< draft-ietf-dane-srv-08.txt   draft-ietf-dane-srv-09.txt >
DNS-Based Authentication of Named Entities (DANE) T. Finch DNS-Based Authentication of Named Entities (DANE) T. Finch
Internet-Draft University of Cambridge Internet-Draft University of Cambridge
Intended status: Standards Track M. Miller Intended status: Standards Track M. Miller
Expires: April 24, 2015 Cisco Systems, Inc. Expires: August 17, 2015 Cisco Systems, Inc.
P. Saint-Andre P. Saint-Andre
&yet &yet
October 21, 2014 February 13, 2015
Using DNS-Based Authentication of Named Entities (DANE) TLSA Records Using DNS-Based Authentication of Named Entities (DANE) TLSA Records
with SRV Records with SRV Records
draft-ietf-dane-srv-08 draft-ietf-dane-srv-09
Abstract Abstract
The DANE specification (RFC 6698) describes how to use TLSA resource 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 records secured by DNSSEC (RFC 4033) to associate a server's
certificate, where the association is secured with DNSSEC. However, connection endpoint with its TLS certificate. However, application
application protocols that use SRV records (RFC 2782) to indirectly protocols that use SRV records (RFC 2782) to indirectly name the
name the target server host names for a service domain cannot apply target server connection endpoints for a service domain cannot apply
the rules from RFC 6698. Therefore this document provides guidelines the rules from RFC 6698. Therefore this document provides guidelines
that enable such protocols to locate and use TLSA records. that enable such protocols to locate and use TLSA records.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 24, 2015. This Internet-Draft will expire on August 17, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. DNS Checks . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. DNS Checks . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. SRV Query . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. SRV Query . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Address Queries . . . . . . . . . . . . . . . . . . . . . 4 3.2. Address Queries . . . . . . . . . . . . . . . . . . . . . 4
3.3. TLSA Queries . . . . . . . . . . . . . . . . . . . . . . 4 3.3. TLSA Queries . . . . . . . . . . . . . . . . . . . . . . 5
3.4. Impact on TLS Usage . . . . . . . . . . . . . . . . . . . 5 3.4. Impact on TLS Usage . . . . . . . . . . . . . . . . . . . 5
4. TLS Checks . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. TLS Checks . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. SRV Records Only . . . . . . . . . . . . . . . . . . . . 5 4.1. SRV Records Only . . . . . . . . . . . . . . . . . . . . 6
4.2. TLSA Records . . . . . . . . . . . . . . . . . . . . . . 6 4.2. TLSA Records . . . . . . . . . . . . . . . . . . . . . . 7
5. Guidance for Protocol Authors . . . . . . . . . . . . . . . . 6 5. Guidance for Protocol Authors . . . . . . . . . . . . . . . . 7
6. Guidance for Server Operators . . . . . . . . . . . . . . . . 7 6. Guidance for Server Operators . . . . . . . . . . . . . . . . 7
7. Guidance for Application Developers . . . . . . . . . . . . . 8 7. Guidance for Application Developers . . . . . . . . . . . . . 8
8. Internationalization Considerations . . . . . . . . . . . . . 8 8. Internationalization Considerations . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10. Security Considerations . . . . . . . . . . . . . . . . . . . 8 10. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10.1. Mixed Security Status . . . . . . . . . . . . . . . . . 8 10.1. Mixed Security Status . . . . . . . . . . . . . . . . . 9
10.2. A Service Domain Trusts its Servers . . . . . . . . . . 8 10.2. Certificate Subject Name Matching . . . . . . . . . . . 9
10.3. Certificate Subject Name Matching . . . . . . . . . . . 9 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9 11.1. Normative References . . . . . . . . . . . . . . . . . . 9
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 11.2. Informative References . . . . . . . . . . . . . . . . . 10
12.1. Normative References . . . . . . . . . . . . . . . . . . 9
12.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 11 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 11
A.1. IMAP . . . . . . . . . . . . . . . . . . . . . . . . . . 11 A.1. IMAP . . . . . . . . . . . . . . . . . . . . . . . . . . 11
A.2. XMPP . . . . . . . . . . . . . . . . . . . . . . . . . . 11 A.2. XMPP . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Appendix B. Rationale . . . . . . . . . . . . . . . . . . . . . 12 Appendix B. Rationale . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction 1. Introduction
The base DANE specification [RFC6698] describes how to use TLSA The base DANE specification [RFC6698] describes how to use TLSA
resource records in the DNS to associate a server's host name with resource records secured by DNSSEC [RFC4033] to associate a target
its TLS certificate, where the association is secured using DNSSEC. server's connection endpoint with its TLS certificate. Some
That document "only relates to securely associating certificates for application protocols locate connection endpoints indirectly via SRV
TLS and DTLS with host names" (see the last paragraph of section 1.2 records [RFC2782]. As a result of this indirection, the rules
of [RFC6698]). specified in [RFC6698] cannot be directly applied to such application
protocols. (Rules for SMTP [RFC5321], which uses MX resource records
Some application protocols do not use host names directly; instead, instead of SRV records, are described in
they use a service domain, and the relevant target server host names [I-D.ietf-dane-smtp-with-dane].)
