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rfc:rfc2136

Network Working Group P. Vixie, Editor Request for Comments: 2136 ISC Updates: 1035 S. Thomson Category: Standards Track Bellcore

                                                           Y. Rekhter
                                                                Cisco
                                                             J. Bound
                                                                  DEC
                                                           April 1997
       Dynamic Updates in the Domain Name System (DNS UPDATE)

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

 The Domain Name System was originally designed to support queries of
 a statically configured database.  While the data was expected to
 change, the frequency of those changes was expected to be fairly low,
 and all updates were made as external edits to a zone's Master File.
 Using this specification of the UPDATE opcode, it is possible to add
 or delete RRs or RRsets from a specified zone.  Prerequisites are
 specified separately from update operations, and can specify a
 dependency upon either the previous existence or nonexistence of an
 RRset, or the existence of a single RR.
 UPDATE is atomic, i.e., all prerequisites must be satisfied or else
 no update operations will take place.  There are no data dependent
 error conditions defined after the prerequisites have been met.

1 - Definitions

 This document intentionally gives more definition to the roles of
 "Master," "Slave," and "Primary Master" servers, and their
 enumeration in NS RRs, and the SOA MNAME field.  In that sense, the
 following server type definitions can be considered an addendum to
 [RFC1035], and are intended to be consistent with [RFC1996]:
    Slave           an authoritative server that uses AXFR or IXFR to
                    retrieve the zone and is named in the zone's NS
                    RRset.

Vixie, et. al. Standards Track [Page 1] RFC 2136 DNS Update April 1997

    Master          an authoritative server configured to be the
                    source of AXFR or IXFR data for one or more slave
                    servers.
    Primary Master  master server at the root of the AXFR/IXFR
                    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.
 A domain name identifies a node within the domain name space tree
 structure.  Each node has a set (possibly empty) of Resource Records
 (RRs).  All RRs having the same NAME, CLASS and TYPE are called a
 Resource Record Set (RRset).
 The pseudocode used in this document is for example purposes only.
 If it is found to disagree with the text, the text shall be
 considered authoritative.  If the text is found to be ambiguous, the
 pseudocode can be used to help resolve the ambiguity.
 1.1 - Comparison Rules
 1.1.1. Two RRs are considered equal if their NAME, CLASS, TYPE,
 RDLENGTH and RDATA fields are equal.  Note that the time-to-live
 (TTL) field is explicitly excluded from the comparison.
 1.1.2. The rules for comparison of character strings in names are
 specified in [RFC1035 2.3.3].
 1.1.3. Wildcarding is disabled.  That is, a wildcard ("*") in an
 update only matches a wildcard ("*") in the zone, and vice versa.
 1.1.4. Aliasing is disabled: A CNAME in the zone matches a CNAME in
 the update, and will not otherwise be followed.  All UPDATE
 operations are done on the basis of canonical names.
 1.1.5. The following RR types cannot be appended to an RRset.  If the
 following comparison rules are met, then an attempt to add the new RR
 will result in the replacement of the previous RR:
    SOA    compare only NAME, CLASS and TYPE -- it is not possible to
           have more than one SOA per zone, even if any of the data
           fields differ.
    WKS    compare only NAME, CLASS, TYPE, ADDRESS, and PROTOCOL
           -- only one WKS RR is possible for this tuple, even if the
           services masks differ.

Vixie, et. al. Standards Track [Page 2] RFC 2136 DNS Update April 1997

    CNAME  compare only NAME, CLASS, and TYPE -- it is not possible
           to have more than one CNAME RR, even if their data fields
           differ.
 1.2 - Glue RRs
 For the purpose of determining whether a domain name used in the
 UPDATE protocol is contained within a specified zone, a domain name
 is "in" a zone if it is owned by that zone's domain name.  See
 section 7.18 for details.
 1.3 - New Assigned Numbers
    CLASS = NONE (254)
    RCODE = YXDOMAIN (6)
    RCODE = YXRRSET (7)
    RCODE = NXRRSET (8)
    RCODE = NOTAUTH (9)
    RCODE = NOTZONE (10)
    Opcode = UPDATE (5)

2 - Update Message Format

 The DNS Message Format is defined by [RFC1035 4.1].  Some extensions
 are necessary (for example, more error codes are possible under
 UPDATE than under QUERY) and some fields must be overloaded (see
 description of CLASS fields below).
 The overall format of an UPDATE message is, following [ibid]:
    +---------------------+
    |        Header       |
    +---------------------+
    |         Zone        | specifies the zone to be updated
    +---------------------+
    |     Prerequisite    | RRs or RRsets which must (not) preexist
    +---------------------+
    |        Update       | RRs or RRsets to be added or deleted
    +---------------------+
    |   Additional Data   | additional data
    +---------------------+

Vixie, et. al. Standards Track [Page 3] RFC 2136 DNS Update April 1997

 The Header Section specifies that this message is an UPDATE, and
 describes the size of the other sections.  The Zone Section names the
 zone that is to be updated by this message.  The Prerequisite Section
 specifies the starting invariants (in terms of zone content) required
 for this update.  The Update Section contains the edits to be made,
 and the Additional Data Section contains data which may be necessary
 to complete, but is not part of, this update.
 2.1 - Transport Issues
 An update transaction may be carried in a UDP datagram, if the
 request fits, or in a TCP connection (at the discretion of the
 requestor).  When TCP is used, the message is in the format described
 in [RFC1035 4.2.2].
 2.2 - Message Header
 The header of the DNS Message Format is defined by [RFC 1035 4.1].
 Not all opcodes define the same set of flag bits, though as a
 practical matter most of the bits defined for QUERY (in [ibid]) are
 identically defined by the other opcodes.  UPDATE uses only one flag
 bit (QR).
 The DNS Message Format specifies record counts for its four sections
 (Question, Answer, Authority, and Additional).  UPDATE uses the same
 fields, and the same section formats, but the naming and use of these
 sections differs as shown in the following modified header, after
 [RFC1035 4.1.1]:
                                    1  1  1  1  1  1
      0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                      ID                       |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |QR|   Opcode  |          Z         |   RCODE   |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    ZOCOUNT                    |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    PRCOUNT                    |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    UPCOUNT                    |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    ADCOUNT                    |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

