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DNS Glue Requirements in Referral Responses
draft-ietf-dnsop-glue-is-not-optional-05

Document Type Active Internet-Draft (dnsop WG)
Authors Mark P. Andrews , Shumon Huque , Paul Wouters , Duane Wessels
Last updated 2022-07-30 (Latest revision 2022-04-22)
Replaces draft-andrews-dnsop-glue-is-not-optional
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draft-ietf-dnsop-glue-is-not-optional-05
DNSOP                                                         M. Andrews
Internet-Draft                                                       ISC
Updates: 1034 (if approved)                                     S. Huque
Intended status: Standards Track                              Salesforce
Expires: 24 October 2022                                      P. Wouters
                                                                   Aiven
                                                              D. Wessels
                                                                Verisign
                                                           22 April 2022

              DNS Glue Requirements in Referral Responses
                draft-ietf-dnsop-glue-is-not-optional-05

Abstract

   The DNS uses glue records to allow iterative clients to find the
   addresses of name servers that are contained within a delegated zone.
   Authoritative Servers are expected to return all available in-domain
   glue records in a referral response.  If message size constraints
   prevent the inclusion of all in-domain glue records, the server MUST
   set the TC flag to inform the client that the response is incomplete,
   and that the client SHOULD use another transport 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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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 24 October 2022.

Copyright Notice

   Copyright (c) 2022 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
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Reserved Words  . . . . . . . . . . . . . . . . . . . . .   3
   2.  Types of Glue in Referral Responses . . . . . . . . . . . . .   3
     2.1.  Glue for In-Domain Name Servers . . . . . . . . . . . . .   3
     2.2.  Glue for Sibling Domain Name Servers  . . . . . . . . . .   4
     2.3.  Glue for Cyclic Sibling Domain Name Servers . . . . . . .   5
     2.4.  Missing Glue  . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Glue for In-Domain Name Servers . . . . . . . . . . . . .   7
     3.2.  Glue for Sibling Domain Name Servers  . . . . . . . . . .   8
     3.3.  Updates to RFC 1034 . . . . . . . . . . . . . . . . . . .   8
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  Changes . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  10
   10. Informative References  . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   The Domain Name System (DNS) [RFC1034], [RFC1035] uses glue records
   to allow iterative clients to find the addresses of name servers 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 in-domain glue records in a referral
   response.  If message size constraints prevent the inclusion of all
   in-domain glue records over the chosen transport, the server MUST set
   the TC (Truncated) flag to inform the client that the response is
   incomplete, and that the client SHOULD use another transport retrieve
   the full response.  This document clarifies that expectation.

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   DNS responses sometimes contain optional data in the additional
   section.  In-domain 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].

   At the time of this writing, addresses (A or AAAA records) for a
   delegation's authoritative name servers are the only type of glue
   defined for the DNS.

   Note that this document only clarifies requirements of name server
   software implementations.  It does not introduce or change any
   requirements on data placed in DNS zones or registries.  In other
   words, this document only makes requirements on "available glue
   records" (i.e., those given in a zone), but does not make
   requirements regarding their presence in a zone.  If some glue
   records are absent from a given zone, an authoritative name server
   may be unable to return a useful referral response for the
   corresponding domain.  The IETF may want to consider a separate
   update to the requirements for including glue in zone data, beyond
   those given in [RFC1034] and [RFC1035].

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 in Referral Responses

   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.  Glue for In-Domain Name Servers

   The following is a simple example of glue records present in the
   delegating zone "test" for the child zone "foo.test".  The name
   servers 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

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   A referral response from "test" for "foo.test" with glue for in-
   domain name servers 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

2.2.  Glue for Sibling Domain Name Servers

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

   Here the delegating zone "test" contains two delegations for the
   child zones "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 glue for sibling
   domain name servers looks like this:

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      ;; 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.  Glue for Cyclic Sibling Domain Name Servers

   The use of sibling domain name servers 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 glue for the sibling domain in a referral response.

