One document matched: draft-ietf-dane-srv-08.xml
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<rfc ipr="trust200902" category="std" docName="draft-ietf-dane-srv-08">
<front>
<title abbrev="TLSA and SRV">
Using DNS-Based Authentication of Named Entities (DANE)
TLSA Records with SRV Records
</title>
<author initials="T." surname="Finch" fullname="Tony Finch">
<organization abbrev="University of Cambridge">
University of Cambridge Computing Service
</organization>
<address>
<postal>
<street>New Museums Site</street>
<street>Pembroke Street</street>
<city>Cambridge</city>
<code>CB2 3QH</code>
<country>ENGLAND</country>
</postal>
<phone>+44 797 040 1426</phone>
<email>dot@dotat.at</email>
<uri>http://dotat.at/</uri>
</address>
</author>
<author initials="M." surname="Miller" fullname="Matthew Miller">
<organization>Cisco Systems, Inc.</organization>
<address>
<postal>
<street>1899 Wynkoop Street, Suite 600</street>
<city>Denver</city>
<region>CO</region>
<code>80202</code>
<country>USA</country>
</postal>
<email>mamille2@cisco.com</email>
</address>
</author>
<author initials="P." surname="Saint-Andre" fullname="Peter Saint-Andre">
<organization>&yet</organization>
<address>
<email>peter@andyet.com</email>
<uri>https://andyet.com/</uri>
</address>
</author>
<date/>
<area>Security</area>
<workgroup>DNS-Based Authentication of Named Entities (DANE)</workgroup>
<abstract>
<t>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.</t>
</abstract>
</front>
<middle>
<section anchor="intro" title="Introduction">
<t>The base DANE specification <xref target="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
<xref target="RFC6698"/>).</t>
<t>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 <xref target="RFC2782"/>.
Because of this intermediate resolution step, the normal DANE rules
specified in <xref target="RFC6698"/> cannot be applied to
protocols that use SRV records. (Rules for SMTP
<xref target='RFC5321'/>, which uses MX records instead of SRV records,
are described in <xref target="I-D.ietf-dane-smtp-with-dane"/>.)</t>
<t>This document describes how to use DANE TLSA records with SRV
records. To summarize:
<list style="symbols">
<t>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).</t>
<t>The TLSA records are located using the port, protocol, and
target server host name fields (not the service domain).</t>
<t>Clients always use TLS when connecting to servers with TLSA
records.</t>
<t>Assuming that the association is secure, the server's
certificate is expected to authenticate the target server host
name, rather than the service domain.</t>
</list></t>
<t>Note: The "CertID" specification <xref target='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".</t>
</section>
<section anchor="terms" title="Terminology">
<t>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
<xref target="RFC2119"/>.</t>
<t>This draft uses the definitions for "secure", "insecure", "bogus", and "indeterminate" from <xref target="RFC4035"/>. This draft uses the acronyms from <xref target="RFC7218"/> for the values of TLSA fields where appropriate.</t>
</section>
<section anchor="dns" title="DNS Checks">
<section title="SRV Query">
<t>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
DNS-related errors; a helpful summary can be found in section 2.1 of
<xref target="I-D.ietf-dane-smtp-with-dane"/>.</t>
<t>For this specification to apply, the entire DNS RRset that is
returned MUST be "secure" according to DNSSSEC validation
(<xref target="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
<xref target="RFC4035"/> section 3.2.3.</t>
<t>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.</t>
<t>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
<xref target="tls"/>). The client can now proceed to making address
queries on the target server host names as described in the next
section.</t>
</section>
<section title="Address Queries">
<t>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:
<list style="symbols">
<t>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.</t>
<t>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.</t>
<t>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.</t>
</list></t>
</section>
<section title="TLSA Queries">
<t>The client SHALL construct the TLSA query name as described in
<xref target="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.</t>
<t>For example, the following SRV record for IMAP (see <xref target='RFC6186'/>)
leads to the TLSA query shown below:</t>
<t><figure><artwork><![CDATA[
_imap._tcp.example.com. 86400 IN SRV 10 0 9143 imap.example.net.
_9143._tcp.imap.example.net. IN TLSA ?