Vixie, et. al. Standards Track [Page 4] RFC 2136 DNS Update April 1997

 These fields are used as follows:
 ID      A 16-bit identifier assigned by the entity that generates any
         kind of request.  This identifier is copied in the
         corresponding reply and can be used by the requestor to match
         replies to outstanding requests, or by the server to detect
         duplicated requests from some requestor.
 QR      A one bit field that specifies whether this message is a
         request (0), or a response (1).
 Opcode  A four bit field that specifies the kind of request in this
         message.  This value is set by the originator of a request
         and copied into the response.  The Opcode value that
         identifies an UPDATE message is five (5).
 Z       Reserved for future use.  Should be zero (0) in all requests
         and responses.  A non-zero Z field should be ignored by
         implementations of this specification.
 RCODE   Response code - this four bit field is undefined in requests
         and set in responses.  The values and meanings of this field
         within responses are as follows:
            Mneumonic   Value   Description
            ------------------------------------------------------------
            NOERROR     0       No error condition.
            FORMERR     1       The name server was unable to interpret
                                the request due to a format error.
            SERVFAIL    2       The name server encountered an internal
                                failure while processing this request,
                                for example an operating system error
                                or a forwarding timeout.
            NXDOMAIN    3       Some name that ought to exist,
                                does not exist.
            NOTIMP      4       The name server does not support
                                the specified Opcode.
            REFUSED     5       The name server refuses to perform the
                                specified operation for policy or
                                security reasons.
            YXDOMAIN    6       Some name that ought not to exist,
                                does exist.
            YXRRSET     7       Some RRset that ought not to exist,
                                does exist.
            NXRRSET     8       Some RRset that ought to exist,
                                does not exist.

Vixie, et. al. Standards Track [Page 5] RFC 2136 DNS Update April 1997

            NOTAUTH     9       The server is not authoritative for
                                the zone named in the Zone Section.
            NOTZONE     10      A name used in the Prerequisite or
                                Update Section is not within the
                                zone denoted by the Zone Section.
 ZOCOUNT The number of RRs in the Zone Section.
 PRCOUNT The number of RRs in the Prerequisite Section.
 UPCOUNT The number of RRs in the Update Section.
 ADCOUNT The number of RRs in the Additional Data Section.
 2.3 - Zone Section
 The Zone Section has the same format as that specified in [RFC1035
 4.1.2], with the fields redefined as follows:
                                    1  1  1  1  1  1
      0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                                               |
    /                     ZNAME                     /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                     ZTYPE                     |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                     ZCLASS                    |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 UPDATE uses this section to denote the zone of the records being
 updated.  All records to be updated must be in the same zone, and
 therefore the Zone Section is allowed to contain exactly one record.
 The ZNAME is the zone name, the ZTYPE must be SOA, and the ZCLASS is
 the zone's class.
 2.4 - Prerequisite Section
 This section contains a set of RRset prerequisites which must be
 satisfied at the time the UPDATE packet is received by the primary
 master server.  The format of this section is as specified by
 [RFC1035 4.1.3].  There are five possible sets of semantics that can
 be expressed here, summarized as follows and then explained below.
    (1)  RRset exists (value independent).  At least one RR with a
         specified NAME and TYPE (in the zone and class specified by
         the Zone Section) must exist.

Vixie, et. al. Standards Track [Page 6] RFC 2136 DNS Update April 1997

    (2)  RRset exists (value dependent).  A set of RRs with a
         specified NAME and TYPE exists and has the same members
         with the same RDATAs as the RRset specified here in this
         Section.
    (3)  RRset does not exist.  No RRs with a specified NAME and TYPE
        (in the zone and class denoted by the Zone Section) can exist.
    (4)  Name is in use.  At least one RR with a specified NAME (in
         the zone and class specified by the Zone Section) must exist.
         Note that this prerequisite is NOT satisfied by empty
         nonterminals.
    (5)  Name is not in use.  No RR of any type is owned by a
         specified NAME.  Note that this prerequisite IS satisfied by
         empty nonterminals.
 The syntax of these is as follows:
 2.4.1 - RRset Exists (Value Independent)
 At least one RR with a specified NAME and TYPE (in the zone and class
 specified in the Zone Section) must exist.
 For this prerequisite, a requestor adds to the section a single RR
 whose NAME and TYPE are equal to that of the zone RRset whose
 existence is required.  RDLENGTH is zero and RDATA is therefore
 empty.  CLASS must be specified as ANY to differentiate this
 condition from that of an actual RR whose RDLENGTH is naturally zero
 (0) (e.g., NULL).  TTL is specified as zero (0).
 2.4.2 - RRset Exists (Value Dependent)
 A set of RRs with a specified NAME and TYPE exists and has the same
 members with the same RDATAs as the RRset specified here in this
 section.  While RRset ordering is undefined and therefore not
 significant to this comparison, the sets be identical in their
 extent.
 For this prerequisite, a requestor adds to the section an entire
 RRset whose preexistence is required.  NAME and TYPE are that of the
 RRset being denoted.  CLASS is that of the zone.  TTL must be
 specified as zero (0) and is ignored when comparing RRsets for
 identity.

