Handling Verification Failures of TSIG-Signed DNS Messages
draft-sury-dnsop-tsig-clarify-00

Domain Name System Operations                                    O. Sury
Internet-Draft                                                       ISC
Updates: RFC8945 (if approved)                                  J. Abley
Intended status: Standards Track                              Cloudflare
Expires: 27 December 2026                                   25 June 2026

       Handling Verification Failures of TSIG-Signed DNS Messages
                    draft-sury-dnsop-tsig-clarify-00

Abstract

   Transation Signatures (TSIG) provide a standard mechanism to sign DNS
   messages, so that the authenticity of messages can be verified by the
   system that receives them.  This document updates the required
   behaviour of a system that receives a signed message that fails
   verification.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   https://ableyjoe.github.io/draft-sury-dnsop-tsig-clarify/draft-sury-
   dnsop-tsig-clarify.html.  Status information for this document may be
   found at https://datatracker.ietf.org/doc/draft-sury-dnsop-tsig-
   clarify/.

   Discussion of this document takes place on the Domain Name System
   Operations Working Group mailing list (mailto:dnsop@ietf.org), which
   is archived at https://mailarchive.ietf.org/arch/browse/dnsop/.
   Subscribe at https://www.ietf.org/mailman/listinfo/dnsop/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ableyjoe/draft-sury-dnsop-tsig-clarify.

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
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Copyright Notice

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Processing of Signed Responses  . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   Transaction Signatures are specified in [RFC8945] and provides a
   mechanism for transaction-level authentication of DNS messages using
   shared secrets and one-way hash functions.  It can be used to
   authenticate messages exchanged between DNS clients and servers, e.g.
   in cases where message integrity and authorisation are important,
   such as DNS Update [RFC2136] and the DNS Zone Transfer Protocol
   [RFC5936].

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2.  Terminology

   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.

   This document assumes familiarity with terminology specific to the
   Domain Name System (DNS) as described in [RFC9499].

3.  Processing of Signed Responses

   The processing of a signed response by a DNS client is described in
   section 5.4 of [RFC8945], which includes the following in the case of
   signed messages that fail verification:

      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.

   There are basically two scenarios how such a message can be received
   by client.  Either the server is misconfigured or the client is under
   attack.

   If we assume that such a response has been sent by misconfigured
   server, e.g. server that doesn't have the correct TSIG keys or isn't
   configured to use TSIG keys to sign messages to this particular
   client then no amount of waiting will allow the DNS communication to
   recover as the server will never send a correct message back.

   If we assume that such a response is malicious, then the possible
   attacks can be categorised as on-path or off-path.

   In the case of an off-path attack, the specified behaviour is not
   ideal since it provides an increased window of opportunity for an
   attacker that has correctly guessed parameters such as source port
   and query id to continue sending responses with different signatures.
   In the case of an on-path attack, there is even less value in waiting
   for a valid response, since the attacker can be assumed to have full
   control over what messages are and are not allowed to reach the
   client, and once again waiting only provides the attacker with more
   opportunity to conduct their attack.

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   This document updates the specification in this case to clarify that
   clients that receive responses that fit the description quoted above
   SHOULD log an error and MUST treat the message as corrupt, MUST
   discard it immediately and MUST NOT continue to wait.

4.  Security Considerations

   This document updates [RFC8945] and provides new guidance in order to
   mitigate on-path and off-path attacks on signed DNS responses.

5.  IANA Considerations

   This document has no IANA actions.

6.  References

6.1.  Normative References

   [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/rfc/rfc2119>.

   [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/rfc/rfc8174>.

   [RFC8945]  Dupont, F., Morris, S., Vixie, P., Eastlake 3rd, D.,
              Gudmundsson, O., and B. Wellington, "Secret Key
              Transaction Authentication for DNS (TSIG)", STD 93,
              RFC 8945, DOI 10.17487/RFC8945, November 2020,
              <https://www.rfc-editor.org/rfc/rfc8945>.

   [RFC9499]  Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
              RFC 9499, DOI 10.17487/RFC9499, March 2024,
              <https://www.rfc-editor.org/rfc/rfc9499>.

6.2.  Informative References

   [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/rfc/rfc2136>.

   [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
              (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
              <https://www.rfc-editor.org/rfc/rfc5936>.

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Acknowledgments

   TODO acknowledge.

Authors' Addresses

   Ondrej Sury
   ISC
   Email: ondrej@isc.org

   Joe Abley
   Cloudflare
   Email: jabley@cloudflare.com

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