draft-ietf-dane-srv-05.txt   draft-ietf-dane-srv-06.txt 
DNS-Based Authentication of Named Entities (DANE) T. Finch DNS-Based Authentication of Named Entities (DANE) T. Finch
Internet-Draft University of Cambridge Internet-Draft University of Cambridge
Intended status: Standards Track M. Miller Intended status: Standards Track M. Miller
Expires: August 17, 2014 Cisco Systems, Inc. Expires: December 12, 2014 Cisco Systems, Inc.
P. Saint-Andre P. Saint-Andre
&yet &yet
February 13, 2014 June 10, 2014
Using DNS-Based Authentication of Named Entities (DANE) TLSA records Using DNS-Based Authentication of Named Entities (DANE) TLSA Records
with SRV and MX records. with SRV Records
draft-ietf-dane-srv-05 draft-ietf-dane-srv-06
Abstract Abstract
The DANE specification (RFC 6698) describes how to use TLSA resource The DANE specification (RFC 6698) describes how to use TLSA resource
records in the DNS to associate a server's host name with its TLS records in the DNS to associate a server's host name with its TLS
certificate. The association is secured with DNSSEC. Some certificate, where the association is secured with DNSSEC. However,
application protocols use SRV records (RFC 2782) to indirectly name application protocols that use SRV records (RFC 2782) to indirectly
the server hosts for a service domain (SMTP uses MX records for the name the target server host names for a service domain cannot apply
same purpose). This specification gives generic instructions for how the rules from RFC 6698. Therefore this document provides guidelines
these application protocols locate and use TLSA records when that enable such protocols to locate and use TLSA records.
technologies such as SRV records are used. Separate documents give
the details that are specific to particular application protocols.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 17, 2014. This Internet-Draft will expire on December 12, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Relation between SRV and MX records . . . . . . . . . . . . . 3 3. DNS Checks . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. DNS Checks for TLSA and SRV Records . . . . . . . . . . . . . 4 3.1. SRV Query . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. SRV Query . . . . . . . . . . . . . . . . . . . . . . . . 4 3.2. Address Queries . . . . . . . . . . . . . . . . . . . . . 4
4.2. TLSA Queries . . . . . . . . . . . . . . . . . . . . . . 5 3.3. TLSA Queries . . . . . . . . . . . . . . . . . . . . . . 5
5. TLS Checks for TLSA and SRV Records . . . . . . . . . . . . . 6 3.4. Impact on TLS Usage . . . . . . . . . . . . . . . . . . . 5
6. Guidance for Application Protocols . . . . . . . . . . . . . 7 4. TLS Checks . . . . . . . . . . . . . . . . . . . . . . . . . 5
7. Guidance for Server Operators . . . . . . . . . . . . . . . . 7 4.1. SRV Records Only . . . . . . . . . . . . . . . . . . . . 5
8. Internationalization Considerations . . . . . . . . . . . . . 8 4.2. TLSA Records . . . . . . . . . . . . . . . . . . . . . . 6
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 5. Guidance for Application Protocols . . . . . . . . . . . . . 7
10. Security Considerations . . . . . . . . . . . . . . . . . . . 8 6. Guidance for Server Operators . . . . . . . . . . . . . . . . 7
10.1. Mixed Security Status . . . . . . . . . . . . . . . . . 8 7. Internationalization Considerations . . . . . . . . . . . . . 8
10.2. A Service Domain Trusts its Servers . . . . . . . . . . 8 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10.3. Certificate Subject Name Matching . . . . . . . . . . . 9 9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9 9.1. Mixed Security Status . . . . . . . . . . . . . . . . . . 8
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 9.2. A Service Domain Trusts its Servers . . . . . . . . . . . 8
12.1. Normative References . . . . . . . . . . . . . . . . . . 9 9.3. Certificate Subject Name Matching . . . . . . . . . . . . 9
12.2. Informative References . . . . . . . . . . . . . . . . . 10 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
Appendix A. Mail Example . . . . . . . . . . . . . . . . . . . . 11 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
Appendix B. XMPP Example . . . . . . . . . . . . . . . . . . . . 11 11.1. Normative References . . . . . . . . . . . . . . . . . . 9
Appendix C. Rationale . . . . . . . . . . . . . . . . . . . . . 12 11.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 11
A.1. IMAP . . . . . . . . . . . . . . . . . . . . . . . . . . 11
A.2. XMPP . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Appendix B. Rationale . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction 1. Introduction
The base DANE specification [RFC6698] describes how to use TLSA The base DANE specification [RFC6698] describes how to use TLSA
resource records in the DNS to associate a server's host name with resource records in the DNS to associate a server's host name with
its TLS certificate. The association is secured using DNSSEC. That its TLS certificate, where the association is secured using DNSSEC.
