. It MAY connect to the target server with or without TLS,
subject to the policies of the application protocol or client subject to the policies of the application protocol or client
implementation. implementation.
4. TLS Checks 4. TLS Checks
When connecting to a server, the client MUST use TLS if the responses When connecting to a server, the client MUST use TLS if the responses
to the SRV and TLSA queries were "secure" as described above. The to the SRV and TLSA queries were "secure" as described above. The
rules described in the next two sections apply to such secure rules described in the next two sections -- Section 4.2 for cases
responses; Section 4.2 where there is at least one usable TLSA where there is at least one usable TLSA record, and Section 4.1
record, and Section 4.1 otherwise. otherwise -- apply to such secure responses.
4.1. SRV Records Only 4.1. SRV Records Only
If the client received zero usable TLSA certificate associations, it If the client received zero usable TLSA certificate associations, it
SHALL validate the server's TLS certificate using the normal PKIX SHALL validate the server's TLS certificate using the normal PKIX
rules [RFC5280] or protocol-specific rules (e.g., following rules [RFC5280] or protocol-specific rules (e.g., following
[RFC6125]) without further input from the TLSA records. In this [RFC6125]) without further input from the TLSA records. In this
case, the client uses the information in the server certificate and case, the client uses the information in the server certificate and
the DNSSEC validation status of the SRV query in its authentication the DNSSEC validation status of the SRV query in its authentication
checks. It SHOULD use the Server Name Indication extension (TLS SNI) checks. It SHOULD use the Server Name Indication extension (TLS SNI)
[RFC6066] or its functional equivalent in the relevant application [RFC6066] or its functional equivalent in the relevant application
protocol (e.g., in XMPP [RFC6120] this is the 'to' address of the protocol (e.g., in the Extensible Messaging and Presence Protocol
initial stream header). The preferred name SHALL be chosen as (XMPP) [RFC6120], this is the 'to' address of the initial stream
follows, and the client SHALL verify the identity asserted by the header). The preferred name SHALL be chosen as follows, and the
server's certificate according to Section 6 of [RFC6125], using a client SHALL verify the identity asserted by the server's certificate
list of reference identifiers constructed as follows (note again that according to Section 6 of [RFC6125], using a list of reference
in RFC 6125 the terms "source domain" and "derived domain" to refer identifiers constructed as follows (note again that in RFC 6125 the
to the same things as "service domain" and "target server host name" terms "source domain" and "derived domain" refer to the same things
in this document). The examples below assume a service domain of as "service domain name" and "target server hostname" in this
"im.example.com" and a target server host name of document). The examples below assume a service domain name of
"im.example.com" and a target server hostname of
"xmpp23.hosting.example.net". "xmpp23.hosting.example.net".
SRV is insecure: The reference identifiers SHALL include the service SRV is insecure: The reference identifiers SHALL include the service
domain and MUST NOT include the SRV target server host name (e.g., domain name and MUST NOT include the SRV target server hostname
include "im.example.com" but not "xmpp23.hosting.example.net"). (e.g., include "im.example.com" but not
The service domain is the preferred name for TLS SNI or its "xmpp23.hosting.example.net"). The service domain name is the
equivalent. preferred name for TLS SNI or its equivalent.
SRV is secure: The reference identifiers SHALL include both the SRV is secure: The reference identifiers SHALL include both the
service domain and the SRV target server host name (e.g., include service domain name and the SRV target server hostname (e.g.,
both "im.example.com" and "xmpp23.hosting.example.net"). The include both "im.example.com" and "xmpp23.hosting.example.net").
target server host name is the preferred name for TLS SNI or its The service domain name is still the preferred name for TLS SNI or
equivalent. its equivalent (this reduces code complexity and the possibility
of interoperability problems).
In the latter case, the client will accept either identity to ensure In the latter case, the client will accept either identity to ensure
compatibility with servers that support this specification as well as compatibility with servers that support this specification as well as
servers that do not support this specification. servers that do not support this specification.
4.2. TLSA Records 4.2. TLSA Records
If the client received one or more usable TLSA certificate If the client received one or more usable TLSA certificate
associations, it SHALL process them as described in Section 2.1 of associations, it SHALL process them as described in Section 2.1 of
[RFC6698]. [RFC6698].
