wdiff draft-ietf-dane-srv-05.txt draft-ietf-dane-srv-06.txt

DNS-Based Authentication of Named Entities (DANE) T. Finch
Internet-Draft University of Cambridge
Intended status: Standards Track M. Miller
Expires: August 17, December 12, 2014 Cisco Systems, Inc.
P. Saint-Andre
&yet
February 13,
June 10, 2014

Using DNS-Based Authentication of Named Entities (DANE) TLSA records Records
with SRV and MX records.
draft-ietf-dane-srv-05 Records
draft-ietf-dane-srv-06

Abstract

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
certificate. The
certificate, where the association is secured with DNSSEC. Some However,
application protocols that use SRV records (RFC 2782) to indirectly
name the target server hosts host names for a service domain (SMTP uses MX records for cannot apply
the
same purpose). This specification gives generic instructions for how
these application rules from RFC 6698. Therefore this document provides guidelines
that enable such protocols to locate and use TLSA records when
technologies such as SRV records are used. Separate documents give
the details that are specific to particular application protocols. records.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on August 17, December 12, 2014.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Relation between SRV and MX records DNS Checks . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. DNS Checks for TLSA and
3.1. SRV Records Query . . . . . . . . . . . . . . . . 4
4.1. SRV Query . . . . . . . . 4
3.2. Address Queries . . . . . . . . . . . . . . . . . . . . . 4
4.2.
3.3. TLSA Queries . . . . . . . . . . . . . . . . . . . . . . 5
5.
3.4. Impact on TLS Usage . . . . . . . . . . . . . . . . . . . 5
4. TLS Checks for TLSA and . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. SRV Records Only . . . . . . . . . . . . . . . . . . . . 5
4.2. TLSA Records . . . . . . . . . . . . . . . . . . . . . . 6
6.
5. Guidance for Application Protocols . . . . . . . . . . . . . 7
7.
6. Guidance for Server Operators . . . . . . . . . . . . . . . . 7
8.
7. Internationalization Considerations . . . . . . . . . . . . . 8
9.
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10.
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10.1.
9.1. Mixed Security Status . . . . . . . . . . . . . . . . . . 8
10.2.
9.2. A Service Domain Trusts its Servers . . . . . . . . . . . 8
10.3.
9.3. Certificate Subject Name Matching . . . . . . . . . . . . 9
11.
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
12.
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
12.1.
11.1. Normative References . . . . . . . . . . . . . . . . . . 9
12.2.
11.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Mail Example Examples . . . . . . . . . . . . . . . . . . . . . . 11
Appendix B.
A.1. IMAP . . . . . . . . . . . . . . . . . . . . . . . . . . 11
A.2. XMPP Example . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Appendix C. B. Rationale . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13

1. Introduction

The base DANE specification [RFC6698] describes how to use TLSA
resource records in the DNS to associate a server's host name with
its TLS certificate. The certificate, where the association is secured using DNSSEC.
That document "only relates to securely associating certificates for
TLS and DTLS with host names" (see the last paragraph of section 1.2
of [RFC6698]).

Some application protocols do not use host names directly; instead,
they use a service domain domain, and the relevant target server host names
are located indirectly via SRV records [RFC2782], or MX records in the case of
SMTP [RFC5321] (Note: in the "CertID" specification [RFC6125], the
source domain and host name are referred to as the "source domain"
and the "derived domain"). [RFC2782]. Because of this
intermediate resolution step, the normal DANE rules specified in
[RFC6698] do not directly
apply cannot be applied to protocols that use SRV or MX records.
(Rules for SMTP [RFC5321], which uses MX records instead of SRV
records, are described in [I-D.ietf-dane-smtp-with-dane].)

This document describes how to use DANE TLSA records with SRV and MX
records. To summarize:

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
are discovered by the SRV or MX query).

o The TLSA records are located using the port, protocol, and target
server host name fields (not the service domain).

o Clients always use TLS when connecting to servers with TLSA
records.

o Assuming that the association is secure, the server's certificate
is expected to authenticate the target server host name, rather
than the service domain.

Separate documents give

Note: The "CertID" specification [RFC6125] does not use the details that are specific terms
"service domain" and "target server host name", but refers to particular
application protocols, such as SMTP [I-D.ietf-dane-smtp-with-dane] the
same entities with the terms "source domain" and XMPP [I-D.ietf-xmpp-dna]. "derived domain".

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this memo are to be interpreted as described in
[RFC2119].

