Network Working Group W. Kumari
Internet-Draft Google
Intended status: Standards Track E. Hunt
Expires: February 10, 2020 ISC
R. Arends
W. Hardaker
D. Lawrence
Oracle + Dyn
August 09, 2019

Extended DNS Errors


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.

Status of This Memo

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Table of Contents

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 a bad hair day?

A good example of issues that would benefit by additional error information are errors caused by DNSSEC validation issues. When a stub resolver queries a DNSSEC bogus name (using a validating resolver), the stub resolver receives only a SERVFAIL in response. Unfortunately, SERVFAIL is used to signal many sorts of DNS errors, and so the stub resolver simply asks the next configured DNS resolver. The result of trying the next resolver is one of two outcomes: either the next resolver also validates, a SERVFAIL is returned again, and the user gets an (largely) incomprehensible error message; or the next resolver is not a validating resolver, and the user is returned a potentially harmful result.

This document specifies a mechanism to extend (or annotate) DNS errors to provide additional information about the cause of the error. These extended error codes are specially useful when received by resolvers, to return to stub resolvers or to downstream resolvers. Authoritative servers MAY parse and use them, but most error codes would make no sense for them. Authoritative servers may need to generate extended error codes though.

1.1. Requirements notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

2. Extended Error EDNS0 option format

This draft uses an EDNS0 ([RFC2671]) 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:

3. Use of the Extended DNS Error option

The Extended DNS Error (EDE) is an EDNS option. It can be included in any response (SERVFAIL, NXDOMAIN, REFUSED, etc) to a query that includes OPT Pseudo-RR [RFC6891]. This document includes a set of initial codepoints (and requests to the IANA to add them to the registry), but is extensible via the IANA registry to allow additional error and information codes to be defined in the future.

The fields of the Extended DNS Error option are defined further in the following sub-sections.

3.1. The INFO-CODE field

This 16-bit value, encoded in network (MSB) byte order, provides the additional context for the RESPONSE-CODE of the DNS message. The INFO-CODE serves as an index to the "Extended DNS Errors" registry Section 5.1.

3.2. The EXTRA-TEXT field

The UTF-8-encoded, EXTRA-TEXT field may be zero-length, or may hold additional information useful to network operators.

4. Defined Extended DNS Errors

This document defines some initial EDE codes. The mechanism is intended to be extensible, and additional code-points can be registered in the "Extended DNS Errors" registry Section 5.1. The INFO-CODE from the EDE EDNS option is used to serve as an index into the "Extended DNS Error codes" IANA registry, the initial values for which are defined in the following sub-sections.

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 a 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 unknown algorithms.

4.3. Extended DNS Error Code 2 - Unsupported DS Algorithm

The resolver attempted to perform DNSSEC validation, but a DS RRSET contained only unknown algorithms.

4.4. Extended DNS Error Code 3 - Stale Answer

The resolver was unable to resolve answer within its time limits and decided to answer with a previously cached data instead of answering with an error. This is typically caused by problems on authoritative side, possibly as result of a DoS attack.

4.5. Extended DNS Error Code 4 - Forged Answer

For policy reasons (legal obligation, or malware filtering, for instance), an answer was forged.

4.6. Extended DNS Error Code 5 - DNSSEC Indeterminate

The resolver attempted to perform DNSSEC validation, but validation ended in the Indeterminate state.

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, a signature in the validation chain was expired.

4.9. Extended DNS Error Code 8 - Signature Not Yet Valid

The resolver attempted to perform DNSSEC validation, but the signatures received were 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 has cached SERVFAIL for this query without additional information.

4.15. Extended DNS Error Code 14 - Not Ready

The server is unable to answer the query as it is not fully up and functional yet.

4.16. Extended DNS Error Code 15 - Blocked

The resolver attempted to perfom a DNS query but the domain is blacklisted due to a security policy implemented on the server being directly talked to.

4.17. Extended DNS Error Code 16 - Censored

The resolver attempted to perfom a DNS query but the domain was blacklisted by a security policy imposed upon the server being talked to. Note that how the imposed policy is applied is irrelevant (in-band DNS somehow, court order, etc).

4.18. Extended DNS Error Code 17 - Prohibited

An authoritative 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, blacklisted IP addresses, local policy, etc.

4.19. Extended DNS Error Code 18 - Stale Answer

The resolver was unable to resolve answer within its time limits and decided to answer with a previously cached NXDOMAIN answer instead of answering with an error. This is typically caused by problems on authoritative side, possibly as result of a DoS attack.

4.20. Extended DNS Error Code 19 - Lame

An authoritative server that receives a query (with the RD bit clear) for a domain for which it is not authoritative SHOULD include this EDE code in the SERVFAIL response. A resolver that receives a query (with the RD bit clear) SHOULD include this EDE code in the REFUSED response.