document "only relates to securely associating certificates for TLS That document "only relates to securely associating certificates for
and DTLS with host names" (see the last paragraph of section 1.2 of TLS and DTLS with host names" (see the last paragraph of section 1.2
[RFC6698]). of [RFC6698]).
Some application protocols do not use host names directly; instead, Some application protocols do not use host names directly; instead,
they use a service domain and the relevant host names are located they use a service domain, and the relevant target server host names
indirectly via SRV records [RFC2782], or MX records in the case of are located indirectly via SRV records [RFC2782]. Because of this
SMTP [RFC5321] (Note: in the "CertID" specification [RFC6125], the intermediate resolution step, the normal DANE rules specified in
source domain and host name are referred to as the "source domain" [RFC6698] cannot be applied to protocols that use SRV records.
and the "derived domain"). Because of this intermediate resolution (Rules for SMTP [RFC5321], which uses MX records instead of SRV
step, the normal DANE rules specified in [RFC6698] do not directly records, are described in [I-D.ietf-dane-smtp-with-dane].)
apply to protocols that use SRV or MX records.
This document describes how to use DANE TLSA records with SRV and MX This document describes how to use DANE TLSA records with SRV
records. To summarize: records. To summarize:
o We rely on DNSSEC to secure the association between the service o We rely on DNSSEC to secure the association between the service
domain and the target server host names (i.e., the host names that domain and the target server host names (i.e., the host names that
are discovered by the SRV or MX query). are discovered by the SRV query).
o The TLSA records are located using the port, protocol, and target o The TLSA records are located using the port, protocol, and target
host name fields (not the service domain). server host name fields (not the service domain).
o Clients always use TLS when connecting to servers with TLSA o Clients always use TLS when connecting to servers with TLSA
records. records.
o Assuming that the association is secure, the server's certificate o Assuming that the association is secure, the server's certificate
is expected to authenticate the target server host name, rather is expected to authenticate the target server host name, rather
than the service domain. than the service domain.
Separate documents give the details that are specific to particular Note: The "CertID" specification [RFC6125] does not use the terms
application protocols, such as SMTP [I-D.ietf-dane-smtp-with-dane] "service domain" and "target server host name", but refers to the
and XMPP [I-D.ietf-xmpp-dna]. same entities with the terms "source domain" and "derived domain".
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 draft uses the definitions for "secure", "insecure", "bogus",
and "indeterminate" from [RFC4035]. This draft uses the acronyms and "indeterminate" from [RFC4033]. This draft uses the acronyms
from [I-D.ietf-dane-registry-acronyms] for the values of TLSA fields from [RFC7218] for the values of TLSA fields where appropriate.
where appropriate.
3. Relation between SRV and MX records
For the purpose of this specification (to avoid cluttering the
description with special cases) we treat each MX record ([RFC5321]
section 5) as being equivalent to an SRV record [RFC2782] with
corresponding fields copied from the MX record and the remaining
fields having fixed values as follows:
Table 1: SRV Fields and MX Equivalents
+---------------+-----------------------------+
| DNS SRV Field | Equivalent MX Value |
+---------------+-----------------------------+
| Service | smtp |
+---------------+-----------------------------+
| Proto | tcp |
+---------------+-----------------------------+
| Name | MX owner name (mail domain) |
+---------------+-----------------------------+
| TTL | MX TTL |
+---------------+-----------------------------+
| Class | MX Class |
+---------------+-----------------------------+
| Priority | MX Priority |
+---------------+-----------------------------+
| Weight | 0 |
+---------------+-----------------------------+
| Port | 25 |
+---------------+-----------------------------+
| Target | MX Target |
+---------------+-----------------------------+
Thus we can treat the following MX record as if it were the SRV
record shown below:
example.com. 86400 IN MX 10 mx.example.net.
_smtp._tcp.example.com. 86400 IN SRV 10 0 25 mx.example.net.
