Internet Engineering Task Force (IETF) S. Huque
Request for Comments: 9824 Salesforce
Updates: 4034, 4035 C. Elmerot
Category: Standards Track Cloudflare
ISSN: 2070-1721 O. Gudmundsson
Retired/Unaffiliated
August 2025
Compact Denial of Existence in DNSSEC
Abstract
This document describes a technique to generate a signed DNS response
on demand for a non-existent nonexistent name by claiming that the name exists but
doesn't have any data for the queried record type. Such answers responses
require only one minimally covering NSEC or NSEC3 record, allow
online signing servers to minimize signing operations and response
sizes, and prevent zone content disclosure.
This document updates RFCs 4034 and 4035.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9824.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction and Motivation
1.1. Requirements Language
2. Distinguishing Non-existent Nonexistent Names
3. Generating Responses with NSEC
3.1. Responses for Non-existent Nonexistent Names
3.2. Responses for Non-existent Nonexistent Types
3.3. Responses for Wildcard Matches
3.4. Responses for Unsigned Referrals
3.5. Responses to Explicit Queries for NXNAME
4. Generating Responses with NSEC3
5. Response Code Restoration
5.1. Signaled Response Code Restoration
6. Operational Implications
7. Updates to RFCs
7.1. Updates to RFC 4034
7.2. Updates to RFC 4035
8. Security Considerations
9. IANA Considerations
10. References
10.1. Normative References
10.2. Informative References
Appendix A. Other Approaches
Appendix B. Historical Implementation Notes
Acknowledgements
Authors' Addresses
1. Introduction and Motivation
One of the functions of DNS 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 for
NSEC in [RFC4470] and for NSEC3 "White Lies" [RFC4470] White Lies in [RFC7129], they are
used to dynamically compute an epsilon function around the queried name. QNAME.
The Type Bit Maps field in the data of the NSEC or NSEC3 record
asserts which resource record (RR) types are present at the name.
The response for a non-existent nonexistent name requires up to two signed NSEC
records or up to three 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) 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 nonexistent name by claiming that the name exists but
has no resource records record sets 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 or NSEC3 record matching
the
queried name. QNAME. 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 or NSEC3 records also prevents
adversaries from enumerating the entire contents of DNS zones by
walking the NSEC chain or performing an offline dictionary attack on
the hashes in the NSEC3 chain.
This document assumes a reasonable level of familiarity with DNS
operations and protocol terms. Much of the terminology is explained
in further detail in "DNS Terminology" [RFC9499].
1.1. Requirements Language
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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Distinguishing Non-existent Nonexistent Names
This method generates NODATA responses for non-existent nonexistent names that
don't match a DNS wildcard. Since there are clearly no record types
for such names, the NSEC Type Bit Maps field in the response will
only contain the NSEC and RRSIG types (and in the case of NSEC3, the
Type Bit Maps field will be empty). Tools that need to accurately
identify non-existent nonexistent 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 Bit Maps field.
This specification defines the use of NXNAME (128), a synthetic RR
type to signal the presence of a non-existent nonexistent name. See Section 9.
The mnemonic for this RR type is NXNAME, chosen to clearly
distinguish it from the response code mnemonic NXDOMAIN.
+========+=======+=============================+
| TYPE | Value | Meaning |
+========+=======+=============================+
| NXNAME | 128 | NXDOMAIN indicator for |
| | | Compact Denial of Existence |
+--------+-------+-----------------------------+
Table 1
This RR type is added to the NSEC Type Bit Maps field for responses
to non-existent nonexistent names, in addition to the required RRSIG and NSEC
types. If NSEC3 is being used, this RR type is the sole entry in the
Type Bit Maps field. It is a "Meta-TYPE", as defined in [RFC6895],
and it stores no data in a DNS zone and cannot be usefully queried.
Section 3.5 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 5.1 ("Signaled Response Code Restoration") to
better restore NXDOMAIN visibility to various applications that may
remain oblivious to the new NXNAME signal.