This draft uses the definitions for "secure", "insecure", "bogus",
and "indeterminate" from [RFC4035]. [RFC4033]. This draft uses the acronyms
from [I-D.ietf-dane-registry-acronyms] [RFC7218] for the values of TLSA fields 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 DNS Checks

To expedite connection to an SRV record [RFC2782] with
corresponding fields copied from the MX record and intended service, where possible 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
queries described in 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 sections SHOULD be performed in
[I-D.ietf-dane-smtp-with-dane].

4. DNS Checks
parallel (this is similar to the "happy eyeballs" approach for TLSA IPv4
and SRV Records

4.1. IPv6 connections described in [RFC6555]).

3.1. SRV Query

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
of CNAME / DNAME aliases referring leading to such a list).

For this specification to apply, all of these the entire DNS RRsets RRset that is
returned MUST be "secure" according to DNSSSEC validation ([RFC4033]
section 5). In the case of aliases, the whole chain of CNAME and
DNAME RRsets MUST be secure as well. This corresponds to the AD bit
being set in the response(s); see [RFC4035] section 3.2.3.

If they are the the entire RRset is not all secure, this protocol has not been
correctly deployed. The client SHOULD fall back to its non-DNSSEC non-DNSSEC,
non-DANE behavior (this corresponds to the AD bit being unset).

If any of the responses are a particular response is "bogus" or "indeterminate" according to
DNSSEC validation, the client MUST abort (This usually corresponds to
a "server failure" response). ignore that target server host
name.

In the successful case, the client now has an authentic list of
target server host names with weight and priority values. It
performs server ordering and selection using the weight and priority
values without regard to the presence or absence of DNSSEC or TLSA
records. It also takes note of the DNSSEC validation status of the
SRV response for use when checking certificate names (see Section 5).

4.2. TLSA Queries

If the SRV response was insecure, the 4).
The client MUST NOT perform any
TLSA queries. If the SRV response is "secure" according can now proceed to DNSSEC
validation, making address queries on the client performs a TLSA query for each SRV target
server host names as described in this the next section.

3.2. Address Queries

For each SRV target server host name, the client makes A / AAAA
queries, performs DNSSEC validation on the address (A, AAAA) response
response, and continues as follows based on the results:

o if If the response is "secure" and usable, the client MUST perform a
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
query for that target; target server host name; the TLSA query will most
likely fail.

o If the response is "bogus" or "indeterminate", the client MUST NOT
connect to this host name; target server; instead it uses the next most
appropriate SRV target.

3.3. TLSA Queries

The client SHALL construct the TLSA query name as described in
[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
TLSA base domain set to the SRV target server host name.

For example, the following SRV record for IMAP (see [RFC6186]) leads
to the TLSA query shown below:

_imap._tcp.example.com. 86400 IN SRV 10 0 143 9143 imap.example.net.

_143._tcp.imap.example.net.

_9143._tcp.imap.example.net. IN TLSA ?

3.4. Impact on TLS Usage

The client SHALL determine if the TLSA record(s) returned in the
previous step are usable according to section 4.1 of [RFC6698]. This
affects SRV handling the use TLS as follows:

o If the TLSA response is "secure", "secure" and usable, then the client MUST
use TLS when connecting to the target server. The TLSA records
are used when validating the server's certificate as described
under Section 5. 4.

o If the TLSA response is "insecure", then the client SHALL proceed
as if
this the target server has had no TLSA records. It MAY connect to
the target server with or without TLS. TLS, subject to the policies of
the application protocol or client implementation.

o If the TLSA response is "bogus" or "indeterminate", then the
client MUST NOT connect to this the target server (the client can still
use other SRV targets).

4. TLS Checks for TLSA and SRV Records

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
rules described in the next two sections apply.

4.1. SRV Records Only

If the client received zero usable TLSA certificate associations, it
SHALL 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
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,

In this case, the client uses the information in the server
certificate and the DNSSEC validation status of the SRV query in its
authentication checks. It SHOULD use the Server Name Indication
extension (TLS SNI) [RFC6066] or its functional equivalent in the
relevant application protocol (e.g., in XMPP [RFC6120] this is the
'to' address of the initial stream header). The preferred name SHALL
be chosen as follows, and the client SHALL verify the identity
asserted by the server's certificate according to [RFC6125] section 6, 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
domain and MUST NOT include the SRV target server host name. name (e.g.,
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
service domain and the SRV target server host name. name (e.g., include
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
is compatible to ensure
compatibility with servers that do and support this specification as well as
servers that do not support this specification.

4.2. TLSA Records

If the client received one or more usable TLSA certificate
associations, it SHALL process them as described in section 2.1 of
[RFC6698].