If the TLS server's certificate -- or the public key of the server's If the TLS server's certificate -- or the public key of the server's
certificate -- matches a usable TLSA record with Certificate Usage certificate -- matches a usable TLSA record with certificate usage
"DANE-EE", the client MUST ignore validation checks from [RFC5280] DANE-EE, the client MUST ignore validation checks from [RFC5280] and
and reference identifier checks from [RFC6125]. The information in reference identifier checks from [RFC6125]. The information in such
such a TLSA record supersedes the non-key information in the a TLSA record supersedes the non-key information in the certificate.
certificate.
5. Guidance for Protocol Authors 5. Guidance for Protocol Authors
This document describes how to use DANE with application protocols in This document describes how to use DANE with application protocols in
which target servers are discovered via SRV records. Although this which target servers are discovered via SRV records. Although this
document attempts to provide generic guidance applying to all such document attempts to provide generic guidance applying to all such
protocols, additional documents for particular application protocols protocols, additional documents for particular application protocols
could cover related topics, such as: could cover related topics, such as:
o Fallback logic in the event that a client is unable to connect o Fallback logic in the event that a client is unable to connect
securely to a target server by following the procedures defined in securely to a target server by following the procedures defined in
this document. this document.
o How clients ought to behave if they do not support SRV lookups, or o How clients ought to behave if (1) they do not support SRV lookups
if clients that support SRV lookups encounter service domains that or (2) they do support SRV lookups and encounter service domain
do not offer SRV records. names that do not offer SRV records.
o Whether the application protocol has a functional equivalent for o Whether or not the application protocol has a functional
TLS SNI that is preferred within that protocol. equivalent for TLS SNI that is preferred within that protocol.
o Use of SRV records with additional discovery technologies, such as o The use of SRV records with additional discovery technologies,
the use of both SRV records and NAPTR records [RFC3403] for such as the use of both SRV records and NAPTR records [RFC3403]
transport selection in the Session Initiation Protocol (SIP). for transport selection in the Session Initiation Protocol (SIP).
For example, [I-D.ietf-xmpp-dna] covers such topics for the For example, [XMPP-DNA] covers such topics for XMPP.
Extensible Messaging and Presence Protocol (XMPP).
6. Guidance for Server Operators 6. Guidance for Server Operators
To conform to this specification, the published SRV records and To conform to this specification, the published SRV records and
subsequent address (A and AAAA) records MUST be secured with DNSSEC. subsequent address (A and AAAA) records MUST be secured with DNSSEC.
There SHOULD also be at least one TLSA record published that There SHOULD also be at least one TLSA record published that
authenticates the server's certificate. authenticates the server's certificate.
When using TLSA records with Certificate Usage "DANE-EE", it is not When using TLSA records with certificate usage DANE-EE, it is not
necessary for the deployed certificate to contain an identifier for necessary for the deployed certificate to contain an identifier for
either the source domain or target server host name. However, either the source domain or target server hostname. However,
operators need to be aware that servers relying solely on validation operators need to be aware that servers relying solely on validation
using Certificate Usage "DANE-EE" TLSA records might prevent clients using certificate usage DANE-EE TLSA records might prevent clients
that do not support this specification from successfully connecting that do not support this specification from successfully connecting
with TLS. with TLS.
For TLSA records with Certificate Usage types other than "DANE-EE", For TLSA records with certificate usage types other than DANE-EE, the
the certificate(s) MUST contain an identifier that matches: certificate(s) MUST contain an identifier that matches:
o the service domain name (the "source domain" in [RFC6125] terms, o the service domain name (the "source domain" in [RFC6125] terms,
which is the SRV query domain); and/or which is the SRV query domain), and/or
o the target server host name (the "derived domain" in [RFC6125] o the target server hostname (the "derived domain" in [RFC6125]
terms, which is the SRV target host name). terms, which is the SRV target hostname).
Servers that support multiple service domains (i.e., so-called Servers that support multiple service domain names (i.e., so-called
"multi-tenanted environments") can implement the Transport Layer "multi-tenanted environments") can implement TLS SNI [RFC6066] or its
Security Server Name Indication (TLS SNI) [RFC6066] or its functional functional equivalent to determine which certificate to offer.
equivalent to determine which certificate to offer. Clients that do Clients that do not support this specification will indicate a
not support this specification will indicate a preference for the preference for the service domain name, while clients that support
service domain name, while clients that support this specification this specification will indicate the target server hostname.
will indicate the target server host name. However, the server However, the server determines what certificate to present in the TLS
determines what certificate to present in the TLS handshake; e.g., handshake; e.g., the presented certificate might only authenticate
the presented certificate might only authenticate the target server the target server hostname.
host name.
