Internet Engineering Task Force                                F. Dupont
Internet-Draft                                                 S. Morris
Obsoletes: 2845, 4635 (if approved)                                  ISC
Intended status: Standards Track                                P. Vixie
Expires: January 11, 2021                                       Farsight
                                                         D. Eastlake 3rd
                                                          O. Gudmundsson
                                                           B. Wellington
                                                           July 10, 2020

          Secret Key Transaction Authentication for DNS (TSIG)


   This document describes a protocol for transaction level
   authentication using shared secrets and one way hashing.  It can be
   used to authenticate dynamic updates to a DNS zone as coming from an
   approved client, or to authenticate responses as coming from an
   approved name server.

   No recommendation is 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.

   This document obsoletes RFC2845 and RFC4635.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   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."

   This Internet-Draft will expire on January 11, 2021.

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Copyright Notice

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Protocol Overview . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Document History  . . . . . . . . . . . . . . . . . . . .   4
   2.  Key Words . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Assigned Numbers  . . . . . . . . . . . . . . . . . . . . . .   5
   4.  TSIG RR Format  . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  TSIG RR Type  . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  TSIG Record Format  . . . . . . . . . . . . . . . . . . .   6
     4.3.  MAC Computation . . . . . . . . . . . . . . . . . . . . .   8
       4.3.1.  Request MAC . . . . . . . . . . . . . . . . . . . . .   8
       4.3.2.  DNS Message . . . . . . . . . . . . . . . . . . . . .   9
       4.3.3.  TSIG Variables  . . . . . . . . . . . . . . . . . . .   9
   5.  Protocol Details  . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Generation of TSIG on Requests  . . . . . . . . . . . . .  10
     5.2.  Server Processing of Request  . . . . . . . . . . . . . .  10
       5.2.1.  Key Check and Error Handling  . . . . . . . . . . . .  11
       5.2.2.  MAC Check and Error Handling  . . . . . . . . . . . .  11
       5.2.3.  Time Check and Error Handling . . . . . . . . . . . .  12

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       5.2.4.  Truncation Check and Error Handling . . . . . . . . .  13
     5.3.  Generation of TSIG on Answers . . . . . . . . . . . . . .  13
       5.3.1.  TSIG on TCP Connections . . . . . . . . . . . . . . .  13
       5.3.2.  Generation of TSIG on Error Returns . . . . . . . . .  14
     5.4.  Client Processing of Answer . . . . . . . . . . . . . . .  15
       5.4.1.  Key Error Handling  . . . . . . . . . . . . . . . . .  15
       5.4.2.  MAC Error Handling  . . . . . . . . . . . . . . . . .  15
       5.4.3.  Time Error Handling . . . . . . . . . . . . . . . . .  15
       5.4.4.  Truncation Error Handling . . . . . . . . . . . . . .  16
     5.5.  Special Considerations for Forwarding Servers . . . . . .  16
   6.  Algorithms and Identifiers  . . . . . . . . . . . . . . . . .  16
   7.  TSIG Truncation Policy  . . . . . . . . . . . . . . . . . . .  17
   8.  Shared Secrets  . . . . . . . . . . . . . . . . . . . . . . .  18
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  19
     10.1.  Issue Fixed in this Document . . . . . . . . . . . . . .  20
     10.2.  Why not DNSSEC?  . . . . . . . . . . . . . . . . . . . .  20
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     11.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  24
   Appendix B.  Change History (to be removed before publication)  .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28

1.  Introduction

1.1.  Background

   The Domain Name System (DNS, [RFC1034], [RFC1035]) is a replicated
   hierarchical distributed database system that provides information
   fundamental to Internet operations, such as name to address
   translation and mail handling information.

   This document specifies use of a message authentication code (MAC),
   generated using certain keyed hash functions, to provide an efficient
   means of point-to-point authentication and integrity checking for DNS
   transactions.  Such transactions include DNS update requests and
   responses for which this can provide a lightweight alternative to the
   secure DNS dynamic update 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 ([RFC4033],
   [RFC4034], [RFC4035]) does not protect glue records and unsigned

   The authentication mechanism proposed here provides a simple and
   efficient authentication between clients and servers, by using shared

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   secret keys to establish a trust relationship between two entities.
   Such keys must be protected in a manner similar to private keys, lest
   a third party masquerade as one of the intended parties (by forging
   the MAC).  The proposal is unsuitable for general server to server
   authentication and 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.

