Glue In DNS Referral Responses Is Not Optional
draft-ietf-dnsop-glue-is-not-optional-03

DNSOP                                                         M. Andrews
Internet-Draft                                                       ISC
Updates: 1034 (if approved)                                     S. Huque
Intended status: Standards Track                              Salesforce
Expires: 14 April 2022                                        P. Wouters
                                                                   Aiven
                                                              D. Wessels
                                                                Verisign
                                                         11 October 2021

             Glue In DNS Referral Responses Is Not Optional
                draft-ietf-dnsop-glue-is-not-optional-03

Abstract

   The DNS uses glue records to allow iterative clients to find the
   addresses of nameservers that are contained within a delegated zone.
   Authoritative Servers are expected to return all available glue
   records in referrals.  If message size constraints prevent the
   inclusion of all glue records in a UDP response, the server MUST set
   the TC flag to inform the client that the response is incomplete, and
   that the client SHOULD use TCP to retrieve the full response.  This
   document updates RFC 1034 to clarify correct server behavior.

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
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   This Internet-Draft will expire on 14 April 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Reserved Words  . . . . . . . . . . . . . . . . . . . . .   3
   2.  Types of Glue . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  In-Domain Glue  . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Sibling Glue  . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Sibling Cyclic Glue . . . . . . . . . . . . . . . . . . .   4
     2.4.  Missing glue  . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  In-Domain Glue  . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Sibling Glue  . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Updates to RFC 1034 . . . . . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Changes . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   8
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The Domain Name System (DNS) [RFC1034], [RFC1035] uses glue records
   to allow iterative clients to find the addresses of nameservers that
   are contained within a delegated zone.  Glue records are added to the
   parent zone as part of the delegation process and returned in
   referral responses, otherwise a resolver following the referral has
   no way of finding these addresses.  Authoritative servers are
   expected to return all available glue records in referrals.  If
   message size constraints prevent the inclusion of all glue records in
   a UDP response, the server MUST set the TC (Truncated) flag to inform
   the client that the response is incomplete, and that the client
   SHOULD use TCP to retrieve the full response.  This document
   clarifies that expectation.

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   DNS responses sometimes contain optional data in the additional
   section.  Glue records however are not optional.  Several other
   protocol extensions, when used, are also not optional.  This includes
   TSIG [RFC2845], OPT [RFC6891], and SIG(0) [RFC2931].

   Note that this document only clarifies requirements of name server
   software implementations.  It does not place any requirements on data
   placed in DNS zones or registries.

1.1.  Reserved Words

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  Types of Glue

   This section describes different types of glue that may be found in
   DNS referral responses.  Note that the type of glue depends on the
   QNAME.  A particular record can be in-domain glue for one response
   and sibling glue for another.

2.1.  In-Domain Glue

   The following is a simple example of glue records present in the
   delegating zone "test" for the child zone "foo.test".  The
   nameservers for foo.test (ns1.foo.test and ns2.foo.test) are both
   below the delegation point.  They are configured as glue records in
   the "test" zone:

      foo.test.                  86400   IN NS      ns1.foo.test.
      foo.test.                  86400   IN NS      ns2.foo.test.
      ns1.foo.test.              86400   IN A       192.0.2.1
      ns2.foo.test.              86400   IN AAAA    2001:db8::2:2

   A referral response from "test" for "foo.test" with in-domain glue
   looks like this:

      ;; QUESTION SECTION:
      ;www.foo.test.       IN      A

      ;; AUTHORITY SECTION:
      foo.test.               86400        IN      NS      ns1.foo.test.
      foo.test.               86400        IN      NS      ns2.foo.test.

      ;; ADDITIONAL SECTION:
      ns1.foo.test.           86400        IN      A       192.0.2.1
      ns2.foo.test.           86400        IN      AAAA    2001:db8::2:2

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2.2.  Sibling Glue

   Sibling glue are glue records that are not contained in the delegated
   zone itself, but in another delegated zone from the same parent.  In
   many cases, these are not strictly required for resolution, since the
   resolver can make follow-on queries to the same zone to resolve the
   nameserver addresses after following the referral to the sibling
   zone.  However, most nameserver implementations today provide them as
   an optimization to obviate the need for extra traffic from iterative
   resolvers.

