Internet Engineering Task Force                           F. Dupont, Ed.
Internet-Draft                                                 S. Morris
Obsoletes: 2845, 4635 (if approved)                                  ISC
Intended status: Standards Track                        October 30, 2017                           March 5, 2018
Expires: May 3, September 6, 2018

          Secret Key Transaction Authentication for DNS (TSIG)


   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. mechanism.

   This document includes revised original TSIG specifications (RFC2845)
   and the its extension for HMAC-SHA (RFC4635).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on May 3, September 6, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Key words . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  New Assigned Numbers  . . . . . . . . . . . . . . . . . . . .   4
   4.  TSIG RR Format  . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  TSIG RR Type  . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  TSIG Calculation  . . . . . . . . . . . . . . . . . . . .   5
     4.3.  TSIG Record Format  . . . . . . . . . . . . . . . . . . .   5
       4.3.1.  TSIG RDATA Wire Format  . . . . . . . . . . . . . . .   6
     4.4.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Protocol Operation  . . . . . . . . . . . . . . . . . . . . .   8   7
     5.1.  Effects of adding TSIG to outgoing message  . . . . . . .   8   7
     5.2.  TSIG processing on incoming messages  . . . . . . . . . .   8
     5.3.  Time values used in TSIG calculations . . . . . . . . . .   8
     5.4.  TSIG Variables and Coverage . . . . . . . . . . . . . . .   9   8
       5.4.1.  DNS Message . . . . . . . . . . . . . . . . . . . . .   9
       5.4.2.  TSIG Variables  . . . . . . . . . . . . . . . . . . .   9
       5.4.3.  Request MAC . . . . . . . . . . . . . . . . . . . . .  10   9
     5.5.  Padding . . . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Protocol Details  . . . . . . . . . . . . . . . . . . . . . .  10
     6.1.  TSIG generation on requests . . . . . . . . . . . . . . .  10
     6.2.  TSIG on Answers . . . . . . . . . . . . . . . . . . . . .  10
     6.3.  TSIG on TSIG Error returns  . . . . . . . . . . . . . . .  11  10
     6.4.  TSIG on zone tranfer over a TCP connection  . . . . . . . . . . . . . . . . .  11
     6.5.  Server TSIG checks  . . . . . . . . . . . . . . . . . . .  12  11
       6.5.1.  Key check and error handling  . . . . . . . . . . . .  12  11
       6.5.2.  Specifying Truncation . . . . . . . . . . . . . . . .  12
       6.5.3.  MAC check and error handling  . . . . . . . . . . . .  13  12
       6.5.4.  Time check and error handling . . . . . . . . . . . .  13
       6.5.5.  Truncation check and error handling . . . . . . . . .  13
     6.6.  Client processing of answer . . . . . . . . . . . . . . .  13
       6.6.1.  Key error handling  . . . . . . . . . . . . . . . . .  14  13
       6.6.2.  MAC error handling  . . . . . . . . . . . . . . . . .  14
       6.6.3.  Time error handling . . . . . . . . . . . . . . . . .  14
       6.6.4.  Truncation error handling . . . . . . . . . . . . . .  14
     6.7.  Special considerations for forwarding servers . . . . . .  14
   7.  Algorithms and Identifiers  . . . . . . . . . . . . . . . . .  15  14
   8.  TSIG Truncation Policy  . . . . . . . . . . . . . . . . . . .  15
   9.  Shared Secrets  . . . . . . . . . . . . . . . . . . . . . . .  16
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17  16
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  17
     11.1.  Issue fixed in this document . . . . . . . . . . . . . .  18
     11.2.  Why not DNSSEC?  . . . . . . . . . . . . . . . . . . . .  18
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     12.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  22  21
   Appendix B.  Change History . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   In 2017, security problems in two nameservers strictly following
   [RFC2845] and [RFC4635] (i.e., TSIG and its HMAC-SHA extension)
   specifications were discovered.  The implementations were fixed but,
   to avoid similar problems in the future, the two documents were
   updated and merged, producing these revised specifications for TSIG.

   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.

