rfc2307.txt

Network Working Group                                          L. Howard
Request for Comments: 2307                        Independent Consultant
Category: Experimental                                        March 1998


      An Approach for Using LDAP as a Network Information Service

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

Abstract

   This document describes an experimental mechanism for mapping
   entities related to TCP/IP and the UNIX system into X.500 [X500]
   entries so that they may be resolved with the Lightweight Directory
   Access Protocol [RFC2251]. A set of attribute types and object
   classes are proposed, along with specific guidelines for interpreting
   them.

   The intention is to assist the deployment of LDAP as an
   organizational nameservice. No proposed solutions are intended as
   standards for the Internet. Rather, it is hoped that a general
   consensus will emerge as to the appropriate solution to such
   problems, leading eventually to the adoption of standards. The
   proposed mechanism has already been implemented with some success.

1. Background and Motivation

   The UNIX (R) operating system, and its derivatives (specifically,
   those which support TCP/IP and conform to the X/Open Single UNIX
   specification [XOPEN]) require a means of looking up entities, by
   matching them against search criteria or by enumeration. (Other
   operating systems that support TCP/IP may provide some means of
   resolving some of these entities. This schema is applicable to those
   environments also.)

   These entities include users, groups, IP services (which map names to
   IP ports and protocols, and vice versa), IP protocols (which map
   names to IP protocol numbers and vice versa), RPCs (which map names
   to ONC Remote Procedure Call [RFC1057] numbers and vice versa), NIS



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   netgroups, booting information (boot parameters and MAC address
   mappings), filesystem mounts, IP hosts and networks, and RFC822 mail
   aliases.

   Resolution requests are made through a set of C functions, provided
   in the UNIX system's C library. For example, the UNIX system utility
   "ls", which enumerates the contents of a filesystem directory, uses
   the C library function getpwuid() in order to map user IDs to login
   names. Once the request is made, it is resolved using a "nameservice"
   which is supported by the client library. The nameservice may be, at
   its simplest, a collection of files in the local filesystem which are
   opened and searched by the C library. Other common nameservices
   include the Network Information Service (NIS) and the Domain Name
   System (DNS). (The latter is typically used for resolving hosts,
   services and networks.) Both these nameservices have the advantage of
   being distributed and thus permitting a common set of entities to be
   shared amongst many clients.

   LDAP is a distributed, hierarchical directory service access protocol
   which is used to access repositories of users and other network-
   related entities. Because LDAP is often not tightly integrated with
   the host operating system, information such as users may need to be
   kept both in LDAP and in an operating system supported nameservice
   such as NIS. By using LDAP as the the primary means of resolving
   these entities, these redundancy issues are minimized and the
   scalability of LDAP can be exploited. (By comparison, NIS services
   based on flat files do not have the scalability or extensibility of
   LDAP or X.500.)

   The object classes and attributes defined below are suitable for
   representing the aforementioned entities in a form compatible with
   LDAP and X.500 directory services.

2. General Issues

2.1. Terminology

   The key words "MUST", "SHOULD", and "MAY" used in this document are
   to be interpreted as described in [RFC2119].

   For the purposes of this document, the term "nameservice" refers to a
   service, such as NIS or flat files, that is used by the operating
   system to resolve entities within a single, local naming context.
   Contrast this with a "directory service" such as LDAP, which supports
   extensible schema and multiple naming contexts.






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   The term "NIS-related entities" broadly refers to entities which are
   typically resolved using the Network Information Service. (NIS was
   previously known as YP.) Deploying LDAP for resolving these entities
   does not imply that NIS be used, as a gateway or otherwise. In
   particular, the host and network classes are generically applicable,
   and may be implemented on any system that wishes to use LDAP or X.500
   for host and network resolution.

   The "DUA" (directory user agent) refers to the LDAP client querying
   these entities, such as an LDAP to NIS gateway or the C library.  The
   "client" refers to the application which ultimately makes use of the
   information returned by the resolution. It is irrelevant whether the
   DUA and the client reside within the same address space. The act of
   the DUA making this information to the client is termed
   "republishing".

