[Docs] [txt|pdf] [Tracker] [WG] [Email] [Nits]

Versions: 00 01

INTERNET-DRAFT                                    Donald E. Eastlake 3rd
UPDATES RFC 2535                                                     IBM

Expires: April 2000                                         October 1999

           DNS Request and Transaction Signatures ( SIG(0)s )
           --- ------- --- ----------- ---------- - ------- -

Status of This Document

   This draft, file name draft-ietf-dnsind-sig-zero-00.txt, is intended
   to become a Proposed Standard RFC updating Proposed Standard [RFC
   2535].  Distribution of this document is unlimited. Comments should
   be sent to the DNS Working Group mailing list
   <namedroppers@internic.net> or to the author.

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months.  Internet-Drafts may be updated, replaced, or obsoleted by
   other documents at any time.  It is not appropriate to use Internet-
   Drafts as reference material or to cite them other than as a
   ``working draft'' or ``work in progress.''

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at


   Extensions to the Domain Name System (DNS) are described in [RFC
   2535] that can provide data origin and transaction integrity and
   authentication to security aware resolvers and applications through
   the use of cryptographic digital signatures.

   Implementation experience has indicated the need for minor but non-
   interoperable changes in Request and Transaction signature resource
   records ( SIG(0)s ).  These changes are documented herein.

Donald E. Eastlake 3rd                                          [Page 1]


INTERNET-DRAFT                 DNS SIG(0)                   October 1999


   The significant contributions and suggestions of the following
   persons (in alphabetic order) to this draft are gratefully

        Olafur Gudmundsson
        Brian Wellington

Table of Contents

   [Table of Contents gets moved here from the end]

Donald E. Eastlake 3rd                                          [Page 2]


INTERNET-DRAFT                 DNS SIG(0)                   October 1999

1. Introduction

   This document makes minor but non-interoperable changes to part of
   [RFC 2535], familiarity with which is assumed, and includes
   additional explanatory text.  These changes concern SIG Resource
   Records that are used to sign DNS requests and transactions /
   responses.  Such a resource record, because it has a type covered
   field of zero, is frequently called a SIG(0). The changes are based
   on implementation and attempted implementation experience with TSIG
   [draft-ietf-dnsind-tsig-*.txt] and the [RFC 2535] specification for

   No changes are made herein related to the KEY or NXT RRs or to the
   processing involved with data origin and denial authentication for
   DNS data.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC 2119].

Donald E. Eastlake 3rd                                          [Page 3]


INTERNET-DRAFT                 DNS SIG(0)                   October 1999

2. SIG(0) Design Rational

   The authenticated data origin and data existence denial services of
   secure DNS protect only data resource records (RRs) or
   authenticatably deny their nonexistence.  These services provide no
   protection for DNS requests, no protection for message headers on
   requests or responses, and no protection of the overall integrity of
   a response.

   If header bits are falsely set by a bad server, there is little that
   can be done.  However, it is possible to add transaction
   authentication.  Such authentication means that a requester can be
   sure it is at least getting messages from the server it thinks it
   queried and that the response is from the request it sent (i.e., that
   these messages have not been diddled in transit).  This is
   accomplished by optionally adding either a TSIG RR [draft-ietf-
   dnsind-tsig-*.txt] or, as described herein, a SIG resource record at
   the end of the response which digitally signs both the server's
   response and the corresponding resolver query.

   Requests can also be authenticated by including a TSIG or, as
   described herein, a special SIG RR at the end of the request.
   Authenticating requests serves no function in older DNS servers.
   Requests with a non-empty additional information section produce
   error returns or may even be ignored by a few older DNS servers.
   However, this syntax for signing requests is defined for
   authenticating dynamic update requests [RFC 2136], TKEY [draft-ietf-
   dnsind-tkey-*.txt], or future requests requiring authentication.

   The private keys used in transaction security belong to the host
   composing the DNS message, not to the zone involved.  Request
   authentication may also involve the private key of the host or other
   entity composing the request or other private keys depending on the
   request authority it is sought to establish. The corresponding public
   key(s) are normally stored in and retrieved from the DNS for

   Because requests and replies are highly variable, message
   authentication SIGs can not be pre-calculated.  Thus it will be
   necessary to keep the private key on-line, for example in software or
   in a directly connected piece of hardware.

2.1 Differences Between TSIG and SIG(0)

   There are significant differences between TSIG and SIG(0).

