Extension Mechanisms for DNS (EDNS0)
draft-ietf-dnsext-rfc2671bis-edns0-02

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Abstract

The Domain Name System's wire protocol includes a number of fixed fields whose range has been or soon will be exhausted and does not allow requestors to advertise their capabilities to responders. This document describes backward compatible mechanisms for allowing the protocol to grow.

This document updates the EDNS0 specification based on 10 years of operational experience.

Table of Contents

1.  Introduction
2.  Requirements Language
3.  EDNS Support Requirement
4.  Affected Protocol Elements
    4.1.  Message Header
    4.2.  Label Types
    4.3.  UDP Message Size
5.  Extended Label Types
6.  OPT pseudo-RR
    6.1.  OPT Record Behavior
    6.2.  OPT Record Format
    6.3.  Requestor's Payload Size
    6.4.  Responder's Payload Size
    6.5.  Payload Size Selection
    6.6.  Middleware Boxes
    6.7.  Extended RCODE
    6.8.  OPT Options Type Allocation Procedure
7.  Transport Considerations
8.  Security Considerations
9.  IANA Considerations
10.  Acknowledgements
11.  References
    11.1.  Normative References
    11.2.  Informative References
§  Authors' Addresses

1.  Introduction

DNS [RFC1035] (Mockapetris, P., “Domain names - implementation and specification,” November 1987.) specifies a Message Format and within such messages there are standard formats for encoding options, errors, and name compression. The maximum allowable size of a DNS Message is fixed. Many of DNS's protocol limits are too small for uses which are or which are desired to become common. There is no way for implementations to advertise their capabilities.

Unextended agents will not know how to interpret the protocol extensions detailed here. In practice, these clients will be upgraded when they have need of a new feature, and only new features will make use of the extensions. Extended agents must be prepared for behaviour of unextended clients in the face of new protocol elements, and fall back gracefully to unextended DNS. [RFC2671] (Vixie, P., “Extension Mechanisms for DNS (EDNS0),” August 1999.) originally proposed extensions to the basic DNS protocol to overcome these deficiencies. This memo refines that specification and obsoletes [RFC2671] (Vixie, P., “Extension Mechanisms for DNS (EDNS0),” August 1999.).

2.  Requirements Language

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 RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

3.  EDNS Support Requirement

EDNS support is manditory in a modern world. DNSSEC requires EDNS support, and many other featres are made possible only by EDNS support to request or advertise them.

4.  Affected Protocol Elements

4.1.  Message Header

The DNS Message Header's (see , section 4.1.1 (Mockapetris, P., “Domain names - implementation and specification,” November 1987.) [RFC1035]) second full 16-bit word is divided into a 4-bit OPCODE, a 4-bit RCODE, and a number of 1-bit flags. The original reserved Z bits have been allocated to various purposes, and most of the RCODE values are now in use. More flags and more possible RCODEs are needed. The OPT pseudo-RR specified below contains subfields that carry a bit field extension of the RCODE field and additional flag bits, respectively.

4.2.  Label Types

The first two bits of a wire format domain label are used to denote the type of the label. ,section 4.1.4 (Mockapetris, P., “Domain names - implementation and specification,” November 1987.) [RFC1035] allocates two of the four possible types and reserves the other two. More label types were proposed in [RFC2671] (Vixie, P., “Extension Mechanisms for DNS (EDNS0),” August 1999.) section 3.

4.3.  UDP Message Size

DNS Messages are limited to 512 octets in size when sent over UDP. While the minimum maximum reassembly buffer size still allows a limit of 512 octets of UDP payload, most of the hosts now connected to the Internet are able to reassemble larger datagrams. Some mechanism must be created to allow requestors to advertise larger buffer sizes to responders. To this end, the OPT pseudo-RR specified below contains a maximum payload size field.

5.  Extended Label Types

The first octet in the on-the-wire representation of a DNS label specifies the label type; the basic DNS specification [RFC1035] (Mockapetris, P., “Domain names - implementation and specification,” November 1987.) dedicates the two most significant bits of that octet for this purpose.

This document reserves DNS label type 0b01 for use as an indication for Extended Label Types. A specific extended label type is selected by the 6 least significant bits of the first octet. Thus, Extended Label Types are indicated by the values 64-127 (0b01xxxxxx) in the first octet of the label.