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 This document describes how to use DANE TLSA records with SRV
records. To summarize: records. To summarize:
o We rely on DNSSEC to secure the association between the service o We rely on DNSSEC to secure SRV records that map the desired
domain and the target server host names (i.e., the host names that service, transport protocol, and service domain to the
are discovered by the SRV query). 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 The TLSA records are located using the port, protocol, and target o The TLSA records for each connection endpoint are located using
server host name fields (not the service domain). the transport protocol, port number, and host name for the target
server (not the service domain).
o Clients always use TLS when connecting to servers with TLSA o When DNSSEC-validated TLSA records are published for a particular
records. connection endpoint, clients always use TLS when connecting (even
if the connection endpoint supports cleartext communication).
o Assuming that the association is secure, the server's certificate o If there is at least one usable TLSA record, the connection
is expected to authenticate the target server host name, rather endpoint's TLS certificate or public key needs to match at least
than the service domain. one of those usable TLSA records.
Note: The "CertID" specification [RFC6125] does not use the terms o If there are no usable TLSA records, the target server host name
"service domain" and "target server host name", but refers to the is used as one of the acceptable reference identifiers, as
same entities with the terms "source domain" and "derived domain". 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].
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this memo are to be interpreted as described in "OPTIONAL" in this memo are to be interpreted as described in
[RFC2119]. [RFC2119].
This draft uses the definitions for "secure", "insecure", "bogus", This draft uses the definitions for "secure", "insecure", "bogus",
and "indeterminate" from [RFC4035]. This draft uses the acronyms and "indeterminate" from [RFC4035]. This draft uses the acronyms
from [RFC7218] for the values of TLSA fields where appropriate. 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. DNS Checks
3.1. SRV Query 3.1. SRV Query
When the client makes an SRV query, a successful result will 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 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 of CNAME / DNAME aliases leading to such a list).
to be aware that unsuccessful results can occur because of various
DNS-related errors; a helpful summary can be found in section 2.1 of NOTE: Implementers need to be aware that unsuccessful results can
[I-D.ietf-dane-smtp-with-dane]. 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 DNS RRset that is For this specification to apply, the entire DNS RRset that is
returned MUST be "secure" according to DNSSSEC validation ([RFC4033] returned MUST be "secure" according to DNSSEC validation (Section 5
section 5). In the case of aliases, the whole chain of CNAME and of [RFC4035]). In the case where the answer is obtained via a chain
DNAME RRsets MUST be secure as well. This corresponds to the AD bit of CNAME and/or DNAME aliases, the whole chain of CNAME and DNAME
being set in the response(s); see [RFC4035] section 3.2.3. RRsets MUST also be secure.
If the the entire RRset is "insecure", this protocol has not been If the lookup result is "insecure" (or no SRV records are located),
correctly deployed. The client SHOULD fall back to its non-DNSSEC, this protocol does not apply and the client SHOULD fall back to its
non-DANE behavior (this corresponds to the AD bit being unset). If non-DNSSEC, non-DANE (and possibly non-SRV) behavior. If the SRV
the entire RRset is "bogus", the client MUST abort the attempt. lookup fails because the RRset is "bogus", the client MUST abort its
attempt to connect to the desired service.