   This document specifies use of a message authentication code (MAC),
   either HMAC-MD5 or HMAC-SHA (keyed hash functions), to provide an
   efficient means of point-to-point authentication and integrity
   checking for transactions.

   The second area where the secret key based MACs specified in this
   document can be used is to authenticate DNS update requests as well
   as transaction responses, providing a lightweight alternative to the
   protocol described by [RFC3007].

   A further use of this mechanism 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 DNSSEC does not
   protect glue records and unsigned records unless SIG(0) (transaction
   signature) is used.

   The authentication mechanism proposed in this document uses shared
   secret keys to establish a 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
   MACs). (by
   forging the MAC).  There is an urgent need to provide simple and
   efficient authentication between clients and local servers and this
   proposal addresses that need.  This  The 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 a few recursive servers.

   A server acting as an indirect caching resolver -- a "forwarder" in
   common usage -- might use transaction-based authentication when
   communicating with its small number of preconfigured "upstream"
   servers.  Other uses of DNS secret key authentication and possible
   systems for automatic secret key distribution may be proposed in
   separate future documents.

   Note that use of TSIG presumes prior agreement between the two
   parties involved (e.g., resolver and server involved server) as to the algorithm and
   key to be used.

   Since the publication of first version of this document ([RFC2845]) a
   mechanism based on asymmetric signatures using the SIG RR was
   specified (SIG(0) [RFC2931]) when whereas this document uses symmetric
   authentication codes calculated by HMAC [RFC2104] using strong hash

2.  Key words

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  New Assigned Numbers

   RRTYPE = TSIG (250)
   ERROR = 0..15 (a DNS RCODE)
   ERROR = 16 (BADSIG)
   ERROR = 17 (BADKEY)

4.  TSIG RR Format

4.1.  TSIG RR Type

   To provide secret key authentication, we use a new RR type whose
   mnemonic is TSIG and whose type code is 250.  TSIG is a meta-RR and
   MUST NOT be cached.  TSIG RRs are 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.

4.2.  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 is
   discarded once it has been used to authenticate a DNS message.
   Recommendations concerning the message digest agorithm can be found
   in Section 7.  All multi-octet integers in the TSIG record are sent
   in network byte order (see [RFC1035] 2.3.2).

4.3.  TSIG Record Format

   NAME  The name of the key used in domain name syntax.  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 and share a key,
         possibilities for the key name include <id>,
         <id>, and <id>  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)


   TTL   0

   RdLen (variable)

4.3.1.  TSIG RDATA Wire Format The RDATA for a TSIG RR consists of an octet stream Algorithm
         Name field, a uint48_t Time Signed field, a uint16_t Fudge
         field, a uint16_t MAC Size field, a octet stream MAC field, a
         uint16_t Original ID, a uint16_t Error field, a uint16_t Other
         Len field and an octet stream of Other Data.

                            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
       /                         Algorithm Name                        /
       |                                                               |
       |          Time Signed          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               |            Fudge              |
       |          MAC Size             |                               /
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+             MAC               /
       /                                                               /
       |          Original ID          |            Error              |
       |          Other Len            |                               /
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+           Other Data          /
       /                                                               /
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  The Algorithm Name Field

         The contents of the RDATA fields are:

         *  Algorithm Name field - identifies the TSIG algorithm name in the
            domain name syntax.  The

         *  Time Signed Field - the The Time Signed field specifies seconds
            since 00:00 on 1970-01-01 UTC.  The

         *  Fudge Field

   The Fudge field - specifies allowed time difference in seconds
            permitted in the Time Signed field.  The

         *  MAC Size Field

   The - the MAC Size field specifies the length of MAC
            field in octets.  Truncation is indicated by a MAC size less
            than the HMAC size.  The

         *  MAC Field

   The - the contents of the MAC field contents are defined by the used Algorithm.  The Error field

   The TSIG
            algorithm used.

         *  Error field - contains the Expanded expanded RCODE covering TSIG
            processing.  The Other Len Field


         *  Other Len field - specifies the length of Other Data the "Other Data" field
            in octets.  The Other Data Field


         *  Other Data - this field is will be empty unless the content of
            the Error == BADTIME. field is BADTIME, in which case it will contain
            the server's current time (see Section 6.5.4).

