| Internet-Draft | Compact Denial of Existence in DNSSEC | March 2024 |
| Huque, et al. | Expires 5 September 2024 | [Page] |
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 minimal NSEC record, allow online signing servers to minimize signing operations and response sizes, and prevent zone content disclosure.¶
This note is to be removed before publishing as an RFC.¶
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One of the functions of the Domain Name System Security Extensions (DNSSEC) [RFC9364] 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 up to 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 Denial of Existence" or "Compact Answers", 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 (RCODE) 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.¶
The use of minimally covering NSEC records also prevents adversaries from enumerating the entire contents of DNS zones by walking NSEC chains.¶
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.¶
Previously, some specific implementations of Compact Answers have tried to 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, and of making the Type Bitmaps field larger than it needs to be. It also has the potential to confuse validators and others tools that infer type existence from the NSEC record.¶
This document defines the use of a synthetic Resource Record type to signal the presence of a non-existent name. The mnemonic for this RR type is "NXNAME", chosen to clearly distinguish it from the response code mnemonic NXDOMAIN.¶
Type Value Meaning
NXNAME [TBD] NXDOMAIN indicator for Compact Denial of Existence
¶
This RR type is added to the NSEC type bitmap for responses to non-existent names (in addition to the required RRSIG and NSEC types). It is a "Meta-Type", as defined in [RFC6895], stores no data in a DNS zone, and cannot be usefully queried. Section 3.4 describes what a DNS resolver or authoritative server should do if it receives an explicit query for NXNAME.¶
No special handling of this RR type is required on the part of DNS resolvers. However, resolvers may optionally implement the behavior described in Section 4.1 (Response Code Restoration) to better restore NXDOMAIN visibility to various applications.¶
An alternative way to distinguish NXDOMAIN from ENT is to define the synthetic Resource Record type for ENTs instead, as specified in [ENT-SENTINEL], and this was already successfully deployed in the field for a period of time. This typically imposes less work on the server since NXDOMAIN responses are a lot more common than ENTs. At the time it was deployed, it also allowed a common bitmap pattern ("NSEC RRSIG") to identify NXDOMAIN across this and other implementations that returned a broad bitmap pattern for Empty Non-Terminals. However, the advantage of the NXNAME RR type is that it explicitly identifies NXDOMAIN responses, and allows them to be distinguished conclusively from potential ENT responses in other online signing NSEC implementations.¶
This section describes various types of answers generated by authoritative servers implementing Compact Denial of Existence. At the current time, the compact denial scheme is only defined for NSEC. While it could support NSEC3 too, there is no benefit in introducing the additional complexity associated with it.¶
When the authoritative server 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 Domain Name field SHOULD be set to the immediate lexicographic successor of the QNAME. The Type Bit Maps field MUST only have the bits set for the following RR Types: RRSIG, NSEC, and NXNAME.¶
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 NXNAME
¶
The NSEC record MUST have corresponding RRSIGs generated.¶
When the authoritative server receives a query for a name that exists, but has no resource record sets associated with the queried type, it generates a NODATA response, with a dynamically constructed signed NSEC record in the Authority Section. The owner name of the NSEC record matches the queried name. The Next Domain Name field is set to the immediate lexicographic successor of the QNAME. The Type Bitmaps field lists the available Resource Record types at the name.¶
An Empty Non-Terminal is a special subset of this category, where the name has no resource record sets of any type (but has descendant names that do). For a query for an Empty Non-Terminal, the NSEC type bitmap will only contain RRSIG and NSEC. (Note that this is substantially different than the ENT response in precomputed NSEC, where the NSEC record has the same type bitmap, but "covers" rather than matches the ENT, and has the Next Domain Name field set to the next lexicographic descendent of the ENT in the zone.)¶
For wildcard matches, the authoritative server will provide a dynamically signed response that claims that the queried name exists explicitly. Specifically, the answer RR set will have an RRSIG record demonstrating an exact match (i.e. the label count in the RRSIG RDATA will be equal to the number of labels in the query name minus the root label). This obviates the need to include an NSEC record in the Authority section of the response that shows that no closer match than the wildcard was possible.¶
For a Wildcard NODATA match (where the queried name matches a wildcard but no data for the queried type exists), a response akin to a non-wildcard NODATA is returned. The Answer section is empty, and the Authority section contains a single NSEC record that matches the query name with a type bitmap representing the list of types available at the wildcard.¶
NXNAME is a meta type which should not appear anywhere in a DNS message apart from the NSEC type bitmap of a Compact Answer response for a non-existent name. If however a DNS server implementing this specification receives an explicit query for the NXNAME RR type, this section describes what the response should be.¶
If an explicit query for the NXNAME RR type is received, the DNS server MUST return a Format Error (response code FORMERR). A resolver should not forward these queries upstream or attempt iterative resolution. Many DNS server implementations already return errors for all types in the meta and q-type range unless they are defined to support queries. This general behavior however is not clearly specified in any existing protocol specification document and should be.¶