Vixie, et. al. Standards Track [Page 7] RFC 2136 DNS Update April 1997

 2.4.3 - RRset Does Not Exist
 No RRs with a specified NAME and TYPE (in the zone and class denoted
 by the Zone Section) can exist.
 For this prerequisite, a requestor adds to the section a single RR
 whose NAME and TYPE are equal to that of the RRset whose nonexistence
 is required.  The RDLENGTH of this record is zero (0), and RDATA
 field is therefore empty.  CLASS must be specified as NONE in order
 to distinguish this condition from a valid RR whose RDLENGTH is
 naturally zero (0) (for example, the NULL RR).  TTL must be specified
 as zero (0).
 2.4.4 - Name Is In Use
 Name is in use.  At least one RR with a specified NAME (in the zone
 and class specified by the Zone Section) must exist.  Note that this
 prerequisite is NOT satisfied by empty nonterminals.
 For this prerequisite, a requestor adds to the section a single RR
 whose NAME is equal to that of the name whose ownership of an RR is
 required.  RDLENGTH is zero and RDATA is therefore empty.  CLASS must
 be specified as ANY to differentiate this condition from that of an
 actual RR whose RDLENGTH is naturally zero (0) (e.g., NULL).  TYPE
 must be specified as ANY to differentiate this case from that of an
 RRset existence test.  TTL is specified as zero (0).
 2.4.5 - Name Is Not In Use
 Name is not in use.  No RR of any type is owned by a specified NAME.
 Note that this prerequisite IS satisfied by empty nonterminals.
 For this prerequisite, a requestor adds to the section a single RR
 whose NAME is equal to that of the name whose nonownership of any RRs
 is required.  RDLENGTH is zero and RDATA is therefore empty.  CLASS
 must be specified as NONE.  TYPE must be specified as ANY.  TTL must
 be specified as zero (0).
 2.5 - Update Section
 This section contains RRs to be added to or deleted from the zone.
 The format of this section is as specified by [RFC1035 4.1.3].  There
 are four possible sets of semantics, summarized below and with
 details to follow.

Vixie, et. al. Standards Track [Page 8] RFC 2136 DNS Update April 1997

    (1) Add RRs to an RRset.
    (2) Delete an RRset.
    (3) Delete all RRsets from a name.
    (4) Delete an RR from an RRset.
 The syntax of these is as follows:
 2.5.1 - Add To An RRset
 RRs are added to the Update Section whose NAME, TYPE, TTL, RDLENGTH
 and RDATA are those being added, and CLASS is the same as the zone
 class.  Any duplicate RRs will be silently ignored by the primary
 master.
 2.5.2 - Delete An RRset
 One RR is added to the Update Section whose NAME and TYPE are those
 of the RRset to be deleted.  TTL must be specified as zero (0) and is
 otherwise not used by the primary master.  CLASS must be specified as
 ANY.  RDLENGTH must be zero (0) and RDATA must therefore be empty.
 If no such RRset exists, then this Update RR will be silently ignored
 by the primary master.
 2.5.3 - Delete All RRsets From A Name
 One RR is added to the Update Section whose NAME is that of the name
 to be cleansed of RRsets.  TYPE must be specified as ANY.  TTL must
 be specified as zero (0) and is otherwise not used by the primary
 master.  CLASS must be specified as ANY.  RDLENGTH must be zero (0)
 and RDATA must therefore be empty.  If no such RRsets exist, then
 this Update RR will be silently ignored by the primary master.
 2.5.4 - Delete An RR From An RRset
 RRs to be deleted are added to the Update Section.  The NAME, TYPE,
 RDLENGTH and RDATA must match the RR being deleted.  TTL must be
 specified as zero (0) and will otherwise be ignored by the primary
 master.  CLASS must be specified as NONE to distinguish this from an
 RR addition.  If no such RRs exist, then this Update RR will be
 silently ignored by the primary master.

Vixie, et. al. Standards Track [Page 9] RFC 2136 DNS Update April 1997

 2.6 - Additional Data Section
 This section contains RRs which are related to the update itself, or
 to new RRs being added by the update.  For example, out of zone glue
 (A RRs referred to by new NS RRs) should be presented here.  The
 server can use or ignore out of zone glue, at the discretion of the
 server implementor.  The format of this section is as specified by
 [RFC1035 4.1.3].

3 - Server Behavior

 A server, upon receiving an UPDATE request, will signal NOTIMP to the
 requestor if the UPDATE opcode is not recognized or if it is
 recognized but has not been implemented.  Otherwise, processing
 continues as follows.
 3.1 - Process Zone Section
 3.1.1. The Zone Section is checked to see that there is exactly one
 RR therein and that the RR's ZTYPE is SOA, else signal FORMERR to the
 requestor.  Next, the ZNAME and ZCLASS are checked to see if the zone
 so named is one of this server's authority zones, else signal NOTAUTH
 to the requestor.  If the server is a zone slave, the request will be
 forwarded toward the primary master.
 3.1.2 - Pseudocode For Zone Section Processing
    if (zcount != 1 || ztype != SOA)
         return (FORMERR)
    if (zone_type(zname, zclass) == SLAVE)
         return forward()
    if (zone_type(zname, zclass) == MASTER)
         return update()
    return (NOTAUTH)
 Sections 3.2 through 3.8 describe the primary master's behaviour,
 whereas Section 6 describes a forwarder's behaviour.
 3.2 - Process Prerequisite Section
 Next, the Prerequisite Section is checked to see that all
 prerequisites are satisfied by the current state of the zone.  Using
 the definitions expressed in Section 1.2, if any RR's NAME is not
 within the zone specified in the Zone Section, signal NOTZONE to the
 requestor.

