Signaling Cryptographic Algorithm Understanding in DNSSEC

The DNS Security Extensions (DNSSEC) was developed to provide origin authentication and integrity protection for DNS data by using digital signatures , and . 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 . 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.

The ENDS0 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 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.

Below shows how the signaling attribute is defined in the RDATA of the OPT RR as specified in : 0 8 16 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | OPTION-CODE (TBD) | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | OPTION-LENGTH | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ALG-CODE1 | ALG-CODE2 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ALG-CODE3 | ... \ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 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. The value in this field is the number of algorithm codes the client wishes to signal as understood. ALG-CODE is the assigned DNSSEC algorithm code(s) that the client indicates as understood. These values SHOULD be in the order of preference. For example, if a validating client understands RSA/SHA-1 and RSA/SHA-256 but prefers to trust RSA/SHA-256 signatures due to local policy, the order of the ALG-CODE would be: 8 (RSA/SHA-256), 5 (RSA/SHA-1).

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 order of the algorithms in the AU option to the preferred order; i.e., most preferred first, followed by the least preferred. A validating end-system resolver SHOULD only list algorithm codes that the client has implemented or has a trust anchor for. Conversely, a validating end-system resolver SHOULD NOT include the algorithm code for cryptographic algorithms for which they have not implemented and do not have an associated trust anchor. The end-system resolver MUST also set the DNSSEC-OK bit to indicate that it wishes to receive DNSSEC RRs in the response.

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 sub 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.

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 list of algorithms in the OPT meta-RR, selects the most preferred by the client and adds the appropriate RRSIGs (as per the rules in )of the algorithms listed in the ALG-CODE list. 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 that does not appear in the ALG-CODE list in the client query the authoritative server SHOULD include any or all RRSIGs in the response regardless of algorithm used to generate them.

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 (, , 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 algorithms found in the query AU list. Forwarders that do not do validation or caching MAY copy the AU option seen in received queries as they represent the wishes of the validating downstream resolver that issued the original query.

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.

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.

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.