Other details that are specific to SMTP are described in 3. DNS Checks
[I-D.ietf-dane-smtp-with-dane].
4. DNS Checks for TLSA and SRV Records To expedite connection to the intended service, where possible the
queries described in the following sections SHOULD be performed in
parallel (this is similar to the "happy eyeballs" approach for IPv4
and IPv6 connections described in [RFC6555]).
4.1. SRV Query 3.1. SRV Query
When the client makes an SRV query, a successful result will be a When the client makes an SRV query, a successful result will
list of one or more SRV records (or possibly a chain of CNAME / DNAME typically be a list of one or more SRV records (or possibly a chain
aliases referring to such a list). of CNAME / DNAME aliases leading to such a list).
For this specification to apply, all of these DNS RRsets MUST be For this specification to apply, the entire DNS RRset that is
"secure" according to DNSSSEC validation ([RFC4033] section 5). In returned MUST be "secure" according to DNSSSEC validation ([RFC4033]
the case of aliases, the whole chain of CNAME and DNAME RRsets MUST section 5). In the case of aliases, the whole chain of CNAME and
be secure as well. This corresponds to the AD bit being set in the DNAME RRsets MUST be secure as well. This corresponds to the AD bit
response(s); see [RFC4035] section 3.2.3. being set in the response(s); see [RFC4035] section 3.2.3.
If they are not all secure, this protocol has not been correctly If the the entire RRset is not secure, this protocol has not been
deployed. The client SHOULD fall back to its non-DNSSEC non-DANE correctly deployed. The client SHOULD fall back to its non-DNSSEC,
behavior (this corresponds to the AD bit being unset). non-DANE behavior (this corresponds to the AD bit being unset).
If any of the responses are "bogus" or "indeterminate" according to If a particular response is "bogus" or "indeterminate" according to
DNSSEC validation, the client MUST abort (This usually corresponds to DNSSEC validation, the client MUST ignore that target server host
a "server failure" response). name.
In the successful case, the client now has an authentic list of In the successful case, the client now has an authentic list of
server host names with weight and priority values. It performs target server host names with weight and priority values. It
server ordering and selection using the weight and priority values performs server ordering and selection using the weight and priority
without regard to the presence or absence of DNSSEC or TLSA records. values without regard to the presence or absence of DNSSEC or TLSA
It takes note of the DNSSEC validation status of the SRV response for records. It also takes note of the DNSSEC validation status of the
use when checking certificate names (see Section 5). SRV response for use when checking certificate names (see Section 4).
The client can now proceed to making address queries on the target
server host names as described in the next section.
4.2. TLSA Queries 3.2. Address Queries
If the SRV response was insecure, the client MUST NOT perform any For each SRV target server host name, the client makes A / AAAA
TLSA queries. If the SRV response is "secure" according to DNSSEC queries, performs DNSSEC validation on the address (A, AAAA)
validation, the client performs a TLSA query for each SRV target as response, and continues as follows based on the results:
described in this section.
For each SRV target host name, the client performs DNSSEC validation o If the response is "secure" and usable, the client MUST perform a
on the address (A, AAAA) response and continues based on the results: TLSA query for that target server host name as described in the
next section.
o if the response is "insecure", the client MUST NOT perform a TLSA o If the response is "insecure", the client MUST NOT perform a TLSA
query for that target; the TLSA query will most likely fail. query for that target server host name; the TLSA query will most
likely fail.
o If the response is "bogus" or "indeterminate", the client MUST NOT o If the response is "bogus" or "indeterminate", the client MUST NOT
connect to this host name; instead it uses the next most connect to this target server; instead it uses the next most
appropriate SRV target. appropriate SRV target.
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
[RFC6698] section 3, based on fields from the SRV record: the port [RFC6698] section 3, based on fields from the SRV record: the port
from the SRV RDATA, the protocol from the SRV query name, and the from the SRV RDATA, the protocol from the SRV query name, and the
TLSA base domain set to the SRV target host name. TLSA base domain set to the SRV target server host name.
For example, the following SRV record leads to the TLSA query shown For example, the following SRV record for IMAP (see [RFC6186]) leads
below: to the TLSA query shown below:
_imap._tcp.example.com. 86400 IN SRV 10 0 143 imap.example.net. _imap._tcp.example.com. 86400 IN SRV 10 0 9143 imap.example.net.