4.4.  Example




   TTL   0

   RdLen As appropriate


                    Field Name     Contents
                    -------------- -------------------
                    Algorithm Name SAMPLE-ALG.EXAMPLE.
                    Time Signed    853804800
                    Fudge          300
                    MAC Size       As appropriate
                    MAC            As appropriate
                    Original ID    As appropriate
                    Error          0 (NOERROR)
                    Other Len      0
                    Other Data     Empty

5.  Protocol Operation

5.1.  Effects of adding TSIG to outgoing message

   Once the outgoing message has been constructed, the keyed message
   digest operation HMAC computation
   can be performed.  The resulting message digest MAC will then be stored in a TSIG
   which is appended to the additional data section (the ARCOUNT is
   incremented to reflect this).  If the TSIG record cannot be added
   without causing the message to be truncated, the server MUST alter
   the response so that a TSIG can be included.  This response consists
   of only the question and a TSIG record, and has the TC bit set and
   RCODE 0 (NOERROR).  The client SHOULD at this point retry the request
   using TCP (per [RFC1035] 4.2.2).

5.2.  TSIG processing on incoming messages

   If an incoming message contains a TSIG record, it MUST be the last
   record in the additional section.  Multiple TSIG records are not
   allowed.  If a TSIG record is present in any other position, the
   packet DNS
   message is dropped and a response with RCODE 1 (FORMERR) MUST be
   returned.  Upon receipt of a message with a correctly placed TSIG RR,
   the TSIG RR is copied to a safe location, removed from the DNS
   Message, and decremented out of the DNS message header's ARCOUNT.  At
   this point the keyed message digest operation HMAC computation is performed: until this operation
   concludes that the signature is valid, the signature MUST be
   considered to be invalid.

   If the algorithm name or key name is unknown to the recipient, or if
   the message digests MACs do not match, the whole DNS message MUST be discarded.  If
   the message is a query, a response with RCODE 9 (NOTAUTH) MUST be
   sent back to the originator with TSIG ERROR 17 (BADKEY) or TSIG ERROR
   16 (BADSIG).  If no key is available to sign this message it MUST be
   sent unsigned (MAC size == 0 and empty MAC).  A message to the system
   operations log SHOULD 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.

5.3.  Time values used in TSIG calculations

   The data digested includes the two timer values in the TSIG header in
   order to defend against replay attacks.  If this were not done, an
   attacker could replay old messages but update the "Time Signed" and
   "Fudge" fields to make the message look new.  This data is named
   "TSIG Timers", and for the purpose of digest MAC calculation they are
   invoked in their "on the wire" format, in the following order: first
   Time Signed, then Fudge.  For example:

     Field Name  Value     Wire Format       Meaning
     ----------- --------- ----------------- ------------------------
     Time Signed 853804800 00 00 32 e4 07 00 Tue Jan 21 00:00:00 1997
     Fudge       300       01 2C             5 minutes

5.4.  TSIG Variables and Coverage

   When generating or verifying the contents of a TSIG record, the
   following data are digested, passed as input to MAC computation, in network
   byte order or wire format, as appropriate:

5.4.1.  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.  If the message ID differs from the original message ID, the
   original message ID is substituted for the message ID.  This could
   happen when forwarding a dynamic update request, for example.

5.4.2.  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 Algorithm Name in canonical wire format
    TSIG RDATA Time Signed    in network byte order
    TSIG RDATA Fudge          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 MAC Length are not included in the hash input to
   MAC computation since they are not guaranteed to be knowable before
   the MAC is generated.

   The Original ID field is not included in this section, as it has
   already been substituted for the message ID in the DNS header and

   For each label type, there must be a defined "Canonical wire format"
   that specifies how to express a label in an unambiguous way.  For
   label type 00, this is defined in [RFC4034], for label type 01, this
   is defined in [RFC6891].  The use of label types other than 00 and 01
   is not defined for this specification.

5.4.3.  Request MAC

   When generating the MAC to be included in a response, the validated
   request MAC MUST be included in the digest. MAC computation.  If the request
   MAC failed to validate, an unsigned error message MUST be returned
   instead.  (Section 6.3).

