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IETF RFC 8914
Last modified on Friday, October 23rd, 2020
 Permanent link to RFC 8914
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Internet Engineering Task Force (IETF) W. Kumari
Request for Comments: 8914 Google
Category: Standards Track E. Hunt
ISSN: 2070-1721 ISC
R. Arends
ICANN
W. Hardaker
USC/ISI
D. Lawrence
Salesforce
October 2020
Extended DNS Errors
 Abstract
This document defines an extensible method to return additional
information about the cause of DNS errors. Though created primarily
to extend SERVFAIL to provide additional information about the cause
of DNS and DNSSEC failures, the Extended DNS Errors option defined in
this document allows all response types to contain extended error
information. Extended DNS Error information does not change the
processing of RCODEs.
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/RFC 8914.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction and Background
1.1. Requirements Notation
2. Extended DNS Error EDNS0 Option Format
3. Extended DNS Error Processing
4. Defined Extended DNS Errors
4.1. Extended DNS Error Code 0 - Other
4.2. Extended DNS Error Code 1 - Unsupported DNSKEY Algorithm
4.3. Extended DNS Error Code 2 - Unsupported DS Digest Type
4.4. Extended DNS Error Code 3 - Stale Answer
4.5. Extended DNS Error Code 4 - Forged Answer
4.6. Extended DNS Error Code 5 - DNSSEC Indeterminate
4.7. Extended DNS Error Code 6 - DNSSEC Bogus
4.8. Extended DNS Error Code 7 - Signature Expired
4.9. Extended DNS Error Code 8 - Signature Not Yet Valid
4.10. Extended DNS Error Code 9 - DNSKEY Missing
4.11. Extended DNS Error Code 10 - RRSIGs Missing
4.12. Extended DNS Error Code 11 - No Zone Key Bit Set
4.13. Extended DNS Error Code 12 - NSEC Missing
4.14. Extended DNS Error Code 13 - Cached Error
4.15. Extended DNS Error Code 14 - Not Ready
4.16. Extended DNS Error Code 15 - Blocked
4.17. Extended DNS Error Code 16 - Censored
4.18. Extended DNS Error Code 17 - Filtered
4.19. Extended DNS Error Code 18 - Prohibited
4.20. Extended DNS Error Code 19 - Stale NXDOMAIN Answer
4.21. Extended DNS Error Code 20 - Not Authoritative
4.22. Extended DNS Error Code 21 - Not Supported
4.23. Extended DNS Error Code 22 - No Reachable Authority
4.24. Extended DNS Error Code 23 - Network Error
4.25. Extended DNS Error Code 24 - Invalid Data
5. IANA Considerations
5.1. A New Extended DNS Error Code EDNS Option
5.2. New Registry for Extended DNS Error Codes
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction and Background
There are many reasons that a DNS query may fail -- some of them
transient, some permanent; some can be resolved by querying another
server, some are likely best handled by stopping resolution.
Unfortunately, the error signals that a DNS server can return are
very limited and are not very expressive. This means that
applications and resolvers often have to "guess" at what the issue
is, e.g., was the answer marked REFUSED because of a lame delegation
or because the nameserver is still starting up and loading zones? Is
a SERVFAIL a DNSSEC validation issue, or is the nameserver
experiencing some other failure? What error messages should be
presented to the user or logged under these conditions?
A good example of issues that would benefit from additional error
information are errors caused by DNSSEC validation issues. When a
stub resolver queries a name that is DNSSEC bogus [RFC 8499] (using a
validating resolver), the stub resolver receives only a SERVFAIL in
response. Unfortunately, the SERVFAIL Response Code (RCODE) is used
to signal many sorts of DNS errors, and so the stub resolver's only
option is to ask the next configured DNS resolver. The result of
trying the next resolver is one of two outcomes: either the next
resolver also validates and a SERVFAIL is returned again or the next
resolver is not a validating resolver and the user is returned a
potentially harmful result. With an Extended DNS Error (EDE) option
enclosed in the response message, the resolver is able to return a
more descriptive reason as to why any failures happened or add
additional context to a message containing a NOERROR RCODE.
This document specifies a mechanism to extend DNS errors to provide
additional information about the cause of an error. The Extended DNS
Error codes described in this document can be used by any system that
sends DNS queries and receives a response containing an EDE option.
