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IETF RFC 9471
Last modified on Saturday, September 30th, 2023
 Permanent link to RFC 9471
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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/RFC 9471.
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
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
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) [RFC 1034] [RFC 1035] 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 [RFC 8945], OPT [RFC 6891], and SIG(0) [RFC 2931].
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 [RFC 1034] and [RFC 1035].
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" [RFC 8499].
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 [RFC 2119] [RFC 8174] 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 [RFC 4033] [RFC 4034] [RFC 4035], 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 [RFC 6891] [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
[RFC 1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC 1034, November 1987,
<https://www.rfc-editor.org/info/RFC 1034>.
[RFC 1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC 1035,
November 1987, <https://www.rfc-editor.org/info/RFC 1035>.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
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/>.
[RFC 2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures
( SIG(0)s )", RFC 2931, DOI 10.17487/RFC 2931, September
2000, <https://www.rfc-editor.org/info/RFC 2931>.
[RFC 4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC 4033, March 2005,
<https://www.rfc-editor.org/info/RFC 4033>.
[RFC 4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC 4034, March 2005,
<https://www.rfc-editor.org/info/RFC 4034>.
[RFC 4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC 4035, March 2005,
<https://www.rfc-editor.org/info/RFC 4035>.
[RFC 6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC 6891, April 2013,
<https://www.rfc-editor.org/info/RFC 6891>.
[RFC 8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC 8499,
January 2019, <https://www.rfc-editor.org/info/RFC 8499>.
[RFC 8945] 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/RFC 8945, November 2020,
<https://www.rfc-editor.org/info/RFC 8945>.
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
RFC TOTAL SIZE: 19469 bytes
PUBLICATION DATE: Saturday, September 30th, 2023
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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