   To avoid confusion, the term "login name" refers to the user's login
   name (being the value of the uid attribute) and the term "user ID"
   refers to he user's integer identification number (being the value of
   the uidNumber attribute).

   The phrases "resolving an entity" and "resolution of entities" refer
   respectively to enumerating NIS-related entities of a given type, and
   matching them against a given search criterion. One or more entities
   are returned as a result of successful "resolutions" (a "match"
   operation will only return one entity).

   The use of the term UNIX does not confer upon this schema the
   endorsement of owners of the UNIX trademark. Where necessary, the
   term "TCP/IP entity" is used to refer to protocols, services, hosts,
   and networks, and the term "UNIX entity" to its complement. (The
   former category does not mandate the host operating system supporting
   the interfaces required for resolving UNIX entities.)

   The OIDs defined below are derived from iso(1) org(3) dod(6)
   internet(1) directory(1) nisSchema(1).

2.2. Attributes

   The attributes and classes defined in this document are summarized
   below.

   The following attributes are defined in this document:

           uidNumber
           gidNumber
           gecos
           homeDirectory



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           loginShell
           shadowLastChange
           shadowMin
           shadowMax
           shadowWarning
           shadowInactive
           shadowExpire
           shadowFlag
           memberUid
           memberNisNetgroup
           nisNetgroupTriple
           ipServicePort
           ipServiceProtocol
           ipProtocolNumber
           oncRpcNumber
           ipHostNumber
           ipNetworkNumber
           ipNetmaskNumber
           macAddress
           bootParameter
           bootFile
           nisMapName
           nisMapEntry

   Additionally, some of the attributes defined in [RFC2256] are
   required.

2.3. Object classes

   The following object classes are defined in this document:

           posixAccount
           shadowAccount
           posixGroup
           ipService
           ipProtocol
           oncRpc
           ipHost
           ipNetwork
           nisNetgroup
           nisMap
           nisObject
           ieee802Device
           bootableDevice

   Additionally, some of the classes defined in [RFC2256] are required.





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2.4. Syntax definitions

   The following syntax definitions [RFC2252] are used by this schema.
   The nisNetgroupTripleSyntax represents NIS netgroup triples:

           ( nisSchema.0.0 NAME 'nisNetgroupTripleSyntax'
             DESC 'NIS netgroup triple' )

   Values in this syntax are represented by the following:

        nisnetgrouptriple = "(" hostname "," username "," domainname ")"
        hostname          = "" / "-" / keystring
        username          = "" / "-" / keystring
        domainname        = "" / "-" / keystring

   X.500 servers may use the following representation of the above
   syntax:

        nisNetgroupTripleSyntax ::= SEQUENCE {
         hostname  [0] IA5String OPTIONAL,
         username  [1] IA5String OPTIONAL,
         domainname  [2] IA5String OPTIONAL
        }

   The bootParameterSyntax syntax represents boot parameters:

           ( nisSchema.0.1 NAME 'bootParameterSyntax'
             DESC 'Boot parameter' )

   where:

        bootparameter     = key "=" server ":" path
        key               = keystring
        server            = keystring
        path              = keystring

   X.500 servers may use the following representation of the above
   syntax:

        bootParameterSyntax ::= SEQUENCE {
         key     IA5String,
         server  IA5String,
         path    IA5String
        }

   Values adhering to these syntaxes are encoded as strings by LDAP
   servers.




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3. Attribute definitions

   This section contains attribute definitions to be implemented by DUAs
   supporting this schema.

