DNSOP                                                         M. Andrews
Internet-Draft                                                       ISC
Updates: 1034 (if approved)                                     S. Huque
Intended status: Standards Track                              Salesforce
Expires: 12 August 2022                                       P. Wouters
                                                                   Aiven
                                                              D. Wessels
                                                                Verisign
                                                         8 February 2022


                     DNS Referral Glue Requirements
                draft-ietf-dnsop-glue-is-not-optional-04

Abstract

   The DNS uses referral 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
   referral glue records in a referral response.  If message size
   constraints prevent the inclusion of all in-domain referral 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
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   This Internet-Draft will expire on 12 August 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 Referral Glue  . . . . . . . . . . . . . . . . . . .   3
     2.1.  In-Domain Referral Glue . . . . . . . . . . . . . . . . .   3
     2.2.  Sibling Referral Glue . . . . . . . . . . . . . . . . . .   4
     2.3.  Cyclic Sibling Referral Glue  . . . . . . . . . . . . . .   5
     2.4.  Missing Referral Glue . . . . . . . . . . . . . . . . . .   5
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  In-Domain Referral Glue . . . . . . . . . . . . . . . . .   6
     3.2.  Sibling Referral Glue . . . . . . . . . . . . . . . . . .   7
     3.3.  Updates to RFC 1034 . . . . . . . . . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Changes . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   9
   10. Informative References  . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The Domain Name System (DNS) [RFC1034], [RFC1035] uses referral glue
   records to allow iterative clients to find the addresses of
   nameservers that are contained within a delegated zone.  Referral
   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
   referral 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 referral 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, referral glue is the only type of glue
   defined for the DNS.  Referral glue records are always addresses (A
   or AAAA records) of a delegation's authoritative name servers.  New
   work underway in the IETF may lead to definitions for other types of
   glue data, with requirements that differ from referral glue.  This
   document only describes requirements for referral glue.  Unless
   stated otherwise, "glue" in the remainder of this document always
   means "referral glue."

   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 Referral 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 Referral 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:





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      ;; 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.  Sibling Referral Glue

   Sibling glue are glue records that are not contained in the delegated
   zone itself, but in another zone delegated from the same parent.  In
   many cases, these are not strictly required for resolution, since the
   resolver can make follow-on queries to the other 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 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 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





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2.3.  Cyclic Sibling Referral 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 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 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

   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 NS
   RRs in a cyclic dependency as above.

2.4.  Missing Referral 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.




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   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.

      % 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
   referral responses.

3.1.  In-Domain Referral 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, or MUST set TC=1 if constrained by message size.



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   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
   65,536 bytes.

3.2.  Sibling Referral Glue

   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 all in-domain glue records, not
   all sibling glue records 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
   only sibling glue when a name servers chooses to omit them 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 in-domain glue RRs 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.










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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.

6.  IANA Considerations

   There are no actions for IANA.

7.  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.

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.



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   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.

   *  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.

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/>.

   [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>.






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   [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

   Email: dwessels@verisign.com



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