1.2.  Protocol Overview

   Secret Key Transaction Authentication makes use of signatures on
   messages sent between the parties involved (e.g. resolver and
   server).  These are known as "transaction signatures", or TSIG.  For
   historical reasons, in this document they are referred to as message
   authentication codes (MAC).

   Use of TSIG presumes prior agreement between the two parties involved
   (e.g., resolver and server) as to any algorithm and key to be used.
   The way that this agreement is reached is outside the scope of the

   A DNS message exchange involves the sending of a query and the
   receipt of one of more DNS messages in response.  For the query, the
   MAC is calculated based on the hash of the contents and the agreed
   TSIG key.  The MAC for the response is similar, but also includes the
   MAC of the query as part of the calculation.  Where a response
   comprises multiple packets, the calculation of the MAC associated
   with the second and subsequent packets includes in its inputs the MAC
   for the preceding packet.  In this way it is possible to detect any
   interruption in the packet sequence, although not its premature

   The MAC is contained in a TSIG resource record included in the
   Additional Section of the DNS message.

1.3.  Document History

   TSIG was originally specified by [RFC2845].  In 2017, two nameserver
   implementations strictly following that document (and the related
   [RFC4635]) were discovered to have security problems related to this
   feature ([CVE-2017-3142], [CVE-2017-3143], [CVE-2017-11104]).  The
   implementations were fixed but, to avoid similar problems in the
   future, the two documents were updated and merged, producing this
   revised specification for TSIG.

   While TSIG implemented according to this RFC provides for enhanced
   security, there are no changes in interoperability.  TSIG is on the

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   wire still the same mechanism described in [RFC2845]; only the
   checking semantics have been changed.  See Section 10.1 for further

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.  Assigned Numbers

   This document defines the following RR type and associated value:

         TSIG (250)

   In addition, the document also defines the following DNS RCODEs and
   associated names:

         16 (BADSIG)
         17 (BADKEY)
         18 (BADTIME)
         22 (BADTRUNC)

   (See [RFC6895] Section 2.3 concerning the assignment of the value 16
   to BADSIG.)

   These RCODES may appear within the "Error" field of a TSIG RR.

4.  TSIG RR Format

4.1.  TSIG RR Type

   To provide secret key authentication, we use an 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.

   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.

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4.2.  TSIG Record Format

   The fields of the TSIG RR are described below.  As is usual, all
   multi-octet integers in the record are sent in network byte order
   (see [RFC1035] 2.3.2).

   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.  For example, 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.  This allows for periodic key rotation as
         per best operational practices, as well as algorithm agility as
         indicated by [BCP201].

         The name may be used as a local index to the key involved but
         it is recommended that it be globally unique.  Where a key is
         just shared between two hosts, its name actually need only be
         meaningful to them but it is recommended that the key name be
         mnemonic and incorporates the names of participating agents or
         resources as suggested above.

   TYPE  This MUST be TSIG (250: Transaction SIGnature)


   TTL   This MUST be 0

   RdLen (variable)

   RDATA The RDATA for a TSIG RR consists of a number of fields,
         described below:

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                            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 contents of the RDATA fields are:

         *  Algorithm Name - a octet sequence identifying the TSIG
            algorithm name in the domain name syntax.  (Allowed names
            are listed in Table 2.)  The name is stored in the DNS name
            wire format as described in [RFC1034].  As per [RFC3597],
            this name MUST NOT be compressed.

         *  Time Signed - an unsigned 48-bit integer containing the time
            the message was signed as seconds since 00:00 on 1970-01-01
            UTC, ignoring leap seconds.

         *  Fudge - an unsigned 16-bit integer specifying the allowed
            time difference in seconds permitted in the Time Signed

         *  MAC Size - an unsigned 16-bit integer giving the length of
            MAC field in octets.  Truncation is indicated by a MAC size
            less than the size of the keyed hash produced by the
            algorithm specified by the Algorithm Name.

