Signaling Cryptographic Algorithm Understanding in DNSSEC
draft-crocker-dnssec-algo-signal-04

Status of This Memo

This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”

The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html.

This Internet-Draft will expire on April 17, 2010.

Copyright Notice

Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document.

Abstract

The DNS Security Extensions (DNSSEC) was developed to provide origin authentication and integrity protection for DNS data by using digital signatures. These digital signatures can be generated using different algorithms. This draft sets out to specify a way for validating end-system resolvers to signal to a server which cryptographic algorithms they support.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

Table of Contents

1.  Introduction

2.  Signaling Algorithm Understood (AU) Using EDNS

3.  Client Considerations
    3.1.  Recommendations for Stub Clients
    3.2.  Recursive Cache Considerations

4.  Intermediate Middlebox Considerations

5.  Server Considerations

6.  Traffic Analysis Considerations

7.  IANA Considerations

8.  Security Considerations

9.  Normative References

1.  Introduction

The DNS Security Extensions (DNSSEC) was developed to provide origin authentication and integrity protection for DNS data by using digital signatures [RFC4033] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” March 2005.), [RFC4034] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Resource Records for the DNS Security Extensions,” March 2005.) and [RFC4035] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.). Each digital signature RR (RRSIG) contains an algorithm code number. These algorithm codes help validators identify which cryptographic algorithm was used to generate the digital signature.

This proposed EDNS option serves to measure the acceptance and use of new digital signing algorithms. This algorithm signaling option can be used by zone administrators as a gauge to measure the successful deployment of code that implements a newly deployed digital signature algorithm used with DNSSEC. A zone administrator may be able to determine when to stop serving the old algorithm when the server sees that all or almost all of its clients signal that they are able to accept the new algorithm.

This draft does not seek to include a formal process for including new algorithms for use with DNSSEC. It also does not address the question of which algorithms are to be included in any official list of mandatory or recommended cryptographic algorithms for use with DNSSEC. Rather, this document specifies a means by which a client query can signal a set of algorithms it implements.

This draft sets out to specify a way for validating end-system resolvers to signal to a server which cryptographic algorithms they support in a DNSSEC response. This is done using the EDNS attribute values in the OPT meta-RR [RFC2671] (Vixie, P., “Extension Mechanisms for DNS (EDNS0),” August 1999.).

2.  Signaling Algorithm Understood (AU) Using EDNS

The ENDS0 specification outlined in [RFC2671] (Vixie, P., “Extension Mechanisms for DNS (EDNS0),” August 1999.) defines a way to include new options using a standardized mechanism. These options are contained in the RDATA of the OPT meta-RR. This document seeks to define a new EDNS0 option for a client to signal which algorithms the client supports.

Below shows how the signaling attribute is defined in the RDATA of the OPT RR as specified in [RFC2671] (Vixie, P., “Extension Mechanisms for DNS (EDNS0),” August 1999.):

    0                       8                      16
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                 OPTION-CODE (TBD)             |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                 OPTION-LENGTH                 |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |       ALG-CODE        |        ...            \
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

OPTION-CODE is the code for the Algorithm Understood (AU) option. Its value is fixed at TBD.

OPTION-LENGTH is the length of the data of the attribute in octets. DNSSEC algorithm codes are 1 octet long so this value is the number of octets.

ALG-CODE is the assigned DNSSEC algorithm codes that the client indicates as understood. The values MUST be in descending order, with the highest algorithm code first, followed by as many other codes as the validator wishes to signal that it prefers. For example, if a validating client implements RSA/SHA-1 and RSA/SHA-256 the value of ALG-CODE would be: 8 (RSA/SHA-256), 5 (RSA/SHA-1) indicating that the validator implements both RSA/SHA-256 and RSA/SHA-1, but does not wish to receive (or does not understand) RRSIGs using DSA or RSA/MD5.

3.  Client Considerations

A validating end-system resolver sets the AU option in the OPT meta-RR when sending a query. The validating end-system resolver sets the value(s) in the order of preference, with highest algorithm value first as described in section 2. The end-system resolver MUST also set the DNSSEC-OK bit [RFC4035] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) to indicate that it wishes to receive DNSSEC RRs in the response.

Note that when including the PRIVATEDNS (253) and/or the PRIVATEOID (254) codes, the client only indicates that it understands one or more private algorithms but does not indicate which algorithms.

3.1.  Recommendations for Stub Clients

Typically, stub resolvers rely on an upstream recursive server (or cache) to provide a response, any algorithm supportence on the stub resolver's side could be overruled by the upstream recursive server. The AU EDNS option is NOT RECOMMENDED for non-validating stub clients.

The exception to the above is that validating stub resolvers which set the CD bit in queries MAY set the AU option. In the most common scenario, the validating stub indicates that it wishes to perform its own validation (via the CD bit) and may therefor wish to indicate which cryptographic algorithm(s) it supports.

3.2.  Recursive Cache Considerations

DNSSEC validating recursive caches MAY set the AU option on any outgoing query from the cache when performing recursion on behalf of a non-DNSSEC aware stub client. If the stub indicates it is DNSSEC-aware, but does not set the AU option in the query, the DNSSEC validating recursive cache SHOULD NOT set the AU option to avoid conflicts.

Forwarders that do not do validation or caching SHOULD copy the AU option seen in received queries as they represent the wishes of the validating downstream resolver that issued the original query.

4.  Intermediate Middlebox Considerations

Intermediate middleboxes SHOULD copy the AU option seen in queries from end system resolvers. If the system is validating, it SHOULD also check for the presence of the CD bit in the query. If present, the intermediate middlebox SHOULD copy the AU option as seen in the query. If not set or if the DNSSEC-OK bit is not set, then the validating intermediate middlebox MAY chose to ignore the AU option in the query and MAY include its own preference as the AU option.

5.  Server Considerations

When an authoritative server sees the AU option in the OPT meta-RR in a request the normal algorithm for servicing requests is followed.

If the AU option is present but the DNSSEC-OK bit is not set, then the authoritative server ignores the ALG-CODE list and does not include any additional DNSSEC RRs in the response.

6.  Traffic Analysis Considerations

Zone administrators that are planning or are in the process of completing a cryptographic algorithm rollover operation should monitor DNS query traffic and record the values of the AU option in queries. This monitoring can measure the deployment of client code that implements (and signals) certain algorithms. Exactly how to capture DNS traffic and measure new algorithm adoption is beyond the scope of this document.

Zone administrators can use this data to set plans for starting an algorithm rollover and when older algorithms can be phased out without disrupting the majority of clients. In order ot keep this disruption to a minimum, zone administrators should wait to complete an algorithm rollover until a large majority of clients signal that they understand the new algorithm. Note that clients that do not implement the AU option may be older implementations which would also not implement any newly deployed algorithm.

7.  IANA Considerations

The algorithm codes used to identify DNSSEC algorithms has already been established by IANA. This document does not seek to alter that registry in any way.

This draft seeks to update the "DNS EDNS0 Options" registry by adding the AU option and referencing this document. The code for the option should be TBD.

8.  Security Considerations

This document specifies a way for a client to signal its digital signature algorithm preference to a cache or server. It is not meant to be a discussion on algorithm superiority. The signal is an optional code contained in the OPT meta-RR used with EDNS0. The goal of this option is to signal new algorithm uptake in client code to allow zone administrators to know when it is possible to complete an algorithm rollover in a DNSSEC signed zone.

9. Normative References

Authors' Addresses