Vixie, et. al. Standards Track [Page 10] RFC 2136 DNS Update April 1997

 3.2.1. For RRs in this section whose CLASS is ANY, test to see that
 TTL and RDLENGTH are both zero (0), else signal FORMERR to the
 requestor.  If TYPE is ANY, test to see that there is at least one RR
 in the zone whose NAME is the same as that of the Prerequisite RR,
 else signal NXDOMAIN to the requestor.  If TYPE is not ANY, test to
 see that there is at least one RR in the zone whose NAME and TYPE are
 the same as that of the Prerequisite RR, else signal NXRRSET to the
 requestor.
 3.2.2. For RRs in this section whose CLASS is NONE, test to see that
 the TTL and RDLENGTH are both zero (0), else signal FORMERR to the
 requestor.  If the TYPE is ANY, test to see that there are no RRs in
 the zone whose NAME is the same as that of the Prerequisite RR, else
 signal YXDOMAIN to the requestor.  If the TYPE is not ANY, test to
 see that there are no RRs in the zone whose NAME and TYPE are the
 same as that of the Prerequisite RR, else signal YXRRSET to the
 requestor.
 3.2.3. For RRs in this section whose CLASS is the same as the ZCLASS,
 test to see that the TTL is zero (0), else signal FORMERR to the
 requestor.  Then, build an RRset for each unique <NAME,TYPE> and
 compare each resulting RRset for set equality (same members, no more,
 no less) with RRsets in the zone.  If any Prerequisite RRset is not
 entirely and exactly matched by a zone RRset, signal NXRRSET to the
 requestor.  If any RR in this section has a CLASS other than ZCLASS
 or NONE or ANY, signal FORMERR to the requestor.
 3.2.4 - Table Of Metavalues Used In Prerequisite Section
 CLASS    TYPE     RDATA    Meaning
 ------------------------------------------------------------
 ANY      ANY      empty    Name is in use
 ANY      rrset    empty    RRset exists (value independent)
 NONE     ANY      empty    Name is not in use
 NONE     rrset    empty    RRset does not exist
 zone     rrset    rr       RRset exists (value dependent)

Vixie, et. al. Standards Track [Page 11] RFC 2136 DNS Update April 1997

 3.2.5 - Pseudocode for Prerequisite Section Processing
    for rr in prerequisites
         if (rr.ttl != 0)
              return (FORMERR)
         if (zone_of(rr.name) != ZNAME)
              return (NOTZONE);
         if (rr.class == ANY)
              if (rr.rdlength != 0)
                   return (FORMERR)
              if (rr.type == ANY)
                   if (!zone_name<rr.name>)
                        return (NXDOMAIN)
              else
                   if (!zone_rrset<rr.name, rr.type>)
                        return (NXRRSET)
         if (rr.class == NONE)
              if (rr.rdlength != 0)
                   return (FORMERR)
              if (rr.type == ANY)
                   if (zone_name<rr.name>)
                        return (YXDOMAIN)
              else
                   if (zone_rrset<rr.name, rr.type>)
                        return (YXRRSET)
         if (rr.class == zclass)
              temp<rr.name, rr.type> += rr
         else
              return (FORMERR)
    for rrset in temp
         if (zone_rrset<rrset.name, rrset.type> != rrset)
              return (NXRRSET)
 3.3 - Check Requestor's Permissions
 3.3.1. Next, the requestor's permission to update the RRs named in
 the Update Section may be tested in an implementation dependent
 fashion or using mechanisms specified in a subsequent Secure DNS
 Update protocol.  If the requestor does not have permission to
 perform these updates, the server may write a warning message in its
 operations log, and may either signal REFUSED to the requestor, or
 ignore the permission problem and proceed with the update.

Vixie, et. al. Standards Track [Page 12] RFC 2136 DNS Update April 1997

 3.3.2. While the exact processing is implementation defined, if these
 verification activities are to be performed, this is the point in the
 server's processing where such performance should take place, since
 if a REFUSED condition is encountered after an update has been
 partially applied, it will be necessary to undo the partial update
 and restore the zone to its original state before answering the
 requestor.
 3.3.3 - Pseudocode for Permission Checking
    if (security policy exists)
         if (this update is not permitted)
              if (local option)
                   log a message about permission problem
              if (local option)
                   return (REFUSED)
 3.4 - Process Update Section
 Next, the Update Section is processed as follows.
 3.4.1 - Prescan
 The Update Section is parsed into RRs and each RR's CLASS is checked
 to see if it is ANY, NONE, or the same as the Zone Class, else signal
 a FORMERR to the requestor.  Using the definitions in Section 1.2,
 each RR's NAME must be in the zone specified by the Zone Section,
 else signal NOTZONE to the requestor.
 3.4.1.2. For RRs whose CLASS is not ANY, check the TYPE and if it is
 ANY, AXFR, MAILA, MAILB, or any other QUERY metatype, or any
 unrecognized type, then signal FORMERR to the requestor.  For RRs
 whose CLASS is ANY or NONE, check the TTL to see that it is zero (0),
 else signal a FORMERR to the requestor.  For any RR whose CLASS is
 ANY, check the RDLENGTH to make sure that it is zero (0) (that is,
 the RDATA field is empty), and that the TYPE is not AXFR, MAILA,
 MAILB, or any other QUERY metatype besides ANY, or any unrecognized
 type, else signal FORMERR to the requestor.