        ( nisSchema.1.0 NAME 'uidNumber'
          DESC 'An integer uniquely identifying a user in an
                administrative domain'
          EQUALITY integerMatch SYNTAX 'INTEGER' SINGLE-VALUE )

        ( nisSchema.1.1 NAME 'gidNumber'
          DESC 'An integer uniquely identifying a group in an
                administrative domain'
          EQUALITY integerMatch SYNTAX 'INTEGER' SINGLE-VALUE )

        ( nisSchema.1.2 NAME 'gecos'
          DESC 'The GECOS field; the common name'
          EQUALITY caseIgnoreIA5Match
          SUBSTRINGS caseIgnoreIA5SubstringsMatch
          SYNTAX 'IA5String' SINGLE-VALUE )

        ( nisSchema.1.3 NAME 'homeDirectory'
          DESC 'The absolute path to the home directory'
          EQUALITY caseExactIA5Match
          SYNTAX 'IA5String' SINGLE-VALUE )

        ( nisSchema.1.4 NAME 'loginShell'
          DESC 'The path to the login shell'
          EQUALITY caseExactIA5Match
          SYNTAX 'IA5String' SINGLE-VALUE )

        ( nisSchema.1.5 NAME 'shadowLastChange'
          EQUALITY integerMatch
          SYNTAX 'INTEGER' SINGLE-VALUE )

        ( nisSchema.1.6 NAME 'shadowMin'
          EQUALITY integerMatch
          SYNTAX 'INTEGER' SINGLE-VALUE )

        ( nisSchema.1.7 NAME 'shadowMax'
          EQUALITY integerMatch
          SYNTAX 'INTEGER' SINGLE-VALUE )

        ( nisSchema.1.8 NAME 'shadowWarning'
          EQUALITY integerMatch
          SYNTAX 'INTEGER' SINGLE-VALUE )

        ( nisSchema.1.9 NAME 'shadowInactive'



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          EQUALITY integerMatch
          SYNTAX 'INTEGER' SINGLE-VALUE )

        ( nisSchema.1.10 NAME 'shadowExpire'
          EQUALITY integerMatch
          SYNTAX 'INTEGER' SINGLE-VALUE )

        ( nisSchema.1.11 NAME 'shadowFlag'
          EQUALITY integerMatch
          SYNTAX 'INTEGER' SINGLE-VALUE )

        ( nisSchema.1.12 NAME 'memberUid'
          EQUALITY caseExactIA5Match
          SUBSTRINGS caseExactIA5SubstringsMatch
          SYNTAX 'IA5String' )

        ( nisSchema.1.13 NAME 'memberNisNetgroup'
          EQUALITY caseExactIA5Match
          SUBSTRINGS caseExactIA5SubstringsMatch
          SYNTAX 'IA5String' )

        ( nisSchema.1.14 NAME 'nisNetgroupTriple'
          DESC 'Netgroup triple'
          SYNTAX 'nisNetgroupTripleSyntax' )

        ( nisSchema.1.15 NAME 'ipServicePort'
          EQUALITY integerMatch
          SYNTAX 'INTEGER' SINGLE-VALUE )

        ( nisSchema.1.16 NAME 'ipServiceProtocol'
          SUP name )

        ( nisSchema.1.17 NAME 'ipProtocolNumber'
          EQUALITY integerMatch
          SYNTAX 'INTEGER' SINGLE-VALUE )

        ( nisSchema.1.18 NAME 'oncRpcNumber'
          EQUALITY integerMatch
          SYNTAX 'INTEGER' SINGLE-VALUE )

        ( nisSchema.1.19 NAME 'ipHostNumber'
          DESC 'IP address as a dotted decimal, eg. 192.168.1.1,
                omitting leading zeros'
          EQUALITY caseIgnoreIA5Match
          SYNTAX 'IA5String{128}' )

        ( nisSchema.1.20 NAME 'ipNetworkNumber'
          DESC 'IP network as a dotted decimal, eg. 192.168,



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                omitting leading zeros'
          EQUALITY caseIgnoreIA5Match
          SYNTAX 'IA5String{128}' SINGLE-VALUE )

        ( nisSchema.1.21 NAME 'ipNetmaskNumber'
          DESC 'IP netmask as a dotted decimal, eg. 255.255.255.0,
                omitting leading zeros'
          EQUALITY caseIgnoreIA5Match
          SYNTAX 'IA5String{128}' SINGLE-VALUE )