   Because TSIG involves secret keys installed at both the requester and
   server the presence of such a key implies that the other party

Donald E. Eastlake 3rd                                          [Page 4]


INTERNET-DRAFT                 DNS SIG(0)                   October 1999

   understands TSIG and likely has the same key installed.  Furthermore,
   TSIG uses keyed hash authentication codes which are relatively
   inexpensive to compute.  Thus it is common to sign requests with TSIG
   and responses are signed with TSIG if the corresponding request is

   SIG(0) on the other hand, uses KEY RRs that are stored in the DNS
   under the host name.  Thus, existence of such a KEY RR does not
   necessarily imply implementation of SIG(0).  In addition, SIG(0)
   involves relatively expensive public key cryptographic operations
   that should be minimized and the verification of a SIG(0) involves
   obtaining and verifying the corresponding KEY which can be an
   expensive and lengthy operation.  Indeed, a policy of using SIG(0) on
   all requests and verifying it before responding would, for some
   configurations, lead to a deadly embrace with the attempt to obtain
   and verify the KEY needed to authenticate the request SIG(0)
   resulting in additional requests accompanied by a SIG(0) leading to
   further requests accompanied by a SIG(0), etc.  Furthermore, omitting
   SIG(0)s when not required on requests halves the number of public key
   operations required by the transaction.

   For these reasons, SIG(0)s MUST only be used on requests when
   necessary to authentication that the requester has some required
   privilege or identity.  SIG(0)s on replies are defined in such a way
   as to not require a SIG(0) on the corresponding request and still
   provide transaction protection.  Some replies, such as those
   involving TKEY [draft-ietf-dnsind-tkey-*.txt], MUST be signed with
   TSIG or SIG(0).  For other replies, whether they are signed by the
   server or required to be signed by the requester SHOULD be a local
   configuration option.

Donald E. Eastlake 3rd                                          [Page 5]


INTERNET-DRAFT                 DNS SIG(0)                   October 1999

3. The SIG(0) Resource Record

   The structure of and type number of SIG resource records (RRs) is
   given in [RFC 2535] Section 4.1.  However all of Section and
   the parts of Sections 4.2 and 4.3 related to SIG(0) should be
   considered replaced by the material below.  Any conflict between [RFC
   2535] and this document concerning SIG(0) RRs should be resolved in
   favor of this document.

   For all SIG(0)s, the signer field MUST be a name of the originating
   server host.  (The inverse IP address mapping name MAY be used if the
   relevant KEY is stored there.)  However, the owner name, class, TTL,
   and original TTL, are meaningless.  The class and TTL fields SHOULD
   be zero.  To conserve space, the owner name SHOULD be root (a single
   zero octet).  When SIG(0) authentication on a response is desired,
   that SIG RR must be considered the highest priority for inclusion in
   the response.

3.1 Calculating Request and Transaction SIGs

   A DNS request may be optionally signed by including one or more SIGs
   at the end of the query additional information section.  Such SIGs
   are identified by having a "type covered" field of zero. They sign
   the preceding DNS request message including DNS header but not
   including the UDP/IP header or any request SIG(0)s or TSIGs  [draft-
   ietf-dnsind-tsig-*.txt] at the end and before the request RR counts
   have been adjusted for the inclusions of any request SIG(0)s or

   They are calculated by using a "data" (see [RFC 2535], Section 4.1.8)
   of (1) the SIG's RDATA section omitting the signature subfield
   itself, (2) the entire DNS query messages, including DNS header, but
   not the UDP/IP header or any SIG(0) or TSIG and before the reply RR
   counts have been adjusted for the inclusion of any SIG(0) or TSIG.
   That is

      data = RDATA | request (- SIG(0)s & TSIGs)

   where "|" is concatenation and RDATA is the RDATA of the SIG(0) being
   calculated less the signature itself.

   Similarly, a SIG(0) can be used to secure a response and the request
   that produced it.  Such transaction signatures are calculated by
   using a "data" of (1) the SIG's RDATA section omitting the signature
   itself, (2) the entire DNS query message that produced this response,
   including the query's DNS header and any SIG(0)s or TSIGs but not its
   UDP/IP header, and (3) the entire DNS response message, including DNS
   header but not the UDP/IP header or any SIG(0) or TSIGs and before

Donald E. Eastlake 3rd                                          [Page 6]


INTERNET-DRAFT                 DNS SIG(0)                   October 1999

   the response RR counts have been adjusted for the inclusion of any
   SIG(0) or TSIG.

   That is

      data = RDATA | full query | response (- SIG(0)s & TSIGs)

   where "|" is concatenation and RDATA is the RDATA of the SIG(0) being
   calculated less the signature itself.

   Verification of a response SIG(0) (which is signed by the server host
   key, not the zone key) by the requesting resolver shows that the
   query and response were not tampered with in transit, that the
   response corresponds to the intended query, and that the response
   comes from the queried server.

