💾 Archived View for gemini.bortzmeyer.org › rfc-mirror › rfc7673.txt captured on 2023-01-29 at 04:58:31.
⬅️ Previous capture (2021-11-30)
-=-=-=-=-=-=-
Internet Engineering Task Force (IETF) T. Finch
Request for Comments: 7673 University of Cambridge
Category: Standards Track M. Miller
ISSN: 2070-1721 Cisco Systems, Inc.
P. Saint-Andre
&yet
October 2015
Using DNS-Based Authentication of Named Entities (DANE)
TLSA Records with SRV Records
Abstract
The DNS-Based Authentication of Named Entities (DANE) specification
(RFC 6698) describes how to use TLSA resource records secured by
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
domain's TLS servers). However, application protocols that use SRV
records (RFC 2782) to indirectly name the target server connection
endpoints for a service domain name cannot apply the rules from RFC
6698. Therefore, this document provides guidelines that enable such
protocols to locate and use TLSA records.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
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.
Finch, et al. Standards Track [Page 1]
RFC 7673 TLSA and SRV October 2015
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(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
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................4
3. DNS Checks ......................................................4
3.1. SRV Query ..................................................4
3.2. Address Queries ............................................5
3.3. TLSA Queries ...............................................6
3.4. Impact on TLS Usage ........................................6
4. TLS Checks ......................................................7
4.1. SRV Records Only ...........................................7
4.2. TLSA Records ...............................................8
5. Guidance for Protocol Authors ...................................8
6. Guidance for Server Operators ...................................8
7. Guidance for Application Developers .............................9
8. Internationalization Considerations .............................9
9. Security Considerations ........................................10
9.1. Mixed Security Status .....................................10
9.2. Certificate Subject Name Matching .........................10
10. References ....................................................11
10.1. Normative References .....................................11
10.2. Informative References ...................................12
Appendix A. Examples ..............................................13
A.1. IMAP .......................................................13
A.2. XMPP .......................................................13
Appendix B. Rationale .............................................14
Acknowledgements ..................................................15
Authors' Addresses ................................................16
Finch, et al. Standards Track [Page 2]
RFC 7673 TLSA and SRV October 2015
1. Introduction
The base DNS-Based Authentication of Named Entities (DANE)
specification [RFC6698] describes how to use TLSA resource records
secured by DNSSEC [RFC4033] to associate a target server's connection
endpoint with its Transport Layer Security (TLS) certificate (thus
enabling administrators of domain names to specify the keys used in
that domain's TLS servers). Some application protocols locate
connection endpoints indirectly via SRV records [RFC2782]. As a
result of this indirection, the rules specified in [RFC6698] cannot
be directly applied to such application protocols. (Rules for SMTP
[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
records. To summarize:
o We rely on DNSSEC to secure SRV records that map the desired
service, transport protocol, and service domain name to the
corresponding target server connection endpoints (i.e., the target
server hostnames and port numbers returned in the SRV records for
that service type).
o Although in accordance with [RFC2782] a service domain name can
advertise a number of SRV records (some of which might map to
connection endpoints that do not support TLS), the intent of this
specification is for a client to securely discover connection
endpoints that support TLS.
o The TLSA records for each connection endpoint are located using
the transport protocol, port number, and hostname for the target
server (not the service domain name).
o When DNSSEC-validated TLSA records are published for a given
connection endpoint, clients always use TLS when connecting (even
if the connection endpoint supports cleartext communication).
o If there is at least one usable TLSA record for a given connection
endpoint, the connection endpoint's TLS certificate or public key
needs to match at least one of those usable TLSA records.
o If there are no usable TLSA records for a given connection
endpoint, the target server hostname is used as one of the
acceptable reference identifiers, as described in [RFC6125].
Other reference identifiers might arise through CNAME expansion of
either the service domain name or target server hostname, as
detailed in [RFC7671].
Finch, et al. Standards Track [Page 3]
RFC 7673 TLSA and SRV October 2015
o If there are no usable TLSA records for any connection endpoint
(and thus the client cannot securely discover a connection
endpoint that supports TLS), the client's behavior is a matter for
the application protocol or client implementation; this might
involve a fallback to non-DANE behavior using the public key
infrastructure [RFC5280].