3. Generating Responses with NSEC
This section describes various types of answers generated by
authoritative servers implementing Compact Denial of Existence using
NSEC. Section 4 describes changes needed to support NSEC3.
3.1. Responses for Non-existent Nonexistent Names
When the authoritative server receives a query for a non-existent nonexistent 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) QNAME placed in the
Authority section.
The Next Domain Name field SHOULD be set to the immediate
lexicographic successor of the QNAME. This is accomplished by adding
a leading label with a single null (zero-value) octet. 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-existent nonexistent name "a.example.com."
would result in the generation of the following NSEC record (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.
3.2. Responses for Non-existent Nonexistent Types
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. QNAME. The Next Domain Name field is set to the
immediate lexicographic successor of the QNAME. The Type Bit Maps
field lists the available RR types at the name.
An Empty Non-Terminal ENT 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, ENT, the NSEC Type Bit Maps field 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 descendant
of the ENT in the zone.)
3.3. Responses for Wildcard Matches
For wildcard matches, the authoritative server will provide a
dynamically signed response that claims that the queried name QNAME exists
explicitly. Specifically, the answer RRset 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 QNAME matches a wildcard but
no data for the queried type exists), a response akin to a non-wildcard 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 Bit Maps field representing the list of types
available at the wildcard.
3.4. Responses for Unsigned Referrals
Per the DNSSEC protocol, a referral to an unsigned subzone includes
an NSEC record whose owner name matches the subzone and proves the
delegation is unsigned by the absence of the Delegation Signer (DS)
RR type bit in the Type Bit Maps field.
With Compact Denial of Existence, the Next Domain Name field for this
NSEC record is computed with a slightly different epsilon function
than the immediate lexicographic successor of the owner name, as that
name would then fall under the delegated subzone. Instead, the Next
Domain Name field is formed by appending a zero octet to the first
label of the owner name.
For example, a referral response for the subzone sub.example.com
would include an NSEC record like the following:
sub.example.com. 3600 IN NSEC sub\000.example.com. NS RRSIG NSEC
3.5. Responses to Explicit Queries for NXNAME
NXNAME is a Meta-TYPE that should not appear anywhere in a DNS
message apart from the NSEC type bitmap of a Compact Answer response
for a non-existent nonexistent name. However, if 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 range for Meta-TYPEs and QTYPEs, except
those types that are already defined to support queries.
Optionally, a DNS server MAY also include the following Extended DNS
Error (EDE) code [RFC8914] in the response:
+===========+====================+
| INFO-CODE | Purpose |
+===========+====================+
| 30 | Invalid (Invalid Query Type |
+-----------+--------------------+
Table 2 Type).
See Section 9.
Note that this EDE code is generally applicable to any RR type that
should not appear in DNS queries.
4. Generating Responses with NSEC3
NSEC3 [RFC5155] uses hashed names to provide zone enumeration
defense. This protection is already better provided by minimally
covering NSEC records. NSEC3's Opt-Out feature also provides no
specific benefit for online signing implementations (minimally
covering NSEC3 records provide no useful Opt-Out span). Hence, there
is no known advantage to implementing Compact Denial of Existence
with NSEC3. However, an existing implementation of traditional NSEC3
online signing migrating to Compact Denial of Existence may find it
simpler to do so with NSEC3 rather than implementing NSEC from
scratch.
For NSEC3, the functional details of the protocol remain as described
in Section 3, with the following changes.
NSEC3 records and their signatures are dynamically generated instead
of NSEC.
The NSEC3 parameters should be set to algorithm 1, a flags field of
0, an additional hash iteration count of 0, and an empty salt. In
DNS presentation format, this is "1 0 0 -".
The owner name of the NSEC3 resource record is the NSEC3 hash of the
relevant domain name, encoded in Base32 with Extended Hex Alphabet
([RFC4648], Section 7), prepended as a single label to the name of
the zone. The Next Hashed Owner Name is the immediate name successor
of the unencoded binary form of the previous hash, which can be
computed by adding one to the binary hash value.
In responses for non-existent nonexistent names, the Type Bit Maps field will
contain only the NXNAME Meta-TYPE. In responses to Empty Non-
Terminal ENT names, the
Type Bit Maps field will be empty.