In the successful case, the client now has an authentic list of When the lookup returns a "secure" RRset (possibly via a chain of
target server host names with weight and priority values. It "secure" CNAME/DNAME records), the client now has an authentic list
performs server ordering and selection using the weight and priority of target server connection endpoints with weight and priority
values without regard to the presence or absence of DNSSEC or TLSA values. It performs server ordering and selection using the weight
records. It also takes note of the DNSSEC validation status of the and priority values without regard to the presence or absence of
SRV response for use when checking certificate names (see Section 4). DNSSEC or TLSA records. It also takes note of the DNSSEC validation
The client can now proceed to making address queries on the target status of the SRV response for use when checking certificate names
server host names as described in the next section. (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 3.2. Address Queries
For each SRV target server host name, the client makes A and AAAA For each SRV target server connnection endpoint, the client makes A
queries, performs DNSSEC validation on the address (A or AAAA) and/or AAAA queries, performs DNSSEC validation on the address (A or
response, and continues as follows based on the results: AAAA) response, and continues as follows based on the results:
o If the response is "secure" and usable, the client MUST perform a o If either the A or AAAA RRSets are "secure", the client MUST
TLSA query for that target server host name as described in the perform a TLSA query for that target server connection endpoint as
next section. described in the next section.
o If the response is "insecure", the client MUST NOT perform a TLSA o If both RRsets are "insecure", the client MUST NOT perform a TLSA
query for that target server host name; the TLSA query will most query for that target server connection endpoint; the TLSA query
likely fail. will most likely fail or produce spurious results.
o If the response is "bogus" or "indeterminate", the client MUST NOT o If the address record lookup fails (this a validation status of
connect to this target server; instead it uses the next most either "bogus" or "indeterminate"), the client MUST NOT connect to
appropriate SRV target. this connection endpoint; instead it uses the next most
appropriate SRV target. This mitigates against downgrade attacks.
3.3. TLSA Queries 3.3. TLSA Queries
The client SHALL construct the TLSA query name as described in The client SHALL construct the TLSA query name as described in
[RFC6698] section 3, based on fields from the SRV record: the port Section 3 of [RFC6698], based on the fields from the SRV record: the
from the SRV RDATA, the protocol from the SRV query name, and the port number from the SRV RDATA, the transport protocol from the SRV
TLSA base domain set to the SRV target server host name. query name, and the TLSA base domain from the SRV target server host
name.
For example, the following SRV record for IMAP (see [RFC6186]) leads For example, the following SRV record for IMAP (see [RFC6186]):
to the TLSA query shown below:
_imap._tcp.example.com. 86400 IN SRV 10 0 9143 imap.example.net. _imap._tcp.example.com. 86400 IN SRV 10 0 9143 imap.example.net.
_9143._tcp.imap.example.net. IN TLSA ? leads to the TLSA query shown below:
_9143._tcp.imap.example.net. IN TLSA ?
3.4. Impact on TLS Usage 3.4. Impact on TLS Usage
The client SHALL determine if the TLSA record(s) returned in the The client SHALL determine if the TLSA records returned in the
previous step are usable according to section 4.1 of [RFC6698]. This previous step are usable according to Section 4.1 of [RFC6698]. This
affects the use TLS as follows: affects the use TLS as follows:
o If the TLSA response is "secure" and usable, then the client MUST o If the TLSA response is "secure" and usable, then the client MUST
use TLS when connecting to the target server. The TLSA records use TLS when connecting to the target server. The TLSA records
are used when validating the server's certificate as described are used when validating the server's certificate as described in
under Section 4. Section 4.
o If the TLSA response is "insecure", then the client SHALL proceed o If the TLSA lookup fails, then the client SHALL proceed as if the
as if the target server had no TLSA records. It MAY connect to target server had no TLSA records. It MAY connect to the target
the target server with or without TLS, subject to the policies of server with or without TLS, subject to the policies of the
the application protocol or client implementation. application protocol or client implementation.
o If the TLSA response is "bogus" or "indeterminate", then the o If the TLSA response is "bogus" or "indeterminate", then the
client MUST NOT connect to the target server (the client can still client MUST NOT connect to the target server (the client can still
use other SRV targets). use other SRV targets).