This document does not describe any specific Extended Label Type.

In practice, Extended Label Types are difficult to use due to support in clients and intermediate gateways. Therefore, the registry of Extended Label Types is requested to be closed. They cause interoperability problems and at present no defined label types are in use.

6.  OPT pseudo-RR

6.1.  OPT Record Behavior

One OPT pseudo-RR (RR type 41) MAY be added to the additional data section of a request. If present in requests, compliant responders which implement EDNS MUST include an OPT record in non-truncated responses, and SHOULD attempt to include them in all responses. An OPT is called a pseudo-RR because it pertains to a particular transport level message and not to any actual DNS data. OPT RRs MUST NOT be cached, forwarded, or stored in or loaded from master files. The quantity of OPT pseudo-RRs per message MUST be either zero or one, but not greater.

6.2.  OPT Record Format

An OPT RR has a fixed part and a variable set of options expressed as {attribute, value} pairs. The fixed part holds some DNS meta data and also a small collection of basic extension elements which we expect to be so popular that it would be a waste of wire space to encode them as {attribute, value} pairs.

The fixed part of an OPT RR is structured as follows:

The variable part of an OPT RR is encoded in its RDATA and is structured as zero or more of the following:

              +0 (MSB)                            +1 (LSB)
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: |                          OPTION-CODE                          |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: |                         OPTION-LENGTH                         |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
4: |                                                               |
   /                          OPTION-DATA                          /
   /                                                               /
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

OPTION-CODE

Assigned by Expert Review.

OPTION-LENGTH

Size (in octets) of OPTION-DATA.

OPTION-DATA

Varies per OPTION-CODE.

Order of appearance of option tuples is never relevant. Any option whose meaning is affected by other options is so affected no matter which one comes first in the OPT RDATA.

Any OPTION-CODE values not understood by a responder or requestor MUST be ignored. Specifications of such options might wish to include some kind of signalled acknowledgement. For example, an option specification might say that if a responder sees option XYZ, it SHOULD include option XYZ in its response.

6.3.  Requestor's Payload Size

The requestor's UDP payload size (which OPT stores in the RR CLASS field) is the number of octets of the largest UDP payload that can be reassembled and delivered in the requestor's network stack. Note that path MTU, with or without fragmentation, may be smaller than this. Values lower than 512 MUST be treated as equal to 512.

Note that a 512-octet UDP payload requires a 576-octet IP reassembly buffer. Choosing 1280 for IPv4 over Ethernet would be reasonable. The consequence of choosing too large a value may be an ICMP message from an intermediate gateway, or even a silent drop of the response message.

The requestor's maximum payload size can change over time, and MUST therefore not be cached for use beyond the transaction in which it is advertised.

6.4.  Responder's Payload Size

The responder's maximum payload size can change over time, but can be reasonably expected to remain constant between two sequential transactions; for example, a meaningless QUERY to discover a responder's maximum UDP payload size, followed immediately by an UPDATE which takes advantage of this size. (This is considered preferrable to the outright use of TCP for oversized requests, if there is any reason to suspect that the responder implements EDNS, and if a request will not fit in the default 512 payload size limit.)

6.5.  Payload Size Selection

Due to transaction overhead, it is unwise to advertise an architectural limit as a maximum UDP payload size. Just because your stack can reassemble 64KB datagrams, don't assume that you want to spend more than about 4KB of state memory per ongoing transaction.

A requestor MAY choose to implement a fallback to smaller advertised sizes to work around firewall or other network limitations. A requestor SHOULD choose to use a fallback mechanism which begins with a large size, such as 4096. If that fails, a fallback around the 1220 byte range SHOULD be tried, as it has a reasonable chance to fit within a single Ethernet frame. Failing that, a requestor MAY choose a 512 byte packet, which with large answers may cause a TCP retry.

6.6.  Middleware Boxes

Middleware boxes MUST NOT limit DNS messages over UDP to 512 bytes.

Middleware boxes which simply forward requests to a recursive resolver MUST NOT modify the OPT record contents in either direction.

6.7.  Extended RCODE

The extended RCODE and flags (which OPT stores in the RR TTL field) are structured as follows:

              +0 (MSB)                            +1 (LSB)
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: |         EXTENDED-RCODE        |            VERSION            |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: | DO|                           Z                               |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

EXTENDED-RCODE

Forms upper 8 bits of extended 12-bit RCODE. Note that EXTENDED-RCODE value "0" indicates that an unextended RCODE is in use (values "0" through "15").