]]></artwork></figure></t>
</section>
<section title="Impact on TLS Usage">
<t>The client SHALL determine if the TLSA record(s) returned in
the previous step are usable according to section 4.1 of
<xref target="RFC6698"/>. This affects the use TLS as follows:</t>
<t>
<list style='symbols'>
<t>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 <xref target="tls"/>.</t>
<t>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.</t>
<t>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).</t>
</list>
</t>
</section>
</section>
<section anchor="tls" title="TLS Checks">
<t>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.</t>
<section title="SRV Records Only">
<t>If the client received zero usable TLSA certificate
associations, it SHALL validate the server's TLS certificate using
the normal PKIX rules <xref target="RFC5280"/> or protocol-specific
rules (e.g., following <xref target='RFC6125'/>) without further
input from the TLSA records. </t>
<t>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) <xref target="RFC6066"/> or its functional
equivalent in the relevant application protocol (e.g., in XMPP
<xref target="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 <xref target="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".</t>
<t>
<list style="hanging">
<t hangText="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.</t>
<t hangText="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.</t>
</list>
</t>
<t>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.</t>
</section>
<section title="TLSA Records">
<t>If the client received one or more
usable TLSA certificate associations, it SHALL process them as
described in section 2.1 of <xref target="RFC6698"/>.</t>
<t>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
<xref target="RFC5280"/> and <xref target="RFC6125"/>
authentication checks (e.g., reference identifier, key usage,
expiration, issuance) MUST be ignored or omitted.</t>
</section>
</section>
<section title="Guidance for Protocol Authors">
<t>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:</t>
<t>
<list style="symbols">
<t>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.</t>
<t>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.</t>
<t>Whether the application protocol has a functional equivalent for
TLS SNI that is preferred within that protocol.</t>
<t>Use of SRV records with additional discovery technologies, such as
the use of both SRV records and NAPTR records <xref target='RFC3403'/>
for transport selection in the Session Initiation Protocol (SIP).</t>
</list>
</t>
<t>For example, <xref target='I-D.ietf-xmpp-dna'/> covers such topics
for the Extensible Messaging and Presence Protocol (XMPP).</t>
</section>
<section title="Guidance for Server Operators">
<t>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.</t>
<t>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.</t>
<t>For TLSA records with Certificate Usage types other than
"DANE-EE", the certificate(s) MUST contain an identifier that
matches:
<list style="symbols">
<t>the service domain name (the "source domain" in
<xref target="RFC6125"/> terms, which is the SRV query
domain); and/or</t>
<t>the target server host name (the "derived domain" in
<xref target="RFC6125"/> terms, which is the SRV target).</t>
</list>
</t>
<t>Servers that support multiple service domains (i.e.,
so-called "multi-tenanted environments") can implement the Transport
Layer Security Server Name Indication (TLS SNI)
<xref target="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.</t>
</section>
<section title="Guidance for Application Developers">
<t>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).</t>
</section>
<section title="Internationalization Considerations">
<t>If any of the DNS queries are for an internationalized domain
name, then they need to use the A-label form
<xref target="RFC5890"/>.</t>
</section>
<section title="IANA Considerations">
<t>No IANA action is required.</t>
</section>
<section anchor="security" title="Security Considerations">
<section title="Mixed Security Status">
<t>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.</t>
<t>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.</t>
</section>
<section title="A Service Domain Trusts its Servers">
<t>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.</t>
</section>
<section title="Certificate Subject Name Matching">
<t>Section 4 of the TLSA specification <xref target="RFC6698"/>
leaves the details of checking names in certificates to higher
level application protocols, though it suggests the use of
<xref target="RFC6125"/>.</t>
<t>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.</t>
<t>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.</t>
</section>
</section>
<section title="Acknowledgements">
<t>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.</t>
</section>
</middle>
<back>
<references title="Normative References">
&rfc2119; <!-- MUSTard MAYonnaise -->
&rfc2782; <!-- DNS SRV -->
&rfc4033; <!-- DNSSEC overview -->
&rfc4035; <!-- DNSSEC protocol -->
&rfc5280; <!-- PKIX -->
&rfc5321; <!-- ESMTP -->
&rfc5890; <!-- IDNA -->
&rfc6066; <!-- TLS SNI -->
&rfc6120; <!-- XMPP -->
&rfc6125; <!-- certificate verification -->
&rfc6186; <!-- email submission discovery -->
&rfc6698; <!-- DANE TLSA -->
&rfc7218; <!-- acronymania -->
</references>
<references title="Informative References">
&I-D.ietf-dane-smtp-with-dane;
&I-D.ietf-xmpp-dna;
&rfc3403; <!-- NAPTR -->
</references>
<section title="Examples">
<t>In the following, most of the DNS resource data is elided for
simplicity.</t>
<section title="IMAP">
<t><figure>
<artwork><![CDATA[
; 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 ...
]]></artwork>
</figure></t>
<t>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.</t>
</section>
<section title="XMPP">
<t><figure>
<artwork><![CDATA[
; 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 ...
]]></artwork>
</figure></t>
<t>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.</t>
</section>
</section>
<section title="Rationale">
<t>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.</t>
<t>There are a number of other reasons for doing it this way:
<list style="symbols">
<t>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.</t>
<t>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.</t>
<t>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.
</t>
<t>The same TLSA records work with this specification, and with
direct connections to the host name in the style of
<xref target="RFC6698"/>.</t>
<t>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 <xref target="I-D.ietf-dane-smtp-with-dane"/> for details.
</t>
<t>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 <xref target="I-D.ietf-dane-smtp-with-dane"/>
for details.</t>
</list></t>
<t>This specification does not provide an option to put TLSA
records under the service domain because that would add complexity
without providing any benefit, and security protocols are best
kept simple. As described above, there are real-world cases where
authenticating the service domain cannot be made to work, so there
would be complicated criteria for when service domain TLSA records
might be used and when they cannot. This is all avoided by
putting the TLSA records under the target server host name.</t>
<t>The disadvantage is that clients which do not complete DNSSEC
validation must, according to <xref target="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.</t>
</section>
</back>
</rfc>
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