4. TLS Checks 4. TLS Checks
When connecting to a server, the client MUST use TLS if the responses 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 to the SRV and TLSA queries were "secure" as described above. The
rules described in the next two sections apply. 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 4.1. SRV Records Only
If the client received zero usable TLSA certificate associations, it If the client received zero usable TLSA certificate associations, it
SHALL validate the server's TLS certificate using the normal PKIX SHALL validate the server's TLS certificate using the normal PKIX
rules [RFC5280] or protocol-specific rules (e.g., following rules [RFC5280] or protocol-specific rules (e.g., following
[RFC6125]) without further input from the TLSA records. [RFC6125]) without further input from the TLSA records. In this
case, the client uses the information in the server certificate and
In this case, the client uses the information in the server the DNSSEC validation status of the SRV query in its authentication
certificate and the DNSSEC validation status of the SRV query in its checks. It SHOULD use the Server Name Indication extension (TLS SNI)
authentication checks. It SHOULD use the Server Name Indication [RFC6066] or its functional equivalent in the relevant application
extension (TLS SNI) [RFC6066] or its functional equivalent in the protocol (e.g., in XMPP [RFC6120] this is the 'to' address of the
relevant application protocol (e.g., in XMPP [RFC6120] this is the initial stream header). The preferred name SHALL be chosen as
'to' address of the initial stream header). The preferred name SHALL follows, and the client SHALL verify the identity asserted by the
be chosen as follows, and the client SHALL verify the identity server's certificate according to Section 6 of [RFC6125], using a
asserted by the server's certificate according to section 6 of list of reference identifiers constructed as follows (note again that
in RFC 6125 the terms "source domain" and "derived domain" to refer
[RFC6125], using a list of reference identifiers constructed as to the same things as "service domain" and "target server host name"
follows (note again that in RFC 6125 the terms "source domain" and in this document). The examples below assume a service domain of
"derived domain" refer to the same things as "service domain" and "im.example.com" and a target server host name of
"target server host name" in this document). The examples below "xmpp23.hosting.example.net".
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 SRV is insecure: The reference identifiers SHALL include the service
domain and MUST NOT include the SRV target server host name (e.g., domain and MUST NOT include the SRV target server host name (e.g.,
include "im.example.com" but not "xmpp23.hosting.example.net"). include "im.example.com" but not "xmpp23.hosting.example.net").
The service domain is the preferred name for TLS SNI or its The service domain is the preferred name for TLS SNI or its
equivalent. equivalent.
SRV is secure: The reference identifiers SHALL include both the SRV is secure: The reference identifiers SHALL include both the
service domain and the SRV target server host name (e.g., include service domain and the SRV target server host name (e.g., include
both "im.example.com" and "xmpp23.hosting.example.net"). The both "im.example.com" and "xmpp23.hosting.example.net"). The
target server host name is the preferred name for TLS SNI or its target server host name is the preferred name for TLS SNI or its
equivalent. equivalent.
In the latter case, the client will accept either identity to ensure In the latter case, the client will accept either identity to ensure
compatibility with servers that support this specification as well as compatibility with servers that support this specification as well as
servers that do not support this specification. servers that do not support this specification.
4.2. TLSA Records 4.2. TLSA Records
If the client received one or more usable TLSA certificate If the client received one or more usable TLSA certificate
associations, it SHALL process them as described in section 2.1 of associations, it SHALL process them as described in Section 2.1 of
[RFC6698]. [RFC6698].
If the TLS server's certificate -- or the public key of the server's If the TLS server's certificate -- or the public key of the server's
certificate -- matches a usable TLSA record with Certificate Usage certificate -- matches a usable TLSA record with Certificate Usage
"DANE-EE", the client MUST consider the server to be authenticated. "DANE-EE", the client MUST ignore expiration checks from [RFC5280]
Because the information in such a TLSA record supersedes the non-key and reference identifier checks from [RFC6125]. The information in
information in the certificate, all other [RFC5280] and [RFC6125] such a TLSA record supersedes the non-key information in the
authentication checks (e.g., reference identifier, key usage, certificate.
expiration, issuance) MUST be ignored or omitted.
