Network Working Group                                            P. Koch
Internet-Draft                                                  DENIC eG
Expires: January 13, 2011                                  July 12, 2010

            DNS Glue RR Survey and Terminology Clarification


   This document presents a survey of the use of the term "glue record"
   in DNS related RFCs and proposes a terminology for the various glue
   policies seen in different top level domains.

Status of this Memo

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   This Internet-Draft will expire on January 13, 2011.

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   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
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Table of Contents

   1.          Introduction . . . . . . . . . . . . . . . . . . . . .  3
   2.          RFC Survey . . . . . . . . . . . . . . . . . . . . . .  3
   3.          Terms  . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.          Name Server Naming Strategies  . . . . . . . . . . . .  4
   5.          Glue Policies  . . . . . . . . . . . . . . . . . . . .  5
   6.          Open Issues  . . . . . . . . . . . . . . . . . . . . .  5
   6.1.        Root Server "Glue" in the Root Zone File . . . . . . .  6
   6.2.        Using Glue records in responses  . . . . . . . . . . .  6
   6.3.        Registry considerations for maintaining Glue RRs . . .  7
   6.4.        Glue RRs for multihomed name servers . . . . . . . . .  8
   6.5.        Grandchild Glue  . . . . . . . . . . . . . . . . . . .  8
   7.          DNSSEC Considerations  . . . . . . . . . . . . . . . .  9
   8.          IPv6 Considerations  . . . . . . . . . . . . . . . . .  9
   9.          Security Considerations  . . . . . . . . . . . . . . .  9
   10.         IANA Considerations  . . . . . . . . . . . . . . . . .  9
   11.         References . . . . . . . . . . . . . . . . . . . . . .  9
   Appendix A. Document Revision History  . . . . . . . . . . . . . . 12
   A.1.        Changes from -03 to -04  . . . . . . . . . . . . . . . 13
   A.2.        Changes from -02 to -03  . . . . . . . . . . . . . . . 13
   A.3.        Changes from -01 to -02  . . . . . . . . . . . . . . . 13
   A.4.        Changes from -00 to -01  . . . . . . . . . . . . . . . 13
               Author's Address . . . . . . . . . . . . . . . . . . . 13

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1.  Introduction

   When delegating zones from a top level domain (TLD) or other DNS
   zone, some additional information is needed when resolving a name
   server's (as per an NS RR) address would involve the particular name
   server itself.  Such a dependency on itself, direct or indirect, may
   effectively shadow a part of a zone's NS RRSet, reducing redundancy,
   or even render the zone completely unresolvable.  This additional
   information is an amendment to the delegation in the form of glue
   address records.  In the real life DNS multiple strategies to
   determine necessity or acceptance of glue records co-exist.  This
   document lists a subset of those approaches.

   This document also tries to clarify when and where to call an address
   record "glue record".

   Comments should be directed at the author.

   Domain names and IP addresses herein are for explanatory purposes
   only and should not be expected to lead to useful information in real
   life [RFC2606],[RFC5735]).

2.  RFC Survey

   To find out more about early motivations and strategies for DNS glue
   records, all existing RFCs were automatically searched for the term
   "glue" (case insensitive, no word boundaries) and those matching were
   inspected on a case by case basis.  Whenever the term was used in a
   DNS context, the RFC was added to the list which can be found in the
   References section.  It turned out that, while all early RFCs are
   consistent in using "glue" only for type A address records for NS RR
   targets, they apply slightly different logic as to when a glue A RR
   should be present.

3.  Terms

   When the term "glue record" was introduced in [RFC0973], it was meant
   to denominate both data origin and purpose.  Data origin is related
   to the zone, although the glue records do not belong to the
   authoritative zone data.  The purpose is constrained to providing
   address information for name servers mentioned in NS RRs, which would
   otherwise not be resolvable.  Glue records are address information
   accompanying a delegation (in the delegating zone).

   There is sometimes confusion when data in a DNS response is also
   called "glue" data, e.g.  [RFC2010] starts speaking of "fetching"

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   glue.  In a DNS response packet (answer or referral) the address
   information for name servers is carried in the additional section.
   This address information might have originated from glue records but
   might also come from cached or authoritative data.  DNS data in
   response packets should only be called "glue data" when it is certain
   and needs to be emphasized that it originates from glue records.

   [RFC4472] introduces the concept of 'critical' and 'courtesy'
   additional data.

