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14 RFC 7673
DNS-Based Authentication of Named T. Finch
Entities (DANE) University of Cambridge
Internet-Draft February 25, 2013
Intended status: Standards Track
Expires: August 29, 2013
Using DNS-Based Authentication of Named Entities (DANE) TLSA records
with SRV and MX records.
draft-ietf-dane-srv-02
Abstract
The 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 with DNSSEC. Some
application protocols can use SRV records [RFC2782] 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.
Separate documents give the details that are specific to particular
application protocols.
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 29, 2013.
Copyright Notice
Copyright (c) 2013 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
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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. Relation between SRV and MX records . . . . . . . . . . . . . 3
3. DNS checks for TLSA and SRV records . . . . . . . . . . . . . 4
3.1. SRV query . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. TLSA queries . . . . . . . . . . . . . . . . . . . . . . . 5
4. TLS checks for TLSA and SRV records . . . . . . . . . . . . . 5
5. Guidance for application protocols . . . . . . . . . . . . . . 6
6. Guidance for server operators . . . . . . . . . . . . . . . . 6
7. Security considerations . . . . . . . . . . . . . . . . . . . 7
7.1. Mixed security status . . . . . . . . . . . . . . . . . . 7
7.2. A service domain trusts its servers . . . . . . . . . . . 7
7.3. Certificate subject name matching . . . . . . . . . . . . 8
7.4. Deliberate omissions . . . . . . . . . . . . . . . . . . . 8
8. Internationalization Considerations . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Normative References . . . . . . . . . . . . . . . . . . . 9
11.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . . 10
Appendix B. Rationale . . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
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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 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, but
instead use a service domain. The domain's servers are located
indirectly via SRV records [RFC2782] (or MX records in the case of
SMTP [RFC5321]). When they do not use host names [RFC6698] does not
direcly apply to these protocols.
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 result of the
SRV or MX query.
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
records.
o The server's certificate is expected to authenticate the server
host name, rather than the service domain.
Separate documents give the details that are specific to particular
application protocols. For examples, see [I-D.ietf-dane-smtp] and
[I-D.ietf-dane-mua].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
memo are to be interpreted as described in [RFC2119].
2. 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 a SRV record [RFC2782] with
corresponding fields copied from the MX record and the remaining
fields having fixed values as follows:
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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
For example this MX record is treated as if it were the following SRV
record:
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
[I-D.ietf-dane-smtp].
3. DNS checks for TLSA and SRV records
3.1. SRV query
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
or more SRV records.
For this specification to take effect, all of these DNS RRsets MUST
be "secure" according to DNSSSEC validation ([RFC4033] section 5).
In the case of aliases, the whole chain MUST be secure as well as the
ultimate target. (This corresponds to the AD bit being set in the
response(s) - see [RFC4035] section 3.2.3.)
If they are not all secure, this protocol has not been fully
deployed. The client SHOULD fall back to its non-DNSSEC non-DANE
behaviour. (This corresponds to the AD bit being unset.)
If any of the responses is "bogus" according to DNSSEC validation the
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client MUST abort. (This usually corresponds to a "server failure"
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 Section 4).
3.2. TLSA queries
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
[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
domain is the SRV target host name.
For example this SRV record leads to the following TLSA query:
_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 in section Section 4.
If the TLSA response is "insecure" or "indeterminate" 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" then the client MUST NOT connect to
the corresponding 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. If the
client received zero usable TLSA certificate associations, it SHALL
validate the server's TLS certificate using the normal PKIX rules
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[RFC5280] 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.
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 SHALL 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 that
it is compatible with servers that do and do not support this
specification.)
5. 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 ought to be covered: (This section is
currently sketchy.)
o SRV fallback logic? In the event of bogus replies etc.
o Compatibility with non-SRV clients.
6. 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
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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.)
7. Security considerations
7.1. Mixed security status
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
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 fall-back 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.
7.2. 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, and publishing a
TLSA record under that name. If these are not correct then
connections from TLSA-aware clients might fail.
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7.3. 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 checking might appear to be unnecessary, since DNSSEC provides a
secure binding between the server name and the TLSA record, which in
turn authenticates the certificate. However this latter step can be
indirect, via a chain of certificates. A usage=0 TLSA record only
authenticates the CA that issued the certificate, and third parties
can obtain certificates from the same CA.
So this specification says that clients check that the server's
certificate matches the server host name, to ensure that the
certificate was issued by the CA to the server that the client is
connecting to. The client always performs this check regardless of
the TLSA usage, to simplify implementation and so that this
specification is less likely to need updating when new TLSA usages
are added.
7.4. Deliberate omissions
We do not specify that clients check the DNSSEC state of the server
address records. This is not necessary since the certificate checks
ensure that the client has connected to the correct server. (The
address records will normally have the same security state as the
TLSA records, but they can differ if there are CNAME or DNAME
indirections.)
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. Acknowledgements
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, Ned Freed, Olafur
Gudmundsson, Paul Hoffman, Phil Pennock, Hector Santos, Jonas
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Schneider, and Alessandro Vesely for helpful suggestions.
11. References
11.1. Normative References
[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.
[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.
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11.2. Informative References
[I-D.ietf-dane-smtp]
Finch, T., "Secure SMTP using DNS-Based Authentication of
Named Entities (DANE) TLSA records.", draft-ietf-dane-smtp
(work in progress), March 2013.
[I-D.ietf-dane-mua]
Finch, T., "Using DNS-Based Authentication of Named
Entities (DANE) with POP, IMAP, and message submission.",
draft-ietf-dane-mua (work in progress), March 2013.
Appendix A. Example
In the following, most of the DNS resource data is elided for
simplicity.
; 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. AAAA 2001:db8:212:8::e:1
; 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
mx.example.net.
Appendix B. 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 and scales up more easily to larger numbers of service
domains.
There are a number of other reasons for doing it this way:
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o The certificate is part of the server configuration, so it makes
sense to associate it with the server 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.
o It is common for SMTP servers to act in multiple roles, 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.
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 Server Name Indication
support is necessary for backwards compatibility.
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Author's Address
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/
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