One document matched: draft-ietf-dane-srv-04.xml
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<rfc ipr="trust200902"
category="std"
docName="draft-ietf-dane-srv-04">
<front>
<title abbrev="TLSA and SRV">
Using DNS-Based Authentication of Named Entities (DANE)
TLSA records with SRV and MX 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>ietf@stpeter.im</email>
</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. The association is secured with DNSSEC. Some
application protocols use SRV records (RFC 2782) to indirectly
name the server hosts for a service domain (SMTP uses MX records
for the same purpose). This specification gives generic
instructions for how these application protocols locate and use
TLSA records when technologies such as SRV records are used. Separate
documents give the details that are specific to particular application
protocols.</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. 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 host names are located
indirectly via SRV records <xref target="RFC2782"/>, or MX records
in the case of SMTP <xref target="RFC5321"/>. (Note: in the "CertID"
specification <xref target='RFC6125'/>, the source domain and host name
are referred to as the "source domain" and the "derived domain".)
Because of this intermediate resolution step, the normal DANE rules
specified in <xref target="RFC6698"/> do not directly apply to
protocols that use SRV or MX records.</t>
<t>This document describes how to use DANE TLSA records with SRV
and MX 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 or MX query).</t>
<t>The TLSA records are located using the port, protocol, and
target 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>Separate documents give the details that are specific to
particular application protocols, such as
SMTP <xref target="I-D.ietf-dane-smtp-with-dane"/> and
XMPP <xref target='I-D.ietf-xmpp-dna'/>.</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>
</section>
<section title="Relation between SRV and MX records">
<t>For the purpose of this specification (to avoid cluttering the
description with special cases) we treat each MX record
(<xref target="RFC5321"/> section 5) as being equivalent to an SRV
record <xref target="RFC2782"/> with corresponding fields copied
from the MX record and the remaining fields having fixed values
as follows:</t>
<figure>
<preamble>Table 1: SRV Fields and MX Equivalents</preamble>
<artwork><![CDATA[
+---------------+-----------------------------+
| DNS SRV Field | Equivalent MX Value |
+---------------+-----------------------------+
| Service | smtp |
+---------------+-----------------------------+
| Proto | tcp |
+---------------+-----------------------------+
| Name | MX owner name (mail domain) |
+---------------+-----------------------------+
| TTL | MX TTL |
+---------------+-----------------------------+
| Class | MX Class |
+---------------+-----------------------------+
| Priority | MX Priority |
+---------------+-----------------------------+
| Weight | 0 |
+---------------+-----------------------------+
| Port | 25 |
+---------------+-----------------------------+
| Target | MX Target |
+---------------+-----------------------------+
]]></artwork>
</figure>
<t>Thus we can treat the following MX record as if it were the SRV record shown below:</t>
<t><figure><artwork><![CDATA[
example.com. 86400 IN MX 10 mx.example.net.
_smtp._tcp.example.com. 86400 IN SRV 10 0 25 mx.example.net.