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 document uses the definitions for "secure", "insecure", "bogus",
and "indeterminate" from Section 4.3 of [RFC4035]. This document
uses the acronyms from [RFC7218] for the values of TLSA fields where
appropriate.
Additionally, this document uses the following terms:
connection endpoint: A tuple of a fully qualified DNS hostname,
transport protocol, and port number that a client uses to
establish a connection to the target server.
service domain name: The fully qualified DNS domain name that
identifies an application service; corresponds to the term "source
domain" from [RFC6125].
This document uses the term "target server hostname" in place of the
term "derived domain" from the so-called CertID specification
[RFC6125].
3. DNS Checks
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 leading to such a list).
NOTE: Implementers need to be aware that unsuccessful results can
occur because of various DNS-related errors; guidance on avoiding
downgrade attacks can be found in Section 2.1 of [RFC7672].
Finch, et al. Standards Track [Page 4]
RFC 7673 TLSA and SRV October 2015
For this specification to apply, the entire chain of DNS RRset(s)
returned MUST be "secure" according to DNSSEC validation (Section 5
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
RRsets MUST also be secure.
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
attempt to connect to the desired service. If the lookup result is
"insecure" (or no SRV records exist), this protocol does not apply
and the client SHOULD fall back to its non-DNSSEC, non-DANE (and
possibly non-SRV) behavior.
When the lookup returns a "secure" RRset (possibly via a chain of
"secure" CNAME/DNAME records), the client now has an authentic list
of target server connection endpoints 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 4). The client can then proceed to making address
queries on the target server hostnames as described in the following
section.
3.2. Address Queries
For each SRV target server connection endpoint, the client makes
A and/or AAAA queries, performs DNSSEC validation on the address
(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
query for that target server connection endpoint, as described in
the next section.
o If no returned RRsets are "secure", the client MUST NOT perform a
TLSA query for that target server connection endpoint; the TLSA
query will most likely fail or produce spurious results.
o If the address record lookup fails (a validation status of either
"bogus" or "indeterminate"), the client MUST NOT connect to this
connection endpoint; instead, it uses the next most appropriate
SRV target. This helps prevent downgrade attacks.
Finch, et al. Standards Track [Page 5]
RFC 7673 TLSA and SRV October 2015
3.3. TLSA Queries
The client SHALL construct the TLSA query name as described in
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
query name, and the TLSA base domain from the SRV target server
hostname.
For example, the following SRV record for IMAP (see [RFC6186])
_imap._tcp.example.com. 86400 IN SRV 10 0 9143 imap.example.net.
leads to the TLSA query shown below:
_9143._tcp.imap.example.net. IN TLSA ?
3.4. Impact on TLS Usage
The client SHALL determine if the TLSA records returned in the
previous step are usable according to Section 4.1 of [RFC6698]. This
affects the use of TLS as follows:
o If the TLSA response is "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 in
Section 4.
o If the TLSA response is "bogus" or "indeterminate" (or the lookup
fails for reasons other than no records), then the client MUST NOT
connect to the target server (the client can still use other SRV
targets).
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
records. It MAY connect to the target server with or without TLS,
subject to the policies of the application protocol or client
implementation.
Finch, et al. Standards Track [Page 6]
RFC 7673 TLSA and SRV October 2015
4. TLS Checks
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 -- Section 4.2 for cases
where there is at least one usable TLSA record, and Section 4.1
otherwise -- apply to such secure responses.
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. 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 the Extensible Messaging and Presence Protocol
(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 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 name" and "target server hostname" in this
document). The examples below assume a service domain name of
"im.example.com" and a target server hostname of
"xmpp23.hosting.example.net".
SRV is insecure: The reference identifiers SHALL include the service
domain name and MUST NOT include the SRV target server hostname
(e.g., include "im.example.com" but not
"xmpp23.hosting.example.net"). The service domain name is the
preferred name for TLS SNI or its equivalent.
SRV is secure: The reference identifiers SHALL include both the
service domain name and the SRV target server hostname (e.g.,
include both "im.example.com" and "xmpp23.hosting.example.net").
