Glue In DNS Referral Responses Is Not Optional

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Table of Contents

1. Introduction

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].¶

2. Glue record example

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

   % 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 ...

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 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."¶

4. Sibling Glue

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.¶

      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.

   ;; 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

5. Promoted (or orphaned) glue

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.¶

6. Security Considerations

This document clarifies correct DNS server behaviour and does not introduce any changes or new security considerations.¶

7. IANA Considerations

There are no actions for IANA.¶

Authors' Addresses