        ( nisSchema.1.22 NAME 'macAddress'
          DESC 'MAC address in maximal, colon separated hex
                notation, eg. 00:00:92:90:ee:e2'
          EQUALITY caseIgnoreIA5Match
          SYNTAX 'IA5String{128}' )

        ( nisSchema.1.23 NAME 'bootParameter'
          DESC 'rpc.bootparamd parameter'
          SYNTAX 'bootParameterSyntax' )

        ( nisSchema.1.24 NAME 'bootFile'
          DESC 'Boot image name'
          EQUALITY caseExactIA5Match
          SYNTAX 'IA5String' )

        ( nisSchema.1.26 NAME 'nisMapName'
          SUP name )

        ( nisSchema.1.27 NAME 'nisMapEntry'
          EQUALITY caseExactIA5Match
          SUBSTRINGS caseExactIA5SubstringsMatch
          SYNTAX 'IA5String{1024}' SINGLE-VALUE )

4. Class definitions

   This section contains class definitions to be implemented by DUAs
   supporting the schema.

   The rfc822MailGroup object class MAY be used to represent a mail
   group for the purpose of alias expansion. Several alternative schemes
   for mail routing and delivery using LDAP directories, which are
   outside the scope of this document.

        ( nisSchema.2.0 NAME 'posixAccount' SUP top AUXILIARY
          DESC 'Abstraction of an account with POSIX attributes'
          MUST ( cn $ uid $ uidNumber $ gidNumber $ homeDirectory )
          MAY ( userPassword $ loginShell $ gecos $ description ) )




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        ( nisSchema.2.1 NAME 'shadowAccount' SUP top AUXILIARY
          DESC 'Additional attributes for shadow passwords'
          MUST uid
          MAY ( userPassword $ shadowLastChange $ shadowMin
                shadowMax $ shadowWarning $ shadowInactive $
                shadowExpire $ shadowFlag $ description ) )

        ( nisSchema.2.2 NAME 'posixGroup' SUP top STRUCTURAL
          DESC 'Abstraction of a group of accounts'
          MUST ( cn $ gidNumber )
          MAY ( userPassword $ memberUid $ description ) )

        ( nisSchema.2.3 NAME 'ipService' SUP top STRUCTURAL
          DESC 'Abstraction an Internet Protocol service.
                Maps an IP port and protocol (such as tcp or udp)
                to one or more names; the distinguished value of
                the cn attribute denotes the service's canonical
                name'
          MUST ( cn $ ipServicePort $ ipServiceProtocol )
          MAY ( description ) )

        ( nisSchema.2.4 NAME 'ipProtocol' SUP top STRUCTURAL
          DESC 'Abstraction of an IP protocol. Maps a protocol number
                to one or more names. The distinguished value of the cn
                attribute denotes the protocol's canonical name'
          MUST ( cn $ ipProtocolNumber $ description )
          MAY description )

        ( nisSchema.2.5 NAME 'oncRpc' SUP top STRUCTURAL
          DESC 'Abstraction of an Open Network Computing (ONC)
               [RFC1057] Remote Procedure Call (RPC) binding.
               This class maps an ONC RPC number to a name.
               The distinguished value of the cn attribute denotes
               the RPC service's canonical name'
          MUST ( cn $ oncRpcNumber $ description )
          MAY description )

        ( nisSchema.2.6 NAME 'ipHost' SUP top AUXILIARY

          DESC 'Abstraction of a host, an IP device. The distinguished
                value of the cn attribute denotes the host's canonical
                name. Device SHOULD be used as a structural class'
          MUST ( cn $ ipHostNumber )
          MAY ( l $ description $ manager ) )

        ( nisSchema.2.7 NAME 'ipNetwork' SUP top STRUCTURAL
          DESC 'Abstraction of a network. The distinguished value of
                the cn attribute denotes the network's canonical name'