   In the case of a DNS message via TCP, a SIG(0) on the first data
   packet is calculated with "data" as above and for each subsequent
   packet, it is calculated as follows:

      data = RDATA | DNS payload (- SIG(0)s & TSIGs) | previous packet

   where RDATA is as above and previous packet is the previous DNS
   payload including DNS header and any SIG(0)s and TSIGs but not the
   TCP/IP header.  Support of TCP SIG(0) for is OPTIONAL.  As an
   alternative, TSIG may be used after, if necessary, setting up a key
   with TKEY [draft-ietf-dnsind-tkey-*.txt].

   Except where needed to authenticate an update, TKEY, or similar
   privileged request, servers are not required to check request SIGs.

3.2 Processing Responses and SIG(0) RRs

   If a SIG RR is at the end of the additional information section of a
   response and has a type covered of zero, it is a transaction
   signature covering the response and the query that produced the
   response.  For all TKEY responses, it MUST be checked and the message
   rejected if the checks fail.  For all other responses, it MAY be
   optionally checked and the message rejected if the checks fail.

   If response SIG(0) checks succeed, such a transaction authentication
   SIG does NOT directly authenticate any data-RRs in the message but
   does authenticate TKEY and other meta-RRs.  (Only a proper SIG RR
   signed by the zone or a key tracing its authority to the zone or to
   static resolver configuration can directly authenticate data-RRs,
   depending on resolver policy.) If a resolver or server does not
   implement transaction and/or request SIGs, it MUST ignore them
   without error where they are optional and treat them as failing where

Donald E. Eastlake 3rd                                          [Page 7]


INTERNET-DRAFT                 DNS SIG(0)                   October 1999

   they are required.

3.3 SIG(0) Lifetime and Expiration

   The inception and expiration times in SIG(0)s are for the purpose of
   resisting replay attacks.  They should be set to form a time bracket
   such that messages outside that bracket can be ignored.  In IP
   networks, this time bracket should not normally extend further than 5
   minutes into the past and 5 minutes into the future.

Donald E. Eastlake 3rd                                          [Page 8]


INTERNET-DRAFT                 DNS SIG(0)                   October 1999

4. Security Considerations

   No additional considerations beyond those in [RFC 2535].  The
   inclusion of the SIG(0) inception and expiration time under the
   signature improves resistance to replay attacks.

5. IANA Considerations

   No new fields are created or field values assigned by the document.


   [RFC 1982] - Robert Elz, Randy Bush, "Serial Number Arithmetic",

   [RFC 2119] - S. Bradner, "Key words for use in RFCs to Indicate
   Requirement Levels", March 1997.

   [RFC 2136] - P. Vixie, S. Thomson, Y. Rekhter, J. Bound, "Dynamic
   Updates in the Domain Name System (DNS UPDATE)", 04/21/1997.

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

   [draft-ietf-dnsind-tsig-*.txt] - P. Vixie, O. Gudmundsson, D.
   Eastlake, B. Wellington, "Secret Key Transaction Signatures for DNS

   [draft-ietf-dnsind-tkey-*.txt] - D. Eastlake, "Secret Key
   Establishment for DNS  (RR)"

Donald E. Eastlake 3rd                                          [Page 9]


INTERNET-DRAFT                 DNS SIG(0)                   October 1999

Author's Address

   Donald E. Eastlake 3rd
   65 Shindegan Hill Road
   Carmel, NY 10512 USA

   Telephone:   +1-914-784-7913(w)
   fax:         +1-914-784-3833(w)
   email:       dee3@us.ibm.com

Expiration and File Name

   This draft expires April 2000.

   Its file name is draft-ietf-dnsind-sig-zero-00.txt.

Appendix: SIG(0) Changes from RFC 2535

   Add explanatory text concerning the differences between TSIG and

   Change the data over which SIG(0) is calculated to include the SIG(0)
   RDATA other than the signature itself to secure the signature
   inception and expiration times and resist replay attacks.  Specify
   SIG(0) for TCP.

   Add discussion of appropriate inception and expiration times for

   Change wording to permit mixing TSIG and SIG(0) RRs.

   Reword some areas for clarity.

Donald E. Eastlake 3rd                                         [Page 10]


INTERNET-DRAFT                 DNS SIG(0)                   October 1999

          Status of This Document....................................1

      Table of Contents..........................................2

      1. Introduction............................................3

      2. SIG(0) Design Rational..................................4
      2.1 Differences Between TSIG and SIG(0)....................4

      3. The SIG(0) Resource Record..............................6
      3.1 Calculating Request and Transaction SIGs...............6
      3.2 Processing Responses and SIG(0) RRs....................7
      3.3 SIG(0) Lifetime and Expiration.........................8

      4. Security Considerations.................................9
      5. IANA Considerations.....................................9

      Author's Address..........................................10
      Expiration and File Name..................................10
      Appendix: SIG(0) Changes from RFC 2535....................10

Donald E. Eastlake 3rd                                         [Page 11]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/