5. Guidance for Protocol Authors 5. Guidance for Protocol Authors
This document describes how to use DANE with application protocols in This document describes how to use DANE with application protocols in
which target servers are discovered via SRV records. Although this which target servers are discovered via SRV records. Although this
document attempts to provide generic guidance applying to all such document attempts to provide generic guidance applying to all such
protocols, additional documents for particular application protocols protocols, additional documents for particular application protocols
could cover related topics, such as: could cover related topics, such as:
o Fallback logic in the event that a client is unable to connect o Fallback logic in the event that a client is unable to connect
skipping to change at page 7, line 41 skipping to change at page 8, line 17
that do not support this specification from successfully connecting that do not support this specification from successfully connecting
with TLS. with TLS.
For TLSA records with Certificate Usage types other than "DANE-EE", For TLSA records with Certificate Usage types other than "DANE-EE",
the certificate(s) MUST contain an identifier that matches: the certificate(s) MUST contain an identifier that matches:
o the service domain name (the "source domain" in [RFC6125] terms, o the service domain name (the "source domain" in [RFC6125] terms,
which is the SRV query domain); and/or which is the SRV query domain); and/or
o the target server host name (the "derived domain" in [RFC6125] o the target server host name (the "derived domain" in [RFC6125]
terms, which is the SRV target). terms, which is the SRV target host name).
Servers that support multiple service domains (i.e., so-called Servers that support multiple service domains (i.e., so-called
"multi-tenanted environments") can implement the Transport Layer "multi-tenanted environments") can implement the Transport Layer
Security Server Name Indication (TLS SNI) [RFC6066] or its functional Security Server Name Indication (TLS SNI) [RFC6066] or its functional
equivalent to determine which certificate to offer. Clients that do equivalent to determine which certificate to offer. Clients that do
not support this specification will indicate a preference for the not support this specification will indicate a preference for the
service domain name, while clients that support this specification service domain name, while clients that support this specification
will indicate the target server host name. However, the server will indicate the target server host name. However, the server
determines what certificate to present in the TLS handshake; e.g., determines what certificate to present in the TLS handshake; e.g.,
the presented certificate might only authenticate the target server the presented certificate might only authenticate the target server
host name. host name.
7. Guidance for Application Developers 7. Guidance for Application Developers
Developers of application clients that depend on DANE-SRV often would Developers of application clients that depend on DANE-SRV often would
like to prepare as quickly as possible for making a connection to the 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 intended service, thus reducing the wait time for end users. To make
this optimization possible, a DNS library might perform the SRV this optimization possible, a DNS library might perform the SRV
queries, address queries, and TLSA queries in parallel (because a queries, address queries, and TLSA queries in parallel. (Because a
TLSA record can be ignored if it turns out that the address record on 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 which it depends is not secure, performing the TLSA queries in
parallel with the SRV queries and address queries is not harmful from parallel with the SRV queries and address queries is not harmful from
a security perspective and can yield some operational benefits). a security perspective and can yield some operational benefits.)
8. Internationalization Considerations 8. Internationalization Considerations
If any of the DNS queries are for an internationalized domain name, If any of the DNS queries are for an internationalized domain name,
then they need to use the A-label form [RFC5890]. then they need to use the A-label form [RFC5890].
9. IANA Considerations 9. IANA Considerations
No IANA action is required. No IANA action is required.
10. Security Considerations 10. Security Considerations
10.1. Mixed Security Status 10.1. Mixed Security Status
We do not specify that clients checking all of a service domain's We do not specify that all of the target server connection endpoints
target server host names are consistent in whether they have or do 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 not have TLSA records. This is so that partial or incremental
deployment does not break the service. Different levels of deployment does not break the service. Different levels of
deployment are likely if a service domain has a third-party fallback deployment are likely if a service domain has a third-party fallback
server, for example. server, for example.
The SRV sorting rules are unchanged; in particular they have not been The SRV sorting rules are unchanged; in particular they have not been
altered in order to prioritize secure servers over insecure servers. altered in order to prioritize secure connection endpoints over
If a site wants to be secure it needs to deploy this protocol insecure connection endpoints. If a site wants to be secure it needs
completely; a partial deployment is not secure and we make no special to deploy this protocol completely; a partial deployment is not
effort to support it. 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.