_143._tcp.imap.example.net. IN TLSA ? _9143._tcp.imap.example.net. IN TLSA ?
The client SHALL determine if the TLSA record(s) are usable according 3.4. Impact on TLS Usage
to section 4.1 of [RFC6698]. This affects SRV handling as follows:
If the TLSA response is "secure", the client MUST use TLS when The client SHALL determine if the TLSA record(s) returned in the
connecting to the server. The TLSA records are used when validating previous step are usable according to section 4.1 of [RFC6698]. This
the server's certificate as described under Section 5. affects the use TLS as follows:
If the TLSA response is "insecure", the client SHALL proceed as if o If the TLSA response is "secure" and usable, then the client MUST
this server has no TLSA records. It MAY connect to the server with use TLS when connecting to the target server. The TLSA records
or without TLS. are used when validating the server's certificate as described
under Section 4.
If the TLSA response is "bogus" or "indeterminate", then the client o If the TLSA response is "insecure", then the client SHALL proceed
MUST NOT connect to this server (the client can still use other SRV as if the target server had no TLSA records. It MAY connect to
targets). the target server with or without TLS, subject to the policies of
the application protocol or client implementation.
5. TLS Checks for TLSA and SRV Records o If the TLSA response is "bogus" or "indeterminate", then the
client MUST NOT connect to the target server (the client can still
use other SRV targets).
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. If the to the SRV and TLSA queries were "secure" as described above. The
client received zero usable TLSA certificate associations, it SHALL rules described in the next two sections apply.
validate the server's TLS certificate using the normal PKIX rules
[RFC5280] or protocol-specific rules (e.g., following [RFC6125])
without further input from the TLSA records. If the client received
one or more usable TLSA certificate associations, it SHALL process
them as described in [RFC6698] section 2.1.
If the TLS server's certificate -- or the public key of the server's 4.1. SRV Records Only
certificate -- matches a usable TLSA record with Certificate Usage
"DANE-EE", the client MUST consider the server to be authenticated.
Because the information in such a TLSA record supersedes the non-key
information in the certificate, all other [RFC5280] and [RFC6125]
authentication checks (e.g., reference identifier, key usage,
expiration, issuance, etc.) MUST be ignored or omitted.
Otherwise, the client uses the information in the server certificate If the client received zero usable TLSA certificate associations, it
and DNSSEC validation status of the SRV query in its authentication SHALL validate the server's TLS certificate using the normal PKIX
checks. It SHOULD use the Server Name Indication extension (TLS SNI) rules [RFC5280] or protocol-specific rules (e.g., following
[RFC6066] or its functional equivalent in the relevant application [RFC6125]) without further input from the TLSA records.
protocol (e.g., in XMPP [RFC6120] this is the 'to' address of the
initial stream header). The preferred name SHALL be chosen as In this case, the client uses the information in the server
follows, and the client SHALL verify the identity asserted by the certificate and the DNSSEC validation status of the SRV query in its
server's certificate according to [RFC6125] section 6, using a list authentication checks. It SHOULD use the Server Name Indication
of reference identifiers constructed as follows (note again that in extension (TLS SNI) [RFC6066] or its functional equivalent in the
RFC 6125 the terms "source domain" and "derived domain" refer to the relevant application protocol (e.g., in XMPP [RFC6120] this is the
same things as "service domain" and "target host name" in this 'to' address of the initial stream header). The preferred name SHALL
document). be chosen as follows, and the client SHALL verify the identity
asserted by the server's certificate according to section 6 of
[RFC6125], using a list of reference identifiers constructed as
follows (note again that in RFC 6125 the terms "source domain" and
"derived domain" refer to the same things as "service domain" and
"target server host name" in this document). The examples below
assume a service domain of "im.example.com" and a target server host
name of "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 host name. The service domain and MUST NOT include the SRV target server host name (e.g.,
domain is the preferred name for TLS SNI or its equivalent. include "im.example.com" but not "xmpp23.hosting.example.net").
The service domain is the 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 host name. The target host name service domain and the SRV target server host name (e.g., include
is the preferred name for TLS SNI or its equivalent. both "im.example.com" and "xmpp23.hosting.example.net"). The
target server host name is the preferred name for TLS SNI or its
equivalent.