7. Guidance for Application Developers 7. Guidance for Application Developers
Developers of application clients that depend on DANE-SRV often would Developers of application clients that depend on DANE-SRV often would
like to prepare as quickly as possible for making a connection to the like to prepare as quickly as possible for making a connection to the
intended service, thus reducing the wait time for end users. To make intended service, thus reducing the wait time for end users. To make
this optimization possible, a DNS library might perform the SRV this optimization possible, a DNS library might perform the address
queries, address queries, and TLSA queries in parallel. (Because a queries and TLSA queries in parallel. (Because a TLSA record can be
TLSA record can be ignored if it turns out that the address record on ignored if it turns out that the address record on which it depends
which it depends is not secure, performing the TLSA queries in is not secure, performing the TLSA queries in parallel with the
parallel with the SRV queries and address queries is not harmful from address queries is not harmful from a security perspective and can
a security perspective and can yield some operational benefits.) yield some operational benefits.)
8. Internationalization Considerations 8. Internationalization Considerations
If any of the DNS queries are for an internationalized domain name, If any of the DNS queries are for an internationalized domain name,
then they need to use the A-label form [RFC5890]. then they need to use the A-label form [RFC5890].
9. IANA Considerations 9. Security Considerations
No IANA action is required.
10. Security Considerations
10.1. Mixed Security Status 9.1. Mixed Security Status
We do not specify that all of the target server connection endpoints We do not specify that all of the target server connection endpoints
for a service domain need to be consistent in whether they have or do for a service domain name need to be consistent in whether they have
not have TLSA records. This is so that partial or incremental or do not have TLSA records. This is so that partial or incremental
deployment does not break the service. Different levels of deployment does not break the service. Different levels of
deployment are likely if a service domain has a third-party fallback deployment are likely if a service domain name has a third-party
server, for example. fallback server, for example.
The SRV sorting rules are unchanged; in particular they have not been The SRV sorting rules are unchanged; in particular, they have not
altered in order to prioritize secure connection endpoints over been altered in order to prioritize secure connection endpoints over
insecure connection endpoints. If a site wants to be secure it needs insecure connection endpoints. If a site wants to be secure, it
to deploy this protocol completely; a partial deployment is not needs to deploy this protocol completely; a partial deployment is not
secure and we make no special effort to support it. secure, and we make no special effort to support it.
10.2. Certificate Subject Name Matching 9.2. Certificate Subject Name Matching
Section 4 of the TLSA specification [RFC6698] leaves the details of Section 4 of the TLSA specification [RFC6698] leaves the details of
checking names in certificates to higher level application protocols, checking names in certificates to higher-level application protocols,
though it suggests the use of [RFC6125]. though it suggests the use of [RFC6125].
Name checks are not necessary if the matching TLSA record is of Name checks are not necessary if the matching TLSA record is of
Certificate Usage "DANE-EE". Because such a record identifies the certificate usage DANE-EE. Because such a record identifies the
specific certificate (or public key of the certificate), additional specific certificate (or public key of the certificate), additional
checks are superfluous and potentially conflicting. checks are superfluous and potentially conflicting.
Otherwise, while DNSSEC provides a secure binding between the server Otherwise, while DNSSEC provides a secure binding between the server
name and the TLSA record, and the TLSA record provides a binding to a name and the TLSA record, and the TLSA record provides a binding to a
certificate, this latter step can be indirect via a chain of certificate, this latter step can be indirect via a chain of
certificates. For example, a Certificate Usage "PKIX-TA" TLSA record certificates. For example, a certificate usage PKIX-TA TLSA record
only authenticates the CA that issued the certificate, and third only authenticates the Certification Authority (CA) that issued the
parties can obtain certificates from the same CA. Therefore, clients certificate, and third parties can obtain certificates from the same
need to check whether the server's certificate matches one of the CA. Therefore, clients need to check to see whether or not the
expected reference identifiers to ensure that the certificate was server's certificate matches one of the expected reference
issued by the CA to the server the client expects (naturally, this is identifiers to ensure that the certificate was issued by the CA to
in addition to standard certificate-related checks as specified in the server the client expects (naturally, this is in addition to
[RFC5280], including but not limited to certificate syntax, standard certificate-related checks as specified in [RFC5280],
certificate extensions such as name constraints and extended key including but not limited to certificate syntax, certificate
usage, and handling of certification paths). extensions such as name constraints and extended key usage, and
handling of certification paths).
