< draft-ietf-dane-srv-00.txt   draft-ietf-dane-srv-01.txt >
DNS-Based Authentication of Named T. Finch DNS-Based Authentication of Named T. Finch
Entities (DANE) University of Cambridge Entities (DANE) University of Cambridge
Internet-Draft February 18, 2013 Internet-Draft February 25, 2013
Intended status: Standards Track Intended status: Standards Track
Expires: August 22, 2013 Expires: August 29, 2013
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 and MX records.
draft-ietf-dane-srv-00 draft-ietf-dane-srv-01
Abstract Abstract
The DANE specification [RFC6698] describes how to use TLSA resource The DANE specification [RFC6698] 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. The association is secured with DNSSEC. Some
application protocols can use SRV records [RFC2782] to indirectly application protocols can use SRV records [RFC2782] to indirectly
name the server hosts for a service domain. (SMTP uses MX records name the server hosts for a service domain. (SMTP uses MX records
for the same purpose.) This specification gives generic instructions for the same purpose.) This specification gives generic instructions
for how these application protocols locate and use TLSA records. for how these application protocols locate and use TLSA records.
skipping to change at page 1, line 40 skipping to change at page 1, line 40
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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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 22, 2013. This Internet-Draft will expire on August 29, 2013.
Copyright Notice Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Using TLSA records with SRV and MX . . . . . . . . . . . . . . 3 2. DNS checks for TLSA and SRV / MX records . . . . . . . . . . . 3
2.1. MX records . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. MX records . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. SRV query . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2. SRV query . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. TLSA queries . . . . . . . . . . . . . . . . . . . . . . . 5 2.3. TLSA queries . . . . . . . . . . . . . . . . . . . . . . . 5
3. Guidelines for application protocols . . . . . . . . . . . . . 5 3. TLS checks for TLSA and SRV / MX records . . . . . . . . . . . 5
4. Security considerations . . . . . . . . . . . . . . . . . . . 5 4. Guidance for application protocols . . . . . . . . . . . . . . 6
4.1. Mixed security status . . . . . . . . . . . . . . . . . . 5 5. Guidance for server operators . . . . . . . . . . . . . . . . 6
4.2. A service domain trusts its servers . . . . . . . . . . . 6 6. Security considerations . . . . . . . . . . . . . . . . . . . 7
4.3. Certificate subject name matching . . . . . . . . . . . . 6 6.1. Mixed security status . . . . . . . . . . . . . . . . . . 7
4.4. Deliberate omissions . . . . . . . . . . . . . . . . . . . 6 6.2. A service domain trusts its servers . . . . . . . . . . . 7
5. Internationalization Considerations . . . . . . . . . . . . . 7 6.3. Certificate subject name matching . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 6.4. Deliberate omissions . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 7. Internationalization Considerations . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 7 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 8 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . . 8 10.1. Normative References . . . . . . . . . . . . . . . . . . . 9
Appendix B. Rationale . . . . . . . . . . . . . . . . . . . . . . 8 10.2. Informative References . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10 Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . . 10
Appendix B. Rationale . . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
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. The association is secured using DNSSEC. That
document "only relates to securely associating certificates for TLS document "only relates to securely associating certificates for TLS
and DTLS with host names" (see the last paragraph of section 1.2 of and DTLS with host names" (see the last paragraph of section 1.2 of
[RFC6698]). [RFC6698]).
Some application protocols do not use host names directly, but Some application protocols do not use host names directly, but
instead use a service domain. The domain's servers are located instead use a service domain. The domain's servers are located
indirectly via SRV records [RFC2782] (or MX records in the case of indirectly via SRV records [RFC2782] (or MX records in the case of
SMTP [RFC5321]). When they do not use host names [RFC6698] does not SMTP [RFC5321]). When they do not use host names [RFC6698] does not
direcly apply to these protocols. direcly apply to these protocols.
This document describes how to use DANE TLSA records with SRV and MX This document describes how to use DANE TLSA records with SRV and MX
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 SRV or MX query. domain and the target server host names, i.e. the result of the
SRV or MX query.
o The TLSA records are located alongside the SRV target host names. o The TLSA records are located using the SRV port, protocol, and
target host name fields.
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 The server's certificate is expected to match the server host o The server's certificate is expected to authenticate the server
name, rather than the service domain. host name, rather than the service domain.
Separate documents give the details that are specific to particular Separate documents give the details that are specific to particular
application protocols. For examples, see [I-D.ietf-dane-smtp] and application protocols. For examples, see [I-D.ietf-dane-smtp] and
[I-D.ietf-dane-mua]. [I-D.ietf-dane-mua].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
memo are to be interpreted as described in [RFC2119]. memo are to be interpreted as described in [RFC2119].
