Signaling Cryptographic Algorithm Understanding in DNSSEC

The DNS Security Extensions (DNSSEC) , and were developed to provide origin authentication and integrity protection for DNS data by using digital signatures. Each digital signature RR (RRSIG) contains an algorithm code number. These algorithm codes tell validators which cryptographic algorithm was used to generate the digital signature. 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 . 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 A zone administrator is able to determine when to stop signing with a superseded algorithm when the server sees that a significant number of its clients signal that they are able to accept the new algorithm.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 before a tipping point is seen. This draft does not seek to introduce another 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 the set of algorithms and hashes which it implements.

The EDNS0 specification outlined in 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.

The figure below shows how each option is defined in the RDATA of the OPT RR specified in : 0 8 16 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | OPTION-CODE (TBD) | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | LIST-LENGTH | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ALG-CODE | ... \ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ OPTION-CODE is the code for the given signaling option. They are: DNSSEC Algorithm Understood (DAU) option for DNSSEC digital signing algorithms. Its value is fixed at TBD1. DS Hash Understood (DHU) option for DS RR hash algorithms. Its value is fixed at TBD2. 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). 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.

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 DNSSEC-OK bit 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.

Typically, stub resolvers rely on an upstream recursive server (or 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.

A validating stub resolver already (usually) sets the DO bit 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.

The DAU, DHU and N3U EDNS options are NOT RECOMMENDED for non-validating stub 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 . 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. 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.

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.

Intermediate proxies that understand DNS are RECOMMENDED to behave like a comparable recursive resolver when dealing with the DAU, DHU and N3U options.

When an authoritative server sees the DAU, DHU and/or N3U option(s) in the OPT meta-RR in a request the normal algorithm for servicing requests is followed. The options MUST NOT trigger any special 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).

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 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 can use the data to determine when older algorithms can be phased out without disrupting a significant number of clients. In order to keep this disruption to a minimum, zone administrators should wait to complete an algorithm rollover until a large majority of clients signal that they recognize the new algorithm. This may be in the order of years rather than months. Note that clients that do not implement these options are likely to be older implementations which would also not implement any newly deployed algorithm.

The algorithm codes used to identify DNSSEC algorithms, DS RR hash algorithms and NSEC3 hash algorithms have already been established by IANA. This document does not seek to alter that registry in any way. This draft seeks to update the "DNS EDNS Options" registry by adding the DAU, DHU and N3U options and referencing this document. The code for these options are TBD1, TBD2 and TBD3 respectively.

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.