   Here the delegating zone "test" contains 2 sub-delegations for the
   subzones "bar.test" and "foo.test":

      bar.test.                  86400   IN NS      ns1.bar.test.
      bar.test.                  86400   IN NS      ns2.bar.test.
      ns1.bar.test.              86400   IN A       192.0.2.1
      ns2.bar.test.              86400   IN AAAA    2001:db8::2:2

      foo.test.                  86400   IN NS      ns1.bar.test.
      foo.test.                  86400   IN NS      ns2.bar.test.

   A referral response from "test" for "foo.test" with sibling glue
   looks like this:

      ;; QUESTION SECTION:
      ;www.foo.test.       IN      A

      ;; AUTHORITY SECTION:
      foo.test.               86400        IN      NS      ns1.bar.test.
      foo.test.               86400        IN      NS      ns2.bar.test.

      ;; ADDITIONAL SECTION:
      ns1.bar.test.           86400        IN      A       192.0.2.1
      ns2.bar.test.           86400        IN      AAAA    2001:db8::2:2

2.3.  Sibling Cyclic Glue

   The use of sibling glue can introduce cyclic dependencies.  This
   happens when one domain specifies name servers from a sibling domain,
   and vice versa.  This type of cyclic dependency can only be broken
   when the delegating name server includes the sibling glue in a
   referral response.

   Here the delegating zone "test" contains 2 sub-delegations for the
   subzones "bar.test" and "foo.test", and each use name servers under
   the other:

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      bar.test.                  86400   IN NS      ns1.foo.test.
      bar.test.                  86400   IN NS      ns2.foo.test.
      ns1.bar.test.              86400   IN A       192.0.2.1
      ns2.bar.test.              86400   IN AAAA    2001:db8::2:2

      foo.test.                  86400   IN NS      ns1.bar.test.
      foo.test.                  86400   IN NS      ns2.bar.test.
      ns1.foo.test.              86400   IN A       192.0.2.3
      ns2.foo.test.              86400   IN AAAA    2001:db8::2:4

   A referral response from "test" for "bar.test" with sibling glue
   looks like this:

      ;; QUESTION SECTION:
      ;www.bar.test.       IN      A

      ;; AUTHORITY SECTION:
      bar.test.               86400        IN      NS      ns1.foo.test.
      bar.test.               86400        IN      NS      ns2.foo.test.

      ;; ADDITIONAL SECTION:
      ns1.foo.test.           86400        IN      A       192.0.2.3
      ns2.foo.test.           86400        IN      AAAA    2001:db8::2:4

2.4.  Missing glue

   An example of missing glue is included here, even though it is not
   really a type of glue.  While not common, real examples of responses
   that lack required glue, and with TC=0, have been shown to occur and
   cause resolution failures.

   The example below is based on a response observed in June 2020.  The
   names have been altered to fall under documentation domains.  It
   shows a case where none of the glue records present in the zone fit
   into the available space of the UDP respose, and TC=1 was not set.
   While this example shows a referral with DNSSEC records [RFC4033],
   [RFC4034], [RFC4035], this behaviour has been seen with plain DNS
   responses as well.  Some records have been truncated for display
   purposes.  Note that at the time of this writing, the servers
   originally responsible for this example have been updated and now
   correctly set the TC=1 flag.