   The request's MAC is digested in wire format, including the following

              Field      Type         Description
              ---------- ------------ ----------------------
              MAC Length uint16_t     in network byte order
              MAC Data   octet stream exactly as transmitted

5.5.  Padding

   Digested components (i.e., inputs to HMAC computation) are fed into
   the hashing function as a continuous octet stream with no interfield

6.  Protocol Details

6.1.  TSIG generation on requests

   Client performs the message digest operation HMAC computation and appends a TSIG record to the
   additional data section and transmits the request to the server.  The
   client MUST store the message digest MAC from the request while awaiting an answer.
   The digest components for a request are:

      DNS Message (request)
      TSIG Variables (request)

   Note that some older name servers will not accept requests with a
   nonempty additional data section.  Clients SHOULD only attempt signed
   transactions with servers who are known to support TSIG and share
   some secret key with the client -- so, this is not a problem in

6.2.  TSIG on Answers

   When a server has generated a response to a signed request, it signs
   the response using the same algorithm and key.  The server MUST NOT
   generate a signed response to an unsigned request or a request that
   fails validation.  The digest components are:

      Request MAC
      DNS Message (response)
      TSIG Variables (response)

6.3.  TSIG on TSIG Error returns

   When a server detects an error relating to the key or MAC, the server
   SHOULD send back an unsigned error message (MAC size == 0 and empty
   MAC).  If an error is detected relating to the TSIG validity period
   or the MAC is too short for the local policy, the server SHOULD send
   back a signed error message.  The digest components are:

      Request MAC (if the request MAC validated)
      DNS Message (response)
      TSIG Variables (response)

   The reason that the request is not included in this digest MAC in some cases
   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 6.2.

6.4.  TSIG on zone tranfer over a TCP connection

   A zone transfer over a DNS TCP session can include multiple DNS envelopes.  This is, for
   example, commonly used by zone transfer.
   messages.  Using TSIG on such a connection can protect the connection
   from hijacking and provide data integrity.  The TSIG MUST be included
   on the first and last DNS
   envelopes.  It can messages, and for new implementations
   SHOULD be optionally placed on any all intermediary
   envelopes.  It is expensive to include it on every envelopes, but it messages.  For backward
   compatibility the client which receives DNS messages and verifies
   TSIG MUST be placed on at least every 100'th envelope. accept up to 99 intermediary messages without a TSIG.  The
   first envelope is processed as a standard answer, and subsequent
   messages have the following digest components:

      Prior Digest MAC (running)
      DNS Messages (any unsigned messages since the last TSIG)
      TSIG Timers (current message)

   This allows the client to rapidly detect when the session has been
   altered; at which point it can close the connection and retry.  If a
   client TSIG verification fails, the client MUST close the connection.
   If the client does not receive TSIG records frequently enough (as
   specified above) it SHOULD assume the connection has been hijacked
   and it SHOULD close the connection.  The client SHOULD treat this the
   same way as they would any other interrupted transfer (although the
   exact behavior is not specified).

6.5.  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 MAC, check Time values, check
   Truncation policy.

6.5.1.  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).  This response MUST be unsigned
   as specified in Section 6.3.  The server SHOULD log the error.

6.5.2.  Specifying Truncation

   When space is at a premium and the strength of the full length of an
   HMAC is not needed, it is reasonable to truncate the HMAC and use the
   truncated value for authentication.  HMAC SHA-1 truncated to 96 bits
   is an option available in several IETF protocols, including IPsec and

   Processing of a truncated MAC follows these rules

   1.  If "MAC size" field is greater than HMAC output length:

       This case MUST NOT be generated and, if received, MUST cause the
       DNS message to be dropped and RCODE 1 (FORMERR) to be returned.

   2.  If "MAC size" field equals HMAC output length:

       The entire output HMAC output is present and used.

