| Internet-Draft | Glue In DNS Referral Responses Is Not Op | October 2021 |
| Andrews, et al. | Expires 14 April 2022 | [Page] |
The DNS uses 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 glue records in referrals. If message size constraints prevent the inclusion of all glue records in a UDP response, the server MUST set the TC flag to inform the client that the response is incomplete, and that the client SHOULD use TCP to retrieve the full response. This document updates RFC 1034 to clarify correct server behavior.¶
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 14 April 2022.¶
Copyright (c) 2021 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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶
The Domain Name System (DNS) [RFC1034], [RFC1035] uses glue records to allow iterative clients to find the addresses of nameservers 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 in referrals. If message size constraints prevent the inclusion of all glue records in a UDP response, the server MUST set the TC (Truncated) flag to inform the client that the response is incomplete, and that the client SHOULD use TCP to retrieve the full response. This document clarifies that expectation.¶
DNS responses sometimes contain optional data in the additional section. 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].¶
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.¶
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].¶
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.¶
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:¶
;; 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¶
Sibling glue are glue records that are not contained in the delegated zone itself, but in another delegated zone from the same parent. In many cases, these are not strictly required for resolution, since the resolver can make follow-on queries to the same 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 2 sub-delegations for the subzones "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¶
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 2 sub-delegations for the subzones "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¶
An example of missing glue is included here, even though it is not really 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 respose, and TC=1 was not set. While this example shows a referral with DNSSEC records [RFC4033], [RFC4034], [RFC4035], this behaviour 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=1 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 ...
¶
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. If all in-domain glue records do not fit in a UDP response, the name server MUST set TC=1.¶
This document clarifies that when a name server generates a referral response, it MUST [SHOULD] include available sibling glue records in the additional section. If all sibling glue records do not fit in a UDP response, the name server MUST [is NOT REQUIRED to] set TC=1.¶
[this doesn't really account for SHOULD on sibling glue...]¶
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 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 do not fit, set TC=1 in the header. Go to step 4."¶
This document clarifies correct DNS server behaviour and does not introduce any changes or new security considerations.¶
There are no actions for IANA.¶
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.¶
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:¶
From -02 to -03:¶