Vixie, et. al. Standards Track [Page 13] RFC 2136 DNS Update April 1997

 3.4.1.3 - Pseudocode For Update Section Prescan
    [rr] for rr in updates
         if (zone_of(rr.name) != ZNAME)
              return (NOTZONE);
         if (rr.class == zclass)
              if (rr.type & ANY|AXFR|MAILA|MAILB)
                   return (FORMERR)
         elsif (rr.class == ANY)
              if (rr.ttl != 0 || rr.rdlength != 0
                  || rr.type & AXFR|MAILA|MAILB)
                   return (FORMERR)
         elsif (rr.class == NONE)
              if (rr.ttl != 0 || rr.type & ANY|AXFR|MAILA|MAILB)
                   return (FORMERR)
         else
              return (FORMERR)
 3.4.2 - Update
 The Update Section is parsed into RRs and these RRs are processed in
 order.
 3.4.2.1. If any system failure (such as an out of memory condition,
 or a hardware error in persistent storage) occurs during the
 processing of this section, signal SERVFAIL to the requestor and undo
 all updates applied to the zone during this transaction.
 3.4.2.2. Any Update RR whose CLASS is the same as ZCLASS is added to
 the zone.  In case of duplicate RDATAs (which for SOA RRs is always
 the case, and for WKS RRs is the case if the ADDRESS and PROTOCOL
 fields both match), the Zone RR is replaced by Update RR.  If the
 TYPE is SOA and there is no Zone SOA RR, or the new SOA.SERIAL is
 lower (according to [RFC1982]) than or equal to the current Zone SOA
 RR's SOA.SERIAL, the Update RR is ignored.  In the case of a CNAME
 Update RR and a non-CNAME Zone RRset or vice versa, ignore the CNAME
 Update RR, otherwise replace the CNAME Zone RR with the CNAME Update
 RR.
 3.4.2.3. For any Update RR whose CLASS is ANY and whose TYPE is ANY,
 all Zone RRs with the same NAME are deleted, unless the NAME is the
 same as ZNAME in which case only those RRs whose TYPE is other than
 SOA or NS are deleted.  For any Update RR whose CLASS is ANY and
 whose TYPE is not ANY all Zone RRs with the same NAME and TYPE are
 deleted, unless the NAME is the same as ZNAME in which case neither
 SOA or NS RRs will be deleted.

Vixie, et. al. Standards Track [Page 14] RFC 2136 DNS Update April 1997

 3.4.2.4. For any Update RR whose class is NONE, any Zone RR whose
 NAME, TYPE, RDATA and RDLENGTH are equal to the Update RR is deleted,
 unless the NAME is the same as ZNAME and either the TYPE is SOA or
 the TYPE is NS and the matching Zone RR is the only NS remaining in
 the RRset, in which case this Update RR is ignored.
 3.4.2.5. Signal NOERROR to the requestor.
 3.4.2.6 - Table Of Metavalues Used In Update Section
 CLASS    TYPE     RDATA    Meaning
 ---------------------------------------------------------
 ANY      ANY      empty    Delete all RRsets from a name
 ANY      rrset    empty    Delete an RRset
 NONE     rrset    rr       Delete an RR from an RRset
 zone     rrset    rr       Add to an RRset
 3.4.2.7 - Pseudocode For Update Section Processing
    [rr] for rr in updates
         if (rr.class == zclass)
              if (rr.type == CNAME)
                   if (zone_rrset<rr.name, ~CNAME>)
                        next [rr]
              elsif (zone_rrset<rr.name, CNAME>)
                   next [rr]
              if (rr.type == SOA)
                   if (!zone_rrset<rr.name, SOA> ||
                       zone_rr<rr.name, SOA>.serial > rr.soa.serial)
                        next [rr]
              for zrr in zone_rrset<rr.name, rr.type>
                   if (rr.type == CNAME || rr.type == SOA ||
                       (rr.type == WKS && rr.proto == zrr.proto &&
                        rr.address == zrr.address) ||
                       rr.rdata == zrr.rdata)
                        zrr = rr
                        next [rr]
              zone_rrset<rr.name, rr.type> += rr
         elsif (rr.class == ANY)
              if (rr.type == ANY)
                   if (rr.name == zname)
                        zone_rrset<rr.name, ~(SOA|NS)> = Nil
                   else
                        zone_rrset<rr.name, *> = Nil
              elsif (rr.name == zname &&
                     (rr.type == SOA || rr.type == NS))
                   next [rr]
              else

Vixie, et. al. Standards Track [Page 15] RFC 2136 DNS Update April 1997

                   zone_rrset<rr.name, rr.type> = Nil
         elsif (rr.class == NONE)
              if (rr.type == SOA)
                   next [rr]
              if (rr.type == NS && zone_rrset<rr.name, NS> == rr)
                   next [rr]
              zone_rr<rr.name, rr.type, rr.data> = Nil
    return (NOERROR)
 3.5 - Stability
 When a zone is modified by an UPDATE operation, the server must
 commit the change to nonvolatile storage before sending a response to
 the requestor or answering any queries or transfers for the modified
 zone.  It is reasonable for a server to store only the update records
 as long as a system reboot or power failure will cause these update
 records to be incorporated into the zone the next time the server is
 started.  It is also reasonable for the server to copy the entire
 modified zone to nonvolatile storage after each update operation,
 though this would have suboptimal performance for large zones.
 3.6 - Zone Identity
 If the zone's SOA SERIAL is changed by an update operation, that
 change must be in a positive direction (using modulo 2**32 arithmetic
 as specified by [RFC1982]).  Attempts to replace an SOA with one
 whose SERIAL is less than the current one will be silently ignored by
 the primary master server.
 If the zone's SOA's SERIAL is not changed as a result of an update
 operation, then the server shall increment it automatically before
 the SOA or any changed name or RR or RRset is included in any
 response or transfer.  The primary master server's implementor might
 choose to autoincrement the SOA SERIAL if any of the following events
 occurs:
 (1)  Each update operation.
 (2)  A name, RR or RRset in the zone has changed and has subsequently
      been visible to a DNS client since the unincremented SOA was
      visible to a DNS client, and the SOA is about to become visible
      to a DNS client.
 (3)  A configurable period of time has elapsed since the last update
      operation.  This period shall be less than or equal to one third
      of the zone refresh time, and the default shall be the lesser of
      that maximum and 300 seconds.