11. References
11.1. Normative References
[I-D.ietf-dane-ops] 10. References
Dukhovni, V. and W. Hardaker, "Updates to and Operational
Guidance for the DANE Protocol", draft-ietf-dane-ops-07
(work in progress), October 2014.
[I-D.ietf-dane-smtp-with-dane] 10.1. Normative References
Dukhovni, V. and W. Hardaker, "SMTP security via
opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-15
(work in progress), March 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782, specifying the location of services (DNS SRV)", RFC 2782,
February 2000. DOI 10.17487/RFC2782, February 2000,
<http://www.rfc-editor.org/info/rfc2782>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC Rose, "DNS Security Introduction and Requirements",
4033, March 2005. RFC 4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005. Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<http://www.rfc-editor.org/info/rfc4035>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008. (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC5890] Klensin, J., "Internationalized Domain Names for [RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework", Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010. RFC 5890, DOI 10.17487/RFC5890, August 2010,
<http://www.rfc-editor.org/info/rfc5890>.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extension Definitions", RFC 6066, January 2011. Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<http://www.rfc-editor.org/info/rfc6066>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509 within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer (PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011. Security (TLS)", RFC 6125, DOI 10.17487/RFC6125,
March 2011, <http://www.rfc-editor.org/info/rfc6125>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS) of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012. Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698,
August 2012, <http://www.rfc-editor.org/info/rfc6698>.
[RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify [RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify
Conversations about DNS-Based Authentication of Named Conversations about DNS-Based Authentication of Named
Entities (DANE)", RFC 7218, April 2014. Entities (DANE)", RFC 7218, DOI 10.17487/RFC7218,
April 2014, <http://www.rfc-editor.org/info/rfc7218>.
11.2. Informative References [RFC7671] Dukhovni, V. and W. Hardaker, "The DNS-Based
Authentication of Named Entities (DANE) Protocol: Updates
and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671,
October 2015, <http://www.rfc-editor.org/info/rfc7671>.
[I-D.ietf-xmpp-dna] [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via
Saint-Andre, P., Miller, M., and P. Hancke, "Domain Name Opportunistic DNS-Based Authentication of Named Entities
Associations (DNA) in the Extensible Messaging and (DANE) Transport Layer Security (TLS)", RFC 7672,
Presence Protocol (XMPP)", draft-ietf-xmpp-dna-10 (work in DOI 10.17487/RFC7672, October 2015,
progress), March 2015. <http://www.rfc-editor.org/info/rfc7672>.
10.2. Informative References
[RFC3403] Mealling, M., "Dynamic Delegation Discovery System (DDDS) [RFC3403] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part Three: The Domain Name System (DNS) Database", RFC Part Three: The Domain Name System (DNS) Database",
3403, October 2002. RFC 3403, DOI 10.17487/RFC3403, October 2002,
<http://www.rfc-editor.org/info/rfc3403>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008. DOI 10.17487/RFC5321, October 2008,
<http://www.rfc-editor.org/info/rfc5321>.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011. Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <http://www.rfc-editor.org/info/rfc6120>.
[RFC6186] Daboo, C., "Use of SRV Records for Locating Email [RFC6186] Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186, March 2011. Submission/Access Services", RFC 6186,
DOI 10.17487/RFC6186, March 2011,
<http://www.rfc-editor.org/info/rfc6186>.
[XMPP-DNA] Saint-Andre, P., Miller, M., and P. Hancke, "Domain Name
Associations (DNA) in the Extensible Messaging and
Presence Protocol (XMPP)", Work in Progress,
draft-ietf-xmpp-dna-11, September 2015.
Appendix A. Examples Appendix A. Examples
In the following, most of the DNS resource data is elided for In the following, most of the DNS resource data is elided for
simplicity. simplicity.
A.1. IMAP A.1. IMAP
; mail domain ; mail domain
_imap._tcp.example.com. SRV 10 0 9143 imap.example.net. _imap._tcp.example.com. SRV 10 0 9143 imap.example.net.
example.com. RRSIG SRV ... example.com. RRSIG SRV ...
; target server host name ; target server hostname
imap.example.net. A 192.0.2.1 imap.example.net. A 192.0.2.1
imap.example.net. RRSIG A ... imap.example.net. RRSIG A ...
imap.example.net. AAAA 2001:db8:212:8::e:1 imap.example.net. AAAA 2001:db8:212:8::e:1
imap.example.net. RRSIG ... imap.example.net. RRSIG ...