4.  Name Server Naming Strategies

   The DNS refers to name servers by their name in NS resource records.
   The servers' names can have different relations to both the
   delegating zone and the zone delegated to them.  The following
   categories describe the naming scheme from the perspective of the
   delegated zone.  They will be illustrated with the help of the
   "example" TLD zone, which delegates the zone "del.example" to the
   respective sets of name servers.

   in domain  A name server can be within the delegated domain, which
      includes the name of the delegated domain itself, e.g.,
      "del.example", "dns.del.example", or "dns.sub.del.example".  In
      all but the first case the name might be part of a subzone of
      "del.example".  This naming scheme is sometimes also called "in-

   sibling domain  A name server's name can be within the delegating but
      outside the delegated domain, e.g., "dns.other.example".  There
      are two sub-cases here, depending on the treatment of the
      "other.example" domain:

      authoritative in sibling  When the name server's name is within
         the parent domain, but in a separate zone, this is a sibling
         zone of the delegated zone.

      authoritative in parent  A name server's name may also appear
         authoritative in the parent zone.

   unrelated  A name server's name may not share its parent with the
      delegated domain, e.g., "".

   The discussion of the advantages and disadvantages of the three
   categories or any combination is beyond the scope of this document.

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5.  Glue Policies

   In the DNS tree different policies are applied with respect to
   registering glue with the delegating zone.  "Registering" in this
   case means that the respective glue information is accepted,
   requested or required and then attached to the zone data so that it
   is available at all authoritative servers, i.e. the glue travels with
   the zone data by AXFR, IXFR or other means.  However, it does not
   make the glue data part of the zone's authoritative data.

   This is a list of existing glue policies:

   "never" or "null":  Glue RRs are never registered.  This currently
      applies to larger parts of the IN-ADDR.ARPA reverse tree.

   "narrow":  Glue RRs are registered if and only if the name server
      resides within or below the delegated (child) zone (that is,
      within the delegated domain).  This was suggested by [RFC1034].

   "wide":  Glue RRs are registered if and only if the name server
      resides below the delegating (parent) zone.  If the name server's
      name belongs to the parent zone, it is part of the authoritative
      data and thus there is no need to register glue RRs.  This was
      suggested by [RFC1033].  It is used for the root zone.

   "case by case":  Glue RRs are registered following the "narrow"
      policy except where there are (circular) dependencies that demand
      additional glue RRs.

   "mandatory":  Glue RRs are always registered for all name servers.
      This was suggested by [RFC0973].

   "other":  Combinations of the above may exist, e.g. if a registry
      runs multiple sibling domains and decides to register glue RRs
      whenever a name server resides in or below one of the siblings.
      This category would also include other policies like "random" or

   Glue RRs are needed only in the delegating zone, regardless of glue
   policy.  See Section 6.1 for a discussion of root zone issues.

   Various RFCs have identified extraneous glue RRs as sources of error
   and confusion ([RFC1713], [RFC1912]).

6.  Open Issues

   Future versions of this document will expand on these topics:

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   o  Software issues when following NS RRs

   o  Mixed IPv4 and IPv6 environments, following the example of

   o  TTL considerations: glue data vs. authoritative data as well as NS
      RRSet TTLs vs A RRSet TTLs

6.1.  Root Server "Glue" in the Root Zone File

   As said before, Glue is meant to be present in the delegating zone
   only.  The only exception seems to be root zone which also contains
   the address records for its authoritative name servers.  However,
   with the current setup the root servers also serve the ARPA domain
   and with the root zone's "wide" glue policy this means that there
   should be glue RRs for this particular set of nameservers, but only
   in their capacity as ARPA TLD servers.

   [The position of the A RRs in the root zone file (which has just
   editorial value) as well as their TTLs suggest that historically
   there will have been a different reason].

   [The upcoming change in the ARPA NS RRSet in the spirit of [RFC5855]
   renders this consideration void.]

   Also, per operational practice, all root servers are authoritative
   for the zone they reside in and that is today also reflected by the
   official delegation for the "" zone.  So, they have
   the authoritative data present and do not need to rely upon the data
   transported with the root zone.

   [To have a complete trust chain available at the root servers leading
   to their own names, it would be useful to have them configured
   authoritative for all intermediate zones.  It has been suggested
   before to move the root server's names to a distinct TLD.  Another
   option would be to move their names to e.g.  ROOT-SERVERS.ARPA

6.2.  Using Glue records in responses

   Some implementations use Glue information not only during additional
   section processing, but also in the answer section of responses.

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   Given an excerpt of the "example" TLD zone file,

     one.example.       NS
                        NS dns.two.example.   A

   what should a name server authoritative for the example TLD do when
   asked for the A RR for  Some implementations will
   put the A RR in the answer section of the response, others will
   respond with a referral and only copy the glue A RR into the
   additional section (the handling of dns.two.example's A RR is not
   considered here).

   Step 4 of the algorithm in 4.3.2 of [RFC1034] suggests that after
   copying the NS RRs into the authority section (in step 3b) the cache
   should be consulted and used to fill the answer section.  Depending
   on whether or not Glue data is considered to reside in the cache (it
   is definitely not authoritative), one or the other response type will
   be preferred.