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          MUST ( cn $ ipNetworkNumber )
          MAY ( ipNetmaskNumber $ l $ description $ manager ) )

        ( nisSchema.2.8 NAME 'nisNetgroup' SUP top STRUCTURAL
          DESC 'Abstraction of a netgroup. May refer to other netgroups'
          MUST cn
          MAY ( nisNetgroupTriple $ memberNisNetgroup $ description ) )

        ( nisSchema.2.09 NAME 'nisMap' SUP top STRUCTURAL
          DESC 'A generic abstraction of a NIS map'
          MUST nisMapName
          MAY description )

        ( nisSchema.2.10 NAME 'nisObject' SUP top STRUCTURAL
          DESC 'An entry in a NIS map'
          MUST ( cn $ nisMapEntry $ nisMapName )
          MAY description )

        ( nisSchema.2.11 NAME 'ieee802Device' SUP top AUXILIARY
          DESC 'A device with a MAC address; device SHOULD be
                used as a structural class'
          MAY macAddress )

        ( nisSchema.2.12 NAME 'bootableDevice' SUP top AUXILIARY
          DESC 'A device with boot parameters; device SHOULD be
                used as a structural class'
          MAY ( bootFile $ bootParameter ) )

5. Implementation details

5.1. Suggested resolution methods

   The preferred means of directing a client application (one using the
   shared services of the C library) to use LDAP as its information
   source for the functions listed in 5.2 is to modify the source code
   to directly query LDAP. As the source to commercial C libraries and
   applications is rarely available to the end-user, one could emulate a
   supported nameservice (such as NIS). (This is also an appropriate
   opportunity to perform caching of entries across process address
   spaces.) In the case of NIS, reference implementations are widely
   available and the RPC interface is well known.

   The means by which the operating system is directed to use LDAP is
   implementation dependent. For example, some operating systems and C
   libraries support end-user extensible resolvers using dynamically
   loadable libraries and a nameservice "switch". The means in which the
   DUA locates LDAP servers is also implementation dependent.




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5.2. Affected library functions

   The following functions are typically found in the C libraries of
   most UNIX and POSIX compliant systems. An LDAP search filter
   [RFC2254] which may be used to satisfy the function call is included
   alongside each function name. Parameters are denoted by %s and %d for
   string and integer arguments, respectively. Long lines are broken.

        getpwnam()              (&(objectClass=posixAccount)(uid=%s))
        getpwuid()              (&(objectClass=posixAccount)
                                (uidNumber=%d))
        getpwent()              (objectClass=posixAccount)

        getspnam()              (&(objectClass=shadowAccount)(uid=%s))
        getspent()              (objectClass=shadowAccount)

        getgrnam()              (&(objectClass=posixGroup)(cn=%s))
        getgrgid()              (&(objectClass=posixGroup)
                                (gidNumber=%d))
        getgrent()              (objectClass=posixGroup)

        getservbyname()         (&(objectClass=ipService)
                                (cn=%s)(ipServiceProtocol=%s))
        getservbyport()         (&(objectClass=ipService)
                                (ipServicePort=%d)
                                (ipServiceProtocol=%s))
        getservent()            (objectClass=ipService)

        getrpcbyname()          (&(objectClass=oncRpc)(cn=%s))
        getrpcbynumber()        (&(objectClass=oncRpc)(oncRpcNumber=%d))
        getrpcent()             (objectClass=oncRpc)

        getprotobyname()        (&(objectClass=ipProtocol)(cn=%s))
        getprotobynumber()      (&(objectClass=ipProtocol)
                                (ipProtocolNumber=%d))
        getprotoent()           (objectClass=ipProtocol)

        gethostbyname()         (&(objectClass=ipHost)(cn=%s))
        gethostbyaddr()         (&(objectClass=ipHost)(ipHostNumber=%s))
        gethostent()            (objectClass=ipHost)

        getnetbyname()          (&(objectClass=ipNetwork)(cn=%s))
        getnetbyaddr()          (&(objectClass=ipNetwork)
                                (ipNetworkNumber=%s))
        getnetent()             (objectClass=ipNetwork)

        setnetgrent()           (&(objectClass=nisNetgroup)(cn=%s))




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5.3. Interpreting user and group entries

   User and group resolution is initiated by the functions prefixed by
   getpw and getgr respectively. The uid attribute contains the user's
   login name. The cn attribute, in posixGroup entries, contains the
   group's name.