4.21. Extended DNS Error Code 20 - Deprecated

The requested operation or query is not supported as its use has been deprecated.

4.22. Extended DNS Error Code 21 - No Reachable Authority

The resolver could not reach any of the authoritative name servers (or they refused to reply).

5. IANA Considerations

5.1. A New Extended Error Code EDNS Option

This document defines a new EDNS(0) option, entitled "Extended DNS Error", assigned a value of TBD1 from the "DNS EDNS0 Option Codes (OPT)" registry [to be removed upon publication: []

Value  Name                 Status    Reference
-----  ----------------     ------    ------------------
 TBD   Extended DNS Error    TBD       [ This document ]

5.2. New Double-Index Registry Table for Extended Error Codes

This document defines a new double-index IANA registry table, where the index value is the INFO-CODE from the Extended DNS Error EDNS option defined in this document. The IANA is requested to create and maintain this "Extended DNS Error codes" registry. The code-point space for each INFO-CODE index is to be broken into 3 ranges:

A starting set of entries, based on the contents of this document, is as follows:

Other Error
Section 4.1

Unsupported DNSKEY Algorithm
Section 4.2

Unsupported DS Algorithm
Section 4.3

Answering with stale/cached data
Section 4.4

Forged Answer
Section 4.5

DNSSEC Indeterminate
Section 4.6

Section 4.7

Signature Expired
Section 4.8

Signature Not Yet Valid
Section 4.9

DNSKEY Missing
Section 4.10

RRSIGs Missing
Section 4.11

No Zone Key Bit Set
Section 4.12

No NSEC records could be obtained
Section 4.13

The SERVFAIL error comes from the cache
Section 4.14

Not Ready.
Section 4.15

Section 4.16

Section 4.17

Section 4.18

Answering with stale/cached NXDOMAIN data
Section 4.19

Section 4.20

Section 4.21

No Reachable Authority
Section 4.21

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 validaion issue will simply ask the next (potentially non-validating) resolver in their list, and thus don't get any of the protections which DNSSEC should provide. This is very similar to a kid asking his mother if he can have another cookie. When the mother says "No, it will ruin your dinner!", going off and asking his (more permissive) father and getting a "Yes, sure, have a cookie!".

This information is unauthenticated information, and an attacker (e.g MITM or malicious recursive server) could insert an extended error response into already untrusted data — ideally clients and resolvers would not trust any unauthenticated information, but until we live in an era where all DNS answers are authenticated via DNSSEC or other mechanisms, there are some tradeoffs. As an example, an attacker who is able to insert the DNSSEC Bogus Extended Error into a packet could instead simply reply with a fictitious address (A or AAAA) record.

7. Acknowledgements

The authors wish to thank Joe Abley, Mark Andrews, Stephane Bortzmeyer, Vladimir Cunat, Ralph Dolmans, Peter DeVries, Peter van Dijk, Donald Eastlake, Bob Harold, Geoff Huston, Shane Kerr, Edward Lewis, Carlos M. Martinez, George Michelson, Michael Sheldon, Puneet Sood, Petr Spacek, Ondrej Sury, Loganaden Velvindron, and Paul Vixie. They also vaguely remember discussing this with a number of people over the years, but have forgotten who all they were -- if you were one of them, and are not listed, please let us know and we'll acknowledge you.

I also want to thank the band "Infected Mushroom" for providing a good background soundtrack (and to see if I can get away with this!) Another author would like to thank the band "Mushroom Infectors". This was funny at the time we wrote it, but we cannot remember why...

8. References

8.1. 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.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671, DOI 10.17487/RFC2671, August 1999.
[RFC6891] Damas, J., Graff, M. and P. Vixie, "Extension Mechanisms for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891, April 2013.

8.2. Informative References

[GeoffValidation] APNIC, G. H., "A quick review of DNSSEC Validation in today’s Internet", June 2016.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D. and B. Wellington, "Secret Key Transaction Authentication for DNS (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000.
[RFC8094] Reddy, T., Wing, D. and P. Patil, "DNS over Datagram Transport Layer Security (DTLS)", RFC 8094, DOI 10.17487/RFC8094, February 2017.

Authors' Addresses

Warren Kumari Google 1600 Amphitheatre Parkway Mountain View, CA, 94043 US EMail:
Evan Hunt ISC 950 Charter St Redwood City, CA, 94063 US EMail:
Roy Arends ICANN EMail:
Wes Hardaker USC/ISI P.O. Box 382 Davis, CA, 95617 US EMail:
David C Lawrence Oracle + Dyn 150 Dow St Manchester, NH, 03101 US EMail: