DNS-Based Authentication of Named Entities (DANE) T. Finch
Internet-Draft University of Cambridge
Intended status: Standards Track M. Miller
Expires: April 24, 2015 Cisco Systems, Inc.
P. Saint-Andre
&yet
October 21, 2014
Using DNS-Based Authentication of Named Entities (DANE) TLSA Records
with SRV Records
draft-ietf-dane-srv-08
Abstract
The DANE specification (RFC 6698) describes how to use TLSA resource
records in the DNS to associate a server's host name with its TLS
certificate, where the association is secured with DNSSEC. However,
application protocols that use SRV records (RFC 2782) to indirectly
name the target server host names for a service domain 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 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
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This Internet-Draft will expire on April 24, 2015.
Copyright Notice
Copyright (c) 2014 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. DNS Checks . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. SRV Query . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Address Queries . . . . . . . . . . . . . . . . . . . . . 4
3.3. TLSA Queries . . . . . . . . . . . . . . . . . . . . . . 4
3.4. Impact on TLS Usage . . . . . . . . . . . . . . . . . . . 5
4. TLS Checks . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. SRV Records Only . . . . . . . . . . . . . . . . . . . . 5
4.2. TLSA Records . . . . . . . . . . . . . . . . . . . . . . 6
5. Guidance for Protocol Authors . . . . . . . . . . . . . . . . 6
6. Guidance for Server Operators . . . . . . . . . . . . . . . . 7
7. Guidance for Application Developers . . . . . . . . . . . . . 8
8. Internationalization Considerations . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . 9
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
12.1. Normative References . . . . . . . . . . . . . . . . . . 9
12.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 11
A.1. IMAP . . . . . . . . . . . . . . . . . . . . . . . . . . 11
A.2. XMPP . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Appendix B. Rationale . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
The base DANE specification [RFC6698] describes how to use TLSA
resource records in the DNS to associate a server's host name with
its TLS certificate, where the association is secured using DNSSEC.
That document "only relates to securely associating certificates for
TLS and DTLS with host names" (see the last paragraph of section 1.2
of [RFC6698]).
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Some application protocols do not use host names directly; instead,
they use a service domain, and the relevant target server host names
are located indirectly via SRV records [RFC2782]. Because of this
intermediate resolution step, the normal DANE rules specified in
[RFC6698] cannot be applied to protocols that use SRV records.
(Rules for SMTP [RFC5321], which uses MX records instead of SRV
records, are described in [I-D.ietf-dane-smtp-with-dane].)
This document describes how to use DANE TLSA records with SRV
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 query).
o The TLSA records are located using the port, protocol, and target
server host name fields (not the service domain).
o Clients always use TLS when connecting to servers with TLSA
records.
o Assuming that the association is secure, the server's certificate
is expected to authenticate the target server host name, rather
than the service domain.
Note: The "CertID" specification [RFC6125] does not use the terms
"service domain" and "target server host name", but refers to the
same entities with the terms "source domain" and "derived domain".
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this memo are to be interpreted as described in
[RFC2119].
This draft uses the definitions for "secure", "insecure", "bogus",
and "indeterminate" from [RFC4035]. This draft uses the acronyms
from [RFC7218] for the values of TLSA fields where appropriate.
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). Implementers need
to be aware that unsuccessful results can occur because of various
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DNS-related errors; a helpful summary can be found in section 2.1 of
[I-D.ietf-dane-smtp-with-dane].
For this specification to apply, the entire DNS RRset that is
returned MUST be "secure" according to DNSSSEC validation ([RFC4033]
section 5). In the case of aliases, the whole chain of CNAME and
DNAME RRsets MUST be secure as well. This corresponds to the AD bit
being set in the response(s); see [RFC4035] section 3.2.3.
If the the entire RRset is "insecure", this protocol has not been
correctly deployed. The client SHOULD fall back to its non-DNSSEC,
non-DANE behavior (this corresponds to the AD bit being unset). If
the entire RRset is "bogus", the client MUST abort the attempt.
In the successful case, the client now has an authentic list of
target server host names with weight and priority values. It
performs server ordering and selection using the weight and priority
values without regard to the presence or absence of DNSSEC or TLSA
records. It also takes note of the DNSSEC validation status of the
SRV response for use when checking certificate names (see Section 4).
The client can now proceed to making address queries on the target
server host names as described in the next section.
3.2. Address Queries
For each SRV target server host name, the client makes A and AAAA
queries, performs DNSSEC validation on the address (A or AAAA)
response, and continues as follows based on the results:
o If the response is "secure" and usable, the client MUST perform a
TLSA query for that target server host name as described in the
next section.
o If the response is "insecure", the client MUST NOT perform a TLSA
query for that target server host name; the TLSA query will most
likely fail.
o If the response is "bogus" or "indeterminate", the client MUST NOT
connect to this target server; instead it uses the next most
appropriate SRV target.