2. Using TLSA records with SRV and MX 2. DNS checks for TLSA and SRV / MX records
2.1. MX records 2.1. MX records
For the purpose of this specification (to avoid cluttering the For the purpose of this specification (to avoid cluttering the
description with special cases) each MX record ([RFC5321] section 5) description with special cases) each MX record ([RFC5321] section 5)
is treated as being equivalent to a SRV record [RFC2782] with is treated as being equivalent to a SRV record [RFC2782] with
corresponding fields copied from the MX record and the remaining corresponding fields copied from the MX record and the remaining
fields having fixed values as follows: fields having fixed values as follows:
Service smtp Service - smtp
Proto tcp Proto - tcp
Name MX owner name Name - MX owner name (mail domain)
TTL MX TTL TTL - MX TTL
Class MX Class Class - MX Class
Priority MX Priority Priority - MX Priority
Weight 0 Weight - 0
Port 25 Port - 25
Target MX Target Target - MX Target
For example this MX record is treated as if it were the following SRV For example this MX record is treated as if it were the following SRV
record: record:
example.com. 86400 IN MX 10 mx.example.net. example.com. 86400 IN MX 10 mx.example.net.
_smtp._tcp.example.com. 86400 IN SRV 10 0 25 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
[I-D.ietf-dane-smtp].
2.2. SRV query 2.2. SRV query
When the client makes a SRV query, a successful result can be a When the client makes a SRV query, a successful result will be (a
(possible chain of CNAME / DNAME aliases referring to a) list of one possible chain of CNAME / DNAME aliases referring to) a list of one
or more SRV records. or more SRV records.
For this specification to take effect, all of these DNS RRsets MUST For this specification to take effect, all of these DNS RRsets MUST
be "secure" according to DNSSSEC validation ([RFC4033] section 5). be "secure" according to DNSSSEC validation ([RFC4033] section 5).
In the case of a (chain of) aliases, the whole chain MUST be secure In the case of aliases, the whole chain MUST be secure as well as the
as well as the ultimate target. (This corresponds to the AD bit ultimate target. (This corresponds to the AD bit being set in the
being set in the response(s) - see [RFC4035] section 3.2.3.) response(s) - see [RFC4035] section 3.2.3.)
If they are not all secure, this protocol has not been fully If they are not all secure, this protocol has not been fully
deployed. The client SHOULD fall back to its non-DNSSEC non-DANE deployed. The client SHOULD fall back to its non-DNSSEC non-DANE
behaviour. (This corresponds to the AD bit being unset.) behaviour. (This corresponds to the AD bit being unset.)
If any of the responses is "bogus" according to DNSSEC validation the If any of the responses is "bogus" according to DNSSEC validation the
client MUST abort. (This usually corresponds to a "server failure" client MUST abort. (This usually corresponds to a "server failure"
response.) response.)
The client now has an authentic list of server host names with weight In the successful case, the client now has an authentic list of
and priority values. It performs server ordering and selection using server host names with weight and priority values. It performs
the weight and priority values without regard to the presence or server ordering and selection using the weight and priority values
absence of DNSSEC or TLSA records. without regard to the presence or absence of DNSSEC or TLSA records.
It takes note of the DNSSEC validation status of the SRV response for
use when checking certificate names (see section Section 3).
2.3. TLSA queries 2.3. TLSA queries
This sub-section applies to each server host name individually. This sub-section applies to each server host name individually,
provided the SRV response was secure according to DNSSEC validation.
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: port (from [RFC6698] section 3, based on fields from the SRV record: port from
the SRV RDATA), protocol (from the SRV query name), and the TLSA base the SRV RDATA, protocol from the SRV query name, and the TLSA base
domain is the SRV target host name. domain is the SRV target host name.
For example this SRV record leads to the following TLSA query: For example this SRV record leads to the following TLSA query:
_imap._tcp.example.com. 86400 IN SRV 10 0 143 imap.example.net. _imap._tcp.example.com. 86400 IN SRV 10 0 143 imap.example.net.
