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Detached Domain Name System (DNS) Information
RFC 2540

Document Type RFC - Experimental (March 1999)
Author Donald E. Eastlake 3rd
Last updated 2013-03-02
RFC stream Internet Engineering Task Force (IETF)
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RFC 2540
Network Working Group                                        D. Eastlake
Request for Comments: 2540                                           IBM
Category: Experimental                                        March 1999

             Detached Domain Name System (DNS) Information

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.


   A standard format is defined for representing detached DNS
   information.  This is anticipated to be of use for storing
   information retrieved from the Domain Name System (DNS), including
   security information, in archival contexts or contexts not connected
   to the Internet.

Table of Contents

   1. Introduction............................................1
   2. General Format..........................................2
   2.1 Binary Format..........................................3
   2.2. Text Format...........................................4
   3. Usage Example...........................................4
   4. IANA Considerations.....................................4
   5. Security Considerations.................................4
   Author's Address...........................................5
   Full Copyright Statement...................................6

1. Introduction

   The Domain Name System (DNS) is a replicated hierarchical distributed
   database system [RFC 1034, 1035] that can provide highly available
   service.  It provides the operational basis for Internet host name to
   address translation, automatic SMTP mail routing, and other basic
   Internet functions.  The DNS has been extended as described in [RFC
   2535] to permit the general storage of public cryptographic keys in

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   the DNS and to enable the authentication of information retrieved
   from the DNS though digital signatures.

   The DNS was not originally designed for storage of information
   outside of the active zones and authoritative master files that are
   part of the connected DNS.  However there may be cases where this is
   useful, particularly in connection with archived security

2. General Format

   The formats used for detached Domain Name System (DNS) information
   are similar to those used for connected DNS information. The primary
   difference is that elements of the connected DNS system (unless they
   are an authoritative server for the zone containing the information)
   are required to count down the Time To Live (TTL) associated with
   each DNS Resource Record (RR) and discard them (possibly fetching a
   fresh copy) when the TTL reaches zero.  In contrast to this, detached
   information may be stored in a off-line file, where it can not be
   updated, and perhaps used to authenticate historic data or it might
   be received via non-DNS protocols long after it was retrieved from
   the DNS.  Therefore, it is not practical to count down detached DNS
   information TTL and it may be necessary to keep the data beyond the
   point where the TTL (which is defined as an unsigned field) would
   underflow.  To preserve information as to the freshness of this
   detached data, it is accompanied by its retrieval time.

   Whatever retrieves the information from the DNS must associate this
   retrieval time with it.  The retrieval time remains fixed thereafter.
   When the current time minus the retrieval time exceeds the TTL for
   any particular detached RR, it is no longer a valid copy within the
   normal connected DNS scheme.  This may make it invalid in context for
   some detached purposes as well.  If the RR is a SIG (signature) RR it
   also has an expiration time.  Regardless of the TTL, it and any RRs
   it signs can not be considered authenticated after the signature
   expiration time.

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2.1 Binary Format

   The standard binary format for detached DNS information is as

                         1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |                      first retrieval time                     |
    |          RR count             |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     Resource Records (RRs)    |
    /                                                               /
    |                       next retrieval time                     |
    |          RR count             |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     Resource Records (RRs)    |
    /                                                               /
    /                              ...                              /
    |     hex 20    |

   Retrieval time - the time that the immediately following information
       was obtained from the connected DNS system.  It is an unsigned
       number of seconds since the start of 1 January 1970, GMT,
       ignoring leap seconds, in network (big-endian) order.  Note that
       this time can not be before the initial proposal of this
       standard.  Therefore, the initial byte of an actual retrieval
       time, considered as a 32 bit unsigned quantity, would always be
       larger than 20 hex.  The end of detached DNS information is
       indicated by a "retrieval time" field initial byte equal to 0x20.
       Use of a "retrieval time" field with a leading unsigned byte of
       zero indicates a 64 bit (actually 8 leading zero bits plus a 56
       bit quantity).  This 64 bit format will be required when
       retrieval time is larger than 0xFFFFFFFF, which is some time in
       the year 2106.  The meaning of retrieval times with an initial
       byte between 0x01 and 0x1F is reserved (see section 5).
       Retrieval times will not generally be 32 bit aligned with respect
       to each other due to the variable length nature of RRs.

   RR count - an unsigned integer number (with bytes in network order)
       of following resource records retrieved at the preceding
       retrieval time.

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   Resource Records - the actual data which is in the same format as if
       it were being transmitted in a DNS response.  In particular, name
       compression via pointers is permitted with the origin at the
       beginning of the particular detached information data section,
       just after the RR count.

2.2. Text Format

   The standard text format for detached DNS information is as
   prescribed for zone master files [RFC 1035] except that the $INCLUDE
   control entry is prohibited and the new $DATE entry is required
   (unless the information set is empty). $DATE is followed by the date
   and time that the following information was obtained from the DNS
   system as described for retrieval time in section 2.1 above.  It is
   in the text format YYYYMMDDHHMMSS where YYYY is the year (which may
   be more than four digits to cover years after 9999), the first MM is
   the month number (01-12), DD is the day of the month (01-31), HH is
   the hour in 24 hours notation (00-23), the second MM is the minute
   (00-59), and SS is the second (00-59).  Thus a $DATE must appear
   before the first RR and at every change in retrieval time through the
   detached information.

3. Usage Example

   A document might be authenticated by a key retrieved from the DNS in
   a KEY resource record (RR).  To later prove the authenticity of this
   document, it would be desirable to preserve the KEY RR for that
   public key, the SIG RR signing that KEY RR, the KEY RR for the key
   used to authenticate that SIG, and so on through SIG and KEY RRs
   until a well known trusted key is reached, perhaps the key for the
   DNS root or some third party authentication service. (In some cases
   these KEY RRs will actually be sets of KEY RRs with the same owner
   and class because SIGs actually sign such record sets.)

   This information could be preserved as a set of detached DNS
   information blocks.

4. IANA Considerations

   Allocation of meanings to retrieval time fields with a initial byte
   of between 0x01 and 0x1F requires an IETF consensus.

5. Security Considerations

   The entirety of this document concerns a means to represent detached
   DNS information.  Such detached resource records may be security
   relevant and/or secured information as described in [RFC 2535].  The
   detached format provides no overall security for sets of detached

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   information or for the association between retrieval time and
   information.  This can be provided by wrapping the detached
   information format with some other form of signature.  However, if
   the detached information is accompanied by SIG RRs, its validity
   period is indicated in those SIG RRs so the retrieval time might be
   of secondary importance.


   [RFC 1034]   Mockapetris, P., "Domain Names - Concepts and
                Facilities", STD 13, RFC 1034, November 1987.

   [RFC 1035]   Mockapetris, P., " Domain Names - Implementation and
                Specifications", STD 13, RFC 1035, November 1987.

   [RFC 2535]   Eastlake, D., "Domain Name System Security Extensions",
                RFC 2535, March 1999.

Author's Address

   Donald E. Eastlake 3rd
   65 Shindegan Hill Road, RR #1
   Carmel, NY 10512

   Phone:   +1-914-276-2668(h)
   Fax:     +1-914-784-3833(w)

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Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

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