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   DNSIND Working Group                              Paul Vixie (Ed.) (ISC)
   INTERNET-DRAFT                                  Olafur Gudmundsson (TIS)
                                            Donald Eastlake 3rd (CyberCash)
   <draft-ietf-dnsind-tsig-05.txt>                                June 1998
   
   Amends: RFC 1035
   
   
               Secret Key Transaction Signatures for DNS (TSIG)
   
   
   Status of this Memo
   
      This document is an Internet-Draft.  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
      and may be updated, replaced, or obsoleted by other documents at any
      time.  It is inappropriate to use Internet-Drafts as reference
      material or to cite them other than as ``work in progress.''
   
      To view the entire list of current Internet-Drafts, please check
      the "1id-abstracts.txt" listing contained in the Internet-Drafts
      Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net
      (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au
      (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu
      (US West Coast).
   
   
   Abstract
   
      This protocol allows for transaction level authentication using
      shared secrets and one way hashing.  It can be used to authenticate
      dynamic updates as coming from an approved client, or to authenticate
      responses as coming from an approved recursive name server.
   
      No provision has been made here for distributing the shared secrets;
      it is expected that a network administrator will statically configure
      name servers and clients using some out of band mechanism such as
      sneaker-net until a secure automated mechanism for key distribution
      is available.
   
   
   
   
   
   
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   1 - Introduction
   
   1.1. The Domain Name System (DNS) [RFC1034, RFC1035] is a replicated
   hierarchical distributed database system that provides information
   fundamental to Internet operations, such as name <=> address translation
   and mail handling information.  DNS has recently been extended [RFC2065]
   to provide for data origin authentication, and public key distribution,
   all based on public key cryptography and public key based digital
   signatures.  To be practical, this form of security generally requires
   extensive local caching of keys and tracing of authentication through
   multiple keys and signatures to a pre-trusted locally configured key.
   
   1.2. One difficulty with the [RFC2065] scheme is that common DNS
   implementations include simple ``stub'' resolvers which do not have
   caches.  Such resolvers typically rely on a caching DNS server on
   another host.  It is impractical for these stub resolvers to perform
   general [RFC2065] authentication and they would naturally depend on
   their caching DNS server to perform such services for them.  To do so
   securely requires secure communication of queries and responses.
   [RFC2065] provides public key transaction signatures to support this but
   such signatures are very expensive computationally to generate.  In
   general, these require the same complex public key logic that is
   impractical for stubs.  This document specifies an efficient secret key
   based transaction signature that avoids these difficulties.
   
   1.3. A second area where use of straight [RFC2065] public key based
   mechanisms may be impractical is authenticating dynamic update [RFC2136]
   requests.  [RFC2065] provides for request signatures but with [RFC2065]
   they, like transaction signatures, require computationally expensive
   public key cryptography and complex authentication logic.  Secure Domain
   Name System Dynamic Update ([RFC2137]) describes how different keys are
   used in dynamically updated zones.  This document's secret key based
   signatures can be used to authenticate DNS update requests as well as
   transaction responses, providing a lightweight alternative to the
   protocol described by [RFC2137].
   
   1.4. A further use of this mechanishm is to protect zone transfers.  In
   this case the data covered would be the whole zone transfer including
   any glue records sent.  The protocol described by [RFC2065] does not
   protect glue records and unsigned records unless SIG(0) (transaction
   signature) is used.
   
   
   
   
   
   
   
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   1.5. The signature mechanism proposed in this document uses shared
   secret keys to establish trust relationship between two entities.  Such
   keys must be protected in a fashion similar to private keys, lest a
   third party masquerade as one of the intended parties (forge
   signatures).  There is an urgent need to provide simple and efficient
   authentication between clients and local servers and this proposal
   addresses that need.  This proposal is unsuitable for general server to
   server authentication for servers which speak with many other servers,
   since key management would become unwieldy with the number of shared
   keys going up quadratically.  But it is suitable for many resolvers on
   hosts that only talk to few recursive servers.
   
   1.6. A server acting as an indirect caching resolver -- a ``forwarder''
   in common usage -- might use transaction signatures when communicating
   with its small number of preconfigured ``upstream'' servers.  Other uses
   of DNS secret key signatures and possible systems for automatic secret
   key distribution may be proposed in separate future documents.
   
