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The DNS Class Field is Not Useful

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Author Andrew Sullivan
Last updated 2016-02-14
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IETF                                                         A. Sullivan
Internet-Draft                                                 Dyn, Inc.
Intended status: Informational                         February 14, 2016
Expires: August 17, 2016

                   The DNS Class Field is Not Useful


   Domain Name System Resource Records are identified in part by their
   class.  The class field is not effective, and it is not used the way
   it appears to have been intended.  This memo makes no recommendation
   about the DNS parameters registry, but urges those defining new
   RRTYPEs to define them for all classes.

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
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   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 August 17, 2016.

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Table of Contents

   1.  Classes in the Domain Name System . . . . . . . . . . . . . .   2
   2.  Why classes are not as useful as they should be . . . . . . .   3
     2.1.  Class data is in the wrong part of a resource
           record  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Not all RRTYPEs are careful about class . . . . . . . . .   4
     2.3.  A new class requires new delegations  . . . . . . . . . .   5
   3.  DNS classes are effectively vestigial . . . . . . . . . . . .   5
   4.  Plea to authors: define new RRTYPEs for all classes . . . . .   5
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Informative References  . . . . . . . . . . . . . . . . .   5
     5.2.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Appendix A.  Discussion Venue . . . . . . . . . . . . . . . . . .   6
   Appendix B.  Change History . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Classes in the Domain Name System

   The Domain Name System (DNS) [RFC1034] [RFC1035] includes two types
   of division: one by class, and one by "cuts".  As [RFC1034] says,

      The database for any class is organized, delegated, and maintained
      separately from all other classes.  Since, by convention, the name
      spaces are the same for all classes, the separate classes can be
      thought of as an array of parallel namespace trees.  Note that the
      data attached to nodes will be different for these different
      parallel classes.  The most common reasons for creating a new
      class are the necessity for a new data format for existing types
      or a desire for a separately managed version of the existing name

   As of this writing, there are only three "ordinary" classes assigned.
   Class 1 is the Internet or IN class.  Class 3 is the Chaos or CH
   class.  Class 4 is the Hesiod or HS class.  Class 2 is noted in
   [RFC1035] as the CSNET or CS class, but the current registry (at [1])
   no longer includes the assignment.

   There are two other assigned classes; these have special purposes.
   Class 255, ANY or *, matches any class [RFC1035].  Class 254, NONE,
   is used in [RFC2136] to specify certain kinds of operations on

   Of the ordinary classes, only IN is found in common use on the
   Internet.  Class CH is now sometimes used to carry certain kinds of
   name server metadata.  It seems new classes might have been useful
   for a number of features that have been delivered in some other way.

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   Yet classes have not been used, which might suggest that classes are
   less useful than they otherwise appear to be.

   Nevertheless, from time to time someone comes up with a suggestion
   for why a new class would solve some problem or other.  The purpose
   of this memo is to offer some considerations for those contemplating
   such innovation.

2.  Why classes are not as useful as they should be

   There are three problems with classes that mean they cannot work as
   hoped.  First, the class field is in the wrong part of the DNS
   message to be useful as a selector.  Second, specification of
   resource record types has not always attended to the handling of
   types across classes, which means that new classes are likely to be,
   at best, of uncertain utility.

   Apart from those technical problems, there is an administrative
   problem.  The Internet name space starts from a common root, which
   means that any resolver needs to start from the same bootstrap
   mechanism no matter what classes it uses.  But in order for that to
   work, the new class would need to be delegated from the root name

2.1.  Class data is in the wrong part of a resource record

   As noted in Section 1, classes are intended to divide the DNS into
   separate trees.  Unfortunately, however, the class field does not
   appear in a convenient location in a DNS resource record (RR) to be
   effective for that purpose.  DNS resource records provide the owner
   name before the class.  As a practical matter, then, the namespace is

   The issues is made plain by considering the matching algorithm for
   name servers, described in [RFC1034] section 4.3.2.  Suppose there
   are two (imaginary) classes, EG and IE.  Imagine, further, that the
   same name has different RDATA in each class: EG CNAME IE CNAME

   In principle, the above should work because, as noted in Section 1,
   each class is supposed to be "delegated separately".  Therefore, when
   the name server for in class EG receives the query for, it already knows which class database to search;
   similarly for in class IE.