The service domain name is still the preferred name for TLS SNI or
its equivalent (this reduces code complexity and the possibility
of interoperability problems).
In the latter case, the client will accept either identity to ensure
compatibility with servers that support this specification as well as
servers that do not support this specification.
Finch, et al. Standards Track [Page 7]
RFC 7673 TLSA and SRV October 2015
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].
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 ignore validation checks from [RFC5280] and
reference identifier checks from [RFC6125]. The information in such
a TLSA record supersedes the non-key information in the certificate.
5. Guidance for Protocol Authors
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 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.
o How clients ought to behave if (1) they do not support SRV lookups
or (2) they do support SRV lookups and encounter service domain
names that do not offer SRV records.
o Whether or not the application protocol has a functional
equivalent for TLS SNI that is preferred within that protocol.
o The use of SRV records with additional discovery technologies,
such as the use of both SRV records and NAPTR records [RFC3403]
for transport selection in the Session Initiation Protocol (SIP).
For example, [XMPP-DNA] covers such topics for XMPP.
6. Guidance for Server Operators
To conform to this specification, the published SRV records and
subsequent address (A and 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 to contain an identifier for
either the source domain or target server hostname. However,
operators need to be aware that servers relying solely on validation
Finch, et al. Standards Track [Page 8]
RFC 7673 TLSA and SRV October 2015
using certificate usage 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 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 hostname (the "derived domain" in [RFC6125]
terms, which is the SRV target hostname).
Servers that support multiple service domain names (i.e., so-called
"multi-tenanted environments") can implement 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 hostname.
However, the server determines what certificate to present in the TLS
handshake; e.g., the presented certificate might only authenticate
the target server hostname.
7. Guidance for Application Developers
Developers of application clients that depend on DANE-SRV often would
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
this optimization possible, a DNS library might perform the address
queries and TLSA queries in parallel. (Because a TLSA record can be
ignored if it turns out that the address record on which it depends
is not secure, performing the TLSA queries in parallel with the
address queries is not harmful from a security perspective and can
yield some operational benefits.)
8. Internationalization Considerations
If any of the DNS queries are for an internationalized domain name,
then they need to use the A-label form [RFC5890].
Finch, et al. Standards Track [Page 9]
RFC 7673 TLSA and SRV October 2015
9. Security Considerations
9.1. Mixed Security Status
We do not specify that all of the target server connection endpoints
for a service domain name need to be 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 name has a third-party
fallback server, for example.
The SRV sorting rules are unchanged; in particular, they have not
been altered in order to prioritize secure connection endpoints over
insecure connection endpoints. 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.
9.2. 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 Certification Authority (CA) that issued the
certificate, and third parties can obtain certificates from the same
CA. Therefore, clients need to check to see whether or not 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 (naturally, this is in addition to
standard certificate-related checks as specified in [RFC5280],
including but not limited to certificate syntax, certificate
extensions such as name constraints and extended key usage, and
handling of certification paths).
Finch, et al. Standards Track [Page 10]
RFC 7673 TLSA and SRV October 2015
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
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
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<http://www.rfc-editor.org/info/rfc2782>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
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.
Rose, "Protocol Modifications for the DNS Security
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.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<http://www.rfc-editor.org/info/rfc5890>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
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
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125,
March 2011, <http://www.rfc-editor.org/info/rfc6125>.
Finch, et al. Standards Track [Page 11]
RFC 7673 TLSA and SRV October 2015
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698,
August 2012, <http://www.rfc-editor.org/info/rfc6698>.
[RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify
Conversations about DNS-Based Authentication of Named
Entities (DANE)", RFC 7218, DOI 10.17487/RFC7218,
April 2014, <http://www.rfc-editor.org/info/rfc7218>.
[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>.
[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via
Opportunistic DNS-Based Authentication of Named Entities
(DANE) Transport Layer Security (TLS)", RFC 7672,
DOI 10.17487/RFC7672, October 2015,
<http://www.rfc-editor.org/info/rfc7672>.
10.2. Informative References
[RFC3403] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part Three: The Domain Name System (DNS) Database",
RFC 3403, DOI 10.17487/RFC3403, October 2002,
<http://www.rfc-editor.org/info/rfc3403>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<http://www.rfc-editor.org/info/rfc5321>.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
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
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.
Finch, et al. Standards Track [Page 12]
RFC 7673 TLSA and SRV October 2015
Appendix A. Examples
In the following, most of the DNS resource data is elided for
simplicity.
A.1. IMAP
; mail domain
_imap._tcp.example.com. SRV 10 0 9143 imap.example.net.
example.com. RRSIG SRV ...
; target server hostname
imap.example.net. A 192.0.2.1
imap.example.net. RRSIG A ...
imap.example.net. AAAA 2001:db8:212:8::e:1
imap.example.net. RRSIG ...
; TLSA resource record
_9143._tcp.imap.example.net. TLSA ...
_9143._tcp.imap.example.net. RRSIG TLSA ...
Mail messages received for addresses at example.com are retrieved via
IMAP at 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.
A.2. XMPP
; XMPP domain
_xmpp-client._tcp.example.com. SRV 1 0 5222 im.example.net.
_xmpp-client._tcp.example.com. RRSIG SRV ...
; target server hostname
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.
Finch, et al. Standards Track [Page 13]
RFC 7673 TLSA and SRV October 2015
Appendix B. Rationale
The long-term goal of this specification is to settle on TLS
certificates that verify the target server hostname rather than the
service domain name, 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 domain names.
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 target server hostname rather than
the service domain name.
o In the absence of TLS SNI, if the certificate identifies the
target server hostname, then it does not need to list all the
possible service domain names.
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 domain names.
o The same TLSA records work with this specification, and with
direct connections to the connection endpoint in the style of
[RFC6698].
o Some application protocols, such as SMTP, allow a client to
perform transactions with multiple service domain names in the
same connection. It is not, in general, feasible for the client
to specify the service domain name using TLS SNI when the
connection is established, and the server might not be able to
present a certificate that authenticates all possible service
domain names. See [RFC7672] 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 [RFC7672] for details.
Finch, et al. Standards Track [Page 14]
RFC 7673 TLSA and SRV October 2015
This specification does not provide an option to put TLSA records
under the service domain name, because that would add complexity
without providing any benefit; security protocols are best kept
simple. As described above, there are real-world cases where
authenticating the service domain name cannot be made to work, so
there would be complicated criteria regarding when service domain
name TLSA records might be used and when they cannot. This is all
avoided by putting the TLSA records under the target server hostname.
The disadvantage is that clients that do not complete DNSSEC
validation must, according to [RFC6125] rules, check the server
certificate against the service domain name, since they have no other
way to authenticate the server. This means that SNI support or its
functional equivalent is necessary for backward compatibility.
Acknowledgements
Thanks to Mark Andrews for arguing that authenticating the target
server hostname is the right thing, and that we ought to rely on
DNSSEC to secure the SRV lookup. Thanks to Stephane Bortzmeyer,
James Cloos, Viktor Dukhovni, Ned Freed, Olafur Gudmundsson, Paul
Hoffman, Phil Pennock, Hector Santos, Jonas Schneider, and Alessandro
Vesely for helpful suggestions.
Carl Wallace completed an insightful review on behalf of the Security
Directorate.
Ben Campbell, Brian Haberman, and Alvaro Retana provided helpful
feedback during IESG review.
The authors gratefully acknowledge the assistance of Olafur
Gudmundsson and Warren Kumari as the working group chairs and Stephen
Farrell as the sponsoring Area Director.
Peter Saint-Andre wishes to acknowledge Cisco Systems, Inc., for
employing him during his work on earlier draft versions of this
document.
Finch, et al. Standards Track [Page 15]
RFC 7673 TLSA and SRV October 2015
Authors' Addresses
Tony Finch
University of Cambridge Information Services
Roger Needham Building
7 JJ Thomson Avenue
Cambridge CB3 0RB
United Kingdom
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
United States
Email: mamille2@cisco.com
Peter Saint-Andre
&yet
Email: peter@andyet.com
URI: https://andyet.com/
Finch, et al. Standards Track [Page 16]