Network Working Group                                        W A Simpson
Internet Draft                                                Daydreamer
expires in six months                                      February 1995

                       ICMP Domain Name Messages

Status of this Memo

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   This document specifies ICMP messages for learning the Fully
   Qualified Domain Name of a target, without laborious maintainance and
   searching of an "inverse" DNS tree.

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DRAFT                       ICMP Domain Name               February 1995

1.  Introduction

   The Domain Name System (DNS) is described in [RFC-1034].  The IN-ADDR
   domain of the DNS is specified [RFC-1035] to perform address to
   domain name mapping, and to facilitate queries to locate all gateways
   (routers) on a particular network in the Internet.

   Neither function has been remarkably successful.  The IN-ADDR domain
   is not reliably populated.

   As multiple routers were used at the boundaries and within networks,
   the IN-ADDR mechanism was found to be inadequate.  The location of
   routers by hosts is now performed using "ICMP Router Discovery
   Messages" [RFC-1256].

   As network numbers migrated to "classless" routing and aggregation,
   the IN-ADDR delegation granularity has fragmented, and requires
   overlapping administration.  This structure is not amenable to
   cooperative secure updating.

   As application servers have appeared which require the Domain Name
   for user interaction and security logging, the IN-ADDR servers have
   been inundated with queries.  This produces long user visible pauses
   at the initiation of sessions.

   Instead, this document proposes that each computer be queried
   directly for its Domain Name.  This has the advantages that the
   mapping is under the same administration as the address assignment,
   and the queries are distributed in the same fashion as IP routing.
   In effect, the routing is used to index the mapping database.

   The datagram format and basic facilities are already defined for ICMP

   Up-to-date values of the ICMP Type field are specified in the most
   recent "Assigned Numbers" [RFC-1700].  This document concerns the
   following values:

       37  Domain Name Request
       38  Domain Name Reply

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DRAFT                       ICMP Domain Name               February 1995

1.1.  Domain Name Request

   |     Type      |     Code      |          Checksum             |
   |           Identifier          |        Sequence Number        |

   Type             37

   Code             0

   Checksum         The ICMP Checksum.

   Identifier       If Code is zero, a value to aid in matching requests
                    and replies.  For example, it might be used like a
                    port in TCP or UDP to identify a session.  May be

   Sequence Number  If Code is zero, a value to aid in matching requests
                    and replies.  For example, the number might be
                    incremented on each request sent.  May be zero.

   A separate Domain Name Request is used for each IP Destination

   An ICMP Domain Name Request received with a broadcast or multicast
   Destination MUST be silently discarded.

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DRAFT                       ICMP Domain Name               February 1995

1.2.  Domain Name Reply

   |     Type      |     Code      |          Checksum             |
   |           Identifier          |        Sequence Number        |
   |   Names ...

   Type             38

   Code             0

   Checksum         The ICMP Checksum.

   Identifier       Copied from the request.

   Sequence Number  Copied from the request.

   Names            zero or more Fully Qualified Domain Names.  The
                    length of this field is determined from the total
                    length of the datagram.

                    Each name is expressed as a sequence of labels.
                    Each label is represented as a one octet length
                    field, followed by that number of octets.  Since
                    every domain name ends with the null label of the
                    root, a domain name is terminated by a length byte
                    of zero.  The high order two bits of every length
                    octet must be zero, and the remaining six bits of
                    the length field limit the label to 63 octets or

                    To simplify implementations, the total length of a
                    domain name (including label octets and label length
                    octets) is restricted to 255 octets or less.

   The Source in a Reply MUST be the same as the Destination of the
   corresponding Request message.

   Every host and router MUST implement an ICMP Domain Name server
   function that receives Domain Name Requests and sends corresponding
   Domain Name Replies.  A host SHOULD also implement an application-
   layer interface for sending a Domain Name Request and receiving a
   Domain Name Reply, for diagnostic purposes.

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Security Considerations

   A primary purpose of this specification is to provide a mechanism for
   updating and learning address to domain name mapping which is more
   secure than IN-ADDR mapping.

   Although the routing infrastructure to the Destination does not
   provide security in and of itself, it is as least as reliable as
   delivery of correspondence for the other sessions with the same peer.

   Although the peer might not be accurate in its reply, this mechanism
   is amenable to establishment of Security Associations for
   authentication and privacy.







Author's Address

   Questions about this memo can also be directed to:

      William Allen Simpson
      Computer Systems Consulting Services
      1384 Fontaine
      Madison Heights, Michigan  48071

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