Internet DRAFT - draft-ietf-dane-srv
draft-ietf-dane-srv
DNS-Based Authentication of Named Entities (DANE) T. Finch
Internet-Draft University of Cambridge
Intended status: Standards Track M. Miller
Expires: October 25, 2015 Cisco Systems, Inc.
P. Saint-Andre
&yet
April 23, 2015
Using DNS-Based Authentication of Named Entities (DANE) TLSA Records
with SRV Records
draft-ietf-dane-srv-14
Abstract
The DANE specification (RFC 6698) describes how to use TLSA resource
records secured by DNSSEC (RFC 4033) to associate a server's
connection endpoint with its TLS certificate (thus enabling
administrators of domain names to specify the keys used in that
domain's TLS servers). However, application protocols that use SRV
records (RFC 2782) to indirectly name the target server connection
endpoints for a service domain 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
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 October 25, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. DNS Checks . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. SRV Query . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Address Queries . . . . . . . . . . . . . . . . . . . . . 5
3.3. TLSA Queries . . . . . . . . . . . . . . . . . . . . . . 5
3.4. Impact on TLS Usage . . . . . . . . . . . . . . . . . . . 6
4. TLS Checks . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. SRV Records Only . . . . . . . . . . . . . . . . . . . . 6
4.2. TLSA Records . . . . . . . . . . . . . . . . . . . . . . 7
5. Guidance for Protocol Authors . . . . . . . . . . . . . . . . 7
6. Guidance for Server Operators . . . . . . . . . . . . . . . . 8
7. Guidance for Application Developers . . . . . . . . . . . . . 9
8. Internationalization Considerations . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10.1. Mixed Security Status . . . . . . . . . . . . . . . . . 9
10.2. Certificate Subject Name Matching . . . . . . . . . . . 9
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 11
A.1. IMAP . . . . . . . . . . . . . . . . . . . . . . . . . . 12
A.2. XMPP . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Appendix B. Rationale . . . . . . . . . . . . . . . . . . . . . 13
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
The base DANE specification [RFC6698] describes how to use TLSA
resource records secured by DNSSEC [RFC4033] to associate a target
server's connection endpoint with its TLS certificate (thus enabling
administrators of domain names to specify the keys used in that
domain's TLS servers). Some application protocols locate connection
endpoints indirectly via SRV records [RFC2782]. As a result of this
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indirection, the rules specified in [RFC6698] cannot be directly
applied to such application protocols. (Rules for SMTP [RFC5321],
which uses MX resource records instead of SRV records, are described
in [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 SRV records that map the desired
service, transport protocol, and service domain to the
corresponding target server connection endpoints (i.e., the target
server host names and port numbers returned in the SRV records for
that service type).
o Although in accordance with [RFC2782] a service domain can
advertise a number of SRV records (some of which might map to
connection endpoints that do not support TLS), the intent of this
specification is for a client to securely discover connection
endpoints that support TLS.
o The TLSA records for each connection endpoint are located using
the transport protocol, port number, and host name for the target
server (not the service domain).
o When DNSSEC-validated TLSA records are published for a given
connection endpoint, clients always use TLS when connecting (even
if the connection endpoint supports cleartext communication).
o If there is at least one usable TLSA record for a given connection
endpoint, the connection endpoint's TLS certificate or public key
needs to match at least one of those usable TLSA records.
o If there are no usable TLSA records for a given connection
endpoint, the target server host name is used as one of the
acceptable reference identifiers, as described in [RFC6125].
Other reference identifiers might arise through CNAME expansion of
either the service domain or target server host name, as detailed
in [I-D.ietf-dane-ops].
o If there are no usable TLSA records for any connection endpoint
(and thus the client cannot securely discover a connection
endpoint that supports TLS), the client's behavior is a matter for
the application protocol or client implementation; this might
involve a fallback to non-DANE behavior using the public key
infrastructure [RFC5280].
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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 Section 4.3 of [RFC4035]. This draft uses
the acronyms from [RFC7218] for the values of TLSA fields where
appropriate.
