DNS Extensions Working Group S. Crocker Internet-Draft Shinkuro Inc. Updates: 4035 (if approved) S. Rose Intended status: Standards Track NIST Expires: July 2, 2009 December 29, 2008 Signaling Cryptographic Algorithm Understanding in DNSSEC draft-crocker-dnssec-algo-signal-02 Status of This Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. 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 July 2, 2009. Copyright Notice Copyright (c) 2008 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. 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 Crocker & Rose Expires July 2, 2009 [Page 1]
Internet-Draft Algorithm-Signal December 2008 authentication and integrity protection for DNS data by using digital signatures. These digital signatures can be generated using different algorithms. Each digital signature added to a response increases the size of the response, which could result in the response message being truncated. This draft sets out to specify a way for validating end-system resolvers to signal to a server which cryptographic algorithms they prefer in a DNSSEC response by defining an EDNS option to list a client's preferred algorithms. 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 [RFC2119]. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Signaling Algorithm Understood (AU) Using EDNS . . . . . . . . 3 3. Client Considerations . . . . . . . . . . . . . . . . . . . . . 4 3.1. Recommendations for Stub Clients . . . . . . . . . . . . . 5 4. Server Considerations . . . . . . . . . . . . . . . . . . . . . 5 5. Cache and Forwarder Considerations . . . . . . . . . . . . . . 5 5.1. Intermediate Proxy Resolvers . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 8. Normative References . . . . . . . . . . . . . . . . . . . . . 6 Crocker & Rose Expires July 2, 2009 [Page 2]
Internet-Draft Algorithm-Signal December 2008 1. Introduction The DNS Security Extensions (DNSSEC) was developed to provide origin authentication and integrity protection for DNS data by using digital signatures [RFC4033], [RFC4034] and [RFC4035]. 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. RRSIG RRs can be fairly large, and increase the size of a response. If multiple algorithms are used, then multiple RRSIGs are returned for each RRset in a response. If the response is too large, it may be truncated, and the client forced to resend the query using TCP. It would be in the client and server's interests if there was a way to limit the number of RRSIGs in a response to only those algorithms the client was interested in (if present). This draft sets out to specify a way for validating end-system resolvers to signal to a server which cryptographic algorithms they prefer in a DNSSEC response. This is done using the EDNS attribute values in the OPT meta-RR [RFC2671]. This option could also be used by servers to advertise which cryptographic algorithms are used in signing a particular zone. An additional reason for having the end-system resolver signal which algorithms it understands is to speed the transition to a new algorithm. A server will be able to determine when to start serving a new algorithm when it sees a sufficient number of its clients are able to accept the new algorithm and it will be able to determine when to stop serving the old algorithm when it sees that all or almost all of its clients are able to accept the new algorithm. Information about clients can also be used to communicate to the operators of those clients and/or the providers of their software that it's time to upgrade. 2. Signaling Algorithm Understood (AU) Using EDNS The ENDS0 specification outlined in [RFC2671] 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 prefers, and the server to advertise which algorithms are used to sign a particular zone. Crocker & Rose Expires July 2, 2009 [Page 3]
Internet-Draft Algorithm-Signal December 2008 Below shows how the signaling attribute is defined in the RDATA of the OPT RR as specified in [RFC2671]: 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 set at 1. ALG-CODE is the assigned DNSSEC algorithm codes that the client indicates as understood. This value SHOULD be the largest algorithm code value understood by the validator (excluding Reserved codes, and values greater than 252). It is assumed that the validator understands all previously defined (and lower) algorithm codes. For example, if a validating client understands RSA/SHA-1 and RSA/SHA-256 the value of ALG-CODE would be: 8 (RSA/SHA-256), indicating that the validator understand both RSA/SHA-256, RSA/SHA-1 and DSA but not ECC because it is currently reserved and not defined. 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 SHOULD set the value to be the largest algorithm code that the validator understands (excluding Reserved codes and values greater than 252). A validating end-system resolver SHOULD only list algorithm codes that the client has implemented. Conversely, a validating end-system resolver SHOULD NOT include the algorithm code for cryptographic algorithms for which they have not implemented. The end-system resolver MUST also set the DNSSEC-OK bit [RFC4035] to indicate that it wishes to receive DNSSEC RRs in the response. Crocker & Rose Expires July 2, 2009 [Page 4]