3.3. TLSA Queries
The client SHALL construct the TLSA query name as described in
[RFC6698] section 3, based on fields from the SRV record: the port
from the SRV RDATA, the protocol from the SRV query name, and the
TLSA base domain set to the SRV target server host name.
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For example, the following SRV record for IMAP (see [RFC6186]) leads
to the TLSA query shown below:
_imap._tcp.example.com. 86400 IN SRV 10 0 9143 imap.example.net.
_9143._tcp.imap.example.net. IN TLSA ?
3.4. Impact on TLS Usage
The client SHALL determine if the TLSA record(s) returned in the
previous step are usable according to section 4.1 of [RFC6698]. This
affects the use 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
under Section 4.
o If the TLSA response is "insecure", 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.
o If the TLSA response is "bogus" or "indeterminate", then the
client MUST NOT connect to the target server (the client can still
use other SRV targets).
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 apply.
4.1. SRV Records Only
If the client received zero usable TLSA certificate associations, it
SHALL validate the server's TLS certificate using the normal PKIX
rules [RFC5280] or protocol-specific rules (e.g., following
[RFC6125]) without further input from the TLSA records.
In this case, the client uses the information in the server
certificate and the DNSSEC validation status of the SRV query in its
authentication checks. It SHOULD use the Server Name Indication
extension (TLS SNI) [RFC6066] or its functional equivalent in the
relevant application protocol (e.g., in XMPP [RFC6120] this is the
'to' address of the initial stream header). The preferred name SHALL
be chosen as follows, and the client SHALL verify the identity
asserted by the server's certificate according to section 6 of
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[RFC6125], using a list of reference identifiers constructed as
follows (note again that in RFC 6125 the terms "source domain" and
"derived domain" refer to the same things as "service domain" and
"target server host name" in this document). The examples below
assume a service domain of "im.example.com" and a target server host
name of "xmpp23.hosting.example.net".
SRV is insecure: The reference identifiers SHALL include the service
domain and MUST NOT include the SRV target server host name (e.g.,
include "im.example.com" but not "xmpp23.hosting.example.net").
The service domain is the preferred name for TLS SNI or its
equivalent.
SRV is secure: The reference identifiers SHALL include both the
service domain and the SRV target server host name (e.g., include
both "im.example.com" and "xmpp23.hosting.example.net"). The
target server host name is the preferred name for TLS SNI or its
equivalent.
In the latter case, the client will accept either identity to ensure
compatibility with servers that support this specification as well as
servers that do not support this specification.
4.2. TLSA Records
If the client received one or more usable TLSA certificate
associations, it SHALL process them as described in section 2.1 of
[RFC6698].
If the TLS server's certificate -- or the public key of the server's
certificate -- matches a usable TLSA record with Certificate Usage
"DANE-EE", the client MUST consider the server to be authenticated.
Because the information in such a TLSA record supersedes the non-key
information in the certificate, all other [RFC5280] and [RFC6125]
authentication checks (e.g., reference identifier, key usage,
expiration, issuance) MUST be ignored or omitted.
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.
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o How clients ought to behave if they do not support SRV lookups, or
if clients that support SRV lookups encounter service domains that
do not offer SRV records.
o Whether the application protocol has a functional equivalent for
TLS SNI that is preferred within that protocol.
o 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, [I-D.ietf-xmpp-dna] covers such topics for the
Extensible Messaging and Presence Protocol (XMPP).
6. Guidance for Server Operators
To conform to this specification, the published SRV records and
subsequent address (A 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 host name. However,
operators need to be aware that servers relying solely on validation
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 host name (the "derived domain" in [RFC6125]
terms, which is the SRV target).
Servers that support multiple service domains (i.e., so-called
"multi-tenanted environments") can implement the Transport Layer
Security Server Name Indication (TLS SNI) [RFC6066] or its functional
equivalent to determine which certificate to offer. Clients that do
not support this specification will indicate a preference for the
service domain name, while clients that support this specification
will indicate the target server host name. However, the server
determines what certificate to present in the TLS handshake; e.g.,
the presented certificate might only authenticate the target server
host name.
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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 SRV
queries, 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 SRV queries and 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].
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
target server host names are consistent in whether they have or do
not have TLSA records. This is so that partial or incremental
deployment does not break the service. Different levels of
deployment are likely if a service domain has a third-party fallback
server, for example.
The SRV 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 (where necessary),
and publishing a TLSA record for that certificate. If these are not
correct then connections from TLSA-aware clients might fail.
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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 "DANE-EE". Because such a record identifies the
specific certificate (or public key of the certificate), additional
checks are superfluous and potentially conflicting.