   Here the delegating zone "test" contains two delegations for the
   child zones "bar.test" and "foo.test", and each use name servers
   under the other:

      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 glue for sibling
   domain name servers 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

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   In late 2021 the authors analyzed zone file data available from
   ICANN's Centralized Zone Data Service [CZDS] and found 222 out of
   approximately 209,000,000 total delegations that had only sibling
   domain NS RRs in a cyclic dependency as above.

2.4.  Missing Glue

   An example of missing glue is included here, even though it can not
   be considered as 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 response, and the TC flag was not
   set.  While this example shows a referral with DNSSEC records
   [RFC4033], [RFC4034], [RFC4035], this behavior 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 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

   This section describes updated requirements for including glue in DNS
   referral responses.

3.1.  Glue for In-Domain Name Servers

   This document clarifies that when a name server generates a referral
   response, it MUST include all available glue records for in-domain
   name servers in the additional section, or MUST set TC=1 if
   constrained by message size.

   At the time of writing, most iterative clients send initial queries
   over UDP and retry over TCP upon receiving a response with the TC
   flag set.  UDP responses are generally limited to between 1232 and
   4096 bytes, due to values commonly used for the EDNS0 UDP Message
   Size field [RFC6891], [FLAGDAY2020].  TCP responses are limited to
   65,535 bytes.

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3.2.  Glue for Sibling Domain Name Servers

   This document clarifies that when a name server generates a referral
   response, it SHOULD include all available glue records in the
   additional section.  If, after adding glue for all in-domain name
   servers, the glue for all sibling domain name servers does not fit
   due to message size constraints, the name server is NOT REQUIRED to
   set TC=1.

   Note that users may experience resolution failures for domains with
   cyclically-dependent sibling name servers when the delegating name
   server chooses to omit the corresponding glue in a referral response.
   As described in Section 2.3, such domains are rare.

3.3.  Updates to RFC 1034

   Replace

   "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 NS 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 for in-domain name
   servers do not fit, set TC=1 in the header.  Go to step 4."

4.  Security Considerations

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

5.  Operational Considerations

   At the time of this writing, the behavior of most DNS server
   implementations is to set the TC flag only if none of the available
   glue records fit in a response over UDP transport.  The updated
   requirements in this document might lead to an increase in the
   fraction of UDP responses with the TC flag set, and consequently an
   increase in the number of queries to over TCP transport.

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6.  IANA Considerations

   There are no actions for IANA.

7.  Acknowledgements

   The authors wish to thank Joe Abley, David Blacka, Brian Dickson,
   Kazunori Fujiwara, Paul Hoffman, Geoff Huston, Jared Mauch, George
   Michaelson, Yasuhiro Orange Morishita, Benno Overeinder, John R
   Levine, Hugo Salgado, Shinta Sato, Puneet Sood, Petr Spacek, Ralf
   Weber, Tim Wicinski, Suzanne Woolf, and other members of the DNSOP
   working group for their input.

8.  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.

   *  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.

   From -03 to -04:

   *  Use "referral glue" on the assumption that other types of glue may
      be defined in the future.

   *  Added Operational Considerations section.

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   *  Note many current implementations set TC=1 only when no glue RRs
      fit.  New requirements may lead to more truncation and TCP.

   *  Sibling glue can be optional.  Only require TC=1 when all in-
      domain glue RRs don't fit.

   *  Avoid talking about requirements for UDP/TCP specifically, and
      talk more generically about message size constraints regardless of
      transport.

   From -04 to -05:

   *  Reverting the -04 change to use the phrase "referral glue".

   *  Rephrase "in-domain glue" as "glue for in-domain name servers".

   *  Rephrase "sibling glue" as "glue for sibling domain name servers".

   *  Expand paragraph noting this document does not make requirements
      about presence of glue in zones.

9.  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>.

10.  Informative References

   [CZDS]     ICANN, "Centralized Zone Data Service", January 2022,
              <https://czds.icann.org/>.

   [FLAGDAY2020]
              Various DNS software and service providers, "DNS Flag Day
              2020", October 2020, <https://dnsflagday.net/2020/>.

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   [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>.

   [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

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   Email: dwessels@verisign.com

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