   3.  "MAC size" field is less than HMAC output length but greater than
       that specified in case 4, below:

       This is sent when the signer has truncated the HMAC output to an
       allowable length, as described in [RFC2104], taking initial
       octets and discarding trailing octets.  TSIG truncation can only
       be to an integral number of octets.  On receipt of a packet DNS message
       with truncation thus indicated, the locally calculated MAC is
       similarly truncated and only the truncated values are compared
       for authentication.  The request MAC used when calculating the
       TSIG MAC for a reply is the truncated request MAC.

   4.  "MAC size" field is less than the larger of 10 (octets) and half
       the length of the hash function in use:

       With the exception of certain TSIG error messages described in
       Section 6.3, where it is permitted that the MAC size be zero,
       this case MUST NOT be generated and, if received, MUST cause the
       DNS message to be dropped and RCODE 1 (FORMERR) to be returned.

6.5.3.  MAC check and error handling

   If a TSIG fails to verify, the server MUST generate an error response
   as specified in Section 6.3 with RCODE 9 (NOTAUTH) and TSIG ERROR 16
   (BADSIG).  This response MUST be unsigned as specified in
   Section 6.3.  The server SHOULD log the error.

6.5.4.  Time check and error handling

   If the server time is outside the time interval specified by the
   request (which is: Time Signed, plus/minus Fudge), the server MUST
   generate an error response with RCODE 9 (NOTAUTH) and TSIG ERROR 18
   (BADTIME).  The server SHOULD also cache the most recent time signed
   value in a message generated by a key, and SHOULD return BADTIME if a
   message received later has an earlier time signed value.  A response
   indicating a BADTIME error MUST be signed by the same key as the
   request.  It MUST include the client's current time in the time
   signed field, the server's current time (a uint48_t) in the other
   data field, and 6 in the other data length field.  This is done so
   that the client can verify a message with a BADTIME error without the
   verification failing due to another BADTIME error.  The data signed
   is specified in Section 6.3.  The server SHOULD log the error.

6.5.5.  Truncation check and error handling

   If a TSIG is received with truncation that is permitted under
   Section 6.5.2 above but the MAC is too short for the local policy in
   force, an RCODE 9 (NOTAUTH) and TSIG ERROR 22 (BADTRUNC) MUST be
   returned.  The server SHOULD log the error.

6.6.  Client processing of answer

   When a client receives a response from a server and expects to see a
   TSIG, 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 MAC in the same way as the server, applying the
   same rules to decide if truncated MAC is valid.  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 if it were a TSIG Error response, as specified in
   Section 6.3.  A message containing an unsigned TSIG record or a TSIG
   record which fails verification SHOULD NOT be considered an
   acceptable response; the client SHOULD log an error and continue to
   wait for a signed response until the request times out.

6.6.1.  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.  The client MAY retry the request
   using the key specified by the server.  This should never occur, as a
   server MUST NOT sign a response with a different key than signed the

6.6.2.  MAC error handling

   If the response RCODE is 9 (NOTAUTH) and TSIG ERROR is 16 (BADSIG),
   this is a MAC 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.  Clients SHOULD keep track of how many MAC errors
   are associated with each key.  Clients SHOULD log this event.

6.6.3.  Time error handling

   If the response RCODE is 9 (NOTAUTH) and the TSIG ERROR is 18
   (BADTIME), or the current time does not fall in the range specified
   in the TSIG record, then this is a Time error.  This is an indication
   that the client and server clocks are not synchronized.  In this case
   the client SHOULD log the event.  DNS resolvers MUST NOT adjust any
   clocks in the client based on BADTIME errors, but the server's time
   in the other data field SHOULD be logged.

6.6.4.  Truncation error handling

   If the response RCODE is 9 (NOTAUTH) and the TSIG ERROR is 22
   (BADTRUNC) the this is a Truncation error.  The client MAY retry with
   lesser truncation up to the full HMAC output (no truncation), using
   the truncation used in the response as a hint for what the server
   policy allowed (Section 8).  Clients SHOULD log this event.