5. Guidance for Application Protocols

Separate documents describe

This document describes how to apply use DANE with application protocols in
which target servers are discovered via SRV records. Although this specification
document attempts to provide generic guidance applying to all such
protocols, additional documents for particular application protocols. Such documents ought to protocols
could cover the
following points: related topics, such as:

o Fallback logic in the event of bogus replies and that a client is unable to connect
securely to a target server by following the like. procedures defined in
this document.

o The use of TLS SNI How clients ought to behave if they do not support SRV lookups, or its functional equivalent.

o Appropriate mappings for non-SRV technologies such as MX.

o Compatibility with
if clients that support SRV lookups encounter service domains that
do not support offer SRV lookups.

7. 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
subsequent address (A, AAAA) records MUST be secured with DNSSEC.
There SHOULD also be at least one TLSA record published that
authenticates the server's certificate.

When using TLSA records with Certificate Usage "DANE-EE", it is not
necessary for the deployed certificate does not need to contain any of the possible
reference identifiers discussed below. Indeed, none of the
certificate's information is necessary an identifier for such certificates.
either the source domain or target server host name. However,
servers that rely solely on validation using Certificate Usage "DANE-EE" "DANE-
EE" TLSA records might prevent clients that do not support this
specification from successfully connecting with TLS.

For TLSA records with Certificate Usage types other than "DANE-EE",
the certificate(s) MUST contain a reference an identifier that matches:

o the service domain name (the "source domain" in [RFC6125] terms,
which is the SRV query domain); and/or

o the target server host name (the "derived domain" in [RFC6125]
terms, which is the SRV target).

Servers that support multiple service domains (i.e., multi-tenant) so-called
"multi-tenanted environments") can implement the Transport Layer
Security Server Name Indicator Indication (TLS SNI) [RFC6066] or its functional
equivalent to determine which certificate to offer. Clients that do
not support this specification will indicate a preference for the
service domain name, while clients that support this specification
will indicate the target server host name. However, the server
determines what certificate to present in the TLS handshake; e.g.,
the presented certificate might only authenticate the target server
host name.

7. Internationalization Considerations

If any of the DNS queries are for an internationalized domain name,
then they need to use the A-label form [RFC5890].

8. IANA Considerations

No IANA action is required.

10.

9. Security Considerations

10.1.

9.1. Mixed Security Status

We do not specify that clients checking all of a service domain's
target server host names are consistent in whether they have or do
not have TLSA records. This is so that partial or incremental
deployment does not break the service. Different levels of
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 not been
altered in order to prioritize secure servers over insecure servers.
If a site wants to be secure it needs to deploy this protocol
completely; a partial deployment is not secure and we make no special
effort to support it.

10.2.

9.2. A Service Domain Trusts its Servers

By signing their zone with DNSSEC, service domain operators
implicitly instruct their clients to check their server TLSA records.
This implies another point in the trust relationship between service
domain holders and their server operators. Most of the setup
requirements for this protocol fall on the server operator:
installing a TLS certificate with the correct name (where necessary),
and publishing a TLSA record for that certificate. If these are not
correct then connections from TLSA-aware clients might fail.

10.3.

9.3. Certificate Subject Name Matching

Section 4 of the TLSA specification [RFC6698] leaves the details of
checking names in certificates to higher level application protocols,
though it suggests the use of [RFC6125].

Name checks are not necessary if the matching TLSA record is of
Certificate Usage "DANE-EE". Because such a record identifies the
specific certificate (or public key of the certificate), additional
checks are superfluous and potentially conflicting.

Otherwise, while DNSSEC provides a secure binding between the server
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
certificates. For example, a Certificate Usage "PKIX-TA" TLSA record
only authenticates the CA that issued the certificate, and third
parties can obtain certificates from the same CA. Therefore, clients
need to check whether the server's certificate matches one of the
expected reference identifiers to ensure that the certificate was
issued by the CA to the server the client expects.

11.

10. Acknowledgements

Thanks to Mark Andrews for arguing that authenticating the target
server host name is the right thing, and that we ought to rely on
DNSSEC to secure the SRV / MX lookup. Thanks to James Cloos, Viktor
Dukhovni, Ned Freed, Olafur Gudmundsson, Paul Hoffman, Phil Pennock,
Hector Santos, Jonas Schneider, and Alessandro Vesely for helpful
suggestions.

12.

11. References

12.1.

11.1. Normative References

[I-D.ietf-dane-registry-acronyms]
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
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.

[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.

[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.

[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.

[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.

[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.

[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.

[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011.

[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, 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
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.

12.2.