For example, a request for the non-existent nonexistent name "a.example.com."
would result in the generation of the following NSEC3 record:
H64KFA4P1ACER2EBPS9QSDK6DNP8B3JQ.example.com. IN NSEC3 1 0 0 - (
H64KFA4P1ACER2EBPS9QSDK6DNP8B3JR NXNAME )
Unlike Compact Denial of Existence with NSEC, no special form of the
next name
Next Hashed Owner Name field for unsigned referrals is needed. The
Next Hashed Owner Name field remains the NSEC3 hash of original owner
name plus one.
5. Response Code Restoration
For non-existent nonexistent names, implementations should try to preserve the
response code value of 3 (NXDOMAIN) whenever possible. This is
generally possible for non-DNSSEC-enabled queries, namely those that
do not set the DNSSEC_OK EDNS flag (DO bit). DO bit ("DNSSEC answer OK") in the EDNS0 OPT header.
For such queries, authoritative servers implementing Compact Denial
of Existence could return a normal NXDOMAIN response. However, there
may be limited benefit to doing this since most modern DNS resolvers
are DNSSEC aware, and per Section 3 of [RFC3225], 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.
5.1. Signaled Response Code Restoration
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
second 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 with a signed NXNAME for a non-existent
nonexistent name, together with the response code field set to
NXDOMAIN (3).
In responses to such queries, a Compact Denial an authoritative server implementing
both Compact Denial of Existence and this signaling scheme will set
the Compact Answers OK EDNS header flag and, for non-existent nonexistent 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.
6. Operational Implications
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 5.1.
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 nonexistent names will need to infer them from
the presence of the NXNAME RR type in the Type Bit Maps field 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 a AAAA record for a
non-existent
nonexistent 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 could 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 Bit Maps field in
responses to accurately determine non-existent nonexistent names. Note that this
is less of a concern with connection functions like Happy Eyeballs
[RFC8305] that typically issue back-to-back DNS queries without
waiting for individual responses.
Protocol optimizations that enable DNS resolvers to synthesize
NXDOMAIN or wildcard responses, like those described in [RFC8020] and
[RFC8198], cannot be realized from responses that use Compact Denial
of Existence. In general, no online signing scheme that employs
minimally covering NSEC or NSEC3 records (including this one) permits
NXDOMAIN or wildcard response synthesis in the style described in
[RFC8198]. Additionally, this protocol also precludes NXDOMAIN
synthesis for DNSSEC-enabled responses in the style described in
[RFC8020].
7. Updates to RFCs
7.1. Updates to RFC 4034
Section 4.1.2 of [RFC4034] ("The Type Bit Maps Field") states the
following:
| Bits representing pseudo-types MUST be clear, as they do not
| appear in zone data. If encountered, they MUST be ignored upon
| being read.
This paragraph is updated to the following:
| Bits representing pseudo-types MUST be clear, as they do not
| appear in zone data. If encountered, they MUST be ignored upon
| being read. There is one exception to this rule for Compact
| Denial of Existence (RFC 9824), where the NXNAME pseudo-type is
| allowed to appear in responses to non-existent nonexistent names.
|
|
Note: As a practical matter, no known resolver insists that
| pseudo-types pseudo-
types must be clear in the NSEC Type Bit Maps, Maps field, so this
|
restriction (prior to its revision) has posed no problem for the
|
deployment of this mechanism.
7.2. Updates to RFC 4035
Section 2.3 of [RFC4035] ("Including NSEC RRs in a Zone") states the
following:
| An NSEC record (and its associated RRSIG RRset) MUST NOT be the
| only RRset at any particular owner name. That is, the signing
| process MUST NOT create NSEC or RRSIG RRs for owner name nodes
| that were not the owner name of any RRset before the zone was
| signed. The main reasons for this are a desire for namespace
| consistency between signed and unsigned versions of the same zone
| and a desire to reduce the risk of response inconsistency in
| security oblivious recursive name servers.
