DNS-Based Authentication of Named Entities (DANE) T. Finch
Internet-Draft University of Cambridge
Intended status: Standards Track M. Miller
15, 17, 2014 Cisco Systems, Inc.
February 11, 13, 2014
Using DNS-Based Authentication of Named Entities (DANE) TLSA records
with SRV and MX records.
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 association is secured with DNSSEC. Some
application protocols use SRV records (RFC 2782) to indirectly name
the server hosts for a service domain (SMTP uses MX records for the
same purpose). This specification gives generic instructions for how
these application protocols 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.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 15, 17, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Relation between SRV and MX records . . . . . . . . . . . . . 3
4. DNS Checks for TLSA and SRV Records . . . . . . . . . . . . . 4
4.1. SRV Query . . . . . . . . . . . . . . . . . . . . . . . . 4
4.2. TLSA Queries . . . . . . . . . . . . . . . . . . . . . . 5
5. TLS Checks for TLSA and SRV Records . . . . . . . . . . . . . 6
6. Guidance for Application Protocols . . . . . . . . . . . . . 6 7
7. Guidance for Server Operators . . . . . . . . . . . . . . . . 7
8. Internationalization Considerations . . . . . . . . . . . . . 7 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 8
10. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10.1. Mixed Security Status . . . . . . . . . . . . . . . . . 8
10.2. A Service Domain Trusts its Servers . . . . . . . . . . 8
10.3. Certificate Subject Name Matching . . . . . . . . . . . 8 9
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
12.1. Normative References . . . . . . . . . . . . . . . . . . 9
12.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Mail Example . . . . . . . . . . . . . . . . . . . . 10 11
Appendix B. XMPP Example . . . . . . . . . . . . . . . . . . . . 10 11
Appendix C. Rationale . . . . . . . . . . . . . . . . . . . . . 11 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 13
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 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
Some application protocols do not use host names directly; instead,
they use a service domain and the relevant host names are located
indirectly via SRV records [RFC2782], or MX records in the case of
SMTP [RFC5321]. [RFC5321] (Note: in the "CertID" specification [RFC6125], the
source domain and host name are referred to as the "source domain"
and the "derived domain".) domain"). Because of this intermediate resolution
step, the normal DANE rules specified in [RFC6698] do not directly
apply to protocols that use SRV or MX records.
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
host name fields (not the service domain).
o Clients always use TLS when connecting to servers with TLSA
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 the details that are specific to particular
application protocols, such as SMTP [I-D.ietf-dane-smtp-with-dane]
and XMPP [I-D.ietf-xmpp-dna].
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
This draft uses the definitions for "secure", "insecure", "bogus",
and "indeterminate" from [RFC4035]. This draft uses the acronyms
from [I-D.ietf-dane-registry-acronyms] for the values of TLSA fields
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
4. DNS Checks for TLSA and SRV Records
4.1. SRV Query
When the client makes an SRV query, a successful result will be a
list of one or more SRV records (or possibly a chain of CNAME / DNAME
aliases referring to such a list).
For this specification to apply, all of these DNS RRsets 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 as the
ultimate target. (This well. This corresponds to the AD bit being set in the
response(s); see [RFC4035] section 3.2.3.) 3.2.3.
If they are not all secure, this protocol has not been fully correctly
deployed. The client SHOULD fall back to its non-DNSSEC non-DANE
behavior (this corresponds to the AD bit being unset.) unset).
If any of the responses is are "bogus" or "indeterminate" according to
DNSSEC validation, the client MUST abort. abort (This usually corresponds to
a "server failure" response.) response).
In the successful case, the client now has an authentic list of
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 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 or indeterminate, insecure, the client MUST NOT perform any
TLSA queries. If the SRV response is secure "secure" according to DNSSEC
validation, the client performs a TLSA query for each SRV target as describes
described in this section.
For each SRV target host name, if the response to client performs DNSSEC validation
on the address (A or (A, AAAA) query response and continues based on the results:
o if the response is insecure or indeterminate, "insecure", the client MUST NOT perform a TLSA
query for that target; the TLSA a query will most likely fail.
o If the response is "bogus" or "indeterminate", the client MUST NOT
connect to this host name; instead it uses the next most
appropriate SRV target.
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 host name.
For example, the following SRV record leads to the TLSA query shown
_imap._tcp.example.com. 86400 IN SRV 10 0 143 imap.example.net.