_143._tcp.imap.example.net. IN TLSA ? _143._tcp.imap.example.net. IN TLSA ?
o A secure answer containing one or more TLSA records, in which case The client SHALL determine if the TLSA record(s) are usable according
the client SHALL proceed as descrbed below. to section 4.1 of [RFC6698]. This affects SRV handling as follows:
o A bogus answer or other failure, which the client MUST treat as a If the TLSA response is "secure" the client MUST use TLS when
temporary error. connecting to the server. If the client receives zero usable
certificate associations, it processes TLS in the normal fashion
without any input from the TLSA records. If the client receives one
or more usable certificate associations, it processes them as
described in [RFC6698].
o If there is no TLSA record or its DNSSEC validation state is If the TLSA response is "insecure" or "indeterminate" the client
insecure or indeterminate, this protocol has not been fully SHALL proceed as if this server has no TLSA records. It MAY connect
deployed. The client SHOULD deliver to this server insecurely to the server with or without TLS.
(which might be over unauthenticated TLS, as described in the
introduction).
3. Guidelines for application protocols If the TLSA response is "bogus" then the client MUST NOT connect to
the corresponding server. (The client can still use other SRV
targets.)
3. TLS checks for TLSA and SRV / MX 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
client SHALL ensure the server's certificate passes the [RFC6698]
checks if there are usable TLSA records.
The client uses the DNSSEC validation status of the SRV query in its
server certificate identity checks. (The TLSA validation status does
not affect the server certificate identity checks.) It MUST use the
Server Name Indication extension (TLS SNI) [RFC6066] with the
preferred name chosen as follows. It SHALL verify the identity
asserted by the server's certificate according to [RFC6125] section
6, using a list of reference identifiers constructed as follows.
SRV is insecure or indeterminate: The reference identifiers SHALL
include the service domain and MUST NOT include the SRV target
host name. The service domain is the preferred name for TLS SNI.
SRV is secure: The reference identifiers SHALL include both the
service domain and the SRV target host name. The target host name
is the preferred name for TLS SNI.
(In the latter case, the client will accept either identity so it is
compatible with servers that do and do not support this
specification.)
4. Guidance for application protocols
Separate documents describe how to apply this specification to Separate documents describe how to apply this specification to
particular application protocols. If you are writing such as particular application protocols. If you are writing such as
document the following points ought to be covered: document the following points ought to be covered: (This section is
currently sketchy.)
o How should the client react to a "bogus" DNSSEC status? o SRV fallback logic? In the event of bogus replies etc.
4. Security considerations o Compatibility with non-SRV clients.
4.1. Mixed security status 5. Guidance for server operators
In order to support this specification, server software MUST
implement the TLS Server Name Indication extension (TLS SNI)
[RFC6066] for selecting the appropriate certificate.
A server that supports TLS and is the target of a SRV record MUST
have a TLS certificate that authenticates the SRV query domain (i.e.
the service domain, or "source domain" in [RFC6125] terms). This is
necessary for clients that cannot perform DNSSEC validation. This
certificate MUST be the default that is presented if the client does
not use TLS SNI.
In order to support this specification, the server SHOULD also have a
certificate that authenticates the SRV target domain (the mail server
hostname). This can be done using a multi-name certificate or by
using the client's TLS SNI to select the appropriate certificate.
The server's TLSA record SHOULD correspond to this certificate.
Note: In some application protocols, there are old non-SRV clients
that expect a server's TLS certificate to authenticate its host name;
they are also unlikely to support SNI. This means that servers for
old clients need a different default certificate from servers that
are the targets of SRV records. If the server does not have a
certificate that authenticates all relevant names, it is necessary to
segregate old and new clients. This can be done by using different
target hosts or non-standard ports in the SRV targets. (The latter
avoids the need for additional certificates.)
6. Security considerations
6.1. Mixed security status
We do not specify that clients check that all of a service domain's We do not specify that clients check that all of a service domain's
server host names are consistent in whether they have or do not have server host names are consistent in whether they have or do not have
TLSA records. This is so that partial or incremental deployment does TLSA records. This is so that partial or incremental deployment does
not break the service. Different levels of deployment are likely if not break the service. Different levels of deployment are likely if
a service domain has a third-party fall-back server, for example. a service domain has a third-party fall-back server, for example.
The SRV and MX sorting rules are unchanged; in particular they have The SRV and MX sorting rules are unchanged; in particular they have
not been altered in order to prioritize secure servers over insecure not been altered in order to prioritize secure servers over insecure
servers. If a site wants to be secure it needs to deploy this servers. If a site wants to be secure it needs to deploy this
protocol completely; a partial deployment is not secure and we make protocol completely; a partial deployment is not secure and we make
no special effort to support it. no special effort to support it.
4.2. A service domain trusts its servers 6.2. 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, and publishing a installing a TLS certificate with the correct name, and publishing a
TLSA record under that name. If these are not correct then TLSA record under that name. If these are not correct then
connections from TLSA-aware clients might fail. connections from TLSA-aware clients might fail.