In the latter case, the client will accept either identity so that it In the latter case, the client will accept either identity to ensure
is compatible with servers that do and do not support this compatibility with servers that support this specification as well as
specification. servers that do not support this specification.
6. Guidance for Application Protocols 4.2. TLSA Records
Separate documents describe how to apply this specification to If the client received one or more usable TLSA certificate
particular application protocols. Such documents ought to cover the associations, it SHALL process them as described in section 2.1 of
following points: [RFC6698].
o Fallback logic in the event of bogus replies and the like. If the TLS server's certificate -- or the public key of the server's
certificate -- matches a usable TLSA record with Certificate Usage
"DANE-EE", the client MUST consider the server to be authenticated.
Because the information in such a TLSA record supersedes the non-key
information in the certificate, all other [RFC5280] and [RFC6125]
authentication checks (e.g., reference identifier, key usage,
expiration, issuance) MUST be ignored or omitted.
o The use of TLS SNI or its functional equivalent. 5. Guidance for Application Protocols
o Appropriate mappings for non-SRV technologies such as MX. This document describes how to use DANE with application protocols in
which target servers are discovered via SRV records. Although this
document attempts to provide generic guidance applying to all such
protocols, additional documents for particular application protocols
could cover related topics, such as:
o Compatibility with clients that do not support SRV lookups. o Fallback logic in the event that a client is unable to connect
securely to a target server by following the procedures defined in
this document.
7. Guidance for Server Operators o How clients ought to behave if they do not support SRV lookups, or
if clients that support SRV lookups encounter service domains that
do not offer SRV records.
o Whether the application protocol has a functional equivalent for
TLS SNI that is preferred within that protocol.
For example, [I-D.ietf-xmpp-dna] covers such topics for the
Extensible Messaging and Presence Protocol (XMPP).
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, AAAA) records MUST be secured with DNSSEC. subsequent address (A, 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", the When using TLSA records with Certificate Usage "DANE-EE", it is not
deployed certificate does not need to contain any of the possible necessary for the deployed certificate to contain an identifier for
reference identifiers discussed below. Indeed, none of the either the source domain or target server host name. However,
certificate's information is necessary for such certificates. servers that rely solely on validation using Certificate Usage "DANE-
However, servers that rely solely on validation using Certificate EE" TLSA records might prevent clients that do not support this
Usage "DANE-EE" TLSA records might prevent clients that do not specification from successfully connecting with TLS.
support this specification from successfully connecting 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 certificate(s) MUST contain a reference identifier that matches: the 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 server host name (the "derived domain" in [RFC6125] terms, o the target server host name (the "derived domain" in [RFC6125]
which is the SRV target). terms, which is the SRV target).
Servers that support multiple service domains (i.e., multi-tenant) Servers that support multiple service domains (i.e., so-called
can implement Server Name Indicator (TLS SNI) [RFC6066] or its "multi-tenanted environments") can implement the Transport Layer
functional equivalent to determine which certificate to offer. Security Server Name Indication (TLS SNI) [RFC6066] or its functional
Clients that do not support this specification will indicate a equivalent to determine which certificate to offer. Clients that do
preference for the service domain name, while clients that support not support this specification will indicate a preference for the
this specification will indicate the server host name. However, the service domain name, while clients that support this specification
server determines what certificate to present in the TLS handshake; will indicate the target server host name. However, the server
e.g., the presented certificate might only authenticate the server determines what certificate to present in the TLS handshake; e.g.,
the presented certificate might only authenticate the target server
host name. host name.
8. Internationalization Considerations 7. 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 8. IANA Considerations
No IANA action is required. No IANA action is required.
10. Security Considerations 9. Security Considerations
10.1. Mixed Security Status 9.1. Mixed Security Status
We do not specify that clients checking all of a service domain's We do not specify that clients checking all of a service domain's
server host names are consistent in whether they have or do not have target server host names are consistent in whether they have or do
TLSA records. This is so that partial or incremental deployment does not have TLSA records. This is so that partial or incremental
not break the service. Different levels of deployment are likely if deployment does not break the service. Different levels of
a service domain has a third-party fallback server, for example. deployment are likely if a service domain has a third-party fallback
server, for example.