Otherwise, while DNSSEC provides a secure binding between the server
name and the TLSA record, and the TLSA record provides a binding to a
certificate, this latter step can be indirect via a chain of
certificates. For example, a Certificate Usage "PKIX-TA" TLSA record
only authenticates the CA that issued the certificate, and third
parties can obtain certificates from the same CA. Therefore, clients
need to check whether the server's certificate matches one of the
expected reference identifiers to ensure that the certificate was
issued by the CA to the server the client expects.
11. Acknowledgements
Thanks to Mark Andrews for arguing that authenticating the target
server host name is the right thing, and that we ought to rely on
DNSSEC to secure the SRV lookup. Thanks to James Cloos, Viktor
Dukhovni, Ned Freed, Olafur Gudmundsson, Paul Hoffman, Phil Pennock,
Hector Santos, Jonas Schneider, and Alessandro Vesely for helpful
suggestions.
12. References
12.1. 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.
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[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011.
[RFC6186] Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186, March 2011.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.
[RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify
Conversations about DNS-Based Authentication of Named
Entities (DANE)", RFC 7218, April 2014.
12.2. Informative References
[I-D.ietf-dane-smtp-with-dane]
Dukhovni, V. and W. Hardaker, "SMTP security via
opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-05
(work in progress), February 2014.
[I-D.ietf-xmpp-dna]
Saint-Andre, P. and M. Miller, "Domain Name Associations
(DNA) in the Extensible Messaging and Presence Protocol
(XMPP)", draft-ietf-xmpp-dna-05 (work in progress),
February 2014.
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[RFC3403] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part Three: The Domain Name System (DNS) Database", RFC
3403, October 2002.
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 host name
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 submitted for addresses at example.com are sent via
IMAP to 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.example.com. SRV 1 0 5222 im.example.net.
_xmpp-client.example.com. RRSIG SRV ...
; target server host name
im.example.net. A 192.0.2.3
im.example.net. RRSIG A ...
im.example.net. AAAA 2001:db8:212:8::e:4
im.example.net. RRSIG AAAA ...
; TLSA resource record
_5222._tcp.im.example.net. TLSA ...
_5222._tcp.im.example.net. RRSIG TLSA ...
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XMPP sessions for addresses at example.com are established at
im.example.net. Connections to im.example.net port 5222 that use
STARTTLS will get a server certificate that authenticates the name
im.example.net.
Appendix B. Rationale
The long-term goal of this specification is to settle on TLS
certificates that verify the target server host name rather than the
service domain, since this is more convenient for servers hosting
multiple domains (so-called "multi-tenanted environments") and scales
up more easily to larger numbers of service domains.
There are a number of other reasons for doing it this way:
o The certificate is part of the server configuration, so it makes
sense to associate it with the server host name rather than the
service domain.
o In the absence of TLS SNI, if the certificate identifies the host
name then it does not need to list all the possible service
domains.
o When the server certificate is replaced it is much easier if there
is one part of the DNS that needs updating to match, instead of an
unbounded number of hosted service domains.
o The same TLSA records work with this specification, and with
direct connections to the host name in the style of [RFC6698].
o Some application protocols, such as SMTP, allow a client to
perform transactions with multiple service domains in the same
connection. It is not in general feasible for the client to
specify the service domain using TLS SNI when the connection is
established, and the server might not be able to present a
certificate that authenticates all possible service domains. See
[I-D.ietf-dane-smtp-with-dane] for details.
o It is common for SMTP servers to act in multiple roles, for
example as outgoing relays or as incoming MX servers, depending on
the client identity. It is simpler if the server can present the
same certificate regardless of the role in which it is to act.
Sometimes the server does not know its role until the client has
authenticated, which usually occurs after TLS has been
established. See [I-D.ietf-dane-smtp-with-dane] for details.
This specification does not provide an option to put TLSA records
under the service domain because that would add complexity without
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providing any benefit, and security protocols are best kept simple.
As described above, there are real-world cases where authenticating
the service domain cannot be made to work, so there would be
complicated criteria for when service domain TLSA records might be
used and when they cannot. This is all avoided by putting the TLSA
records under the target server host name.
The disadvantage is that clients which do not complete DNSSEC
validation must, according to [RFC6125] rules, check the server
certificate against the service domain, since they have no other way
to authenticate the server. This means that SNI support or its
functional equivalent is necessary for backward compatibility.
Authors' Addresses
Tony Finch
University of Cambridge Computing Service
New Museums Site
Pembroke Street
Cambridge CB2 3QH
ENGLAND
Phone: +44 797 040 1426
Email: dot@dotat.at
URI: http://dotat.at/
Matthew Miller
Cisco Systems, Inc.
1899 Wynkoop Street, Suite 600
Denver, CO 80202
USA
Email: mamille2@cisco.com
Peter Saint-Andre
&yet
Email: peter@andyet.com
URI: https://andyet.com/
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