]]></artwork></figure></t>
<t>Other details that are specific to SMTP are described in
<xref target="I-D.ietf-dane-smtp-with-dane"/>.</t>
</section>
<section anchor="dns" title="DNS Checks for TLSA and SRV Records">
<section title="SRV Query">
<t>When the client makes an SRV query, a successful result will be
a list of one or more SRV records (or possibly a chain of
CNAME / DNAME aliases referring to such a list).</t>
<t>For this specification to apply, all of these DNS RRsets
MUST be "secure" according to DNSSSEC validation
(<xref target="RFC4033"/> section 5). In the case of aliases, the
whole chain MUST be secure as well as the ultimate target. (This
corresponds to the AD bit being set in the response(s) - see
<xref target="RFC4035"/> section 3.2.3.)</t>
<t>If they are not all secure, this protocol has not been fully
deployed. The client SHOULD fall back to its non-DNSSEC non-DANE
behavior. (This corresponds to the AD bit being unset.)</t>
<t>If any of the responses is "bogus" according to DNSSEC
validation, the client MUST abort. (This usually corresponds to a
"server failure" response.)</t>
<t>In the successful case, the client now has an authentic list of
server host names with weight and priority values. It performs
server ordering and selection using the weight and priority
values without regard to the presence or absence of DNSSEC or
TLSA records. It takes note of the DNSSEC validation status of
the SRV response for use when checking certificate names (see
<xref target="tls"/>).</t>
</section>
<section title="TLSA Queries">
<t>If the SRV response was insecure or indeterminate, the client
MUST NOT perform any TLSA queries. If the SRV response is secure
according to DNSSEC validation, the client performs a TLSA query
for each SRV target as describes in this section.</t>
<t>For each SRV target host name, if the response to the address
(A or AAAA) query is insecure or indeterminate, the client MUST
NOT perform a TLSA query for that target; the TLSA a query will
most likely fail.</t>
<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 host
name.</t>
<t>For example, the following SRV record leads to the TLSA
query shown below:
<figure><artwork><![CDATA[
_imap._tcp.example.com. 86400 IN SRV 10 0 143 imap.example.net.
_143._tcp.imap.example.net. IN TLSA ?
]]></artwork></figure></t>
<t>The client SHALL determine if the TLSA record(s) are usable
according to section 4.1 of <xref target="RFC6698"/>. This
affects SRV handling as follows:</t>
<t>If the TLSA response is "secure", the client MUST use TLS when
connecting to the server. The TLSA records are used when
validating the server's certificate as described under
<xref target="tls"/>.</t>
<t>If the TLSA response is "insecure" or "indeterminate", the
client SHALL proceed as if this server has no TLSA records. It
MAY connect to the server with or without TLS.</t>
<t>If the TLSA response is "bogus", then the client MUST NOT
connect to the corresponding server. (The client can still use
other SRV targets.)</t>
</section>
</section>
<section anchor="tls" title="TLS Checks for TLSA and SRV Records">
<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. 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. If the client received one or more
usable TLSA certificate associations, it SHALL process them as
described in <xref target="RFC6698"/> section 2.1.</t>
<t>If a usable TLSA record with Certificate Usage "3" matches the
TLS server's certificate, or public key for the certificate, all
other validation and verification checks MAY be ignored (e.g.,
reference identifier, key usage, expiration, issuance, etc.).</t>
<t>Otherwise, the client uses the DNSSEC validation status of the
SRV query in its server certificate identity 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 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 <xref target="RFC6125"/> section 6, 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 host name" in this document.)
<list style="hanging">
<t hangText="SRV is insecure or indeterminate:">
The reference identifiers SHALL include the service domain and
MUST NOT include the SRV target host name. The service domain is
the preferred name for TLS SNI or its equivalent.</t>
<t hangText="SRV is secure:">
The reference identifiers SHALL include both the service domain
and the SRV target host name. The target host name is the
preferred name for TLS SNI or its equivalent.</t>
</list></t>
<t>(In the latter case, the client will accept either identity so
that it is compatible with servers that do and do not support this
specification.)</t>
</section>
<section title="Guidance for Application Protocols">
<t>Separate documents describe how to apply this specification to
particular application protocols. If you are writing such as
document the following points ought to be covered:
<list style="symbols">
<t>Fallback logic in the event of bogus replies and the like.</t>
<t>Compatibility with clients that do not support SRV lookups.</t>
</list>
</t>
</section>
<section title="Guidance for Server Operators">
<t>To conform to this specification, the published SRV records and
subsequent address (A, AAAA) records MUST be secured with DNSSEC.