; TLSA resource record ; TLSA resource record
_9143._tcp.imap.example.net. TLSA ... _9143._tcp.imap.example.net. TLSA ...
_9143._tcp.imap.example.net. RRSIG TLSA ... _9143._tcp.imap.example.net. RRSIG TLSA ...
skipping to change at page 12, line 33 skipping to change at page 13, line 38
IMAP at imap.example.net. Connections to imap.example.net port 9143 IMAP at imap.example.net. Connections to imap.example.net port 9143
that use STARTTLS will get a server certificate that authenticates that use STARTTLS will get a server certificate that authenticates
the name imap.example.net. the name imap.example.net.
A.2. XMPP A.2. XMPP
; XMPP domain ; XMPP domain
_xmpp-client._tcp.example.com. SRV 1 0 5222 im.example.net. _xmpp-client._tcp.example.com. SRV 1 0 5222 im.example.net.
_xmpp-client._tcp.example.com. RRSIG SRV ... _xmpp-client._tcp.example.com. RRSIG SRV ...
; target server host name ; target server hostname
im.example.net. A 192.0.2.3 im.example.net. A 192.0.2.3
im.example.net. RRSIG A ... im.example.net. RRSIG A ...
im.example.net. AAAA 2001:db8:212:8::e:4 im.example.net. AAAA 2001:db8:212:8::e:4
im.example.net. RRSIG AAAA ... im.example.net. RRSIG AAAA ...
; TLSA resource record ; TLSA resource record
_5222._tcp.im.example.net. TLSA ... _5222._tcp.im.example.net. TLSA ...
_5222._tcp.im.example.net. RRSIG TLSA ... _5222._tcp.im.example.net. RRSIG TLSA ...
XMPP sessions for addresses at example.com are established at XMPP sessions for addresses at example.com are established at
im.example.net. Connections to im.example.net port 5222 that use im.example.net. Connections to im.example.net port 5222 that use
STARTTLS will get a server certificate that authenticates the name STARTTLS will get a server certificate that authenticates the name
im.example.net. im.example.net.
Appendix B. Rationale Appendix B. Rationale
The long-term goal of this specification is to settle on TLS The long-term goal of this specification is to settle on TLS
certificates that verify the target server host name rather than the certificates that verify the target server hostname rather than the
service domain, since this is more convenient for servers hosting service domain name, since this is more convenient for servers
multiple domains (so-called "multi-tenanted environments") and scales hosting multiple domains (so-called "multi-tenanted environments")
up more easily to larger numbers of service domains. and scales up more easily to larger numbers of service domain names.
There are a number of other reasons for doing it this way: There are a number of other reasons for doing it this way:
o The certificate is part of the server configuration, so it makes o The certificate is part of the server configuration, so it makes
sense to associate it with the server host name rather than the sense to associate it with the target server hostname rather than
service domain. the service domain name.
o In the absence of TLS SNI, if the certificate identifies the o In the absence of TLS SNI, if the certificate identifies the
target server host name then it does not need to list all the target server hostname, then it does not need to list all the
possible service domains. possible service domain names.
o When the server certificate is replaced it is much easier if there o When the server certificate is replaced, it is much easier if
is one part of the DNS that needs updating to match, instead of an there is one part of the DNS that needs updating to match, instead
unbounded number of hosted service domains. of an unbounded number of hosted service domain names.
o The same TLSA records work with this specification, and with o The same TLSA records work with this specification, and with
direct connections to the connection endpoint in the style of direct connections to the connection endpoint in the style of
[RFC6698]. [RFC6698].
o Some application protocols, such as SMTP, allow a client to o Some application protocols, such as SMTP, allow a client to
perform transactions with multiple service domains in the same perform transactions with multiple service domain names in the
connection. It is not in general feasible for the client to same connection. It is not, in general, feasible for the client
specify the service domain using TLS SNI when the connection is to specify the service domain name using TLS SNI when the
established, and the server might not be able to present a connection is established, and the server might not be able to
certificate that authenticates all possible service domains. See present a certificate that authenticates all possible service
[I-D.ietf-dane-smtp-with-dane] for details. domain names. See [RFC7672] for details.
o It is common for SMTP servers to act in multiple roles, for o It is common for SMTP servers to act in multiple roles -- for
example as outgoing relays or as incoming MX servers, depending on example, as outgoing relays or as incoming MX servers, depending
the client identity. It is simpler if the server can present the on the client identity. It is simpler if the server can present
same certificate regardless of the role in which it is to act. the same certificate regardless of the role in which it is to act.