Vixie, et. al. Standards Track [Page 16] RFC 2136 DNS Update April 1997

 (4)  A configurable number of updates has been applied since the last
      SOA change.  The default value for this configuration parameter
      shall be one hundred (100).
 It is imperative that the zone's contents and the SOA's SERIAL be
 tightly synchronized.  If the zone appears to change, the SOA must
 appear to change as well.
 3.7 - Atomicity
 During the processing of an UPDATE transaction, the server must
 ensure atomicity with respect to other (concurrent) UPDATE or QUERY
 transactions.  No two transactions can be processed concurrently if
 either depends on the final results of the other; in particular, a
 QUERY should not be able to retrieve RRsets which have been partially
 modified by a concurrent UPDATE, and an UPDATE should not be able to
 start from prerequisites that might not still hold at the completion
 of some other concurrent UPDATE.  Finally, if two UPDATE transactions
 would modify the same names, RRs or RRsets, then such UPDATE
 transactions must be serialized.
 3.8 - Response
 At the end of UPDATE processing, a response code will be known.  A
 response message is generated by copying the ID and Opcode fields
 from the request, and either copying the ZOCOUNT, PRCOUNT, UPCOUNT,
 and ADCOUNT fields and associated sections, or placing zeros (0) in
 the these "count" fields and not including any part of the original
 update.  The QR bit is set to one (1), and the response is sent back
 to the requestor.  If the requestor used UDP, then the response will
 be sent to the requestor's source UDP port.  If the requestor used
 TCP, then the response will be sent back on the requestor's open TCP
 connection.

4 - Requestor Behaviour

 4.1. From a requestor's point of view, any authoritative server for
 the zone can appear to be able to process update requests, even
 though only the primary master server is actually able to modify the
 zone's master file.  Requestors are expected to know the name of the
 zone they intend to update and to know or be able to determine the
 name servers for that zone.

Vixie, et. al. Standards Track [Page 17] RFC 2136 DNS Update April 1997

 4.2. If update ordering is desired, the requestor will need to know
 the value of the existing SOA RR.  Requestors who update the SOA RR
 must update the SOA SERIAL field in a positive direction (as defined
 by [RFC1982]) and also preserve the other SOA fields unless the
 requestor's explicit intent is to change them.  The SOA SERIAL field
 must never be set to zero (0).
 4.3. If the requestor has reasonable cause to believe that all of a
 zone's servers will be equally reachable, then it should arrange to
 try the primary master server (as given by the SOA MNAME field if
 matched by some NS NSDNAME) first to avoid unnecessary forwarding
 inside the slave servers.  (Note that the primary master will in some
 cases not be reachable by all requestors, due to firewalls or network
 partitioning.)
 4.4. Once the zone's name servers been found and possibly sorted so
 that the ones more likely to be reachable and/or support the UPDATE
 opcode are listed first, the requestor composes an UPDATE message of
 the following form and sends it to the first name server on its list:
    ID:                        (new)
    Opcode:                    UPDATE
    Zone zcount:               1
    Zone zname:                (zone name)
    Zone zclass:               (zone class)
    Zone ztype:                T_SOA
    Prerequisite Section:      (see previous text)
    Update Section:            (see previous text)
    Additional Data Section:   (empty)
 4.5. If the requestor receives a response, and the response has an
 RCODE other than SERVFAIL or NOTIMP, then the requestor returns an
 appropriate response to its caller.
 4.6. If a response is received whose RCODE is SERVFAIL or NOTIMP, or
 if no response is received within an implementation dependent timeout
 period, or if an ICMP error is received indicating that the server's
 port is unreachable, then the requestor will delete the unusable
 server from its internal name server list and try the next one,
 repeating until the name server list is empty.  If the requestor runs
 out of servers to try, an appropriate error will be returned to the
 requestor's caller.

Vixie, et. al. Standards Track [Page 18] RFC 2136 DNS Update April 1997

5 - Duplicate Detection, Ordering and Mutual Exclusion

 5.1. For correct operation, mechanisms may be needed to ensure
 idempotence, order UPDATE requests and provide mutual exclusion.  An
 UPDATE message or response might be delivered zero times, one time,
 or multiple times.  Datagram duplication is of particular interest
 since it covers the case of the so-called "replay attack" where a
 correct request is duplicated maliciously by an intruder.
 5.2. Multiple UPDATE requests or responses in transit might be
 delivered in any order, due to network topology changes or load
 balancing, or to multipath forwarding graphs wherein several slave
 servers all forward to the primary master.  In some cases, it might
 be required that the earlier update not be applied after the later
 update, where "earlier" and "later" are defined by an external time
 base visible to some set of requestors, rather than by the order of
 request receipt at the primary master.
 5.3. A requestor can ensure transaction idempotence by explicitly
 deleting some "marker RR" (rather than deleting the RRset of which it
 is a part) and then adding a new "marker RR" with a different RDATA
 field.  The Prerequisite Section should specify that the original
 "marker RR" must be present in order for this UPDATE message to be
 accepted by the server.
 5.4. If the request is duplicated by a network error, all duplicate
 requests will fail since only the first will find the original
 "marker RR" present and having its known previous value.  The
 decisions of whether to use such a "marker RR" and what RR to use are
 left up to the application programmer, though one obvious choice is
 the zone's SOA RR as described below.
 5.5. Requestors can ensure update ordering by externally
 synchronizing their use of successive values of the "marker RR."
 Mutual exclusion can be addressed as a degenerate case, in that a
 single succession of the "marker RR" is all that is needed.
 5.6. A special case where update ordering and datagram duplication
 intersect is when an RR validly changes to some new value and then
 back to its previous value.  Without a "marker RR" as described
 above, this sequence of updates can leave the zone in an undefined
 state if datagrams are duplicated.
 5.7. To achieve an atomic multitransaction "read-modify-write" cycle,
 a requestor could first retrieve the SOA RR, and build an UPDATE
 message one of whose prerequisites was the old SOA RR.  It would then
 specify updates that would delete this SOA RR and add a new one with
 an incremented SOA SERIAL, along with whatever actual prerequisites

Vixie, et. al. Standards Track [Page 19] RFC 2136 DNS Update April 1997

 and updates were the object of the transaction.  If the transaction
 succeeds, the requestor knows that the RRs being changed were not
 otherwise altered by any other requestor.

