Internet Engineering Task Force (IETF)                         F. Dupont
Request for Comments: 8945                                           ISC
STD: 93                                                        S. Morris
Obsoletes: 2845, 4635                                       Unaffiliated
Category: Standards Track                                       P. Vixie
ISSN: 2070-1721                                                 Farsight
                                                         D. Eastlake 3rd
                                                               Futurewei
                                                          O. Gudmundsson
                                                              Cloudflare
                                                           B. Wellington
                                                                  Akamai
                                                           November 2020


          Secret Key Transaction Authentication for DNS (TSIG)

Abstract

   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 RFCs 2845 and 4635.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8945.

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   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Contributions published or made publicly available before November
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   Without obtaining an adequate license from the person(s) controlling
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   than English.

Table of Contents

   1.  Introduction
     1.1.  Background
     1.2.  Protocol Overview
     1.3.  Document History
   2.  Key Words
   3.  Assigned Numbers
   4.  TSIG RR Format
     4.1.  TSIG RR Type
     4.2.  TSIG Record Format
     4.3.  MAC Computation
       4.3.1.  Request MAC
       4.3.2.  DNS Message
       4.3.3.  TSIG Variables
   5.  Protocol Details
     5.1.  Generation of TSIG on Requests
     5.2.  Server Processing of Request
       5.2.1.  Key Check and Error Handling
       5.2.2.  MAC Check and Error Handling
       5.2.3.  Time Check and Error Handling
       5.2.4.  Truncation Check and Error Handling
     5.3.  Generation of TSIG on Answers
       5.3.1.  TSIG on TCP Connections
       5.3.2.  Generation of TSIG on Error Returns
     5.4.  Client Processing of Answer
       5.4.1.  Key Error Handling
       5.4.2.  MAC Error Handling
       5.4.3.  Time Error Handling
       5.4.4.  Truncation Error Handling
     5.5.  Special Considerations for Forwarding Servers
   6.  Algorithms and Identifiers
   7.  TSIG Truncation Policy
   8.  Shared Secrets
   9.  IANA Considerations
   10. Security Considerations
     10.1.  Issue Fixed in This Document
     10.2.  Why Not DNSSEC?
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Acknowledgements
   Authors' Addresses

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 records.

   The authentication mechanism proposed here provides a simple and
   efficient authentication between clients and servers, by using shared
   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 that 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 (MACs).

   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
   document.

   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
   termination.

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

2.  Key Words

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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 Resource Record (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 Section 2.3 of [RFC6895] 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.

4.2.  TSIG Record Format

   The fields of the TSIG RR are described below.  All multi-octet
   integers in the record are sent in network byte order (see
   Section 2.3.2 of [RFC1035]).

   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 A.site.example and B.example.net
      share a key, possibilities for the key name include
      <id>.A.site.example, <id>.B.example.net, and
      <id>.A.site.example.B.example.net.  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 [RFC7696].

      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 incorporate the names of participating agents or
      resources as suggested above.

   TYPE:  This MUST be TSIG (250: Transaction SIGnature).

   CLASS:  This MUST be ANY.

   TTL:  This MUST be 0.

   RDLENGTH:  (variable)

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

                            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:
      an octet sequence identifying the TSIG algorithm in the domain
      name syntax.  (Allowed names are listed in Table 3.)  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 field.

   MAC Size:
      an unsigned 16-bit integer giving the length of the 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 the 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 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 interfield
   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 Size | Unsigned 16-bit integer | in network byte order  |
      +----------+-------------------------+------------------------+
      | MAC Data | octet sequence          | exactly as transmitted |
      +----------+-------------------------+------------------------+

                           Table 1: Request's MAC

   Special considerations apply to the TSIG calculation for the second
   and subsequent messages in a response that consists of multiple DNS
   messages (e.g., a zone transfer).  These are described in
   Section 5.3.1.

4.3.2.  DNS Message

   In the MAC computation, the whole/complete DNS message in wire format
   is used.

   When creating an outgoing message, the TSIG is based on the message
   content before the TSIG RR has been added to the additional section
   and before the DNS Message Header's ARCOUNT has been incremented to
   include the TSIG RR.

   When verifying an incoming message, the TSIG is checked against the
   message after the TSIG RR has been removed, the ARCOUNT decremented,
   and the message ID replaced by the original message ID from the TSIG
   if those IDs differ.  (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          | MUST be ANY                        |
   +------------+----------------+------------------------------------+
   | TSIG RR    | TTL            | MUST be 0                          |
   +------------+----------------+------------------------------------+
   | 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 2: TSIG Variables

   The RR RDLENGTH and RDATA MAC Size 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
   hashed.

   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 Section 6.2 of [RFC4034].  The use
   of label types other than 00 is not defined for this specification.

4.3.3.1.  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.  The two fields are
   collectively named "TSIG Timers", and for the purpose of MAC
   calculation, they are hashed in their 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 (Hashed Message Authentication Code (HMAC))
   computation, appends a TSIG record with the calculated MAC to the
   additional 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 the 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:

   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.

5.2.2.1.  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:

   If the MAC Size field is greater than the 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.

   If the MAC Size field equals the keyed hash output length:  The
      entire keyed hash output is present and used.

   If the 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.

   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 the Time Signed value plus/minus the Fudge value),
   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 value
   plus the Fudge value has passed.  If implementations do retry
   requests in these cases, a limit SHOULD be placed on the maximum
   number of retries.

5.2.4.  Truncation Check and Error Handling

   If a TSIG is received with truncation that is permitted per
   Section 5.2.2.1 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
   cases.

   It also MUST 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 contains only the question and a TSIG
   record, has the TC bit set, and has an RCODE of 0 (NOERROR).  At this
   point, the client SHOULD retry the request using TCP (as per
   Section 4.2.2 of [RFC1035]).

