--- 1/draft-ietf-dnsext-dnssec-algo-signal-07.txt 2012-08-14 20:14:09.817843393 +0200 +++ 2/draft-ietf-dnsext-dnssec-algo-signal-08.txt 2012-08-14 20:14:09.837843485 +0200 @@ -1,19 +1,19 @@ DNS Extensions Working Group S. Crocker Internet-Draft Shinkuro Inc. Intended status: Standards Track S. Rose -Expires: December 16, 2012 NIST - June 14, 2012 +Expires: February 15, 2013 NIST + August 14, 2012 Signaling Cryptographic Algorithm Understanding in DNSSEC - draft-ietf-dnsext-dnssec-algo-signal-07 + draft-ietf-dnsext-dnssec-algo-signal-08 Abstract The DNS Security Extensions (DNSSEC) were 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 digital signature and hash algorithms they support. @@ -32,21 +32,21 @@ 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/. 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." - This Internet-Draft will expire on December 16, 2012. + This Internet-Draft will expire on February 15, 2013. Copyright Notice Copyright (c) 2012 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 @@ -94,21 +94,22 @@ Likewise, Delegation Signer (DS) RRs and NSEC3 RRs use a hashed value as part of their RDATA and, like digital signature algorithms, these hash algorithms have code numbers. All three algorithm codes (RRSIG/ DNSKEY, DS and NSEC3) are maintained in unique IANA registries. This draft sets out to specify a way for validating end-system resolvers to tell a server in a DNS query which digital signature and/or hash algorithms they support. This is done using the new EDNS options specified below in Section 2 for use in the OPT meta-RR - [I-D.ietf-dnsext-rfc2671bis-edns0]. + [I-D.ietf-dnsext-rfc2671bis-edns0]. These three new EDNS option + codes are all OPTIONAL to implement and use. These proposed EDNS options serve to measure the acceptance and use of new digital signing algorithms. These signaling options can be used by zone administrators as a gauge to measure the successful deployment of code that implements newly deployed digital signature algorithm, DS hash and NSEC3 hash algorithm used with DNSSEC. A zone administrator is able to determine when to stop signing with a superseded algorithm when the server sees that a significant number of its clients signal that they are able to accept the new algorithm. Note that this survey may be conducted over the period of years @@ -125,21 +126,21 @@ 2. Signaling DNSSEC Algorithm Understood (DAU), DS Hash Understood (DHU) and NSEC3 Hash Understood (N3U) Using EDNS The EDNS0 specification outlined in [I-D.ietf-dnsext-rfc2671bis-edns0] 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 defines three new EDNS options for a client to signal which digital signature and/or hash algorithms the client supports. These options can be used independently of each other and MAY appear in any order in the OPT - RR. + RR. Each option code can appear only once in an OPT RR. The figure below shows how each option is defined in the RDATA of the OPT RR specified in [I-D.ietf-dnsext-rfc2671bis-edns0]: 0 8 16 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | OPTION-CODE (TBD) | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | LIST-LENGTH | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ @@ -157,40 +158,36 @@ o NSEC3 Hash Understood (N3U) option for NSEC3 hash algorithms. Its value is fixed at TBD3. LIST-LENGTH is the length of the list of digital signature or hash algorithm codes in octets. Each algorithm code occupies a single octet. ALG-CODE is the list of assigned values of DNSSEC zone signing algorithms, DS hash algorithms, or NSEC3 hash algorithms (depending on the OPTION-CODE in use) that the client declares to be supported. - The values are listed in descending order of preference, with the - most preferred algorithm first. For example, if a validating client - signals the DAU option and RSA/SHA-1, RSA/SHA-256 and prefers the - latter, the values of ALG-CODE would be: 8 (RSA/SHA-256), 5 (RSA/ - SHA-1). + The order of the code values can be arbitrary and SHOULD NOT be used + to infer preference. If all three options are included in the OPT RR, there is a potential for the OPT RR to take up considerable size in the DNS message. However, in practical terms, including all three options is likely to take up 22-32 octets (average of 6-10 digital signature algorithms, 3-5 DS hash algorithms and 1-5 NSEC3 hash algorithms) including the EDNS option codes and option lengths in a potential future example. 3. Client Considerations A validating end-system resolver sets the DAU, DHU and/or N3U option, or combination thereof in the OPT meta-RR when sending a query. The - validating end-system resolver sets the value(s) in the order of - preference, with the most preferred algorithm(s) first as described - in section 2. The validating end-system resolver MUST also set the + validating end-system resolver sets the value(s) in any arbitrary + order. The validating end-system resolver MUST also set the DNSSEC-OK bit [RFC4035] to indicate that it wishes to receive DNSSEC RRs in the response. Note that the PRIVATEDNS (253) and/or the PRIVATEOID (254) digital signature codes both cover a potentially wide range of algorithms and are likely not useful to a server. There is no compelling reason for a client to include these codes in its list of the DAU. Likewise, clients MUST NOT include RESERVED codes in any of the options. 