Different codes are useful in different circumstances, and thus
different systems (stub resolvers, recursive resolvers, and
authoritative resolvers) might receive and use them.
1.1. Requirements Notation
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 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
2. Extended DNS Error EDNS0 Option Format
This document uses an Extended Mechanism for DNS (EDNS0) [RFC 6891]
option to include Extended DNS Error (EDE) information in DNS
messages. The option is structured as follows:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: | OPTION-CODE |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: | OPTION-LENGTH |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
4: | INFO-CODE |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
6: / EXTRA-TEXT ... /
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Field definition details:
OPTION-CODE:
2 octets / 16 bits (defined in [RFC 6891]) contains the value 15
for EDE.
OPTION-LENGTH:
2 octets / 16 bits (defined in [RFC 6891]) contains the length of
the payload (everything after OPTION-LENGTH) in octets and should
be 2 plus the length of the EXTRA-TEXT field (which may be a zero-
length string).
INFO-CODE:
16 bits, which is the principal contribution of this document.
This 16-bit value, encoded in network most significant bit (MSB)
byte order, provides the additional context for the RESPONSE-CODE
of the DNS message. The INFO-CODE serves as an index into the
"Extended DNS Errors" registry, defined and created in
Section 5.2.
EXTRA-TEXT:
a variable-length, UTF-8-encoded [RFC 5198] text field that may
hold additional textual information. This information is intended
for human consumption (not automated parsing). EDE text may be
null terminated but MUST NOT be assumed to be; the length MUST be
derived from the OPTION-LENGTH field. The EXTRA-TEXT field may be
zero octets in length, indicating that there is no EXTRA-TEXT
included. Care should be taken not to include private information
in the EXTRA-TEXT field that an observer would not otherwise have
access to, such as account numbers.
The Extended DNS Error (EDE) option can be included in any response
(SERVFAIL, NXDOMAIN, REFUSED, even NOERROR, etc.) to a query that
includes an OPT pseudo-RR [RFC 6891]. This document includes a set of
initial codepoints but is extensible via the IANA registry defined
and created in Section 5.2.
3. Extended DNS Error Processing
When the response grows beyond the requestor's UDP payload size
[RFC 6891], servers SHOULD truncate messages by dropping EDE options
before dropping other data from packets. Implementations SHOULD set
the truncation bit when dropping EDE options. Because long EXTRA-
TEXT fields may trigger truncation (which is undesirable given the
supplemental nature of EDE), implementers and operators creating EDE
options SHOULD avoid lengthy EXTRA-TEXT contents.
When a resolver or forwarder receives an EDE option, whether or not
(and how) to pass along EDE information on to their original client
is implementation dependent. Implementations MAY choose to not
forward information, or they MAY choose to create a new EDE option(s)
that conveys the information encoded in the received EDE. When doing
so, the source of the error SHOULD be attributed in the EXTRA-TEXT
field, since an EDNS0 option received by the original client will
appear to have come from the resolver or forwarder sending it.
This document does not allow or prohibit any particular extended
error codes and information to be matched with any particular RCODEs.
Some combinations of extended error codes and RCODEs may seem
nonsensical (such as resolver-specific extended error codes received
in responses from authoritative servers), so systems interpreting the
extended error codes MUST NOT assume that a combination will make
sense. Receivers MUST be able to accept EDE codes and EXTRA-TEXT in
all messages, including those with a NOERROR RCODE but need not act
on them. Applications MUST continue to follow requirements from
applicable specifications on how to process RCODEs no matter what EDE
values are also received. Senders MAY include more than one EDE
option and receivers MUST be able to accept (but not necessarily
process or act on) multiple EDE options in a DNS message.
4. Defined Extended DNS Errors
This document defines some initial EDE codes. The mechanism is
intended to be extensible, and additional codepoints can be
registered in the "Extended DNS Errors" registry (Section 5.2). The
INFO-CODE from the EDE EDNS option is used to serve as an index into
the "Extended DNS Error" IANA registry, the initial values for which
are defined in the following subsections.
4.1. Extended DNS Error Code 0 - Other
The error in question falls into a category that does not match known
extended error codes. Implementations SHOULD include an EXTRA-TEXT
value to augment this error code with additional information.
4.2. Extended DNS Error Code 1 - Unsupported DNSKEY Algorithm
The resolver attempted to perform DNSSEC validation, but a DNSKEY
RRset contained only unsupported DNSSEC algorithms.