10.3. Certificate Subject Name Matching 10.2. Certificate Subject Name Matching
Section 4 of the TLSA specification [RFC6698] leaves the details of Section 4 of the TLSA specification [RFC6698] leaves the details of
checking names in certificates to higher level application protocols, checking names in certificates to higher level application protocols,
though it suggests the use of [RFC6125]. though it suggests the use of [RFC6125].
Name checks are not necessary if the matching TLSA record is of Name checks are not necessary if the matching TLSA record is of
Certificate Usage "DANE-EE". Because such a record identifies the Certificate Usage "DANE-EE". Because such a record identifies the
specific certificate (or public key of the certificate), additional specific certificate (or public key of the certificate), additional
checks are superfluous and potentially conflicting. checks are superfluous and potentially conflicting.
Otherwise, while DNSSEC provides a secure binding between the server Otherwise, while DNSSEC provides a secure binding between the server
name and the TLSA record, and the TLSA record provides a binding to a 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 certificate, this latter step can be indirect via a chain of
certificates. For example, a Certificate Usage "PKIX-TA" TLSA record certificates. For example, a Certificate Usage "PKIX-TA" TLSA record
only authenticates the CA that issued the certificate, and third only authenticates the CA that issued the certificate, and third
parties can obtain certificates from the same CA. Therefore, clients parties can obtain certificates from the same CA. Therefore, clients
need to check whether the server's certificate matches one of the need to check whether the server's certificate matches one of the
expected reference identifiers to ensure that the certificate was expected reference identifiers to ensure that the certificate was
issued by the CA to the server the client expects. issued by the CA to the server the client expects.
11. Acknowledgements 11. References
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 11.1. Normative References
12.1. 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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782, specifying the location of services (DNS SRV)", RFC 2782,
February 2000. February 2000.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC Rose, "DNS Security Introduction and Requirements", RFC
skipping to change at page 10, line 10 skipping to change at page 10, line 25
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005. Extensions", RFC 4035, March 2005.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008. (CRL) Profile", RFC 5280, May 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC5890] Klensin, J., "Internationalized Domain Names for [RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework", Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010. RFC 5890, August 2010.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011. 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 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509 within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer (PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011. Security (TLS)", RFC 6125, March 2011.
[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 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS) of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012. Protocol: TLSA", RFC 6698, August 2012.
[RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify [RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify
Conversations about DNS-Based Authentication of Named Conversations about DNS-Based Authentication of Named
Entities (DANE)", RFC 7218, April 2014. Entities (DANE)", RFC 7218, April 2014.
12.2. Informative References 11.2. Informative References
[I-D.ietf-dane-smtp-with-dane] [I-D.ietf-dane-smtp-with-dane]
Dukhovni, V. and W. Hardaker, "SMTP security via Dukhovni, V. and W. Hardaker, "SMTP security via
opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-05 opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-10
(work in progress), February 2014. (work in progress), May 2014.
[I-D.ietf-xmpp-dna] [I-D.ietf-xmpp-dna]
Saint-Andre, P. and M. Miller, "Domain Name Associations Saint-Andre, P. and M. Miller, "Domain Name Associations
(DNA) in the Extensible Messaging and Presence Protocol (DNA) in the Extensible Messaging and Presence Protocol
(XMPP)", draft-ietf-xmpp-dna-05 (work in progress), (XMPP)", draft-ietf-xmpp-dna-05 (work in progress),
February 2014. February 2014.
[RFC3403] Mealling, M., "Dynamic Delegation Discovery System (DDDS) [RFC3403] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part Three: The Domain Name System (DNS) Database", RFC Part Three: The Domain Name System (DNS) Database", RFC
3403, October 2002. 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 Appendix A. Examples
In the following, most of the DNS resource data is elided for In the following, most of the DNS resource data is elided for
simplicity. simplicity.
A.1. IMAP A.1. IMAP
; mail domain ; mail domain
_imap._tcp.example.com. SRV 10 0 9143 imap.example.net. _imap._tcp.example.com. SRV 10 0 9143 imap.example.net.
example.com. RRSIG SRV ... example.com. RRSIG SRV ...
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imap.example.net. A 192.0.2.1 imap.example.net. A 192.0.2.1
imap.example.net. RRSIG A ... imap.example.net. RRSIG A ...
imap.example.net. AAAA 2001:db8:212:8::e:1 imap.example.net. AAAA 2001:db8:212:8::e:1
imap.example.net. RRSIG ... imap.example.net. RRSIG ...