VERSION

Indicates the implementation level of whoever sets it. Full conformance with this specification is indicated by version ``0.'' Requestors are encouraged to set this to the lowest implemented level capable of expressing a transaction, to minimize the responder and network load of discovering the greatest common implementation level between requestor and responder. A requestor's version numbering strategy MAY ideally be a run time configuration option.
If a responder does not implement the VERSION level of the request, then it answers with RCODE=BADVERS. All responses MUST be limited in format to the VERSION level of the request, but the VERSION of each response SHOULD be the highest implementation level of the responder. In this way a requestor will learn the implementation level of a responder as a side effect of every response, including error responses and including RCODE=BADVERS.

DO

DNSSEC OK bit as defined by [RFC3225] (Conrad, D., “Indicating Resolver Support of DNSSEC,” December 2001.).

Z

Set to zero by senders and ignored by receivers, unless modified in a subsequent specification.

6.8.  OPT Options Type Allocation Procedure

Allocations assigned by expert review. TBD

7.  Transport Considerations

The presence of an OPT pseudo-RR in a request should be taken as an indication that the requestor fully implements the given version of EDNS, and can correctly understand any response that conforms to that feature's specification.

Lack of presence of an OPT record in a request MUST be taken as an indication that the requestor does not implement any part of this specification and that the responder MUST NOT use any protocol extension described here in its response.

Responders who do not implement these protocol extensions MUST respond with FORMERR messages without any OPT record.

If there is a problem with processing the OPT record itself, such as an option value that is badly formatted or includes out of range values, a FORMERR MAY be retured. If this occurs the response MUST include an OPT record. This MAY be used to distinguish between servers whcih do not implement EDNS and format errors within EDNS.

If EDNS is used in a request, and the response arrives with TC set and with no EDNS OPT RR, a requestor SHOULD assume that truncation prevented the OPT RR from being appended by the responder, and further, that EDNS is not used in the response. Correspondingly, an EDNS responder who cannot fit all necessary elements (including an OPT RR) into a response, SHOULD respond with a normal (unextended) DNS response, possibly setting TC if the response will not fit in the unextended response message's 512-octet size.

8.  Security Considerations

Requestor-side specification of the maximum buffer size may open a new DNS denial of service attack if responders can be made to send messages which are too large for intermediate gateways to forward, thus leading to potential ICMP storms between gateways and responders.

Announcing very large UDP buffer sizes may result in dropping by firewalls. This could cause retransmissions with no hope of success. Some devices reject fragmented UDP packets.

Announcing too small UDP buffer sizes may result in fallback to TCP. This is especially important with DNSSEC, where answers are much larger.

9.  IANA Considerations

The IANA has assigned RR type code 41 for OPT.

[RFC2671] (Vixie, P., “Extension Mechanisms for DNS (EDNS0),” August 1999.) specified a number of IANA sub-registries within "DOMAIN NAME SYSTEM PARAMETERS:" "EDNS Extended Label Type", "EDNS Option Codes", "EDNS Version Numbers", and "Domain System Response Code." IANA is advised to re-parent these subregistries to this document.

RFC 2671 created an extended label type registry. We request that this registry be closed.

This document assigns extended label type 0bxx111111 as "Reserved for future extended label types." We request that IANA record this assignment.

This document assigns option code 65535 to "Reserved for future expansion."

This document expands the RCODE space from 4 bits to 12 bits. This will allow IANA to assign more than the 16 distinct RCODE values allowed in RFC 1035 (Mockapetris, P., “Domain names - implementation and specification,” November 1987.) [RFC1035].

This document assigns EDNS Extended RCODE "16" to "BADVERS".

IESG approval should be required to create new entries in the EDNS Extended Label Type or EDNS Version Number registries, while any published RFC (including Informational, Experimental, or BCP) should be grounds for allocation of an EDNS Option Code.

10.  Acknowledgements

Paul Mockapetris, Mark Andrews, Robert Elz, Don Lewis, Bob Halley, Donald Eastlake, Rob Austein, Matt Crawford, Randy Bush, and Thomas Narten were each instrumental in creating and refining this specification.

11.  References

11.1. Normative References

11.2. Informative References

Authors' Addresses