3.1. Stub Resolvers @@ -199,52 +196,46 @@ cache) to provide a response. So optimal setting of the DAU, DSU and N3U options depends on whether the stub resolver elects to perform its own validation. 3.1.1. Validating Stub Resolvers A validating stub resolver already (usually) sets the DO bit [RFC4035] to indicate that it wishes to receive additional DNSSEC RRs (i.e. RRSIG RRs) in the response. Such validating resolvers SHOULD include the DAU, DHU and/or the N3U option(s) in the OPT RR when - sending a query. The way the validating stub resolver indicates - which cryptographic algorithm(s) it supports by setting the values in - the order of preference, with the most preferred algorithm first as - described in Section 2. + sending a query. 3.1.2. Non-Validating Stub Resolvers The DAU, DHU and N3U EDNS options are NOT RECOMMENDED for non- validating stub resolvers. 3.2. Recursive Resolvers 3.2.1. Validating Recursive Resolvers A validating recursive resolver sets the DAU, DHU and/or N3U - option(s) when performing recursion based on the DO and CD flags in - the client request [RFC4035]. If the client of the recursive - resolver did not include the DO bit in the query the recursive - resolver MAY include the option(s) according to its own local policy. + option(s) when performing recursion based on its list of algorithms + and any DAU, DHU and/or N3U option lists in the stub client query. + When the recursive server receives a query with one or more of the + options set, the recursive server MUST set the algorithm list to a + union of the stub client's list and the validating recursive + resolver's list. For example, if the recursive resolver's algorithm + list for the DAU option is (3, 5, 7) and the stub's algorithm list is + (7, 8), the final DAU algorithm list would be (3, 5, 7, 8). If the client did include the DO and CD bits, but did not include the DAU, DHU and/or N3U option(s) in the query, the validating recursive - resolver MUST NOT include the option(s) to avoid conflicts. - - If the client did set the DO bit and the option(s) in the query, the - validating recursive resolver MUST include the option(s) based on the - setting of the CD bit. If the CD bit is set, the validating - recursive resolver MUST include the option(s) based on the client - query or a superset of the client option(s) list and the validator's - own list (if different). If the CD bit is not set, the validating - recursive resolver MAY copy the client option(s) or substitute its - own option list. + resolver MAY include the option(s) with its own list in full. If one + or more of the options are missing, the validating recursive resolver + MAY include the missing options with its own list in full. 3.2.2. Non-validating Recursive Resolvers Recursive resolvers that do not do validation MUST copy the DAU, DHU and/or N3U option(s) seen in received queries as they represent the wishes of the validating downstream resolver that issued the original query. 4. Intermediate System Considerations @@ -260,21 +251,21 @@ processing (e.g. RRSIG filtering in responses) on the server side. If the options are present but the DNSSEC-OK (OK) bit is not set, the server does not do any DNSSEC processing, including any recording of the option(s). 6. Traffic Analysis Considerations Zone administrators that are planning or are in the process of a cryptographic algorithm rollover operation should monitor DNS query - traffic and record the number of queries, the presense of the OPT RR + traffic and record the number of queries, the presence of the OPT RR in queries and the values of the DAU/DHU/N3U option(s) (if present). This monitoring can be used to measure the deployment of client code that implements (and signals) specific algorithms. Description of the techniques used to capture DNS traffic and measure new algorithm adoption is beyond the scope of this document. Zone administrators that need to comply with changes to their organization's security policy (with regards to cryptographic algorithm use) can use this data to set milestone dates for performing an algorithm rollover. For example, zone administrators @@ -302,27 +293,35 @@ 8. Security Considerations This document specifies a way for a client to signal its digital signature and hash algorithm knowledge to a cache or server. It is not meant to be a discussion on algorithm superiority. The signals are optional codes contained in the OPT meta-RR used with EDNS. The goal of these options are 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. + There is a possibility that an eavesdropper or server could infer the + validator in use by a client by the presence of the AU options and/or + algorithm code list. This information leakage in itself is not very + useful to a potential attacker but it could be used to identify the + validator or narrow down the possible validator implementations in + use by a client, which could have a known vulnerability that could be + exploited by the attacker. + 9. Normative References [I-D.ietf-dnsext-rfc2671bis-edns0] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms for DNS (EDNS0)", draft-ietf-dnsext- - rfc2671bis-edns0-08 (work in - progress), February 2012. + rfc2671bis-edns0-09 (work in + progress), August 2012. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033,