Internet-Draft Algorithm-Signal December 2008 3.1. Recommendations for Stub Clients Stub resolvers rely on an upstream recursive server (or cache) to provide a response, any algorithm preference on the stub resolver's side can be overruled by the upstream recursive server. The AU EDNS option is NOT RECOMMENDED for non-validating stub clients. The only exception is for validating stub resolvers, which set the CD bit in queries. In this scenario, the validating stub indicates that it wishes to perform its own validation and may wish to indicate which cryptographic algorithm it prefers. 4. 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. The only difference is what DNSSEC RRs are included in the final response. If the AU option is present but the DNSSEC-OK bit is not set, then the authoritative server does not include any additional DNSSEC RRs in the response. If the DNSSEC-OK bit is set, the authoritative server looks at the ALG-CODE value in the OPT meta-RR, selects the RRSIGs with the algorithm code equal to or lower (whichever is closest) to be included in the response (as per the rules in [RFC4035]). If the zone containing the QNAME is not signed, the authoritative server sends a traditional non-DNSSEC response. If the zone containing the QNAME is signed with a cryptographic algorithm(s) that are all greater than the ALG-CODE value in the client query the authoritative server SHOULD include any or all RRSIGs in the response regardless of algorithm used to generate them. 5. Cache and Forwarder Considerations Caches MUST NOT set the AU option on any outgoing query from the cache when performing recursion on behalf of a stub client. A cache MUST follow the guidelines in the DNSSEC specification ([RFC4033], [RFC4034], [RFC4035] and any updating documents). If a cache receives a query with the AU option set and the response can be answered by data out of the cache, the cache SHOULD follow the AU option request and only include the RRSIGs generated using the algorithm equal to or less than the value in ALG-CODE. Forwarders that do not do validation or caching MAY copy the AU option seen in received queries as they represent the wishes of the Crocker & Rose Expires July 2, 2009 [Page 5]
Internet-Draft Algorithm-Signal December 2008 validating downstream resolver that issued the original query. 5.1. Intermediate Proxy Resolvers Intermediate resolvers SHOULD copy the AU option seen in queries from end- system resolvers. If the intermediate resolver is validating, it SHOULD also check for the presence of the CD bit in the query. If present, the intermediate resolvers SHOULD copy the AU option as seen in the query. If not or if the DNSSEC-OK bit is not set, then the validating intermediate resolver MAY chose to ignore the AU option in the query and MAY include its own preference as the AU option. 6. 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. 7. 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 reduce response size by having the client signal with digital signature algorithms it prefers and that it may not care about other algorithms used to sign zone data. It is possible that an attacker can attempt to conduct a downgrade attack by intercepting the query and altering the AU option code. An attacker could alter the algorithm list to force the client to rely on a weaker digital signature algorithm even though the zone is signed using a stronger algorithm the client prefers. In these cases a client might be able to detect an attack if the target zone has a DS RR in its delegating parent with the desired algorithm. The DS cannot be deleted without making the parent's RRSIG over that RRset invalid. 8. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", Crocker & Rose Expires July 2, 2009 [Page 6]
Internet-Draft Algorithm-Signal December 2008 RFC 2671, August 1999. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005. [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, 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. Authors' Addresses Steve Crocker Shinkuro Inc. 5110 Edgemoor Lane Bethesda, MD 20814 USA EMail: steve@shinkuro.com Scott Rose NIST 100 Bureau Dr. Gaithersburg, MD 20899 USA Phone: +1-301-975-8439 EMail: scott.rose@nist.gov Crocker & Rose Expires July 2, 2009 [Page 7]