USC/ISI

ietf@hardakers.net

Bloomberg, L.P.

ietf-dane@dukhovni.org

NSEC3 is a DNSSEC mechanism providing proof of non-existence by promising there are no names that exist between two domainnames within a zone. Unlike its counterpart NSEC, NSEC3 avoids directly disclosing the bounding domainname pairs. This document provides guidance on setting NSEC3 parameters based on recent operational deployment experience.

As with NSEC , NSEC3 provides proof of non-existence that consists of signed DNS records establishing the non-existence of a given name or associated Resource Record Type (RRTYPE) in a DNSSEC signed zone. In the case of NSEC3, however, the names of valid nodes in the zone are obfuscated through (possibly multiple iterations of) hashing via SHA-1. (currently only SHA-1 is in use within the Internet). NSEC3 also provides “opt-out support”, allowing for blocks of unsigned delegations to be covered by a single NSEC3 record. Opt-out blocks allow large registries to only sign as many NSEC3 records as there are signed DS or other RRsets in the zone – with opt-out, unsigned delegations don’t require additional NSEC3 records. This sacrifices the tamper-resistance proof of non-existence offered by NSEC3 in order to reduce memory and CPU overheads. NSEC3 records have a number of tunable parameters that are specified via an NSEC3PARAM record at the zone apex. These parameters are the Hash Algorithm, processing Flags, the number of hash Iterations and the Salt. Each of these has security and operational considerations that impact both zone owners and validating resolvers. This document provides some best-practice recommendations for setting the NSEC3 parameters.

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

The following sections describe recommendations for setting parameters for NSEC3 and NSEC3PARAM.

The algorithm field is not discussed by this document.

The flags field currently contains a single flag, that of the “Opt-Out” flag , which specifies whether or not NSEC3 records provide proof of non-existence or not. In general, NSEC3 with the Opt-Out flag enabled should only be used in large, highly dynamic zones with a small percentage of signed delegations. Operationally, this allows for less signature creations when new delegations are inserted into a zone. This is typically only necessary for extremely large registration points providing zone updates faster than real-time signing allows. Smaller zones, or large but relatively static zones, are encouraged to use a Flags value of 0 (zero) and take advantage of DNSSEC’s proof-of-non-existence support.

Generally increasing the number of iterations offers little improved protections for modern machinery. Although Section 10.3 of specifies upper bounds for the number hash iterations to use, there is no published guidance on good values to select. Because hashing provides only moderate protection, as shown recently in academic studies of NSEC3 protected zones (tbd: insert ref), this document recommends using an iteration value of 0 (zero). This leaves the creating and verifying hashes with just one application of the hashing algorithm.

Salts add yet another layer of protection against offline, stored dictionary attacks by using randomly generated values when creating new records. The length and usage of a salt value has little operational concerns beyond bandwidth requirements for transmitting the salt. Thus, the primary consideration is whether or not there is a security benefit to deploying signed zones with salt values. Operators may choose to use a salt for this reason, though it should be noted that the use of salts doesn’t prevent against guess based approaches in offline attacks – only against memorization hash based lookups. Thus, the added value is minimal enough that operators may wish to deploy zones without a hash value at all.

Because there has been a large growth of open (public) DNSSEC validating resolvers that are subject to compute resource constraints when handling requests from anonymous clients, this document recommends that validating resolvers should change their behaviour with respect to large iteration values. Validating resolvers SHOULD return a SERVFAIL when processing NSEC3 records with iterations larger than 100. Note that this significantly decreases the requirements originally specified in Section 10.3 of .

This entire document discusses security considerations with various parameters selections of NSEC3 and NSEC3PARAM fields.

This entire document discusses operational considerations with various parameters selections of NSEC3 and NSEC3PARAM fields.

In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The Domain Name System Security (DNSSEC) Extensions introduced the NSEC resource record (RR) for authenticated denial of existence. This document introduces an alternative resource record, NSEC3, which similarly provides authenticated denial of existence. However, it also provides measures against zone enumeration and permits gradual expansion of delegation-centric zones. [STANDARDS-TRACK] This document is part of a family of documents that describe the DNS Security Extensions (DNSSEC). The DNS Security Extensions are a collection of new resource records and protocol modifications that add data origin authentication and data integrity to the DNS. This document describes the DNSSEC protocol modifications. This document defines the concept of a signed zone, along with the requirements for serving and resolving by using DNSSEC. These techniques allow a security-aware resolver to authenticate both DNS resource records and authoritative DNS error indications. This document obsoletes RFC 2535 and incorporates changes from all updates to RFC 2535. [STANDARDS-TRACK] RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.

dns-operations discussion participants

While this document is under development, it can be viewed, tracked, issued, pushed with PRs, … here: https://github.com/hardaker/draft-hardaker-dnsop-nsec3-guidance