Additionally, this document uses the following terms:
connection endpoint: A tuple of a fully qualified DNS host name,
transport protocol, and port number that a client uses to
establish a connection to the target server.
service domain: The fully qualified DNS domain name that identifies
an application service; corresponds to the term "source domain"
from [RFC6125].
This document uses the term "target server host name" in place of the
term "derived domain" from the CertID specification [RFC6125].
3. DNS Checks
3.1. SRV Query
When the client makes an SRV query, a successful result will
typically be a list of one or more SRV records (or possibly a chain
of CNAME / DNAME aliases leading to such a list).
NOTE: Implementers need to be aware that unsuccessful results can
occur because of various DNS-related errors; guidance on avoiding
downgrade attacks can be found in Section 2.1 of
[I-D.ietf-dane-smtp-with-dane].
For this specification to apply, the entire chain of DNS RRset(s)
returned MUST be "secure" according to DNSSEC validation (Section 5
of [RFC4035]). In the case where the answer is obtained via a chain
of CNAME and/or DNAME aliases, the whole chain of CNAME and DNAME
RRsets MUST also be secure.
If the SRV lookup fails because the RRset is "bogus" (or the lookup
fails for reasons other than no records), the client MUST abort its
attempt to connect to the desired service. If the lookup result is
"insecure" (or no SRV records exist), this protocol does not apply
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and the client SHOULD fall back to its non-DNSSEC, non-DANE (and
possibly non-SRV) behavior.
When the lookup returns a "secure" RRset (possibly via a chain of
"secure" CNAME/DNAME records), the client now has an authentic list
of target server connection endpoints with weight and priority
values. It performs server ordering and selection using the weight
and priority values without regard to the presence or absence of
DNSSEC or TLSA records. It also takes note of the DNSSEC validation
status of the SRV response for use when checking certificate names
(see Section 4). The client can then proceed to making address
queries on the target server host names as described in the following
section.
3.2. Address Queries
For each SRV target server connnection endpoint, the client makes A
and/or AAAA queries, performs DNSSEC validation on the address (A or
AAAA) response, and continues as follows based on the results:
o If a returned RRSet is "secure", the client MUST perform a TLSA
query for that target server connection endpoint as described in
the next section.
o If no returned RRsets are "secure", the client MUST NOT perform a
TLSA query for that target server connection endpoint; the TLSA
query will most likely fail or produce spurious results.
o If the address record lookup fails (this a validation status of
either "bogus" or "indeterminate"), the client MUST NOT connect to
this connection endpoint; instead it uses the next most
appropriate SRV target. This mitigates against downgrade attacks.
3.3. TLSA Queries
The client SHALL construct the TLSA query name as described in
Section 3 of [RFC6698], based on the fields from the SRV record: the
port number from the SRV RDATA, the transport protocol from the SRV
query name, and the TLSA base domain from the SRV target server host
name.
For example, the following SRV record for IMAP (see [RFC6186]):
_imap._tcp.example.com. 86400 IN SRV 10 0 9143 imap.example.net.
leads to the TLSA query shown below:
_9143._tcp.imap.example.net. IN TLSA ?
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3.4. Impact on TLS Usage
The client SHALL determine if the TLSA records returned in the
previous step are usable according to Section 4.1 of [RFC6698]. This
affects the use of TLS as follows:
o If the TLSA response is "secure" and usable, then the client MUST
use TLS when connecting to the target server. The TLSA records
are used when validating the server's certificate as described in
Section 4.
o If the TLSA response is "bogus" or "indeterminate" (or the lookup
fails for reasons other than no records), then the client MUST NOT
connect to the target server (the client can still use other SRV
targets).
o If the TLSA response is "insecure" (or no TLSA records exist),
then the client SHALL proceed as if the target server had no TLSA
records. It MAY connect to the target server with or without TLS,
subject to the policies of the application protocol or client
implementation.
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 to such secure
responses; Section 4.2 where there is at least one usable TLSA
record, and Section 4.1 otherwise.
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 [RFC6125], using a
list of reference identifiers constructed as follows (note again that
in RFC 6125 the terms "source domain" and "derived domain" to refer
to the same things as "service domain" and "target server host name"
in this document). The examples below assume a service domain of
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"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 ignore validation checks from [RFC5280]
and reference identifier checks from [RFC6125]. The information in
such a TLSA record supersedes the non-key information in the
certificate.