[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]
Dukhovni, V. and W. Hardaker, "SMTP security via
opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-05
(work in progress), February 2014.

[I-D.ietf-xmpp-dna]
Saint-Andre, P. and M. Miller, "Domain Name Associations
(DNA) in the Extensible Messaging and Presence Protocol
(XMPP)", draft-ietf-xmpp-dna-05 (work in progress),
February 2014.

[RFC6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
Dual-Stack Hosts", RFC 6555, April 2012.

Appendix A. Mail Example Examples

In the following, most of the DNS resource data is elided for
simplicity.

A.1. IMAP

; mail domain
example.com. MX 1 mx.example.net.
_imap._tcp.example.com. SRV 10 0 9143 imap.example.net.
example.com. RRSIG MX SRV ...

; SMTP target server host name
mx.example.net.
imap.example.net. A 192.0.2.1
mx.example.net.
imap.example.net. RRSIG A ...

mx.example.net.

imap.example.net. AAAA 2001:db8:212:8::e:1
mx.example.net.
imap.example.net. RRSIG ...

; TLSA resource record
_25._tcp.mx.example.net.
_9143._tcp.imap.example.net. TLSA ...
_25._tcp.mx.example.net.
_9143._tcp.imap.example.net. RRSIG TLSA ...

Mail messages submitted for addresses at example.com is delivered by SMTP are sent via
IMAP to
mx.example.net. imap.example.net. Connections to mx.example.net imap.example.net port 25 9143
that use STARTTLS will get a server certificate that authenticates
the name
mx.example.net.

Appendix B. imap.example.net.

A.2. XMPP Example

In the following, most of the DNS resource data is elided for
simplicity.

; XMPP domain
_xmpp-client.example.com. SRV 1 0 5222 im.example.net.
_xmpp-client.example.com. RRSIG SRV ...

; XMPP target server host name
im.example.net. A 192.0.2.3
im.example.net. RRSIG A ...

im.example.net. AAAA 2001:db8:212:8::e:4
im.example.net. RRSIG AAAA ...

; TLSA resource record
_5222._tcp.im.example.net. TLSA ...
_5222._tcp.im.example.net. RRSIG TLSA ...

XMPP sessions for addresses at example.com are established at
im.example.net. Connections to im.example.net port 5222 that use
STARTTLS will get a server certificate that authenticates the name
im.example.net.

Appendix C. B. Rationale

The long-term goal of this specification is to settle on TLS
certificates that verify the target server host name rather than the
service domain, since this is more convenient for servers hosting
multiple domains (so-called "multi-tenanted environments") and scales
up more easily to larger numbers of service domains.

There are a number of other reasons for doing it this way:

o The certificate is part of the server configuration, so it makes
sense to associate it with the server host name rather than the
service domain.

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
domains.

o When the server certificate is replaced it is much easier if there
is one part of the DNS that needs updating to match, instead of an
unbounded number of hosted service domains.

o The same TLSA records work with this specification, and with
direct connections to the host name in the style of [RFC6698].

o Some application protocols, such as SMTP, allow a client to
perform transactions with multiple service domains in the same
connection. It is not in general feasible for the client to
specify the service domain using TLS SNI when the connection is
established, and the server might not be able to present a
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
example as outgoing relays or as incoming MX servers, depending on
the client identity. It is simpler if the server can present 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
authenticated, which usually occurs after TLS has been
established. See [I-D.ietf-dane-smtp-with-dane] for details.

This specification does not provide an option to put TLSA records
under the service domain because that would add complexity without
providing any benefit, and security protocols are best kept simple.
As described above, there are real-world cases where authenticating
the service domain cannot be made to work, so there would be
complicated criteria for when service domain TLSA records might be
used and when they cannot. This is all avoided by putting the TLSA
records under the target server host name.

The disadvantage is that clients which do not do complete DNSSEC
validation must, according to [RFC6125] rules, check the server
certificate against the service domain, since they have no other way
to authenticate the server. This means that SNI support or its
functional equivalent is necessary for backward compatibility.

Authors' Addresses

Tony Finch
University of Cambridge Computing Service
New Museums Site
Pembroke Street
Cambridge CB2 3QH
ENGLAND

Phone: +44 797 040 1426
Email: dot@dotat.at
URI: http://dotat.at/

Matthew Miller
Cisco Systems, Inc.
1899 Wynkoop Street, Suite 600
Denver, CO 80202
USA

Email: mamille2@cisco.com

Peter Saint-Andre
&yet
P.O. Box 787
Parker, CO 80134
USA

Email: ietf@stpeter.im peter@andyet.com