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Network Working Group                                           P. Vixie
Request for Comments: 1996                                           ISC
Updates: 1035                                                August 1996
Category: Standards Track


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

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This memo 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.

   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].









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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).

   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



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   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.

   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.



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   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 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.

   3.12. If a NOTIFY request is received by a slave who does not
   implement the NOTIFY opcode, it will respond with a NOTIMP
   (unimplemented feature error) message.  A master server who receives
   such a NOTIMP should consider the NOTIFY transaction complete for



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   that slave.

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.

   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.












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   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.

   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.





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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.

6. References

   [RFC1035]
      Mockapetris, P., "Domain Names - Implementation and
      Specification", STD 13, RFC 1035, November 1987.

   [IXFR]
      Ohta, M., "Incremental Zone Transfer", RFC 1995, August 1996.

7. Author's Address

   Paul Vixie
   Internet Software Consortium
   Star Route Box 159A
   Woodside, CA 94062

   Phone: +1 415 747 0204
   EMail: paul@vix.com



















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