; TLSA resource record ; TLSA resource record
_9143._tcp.imap.example.net. TLSA ... _9143._tcp.imap.example.net. TLSA ...
_9143._tcp.imap.example.net. RRSIG TLSA ... _9143._tcp.imap.example.net. RRSIG TLSA ...
Mail messages submitted for addresses at example.com are sent via Mail messages received for addresses at example.com are retrieved via
IMAP to imap.example.net. Connections to imap.example.net port 9143 IMAP at imap.example.net. Connections to imap.example.net port 9143
that use STARTTLS will get a server certificate that authenticates that use STARTTLS will get a server certificate that authenticates
the name imap.example.net. the name imap.example.net.
A.2. XMPP A.2. XMPP
; XMPP domain ; XMPP domain
_xmpp-client.example.com. SRV 1 0 5222 im.example.net. _xmpp-client.example.com. SRV 1 0 5222 im.example.net.
_xmpp-client.example.com. RRSIG SRV ... _xmpp-client.example.com. RRSIG SRV ...
; target server host name ; target server host name
skipping to change at page 12, line 24 skipping to change at page 12, line 41
service domain, since this is more convenient for servers hosting service domain, since this is more convenient for servers hosting
multiple domains (so-called "multi-tenanted environments") and scales multiple domains (so-called "multi-tenanted environments") and scales
up more easily to larger numbers of service domains. up more easily to larger numbers of service domains.
There are a number of other reasons for doing it this way: There are a number of other reasons for doing it this way:
o The certificate is part of the server configuration, so it makes o The certificate is part of the server configuration, so it makes
sense to associate it with the server host name rather than the sense to associate it with the server host name rather than the
service domain. service domain.
o In the absence of TLS SNI, if the certificate identifies the host o In the absence of TLS SNI, if the certificate identifies the
name then it does not need to list all the possible service target server host name then it does not need to list all the
domains. possible service domains.
o When the server certificate is replaced it is much easier if there 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 is one part of the DNS that needs updating to match, instead of an
unbounded number of hosted service domains. unbounded number of hosted service domains.
o The same TLSA records work with this specification, and with o The same TLSA records work with this specification, and with
direct connections to the host name in the style of [RFC6698]. direct connections to the connection endpoint in the style of
[RFC6698].
o Some application protocols, such as SMTP, allow a client to o Some application protocols, such as SMTP, allow a client to
perform transactions with multiple service domains in the same perform transactions with multiple service domains in the same
connection. It is not in general feasible for the client to connection. It is not in general feasible for the client to
specify the service domain using TLS SNI when the connection is specify the service domain using TLS SNI when the connection is
established, and the server might not be able to present a established, and the server might not be able to present a
certificate that authenticates all possible service domains. See certificate that authenticates all possible service domains. See
[I-D.ietf-dane-smtp-with-dane] for details. [I-D.ietf-dane-smtp-with-dane] for details.
o It is common for SMTP servers to act in multiple roles, for o It is common for SMTP servers to act in multiple roles, for
skipping to change at page 13, line 17 skipping to change at page 13, line 40
complicated criteria for when service domain TLSA records might be complicated criteria for when service domain TLSA records might be
used and when they cannot. This is all avoided by putting the TLSA used and when they cannot. This is all avoided by putting the TLSA
records under the target server host name. records under the target server host name.
The disadvantage is that clients which do not complete DNSSEC The disadvantage is that clients which do not complete DNSSEC
validation must, according to [RFC6125] rules, check the server validation must, according to [RFC6125] rules, check the server
certificate against the service domain, since they have no other way certificate against the service domain, since they have no other way
to authenticate the server. This means that SNI support or its to authenticate the server. This means that SNI support or its
functional equivalent is necessary for backward compatibility. 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.
Authors' Addresses Authors' Addresses
Tony Finch Tony Finch
University of Cambridge Computing Service University of Cambridge Computing Service
New Museums Site New Museums Site
Pembroke Street Pembroke Street
Cambridge CB2 3QH Cambridge CB2 3QH
ENGLAND ENGLAND
Phone: +44 797 040 1426 Phone: +44 797 040 1426
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