5. Guidance for Protocol Authors
This document describes how to use DANE with application protocols in
which target servers are discovered via SRV records. Although this
document attempts to provide generic guidance applying to all such
protocols, additional documents for particular application protocols
could cover related topics, such as:
o Fallback logic in the event that a client is unable to connect
securely to a target server by following the procedures defined in
this document.
o How clients ought to behave if they do not support SRV lookups, or
if clients that support SRV lookups encounter service domains that
do not offer SRV records.
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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 host name).
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 all of the target server connection endpoints
for a service domain need to be consistent in whether they have or do
not have TLSA records. This is so that partial or incremental
deployment does not break the service. Different levels of
deployment are likely if a service domain has a third-party fallback
server, for example.
The SRV sorting rules are unchanged; in particular they have not been
altered in order to prioritize secure connection endpoints over
insecure connection endpoints. If a site wants to be secure it needs
to deploy this protocol completely; a partial deployment is not
secure and we make no special effort to support it.
10.2. Certificate Subject Name Matching
Section 4 of the TLSA specification [RFC6698] leaves the details of
checking names in certificates to higher level application protocols,
though it suggests the use of [RFC6125].
Name checks are not necessary if the matching TLSA record is of
Certificate Usage "DANE-EE". Because such a record identifies the
specific certificate (or public key of the certificate), additional
checks are superfluous and potentially conflicting.
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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 (naturally, this is
in addition to standard certificate-related checks as specified in
[RFC5280], including but not limited to certificate syntax,
certificate extensions such as name constraints and extended key
usage, and handling of certification paths).
11. References
11.1. Normative References
[I-D.ietf-dane-ops]
Dukhovni, V. and W. Hardaker, "Updates to and Operational
Guidance for the DANE Protocol", draft-ietf-dane-ops-07
(work in progress), October 2014.
[I-D.ietf-dane-smtp-with-dane]
Dukhovni, V. and W. Hardaker, "SMTP security via
opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-15
(work in progress), March 2015.
[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.
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[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.
[RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify
Conversations about DNS-Based Authentication of Named
Entities (DANE)", RFC 7218, April 2014.
11.2. Informative References
[I-D.ietf-xmpp-dna]
Saint-Andre, P., Miller, M., and P. Hancke, "Domain Name
Associations (DNA) in the Extensible Messaging and
Presence Protocol (XMPP)", draft-ietf-xmpp-dna-10 (work in
progress), March 2015.
[RFC3403] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part Three: The Domain Name System (DNS) Database", RFC
3403, October 2002.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011.
[RFC6186] Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186, March 2011.
Appendix A. Examples
In the following, most of the DNS resource data is elided for
simplicity.
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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 received for addresses at example.com are retrieved via
IMAP at imap.example.net. Connections to imap.example.net port 9143
that use STARTTLS will get a server certificate that authenticates
the name imap.example.net.
A.2. XMPP
; XMPP domain
_xmpp-client._tcp.example.com. SRV 1 0 5222 im.example.net.
_xmpp-client._tcp.example.com. RRSIG SRV ...
; target server 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 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.
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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
target server 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 connection endpoint 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
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
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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.
Appendix C. 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 Stephane Bortzmeyer,
James Cloos, Viktor Dukhovni, Ned Freed, Olafur Gudmundsson, Paul
Hoffman, Phil Pennock, Hector Santos, Jonas Schneider, and Alessandro
Vesely for helpful suggestions.
Carl Wallace completed an insightful review on behalf of the Security
Directorate.
Ben Campbell, Brian Haberman, and Alvaro Retana provided helpful
feedback during IESG review.
The authors gratefully acknowledge the assistance of Olafur
Gudmundsson and Warren Kumari as the working group chairs and Stephen
Farrell as the sponsoring Area Director.
Peter Saint-Andre wishes to acknowledge Cisco Systems, Inc., for
employing him during his work on earlier versions of this document.
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/
Finch, et al. Expires October 25, 2015 [Page 14]
Internet-Draft TLSA and SRV April 2015
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/
Finch, et al. Expires October 25, 2015 [Page 15]