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      % dig +norec +dnssec +bufsize=512 +ignore @ns.example.net \
             rh202ns2.355.foo.example

      ; <<>> DiG 9.15.4 <<>> +norec +dnssec +bufsize +ignore \
             @ns.example.net rh202ns2.355.foo.example
      ; (2 servers found)
      ;; global options: +cmd
      ;; Got answer:
      ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 8798
      ;; flags: qr; QUERY: 1, ANSWER: 0, AUTHORITY: 9, ADDITIONAL: 1

      ;; OPT PSEUDOSECTION:
      ; EDNS: version: 0, flags: do; udp: 4096
      ;; QUESTION SECTION:
      ;rh202ns2.355.foo.example.         IN A

      ;; AUTHORITY SECTION:
      foo.example.          86400   IN NS      rh120ns2.368.foo.example.
      foo.example.          86400   IN NS      rh202ns2.355.foo.example.
      foo.example.          86400   IN NS      rh120ns1.368.foo.example.
      foo.example.          86400   IN NS      rh202ns1.355.foo.example.
      foo.example.          3600    IN DS      51937 8 1 ...
      foo.example.          3600    IN DS      635 8 2 ...
      foo.example.          3600    IN DS      51937 8 2 ...
      foo.example.          3600    IN DS      635 8 1 ...
      foo.example.          3600    IN RRSIG   DS 8 2 3600 ...

3.  Requirements

3.1.  In-Domain Glue

   This document clarifies that when a name server generates a referral
   response, it MUST include all available in-domain glue records in the
   additional section.  If all in-domain glue records do not fit in a
   UDP response, the name server MUST set TC=1.

3.2.  Sibling Glue

   This document clarifies that when a name server generates a referral
   response, it MUST [SHOULD] include available sibling glue records in
   the additional section.  If all sibling glue records do not fit in a
   UDP response, the name server MUST [is NOT REQUIRED to] set TC=1.

3.3.  Updates to RFC 1034

   [this doesn't really account for SHOULD on sibling glue...]

   Replace

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   "Copy the NS RRs for the subzone into the authority section of the
   reply.  Put whatever addresses are available into the additional
   section, using glue RRs if the addresses are not available from
   authoritative data or the cache.  Go to step 4."

   with

   "Copy the NS RRs for the subzone into the authority section of the
   reply.  Put whatever addresses are available into the additional
   section, using glue RRs if the addresses are not available from
   authoritative data or the cache.  If all glue RRs do not fit, set
   TC=1 in the header.  Go to step 4."

4.  Security Considerations

   This document clarifies correct DNS server behaviour and does not
   introduce any changes or new security considerations.

5.  IANA Considerations

   There are no actions for IANA.

6.  Acknowledgements

   The authors wish to thank Joe Abley, Brian Dickson, Geoff Huston,
   Jared Mauch, George Michaelson, Benno Overeinder, John R Levine,
   Shinta Sato, Puneet Sood, Ralf Weber, Tim Wicinski, Suzanne Woolf,
   and other members of the DNSOP working group for their input.

7.  Changes

   RFC Editor: Please remove this section before publication.

   This section lists substantial changes to the document as it is being
   worked on.

   From -01 to -02:

   *  Clarified that "servers" means "authoritative servers".

   *  Clarified that "available glue" means "all available glue".

   *  Updated examples and placed before RFC 1034 update.

   From -02 to -03:

   *  Clarified scope to focus only on name server responses, and not
      zone/registry data.

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   *  Reorganized with section 2 as Types of Glue and section 3 as
      Requirements.

   *  Removed any discussion of promoted / orphan glue.

   *  Use appropriate documentation addresses and domain names.

   *  Added Sibling Cyclic Glue example.

8.  Normative References

   [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>.

9.  Informative References

   [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>.

   [RFC2931]  Eastlake 3rd, D., "DNS Request and Transaction Signatures
              ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September
              2000, <https://www.rfc-editor.org/info/rfc2931>.

   [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>.

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   [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>.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,
              <https://www.rfc-editor.org/info/rfc6891>.

Authors' Addresses

   M. Andrews
   ISC

   Email: marka@isc.org

   Shumon Huque
   Salesforce

   Email: shuque@gmail.com

   Paul Wouters
   Aiven

   Email: paul.wouters@aiven.io

   Duane Wessels
   Verisign

   Email: dwessels@verisign.com

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