         *  MAC - a sequence of octets whose contents are defined by the
            TSIG algorithm used, possibly truncated as specified by MAC
            Size.  The length of this field is given by the Mac Size.
            Calculation of the MAC is detailed in Section 4.3.

         *  Original ID - An unsigned 16-bit integer holding the message
            ID of the original request message.  For a TSIG RR on a

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            request, it is set equal to the DNS message ID.  In a TSIG
            attached to a response - or in cases such as the forwarding
            of a dynamic update request - the field contains the ID of
            the original DNS request.

         *  Error - in responses, an unsigned 16-bit integer containing
            the extended RCODE covering TSIG processing.  In requests,
            this MUST be zero.

         *  Other Len - an unsigned 16-bit integer specifying the length
            of the "Other Data" field in octets.

         *  Other Data - additional data relevant to the TSIG record.
            In responses, this will be empty (i.e.  "Other Len" will be
            zero) unless the content of the Error field is BADTIME, in
            which case it will be a 48-bit unsigned integer containing
            the server's current time as the number of seconds since
            00:00 on 1970-01-01 UTC, ignoring leap seconds (see
            Section 5.2.3).  This document assigns no meaning to its
            contents in requests.

4.3.  MAC Computation

   When generating or verifying the contents of a TSIG record, the data
   listed in the rest of this section are passed, in the order listed
   below, as input to MAC computation.  The data are passed in network
   byte order or wire format, as appropriate, and are fed into the
   hashing function as a continuous octet sequence with no inter-field
   separator or padding.

4.3.1.  Request MAC

   Only included in the computation of a MAC for a response message (or
   the first message in a multi-message response), the validated request
   MAC MUST be included in the MAC computation.  If the request MAC
   failed to validate, an unsigned error message MUST be returned
   instead.  (Section 5.3.2).

   The request's MAC, comprising the following fields, is digested in
   wire format:

         Field      Type                    Description
         ---------- ----------------------- ----------------------
         MAC Length Unsigned 16-bit integer in network byte order
         MAC Data   octet sequence          exactly as transmitted

   Special considerations apply to the TSIG calculation for the second
   and subsequent messages a response that consists of multiple DNS

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   messages (e.g. a zone transfer).  These are described in
   Section 5.3.1.

4.3.2.  DNS Message

   The DNS message used in the MAC computation is a whole and complete
   DNS message in wire format.

   When creating a TSIG, it is the message 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.

   When verifying an incoming message, it is the message after the TSIG
   RR has been removed and the ARCOUNT field decremented.  If the
   message ID differs from the original message ID, the original message
   ID is substituted for the message ID.  (This could happen, for
   example, when forwarding a dynamic update request.)

4.3.3.  TSIG Variables

   Also included in the digest is certain information present in the
   TSIG RR.  Adding this data provides further protection against an
   attempt to interfere with the message.

    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

                                  Table 1

   The RR RDLEN and RDATA MAC Length are not included in the 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

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   label type 00, this is defined in [RFC4034] Section 6.2.  The use of
   label types other than 00 is not defined for this specification.  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 MAC calculation, they are
   hashed in their "on the wire" format, in the following order: first
   Time Signed, then Fudge.

5.  Protocol Details

5.1.  Generation of TSIG on Requests

   Once the outgoing record has been constructed, the client performs
   the keyed hash (HMAC) computation, appends a TSIG record with the
   calculated MAC to the Additional Data section (incrementing the
   ARCOUNT to reflect the additional RR), and transmits the request to
   the server.  This TSIG record MUST be the only TSIG RR in the message
   and MUST be last record in the Additional Data section.  The client
   MUST store the MAC and the key name from the request while awaiting
   an answer.

   The digest components for a request are:

      DNS Message (request)
      TSIG Variables (request)

5.2.  Server Processing of Request

   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 multiple TSIG records are detected or a TSIG record is
   present in any other position, the DNS message is dropped and a
   response with RCODE 1 (FORMERR) MUST be returned.  Upon receipt of a
   message with exactly one correctly placed TSIG RR, a copy of the TSIG
   RR is stored, and the TSIG RR is removed from the DNS Message, and
   decremented out of the DNS message header's ARCOUNT.

   If the TSIG RR cannot be interpreted, the server MUST regard the
   message as corrupt and return a FORMERR to the server.  Otherwise the
   server is REQUIRED to return a TSIG RR in the response.