Sometimes the server does not know its role until the client has Sometimes the server does not know its role until the client has
authenticated, which usually occurs after TLS has been authenticated, which usually occurs after TLS has been
established. See [I-D.ietf-dane-smtp-with-dane] for details. established. See [RFC7672] for details.
This specification does not provide an option to put TLSA records This specification does not provide an option to put TLSA records
under the service domain because that would add complexity without under the service domain name, because that would add complexity
providing any benefit, and security protocols are best kept simple. without providing any benefit; security protocols are best kept
As described above, there are real-world cases where authenticating simple. As described above, there are real-world cases where
the service domain cannot be made to work, so there would be authenticating the service domain name cannot be made to work, so
complicated criteria for when service domain TLSA records might be there would be complicated criteria regarding when service domain
used and when they cannot. This is all avoided by putting the TLSA name TLSA records might be used and when they cannot. This is all
records under the target server host name. avoided by putting the TLSA records under the target server hostname.
The disadvantage is that clients which do not complete DNSSEC The disadvantage is that clients that do not complete DNSSEC
validation must, according to [RFC6125] rules, check the server validation must, according to [RFC6125] rules, check the server
certificate against the service domain, since they have no other way certificate against the service domain name, since they have no other
to authenticate the server. This means that SNI support or its way to authenticate the server. This means that SNI support or its
functional equivalent is necessary for backward compatibility. functional equivalent is necessary for backward compatibility.
Appendix C. Acknowledgements Acknowledgements
Thanks to Mark Andrews for arguing that authenticating the target Thanks to Mark Andrews for arguing that authenticating the target
server host name is the right thing, and that we ought to rely on server hostname is the right thing, and that we ought to rely on
DNSSEC to secure the SRV lookup. Thanks to Stephane Bortzmeyer, DNSSEC to secure the SRV lookup. Thanks to Stephane Bortzmeyer,
James Cloos, Viktor Dukhovni, Ned Freed, Olafur Gudmundsson, Paul James Cloos, Viktor Dukhovni, Ned Freed, Olafur Gudmundsson, Paul
Hoffman, Phil Pennock, Hector Santos, Jonas Schneider, and Alessandro Hoffman, Phil Pennock, Hector Santos, Jonas Schneider, and Alessandro
Vesely for helpful suggestions. Vesely for helpful suggestions.
Carl Wallace completed an insightful review on behalf of the Security Carl Wallace completed an insightful review on behalf of the Security
Directorate. Directorate.
Ben Campbell, Brian Haberman, and Alvaro Retana provided helpful Ben Campbell, Brian Haberman, and Alvaro Retana provided helpful
feedback during IESG review. feedback during IESG review.
The authors gratefully acknowledge the assistance of Olafur The authors gratefully acknowledge the assistance of Olafur
Gudmundsson and Warren Kumari as the working group chairs and Stephen Gudmundsson and Warren Kumari as the working group chairs and Stephen
Farrell as the sponsoring Area Director. Farrell as the sponsoring Area Director.
Peter Saint-Andre wishes to acknowledge Cisco Systems, Inc., for Peter Saint-Andre wishes to acknowledge Cisco Systems, Inc., for
employing him during his work on earlier versions of this document. employing him during his work on earlier draft versions of this
document.
Authors' Addresses Authors' Addresses
Tony Finch Tony Finch
University of Cambridge Computing Service University of Cambridge Information Services
New Museums Site Roger Needham Building
Pembroke Street 7 JJ Thomson Avenue
Cambridge CB2 3QH Cambridge CB3 0RB
ENGLAND United Kingdom
Phone: +44 797 040 1426 Phone: +44 797 040 1426
Email: dot@dotat.at Email: dot@dotat.at
URI: http://dotat.at/ URI: http://dotat.at/
Matthew Miller Matthew Miller
Cisco Systems, Inc. Cisco Systems, Inc.
1899 Wynkoop Street, Suite 600 1899 Wynkoop Street, Suite 600
Denver, CO 80202 Denver, CO 80202
USA United States
Email: mamille2@cisco.com Email: mamille2@cisco.com
Peter Saint-Andre Peter Saint-Andre
&yet &yet
Email: peter@andyet.com Email: peter@andyet.com
URI: https://andyet.com/ URI: https://andyet.com/
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