6.7.  Special considerations for forwarding servers

   A server acting as a forwarding server of a DNS message SHOULD check
   for the existence of a TSIG record.  If the name on the TSIG is not
   of a secret that the server shares with the originator the server
   MUST forward the message unchanged including the TSIG.  If the name
   of the TSIG is of a key this server shares with the originator, it
   MUST process the TSIG.  If the TSIG passes all checks, the forwarding
   server MUST, if possible, include a TSIG of his own, to the
   destination or the next forwarder.  If no transaction security is
   available to the destination and the response has the AD flag (see
   [RFC4035]), the forwarder MUST unset the AD flag before adding the
   TSIG to the answer.

7.  Algorithms and Identifiers

   The only message digest algorithm specified in the first version of
   these specifications [RFC2845] was "HMAC-MD5" (see [RFC1321],
   [RFC2104]).  The "HMAC-MD5" algorithm is mandatory to implement for

   The use of SHA-1 [FIPS180-4], [RFC3174], (which is a 160-bit hash as
   compared to the 128 bits for MD5), and additional hash algorithms in
   the SHA family [FIPS180-4], [RFC3874], [RFC6234] with 224, 256, 384,
   and 512 bits may be preferred in some cases.  This is because
   increasingly successful cryptanalytic attacks are being made on the
   shorter hashes.

   Use of TSIG between a two DNS resolver and server agents is by mutual agreement.  That
   agreement can include the support of additional algorithms and
   criteria as to which algorithms and truncations are acceptable,
   subject to the restriction and guidelines in Section 6.5.2 above.
   Key agreement can be by the TKEY mechanism [RFC2930] or some other
   mutually agreeable method.

   The current HMAC-MD5.SIG-ALG.REG.INT and gss-tsig identifiers are
   included in the table below for convenience.  Implementations that
   support TSIG MUST also implement HMAC SHA1 and HMAC SHA256 and MAY
   implement gss-tsig and the other algorithms listed below.

                   Requirement Name
                   ----------- ------------------------
                   Mandatory   HMAC-MD5.SIG-ALG.REG.INT
                   Optional    gss-tsig
                   Mandatory   hmac-sha1
                   Optional    hmac-sha224
                   Mandatory   hmac-sha256
                   Optional    hmac-sha384
                   Optional    hmac-sha512

   SHA-1 truncated to 96 bits (12 octets) SHOULD be implemented.

8.  TSIG Truncation Policy

   Use of TSIG is by mutual agreement between two DNS agents, e.g., a
   resolver and server.  Implicit in such an "agreement" are criteria as
   to acceptable keys and algorithms and, with the extensions in this
   document, truncations.  Note that it is common for implementations to
   bind the TSIG secret key or keys that may be in place at a resolver and server two parties
   to particular algorithms.  Thus, such implementations only permit the
   use of an algorithm if there is an associated key in place.  Receipt
   of an unknown, unimplemented, or disabled algorithm typically results
   in a BADKEY error.

   Local policies MAY require the rejection of TSIGs, even though they
   use an algorithm for which implementation is mandatory.

   When a local policy permits acceptance of a TSIG with a particular
   algorithm and a particular non-zero amount of truncation, it SHOULD
   also permit the use of that algorithm with lesser truncation (a
   longer MAC) up to the full HMAC output.

   Regardless of a lower acceptable truncated MAC length specified by
   local policy, a reply SHOULD be sent with a MAC at least as long as
   that in the corresponding request.  Note if the request specified a
   MAC length longer than the HMAC output it will be rejected by
   processing rules Section 6.5.2 case 1.

   Implementations permitting multiple acceptable algorithms and/or
   truncations SHOULD permit this list to be ordered by presumed
   strength and SHOULD allow different truncations for the same
   algorithm to be treated as separate entities in this list.  When so
   implemented, policies SHOULD accept a presumed stronger algorithm and
   truncation than the minimum strength required by the policy.

9.  Shared Secrets

   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
   often run unprivileged, which means all users of a host would be able
   to see whatever configuration data is used by the resolver.

   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-based authentication 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.

   Use of strong random shared secrets is essential to the security of
   TSIG.  See [RFC4086] for a discussion of this issue.  The secret
   SHOULD be at least as long as the keyed message digest, HMAC output, i.e., 16 bytes for
   HMAC-MD5 or 20 bytes for HMAC-SHA1.