   The account object class provides a convenient structural class for
   posixAccount, and SHOULD be used where additional attributes are not
   required.

   It is suggested that uid and cn are used as the RDN attribute type
   for posixAccount and posixGroup entries, respectively.

   An account's GECOS field is preferably determined by a value of the
   gecos attribute. If no gecos attribute exists, the value of the cn
   attribute MUST be used. (The existence of the gecos attribute allows
   information embedded in the GECOS field, such as a user's telephone
   number, to be returned to the client without overloading the cn
   attribute. It also accommodates directories where the common name
   does not contain the user's full name.)

   An entry of class posixAccount, posixGroup, or shadowAccount without
   a userPassword attribute MUST NOT be used for authentication. The
   client should be returned a non-matchable password such as "x".

   userPassword values MUST be represented by following syntax:

        passwordvalue          = schemeprefix encryptedpassword
        schemeprefix           = "{" scheme "}"
        scheme                 = "crypt" / "md5" / "sha" / altscheme
        altscheme              = "x-" keystring
        encryptedpassword      = encrypted password

   The encrypted password contains of a plaintext key hashed using the
   algorithm scheme.

   userPassword values which do not adhere to this syntax MUST NOT be
   used for authentication. The DUA MUST iterate through the values of
   the attribute until a value matching the above syntax is found. Only
   if encryptedpassword is an empty string does the user have no
   password. DUAs are not required to consider encryption schemes which
   the client will not recognize; in most cases, it may be sufficient to
   consider only "crypt".

   Below is an example of a userPassword attribute:

                    userPassword: {crypt}X5/DBrWPOQQaI



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   A future standard may specify LDAP v3 attribute descriptions to
   represent hashed userPasswords, as noted below. This schema MUST NOT
   be used with LDAP v2 DUAs and DSAs.

        attributetype           = attributename sep attributeoption
        attributename           = "userPassword"
        sep                     = ";"
        attributeoption         = schemeclass "-" scheme
        schemeclass             = "hash" / altschemeclass
        scheme                  = "crypt" / "md5" / "sha" / altscheme
        altschemeclass          = "x-" keystring
        altscheme               = keystring


   Below is an example of a userPassword attribute, represented with an
   LDAP v3 attribute description:

           userPassword;hash-crypt: X5/DBrWPOQQaI


   A DUA MAY utilise the attributes in the shadowAccount class to
   provide shadow password service (getspnam() and getspent()). In such
   cases, the DUA MUST NOT make use of the userPassword attribute for
   getpwnam() et al, and MUST return a non-matchable password (such as
   "x") to the client instead.

5.4. Interpreting hosts and networks

   The ipHostNumber and ipNetworkNumber attributes are defined in
   preference to dNSRecord (defined in [RFC1279]), in order to simplify
   the DUA's role in interpreting entries in the directory. A dNSRecord
   expresses a complete resource record, including time to live and
   class data, which is extraneous to this schema.

   Additionally, the ipHost and ipNetwork classes permit a host or
   network (respectively) and all its aliases to be represented by a
   single entry in the directory. This is not necessarily possible if a
   DNS resource record is mapped directly to an LDAP entry.
   Implementations that wish to use LDAP to master DNS zone information
   are not precluded from doing so, and may simply avoid the ipHost and
   ipNetwork classes.

   This document redefines, although not exclusively, the ipNetwork
   class defined in [RFC1279], in order to achieve consistent naming
   with ipHost. The ipNetworkNumber attribute is also used in the
   siteContact object class [ROSE].