   With DNSSEC an A RRSet response originating from glue data will
   always miss the appropriate signature, because neither does the
   delegating zone sign the glue RRSet nor does a glue RRSIG (child's
   signature covering the address RRSet) exist in that delegating zone.

   [discuss levels of indirection and operational reasons that lead to
   the "gluepot response"]

6.3.  Registry considerations for maintaining Glue RRs

   As explained in [RFC5936], Glue address records are "registered with"
   a zone, but since they fall underneath (or, sometimes, onto) the next
   zone cut, they are not part of that zone.  Depending on the data
   model and glue policy in use for a TLD ([RFC5731], [RFC5732]),
   different side effects may be the consequence of undelegating a
   domain.  The standard zone file format does not allow for the
   explicit dedication of address records as glue information.  Instead,
   the distinction is made based on the presence or absence of zone
   cuts.  If, for example, the "del.example" domain was delegated to,
   amongst others, the "dns.del.example" name server an address record
   for "dns.del.example" in the "example" TLD zone will be interpreted
   as glue record.  After deleting the "del.example" NS RRSet (the
   delegation) from the "example" zone, the corresponding address record
   would have to be deleted, as well.  Should it remain, it would be
   elevated to authoritative data, since there no longer is a zone cut.
   Such an effect might be highly undesirable and should be avoided.
   Custom or proprietary name server software may be able to keep
   delegation and glue data separate from the delegation data so that

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   "dns.del.example" would still exist but would not be elevated to
   authoritative status.  However, this effect is similarily undesirable
   since the address might be used to fill the additional section for
   referrals containing NS RRs pointing to "dns.del.example", as if
   "wide" glue policy was in effect.

   With the "wide" glue policy, a glue address record registered with
   some delegation might not even be related to the delegation of its
   own second level domain, i.e., the corresponding name server does not
   have to be part of the NS RRSet for that domain.  Therefore, broader
   checks have to be applied to avoid the aforementioned undesired

6.4.  Glue RRs for multihomed name servers

   Some name server names resolve to A or AAAA RRSets consisting of more
   than one record, i.e. they have multiple addresses.  It is
   recommended that these RRSets be consistent between the child and the

   Research is needed to evaluate the effective difference between
   multiple names and multiple addresses for a name server.  These
   effects heavily depend on server selection algorithms in resolvers.

6.5.  Grandchild Glue

   When a name server resides within the delegated domain, the
   delegation needs a glue record with both the "wide" and the "narrow"
   glue policy.  However, the server does not necessarily have its name
   within the delegated zone since it may belong to a child or
   grandchild zone of the delegated one.

   This is a delegation in the example TLD:

     one.example.       NS one.example.

   Only the first name server is known to have its name in the delegated
   zone, where the second and third could both be in separate zones.
   NB: even could be a zone delegated from one.example.

   As a consequence, it cannot be concluded that any such name server is
   able to authoritatively serve its own name, e.g., if it does not
   serve the grandchild zone.

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7.  DNSSEC Considerations

   DNSSEC signatures do not cover glue records [RFC3833], [RFC4033].

   Using the gluepot to fill the answer section is discouraged with
   DNSSEC, see Section 6.2.

8.  IPv6 Considerations

   While this document makes no explicit statements about AAAA RRs,
   similar logic applies except in cases where A and AAAA glue RR
   interaction requires specific consideration (response packet size,
   TTL consistency, namespace fragmentation).

   The specification of the A6 RR [RFC2874] contains, in section 5.1.2,
   a detailed discussion of glue issues due to the variable
   representation of IPv6 addresses in A6.

9.  Security Considerations

   This section needs more work

10.  IANA Considerations

   This section needs more work

11.  References

              Koch, P. and M. Larson, "Initializing a DNS Resolver with
              Priming Queries", draft-ietf-dnsop-resolver-priming-02
              (work in progress), October 2009.

              Vixie, P. and A. Kato, "DNS Referral Response Size
              Issues", draft-ietf-dnsop-respsize-11 (work in progress),
              July 2008.

              Minda, M., "Using In-Bailiwick Namesevers in .ARPA",
              draft-minda-dnsop-using-in-bailiwick-nameservers-01 (work
              in progress), July 2005.

   [RFC0973]  Mockapetris, P., "Domain system changes and observations",

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              RFC 973, January 1986.

   [RFC1033]  Lottor, M., "Domain administrators operations guide",
              RFC 1033, November 1987.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC1207]  Malkin, G., Marine, A., and J. Reynolds, "FYI on Questions
              and Answers: Answers to commonly asked "experienced
              Internet user" questions", RFC 1207, February 1991.

   [RFC1386]  Cooper, A. and J. Postel, "The US Domain", RFC 1386,
              December 1992.

   [RFC1537]  Beertema, P., "Common DNS Data File Configuration Errors",
              RFC 1537, October 1993.