   To validate the received TSIG RR, the server MUST perform the
   following checks in the following order:

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      1.  Check KEY
      2.  Check MAC
      3.  Check TIME values
      4.  Check Truncation policy

5.2.1.  Key Check and Error Handling

   If a non-forwarding server does not recognize the key or algorithm
   used by the client (or recognizes the algorithm but does not
   implement it), 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 5.3.2.  The server SHOULD log the
   error.  (Special considerations apply to forwarding servers, see
   Section 5.5.)

5.2.2.  MAC Check and Error Handling

   Using the information in the TSIG, the server MUST verify the MAC by
   doing its own calculation and comparing the result with the MAC
   received.  If the MAC fails to verify, the server MUST generate an
   error response as specified in Section 5.3.2 with RCODE 9 (NOTAUTH)
   and TSIG ERROR 16 (BADSIG).  This response MUST be unsigned as
   specified in Section 5.3.2.  The server SHOULD log the error.  MAC Truncation

   When space is at a premium and the strength of the full length of a
   MAC is not needed, it is reasonable to truncate the keyed hash 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 TLS.  However, while this option is kept for backwards
   compatibility, it may not provide a security level appropriate for
   all cases in the modern environment.  In these cases, it is
   preferable to use a hashing algorithm such as SHA-256-128, SHA-
   384-192 or SHA-512-256 [RFC4868].

   Processing of a truncated MAC follows these rules:

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

       The entire output keyed hash output is present and used.

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   3.  "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 5.3.2, 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.

   4.  Otherwise:

       This is sent when the signer has truncated the keyed hash 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 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.

5.2.3.  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 (an unsigned 48-bit integer)
   in the Other Data field, and 6 in the Other Len 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.  In addition,
   the Fudge field MUST be set to the fudge value received from the
   client.  The data signed is specified in Section 5.3.2.  The server
   SHOULD log the error.

   Caching the most recent Time Signed value and rejecting requests with
   an earlier one could lead to valid messages being rejected if transit
   through the network led to UDP packets arriving in a different order
   to the one in which they were sent.  Implementations should be aware
   of this possibility and be prepared to deal with it, e.g. by
   retransmitting the rejected request with a new TSIG once outstanding
   requests have completed or the time given by their Time Signed plus
   fudge value has passed.  If implementations do retry requests in
   these cases, a limit SHOULD be placed on the maximum number of

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5.2.4.  Truncation Check and Error Handling

   If a TSIG is received with truncation that is permitted under
   Section 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.

5.3.  Generation of 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 a request if either the KEY is invalid
   (e.g. key name or algorithm name are unknown), or the MAC fails
   validation: see Section 5.3.2 for details of responding in these

   It also MUST NOT not generate a signed response to an unsigned
   request, except in the case of a response to a client's unsigned TKEY
   request if the secret key is established on the server side after the
   server processed the client's request.  Signing responses to unsigned
   TKEY requests MUST be explicitly specified in the description of an
   individual secret key establishment algorithm [RFC3645].

   The digest components used to generate a TSIG on a response are:

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

   (This calculation is different for the second and subsequent message
   in a multi-message answer, see below.)

   If addition of the TSIG record will cause 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 an RCODE of 0 (NOERROR).  The
   client SHOULD at this point retry the request using TCP (as per
   [RFC1035] 4.2.2).

5.3.1.  TSIG on TCP Connections

   A DNS TCP session such as a zone transfer can include multiple DNS
   messages.  Using TSIG on such a connection can protect the connection
   from attack and provide data integrity.  The TSIG MUST be included on
   all DNS messages in the response.  For backward compatibility, a
   client which receives DNS messages and verifies TSIG MUST accept up
   to 99 intermediary messages without a TSIG and MUST verify that both
   the first and last message contain a TSIG.

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   The first message is processed as a standard answer (see Section 5.3)
   but subsequent messages have the following digest components:

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

   The "Prior MAC" is the MAC from the TSIG attached to the last message
   containing a TSIG.  "DNS Messages" comprises the concatenation (in
   message order) of all messages after the last message that included a
   TSIG and includes the current message.  "TSIG timers" comprises the
   "Time Signed" and "Fudge" fields (in that order) pertaining to the
   message for which the TSIG is being created: this means that the
   successive TSIG records in the stream will have non-decreasing "Time
   Signed" fields.  Note that only the timers are included in the second
   and subsequent messages, not all the TSIG variables.