10.  IANA Considerations

   IANA maintains a registry of algorithm names to be used as "Algorithm
   Names" as defined in Section 4.3.  Algorithm names are text strings
   encoded using the syntax of a domain name.  There is no structure
   required other than names for different algorithms must be unique
   when compared as DNS names, i.e., comparison is case insensitive.
   Previous specifications [RFC2845] and [RFC4635] defined values for
   HMAC MD5 and SHA.  IANA has also registered "gss-tsig" as an
   identifier for TSIG authentication where the cryptographic operations
   are delegated to the Generic Security Service (GSS) [RFC3645].

   New algorithms are assigned using the IETF Consensus policy defined
   in [RFC8126].  The algorithm name HMAC-MD5.SIG-ALG.REG.INT looks like
   a fully-qualified domain name for historical reasons; other algorithm
   names are simple (i.e., single-component) names.

   IANA maintains a registry of "TSIG Error values" to be used for
   "Error" values as defined in Section 4.3.  Initial values should be
   those defined in Section 3.  New TSIG error codes for the TSIG error
   field are assigned using the IETF Consensus policy defined in

11.  Security Considerations

   The approach specified here is computationally much less expensive
   than the signatures specified in DNSSEC.  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.

   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.  Secrets should never be shared by more than two

   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 DNSSEC security checks, then only security checked data will
   be available to the client.

   A fudge value that is too large may leave the server open to replay
   attacks.  A fudge value that is too small may cause failures if
   machines are not time synchronized or there are unexpected network
   delays.  The recommended value in most situation is 300 seconds.

   For all of the message authentication code algorithms listed in this
   document, those producing longer values are believed to be stronger;
   however, while there have been some arguments that mild truncation
   can strengthen a MAC by reducing the information available to an
   attacker, excessive truncation clearly weakens authentication by
   reducing the number of bits an attacker has to try to break the
   authentication by brute force [RFC2104].

   Significant progress has been made recently in cryptanalysis of hash
   functions of the types used here, all of which ultimately derive from
   the design of MD4.  While the results so far should not effect HMAC,
   the stronger SHA-1 and SHA-256 algorithms are being made mandatory
   due to caution.  Note that today SHA-3 [FIPS202] is available as an
   alternative to SHA-2 using a very different design.

   See also the Security Considerations section of [RFC2104] from which
   the limits on truncation in this RFC were taken.

11.1.  Issue fixed in this document

   To bind an answer with

   When signing a DNS reply message using TSIG, its corresponding request MAC computation uses
   the request message's MAC of as an input to cryptographically relate the
   answer is computed using
   reply to the MAC request.  Unfortunately  Unfortunately, the original
   specifications TSIG specification
   [RFC2845] failed to clearly require the MAC request MAC to be
   successfully validated. validated before using it.

   This document proposes the principle that the MAC must be considered
   to be invalid until it was validated.  This leads to the requirement
   that only a validated request MAC is included in a signed answer.  Or
   with other words when the request MAC was not validated the answer
   must be unsigned with a BADKEY or BADSIG TSIG error.

11.2.  Why not DNSSEC?

   This section from the original document [RFC2845] analyzes DNSSEC in
   order to justify the introduction of TSIG.

   DNS has recently been extended by DNSSEC ([RFC4033], [RFC4034] and
   [RFC4035]) 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.

   One difficulty with the DNSSEC 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 DNSSEC 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.
   DNSSEC 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.

   A second area where use of straight DNSSEC public key based
   mechanisms may be impractical is authenticating dynamic update
   [RFC2136] requests.  DNSSEC provides for request signatures but with
   DNSSEC they, like transaction signatures, require computationally
   expensive public key cryptography and complex authentication logic.
   Secure Domain Name System Dynamic Update ([RFC3007]) describes how
   different keys are used in dynamically updated zones.