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   The trailing zeros in a network address MUST be omitted. CIDR-style
   network addresses (eg. 192.168.1/24) MAY be used.

   Hosts with IPv6 addresses MUST be written in their "preferred" form
   as defined in section 2.2.1 of [RFC1884], such that all components of
   the address are indicated and leading zeros are omitted. This
   provides a consistent means of resolving ipHosts by address.

5.5. Interpreting other entities

   In general, a one-to-one mapping between entities and LDAP entries is
   proposed, in that each entity has exactly one representation in the
   DIT. In some cases this is not feasible; for example, a service which
   is represented in more than one protocol domain. Consider the
   following entry:

           dn: cn=domain, dc=aja, dc=com
           cn: domain
           cn: nameserver
           objectClass: top
           objectClass: ipService
           ipServicePort: 53
           ipServiceProtocol: tcp
           ipServiceProtocol: udp

   This entry MUST map to the following two (2) services entities:

           domain  53/tcp  nameserver
           domain  53/udp  nameserver

   While the above two entities may be represented as separate LDAP
   entities, with different distinguished names (such as
   cn=domain+ipServiceProtocol=tcp, ... and
   cn=domain+ipServiceProtocol=udp, ...) it is convenient to represent
   them as a single entry. (If a service is represented in multiple
   protocol domains with different ports, then multiple entries are
   required; multivalued RDNs may be used to distinguish them.)

   With the exception of userPassword values, which are parsed according
   to the syntax considered in section 5.2, any empty values (consisting
   of a zero length string) are returned by the DUA to the client. The
   DUA MUST reject any entries which do not conform to the schema
   (missing mandatory attributes). Non-conforming entries SHOULD be
   ignored while enumerating entries.

   The nisObject object class MAY be used as a generic means of
   representing NIS entities. Its use is not encouraged; where support
   for entities not described in this schema is desired, an appropriate



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   schema should be devised. Implementors are strongly advised to
   support end-user extensible mappings between NIS entities and object
   classes. (Where the nisObject class is used, the nisMapName attribute
   may be used as a RDN.)

5.6. Canonicalizing entries with multi-valued naming attributes

   For entities such as hosts, services, networks, protocols, and RPCs,
   where there may be one or more aliases, the respective entry's
   relative distinguished name SHOULD be used to determine the canonical
   name.  Any other values for the same attribute are used as aliases.
   For example, the service described in section 5.5 has the canonical
   name "domain" and exactly one alias, "nameserver".

   The schema in this document generally only defines one attribute per
   class which is suitable for distinguishing an entity (excluding any
   attributes with integer syntax; it is assumed that entries will be
   distinguished on name). Usually, this is the common name (cn)
   attribute.  This aids the DUA in determining the canonical name of an
   entity, as it can examine the value of the relative distinguished
   name. Aliases are thus any values of the distinguishing attribute
   (such as cn) which do not match the canonical name of the entity.

   In the event that a different attribute is used to distinguish the
   entry, as may be the case where these object classes are used as
   auxiliary classes, the entry's canonical name may not be present in
   the RDN. In this case, the DUA MUST choose one of the non-
   distinguished values to represent the entity's canonical name. As the
   directory server guarantees no ordering of attribute values, it may
   not be possible to distinguish an entry deterministically. This
   ambiguity SHOULD NOT be resolved by mapping one directory entry into
   multiple entities.

6. Implementation focus

   A NIS server which uses LDAP instead of local files has been
   developed which supports the schema defined in this document.

   A reference implementation of the C library resolution code has been
   written for the Free Software Foundation. It may support other C
   libraries which support the Name Service Switch (NSS) or the
   Information Retrieval Service (IRS).

   The author has made available a freely distributable set of scripts
   which parses local databases such as /etc/passwd and /etc/hosts into
   a form suitable for loading into an LDAP server.