   1.7. New Assigned Numbers
   
      RRTYPE = TSIG (250)
      ERROR = 0..15 (a DNS RCODE)
      ERROR = 16 (BADSIG)
      ERROR = 17 (BADKEY)
      ERROR = 18 (BADTIME)
   
   
   2 - TSIG RR Format
   
   
   2.1 TSIG RR Type
   
   To provide secret key signatures, we use a new RR type whose mnemonic is
   TSIG and whose type code is 250.  TSIG is a meta-RR and can not be
   stored.  TSIG RRs can be used for authentication between DNS entities
   that have established a shared secret key.  TSIG RRs are dynamically
   computed to cover a particular DNS transaction and are not DNS RRs in
   the usual sense.
   
   
   
   
   
   
   
   
   
   
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   2.2 TSIG Calculation
   
   As the TSIG RRs are related to one DNS request/response, there is no
   value in storing or retransmitting them, thus the TSIG RR should be
   discarded once it has been used to authenticate DNS message.  The only
   Message Digest algorithm specified in this document is ``HMAC-MD5'' (see
   [RFC1321], [RFC2104]).  Other algorithms can be specified at later date.
   Names and definitions of new algorithms should be registered with IANA.
   All multi-octet integers in TSIG Record are sent in network byte order
   (see [RFC1035 2.3.2]).
   
   
   2.3. Record Format
   
      NAME   A domain-like name of the key used.  The name should reflect
             the names of the hosts and uniquely identify the key among a
             set of keys these two hosts may share at any given time.  If
             hosts A and B in same zone share a key then the key name could
             be A-B-<id>.<zone>.  If two hosts in different zones share the
             key the key name could be <id>.A.<Azone>.B.<Bzone> It should
             be possible for more than one key to be in simultaneous use
             among a set of interacting hosts.  The name only needs to be
             meaningful to the communicating hosts but a meaningful
             mnemonic name as above is strongly recommended.
   
             The name may be used as a local index to the key involved and
             it is recommended that it be globally unique.  Where a key is
             just shared between two hosts, its name actually only need
             only be meaningful to them but it is recommended that the key
             name be mnemonic and incorporate the resolver and server host
             names in that order.
   
      TYPE   TSIG (250: Transaction SIGnature)
   
      CLASS  ANY
   
      TTL    0
   
      RdLen  (variable)
   
   
   
   
   
   
   
   
   
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      RDATA
   
             Field Name       Data Type      Notes
             ------------------------------------------------------------------
             Algorithm Name   domain-name    points to an RP RR giving the
                                             algorithm definer's address.
             Time Signed      u_int32_t      seconds since 1-Jan-70 UTC.
             Expire           u_int32_t      Time Signed + estimate of
                                             RTT/2+fudge.
             Signature Size   u_int16_t      number of octets in Signature.
             Signature        octet stream   defined by Algorithm Name.
             Error            u_int16_t      expanded RCODE covering
                                             signature processing.
             Other Len        u_int16_t      length, in octets, of Other Data.
             Other Data       octet stream   undefined by this protocol.
   
   
   2.4. Example
   
   
      NAME   GW-DENVAX-0042.HOME.VIX.COM.
   
      TYPE   TSIG
   
      CLASS  ANY
   
      TTL    0
   
      RdLen  as appropriate
   
      RDATA
   
             Field Name       Contents
             -------------------------------------------
             Algorithm Name   HMAC-MD5.SIG-ALG.REG.INT.
             Time Signed      853804800
             Expire           853805400
             Signature Size   as appropriate
             Signature        as appropriate
             Error            0 (NOERROR)
             Other Len        0
             Other Data       empty
   
   
   
   
   
   
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   3 - Protocol Operation
   
   
   3.1. Effects of adding TSIG to outgoing message
   
   Once the outgoing message has been constructed, the keyed message digest
   operation can be performed.  The resulting message digest will then be
   stored in a TSIG which is appended to the additional data section.
   Appending a transaction signature to an DNS message is not allowed to
   result in a truncated response; a TCP connection must be used to prevent
   the truncation.  To force a TCP connection, the server is permitted to
   return an answer with no data only TSIG attached and TC bit set and
   RCODE 0 (NOERROR).  The client should at this point retry the request
   using TCP (per [RFC1035 4.2.2]).
   
   3.2. TSIG processing on incoming messages
   
   Upon receipt of a message with a TSIG RR, the TSIG RR is copied to a
   safe location, removed from the DNS Message, and decremented out of the
   DNS Message Headers ARCOUNT.  At this point the keyed message digest
   operation is performed.  If the algorithm name or key name is unknown to
   the recipient, or if the message digests do not match, the whole DNS
   Message must be discarded.  A response with RCODE 9 (NOTAUTH) should be
   sent back to the originator with TSIG ERROR 17 (BADKEY).  A message to
   the system operations log should to be generated, to warn the operations
   staff of a possible security incident in progress.  Care should be taken
   to ensure that logging of this type of event does not open the system to
   a denial of service attack.
   