This paragraph is updated to the following:
| An NSEC record (and its associated RRSIG RRset) MUST NOT be the
| only RRset at any particular owner name. That is, the signing
| process MUST NOT create NSEC or RRSIG RRs for owner name nodes
| that were not the owner name of any RRset before the zone was
| signed. The main reasons for this are a desire for namespace
| consistency between signed and unsigned versions of the same zone
| and a desire to reduce the risk of response inconsistency in
| security oblivious recursive name servers. This concern only
| applies to implementations of DNSSEC that employ precomputed
| signatures. There is an exception to this rule for online signing
| implementations of DNSSEC (e.g., minimally covering NSEC and
| Compact Denial of Existence), where dynamically generated NSEC
| records can be produced for owner names that don't exist or are
| empty non-terminals. ENTs.
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 precomputed 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 precomputed 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 nonexistent domain names by
examining the response code field of DNS response messages. A
NOERROR (rather than NXDOMAIN) code in that field 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.
Because this method does not allow for aggressive negative caching
among resolvers, it allows for certain types of denial-of-service
attacks to be more effective. This includes so-called Pseudorandom
Subdomain Attacks [PRSD-ATTACK], where random names are queried,
either directly via botnets or across a wide range of public resolver
services, in order to intentionally generate non-existent nonexistent responses
from the authoritative servers for a domain. If the resolver cannot
synthesize NXDOMAIN responses from NSEC records, it must pass all
queries on to the domain's authority servers, making resource
exhaustion more likely at those latter servers if they do not have
the capacity to absorb those additional queries.
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
precomputed signatures) and avoid imposing the challenges of NXDOMAIN
visibility.
9. IANA Considerations
IANA has allocated the following in the "Resource Record (RR) TYPEs"
registry in the "Domain Name System (DNS) Parameters" registry group,
from the range for Meta-TYPEs. A lower number in this range lowers
the size of the Type Bit Maps field, which reduces the overall size
of the DNS response message.
+========+=======+=============================+===========+
| Type | Value | Meaning | Reference |
+========+=======+=============================+===========+
| NXNAME | 128 | NXDOMAIN indicator for | RFC 9824 |
| | | Compact Denial of Existence | |
+--------+-------+-----------------------------+-----------+
Table 3 1
IANA has also allocated the following flag in the "EDNS Header Flags
(16 bits)" registry in the "Domain Name System (DNS) Parameters"
registry group. This flag is described in Section 5.1.
+=======+======+====================+===========+
| Bit | Flag | Description | Reference |
+=======+======+====================+===========+
| Bit 1 | CO | Compact Answers OK | RFC 9824 |
+-------+------+--------------------+-----------+
Table 4 2
Last, IANA has allocated the following code point in the "Extended
DNS Error Codes" registry in the "Domain Name System (DNS)
Parameters" registry group. This code point is described in
Section 3.5.
+===========+====================+===========+
| INFO-CODE | Purpose | Reference |
+===========+====================+===========+
| 30 | Invalid Query Type | RFC 9824 |
+-----------+--------------------+-----------+
Table 5 3
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC",
RFC 3225, DOI 10.17487/RFC3225, December 2001,
<https://www.rfc-editor.org/info/rfc3225>.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<https://www.rfc-editor.org/info/rfc4034>.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<https://www.rfc-editor.org/info/rfc4035>.
[RFC4470] Weiler, S. and J. Ihren, "Minimally Covering NSEC Records
and DNSSEC On-line Signing", RFC 4470,
DOI 10.17487/RFC4470, April 2006,
<https://www.rfc-editor.org/info/rfc4470>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
<https://www.rfc-editor.org/info/rfc5155>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013,
<https://www.rfc-editor.org/info/rfc6891>.
[RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA
Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
April 2013, <https://www.rfc-editor.org/info/rfc6895>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8914] Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
Lawrence, "Extended DNS Errors", RFC 8914,
DOI 10.17487/RFC8914, October 2020,
<https://www.rfc-editor.org/info/rfc8914>.
