A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY)
draft-ietf-dnsind-notify-07

DNSIND Working Group                                   Paul Vixie (ISC)
   INTERNET-DRAFT                                              March, 1996
   <draft-ietf-dnsind-notify-07.txt>

   Updates: RFC 1035

       A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY)

   Status of this Memo

      This document is an Internet-Draft.  Internet-Drafts are working
      documents of the Internet Engineering Task Force (IETF), its areas,
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   Abstract

      This draft describes the NOTIFY opcode for DNS, by which a master
      server advises a set of slave servers that the master's data has been
      changed and that a query should be initiated to discover the new
      data.

   1 - Rationale and Scope

   1.1. Slow propagation of new and changed data in a DNS zone can be due
   to a zone's relatively long refresh times.  Longer refresh times are
   beneficial in that they reduce load on the master servers, but that
   benefit comes at the cost of long intervals of incoherence among
   authority servers whenever the zone is updated.

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   1.2. The DNS NOTIFY transaction allows master servers to inform slave
   servers when the zone has changed -- an interrupt as opposed to poll
   model -- which it is hoped will reduce propagation delay while not
   unduly increasing the masters' load.  This specification only allows
   slaves to be notified of SOA RR changes, but the architechture of NOTIFY
   is intended to be extensible to other RR types.

   1.3. This document intentionally gives more definition to the roles of
   ``Master,'' ``Slave'' and ``Stealth'' servers, their enumeration in NS
   RRs, and the SOA MNAME field.  In that sense, this document can be
   considered an addendum to [RFC1035].

   2 - Definitions and Invariants

   2.1. The following definitions are used in this document:

      Slave           an authoritative server which uses zone transfer to
                      retrieve the zone.  All slave servers are named in
                      the NS RRs for the zone.

      Master          any authoritative server configured to be the source
                      of zone transfer for one or more slave servers.

      Primary Master  master server at the root of the zone transfer
                      dependency graph.  The primary master is named in the
                      zone's SOA MNAME field and optionally by an NS RR.
                      There is by definition only one primary master server
                      per zone.

      Stealth         like a slave server except not listed in an NS RR for
                      the zone.  A stealth server, unless explicitly
                      configured to do otherwise, will set the AA bit in
                      responses and be capable of acting as a master.  A
                      stealth server will only be known by other servers if
                      they are given static configuration data indicating
                      its existence.

      Notify Set      set of servers to be notified of changes to some
                      zone.  Default is all servers named in the NS RRset,
                      except for any server also named in the SOA MNAME.
                      Some implementations will permit the name server
                      administrator to override this set or add elements to
                      it (such as, for example, stealth servers).

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   2.2. The zone's servers must be organized into a dependency graph such
   that there is a primary master, and all other servers must use AXFR or
   IXFR either from the primary master or from some slave which is also a
   master.  No loops are permitted in the AXFR dependency graph.

   3 - NOTIFY Message

   3.1. When a master has updated one or more RRs in which slave servers
   may be interested, the master may send the changed RR's name, class,
   type, and optionally, new RDATA(s), to each known slave server using a
   best efforts protocol based on the NOTIFY opcode.

   3.2. NOTIFY uses the DNS Message Format, although it uses only a subset
   of the available fields.  Fields not otherwise described herein are to
   be filled with binary zero (0), and implementations must ignore all
   messages for which this is not the case.

   3.3. NOTIFY is similar to QUERY in that it has a request message with
   the header QR flag ``clear'' and a response message with QR ``set''.
   The response message contains no useful information, but its reception
   by the master is an indication that the slave has received the NOTIFY
   and that the master can remove the slave from any retry queue for this
   NOTIFY event.

   3.4. The transport protocol used for a NOTIFY transaction will be UDP
   unless the master has reason to believe that TCP is necessary; for
   example, if a firewall has been installed between master and slave, and
   only TCP has been allowed; or, if the changed RR is too large to fit in
   a UDP/DNS datagram.