   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).

5.3.2.  Generation of TSIG on Error Returns

   When a server detects an error relating to the key or MAC in the
   incoming request, the server SHOULD send back an unsigned error
   message (MAC size == 0 and empty MAC).  It MUST NOT send back a
   signed error message.

   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 MAC is not included in this 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 5.3.

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5.4.  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.  If
   not, the response is treated as having a format error and is

   If the TSIG RR is present, the client performs the same checks as
   described in Section 5.2.  If the TSIG RR is unsigned as specified in
   Section 5.3.2 or does not validate, the message MUST be discarded
   unless the RCODE is 9 (NOAUTH).  In this case, the client SHOULD
   attempt to verify the response as if it were a TSIG error, as
   described in the following subsections.

   Regardless of the RCODE, a message containing a TSIG RR that is
   unsigned as specified in Section 5.3.2 or which fails verification
   SHOULD NOT be considered an acceptable response as it may have been
   spoofed or manipulated.  Instead, the client SHOULD log an error and
   continue to wait for a signed response until the request times out.

5.4.1.  Key Error Handling

   If an RCODE on a response is 9 (NOTAUTH), but the response TSIG
   validates and the TSIG key is recognized by the client but different
   from that used on the request, then this is a Key Error.  The client
   MAY retry the request using the key specified by the server.
   However, this should never occur, as a server MUST NOT sign a
   response with a different key to that used to sign the request.

5.4.2.  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.

5.4.3.  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.

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5.4.4.  Truncation Error Handling

   If the response RCODE is 9 (NOTAUTH) and the TSIG ERROR is 22
   (BADTRUNC) then this is a Truncation error.  The client MAY retry
   with a 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 7).  Clients SHOULD log this event.

5.5.  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 its own, to the
   destination or the next forwarder.  If no transaction security is
   available to the destination and the message is a query then, if the
   corresponding response has the AD flag (see [RFC4035]) set, the
   forwarder MUST clear the AD flag before adding the TSIG to the
   response and returning the result to the system from which it
   received the query.

6.  Algorithms and Identifiers

   The only message digest algorithm specified in the first version of
   these specifications [RFC2845] was "HMAC-MD5" (see [RFC1321],
   [RFC2104]).  Although a review of its security some years ago
   [RFC6151] concluded that "it may not be urgent to remove HMAC-MD5
   from the existing protocols", with the availability of more secure
   alternatives the opportunity has been taken to make the
   implementation of this algorithm optional.

   [RFC4635] added mandatory support in TSIG for SHA-1 [FIPS180-4],
   [RFC3174].  SHA-1 collisions have been demonstrated [SHA1SHAMBLES] so
   the MD5 security considerations described in section 2 of [RFC6151]
   apply to SHA-1 in a similar manner.  Although support for hmac-sha1
   in TSIG is still mandatory for compatibility reasons, existing uses
   SHOULD be replaced with hmac-sha256 or other SHA-2 digest algorithms
   [FIPS180-4], [RFC3874], [RFC6234].

   Use of TSIG between two DNS 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 above.
   Key agreement can be by the TKEY mechanism [RFC2930] or some other
   mutually agreeable method.

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   Implementations that support TSIG MUST also implement HMAC SHA1 and
   HMAC SHA256 and MAY implement gss-tsig and the other algorithms
   listed below.  SHA-1 truncated to 96 bits (12 octets) SHOULD be

          Name                     Implementation Use
          ------------------------ -------------- ---------------
          HMAC-MD5.SIG-ALG.REG.INT MAY            MUST NOT
          gss-tsig                 MAY            MAY
          hmac-sha1                MUST           NOT RECOMMENDED
          hmac-sha224              MAY            MAY
          hmac-sha256              MUST           RECOMMENDED
          hmac-sha256-128          MAY            MAY
          hmac-sha384              MAY            MAY
          hmac-sha384-192          MAY            MAY
          hmac-sha512              MAY            MAY
          hmac-sha512-256          MAY            MAY

                                  Table 2

7.  TSIG Truncation Policy

   As noted above, two DNS agents (e.g., resolver and server) must
   mutually agree to use TSIG.  Implicit in such an "agreement" are
   criteria as to acceptable keys and algorithms and, with the
   extensions in this document, truncations.  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 keyed hash 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 keyed hash output it will be rejected by
   processing rules Section 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.