4.3. Extended DNS Error Code 2 - Unsupported DS Digest Type
The resolver attempted to perform DNSSEC validation, but a DS RRset
contained only unsupported Digest Types.
4.4. Extended DNS Error Code 3 - Stale Answer
The resolver was unable to resolve the answer within its time limits
and decided to answer with previously cached data instead of
answering with an error. This is typically caused by problems
communicating with an authoritative server, possibly as result of a
denial of service (DoS) attack against another network. (See also
Code 19.)
4.5. Extended DNS Error Code 4 - Forged Answer
For policy reasons (legal obligation or malware filtering, for
instance), an answer was forged. Note that this should be used when
an answer is still provided, not when failure codes are returned
instead. See Blocked (15), Censored (16), and Filtered (17) for use
when returning other response codes.
4.6. Extended DNS Error Code 5 - DNSSEC Indeterminate
The resolver attempted to perform DNSSEC validation, but validation
ended in the Indeterminate state [RFC 4035].
4.7. Extended DNS Error Code 6 - DNSSEC Bogus
The resolver attempted to perform DNSSEC validation, but validation
ended in the Bogus state.
4.8. Extended DNS Error Code 7 - Signature Expired
The resolver attempted to perform DNSSEC validation, but no
signatures are presently valid and some (often all) are expired.
4.9. Extended DNS Error Code 8 - Signature Not Yet Valid
The resolver attempted to perform DNSSEC validation, but no
signatures are presently valid and at least some are not yet valid.
4.10. Extended DNS Error Code 9 - DNSKEY Missing
A DS record existed at a parent, but no supported matching DNSKEY
record could be found for the child.
4.11. Extended DNS Error Code 10 - RRSIGs Missing
The resolver attempted to perform DNSSEC validation, but no RRSIGs
could be found for at least one RRset where RRSIGs were expected.
4.12. Extended DNS Error Code 11 - No Zone Key Bit Set
The resolver attempted to perform DNSSEC validation, but no Zone Key
Bit was set in a DNSKEY.
4.13. Extended DNS Error Code 12 - NSEC Missing
The resolver attempted to perform DNSSEC validation, but the
requested data was missing and a covering NSEC or NSEC3 was not
provided.
4.14. Extended DNS Error Code 13 - Cached Error
The resolver is returning the SERVFAIL RCODE from its cache.
4.15. Extended DNS Error Code 14 - Not Ready
The server is unable to answer the query, as it was not fully
functional when the query was received.
4.16. Extended DNS Error Code 15 - Blocked
The server is unable to respond to the request because the domain is
on a blocklist due to an internal security policy imposed by the
operator of the server resolving or forwarding the query.
4.17. Extended DNS Error Code 16 - Censored
The server is unable to respond to the request because the domain is
on a blocklist due to an external requirement imposed by an entity
other than the operator of the server resolving or forwarding the
query. Note that how the imposed policy is applied is irrelevant
(in-band DNS filtering, court order, etc.).
4.18. Extended DNS Error Code 17 - Filtered
The server is unable to respond to the request because the domain is
on a blocklist as requested by the client. Functionally, this
amounts to "you requested that we filter domains like this one."
4.19. Extended DNS Error Code 18 - Prohibited
An authoritative server or recursive resolver that receives a query
from an "unauthorized" client can annotate its REFUSED message with
this code. Examples of "unauthorized" clients are recursive queries
from IP addresses outside the network, blocklisted IP addresses,
local policy, etc.
4.20. Extended DNS Error Code 19 - Stale NXDOMAIN Answer
The resolver was unable to resolve an answer within its configured
time limits and decided to answer with a previously cached NXDOMAIN
answer instead of answering with an error. This may be caused, for
example, by problems communicating with an authoritative server,
possibly as result of a denial of service (DoS) attack against
another network. (See also Code 3.)
4.21. Extended DNS Error Code 20 - Not Authoritative
An authoritative server that receives a query with the Recursion
Desired (RD) bit clear, or when it is not configured for recursion
for a domain for which it is not authoritative, SHOULD include this
EDE code in the REFUSED response. A resolver that receives a query
with the RD bit clear SHOULD include this EDE code in the REFUSED
response.
4.22. Extended DNS Error Code 21 - Not Supported
The requested operation or query is not supported.
