DNS-Based Authentication of Named Entities (DANE) T. Finch
Internet-Draft University of Cambridge
Intended status: Standards Track M. Miller
Expires: August 15, 2014 Cisco Systems, Inc.
P. Saint-Andre
&yet
February 11, 2014

Using DNS-Based Authentication of Named Entities (DANE) TLSA records with SRV and MX records.
draft-ietf-dane-srv-04

Abstract

The DANE specification (RFC 6698) describes how to use TLSA resource records in the DNS to associate a server's host name with its TLS certificate. 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.

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/.

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This Internet-Draft will expire on August 15, 2014.

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Table of Contents

1. Introduction

The base DANE specification [RFC6698] describes how to use TLSA resource records in the DNS to associate a server's host name with its TLS certificate. The association is secured using DNSSEC. That document "only relates to securely associating certificates for TLS and DTLS with host names" (see the last paragraph of section 1.2 of [RFC6698]).

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 [RFC2782], or MX records in the case of SMTP [RFC5321]. (Note: in the "CertID" specification [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 [RFC6698] do not directly apply to protocols that use SRV or MX records.

This document describes how to use DANE TLSA records with SRV and MX records. To summarize:

Separate documents give the details that are specific to particular application protocols, such as SMTP [I-D.ietf-dane-smtp-with-dane] and XMPP [I-D.ietf-xmpp-dna].

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].

3. Relation between SRV and MX records

For the purpose of this specification (to avoid cluttering the description with special cases) we treat each MX record ([RFC5321] section 5) as being equivalent to an SRV record [RFC2782] with corresponding fields copied from the MX record and the remaining fields having fixed values as follows:

Table 1: SRV Fields and MX Equivalents

   +---------------+-----------------------------+
   | 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                   |  
   +---------------+-----------------------------+
      

Thus we can treat the following MX record as if it were the SRV record shown below:

   example.com.            86400 IN MX  10      mx.example.net.

   _smtp._tcp.example.com. 86400 IN SRV 10 0 25 mx.example.net.
     

Other details that are specific to SMTP are described in [I-D.ietf-dane-smtp-with-dane].

4. DNS Checks for TLSA and SRV Records

4.1. SRV Query

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).

For this specification to apply, all of these DNS RRsets MUST be "secure" according to DNSSSEC validation ([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 [RFC4035] section 3.2.3.)

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.)

If any of the responses is "bogus" according to DNSSEC validation, the client MUST abort. (This usually corresponds to a "server failure" response.)

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 Section 5).

4.2. TLSA Queries

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.

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.

The client SHALL construct the TLSA query name as described in [RFC6698] section 3, based on fields from the SRV record: the port from the SRV RDATA, the protocol from the SRV query name, and the TLSA base domain set to the SRV target host name.

_imap._tcp.example.com. 86400 IN SRV 10 0 143 imap.example.net.

_143._tcp.imap.example.net. IN TLSA ?
     

For example, the following SRV record leads to the TLSA query shown below:

The client SHALL determine if the TLSA record(s) are usable according to section 4.1 of [RFC6698]. This affects SRV handling as follows:

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 Section 5.

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.

If the TLSA response is "bogus", then the client MUST NOT connect to the corresponding server. (The client can still use other SRV targets.)

5. TLS Checks for TLSA and SRV Records

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 [RFC5280] or protocol-specific rules (e.g., following [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 [RFC6698] section 2.1.

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.).

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) [RFC6066] or its functional equivalent in the relevant application protocol (e.g., in XMPP [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 [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.)

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.
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.

(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.)

6. Guidance for Application Protocols

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:

7. Guidance for Server Operators

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:

Servers that support multiple service domains (i.e., multi-tenant) can implement Server Name Identifier (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 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.

8. Internationalization Considerations

If any of the DNS queries are for an internationalized domain name, then they need to use the A-label form [RFC5890].

9. IANA Considerations

No IANA action is required.

10. Security Considerations

10.1. Mixed Security Status

We do not specify that clients checking all of a service domain's server host names are consistent in whether they have or do not have TLSA records. This is so that partial or incremental deployment does not break the service. Different levels of deployment are likely if a service domain has a third-party fallback server, for example.

The SRV 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.

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, and publishing a TLSA record under that name. If these are not correct then connections from TLSA-aware clients might fail.

10.3. Certificate Subject Name Matching

Section 4 of the TLSA specification [RFC6698] leaves the details of checking names in certificates to higher level application protocols, though it suggests the use of [RFC6125].

Name checks are not necessary if the matching TLSA record is of Certificate Usage "3". Because such a record identifies the specific certificate (or public key of the certificate), additional checks are superfluous and potentially conflicting.

Otherwise, while DNSSEC provides a secure binding between the server name and the TLSA record, and the TLSA record provides a binding to a certificate, this latter step can be indirect via a chain of certificates. For example, a Certificate Usage "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.

11. Acknowledgements

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.

12. References

12.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2782] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, March 2005.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R. and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, October 2008.
[RFC5890] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, August 2010.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, January 2011.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Core", RFC 6120, March 2011.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, March 2011.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA", RFC 6698, August 2012.

12.2. Informative References

[I-D.ietf-dane-smtp-with-dane] Dukhovni, V. and W. Hardaker, "SMTP security via opportunistic DANE TLS", Internet-Draft draft-ietf-dane-smtp-with-dane-05, February 2014.
[I-D.ietf-xmpp-dna] Saint-Andre, P. and M. Miller, "Domain Name Associations (DNA) in the Extensible Messaging and Presence Protocol (XMPP)", Internet-Draft draft-ietf-xmpp-dna-05, February 2014.

Appendix A. Mail Example

In the following, most of the DNS resource data is elided for simplicity.

; 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 ...
    

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.

Appendix B. XMPP Example

In the following, most of the DNS resource data is elided for simplicity.

; 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 ...
    

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.

Appendix C. Rationale

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.

There are a number of other reasons for doing it this way:

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.

The disadvantage is that clients which do not do DNSSEC validation must, according to [RFC6125] rules, check the server certificate against the service domain, since they have no other way to authenticate the server. This means that SNI support or its functional equivalent is necessary for backward compatibility.

Authors' Addresses

Tony Finch University of Cambridge Computing Service New Museums Site Pembroke Street Cambridge, CB2 3QH ENGLAND Phone: +44 797 040 1426 EMail: dot@dotat.at URI: http://dotat.at/
Matthew Miller Cisco Systems, Inc. 1899 Wynkoop Street, Suite 600 Denver, CO 80202 USA EMail: mamille2@cisco.com
Peter Saint-Andre &yet EMail: ietf@stpeter.im