Extended DNS Errors

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.

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 .

This draft uses an EDNS0 () option to include Extended DNS Error (EDE) information in DNS messages. The option is structured as follows:

Field definition details: OPTION-CODE, 2 octets (defined in ]), for EDE is TBD. [RFC Editor: change TBD to the proper code once assigned by IANA.] OPTION-LENGTH, 2 octets ((defined in ]) contains the length of the payload (everything after OPTION-LENGTH) in octets and should be 4 plus the length of the EXTRA-TEXT section (which may be a zero-length string). INFO-CODE, 16-bits, which is the principal contribution of this document. EXTRA-TEXT, a variable length, UTF-8 encoded, text field that may hold additional textual information. Note: EXTRA-TEXT may be zero octets in length, indicating there is no EXTRA-TEXT included.

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 . 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.

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 .

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

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 . 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.

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.

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

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

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.

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

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

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

The resolver attempted to perform DNSSEC validation, a signature in the validation chain was expired.

The resolver attempted to perform DNSSEC validation, but the signatures received were not yet valid.

A DS record existed at a parent, but no supported matching DNSKEY record could be found for the child.

The resolver attempted to perform DNSSEC validation, but no RRSIGs could be found for at least one RRset where RRSIGs were expected.

The resolver attempted to perform DNSSEC validation, but no Zone Key Bit was set in a DNSKEY.

The resolver attempted to perform DNSSEC validation, but the requested data was missing and a covering NSEC or NSEC3 was not provided.

The resolver has cached SERVFAIL for this query without additional information.

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

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.

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).

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.

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.

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.

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

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

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: [http://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml#dns-parameters-11]

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: 0 - : Specification required. 65023 - 65279: First come, first served. 65280 - 32767: Experimental / Private use A starting set of entries, based on the contents of this document, is as follows: 0 Other Error 1 Unsupported DNSKEY Algorithm 2 Unsupported DS Algorithm 3 Answering with stale/cached data 4 Forged Answer 5 DNSSEC Indeterminate 6 DNSSEC Bogus 7 Signature Expired 8 Signature Not Yet Valid 9 DNSKEY Missing 10 RRSIGs Missing 11 No Zone Key Bit Set 12 No NSEC records could be obtained 13 The SERVFAIL error comes from the cache 14 Not Ready. 15 Blocked 16 Censored 17 Prohibited 18 Answering with stale/cached NXDOMAIN data 19 Lame 20 Deprecated 21 No Reachable Authority

Though DNSSEC continues to be deployed, unfortunately a significant number of clients (~11% according to ) 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.

The authors wish to thank Joe Abley, Mark Andrews, Stephane Bortzmeyer, Vladimir Cunat, Peter DeVries, Peter van Dijk, Donald Eastlake, Bob Harold, Geoff Huston, Shane Kerr, Edward Lewis, Carlos M. Martinez, George Michelson, Michael Sheldon, 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...