4.3. Certificate subject name matching 6.3. 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 checking might appear to be unnecessary, since DNSSEC provides a Name checking might appear to be unnecessary, since DNSSEC provides a
secure binding between the server name and the TLSA record, which in secure binding between the server name and the TLSA record, which in
turn authenticates the certificate. However this latter step can be turn authenticates the certificate. However this latter step can be
indirect, via a chain of certificates. A usage=0 TLSA record only indirect, via a chain of certificates. A usage=0 TLSA record only
authenticates the CA that issued the certificate, and third parties authenticates the CA that issued the certificate, and third parties
can obtain certificates from the same CA. can obtain certificates from the same CA.
So this specification says that clients check that the server's So this specification says that clients check that the server's
certificate matches the server host name, to ensure that the certificate matches the server host name, to ensure that the
certificate was issued by the CA to the server that the client is certificate was issued by the CA to the server that the client is
connecting to. The client always performs this check regardless of connecting to. The client always performs this check regardless of
the TLSA usage, to simplify implementation and so that this the TLSA usage, to simplify implementation and so that this
specification is less likely to need updating when new TLSA usages specification is less likely to need updating when new TLSA usages
are added. are added.
4.4. Deliberate omissions 6.4. Deliberate omissions
We do not specify that clients check the DNSSEC state of the server We do not specify that clients check the DNSSEC state of the server
address records. This is not necessary since the certificate checks address records. This is not necessary since the certificate checks
ensure that the client has connected to the correct server. (The ensure that the client has connected to the correct server. (The
address records will normally have the same security state as the address records will normally have the same security state as the
TLSA records, but they can differ if there are CNAME or DNAME TLSA records, but they can differ if there are CNAME or DNAME
indirections.) indirections.)
5. 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].
6. IANA Considerations 8. IANA Considerations
No IANA action is required. No IANA action is required.
7. Acknowledgements 9. Acknowledgements
Thanks to Mark Andrews for arguing that authenticating the server Thanks to Mark Andrews for arguing that authenticating the server
host name is the right thing, and that we ought to rely on DNSSEC to host name is the right thing, and that we ought to rely on DNSSEC to
secure the SRV / MX lookup. Thanks to James Cloos, Ned Freed, Olafur secure the SRV / MX lookup. Thanks to James Cloos, Ned Freed, Olafur
Gudmundsson, Paul Hoffman, Phil Pennock, Hector Santos, Jonas Gudmundsson, Paul Hoffman, Phil Pennock, Hector Santos, Jonas
Schneider, and Alessandro Vesely for helpful suggestions. Schneider, and Alessandro Vesely for helpful suggestions.
8. References 10. References
10.1. Normative References
8.1. Normative References
[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", Rose, "DNS Security Introduction and Requirements",
skipping to change at page 7, line 48 skipping to change at page 9, line 28
Rose, "Protocol Modifications for the DNS Security Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005. Extensions", RFC 4035, March 2005.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008. October 2008.
[RFC5890] Klensin, J., "Internationalized Domain Names for [RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework", Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010. RFC 5890, August 2010.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 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.
[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.
8.2. Informative References 10.2. Informative References
[I-D.ietf-dane-smtp] [I-D.ietf-dane-smtp]
Finch, T., "DNS-Based Authentication of Named Entities Finch, T., "DNS-Based Authentication of Named Entities
(DANE) for secure SMTP", draft-ietf-dane-smtp (work in (DANE) for secure SMTP", draft-ietf-dane-smtp (work in
progress), March 2013. progress), March 2013.
[I-D.ietf-dane-mua] [I-D.ietf-dane-mua]
Finch, T., "DNS-Based Authentication of Named Entities Finch, T., "DNS-Based Authentication of Named Entities
(DANE) for POP, IMAP, and message submission", (DANE) for POP, IMAP, and message submission",
draft-ietf-dane-mua (work in progress), March 2013. draft-ietf-dane-mua (work in progress), March 2013.
skipping to change at page 9, line 47 skipping to change at page 11, line 26
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.
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 puttling the TLSA used and when they cannot. This is all avoided by putting the TLSA
records under the server host name. records under the server host name.
The disadvantage is that clients which do not do DNSSEC validation The disadvantage is that clients which do not do DNSSEC validation
must, according to [RFC6125] rules, check the server certificate must, according to [RFC6125] rules, check the server certificate
against the service domain, since they have no other way to against the service domain, since they have no other way to
authenticate the server. This means that Server Name Indication authenticate the server. This means that Server Name Indication
support is necessary for backwards compatibility. support is necessary for backwards compatibility.
Author's Address Author's Address
 End of changes. 43 change blocks. 
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