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8.  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 hash output [RFC2104].

9.  IANA Considerations

   IANA maintains a registry of algorithm names to be used as "Algorithm
   Names" as defined in Section 4.2.  Algorithm names are text strings
   encoded using the syntax of a domain name.  There is no structure to
   the names, and algorithm names are compared as if they were DNS
   names, i.e., comparison is case insensitive.  Previous specifications
   [RFC2845] and [RFC4635] defined values for the HMAC-MD5 and some
   HMAC-SHA algorithms.  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].  This
   document adds to allowed algorithms, and the registry should be
   updated with the names listed in Table 2.

   New algorithms are assigned using the IETF Review 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, single-component names.

   IANA maintains a registry of RCODES (error codes), including "TSIG
   Error values" to be used for "Error" values as defined in
   Section 4.2.  New error codes are assigned and specified as in

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10.  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
   between two DNS systems, e.g., for the last hop from a local name
   server to the user resolver, or between primary and secondary

   Recommendations for choosing and maintaining secret keys can be found
   in [RFC2104].  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 except that
   it mentions the possibilities of manual configuration and the use of
   TKEY [RFC2930].  Secrets SHOULD NOT be shared by more than two
   entities; any such additional sharing would allow any party knowing
   the key to impersonate any other such party to members of the group.

   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 situations is 300 seconds.

   To prevent cross-algorithm attacks, there SHOULD only be one
   algorithm associated with any given key name.

   In several cases where errors are detected, an unsigned error message
   must be returned.  This can allow for an attacker to spoof or
   manipulate these responses.  Section 5.4 recommends logging these as
   errors and continuing to wait for a signed response until the request
   times out.

   Although the strength of an algorithm determines its security, there
   have been some arguments that mild truncation can strengthen a MAC by
   reducing the information available to an attacker.  However,
   excessive truncation clearly weakens authentication by reducing the
   number of bits an attacker has to try to break the authentication by
   brute force [RFC2104].

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   Significant progress has been made recently in cryptanalysis of hash
   functions of the types used here.  While the results so far should
   not affect HMAC, the stronger SHA-256 algorithm is being made
   mandatory as a precaution.

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

10.1.  Issue Fixed in this Document

   When signing a DNS reply message using TSIG, the MAC computation uses
   the request message's MAC as an input to cryptographically relate the
   reply to the request.  The original TSIG specification [RFC2845]
   required that the TIME values be checked before the request's MAC.
   If the TIME was invalid, some implementations failed to carry out
   further checks and could use an invalid request MAC in the signed

   This document makes it a mandatory that the request MAC is considered
   to be invalid until it has been validated: until then, any answer
   must be unsigned.  For this reason, the request MAC is now checked
   before the TIME value.

10.2.  Why not DNSSEC?

   These extracts from the original document [RFC2845] (updated to
   reference current standards) analyze 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.

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      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.

11.  References

11.1.  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,

   [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
              (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
              2003, <>.

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

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

11.2.  Informative References

   [BCP201]   Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,

              Common Vulnerabilities and Exposures, "CVE-2017-11104:
              Improper TSIG validity period check can allow TSIG
              forgery", June 2017, <

              Common Vulnerabilities and Exposures, "CVE-2017-3142: An
              error in TSIG authentication can permit unauthorized zone
              transfers", June 2017, <

              Common Vulnerabilities and Exposures, "CVE-2017-3143: An
              error in TSIG authentication can permit unauthorized
              dynamic updates", June 2017, <

   [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,

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   [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,

   [RFC4868]  Kelly, S. and S. Frankel, "Using HMAC-SHA-256, HMAC-SHA-
              384, and HMAC-SHA-512 with IPsec", RFC 4868,
              DOI 10.17487/RFC4868, May 2007,

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, DOI 10.17487/RFC6151, March 2011,

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   [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,

   [RFC6895]  Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
              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,

              Leurent, G. and T. Peyrin, "SHA-1 is a Shambles", January
              2020, <>.