6 - Forwarding

 When a zone slave forwards an UPDATE message upward toward the zone's
 primary master server, it must allocate a new ID and prepare to enter
 the role of "forwarding server," which is a requestor with respect to
 the forward server.
 6.1. The set of forward servers will be same as the set of servers
 this zone slave would use as the source of AXFR or IXFR data.  So,
 while the original requestor might have used the zone's NS RRset to
 locate its update server, a forwarder always forwards toward its
 designated zone master servers.
 6.2. If the original requestor used TCP, then the TCP connection from
 the requestor is still open and the forwarder must use TCP to forward
 the message.  If the original requestor used UDP, the forwarder may
 use either UDP or TCP to forward the message, at the whim of the
 implementor.
 6.3. It is reasonable for forward servers to be forwarders
 themselves, if the AXFR dependency graph being followed is a deep one
 involving firewalls and multiple connectivity realms.  In most cases
 the AXFR dependency graph will be shallow and the forward server will
 be the primary master server.
 6.4. The forwarder will not respond to its requestor until it
 receives a response from its forward server.  UPDATE transactions
 involving forwarders are therefore time synchronized with respect to
 the original requestor and the primary master server.
 6.5. When there are multiple possible sources of AXFR data and
 therefore multiple possible forward servers, a forwarder will use the
 same fallback strategy with respect to connectivity or timeout errors
 that it would use when performing an AXFR.  This is implementation
 dependent.
 6.6. When a forwarder receives a response from a forward server, it
 copies this response into a new response message, assigns its
 requestor's ID to that message, and sends the response back to the
 requestor.

Vixie, et. al. Standards Track [Page 20] RFC 2136 DNS Update April 1997

7 - Design, Implementation, Operation, and Protocol Notes

 Some of the principles which guided the design of this UPDATE
 specification are as follows.  Note that these are not part of the
 formal specification and any disagreement between this section and
 any other section of this document should be resolved in favour of
 the other section.
 7.1. Using metavalues for CLASS is possible only because all RRs in
 the packet are assumed to be in the same zone, and CLASS is an
 attribute of a zone rather than of an RRset.  (It is for this reason
 that the Zone Section is not optional.)
 7.2. Since there are no data-present or data-absent errors possible
 from processing the Update Section, any necessary data-present and
 data- absent dependencies should be specified in the Prerequisite
 Section.
 7.3. The Additional Data Section can be used to supply a server with
 out of zone glue that will be needed in referrals.  For example, if
 adding a new NS RR to HOME.VIX.COM specifying a nameserver called
 NS.AU.OZ, the A RR for NS.AU.OZ can be included in the Additional
 Data Section.  Servers can use this information or ignore it, at the
 discretion of the implementor.  We discourage caching this
 information for use in subsequent DNS responses.
 7.4. The Additional Data Section might be used if some of the RRs
 later needed for Secure DNS Update are not actually zone updates, but
 rather ancillary keys or signatures not intended to be stored in the
 zone (as an update would be), yet necessary for validating the update
 operation.
 7.5. It is expected that in the absence of Secure DNS Update, a
 server will only accept updates if they come from a source address
 that has been statically configured in the server's description of a
 primary master zone.  DHCP servers would be likely candidates for
 inclusion in this statically configured list.
 7.6. It is not possible to create a zone using this protocol, since
 there is no provision for a slave server to be told who its master
 servers are.  It is expected that this protocol will be extended in
 the future to cover this case.  Therefore, at this time, the addition
 of SOA RRs is unsupported.  For similar reasons, deletion of SOA RRs
 is also unsupported.

Vixie, et. al. Standards Track [Page 21] RFC 2136 DNS Update April 1997

 7.7. The prerequisite for specifying that a name own at least one RR
 differs semantically from QUERY, in that QUERY would return
 <NOERROR,ANCOUNT=0> rather than NXDOMAIN if queried for an RRset at
 this name, while UPDATE's prerequisite condition [Section 2.4.4]
 would NOT be satisfied.
 7.8. It is possible for a UDP response to be lost in transit and for
 a request to be retried due to a timeout condition.  In this case an
 UPDATE that was successful the first time it was received by the
 primary master might ultimately appear to have failed when the
 response to a duplicate request is finally received by the requestor.
 (This is because the original prerequisites may no longer be
 satisfied after the update has been applied.)  For this reason,
 requestors who require an accurate response code must use TCP.
 7.9. Because a requestor who requires an accurate response code will
 initiate their UPDATE transaction using TCP, a forwarder who receives
 a request via TCP must forward it using TCP.
 7.10. Deferral of SOA SERIAL autoincrements is made possible so that
 serial numbers can be conserved and wraparound at 2**32 can be made
 an infrequent occurance.  Visible (to DNS clients) SOA SERIALs need
 to differ if the zone differs.  Note that the Authority Section SOA
 in a QUERY response is a form of visibility, for the purposes of this
 prerequisite.
 7.11. A zone's SOA SERIAL should never be set to zero (0) due to
 interoperability problems with some older but widely installed
 implementations of DNS.  When incrementing an SOA SERIAL, if the
 result of the increment is zero (0) (as will be true when wrapping
 around 2**32), it is necessary to increment it again or set it to one
 (1).  See [RFC1982] for more detail on this subject.
 7.12. Due to the TTL minimalization necessary when caching an RRset,
 it is recommended that all TTLs in an RRset be set to the same value.
 While the DNS Message Format permits variant TTLs to exist in the
 same RRset, and this variance can exist inside a zone, such variance
 will have counterintuitive results and its use is discouraged.
 7.13. Zone cut management presents some obscure corner cases to the
 add and delete operations in the Update Section.  It is possible to
 delete an NS RR as long as it is not the last NS RR at the root of a
 zone.  If deleting all RRs from a name, SOA and NS RRs at the root of
 a zone are unaffected.  If deleting RRsets, it is not possible to
 delete either SOA or NS RRsets at the top of a zone.  An attempt to
 add an SOA will be treated as a replace operation if an SOA already
 exists, or as a no-op if the SOA would be new.