For non-existent names, implementations should try wherever possible, to preserve the response code value of 3 (NXDOMAIN). This is generally possible for non-DNSSEC enabled queries, namely those which do not set the DNSSEC_OK EDNS flag (DO bit). For such queries, authoritative servers implementing Compact Denial of Existence could return a normal NXDOMAIN response. There may be limited benefit to doing this however, since most modern DNS resolvers are DNSSEC-aware, and by [RFC3225] Section 3, DNSSEC-aware recursive servers are required to set the DO bit on outbound queries, regardless of the status of the DO bit on incoming requests.¶
A validating resolver that understands the NXNAME signal from an authoritative server could modify the response code from NOERROR to NXDOMAIN in responses they return to downstream queriers that have not set the DO bit in their requests.¶
This section describes an optional but recommended scheme to permit signaled restoration of the NXDOMAIN RCODE for DNSSEC enabled responses. A new EDNS0 [RFC6891] header flag is defined in the 2nd most significant bit of the flags field in the EDNS0 OPT header. This flag is referred to as the "Compact Answers OK (CO)" flag.¶
+0 (MSB) +1 (LSB)
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
0: | EXTENDED-RCODE | VERSION |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
2: |DO|CO| Z |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
¶
When this flag is sent in a query by a resolver, it indicates that the resolver will accept a signed NXNAME enhanced NODATA response for a non-existent name together with the response code field set to NXDOMAIN (3).¶
In responses to such queries, a Compact Denial authoritative implementing this signaling scheme, will set the Compact Answers OK EDNS header flag, and for non-existent names will additionally set the response code field to NXDOMAIN.¶
EDNS is a hop by hop signal, so resolvers will need to record the presence of this flag in associated cache data and respond to downstream DNSSEC enabled queriers appropriately. If the querier does not set the Compact Answers OK flag, the resolver will need to reset the response code back to NOERROR (0) for an NXNAME response.¶
For DNSSEC enabled queries, a signed zone at an authoritative server implementing Compact Answers will never return a response with a response code of NXDOMAIN, unless they have implemented the optional response code restoration feature described in Section 4.1. Similarly, resolvers not implementing this feature will also not be able to return NXDOMAIN. In the absence of this, tools that rely on accurately determining non-existent names will need to infer them from the presence of the NXNAME RR type in the type bitmap of the NSEC record in NODATA responses from these servers.¶
Address lookup functions typically invoked by applications may need to do more work when dealing with implementations of Compact Answers. For example, a NODATA response to the lookup of an AAAA record for a non-existent name, can cause such functions to issue another query at the same name for an A record. Whereas an NXDOMAIN response to the first query would suppress additional queries for other types at that name. Address lookup functions could be enhanced to issue DNSSEC enabled queries and to examine the NSEC type bitmaps in responses to accurately determine non-existent names.¶
[RFC4034] Section 4.1.2, The Type Bit Maps Field, states the following:¶
This paragraph is updated to the following:¶
Note: as a practical matter, no known resolver insists that pseudo-types must be clear in the NSEC type bitmap, so this restriction (prior to its revision) has posed no problem for the deployment of this mechanism.¶
[RFC4035] Section 2.3, Including NSEC RRs in a Zone, states the following:¶
This paragraph is updated to the following::¶
Cloudflare, NS1, and Amazon Route53 currently implement the Compact Denial of Existence method. From early 2021 until November 2023, NS1 had deployed the Empty Non-Terminal distinguisher using the private RR type code 65281. At the present time Cloudflare (since July 2023) and NS1 (since November 2023) both implement the NXNAME distinguisher using the private RR type code 65283. Amazon Route53 is expected to implement NXNAME after the specification is finalized and an official code point is allocated. At the current time, there are only prototype implementations of the signaled rcode restoration scheme.¶
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 Answers 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.¶
Some security tools rely on detection of non-existent domain names by examining the response code field of DNS response messages. A NOERROR code in that field rather than NXDOMAIN will impact such tools. Implementation of the optional response code restoration scheme will help recover NXDOMAIN visibility for these tools. Note that the DNS header is not cryptographically protected, so the response code field cannot be authenticated. Thus inferring the status of a response from signed data in the body of the DNS message is actually more secure. These tools could be enhanced to recognize the (signed) NXNAME signal.¶
If the motivating aspects of this specification (minimizing response size and computational costs) are not a concern, DNSSEC deployments can opt for a different method (e.g. traditional online signing or pre-computed signatures), and avoid imposing the challenges of NXDOMAIN visibility.¶
The Compact Answers technique (then called "Black Lies") was originally proposed in [BLACK-LIES] by Filippo Valsorda and Olafur Gudmundsson, and implemented by Cloudflare. The Empty Non-Terminal distinguisher RR type was originally proposed in [ENT-SENTINEL] by Shumon Huque, and deployed by NS1. The NXNAME type is based on the FDOM type proposed in [NXDOMAIN-TYPE] by Gudmundsson and Valsorda.¶
Especially detailed discussions on many technical aspects of this document were conducted with Paul Vixie, Jan Vcelak, Viktor Dukhovni, and Ed Lewis. The authors would also like to thank the many other members of the IETF DNS Operations working group for helpful comments and discussions.¶
IANA is requested to allocate a new DNS Resource Record type code for NXNAME in the DNS parameters registry, from the meta type range. Specifically, the lowest available number (currently 128) in the meta range is requested to be allocated. A lower number lowers the size of the type bitmap, which reduces the size of the DNS response message.¶
Type Value Meaning
NXNAME [TBD] NXDOMAIN indicator for Compact Denial of Existence
¶
IANA is additionally requested to allocate the "Compact Answers OK" flag in the EDNS header, as described in Section 4.1¶