   3.3. Expire value considerations
   
   If the value of expire field is in the past this transaction should be
   assumed to be an replay attack, and RCODE 9 (NOTAUTH) should returned
   with TSIG ERROR 18 (BADTIME).  For expire to work correctly it is
   recommended that all machines using TSIG signatures use clock
   synchronization.  If clock synchronization is not used the ``Fudge'' has
   to be larger but ``Fudge'' should always be less than or equal to the
   smallest TTL of the RR in the message.  If the smallest TTL is 0 or
   other very small value fudge may be set to a larger value but not
   greater than 300.  In any case ``Fudge'' should always be set to a value
   that is less than half the reuse time of request ID's.
   
   
   
   
   
   
   
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   3.4. Time values used in TSIG calculations
   
   The data digested includes the two timer values in the TSIG header in
   order to prevent replay attacks.  If this were not done an attacker
   could replay old messages but update the ``Time Signed'' and ``Expire''
   fields to make the message look new.  This data is named ``TSIG
   Timers'', and for the purpose of digest calculation they are invoked in
   their ``on the wire'' format, in the following order: first Time Signed,
   then Expire.  For example:
   
   Field Name    Decimal Value   Wire Format   Actual Date
   -------------------------------------------------------------------------
   Time Signed   853804800       32 e4 07 00   Tue Jan 21 00:00:00 1997
   Expire        853805400       32 e4 09 58   Tue Jan 21 00:10:00 1997
   
   
   3.5. TSIG Variables and Coverage
   
   When generating or verifying a transaction signature, the following data
   are digested, in network byte order or wire format, as appropriate:
   
   DNS Message
      A whole and complete DNS message in wire format, before the TSIG RR
      has been added to the additional data section and before the DNS
      Message Header's ARCOUNT field has been incremented to contain the
      TSIG RR, but substituting 0x0000 as the request ID (due to possible
      request forwarding).
   
   TSIG Variables
   
      Source       Field Name       Notes
      ------------------------------------------------------------------------
      TSIG RR      NAME             Key name, in canonical wire format
      TSIG RR      CLASS            (Always ANY in the current specification)
      TSIG RR      TTL              (Always 0 in the current specification)
      TSIG RDATA   Algorythm Name   in canonical wire format
      TSIG RDATA   Time Signed      in network byte order
      TSIG RDATA   Time Expired     in network byte order
      TSIG RDATA   Error            in network byte order
      TSIG RDATA   Other Len        in network byte order
      TSIG RDATA   Other Data       exactly as transmitted
   
      The RR RDLEN and RDATA Signature Length are not included in the hash
      since they are not guaranteed to be knowable before the signature is
      generated.
   
   
   
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      ``Canonical wire format'' means uncompressed labels shifted to lower
      case.  The use of label types other than 00 is not defined for this
      specification.
   
   Request Signature
      Response signatures will include the request signature in their
      digest.  The request's signature is digested in wire format,
      including the following fields:
   
      Field              Type           Description
      ---------------------------------------------------------
      Signature Length   u_int16_t      in network byte order
      Signature Data     octet stream   exactly as transmitted
   
   
   Digested components are fed into the hashing function as a continuous
   octet stream with no interfield padding.
   
   4 - Protocol Details
   
   
   4.1. TSIG generation on requests
   
   Client performs the message digest operation and appends TSIG to
   additional data section and transmits request to server.   The client
   must store the message digest from the request while awaiting an answer.
   Digest components for requests are:
   
      DNS Message (request)
      TSIG Variables (response)
   
   
   Note that some older name servers will not accept requests with a
   nonempty additional data section, but clients should only attempt signed
   transactions against servers who are known to support TSIG and share
   some secret key with the client -- so, this is not a problem in
   practice.
   
