Glue In DNS Referral Responses Is Not Optional
draft-andrews-dnsop-glue-is-not-optional-00

Abstract

The DNS uses glue records to allow iterative clients to find the addresses of nameservers that live within the delegated zone. Glue records are expected to be returned as part of a referral and if they cannot be fitted into the UDP response, TC=1 MUST be set to inform the client that the response is incomplete and that TCP SHOULD be used to retrieve the full response.

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 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 October 16, 2020.

Copyright Notice

Copyright (c) 2020 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.

Table of Contents

• 1. Introduction
• 1.1. Reserved Words
• 2. Modifications to RFC 1034 to reinforce the requirement
• 3. Security Considerations
• 4. IANA Considerations
• 5. References
• 5.1. Normative References
• 5.2. Informative References
• Author's Address

1. Introduction

The DNS [RFC1034], [RFC1035] uses glue records to allow iterative clients to find the addresses of nameservers that live within the delegated zone. Glue records are expected to be returned as part of a referral and if they cannot be fitted into the UDP response, TC=1 MUST be set to inform the client that the response is incomplete and that TCP SHOULD be used to retrieve the full response.

% 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 ...

;; Query time: 226 msec
;; SERVER: 69.36.157.30#53(69.36.157.30)
;; WHEN: Wed Apr 15 13:34:43 AEST 2020
;; MSG SIZE  rcvd: 500

% 
          

While not common real life examples servers that fail to set TC=1 when glue records are available exist and they do cause resolution failures. The example below shows a case where none of the glue records fitted into the available space and TC=1 was not set in the response. While this example show 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.

This is almost certainly due a wide spread misbelief that all additional section records are optional. This has never been the case with respect to glue records and later protocol extension have added more cases where records in the additional section are not optional in the response. This includes TSIG [RFC2845], OPT [RFC6891], and SIG(0) [RFC2931].

Glue records are added to the parent zone as part of the delegation process. They are expected to be returned as part of a referral and if they can't fit in a UDP response TC=1 MUST be set to signal to the client to retry over TCP. This document reinforces that expectation.

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. Modifications to RFC 1034 to reinforce the requirement

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 glue RRs do not fit set TC=1 in the header. Go to step 4."

3. Security Considerations

This document reinforces DNS server behaviour expectations and does not introduce new security considerations.

4. IANA Considerations

There are no actions for IANA.

5. References

5.1. Normative References

5.2. Informative References

Author's Address