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.¶

2. Extended DNS Error EDNS0 Option Format

This document uses an Extended Mechanism for DNS (EDNS0) [RFC6891] 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 [RFC6891]) contains the value 15 for EDE.¶

OPTION-LENGTH:

2 octets / 16 bits (defined in [RFC6891]) 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 [RFC5198] 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 [RFC6891]. 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 [RFC6891], 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.¶

5. IANA Considerations

7. References

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...¶