The SRV and MX sorting rules are unchanged; in particular they have The SRV sorting rules are unchanged; in particular they have not been
not been altered in order to prioritize secure servers over insecure altered in order to prioritize secure servers over insecure servers.
servers. If a site wants to be secure it needs to deploy this If a site wants to be secure it needs to deploy this protocol
protocol completely; a partial deployment is not secure and we make completely; a partial deployment is not secure and we make no special
no special effort to support it. effort to support it.
10.2. A Service Domain Trusts its Servers 9.2. A Service Domain Trusts its Servers
By signing their zone with DNSSEC, service domain operators By signing their zone with DNSSEC, service domain operators
implicitly instruct their clients to check their server TLSA records. implicitly instruct their clients to check their server TLSA records.
This implies another point in the trust relationship between service This implies another point in the trust relationship between service
domain holders and their server operators. Most of the setup domain holders and their server operators. Most of the setup
requirements for this protocol fall on the server operator: requirements for this protocol fall on the server operator:
installing a TLS certificate with the correct name (where necessary), installing a TLS certificate with the correct name (where necessary),
and publishing a TLSA record for that certificate. If these are not and publishing a TLSA record for that certificate. If these are not
correct then connections from TLSA-aware clients might fail. correct then connections from TLSA-aware clients might fail.
10.3. Certificate Subject Name Matching 9.3. 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 CA that issued the certificate, and third
parties can obtain certificates from the same CA. Therefore, clients parties can obtain certificates from the same CA. Therefore, clients
need to check whether the server's certificate matches one of the need to check whether the server's certificate matches one of the
expected reference identifiers to ensure the certificate was issued expected reference identifiers to ensure that the certificate was
by the CA to the server the client expects. issued by the CA to the server the client expects.
11. Acknowledgements 10. Acknowledgements
Thanks to Mark Andrews for arguing that authenticating the server Thanks to Mark Andrews for arguing that authenticating the target
host name is the right thing, and that we ought to rely on DNSSEC to server host name is the right thing, and that we ought to rely on
secure the SRV / MX lookup. Thanks to James Cloos, Viktor Dukhovni, DNSSEC to secure the SRV lookup. Thanks to James Cloos, Viktor
Ned Freed, Olafur Gudmundsson, Paul Hoffman, Phil Pennock, Hector Dukhovni, Ned Freed, Olafur Gudmundsson, Paul Hoffman, Phil Pennock,
Santos, Jonas Schneider, and Alessandro Vesely for helpful Hector Santos, Jonas Schneider, and Alessandro Vesely for helpful
suggestions. suggestions.
12. References 11. References
12.1. Normative References
[I-D.ietf-dane-registry-acronyms] 11.1. Normative References
Gudmundsson, O., "Adding acronyms to simplify DANE
conversations", draft-ietf-dane-registry-acronyms-03 (work
in progress), January 2014.
[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, March 1997.
[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. February 2000.
[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", RFC
skipping to change at page 10, line 37 skipping to change at page 10, line 29
[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, March 2011.
[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, March 2011.
[RFC6186] Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186, March 2011.
[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, August 2012.
12.2. Informative References [RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify
Conversations about DNS-Based Authentication of Named
Entities (DANE)", RFC 7218, April 2014.
11.2. Informative References
[I-D.ietf-dane-smtp-with-dane] [I-D.ietf-dane-smtp-with-dane]
Dukhovni, V. and W. Hardaker, "SMTP security via Dukhovni, V. and W. Hardaker, "SMTP security via
opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-05 opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-05
(work in progress), February 2014. (work in progress), February 2014.
[I-D.ietf-xmpp-dna] [I-D.ietf-xmpp-dna]
Saint-Andre, P. and M. Miller, "Domain Name Associations Saint-Andre, P. and M. Miller, "Domain Name Associations
(DNA) in the Extensible Messaging and Presence Protocol (DNA) in the Extensible Messaging and Presence Protocol
(XMPP)", draft-ietf-xmpp-dna-05 (work in progress), (XMPP)", draft-ietf-xmpp-dna-05 (work in progress),
February 2014. February 2014.
Appendix A. Mail Example [RFC6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
Dual-Stack Hosts", RFC 6555, April 2012.
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
; mail domain ; mail domain
example.com. MX 1 mx.example.net. _imap._tcp.example.com. SRV 10 0 9143 imap.example.net.
example.com. RRSIG MX ... example.com. RRSIG SRV ...