   
   
   
   
   
   
   
   
   
   
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   4.2. TSIG on Answers
   
   When a server has generated a response to a signed request, it signs the
   response using the same algorythm and key.  Digest components are:
   
      Request Signature
      DNS Message (response)
      TSIG Variables (response)
   
   
   4.3. TSIG on TSIG Error returns
   
   When a server detects an error in TSIG checks, and the server shares a
   secret with the client, the server should send back a signed error
   message.  Digest components are:
   
      DNS Message (response)
      TSIG Variables (response)
   
   The reason that the request is not included in this digest is to make it
   possible for the client to verify the error.  If the error is not a TSIG
   error the response MUST be generated as specified in section in 4.2.  If
   the server shares multiple secrets with the client, the server should
   use the secret the client used if the problem was not a signature error
   in the request, or any available shared secret otherwise.
   
   4.4. TSIG on TCP connection
   
   A DNS TCP session can include multiple DNS headers.  This is, for
   example commonly used by AXFR.  TSIG on such a connection can be used to
   protect the connection from hijacking and provide data integrity.  The
   TSIG MUST be included on the first and last DNS header.  It can be
   optionally placed on any intermediary header.  It is expensive to
   include it on every header, but it SHOULD be placed on at least every
   100'th header.  The first envelope is processed as a standard answer,
   and subject messages have the following digests components:
   
      Prior Digest (running)
      DNS Message (current message)
      TSIG Timers (current message)
   
   This allows client to rapidly detect when a transfer has been altered
   and it can close the connection at that point and retry.  Once client
   TSIG check fails, the client MUST close the connection.  If the client
   does not get TSIG frequently enough it SHOULD assume the connection has
   
   
   
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   been hijacked and it SHOULD close the connection.  Client should treat
   this the same way as any other interrupted transfer.
   
   4.5. Server TSIG checks
   
   Upon receipt of a message, server will check if there is a TSIG RR. If
   one exists, the server is required to return a TSIG RR in the response.
   The server MUST perform the following checks in the following order,
   check KEY, check TIME values, check Signature.
   
   4.5.1. KEY check and error handling
   
   If a non-forwarding server does not recognize the key used by the client
   the server MUST generate an error response with RCODE 9 (NOTAUTH) and
   TSIG ERROR 17 (BADKEY).  If the server and client share another key the
   server should use the one of the other keys to sign the error message as
   specified in section 4.3.  If server does not share any secret with
   client the server should log the error.
   
   4.5.2. TIME check and Error handling
   
   If the server time is outside the time interval in the request, the
   server MUST generate an error response with RCODE 9 (NOTAUTH) and TSIG
   ERROR 18 (BADTIME).  This response MUST be signed by the same key and
   MUST include the servers current time in the time signed field.  The
   data signed is specified in section 4.3.
   
   4.5.3. Signature check and error handling
   
   If TSIG fails to verify, the server MUST generate an error response as
   specified in section 4.3 with RCODE of 9 (NOTAUTH) and TSIG ERROR 16
   (BADSIG).  The server MUST sign this error response with the same key
   the client used.
   
   4.6. Client processing of answer
   
   When a client receives a response from a server it expects a TSIG from,
   it first checks if the TSIG RR is present in the response.  Otherwise
   the response is treated as having a format error and discarded.  The
   client then extracts the TSIG, adjusts the ARCOUNT, and calculates the
   keyed digest in the same way as the server.  If the TSIG does not
   validate, that response must be discarded, unless the RCODE is 9
   (NOTAUTH), in which case the client should attempt to verify the
   response as it was TSIG error as specified in section 4.3.
   
   
   
   
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   4.6.1. Key error handling
   
   If an RCODE on a response is 9 (NOTAUTH), and the response TSIG
   validates, and the TSIG key is different from the key used on the
   request, then this is a key error.  Client should retry the request
   using the key specified by server.
   
   4.6.2. Time error handling
   
   If the response RCODE is 9 (NOTAUTH), and TSIG ERROR is 18 (BADTIME) or
   the TSIG times in request and answer do not overlap, then this is a TIME
   error.  This is an indication that client and server are not clock
   synchronized.  In this case the client should log the event.  DNS
   resolvers MUST not adjust any clocks in the client based on BADTIME
   errors.
   
   4.6.3. Signature error handling
   
   If the response RCODE is 9 (NOTAUTH) and TSIG ERROR is 16 (BADSIG), this
   is a signature error, and client MAY retry the request with a new
   request ID but it would be better to try a different shared key if one
   is available.  Client SHOULD keep track of how many times each key has
   Signature errors.  Clients should log this event.
   