_143._tcp.imap.example.net. IN TLSA ?
The client SHALL determine if the TLSA record(s) are usable according
to section 4.1 of [RFC6698]. This affects SRV handling as follows:
If the TLSA response is "secure", the client MUST use TLS when
connecting to the server. The TLSA records are used when validating
the server's certificate as described under Section 5.
If the TLSA response is "insecure" or "indeterminate", "insecure", the client SHALL proceed as if
this server has no TLSA records. It MAY connect to the server with
or without TLS.
If the TLSA response is "bogus", "bogus" or "indeterminate", then the client
MUST NOT connect to
the corresponding server. (The this server (the client can still use other SRV
5. 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. 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 "3" matches
"DANE-EE", the TLS
server's certificate, or public key for 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
validation [RFC5280] and verification [RFC6125]
authentication checks MAY be ignored (e.g., reference identifier, key usage,
expiration, issuance, etc.). etc.) MUST be ignored or omitted.
Otherwise, the client uses the information in the server certificate
and DNSSEC validation status of the SRV query in its server certificate identity 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 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, using a list
of reference identifiers constructed as follows. (Note follows (note again that in
RFC 6125 the terms "source domain" and "derived domain" refer to the
same things as "service domain" and "target host name" in this document.)
SRV is insecure or indeterminate: insecure: 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 or its equivalent.
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 or its equivalent.
In the latter case, the client will accept either identity so that it
is compatible with servers that do and do not support this
6. Guidance for Application Protocols
Separate documents describe how to apply this specification to
particular application protocols. If you are writing such as
document the following points Such documents ought to be covered: cover the
o Fallback logic in the event of bogus replies and the like.
o The use of TLS SNI or its functional equivalent.
o Appropriate mappings for non-SRV technologies such as MX.
o Compatibility with clients that do not support SRV lookups.
7. 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. Except for
When using TLSA records with Certificate Usage
"3", "DANE-EE", the
deployed certificate authenticated by does not need to contain any of the possible
reference identifiers discussed below. Indeed, none of the
certificate's information is necessary for such certificates.
However, servers that rely solely on validation using Certificate
Usage "DANE-EE" TLSA record(s) 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 identifier that matches:
o the service domain name (the "source domain" in [RFC6125] terms,
which is the SRV query domain); and/or
o the server host name (the "derived domain" in [RFC6125] terms,
which is the SRV target).
Servers that support multiple service domains (i.e., multi-tenant)
can implement Server Name Identifier Indicator (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 server host name. However, the
server determines what certificate to present in the TLS handshake;
e.g., the presented certificate might only authenticate the server
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].
9. IANA Considerations
No IANA action is required.
10. Security Considerations
10.1. Mixed Security Status
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
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. 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, name (where necessary),
and publishing a TLSA record under for that name. certificate. If these are not
correct then connections from TLSA-aware clients might fail.
10.3. 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 "3". "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 "0" "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 the certificate was issued
by the CA to the server the client expects.
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
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
12.1. 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
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,
[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,
[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.
[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. Informative References
Dukhovni, V. and W. Hardaker, "SMTP security via
opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-05
(work in progress), February 2014.
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),
Appendix A. Mail Example
In the following, most of the DNS resource data is elided for
; mail domain
example.com. MX 1 mx.example.net.
example.com. RRSIG MX ...
; SMTP server host name
mx.example.net. A 192.0.2.1
mx.example.net. RRSIG A ...
mx.example.net. AAAA 2001:db8:212:8::e:1
mx.example.net. RRSIG ...
; TLSA resource record
_25._tcp.mx.example.net. TLSA ...
_25._tcp.mx.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
Appendix B. XMPP Example
In the following, most of the DNS resource data is elided for
; XMPP domain
_xmpp-client.example.com. SRV 1 0 5222 im.example.net.
_xmpp-client.example.com. RRSIG SRV ...
; XMPP 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 is delivered by SMTP to
mx.example.net. are established at
im.example.net. Connections to mx.example.net im.example.net port 25 5222 that use
STARTTLS will get a server certificate that authenticates the name
Appendix C. Rationale
The long-term goal of this specification is to settle on TLS
certificates that verify the 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
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
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.
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
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 server host name.
The disadvantage is that clients which do not do 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.
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