



Internet Engineering Task Force (IETF)                        M. Andrews
Request for Comments: 9471                                           ISC
Updates: 1034                                                   S. Huque
Category: Standards Track                                     Salesforce
ISSN: 2070-1721                                               P. Wouters
                                                                   Aiven
                                                              D. Wessels
                                                                Verisign
                                                          September 2023


              DNS Glue Requirements in Referral Responses

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 glue
   records for in-domain name servers in a referral response.  If
   message size constraints prevent the inclusion of all glue records
   for in-domain name servers, 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 to retrieve the full
   response.  This document updates RFC 1034 to clarify correct server
   behavior.

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/rfc9471.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language
   2.  Types of Glue in Referral Responses
     2.1.  Glue for In-Domain Name Servers
     2.2.  Glue for Sibling Domain Name Servers
     2.3.  Glue for Cyclic Sibling Domain Name Servers
     2.4.  Missing Glue
   3.  Requirements
     3.1.  Glue for In-Domain Name Servers
     3.2.  Glue for Sibling Domain Name Servers
     3.3.  Update to RFC 1034
   4.  Security Considerations
   5.  Operational Considerations
   6.  IANA Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgements
   Authors' Addresses

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 glue records for in-domain name
   servers in a referral response.  If message size constraints prevent
   the inclusion of all glue records for in-domain name servers 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 to retrieve the full response.  This
   document clarifies that expectation.

   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 [RFC8945], 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 for name server
   software implementations.  It does not introduce or change any
   requirements regarding data placed in DNS zones or registries.  In
   other words, this document only makes requirements regarding
   "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].

   This document assumes a reasonable level of familiarity with DNS
   operations and protocol terms.  Much of the terminology is explained
   in further detail in "DNS Terminology" [RFC8499].

1.1.  Requirements Language

   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.

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 name server (and its corresponding glue record)
   can be in-domain for one response and in a sibling domain 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

   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 rather are contained in another zone
   delegated from the same parent.  In many cases, glue for sibling
   domain name servers is 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:

      ;; 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 uses 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

   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 Resource Records (RRs) in a cyclic dependency as above.

2.4.  Missing Glue

   An example of missing glue is included here, even though it cannot 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, from the dig command [DIG], 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.

      % 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 this 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.

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 MAY set TC=1 but is
   not obligated to do so.

   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.  Update to RFC 1034

   OLD:

   |  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.

   NEW:

   |  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 received over TCP transport.

6.  IANA Considerations

   This document has no IANA actions.

7.  References

7.1.  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>.

   [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>.

7.2.  Informative References

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

   [DIG]      Wikipedia, "dig (command)", September 2023,
              <https://en.wikipedia.org/wiki/Dig_(command)>.

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

   [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>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

   [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/info/rfc8945>.

Acknowledgements

   The authors wish to thank Joe Abley, David Blacka, Brian Dickson,
   Kazunori Fujiwara, Paul Hoffman, Geoff Huston, John R. Levine, Jared
   Mauch, George Michaelson, Yasuhiro Orange Morishita, Benno
   Overeinder, 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.

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