Appendix A.  Acknowledgments

   This document 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

   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 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.  Mukund Sivaraman, Martin Hoffman and Tony
   Finch made extremely helpful suggestions concerning the structure and
   wording of the updated document.

Appendix B.  Change History (to be removed before publication)

   RFC EDITOR: Please remove this appendix before publication.


      [RFC4635] was merged.

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      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
           MUST NOT generate a signed response.

      4.   Added note about MAC too short for the local policy to
           Section 5.3.2.

      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 5.2.4.  Moved from RCODE 22 to RCODE 9 and TSIG ERROR
           22, i.e., aligned with other TSIG error cases.

      8.   Added Section 5.4.4 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 to the first processing rule in

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      10.  Promoted the requirement that a secret length should be at
           least as long as the 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 5.3.1) to "TSIG on zone transfer over a TCP connection".
      Added a SHOULD for a TSIG in each message (was envelope) for new


      Adopted by the IETF DNSOP working group: title updated and version
      counter reset to 00.


      Relationship between protocol change and principle of assuming the
      request MAC is invalid until validated clarified.  (Jinmei Tatuya)

      Cross reference to considerations for forwarding servers added.
      (Bob Harold)

      Added text from [RFC3645] concerning the signing behavior if a
      secret key is added during a multi-message exchange.

      Added reference to [RFC6895].

      Many improvements in the wording.

      Added RFC 2845 authors as co-authors of this document.


      Added a recommendation to copy time fields in BADKEY errors.
      (Mark Andrews)


      Further changes as a result of comments by Mukund Sivaraman.

      Miscellaneous changes to wording.

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      Major restructuring as a result of comprehensive review by Martin
      Hoffman.  Amongst the more significant changes:

      *  More comprehensive introduction.

      *  Merged "Protocol Description" and "Protocol Details" sections.

      *  Reordered sections so as to follow message exchange through
         "client "sending", "server receipt", "server sending", "client

      *  Added miscellaneous clarifications.


      Make implementation of HMAC-MD5 optional.

      Require that the Fudge field in BADTIME response be equal to the
      Fudge field received from the client.

      Added comment concerning the handling of BADTIME messages due to
      out of order packet reception.


      Wording changes and minor corrections after feedback.


      Updated text about use of hmac-sha1 using suggestion from Tony

      Corrected name of review policy used for new algorithms.


      Addressed comments from IESG review.  These can be found at
      ballot.  Significant changes are:

      *  Added references to CVEs that initiated this draft.

      *  Added reference to paper describing SHA1 collisions.

      *  Modified some paragraphs to remove language that has not "aged

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      *  Mentioned that multiple keys allows for periodic key rotation.

      *  Noted that TSIG detects interruption of packet sequence but not
         premature termination.

      *  Added new algorithms to the algorithm list.

      *  Marked hmac-sha224 as NOT RECOMMENDED.

      *  Added recommendation that there should only be one algorithm
         for each key.

      *  Added some paragraphs to the security recommendations section.

      Other changes:

      *  Explicitly define contents Error field in requests.  State that
         "Other Data" currently has no meaning in requests.

      *  Reworked the section on client processing of response to remove

      *  Section on TSIG over TCP now mentions zone transfer as an
         example, rather than the entire section being about zone

      *  Note that quote from RFC2845 in "What is DNSSEC?" section has
         been edited to refer to the latest standards.


      Change use of hmac-224 from NOT RECOMMENDED to MAY.

Authors' Addresses

   Francis Dupont
   Internet Systems Consortium, Inc.
   PO Box 360
   Newmarket, NH  03857
   United States of America


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   Stephen Morris
   Internet Systems Consortium, Inc.
   PO Box 360
   Newmarket, NH  03857
   United States of America


   Paul Vixie
   Farsight Security Inc
   177 Bovet Road, Suite 180
   San Mateo, CA  94402
   United States of America


   Donald E. Eastlake 3rd
   Futurewei Technologies
   2386 Panoramic Circle
   Apopka, FL  32703
   United States of America


   Olafur Gudmundsson
   San Francisco, CA  94107
   United States of America


   Brian Wellington
   United States of America


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