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.
Abstract
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
Abstract...................................................1
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
References.................................................5
Author's Address...........................................5
Full Copyright Statement...................................6
1. Introduction
The Domain Name System (DNS) is a replicated hierarchical distributed
database system [RFC1034, 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
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
information.
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.
2.1 Binary Format
The standard binary format for detached DNS information is as
follows:
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.
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 [RFC1035] 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 [RFC2535]. The
detached format provides no overall security for sets of detached
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.
References
[RFC1034] Mockapetris, P., "Domain Names - Concepts and
Facilities", STD 13, RFC1034, November 1987.
[RFC1035] Mockapetris, P., " Domain Names - Implementation and
Specifications", STD 13, RFC1035, November 1987.
[RFC2535] Eastlake, D., "Domain Name System Security Extensions",
RFC2535, March 1999.
Author's Address
Donald E. Eastlake 3rd
IBM
65 Shindegan Hill Road, RR #1
Carmel, NY 10512
Phone: +1-914-276-2668(h)
+1-914-784-7913(w)
Fax: +1-914-784-3833(w)
EMail: dee3@us.ibm.com
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