   4.7. Special considerations for forwarding servers
   
   A server acting as a Forwarding Server of a DNS message, should check
   for the existence of the TSIG record.  If the name on the TSIG is not of
   a secret that the server shares with the originator the server will
   forward the message unchanged including the TSIG.  If the name of the
   TSIG is of a key this server shares with the originator it processes the
   TSIG.  If the TSIG passes all checks, the forwarding server has the
   obligation of including a TSIG of his own, to the destination or the
   next forwarder.  If no transaction security is available to the
   destination and response has the AD flag (see [RFC2065]), the forwarder
   MUST unset the AD flag before adding the TSIG to the answer.
   
   
   
   
   
   
   
   
   
   
   
   
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   5 - Shared Secrets
   
   5.1. Secret keys are very sensitive information and all available steps
   should be taken to protect them on every host on which they are stored.
   Generally such hosts need to be physically protected.  If they are
   multi-user machines, great care should be taken that unprivileged users
   have no access to keying material.  Resolvers usually run unprivileged,
   which means all users of a host will usually be able to see whatever
   configuration data is used by the resolver.
   
   5.2. A name server usually runs privileged, which means its
   configuration data need not be visible to all users of the host.  For
   this reason, a host that implements transaction signatures should
   probably be configured with a ``stub resolver'' and a local caching and
   forwarding name server.  This presents a special problem for [RFC2136]
   which otherwise depends on clients to communicate only with a zone's
   authoritative name servers.
   
   5.3. Use of strong random shared secrets is essential to the security of
   TSIG.  See [RFC1750] for a discussion of this issue.  The secret should
   be at least as long as the keyed message digest digest, i.e., 16 bytes
   for HMAC-MD5 or 20 bytes for HMAC-SHA1.
   
   6 - Security Considerations
   
   6.1. The approach specified here is computationally much less expensive
   than the signatures specified in [RFC2065].  As long as the shared
   secret key is not compromised, strong authentication is provided for the
   last hop from a local name server to the user resolver.
   
   6.2. Secret keys should be changed periodically.  If the client host has
   been compromised, the server should suspend the use of all secrets known
   to that client.  If possible, secrets should be stored in encrypted
   form.  Secrets should never be transmitted in the clear over any
   network.  This document does not address the issue on how to distribute
   secrets.
   
   6.3. This mechanism does not authenticate source data, only its
   transmission between two parties who share some secret.  The original
   source data can come from a compromised zone master or can be corrupted
   during transit from an authentic zone master to some ``caching
   forwarder.''  However, if the server is faithfully performing the full
   [RFC2065] security checks, then only security checked data will be
   available to the client.
   
   
   
   
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   7 - References
   
   [RFC1034]  P. Mockapetris, ``Domain Names - Concepts and Facilities,''
              RFC 1034, ISI, November 1987.
   
   [RFC1035]  P. Mockapetris, ``Domain Names - Implementation and
              Specification,'' RFC 1034, ISI, November 1987.
   
   [RFC1321]  R. Rivest, ``The MD5 Message-Digest Algorithm,'' RFC 1321,
              MIT LCS & RSA Data Security, Inc., April 1992.
   
   [RFC1750]  D. Eastlake, S. Crocker, J. Schiller, ``Randomness
              Recommendations for Security,'' RFC 1750, DEC, CyberCash &
              MIT, December 1995.
   
   [RFC2065]  D. Eastlake , C Kaufman, ``Domain Name System Security
              Extensions,'' RFC 2065, CyberCash & Iris, January 1997.
   
   [RFC2104]  H. Krawczyk, M. Bellare, R. Canetti, ``HMAC-MD5: Keyed-MD5
              for Message Authentication,'' RFC 2104 , IBM, UCSD & IBM,
              February 1997.
   
   [RFC2136]  P. Vixie (Ed.), S. Thomson, Y. Rekhter, J. Bound ``Dynamic
              Updates in the Domain Name System,'' RFC 2136, ISC & Bellcore
              & Cisco & DEC, April 1997.
   
   [RFC2137]  D. Eastlake 3rd ``Secure Domain Name System Dynamic Update,''
              CyberCash, April 1997.
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
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   9 - Author's Addresses
   
   
      Paul Vixie                          Olafur Gudmundsson
         Internet Software Consortium        Trusted Information Systems
         950 Charter Street                  3060 Washington Road, Route 97
         Redwood City, CA 94063              Glenwood, MD 21738
         +1 650 779 7001                     +1 301 854 6889
         <paul@vix.com>                      <ogud@tis.com>
   
      Donald E. Eastlake 3rd.
         CyberCash, Inc.
         318 Acton Street
         Carlisle, MA 01741
         +1 978 287 4877
         <dee@cybercash.com>
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
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