4.23. Extended DNS Error Code 22 - No Reachable Authority
The resolver could not reach any of the authoritative name servers
(or they potentially refused to reply).
4.24. Extended DNS Error Code 23 - Network Error
An unrecoverable error occurred while communicating with another
server.
4.25. Extended DNS Error Code 24 - Invalid Data
The authoritative server cannot answer with data for a zone it is
otherwise configured to support. Examples of this include its most
recent zone being too old or having expired.
5. IANA Considerations
5.1. A New Extended DNS Error Code EDNS Option
This document defines a new EDNS(0) option, entitled "Extended DNS
Error", with the assigned value of 15 from the "DNS EDNS0 Option
Codes (OPT)" registry:
+=======+====================+==========+===========+
| Value | Name | Status | Reference |
+=======+====================+==========+===========+
| 15 | Extended DNS Error | Standard | RFC 8914 |
+-------+--------------------+----------+-----------+
Table 1
5.2. New Registry for Extended DNS Error Codes
IANA has created and will maintain a new registry called "Extended
DNS Error Codes" on the "Domain Name System (DNS) Parameters" web
page as follows:
+===============+=========================+
| Range | Registration Procedures |
+===============+=========================+
| 0 - 49151 | First Come First Served |
+---------------+-------------------------+
| 49152 - 65535 | Private Use |
+---------------+-------------------------+
Table 2
The "Extended DNS Error Codes" registry is a table with three
columns: INFO-CODE, Purpose, and Reference. The initial content is
as below.
+=============+==============================+===============+
| INFO-CODE | Purpose | Reference |
+=============+==============================+===============+
| 0 | Other Error | Section 4.1 |
+-------------+------------------------------+---------------+
| 1 | Unsupported DNSKEY Algorithm | Section 4.2 |
+-------------+------------------------------+---------------+
| 2 | Unsupported DS Digest Type | Section 4.3 |
+-------------+------------------------------+---------------+
| 3 | Stale Answer | Section 4.4 |
| | | and [RFC 8767] |
+-------------+------------------------------+---------------+
| 4 | Forged Answer | Section 4.5 |
+-------------+------------------------------+---------------+
| 5 | DNSSEC Indeterminate | Section 4.6 |
+-------------+------------------------------+---------------+
| 6 | DNSSEC Bogus | Section 4.7 |
+-------------+------------------------------+---------------+
| 7 | Signature Expired | Section 4.8 |
+-------------+------------------------------+---------------+
| 8 | Signature Not Yet Valid | Section 4.9 |
+-------------+------------------------------+---------------+
| 9 | DNSKEY Missing | Section 4.10 |
+-------------+------------------------------+---------------+
| 10 | RRSIGs Missing | Section 4.11 |
+-------------+------------------------------+---------------+
| 11 | No Zone Key Bit Set | Section 4.12 |
+-------------+------------------------------+---------------+
| 12 | NSEC Missing | Section 4.13 |
+-------------+------------------------------+---------------+
| 13 | Cached Error | Section 4.14 |
+-------------+------------------------------+---------------+
| 14 | Not Ready | Section 4.15 |
+-------------+------------------------------+---------------+
| 15 | Blocked | Section 4.16 |
+-------------+------------------------------+---------------+
| 16 | Censored | Section 4.17 |
+-------------+------------------------------+---------------+
| 17 | Filtered | Section 4.18 |
+-------------+------------------------------+---------------+
| 18 | Prohibited | Section 4.19 |
+-------------+------------------------------+---------------+
| 19 | Stale NXDomain Answer | Section 4.20 |
+-------------+------------------------------+---------------+
| 20 | Not Authoritative | Section 4.21 |
+-------------+------------------------------+---------------+
| 21 | Not Supported | Section 4.22 |
+-------------+------------------------------+---------------+
| 22 | No Reachable Authority | Section 4.23 |
+-------------+------------------------------+---------------+
| 23 | Network Error | Section 4.24 |
+-------------+------------------------------+---------------+
| 24 | Invalid Data | Section 4.25 |
+-------------+------------------------------+---------------+
| 25-49151 | Unassigned | |
+-------------+------------------------------+---------------+
| 49152-65535 | Reserved for Private Use | Section 5.2 |
+-------------+------------------------------+---------------+
Table 3
6. Security Considerations
Though DNSSEC continues to be deployed, unfortunately a significant
number of clients (~11% according to [GeoffValidation]) that receive
a SERVFAIL from a validating resolver because of a DNSSEC validation
issue will simply ask the next (potentially non-validating) resolver
in their list and thus don't get the protections that DNSSEC should
provide.