   3.5. If TCP is used, both master and slave must continue to offer name
   service during the transaction, even when the TCP transaction is not
   making progress.  The NOTIFY request is sent once, and a ``timeout'' is
   said to have occurred if no NOTIFY response is received within a
   reasonable interval.

   3.6. If UDP is used, a master periodically sends a NOTIFY request to a
   slave until either too many copies have been sent (a ``timeout''), an
   ICMP message indicating that the port is unreachable, or until a NOTIFY
   response is received from the slave with a matching query ID, QNAME, IP
   source address, and UDP source port number.

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   Note:
      The interval between transmissions, and the total number of
      retransmissions, should be operational parameters specifiable by the
      name server administrator, perhaps on a per-zone basis.  Reasonable
      defaults are a 60 second interval (or timeout if using TCP), and a
      maximum of 5 retransmissions (for UDP).  It is considered reasonable
      to use additive or exponential backoff for the retry interval.

   3.7. A NOTIFY request has QDCOUNT>0, ANCOUNT>=0, AUCOUNT>=0, ADCOUNT>=0.
   If ANCOUNT>0, then the answer section represents an unsecure hint at the
   new RRset for this <QNAME,QCLASS,QTYPE>.  A slave receiving such a hint
   is free to treat equivilence of this answer section with its local data
   as a ``no further work needs to be done'' indication.  If ANCOUNT=0, or
   ANCOUNT>0 and the answer section differs from the slave's local data,
   then the slave should query its known masters to retrieve the new data.

   3.8. In no case shall the answer section of a NOTIFY request be used to
   update a slave's local data, or to indicate that a zone transfer needs
   to be undertaken, or to change the slave's zone refresh timers.  Only a
   ``data present; data same'' condition can lead a slave to act
   differently if ANCOUNT>0 than it would if ANCOUNT=0.

   3.9. This version of the NOTIFY specification makes no use of the
   authority or additional data sections, and so conforming implementations
   should set AUCOUNT=0 and ADCOUNT=0 when transmitting requests.  Since a
   future revision of this specification may define a backwards compatible
   use for either or both of these sections, current implementations must
   ignore these sections, but not the entire message, if AUCOUNT>0 and/or
   ADCOUNT>0.

   3.10. If a slave receives a NOTIFY request from a host that is not a
   known master for the zone containing the QNAME, it should ignore the
   request and produce an error message in its operations log.

   Note:
      This implies that slaves of a multihomed master must either know
      their master by the ``closest'' of the master's interface addresses,
      or must know all of the master's interface addresses.  Otherwise, a
      valid NOTIFY request might come from an address that is not on the
      slave's state list of masters for the zone, which would be an error.

   3.11. The only defined NOTIFY event at this time is that the SOA RR has
   changed.  Upon completion of a NOTIFY transaction for QTYPE=SOA, the
   slave should behave as though the zone given in the QNAME had reached
   its REFRESH interval (see [RFC1035]), i.e., it should query its masters

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   for the SOA of the zone given in the NOTIFY QNAME, and check the answer
   to see if the SOA SERIAL has been incremented since the last time the
   zone was fetched.  If so, a zone transfer (either AXFR or IXFR) should
   be initiated.

   Note:
      Because a deep server dependency graph may have multiple paths from
      the primary master to any given slave, it is possible that a slave
      will receive a NOTIFY from one of its known masters even though the
      rest of its known masters have not yet updated their copies of the
      zone.  Therefore, when issuing a QUERY for the zone's SOA, the query
      should be directed at the known master who was the source of the
      NOTIFY event, and not at any of the other known masters.  This
      represents a departure from [RFC1035], which specifies that upon
      expiry of the SOA REFRESH interval, all known masters should be
      queried in turn.

   4 - Details and Examples

   4.1. Retaining query state information across host reboots is optional,
   but it is reasonable to simply execute an SOA NOTIFY transaction on each
   authority zone when a server first starts.

