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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  to this document.  Code Components extracted from this document must
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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: [email protected]


  Shumon Huque
  Salesforce
  Email: [email protected]


  Paul Wouters
  Aiven
  Email: [email protected]


  Duane Wessels
  Verisign
  Email: [email protected]

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