; SMTP server host name ; target server host name
mx.example.net. A 192.0.2.1 imap.example.net. A 192.0.2.1
mx.example.net. RRSIG A ... imap.example.net. RRSIG A ...
mx.example.net. AAAA 2001:db8:212:8::e:1 imap.example.net. AAAA 2001:db8:212:8::e:1
mx.example.net. RRSIG ... imap.example.net. RRSIG ...
; TLSA resource record ; TLSA resource record
_25._tcp.mx.example.net. TLSA ... _9143._tcp.imap.example.net. TLSA ...
_25._tcp.mx.example.net. RRSIG TLSA ... _9143._tcp.imap.example.net. RRSIG TLSA ...
Mail for addresses at example.com is delivered by SMTP to
mx.example.net. Connections to mx.example.net port 25 that use
STARTTLS will get a server certificate that authenticates the name
mx.example.net.
Appendix B. XMPP Example Mail messages submitted for addresses at example.com are sent via
IMAP to imap.example.net. Connections to imap.example.net port 9143
that use STARTTLS will get a server certificate that authenticates
the name imap.example.net.
In the following, most of the DNS resource data is elided for A.2. XMPP
simplicity.
; XMPP domain ; XMPP domain
_xmpp-client.example.com. SRV 1 0 5222 im.example.net. _xmpp-client.example.com. SRV 1 0 5222 im.example.net.
_xmpp-client.example.com. RRSIG SRV ... _xmpp-client.example.com. RRSIG SRV ...
; XMPP server host name ; target server host name
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 C. 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 server host name rather than the service certificates that verify the target server host name rather than the
domain, since this is more convenient for servers hosting multiple service domain, since this is more convenient for servers hosting
domains (so-called "multi-tenanted environments") and scales up more multiple domains (so-called "multi-tenanted environments") and scales
easily to larger numbers of service domains. up more easily to larger numbers of service domains.
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 server host name rather than the
service domain. service domain.
o In the absence of TLS SNI, if the certificate identifies the host o In the absence of TLS SNI, if the certificate identifies the host
name then it does not need to list all the possible service name then it does not need to list all the possible service
domains. domains.
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unbounded number of hosted service domains. unbounded number of hosted service domains.
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 host name in the style of [RFC6698]. direct connections to the host name in the style of [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 domains in the same
connection. It is not in general feasible for the client to connection. It is not in general feasible for the client to
specify the service domain using TLS SNI when the connection is specify the service domain using TLS SNI when the connection is
established, and the server might not be able to present a established, and the server might not be able to present a
certificate that authenticates all possible service domains. certificate that authenticates all possible service domains. See
[I-D.ietf-dane-smtp-with-dane] 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 on
the client identity. It is simpler if the server can present the the client identity. It is simpler if the server can present the
same certificate regardless of the role in which it is to act. 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. established. See [I-D.ietf-dane-smtp-with-dane] 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 because that would add complexity without
providing any benefit, and security protocols are best kept simple. providing any benefit, and security protocols are best kept simple.
As described above, there are real-world cases where authenticating As described above, there are real-world cases where authenticating
the service domain cannot be made to work, so there would be the service domain cannot be made to work, so there would be
complicated criteria for when service domain TLSA records might be complicated criteria for when service domain TLSA records might be
used and when they cannot. This is all avoided by putting the TLSA used and when they cannot. This is all avoided by putting the TLSA
records under the server host name. records under the target server host name.
The disadvantage is that clients which do not do DNSSEC validation The disadvantage is that clients which do not complete DNSSEC
must, according to [RFC6125] rules, check the server certificate validation must, according to [RFC6125] rules, check the server
against the service domain, since they have no other way to certificate against the service domain, since they have no other way
authenticate the server. This means that SNI support or its 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.
Authors' Addresses Authors' Addresses
Tony Finch Tony Finch
University of Cambridge Computing Service University of Cambridge Computing Service
New Museums Site New Museums Site
Pembroke Street Pembroke Street
Cambridge CB2 3QH Cambridge CB2 3QH
ENGLAND ENGLAND
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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 USA
Email: mamille2@cisco.com Email: mamille2@cisco.com
Peter Saint-Andre Peter Saint-Andre
&yet &yet
P.O. Box 787
Parker, CO 80134
USA
Email: ietf@stpeter.im Email: peter@andyet.com
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