EDE information is unauthenticated information, unless secured by a
form of secured DNS transaction, such as [RFC 2845], [RFC 2931],
[RFC 8094], or [RFC 8484]. An attacker (e.g., a man in the middle
(MITM) or malicious recursive server) could insert an extended error
response into untrusted data -- although, ideally, clients and
resolvers would not trust any unauthenticated information. As such,
EDE content should be treated only as diagnostic information and MUST
NOT alter DNS protocol processing. Until all DNS answers are
authenticated via DNSSEC or the other mechanisms mentioned above,
there are some trade-offs. As an example, an attacker who is able to
insert the DNSSEC Bogus Extended Error into a DNS message could
instead simply reply with a fictitious address (A or AAAA) record.
Note that DNS RCODEs also contain no authentication and can be just
as easily manipulated.
By design, EDE potentially exposes additional information via DNS
resolution processes that may leak information. An example of this
is the Prohibited EDE code (18), which may leak the fact that the
name is on a blocklist.
7. References
7.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC 4035, March 2005,
<https://www.rfc-editor.org/info/RFC 4035>.
[RFC 5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, DOI 10.17487/RFC 5198, March 2008,
<https://www.rfc-editor.org/info/RFC 5198>.
[RFC 6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC 6891, April 2013,
<https://www.rfc-editor.org/info/RFC 6891>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
[RFC 8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC 8499,
January 2019, <https://www.rfc-editor.org/info/RFC 8499>.
[RFC 8767] Lawrence, D., Kumari, W., and P. Sood, "Serving Stale Data
to Improve DNS Resiliency", RFC 8767,
DOI 10.17487/RFC 8767, March 2020,
<https://www.rfc-editor.org/info/RFC 8767>.
7.2. Informative References
[GeoffValidation]
Huston, G., "A quick review of DNSSEC Validation in
today's Internet", June 2016, <http://www.potaroo.net/
presentations/2016-06-27-dnssec.pdf>.
[RFC 2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, DOI 10.17487/RFC 2845, May 2000,
<https://www.rfc-editor.org/info/RFC 2845>.
[RFC 2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures
( SIG(0)s )", RFC 2931, DOI 10.17487/RFC 2931, September
2000, <https://www.rfc-editor.org/info/RFC 2931>.
[RFC 8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
Transport Layer Security (DTLS)", RFC 8094,
DOI 10.17487/RFC 8094, February 2017,
<https://www.rfc-editor.org/info/RFC 8094>.
[RFC 8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC 8484, October 2018,
<https://www.rfc-editor.org/info/RFC 8484>.
Acknowledgements
The authors wish to thank Joe Abley, Mark Andrews, Tim April,
Vittorio Bertola, Stephane Bortzmeyer, Vladimir Cunat, Ralph Dolmans,
Peter DeVries, Peter van Dijk, Mats Dufberg, Donald Eastlake, Bob
Harold, Paul Hoffman, Geoff Huston, Shane Kerr, Edward Lewis, Carlos
M. Martinez, George Michelson, Eric Orth, Michael Sheldon, Puneet
Sood, Petr Spacek, Ondrej Sury, John Todd, Loganaden Velvindron, and
Paul Vixie. They also vaguely remember discussing this with a number
of people over the years but have forgotten who all of them were.
Apologies if we forgot to acknowledge your contributions.
One author also wants to thank the band Infected Mushroom for
providing a good background soundtrack. Another author would like to
thank the band Mushroom Infectors. This was funny at the time we
wrote it, but we cannot remember why...
Authors' Addresses
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America
Email: warren@kumari.net
Evan Hunt
ISC
950 Charter St
Redwood City, CA 94063
United States of America
Email: each@isc.org
Roy Arends
ICANN
Email: roy.arends@icann.org
Wes Hardaker
USC/ISI
P.O. Box 382
Davis, CA 95617
United States of America
Email: ietf@hardakers.net
David C Lawrence
Salesforce
415 Mission St
San Francisco, CA 94105
United States of America
Email: tale@dd.org
RFC TOTAL SIZE: 29242 bytes
PUBLICATION DATE: Friday, October 23rd, 2020
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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