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 an attack and provide data integrity.  The TSIG MUST be included
   on all DNS messages in the response.  For backward compatibility, a
   client that 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.

   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 was created; this means that the
   successive TSIG records in the stream will have non-decreasing Time
   Signed values.  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.

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
   discarded.

   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 that 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 is
   different from that used on the request, then this is a key-related
   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-related error, and clients 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-related 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.

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-related 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, and 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] and
   [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 5.2.2.1.  Key
   agreement can be by the TKEY mechanism [RFC2930] or some other
   mutually agreeable method.

   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
   implemented.

      +==========================+================+=================+
      | Algorithm 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 3: Algorithms for Implementations Supporting TSIG

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, 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 5.2.2.1, 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.

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 so 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 are 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 [IANA-TSIG].  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 the allowed
   algorithms, and the registry has been updated with the names listed
   in Table 3.

   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) (see [IANA-RCODEs],
   including "TSIG Error values" to be used for "Error" values, as
   defined in Section 4.2.  This document defines the RCODEs as
   described in Section 3.  New error codes are assigned and specified
   as in [RFC6895].

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 name
   servers.

   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 an 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].

   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
   reply.

   This document makes it 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 values.

10.2.  Why Not DNSSEC?

   DNS has 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.

11.  References

11.1.  Normative References

   [FIPS180-4]
              National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", FIPS PUB 180-4,
              DOI 10.6028/NIST.FIPS.180-4, August 2015,
              <https://doi.org/10.6028/NIST.FIPS.180-4>.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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,
              <https://www.rfc-editor.org/info/rfc2845>.

   [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
              (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
              2003, <https://www.rfc-editor.org/info/rfc3597>.

   [RFC4635]  Eastlake 3rd, D., "HMAC SHA (Hashed Message Authentication
              Code, Secure Hash Algorithm) TSIG Algorithm Identifiers",
              RFC 4635, DOI 10.17487/RFC4635, August 2006,
              <https://www.rfc-editor.org/info/rfc4635>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

11.2.  Informative References

   [CVE-2017-11104]
              Common Vulnerabilities and Exposures, "CVE-2017-11104:
              Improper TSIG validity period check can allow TSIG
              forgery", June 2017, <https://cve.mitre.org/cgi-bin/
              cvename.cgi?name=CVE-2017-11104>.

   [CVE-2017-3142]
              Common Vulnerabilities and Exposures, "CVE-2017-3142: An
              error in TSIG authentication can permit unauthorized zone
              transfers", June 2017, <https://cve.mitre.org/cgi-bin/
              cvename.cgi?name=CVE-2017-3142>.

   [CVE-2017-3143]
              Common Vulnerabilities and Exposures, "CVE-2017-3143: An
              error in TSIG authentication can permit unauthorized
              dynamic updates", June 2017, <https://cve.mitre.org/cgi-
              bin/cvename.cgi?name=CVE-2017-3143>.

   [IANA-RCODEs]
              IANA, "DNS RCODEs",
              <https://www.iana.org/assignments/dns-parameters/>.

   [IANA-TSIG]
              IANA, "TSIG Algorithm Names",
              <https://www.iana.org/assignments/tsig-algorithm-names/>.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,
              <https://www.rfc-editor.org/info/rfc1321>.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [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,
              <https://www.rfc-editor.org/info/rfc2136>.

   [RFC2930]  Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY
              RR)", RFC 2930, DOI 10.17487/RFC2930, September 2000,
              <https://www.rfc-editor.org/info/rfc2930>.

   [RFC3007]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
              Update", RFC 3007, DOI 10.17487/RFC3007, November 2000,
              <https://www.rfc-editor.org/info/rfc3007>.

   [RFC3174]  Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,
              <https://www.rfc-editor.org/info/rfc3174>.

   [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,
              <https://www.rfc-editor.org/info/rfc3645>.

   [RFC3874]  Housley, R., "A 224-bit One-way Hash Function: SHA-224",
              RFC 3874, DOI 10.17487/RFC3874, September 2004,
              <https://www.rfc-editor.org/info/rfc3874>.

   [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,
              <https://www.rfc-editor.org/info/rfc4033>.

   [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,
              <https://www.rfc-editor.org/info/rfc4034>.

   [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,
              <https://www.rfc-editor.org/info/rfc4035>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.

   [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,
              <https://www.rfc-editor.org/info/rfc4868>.

   [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,
              <https://www.rfc-editor.org/info/rfc6151>.

   [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,
              <https://www.rfc-editor.org/info/rfc6234>.

   [RFC6895]  Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
              April 2013, <https://www.rfc-editor.org/info/rfc6895>.

   [RFC7696]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
              <https://www.rfc-editor.org/info/rfc7696>.

   [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,
              <https://www.rfc-editor.org/info/rfc8126>.

   [SHA1SHAMBLES]
              Leurent, G. and T. Peyrin, "SHA-1 is a Shambles", January
              2020, <https://eprint.iacr.org/2020/014.pdf>.

Acknowledgements

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

   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.

   Stephen Morris would like to thank Internet Systems Consortium for
   its support of his participation in the creation of this document.

Authors' Addresses

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

   Email: Francis.Dupont@fdupont.fr


   Stephen Morris
   Unaffiliated
   United Kingdom

   Email: sa.morris8@gmail.com


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

   Email: paul@redbarn.org


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

   Email: d3e3e3@gmail.com


   Olafur Gudmundsson
   Cloudflare
   United States of America

   Email: olafur+ietf@cloudflare.com


   Brian Wellington
   Akamai
   United States of America

   Email: bwelling@akamai.com