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   Yet while the class delegations are defined to be separate, there is
   of course no way to ensure that the NS record RDATA for
   in class IE, and the NS record RDATA for in class EG, are
   always different.  Indeed, if the different classes of name space are
   truly managed separately, but the name space is by convention
   parallel, it would not be surprising that some name server ended up
   authoritative for the same name in different classes.  (In [RFC1034],
   section 3.6.1, there is an ambiguous example that appears to contain
   two classes from the same master file and for the same name.)
   Therefore, the name server for in every class needs to be
   able to identify the class for the QNAME before beginning to follow
   the resolution algorithm.  But given the DNS message format, the name
   server cannot find the class until it knows the name.  In effect, a
   multi-class name server needs to look at the QCLASS in the query it
   gets, and then select the class in question, before it starts trying
   to match name by name.

   Once one describes the resolution pattern this way, it seems obvious
   that a class needs to be selected in order to resolve a QNAME.  But
   in that case, it is rather inconvenient that the class field does not
   come first in a resource record; and it is not too surprising that,
   given that the IN class is so widely used and others so rarely, some
   naive implementations simply assume IN for every resource record.
   That assumption, of course, makes the class division in the DNS again
   less useful.

2.2.  Not all RRTYPEs are careful about class

   Some RRTYPEs are defined in a class-dependent way.  For instance, the
   A record (type 1) is defined in [RFC1035] to be for class IN only.
   Perhaps unfortunately, the IANA registry for RRTYPEs (at [2]) does
   not include an indicator for the class in which the RRTYPE is

   In addition, not every RRTYPE specification is clear about its
   definition under various classes.  For instance, the specification of
   type 55, HIP, appears not to state the class(es) for which it is
   defined [RFC5205].  The specification of LOC, type 29 (in [RFC1876]),
   says that the type is defined for classes other than IN, but also
   says, "The semantics of such use are not defined by this memo."

   One might argue that this issue is resolved by [RFC3597], because it
   specifies that an unknown class and type combination is to be handled
   as unknown.  Formally, of course, that means that every type can be
   handled regardless of class.  But it would appear to reduce the
   utility of classes yet further, by increasing the probability that
   every RR in every class except IN will be treated as unknown.

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2.3.  A new class requires new delegations

   The Internet's DNS system is part of the common name space of the
   Internet, and that common name space starts from a common root (see
   [RFC2826] for the arguments about why this must be true).  In order
   to provide for the resolution of a new class, the root name servers
   would need to respond to resolution requests for that class and
   provide the delegation data.  Current policies about domain name
   delegation in the root zone appear to apply to the IN class, and it
   is not clear where responsibility would lie for the policies about a
   new class.  At the very least, a new policy of this sort would need
   to be worked out.

3.  DNS classes are effectively vestigial

   Given the considerations above, it is plain that DNS classes are
   unlikely to be useful in the future.  Designers of new name systems
   should consider the design of classes in the DNS.  If a similar
   feature is desirable, its design needs to be different in order to be
   useful.  Given the the way the DNS has managed to thrive effectively
   without classes, however, it would be worth asking whether the
   feature is useful at all.

4.  Plea to authors: define new RRTYPEs for all classes

   When defining a new RRTYPE, it is best to ensure that there is a
   clear definition of the class for which the RRTYPE is defined.  In
   the absence of strong reasons to do otherwise, new types should be
   defined for all classes.

5.  References

5.1.  Informative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC1876]  Davis, C., Vixie, P., Goodwin, T., and I. Dickinson, "A
              Means for Expressing Location Information in the Domain
              Name System", RFC 1876, DOI 10.17487/RFC1876, January
              1996, <>.

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   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,

   [RFC2826]  Internet Architecture Board, "IAB Technical Comment on the
              Unique DNS Root", RFC 2826, DOI 10.17487/RFC2826, May
              2000, <>.

   [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
              (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
              2003, <>.

   [RFC5205]  Nikander, P. and J. Laganier, "Host Identity Protocol
              (HIP) Domain Name System (DNS) Extensions", RFC 5205, DOI
              10.17487/RFC5205, April 2008,

5.2.  URIs



Appendix A.  Discussion Venue

   This Internet-Draft is discussed on the IAB Internet Names and
   Identifiers Program public list:

Appendix B.  Change History


      *  Initial version

Author's Address

   Andrew Sullivan
   Dyn, Inc.
   150 Dow St
   Manchester, NH  03101


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