12.  References

12.1.  Normative References

              National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", FIPS PUB 180-4, August 2015.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2845]  Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
              Wellington, "Secret Key Transaction Authentication for DNS
              (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000,

   [RFC4635]  Eastlake 3rd, D., "HMAC SHA (Hashed Message Authentication
              Code, Secure Hash Algorithm) TSIG Algorithm Identifiers",
              RFC 4635, DOI 10.17487/RFC4635, August 2006,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

12.2.  Informative References

   [FIPS202]  National Institute of Standards and Technology, "SHA-3
              Standard", FIPS PUB 202, August 2015.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,

   [RFC2930]  Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY
              RR)", RFC 2930, DOI 10.17487/RFC2930, September 2000,

   [RFC2931]  Eastlake 3rd, D., "DNS Request and Transaction Signatures
              ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September
              2000, <>.

   [RFC3007]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
              Update", RFC 3007, DOI 10.17487/RFC3007, November 2000,

   [RFC3174]  Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,

   [RFC3645]  Kwan, S., Garg, P., Gilroy, J., Esibov, L., Westhead, J.,
              and R. Hall, "Generic Security Service Algorithm for
              Secret Key Transaction Authentication for DNS (GSS-TSIG)",
              RFC 3645, DOI 10.17487/RFC3645, October 2003,

   [RFC3874]  Housley, R., "A 224-bit One-way Hash Function: SHA-224",
              RFC 3874, DOI 10.17487/RFC3874, September 2004,

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

Appendix A.  Acknowledgments

   This document just consolidates and updates the earlier documents by
   the authors of [RFC2845] (Paul Vixie, Olafur Gudmundsson, Donald E.
   Eastlake 3rd and Brian Wellington) and [RFC4635] (Donald E.  Eastlake
   3rd).  It would not be possible without their original work.

   The security problem addressed by this document was reported by
   Clement Berthaux from Synacktiv.

   Note for the RFC Editor (to be removed before publication): the first
   'e' in Clement is a fact a small 'e' with acute, unicode code U+00E9.
   I do not know if xml2rfc supports non ASCII characters so I prefer to
   not experiment with it.  BTW I am French too too so I can help if you
   have questions like correct spelling...

   Peter van Dijk, Benno Overeinder, Willem Toroop, Ondrej Sury, Mukund
   Sivaraman and Ralph Dolmans participated in the discussions that
   prompted this document.

Appendix B.  Change History


      [RFC4635] was merged.

      Authors of original documents were moved to Acknowledgments
      (Appendix A).

      Section 2 was updated to [RFC8174] style.

      Spit references into normative and informative references and
      updated them.

      Added a text explaining why this document was written in the
      Abstract and at the beginning of the introduction.

      Clarified the layout of TSIG RDATA.

      Moved the text about using DNSSEC from the Introduction to the end
      of Security Considerations.

      Added the security clarifications:

      1.   Emphasized that MAC is invalid until it is successfully

      2.   Added requirement that a request MAC that has not been
           successfully validated MUST NOT be included into a response.

      3.   Added requirement that a request that has not been validated
           to the MUST NOT generate a signed response.

      4.   Added note about MAC too short for the local policy to the
           Section 6.3.

      5.   Changed the order of server checks and swapped corresponding

      6.   Removed the truncation size limit "also case" as it does not
           apply and added confusion.

      7.   Relocated the error provision for TSIG truncation to the new
           Section 6.5.5.  Moved from RCODE 22 to RCODE 9 and TSIG ERROR
           22, i.e., aligned with other TSIG error cases.

      8.   Added Section 6.6.4 about truncation error handling by

      9.   Removed the limit to HMAC output in replies as a request
           which specified a MAC length longer than the HMAC output is
           invalid according the the first processing rule in
           Section 6.5.2.

      10.  Promoted the requirement that a secret length should be at
           least as long as the keyed message digest HMAC output to a SHOULD [RFC2119] key

      11.  Added a short text to explain the security issue.


      Improved wording (post-publication comments).

      Specialized and renamed the "TSIG on TCP connection" (Section 6.4)
      to "TSIG on zone tranfer over a TCP connection".  Added a SHOULD
      for a TSIG in each message (was envelope) for new implementations.

Authors' Addresses

   Francis Dupont (editor)
   Internet Software Consortium
   950 Charter Street
   Redwood City, CA  94063
   United States


   Stephen Morris
   Internet Software Consortium
   950 Charter Street
   Redwood City, CA  94063
   United States