| Internet-Draft | Glue In DNS Referral Responses Is Not Op | July 2021 |
| Andrews, et al. | Expires 27 January 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.¶
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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].¶
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].¶
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 192.0.1.1
ns2.foo.test. 86400 IN A 192.0.1.2
¶
Referral responses from "test" for "foo.test" must include the glue records in the additional section (and set TC=1 if they do not fit):¶
;; 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 192.0.1.1 ns2.foo.test. 86400 IN A 192.0.1.2¶
While not common, real life examples of servers that fail to set TC=1 when glue records are available, exist and they do cause resolution failures.¶
The example below from June 2020 shows a case where none of the glue records, present in the zone, fitted into the available space and TC=1 was not set in the response. While this example shows an DNSSEC [RFC4033], [RFC4034], [RFC4035] referral response, this behaviour has also been seen with plain DNS responses as well. The records have been truncated for display purposes. Note that at the time of this writing, this configuration has been corrected and the response correctly sets the TC=1 flag.¶
% dig +norec +dnssec +bufsize=512 +ignore @a.gov-servers.net \
rh202ns2.355.dhhs.gov
; <<>> DiG 9.15.4 <<>> +norec +dnssec +bufsize +ignore \
@a.gov-servers.net rh202ns2.355.dhhs.gov
; (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.dhhs.gov. IN A
;; AUTHORITY SECTION:
dhhs.gov. 86400 IN NS rh120ns2.368.dhhs.gov.
dhhs.gov. 86400 IN NS rh202ns2.355.dhhs.gov.
dhhs.gov. 86400 IN NS rh120ns1.368.dhhs.gov.
dhhs.gov. 86400 IN NS rh202ns1.355.dhhs.gov.
dhhs.gov. 3600 IN DS 51937 8 1 ...
dhhs.gov. 3600 IN DS 635 8 2 ...
dhhs.gov. 3600 IN DS 51937 8 2 ...
dhhs.gov. 3600 IN DS 635 8 1 ...
dhhs.gov. 3600 IN RRSIG DS 8 2 3600 ...
¶
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."¶
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.¶
This document clarifies that sibling glue (being part of all available glue records) MUST be returned in referral responses, and that the requirement to set TC=1 applies to sibling glue that cannot fit in the response too.¶
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 192.0.1.1
ns2.bar.test. 86400 IN A 192.0.1.2
foo.test. 86400 IN NS ns1.bar.test.
foo.test. 86400 IN NS ns2.bar.test.
¶
Referral responses from "test" for "foo.test" must include the sibling glue (and set TC=1 if they do not fit):¶
;; 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 192.0.1.1 ns2.bar.test. 86400 IN A 192.0.1.2¶
When a zone is deleted but the parent notices that its NS glue records are required for other zones, it MAY opt to take these (now orphaned) glue records into its own zone to ensure that other zones depending on this glue are not broken. Technically, these address records are no longer glue records, but authoritative data of the parent zone, and should be added to the DNS response similarly to regular glue records.¶
This document clarifies correct DNS server behaviour and does not introduce any changes or new security considerations.¶
There are no actions for IANA.¶