   [RFC1637]  Manning, B. and R. Colella, "DNS NSAP Resource Records",
              RFC 1637, June 1994.

   [RFC1713]  Romao, A., "Tools for DNS debugging", RFC 1713,
              November 1994.

   [RFC1912]  Barr, D., "Common DNS Operational and Configuration
              Errors", RFC 1912, February 1996.

   [RFC2010]  Manning, B. and P. Vixie, "Operational Criteria for Root
              Name Servers", RFC 2010, October 1996.

   [RFC2065]  Eastlake, D. and C. Kaufman, "Domain Name System Security
              Extensions", RFC 2065, January 1997.

   [RFC2136]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, April 1997.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

   [RFC2535]  Eastlake, D., "Domain Name System Security Extensions",
              RFC 2535, March 1999.

   [RFC2606]  Eastlake, D. and A. Panitz, "Reserved Top Level DNS
              Names", BCP 32, RFC 2606, June 1999.

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   [RFC2672]  Crawford, M., "Non-Terminal DNS Name Redirection",
              RFC 2672, August 1999.

   [RFC2845]  Vixie, P., Gudmundsson, O., Eastlake, D., and B.
              Wellington, "Secret Key Transaction Authentication for DNS
              (TSIG)", RFC 2845, May 2000.

   [RFC2874]  Crawford, M. and C. Huitema, "DNS Extensions to Support
              IPv6 Address Aggregation and Renumbering", RFC 2874,
              July 2000.

   [RFC2931]  Eastlake, D., "DNS Request and Transaction Signatures (
              SIG(0)s)", RFC 2931, September 2000.

   [RFC3363]  Bush, R., Durand, A., Fink, B., Gudmundsson, O., and T.
              Hain, "Representing Internet Protocol version 6 (IPv6)
              Addresses in the Domain Name System (DNS)", RFC 3363,
              August 2002.

   [RFC3364]  Austein, R., "Tradeoffs in Domain Name System (DNS)
              Support for Internet Protocol version 6 (IPv6)", RFC 3364,
              August 2002.

   [RFC3375]  Hollenbeck, S., "Generic Registry-Registrar Protocol
              Requirements", RFC 3375, September 2002.

   [RFC3658]  Gudmundsson, O., "Delegation Signer (DS) Resource Record
              (RR)", RFC 3658, December 2003.

   [RFC3731]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
              Domain Name Mapping", RFC 3731, March 2004.

   [RFC3732]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
              Host Mapping", RFC 3732, March 2004.

   [RFC3833]  Atkins, D. and R. Austein, "Threat Analysis of the Domain
              Name System (DNS)", RFC 3833, August 2004.

   [RFC3845]  Schlyter, J., "DNS Security (DNSSEC) NextSECure (NSEC)
              RDATA Format", RFC 3845, August 2004.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

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   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

   [RFC4183]  Warnicke, E., "A Suggested Scheme for DNS Resolution of
              Networks and Gateways", RFC 4183, September 2005.

   [RFC4472]  Durand, A., Ihren, J., and P. Savola, "Operational
              Considerations and Issues with IPv6 DNS", RFC 4472,
              April 2006.

   [RFC4697]  Larson, M. and P. Barber, "Observed DNS Resolution
              Misbehavior", BCP 123, RFC 4697, October 2006.

   [RFC4871]  Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
              J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
              Signatures", RFC 4871, May 2007.

   [RFC4931]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
              Domain Name Mapping", RFC 4931, May 2007.

   [RFC4932]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
              Host Mapping", RFC 4932, May 2007.

   [RFC5731]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
              Domain Name Mapping", STD 69, RFC 5731, August 2009.

   [RFC5732]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
              Host Mapping", STD 69, RFC 5732, August 2009.

   [RFC5735]  Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
              BCP 153, RFC 5735, January 2010.

   [RFC5855]  Abley, J. and T. Manderson, "Nameservers for IPv4 and IPv6
              Reverse Zones", BCP 155, RFC 5855, May 2010.

   [RFC5936]  Lewis, E. and A. Hoenes, "DNS Zone Transfer Protocol
              (AXFR)", RFC 5936, June 2010.

Appendix A.  Document Revision History

   This section is to be removed should the draft be published.

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A.1.  Changes from -03 to -04

   Adjusted boilerplat text

   Added section about registry considerations

   Extended RFC survey, maintained references

A.2.  Changes from -02 to -03

   Added text about name server naming

   Maintenance of references, minor edits

A.3.  Changes from -01 to -02

   Added text about grandchild glue

   Maintenance of references, minor edits

A.4.  Changes from -00 to -01

   Mentioned RFC survey

   Added text about root server glue

   New text for using glue in responses

Author's Address

   Peter Koch
   Kaiserstrasse 75-77
   Frankfurt  60329

   Phone: +49 69 27235 0
   Email: pk@DENIC.DE

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