Compact Denial of Existence in DNSSEC

Abstract

This document describes a technique to generate a signed DNS response on demand for a non-existent name by claiming that the name exists but doesn't have any data for the queried record type. Such answers require only one NSEC record and allow online signing servers to minimize signing operations and response sizes.¶

1. Introduction and Motivation

One of the functions of the Domain Name System Security Extensions (DNSSEC) [RFC4033] [RFC4034] [RFC4035] [RFC5155] is "Authenticated Denial of Existence", i.e. proving that a DNS name or record type does not exist. Normally, this is done by means of signed NSEC or NSEC3 records. In the precomputed signature model, these records chain together existing names, or cryptographic hashes of them in the zone. In the online signing model, described in NSEC and NSEC3 "White Lies" [RFC4470] [RFC7129], they are used to dynamically compute an epsilon function around the queried name. A 'type bitmap' in the data field of the NSEC or NSEC3 record asserts which resource record types are present at the name.¶

The response for a non-existent name requires upto 2 signed NSEC records or up to 3 signed NSEC3 records (and for online signers, the associated cryptographic computation), to prove that (1) the name did not explicitly exist in the zone, and (2) that it could not have been synthesized by a wildcard.¶

This document describes an alternative technique, "Compact Lies", to generate a signed DNS response on demand for a non-existent name by claiming that the name exists but has no resource records associated with the queried type, i.e. it returns a NODATA response rather than an NXDOMAIN response. A NODATA response (which has a response code of NOERROR, and an empty ANSWER section) requires only one NSEC record matching the queried name. This has two advantages: the DNS response size is smaller, and it reduces the online cryptographic work involved in generating the response.¶

2. Distinguishing NXDOMAIN from Empty Non-Terminal Names

Since NODATA responses are generated for non-existent names, and there are no defined record types for the name, the NSEC type bitmap in the response will only contain "NSEC" and "RRSIG". Tools that need to accurately identify non-existent names in responses cannot rely on this specific type bitmap because Empty Non-Terminal (ENT) names (which positively exist) also have no record types at the name and will return exactly the same type bitmap.¶

Today, some specific implementations of Compact Lies avoid the NXDOMAIN identification problem by synthesizing the NSEC type bitmap for ENTs to include all record types supported except for the queried type. This has the undesirable effect of no longer being able to reliably determine the existence of ENTs.¶

This document defines the use of a synthetic Resource Record type to signal the presence of an Empty Non-Terminal name. The mnemonic for this RR type is "ENT" and its type code is [TBD]. This RR type is added to the NSEC type bitmap for responses to ENTs. No special handling of this RR type is needed on the part of DNS resolvers.¶

3. Responses for Non-Existent Names

When an authoritative server implementing Compact Lies receives a query for a non-existent name in a zone that it serves, a NODATA response (response code NOERROR, empty Answer section) is generated with a dynamically constructed NSEC record with the owner name matching the queried name (QNAME).¶

The next name SHOULD be set to the immediate lexicographic successor of the QNAME. The generated NSEC record's type bitmap MUST have only the RRSIG and NSEC bits set.¶

For example, a request for the non-existing name a.example.com would cause the following NSEC record to be generated (in DNS presentation format):¶

          a.example.com. 3600 IN NSEC \000.a.example.com. RRSIG NSEC

The NSEC record MUST have corresponding RRSIGs generated.¶

4. Responses for Wildcard Matches

For wildcard matches, a Compact Lies implementation will typically provide a dynamically signed response that claims that the queried name exists explicitly. This obviates the need to include an NSEC record in the Additional section of the response that shows that no closer match than the wildcard was possible.¶

Similarly, Wildcard NODATA responses (where the queried name matches a wildcard but no data for the queried type exists), a response akin to a regular NODATA is returned. The Answer section is empty, and the Additional section contains a single NSEC record that matches the query name with a type bitmap representing the list of types available at the wildcard.¶

5. Responses for Empty Non-Terminals

When an authoritative server implementing Compact Lies receives a query for an Empty Non-Terminal name, it generates a NODATA response in the usual way, except that it adds the additional Empty Non-Terminal distinguisher RR type code in the NSEC type bitmap field. Corresponding RRSIG signature record(s) MUST also be generated.¶

          ent1.example.net.  3600 IN NSEC \000.ent.example.net. RRSIG NSEC ENT

Note that this RR type only appears in the type bitmap of an NSEC record in response to a query for an Empty Non-Terminal name. It causes no special processing on the part of validating resolvers. Queries for the ENT RR type itself at an actual Empty Non-Terminal MUST also elicit the response described in this section.¶

6. Operational Implications

A signed zone at an authoritative server implementing Compact Lies will never return a response with a response code of NXDOMAIN. Tools that rely on accurately determining non-existent names will need to infer them from the NSEC type bitmap pattern of "NSEC RRSIG".¶

Address lookup functions typically invoked by applications won't see a practical impact from this indistinguishability. For a non-existent name, the getaddrinfo() function for example will return an exit code of EAI_NODATA rather than EAI_NONAME. But either way the effect on the caller is the same: it will obtain a response with a non-zero exit code and no available addresses.¶

7. Implementation Status

Cloudflare, NS1, and Amazon Route53 currently implement the base Compact Lies scheme. NS1 additonally implements the Empty Non-Terminal distinguisher in the NSEC type bitmap, using the private RR type code 65281.¶

8. Security Considerations

Online signing of DNS records requires authoritative servers for the DNS zone to have access to the private signing keys. Exposing signing keys on Internet reachable servers makes them more vulnerable to attack.¶

Additionally, generating signatures on-demand is more computationally intensive than returning pre-computed signatures. Although the Compact Lies scheme reduces the number of online signing operations compared to previous techniques like White Lies, it still may make authoritative servers more vulnerable to computational denial of service attacks than pre-computed signatures. The use of signature algorithms (like those based on Elliptic Curves) that have a comparatively low cost for signing is recommended.¶

11. References