There SHOULD also be at least one TLSA record published that
authenticates the server's certificate. Except for Certificate
Usage "3", the certificate authenticated by the TLSA record(s)
MUST contain a reference 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<vspace blankLines="1"/></t>
<t>the 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.,
multi-tenant) can implement Server Name Identifier (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 server host name. However, the server determines what
certificate to present in the TLS handshake; e.g., the presented
certificate might only authenticate the server host name.</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 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 and MX sorting rules are unchanged; in particular they
have not been altered in order to prioritize secure servers over
insecure servers. If a site wants to be secure it needs to deploy
this protocol completely; a partial deployment is not secure and
we make no special effort to support it.</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, and
publishing a TLSA record under that name. 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 "3". 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 "0" 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 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
server host name is the right thing, and that we ought to rely on
DNSSEC to secure the SRV / MX lookup. Thanks to James Cloos, 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 -->
&rfc6698; <!-- DANE TLSA -->
</references>
<references title="Informative References">
<reference anchor="I-D.ietf-dane-smtp-with-dane">
<front>
<title>SMTP security via opportunistic DANE TLS</title>
<author initials="V" surname="Dukhovni" fullname="Viktor Dukhovni" />
<author initials="W H." surname="Hardaker" fullname="Wes Hardaker" />
<date month="February" year="2014" />
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-dane-smtp-with-dane-05" />
<format type="TXT" target="http://www.ietf.org/internet-drafts/draft-ietf-dane-smtp-with-dane-05"/>
</reference>
<reference anchor='I-D.ietf-xmpp-dna'>
<front>
<title>Domain Name Associations (DNA) in the Extensible Messaging and Presence Protocol (XMPP)</title>
<author initials='P' surname='Saint-Andre' fullname='Peter Saint-Andre'>
<organization />
</author>
<author initials='M' surname='Miller' fullname='Matthew Miller'>
<organization />
</author>
<date month='February' year='2014' />
<abstract><t>This document improves the security of the Extensible Messaging and Presence Protocol (XMPP) in two ways. First, it specifies how "prooftypes" can establish a strong association between a domain name and an XML stream. Second, it describes how to securely delegate a source domain to a derived domain, which is especially important in virtual hosting environments.</t></abstract>
</front>
<seriesInfo name='Internet-Draft' value='draft-ietf-xmpp-dna-05' />
<format type='TXT'
target='http://www.ietf.org/internet-drafts/draft-ietf-xmpp-dna-05.txt' />
</reference>
</references>
<section title="Mail Example">
<t>In the following, most of the DNS resource data is elided for
simplicity.</t>
<t><figure>
<artwork><![CDATA[
; mail domain
example.com. MX 1 mx.example.net.
example.com. RRSIG MX ...
; SMTP server host name
mx.example.net. A 192.0.2.1
mx.example.net. RRSIG A ...
mx.example.net. AAAA 2001:db8:212:8::e:1
mx.example.net. RRSIG ...
; TLSA resource record
_25._tcp.mx.example.net. TLSA ...
_25._tcp.mx.example.net. RRSIG TLSA ...
]]></artwork>
</figure></t>
<t>Mail for addresses at example.com is delivered by SMTP to
mx.example.net. Connections to mx.example.net port 25 that use
STARTTLS will get a server certificate that authenticates the name
mx.example.net.</t>
</section>
<section title="XMPP Example">
<t>In the following, most of the DNS resource data is elided for
simplicity.</t>
<t><figure>
<artwork><![CDATA[
; XMPP domain
_xmpp-client.example.com. SRV 1 0 5222 im.example.net.
_xmpp-clientexample.com. RRSIG SRV ...
; XMPP 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>Mail for addresses at example.com is delivered by SMTP to
mx.example.net. Connections to mx.example.net port 25 that use
STARTTLS will get a server certificate that authenticates the name
mx.example.net.</t>
</section>
<section title="Rationale">
<t>The long-term goal of this specification is to settle on TLS
certificates that verify the server host name rather than the
service domain, since this is more convenient for servers hosting
multiple domains (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.</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.</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 server host name.</t>
<t>The disadvantage is that clients which do not do 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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