[RFC9364] Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237,
RFC 9364, DOI 10.17487/RFC9364, February 2023,
<https://www.rfc-editor.org/info/rfc9364>.
10.2. Informative References
[BLACK-LIES]
Valsorda, F. and O. Gudmundsson, "Compact DNSSEC Denial of
Existence or Black Lies", Work in Progress, Internet-
Draft, draft-valsorda-dnsop-black-lies-00, 21 March 2016,
<https://datatracker.ietf.org/doc/html/draft-valsorda-
dnsop-black-lies-00>.
[ENT-SENTINEL]
Huque, S., "Empty Non-Terminal Sentinel for Black Lies",
Work in Progress, Internet-Draft, draft-huque-dnsop-
blacklies-ent-01, 27 July 2021,
<https://datatracker.ietf.org/doc/html/draft-huque-dnsop-
blacklies-ent-01>.
[NXDOMAIN-TYPE]
Gudmundsson, O. and F. Valsorda, "Signaling NSEC record
owner name nonexistence", Work in Progress, Internet-
Draft, draft-ogud-fake-nxdomain-type-00, 7 May 2015,
<https://datatracker.ietf.org/doc/html/draft-ogud-fake-
nxdomain-type-00>.
[PRSD-ATTACK]
Nishida, K., "Water Torture: A Slow Drip DNS DDoS Attack
on QTNet", <https://conference.apnic.net/data/39/
dnswatertortureonqtnet_1425130417_1425507043.pptx>.
[RFC7129] Gieben, R. and W. Mekking, "Authenticated Denial of
Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129,
February 2014, <https://www.rfc-editor.org/info/rfc7129>.
[RFC8020] Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is
Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020,
November 2016, <https://www.rfc-editor.org/info/rfc8020>.
[RFC8198] Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
July 2017, <https://www.rfc-editor.org/info/rfc8198>.
[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.
[RFC9499] Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
RFC 9499, DOI 10.17487/RFC9499, March 2024,
<https://www.rfc-editor.org/info/rfc9499>.
Appendix A. Other Approaches
In the past, some implementations of Compact Denial of Existence have
used other means to differentiate NXDOMAIN from Empty Non-Terminals. ENTs.
One method employed by both Cloudflare and Amazon Route53 for a
period of time was the following: For responses to Empty Non-
Terminals, ENTs, they
synthesized the NSEC Type Bit Maps field to include all record types
supported except for the queried type. This method has the
undesirable effect of no longer being able to reliably determine the
existence of ENTs and of making the Type Bit Maps 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.
Another way to distinguish NXDOMAIN from ENT is to define the
synthetic RR type for ENTs instead, as specified in [ENT-SENTINEL].
This method was successfully deployed in the field by NS1 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. ENTs. 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.
Appendix B. Historical Implementation Notes
At the time of publication, the basic Compact Denial of Existence
method using NSEC is implemented by Cloudflare, NS1, Amazon Route53,
and Knot DNS's optional online signing module. From early 2021 until
November 2023, NS1 had deployed the Empty Non-Terminal ENT distinguisher [ENT-SENTINEL]
using the private RR type code 65281. A version of the NXNAME
distinguisher using the private RR type code 65238 was deployed by
both Cloudflare (from July 2023) and NS1 (from November 2023) until
roughly September 2024. Since September 2024, both Cloudflare and
NS1 have deployed NXNAME using the officially allocated code point of
128. Oracle Cloud Initiative implemented Compact Denial of Existence
using NSEC3 in October 2024.
Acknowledgements
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 ENT 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 Včelák, Viktor Dukhovni,
Ed Lewis, and John Levine. The authors would also like to thank the
many other members of the IETF DNS Operations Working Group for
helpful comments and discussions.
Authors' Addresses
Shumon Huque
Salesforce
415 Mission Street, 3rd Floor
San Francisco, CA 94105
United States of America
Email: shuque@gmail.com
Christian Elmerot
Cloudflare
101 Townsend St.
San Francisco, CA 94107
United States of America
Email: elmerot@cloudflare.com
Olafur Gudmundsson
Retired/Unaffiliated
Email: ogud@ogud.com