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7. Security Considerations

   The entirety of related security considerations are outside the scope
   of this document. It is noted that making passwords encrypted with a
   widely understood hash function (such as crypt()) available to non-
   privileged users is dangerous because it exposes them to dictionary
   and brute-force attacks.  This is proposed only for compatibility
   with existing UNIX system implementations. Sites where security is
   critical SHOULD consider using a strong authentication service for
   user authentication.

   Alternatively, the encrypted password could be made available only to
   a subset of privileged DUAs, which would provide "shadow" password
   service to client applications. This may be difficult to enforce.

   Because the schema represents operating system-level entities, access
   to these entities SHOULD be granted on a discretionary basis. (There
   is little point in restricting access to data which will be
   republished without restriction, however.) It is particularly
   important that only administrators can modify entries defined in this
   schema, with the exception of allowing a principal to change their
   password (which may be done on behalf of the user by a client bound
   as a superior principal, such that password restrictions may be
   enforced). For example, if a user were allowed to change the value of
   their uidNumber attribute, they could subvert security by
   equivalencing their account with the superuser account.

   A subtree of the DIT which is to be republished by a DUA (such as a
   NIS gateway) SHOULD be within the same administrative domain that the
   republishing DUA represents. (For example, principals outside an
   organization, while conceivably part of the DIT, should not be
   considered with the same degree of authority as those within the
   organization.)

   Finally, care should be exercised with integer attributes of a
   sensitive nature (particularly the uidNumber and gidNumber
   attributes) which contain zero-length values. DUAs MAY treat such
   values as corresponding to the "nobody" or "nogroup" user and group,
   respectively.

8. Acknowledgements

   Thanks to Leif Hedstrom of Netscape Communications Corporation,
   Michael Grant and Rosanna Lee of Sun Microsystems Inc., Ed Reed of
   Novell Inc., and Mark Wahl of Critical Angle Inc. for their valuable
   contributions to the development of this schema. Thanks to Andrew
   Josey of The Open Group for clarifying the use of the UNIX trademark,
   and to Tim Howes and Peter J. Cherny for their support.



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   UNIX is a registered trademark of The Open Group.

9. References

   [RFC1057]
        Sun Microsystems, Inc., "RPC: Remote Procedure Call: Protocol
        Specification Version 2", RFC 1057, June 1988.

   [RFC1279]
        Kille, S., "X.500 and Domains", RFC 1279, November 1991.

   [RFC1884]
        Hinden, R., and S. Deering, "IP Version 6 Addressing
        Architecture", RFC 1884, December 1995.

   [RFC2119]
        Bradner, S., "Key Words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [RFC2251]
        Wahl, M., Howes, T., and S. Kille, "Lightweight Directory Access
        Protocol (v3)", RFC 2251, December 1997.

   [RFC2252]
        Wahl, M., Coulbeck, A., Howes, T., and S. Kille, "Lightweight
        Directory Access Protocol (v3): Attribute Syntax Definitions",
        RFC 2252, December 1997.

   [RFC2254]
        Howes, T., "The String Representation of LDAP Search Filters",
        RFC 2254, December 1997.

   [RFC2256]
        Wahl, M., "A Summary of the X.500(96) User Schema for use with
        LDAPv3", RFC 2256, December 1997.

   [ROSE]
        M. T. Rose, "The Little Black Book: Mail Bonding with OSI
        Directory Services", ISBN 0-13-683210-5, Prentice-Hall, Inc.,
        1992.

   [X500]
        "Information Processing Systems - Open Systems Interconnection -
        The Directory: Overview of Concepts, Models and Service",
        ISO/IEC JTC 1/SC21, International Standard 9594-1, 1988.






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   [XOPEN]
        ISO/IEC 9945-1:1990, Information Technology - Portable Operating
        Systems Interface (POSIX) - Part 1: Systems Application
        Programming Interface (API) [C Language]

10. Author's Address

   Luke Howard
   PO Box 59
   Central Park Vic 3145
   Australia

   EMail: lukeh@xedoc.com