Network Working Group                                    D. Eastlake 3rd
Request for Comments: 2931                                      Motorola
Updates: 2535                                             September 2000
Category: Standards Track

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

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.


   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.


   The contributions and suggestions of the following persons (in
   alphabetic order) to this memo are gratefully acknowledged:

         Olafur Gudmundsson

         Ed Lewis

         Erik Nordmark

         Brian Wellington

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RFC 2931                       DNS SIG(0)                 September 2000

Table of Contents

   1. Introduction.................................................  2
   2. SIG(0) Design Rationale......................................  3
   2.1 Transaction Authentication..................................  3
   2.2 Request Authentication......................................  3
   2.3 Keying......................................................  3
   2.4 Differences Between TSIG and SIG(0).........................  4
   3. The SIG(0) Resource Record...................................  4
   3.1 Calculating Request and Transaction SIGs....................  5
   3.2 Processing Responses and SIG(0) RRs.........................  6
   3.3 SIG(0) Lifetime and Expiration..............................  7
   4. Security Considerations......................................  7
   5. IANA Considerations..........................................  7
   References......................................................  7
   Author's Address................................................  8
   Appendix: SIG(0) Changes from RFC 2535..........................  9
   Full Copyright Statement........................................ 10

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 (RRs) that are used to digitally 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 [RFC 2845] and the [RFC 2535] specification for

   Sections of [RFC 2535] updated are all of and parts of 4.2
   and 4.3.  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].

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2. SIG(0) Design Rationale

   SIG(0) provides protection for DNS transactions and requests that is
   not provided by the regular SIG, KEY, and NXT RRs specified in [RFC
   2535].  The authenticated data origin services of secure DNS either
   provide protected data resource records (RRs) or authenticatably deny
   their nonexistence.  These services provide no protection for glue
   records, DNS requests, no protection for message headers on requests
   or responses, and no protection of the overall integrity of a

2.1 Transaction Authentication

   Transaction authentication means that a requester can be sure it is
   at least getting the messages from the server it queried and that the
   received messages are in response to the query it sent.  This is
   accomplished by optionally adding either a TSIG RR [RFC 2845] or, as
   described herein, a SIG(0) resource record at the end of the response
   which digitally signs the concatenation of the server's response and
   the corresponding resolver query.

2.2 Request Authentication

   Requests can also be authenticated by including a TSIG or, as
   described herein, a special SIG(0) RR at the end of the request.
   Authenticating requests serves no function in DNS servers that
   predate the specification of dynamic update.  Requests with a non-
   empty additional information section produce error returns or may
   even be ignored by a few such older DNS servers. However, this syntax
   for signing requests is defined for authenticating dynamic update
   requests [RFC 2136], TKEY requests [RFC 2930], or future requests
   requiring authentication.

2.3 Keying

   The private keys used in transaction security belong to the host
   composing the DNS response message, not to the zone involved.
   Request authentication may also involve the private key of the host
   or other entity composing the request or of a zone to be affected by
   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 verification as KEY
   RRs with a protocol byte of 3 (DNSSEC) or 255 (ANY).

   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.

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2.4 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
   understands TSIG and very likely has the same key installed.
   Furthermore, TSIG uses keyed hash authentication codes which are
   relatively inexpensive to compute.  Thus it is common to authenticate
   requests with TSIG and responses are authenticated with TSIG if the
   corresponding request is authenticated.

   SIG(0) on the other hand, uses public key authentication, where the
   public keys are stored in DNS as KEY RRs and a private key is stored
   at the signer.  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 SHOULD only be used on requests when
   necessary to authenticate 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.  For other replies, whether they are
   authenticated by the server or required to be authenticated by the
   requester SHOULD be a local configuration option.

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 transaction SIG(0)s, the signer field MUST be a name of the
   originating host and there MUST be a KEY RR at that name with the
   public key corresponding to the private key used to calculate the

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   signature.  (The host domain name used may be the inverse IP address
   mapping name for an IP address of the host if the relevant KEY is
   stored there.)

   For all SIG(0) RRs, the owner name, class, TTL, and original TTL, are
   meaningless.  The TTL fields SHOULD be zero and the CLASS field
   SHOULD be ANY.  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 of any
   additional information for inclusion in the response. If the SIG(0)
   RR cannot be added without causing the message to be truncated, the
   server MUST alter the response so that a SIG(0) can be included.
   This response consists of only the question and a SIG(0) record, and
   has the TC bit set and RCODE 0 (NOERROR).  The client should at this
   point retry the request using TCP.

3.1 Calculating Request and Transaction SIGs

   A DNS request may be optionally signed by including one SIG(0)s at
   the end of the query additional information section.  Such a SIG is
   identified by having a "type covered" field of zero. It signs the
   preceding DNS request message including DNS header but not including
   the UDP/IP header and before the request RR counts have been adjusted
   for the inclusions of the request SIG(0).

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

      data = RDATA | request - SIG(0)

   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 but not its UDP/IP header, and (3)
   the entire DNS response message, including DNS header but not the
   UDP/IP header and before the response RR counts have been adjusted
   for the inclusion of the SIG(0).

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   That is

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

   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) | previous packet

   where "|" is concatenations, RDATA is as above, and previous packet
   is the previous DNS payload including DNS header and the SIG(0) but
   not the TCP/IP header.  Support of SIG(0) for TCP is OPTIONAL.  As an
   alternative, TSIG may be used after, if necessary, setting up a key
   with TKEY [RFC 2930].

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

   Note: requests and responses can either have a single TSIG or one
   SIG(0) but not both a TSIG and a SIG(0).

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 TKEY responses, it MUST be checked and the message
   rejected if the checks fail unless otherwise specified for the TKEY
   mode in use.  For all other responses, it MAY be checked and the
   message rejected if the checks fail.

   If a response's SIG(0) check succeed, such a transaction
   authentication SIG does NOT directly authenticate the validity any
   data-RRs in the message.  However, it authenticates that they were
   sent by the queried server and have not been diddled.  (Only a proper
   SIG(0) RR signed by the zone or a key tracing its authority to the
   zone or to static resolver configuration can directly authenticate

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   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
   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.

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 parameters are created or parameter values assigned by this


   [RFC 1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
              September 1996.

   [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

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

   [RFC 2845] Vixie, P., Gudmundsson, O., Eastlake, D. and B.
              Wellington, "Secret Key Transaction Signatures for DNS
              (TSIG)", RFC 2845, May 2000.

   [RFC 2930] Eastlake, D., "Secret Key Establishment for DNS (RR)", RFC
              2930, September 2000.

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RFC 2931                       DNS SIG(0)                 September 2000

Author's Address

   Donald E. Eastlake 3rd
   140 Forest Avenue
   Hudson, MA 01749 USA

   Phone: +1-978-562-2827(h)
   Fax:   +1 978-567-7941(h)
   EMail: Donald.Eastlake@motorola.com

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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 so as 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

   Add wording to indicate that either a TSIG or one or more SIG(0)s may
   be present but not both.

   Reword some areas for clarity.

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Full Copyright Statement

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

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   Internet Society.

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