   4.2. Each slave is likely to receive several copies of the same NOTIFY
   request:  One from the primary master, and one from each other slave as
   that slave transfers the new zone and notifies its potential peers.  The
   NOTIFY protocol supports this multiplicity by requiring that NOTIFY be
   sent by a slave/master only AFTER it has updated the SOA RR or has
   determined that no update is necessary, which in practice means after a
   successful zone transfer.  Thus, barring delivery reordering, the last
   NOTIFY any slave receives will be the one indicating the latest change.
   Since a slave always requests SOAs and AXFR/IXFRs only from its known
   masters, it will have an opportunity to retry its QUERY for the SOA
   after each of its masters have completed each zone update.

   4.3. If a master server seeks to avoid causing a large number of
   simultaneous outbound zone transfers, it may delay for an arbitrary
   length of time before sending a NOTIFY message to any given slave.  It
   is expected that the time will be chosen at random, so that each slave
   will begin its transfer at a unique time.  The delay shall not in any
   case be longer than the SOA REFRESH time.

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   Note:
      This delay should be a parameter that each primary master name server
      can specify, perhaps on a per-zone basis.  Random delays of between
      30 and 60 seconds would seem adequate if the servers share a LAN and
      the zones are of moderate size.

   4.4. A slave which receives a valid NOTIFY should defer action on any
   subsequent NOTIFY with the same <QNAME,QCLASS,QTYPE> until it has
   completed the transaction begun by the first NOTIFY.  This duplicate
   rejection is necessary to avoid having multiple notifications lead to
   pummeling the master server.

   4.5 - Zone has Updated on Primary Master

   Primary master sends a NOTIFY request to all servers named in Notify
   Set.  The NOTIFY request has the following characteristics:

   query ID:   (new)
   op:         NOTIFY (4)
   resp:       NOERROR
   flags:      AA
   qcount:     1
   qname:      (zone name)
   qclass:     (zone class)
   qtype:      T_SOA

   4.6 - Zone has Updated on a Slave that is also a Master

   As above in 4.5, except that this server's Notify Set may be different
   from the Primary Master's due to optional static specification of local
   stealth servers.

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   4.7 - Slave Receives a NOTIFY Request from a Master

   When a slave server receives a NOTIFY request from one of its locally
   designated masters for the zone enclosing the given QNAME, with
   QTYPE=SOA and QR=0, it should enter the state it would if the zone's
   refresh timer had expired.  It will also send a NOTIFY response back to
   the NOTIFY request's source, with the following characteristics:

   query ID:   (same)
   op:         NOTIFY (4)
   resp:       NOERROR
   flags:      QR AA
   qcount:     1
   qname:      (zone name)
   qclass:     (zone class)
   qtype:      T_SOA

   This is intended to be identical to the NOTIFY request, except that the
   QR bit is also set.  The query ID of the response must be the same as
   was received in the request.

   4.8 - Master Receives a NOTIFY Response from Slave

   When a master server receives a NOTIFY response, it deletes this query
   from the retry queue, thus completing the ``notification process'' of
   ``this'' RRset change to ``that'' server.

   5 - Security Considerations

   We believe that the NOTIFY operation's only security considerations are:

   1. That a NOTIFY request with a forged IP/UDP source address can cause a
      slave to send spurious SOA queries to its masters, leading to a
      benign denial of service attack if the forged requests are sent very
      often.

   2. That TCP spoofing could be used against a slave server given NOTIFY
      as a means of synchronizing an SOA query and UDP/DNS spoofing as a
      means of forcing a zone transfer.

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   6 - References

   [RFC1035]
      P. Mockapetris, "Domain Names - Implementation and Specification",
      RFC 1035, USC/Information Sciences Institute, November 1987.

   [IXFR]
      M. Ohta, "Incremental Zone Transfer", Internet Draft, February 1996,
      <draft-ietf-dnsind-ixfr-06.txt>.

   7 - Author's Address

         Paul Vixie
            Internet Software Consortium
            Star Route Box 159A
            Woodside, CA 94062
            +1 415 747 0204
            <paul@vix.com>

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