Vixie, et. al. Standards Track [Page 22] RFC 2136 DNS Update April 1997

 7.14. No semantic checking is required in the primary master server
 when adding new RRs.  Therefore a requestor can cause CNAME or NS or
 any other kind of RR to be added even if their target name does not
 exist or does not have the proper RRsets to make the original RR
 useful.  Primary master servers that DO implement this kind of
 checking should take great care to avoid out-of-zone dependencies
 (whose veracity cannot be authoritatively checked) and should
 implement all such checking during the prescan phase.
 7.15. Nonterminal or wildcard CNAMEs are not well specified by
 [RFC1035] and their use will probably lead to unpredictable results.
 Their use is discouraged.
 7.16. Empty nonterminals (nodes with children but no RRs of their
 own) will cause <NOERROR,ANCOUNT=0> responses to be sent in response
 to a query of any type for that name.  There is no provision for
 empty terminal nodes -- so if all RRs of a terminal node are deleted,
 the name is no longer in use, and queries of any type for that name
 will result in an NXDOMAIN response.
 7.17. In a deep AXFR dependency graph, it has not historically been
 an error for slaves to depend mutually upon each other.  This
 configuration has been used to enable a zone to flow from the primary
 master to all slaves even though not all slaves have continuous
 connectivity to the primary master.  UPDATE's use of the AXFR
 dependency graph for forwarding prohibits this kind of dependency
 loop, since UPDATE forwarding has no loop detection analagous to the
 SOA SERIAL pretest used by AXFR.
 7.18. Previously existing names which are occluded by a new zone cut
 are still considered part of the parent zone, for the purposes of
 zone transfers, even though queries for such names will be referred
 to the new subzone's servers.  If a zone cut is removed, all parent
 zone names that were occluded by it will again become visible to
 queries.  (This is a clarification of [RFC1034].)
 7.19. If a server is authoritative for both a zone and its child,
 then queries for names at the zone cut between them will be answered
 authoritatively using only data from the child zone.  (This is a
 clarification of [RFC1034].)

Vixie, et. al. Standards Track [Page 23] RFC 2136 DNS Update April 1997

 7.20. Update ordering using the SOA RR is problematic since there is
 no way to know which of a zone's NS RRs represents the primary
 master, and the zone slaves can be out of date if their SOA.REFRESH
 timers have not elapsed since the last time the zone was changed on
 the primary master.  We recommend that a zone needing ordered updates
 use only servers which implement NOTIFY (see [RFC1996]) and IXFR (see
 [RFC1995]), and that a client receiving a prerequisite error while
 attempting an ordered update simply retry after a random delay period
 to allow the zone to settle.

8 - Security Considerations

 8.1. In the absence of [RFC2137] or equivilent technology, the
 protocol described by this document makes it possible for anyone who
 can reach an authoritative name server to alter the contents of any
 zones on that server.  This is a serious increase in vulnerability
 from the current technology.  Therefore it is very strongly
 recommended that the protocols described in this document not be used
 without [RFC2137] or other equivalently strong security measures,
 e.g. IPsec.
 8.2. A denial of service attack can be launched by flooding an update
 forwarder with TCP sessions containing updates that the primary
 master server will ultimately refuse due to permission problems.
 This arises due to the requirement that an update forwarder receiving
 a request via TCP use a synchronous TCP session for its forwarding
 operation.  The connection management mechanisms of [RFC1035 4.2.2]
 are sufficient to prevent large scale damage from such an attack, but
 not to prevent some queries from going unanswered during the attack.

Acknowledgements

 We would like to thank the IETF DNSIND working group for their input
 and assistance, in particular, Rob Austein, Randy Bush, Donald
 Eastlake, Masataka Ohta, Mark Andrews, and Robert Elz.  Special
 thanks to Bill Simpson, Ken Wallich and Bob Halley for reviewing this
 document.

Vixie, et. al. Standards Track [Page 24] RFC 2136 DNS Update April 1997

References

 [RFC1035]
    Mockapetris, P., "Domain Names - Implementation and
    Specification", STD 13, RFC 1035, USC/Information Sciences
    Institute, November 1987.
 [RFC1982]
    Elz, R., "Serial Number Arithmetic", RFC 1982, University of
    Melbourne, August 1996.
 [RFC1995]
    Ohta, M., "Incremental Zone Transfer", RFC 1995, Tokyo Institute
    of Technology, August 1996.
 [RFC1996]
    Vixie, P., "A Mechanism for Prompt Notification of Zone Changes",
    RFC 1996, Internet Software Consortium, August 1996.
 [RFC2065]
    Eastlake, D., and C. Kaufman, "Domain Name System Protocol
    Security Extensions", RFC 2065, January 1997.
 [RFC2137]
    Eastlake, D., "Secure Domain Name System Dynamic Update", RFC
    2137, April 1997.

Authors' Addresses

 Yakov Rekhter
 Cisco Systems
 170 West Tasman Drive
 San Jose, CA 95134-1706
 Phone: +1 914 528 0090
 EMail: yakov@cisco.com
 Susan Thomson
 Bellcore
 445 South Street
 Morristown, NJ 07960
 Phone: +1 201 829 4514
 EMail: set@thumper.bellcore.com

Vixie, et. al. Standards Track [Page 25] RFC 2136 DNS Update April 1997

 Jim Bound
 Digital Equipment Corp.
 110 Spitbrook Rd ZK3-3/U14
 Nashua, NH 03062-2698
 Phone: +1 603 881 0400
 EMail: bound@zk3.dec.com
 Paul Vixie
 Internet Software Consortium
 Star Route Box 159A
 Woodside, CA 94062
 Phone: +1 415 747 0204
 EMail: paul@vix.com

Vixie, et. al. Standards Track [Page 26]

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