Application Working Group L. Howard
INTERNET-DRAFT Independent Consultant
Expires in six months from 17 November 1997
Intended Category: Experimental
An Approach for Using LDAP as a Network Information Service
<draft-howard-nis-schema-01.txt>
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
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Distribution of this document is unlimited.
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 [LDAPV3]. 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.
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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
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
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2.1. Terminology
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.
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
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below.
The following attributes are defined in this document:
uidNumber
gidNumber
gecos
homeDirectory
loginShell
shadowLastChange
shadowMin
shadowMax
shadowWarning
shadowInactive
shadowExpire
shadowFlag
memberUid
memberNisNetgroup
nisNetgroupTriple
ipServicePort
ipServiceProtocol
ipProtocolNumber
oncRpcNumber
ipHostNumber
ipNetworkNumber
ipNetmaskNumber
macAddress
bootParameter
bootFile
automountInformation
nisMapName
nisMapEntry
Additionally, some of the attributes defined in [LDAPATTRS] and
[LDAPDOMAINS] are required.
2.3. Object classes
The following object classes are defined in this document:
posixAccount
shadowAccount
posixGroup
ipService
ipProtocol
oncRpc
ipHost
ipNetwork
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nisNetgroup
automount
nisObject
Additionally, some of the classes defined in [LDAPATTRS] and
[LDAPDOMAINS] are required.
2.4. Syntax definitions
The following syntax definitions [LDAPATTRS] are used by this schema.
The nisNetgroupTripleSyntax encodes 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
The bootParameterSyntax syntax encodes boot parameters:
( nisSchema.0.1 NAME 'bootParameterSyntax'
DESC 'Boot parameter' )
where:
bootparameter = key "=" value
key = keystring
value = keystring
Values adhering to these syntaxes are encoded as strings.
3. Attribute definitions
This section contains attribute definitions which must be implemented
by DUAs supporting the 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'
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EQUALITY integerMatch SYNTAX 'INTEGER' SINGLE-VALUE )
( nisSchema.1.2 NAME 'gecos'
DESC 'The GECOS field; the user's common name'
EQUALITY caseIgnoreIA5Match
SUBSTRINGS caseIgnoreIA5SubstringsMatch
SYNTAX 'IA5String' SINGLE-VALUE )
( nisSchema.1.3 NAME 'homeDirectory'
DESC 'The absolute path of the user's home directory'
EQUALITY caseExactIA5Match
SYNTAX 'IA5String' SINGLE-VALUE )
( nisSchema.1.4 NAME 'loginShell'
DESC 'The absolute path of the user's 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'
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
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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'
EQUALITY caseIgnoreIA5Match
SYNTAX 'IA5String' )
( 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'
EQUALITY caseIgnoreIA5Match
SYNTAX 'IA5String{128}' )
( nisSchema.1.20 NAME 'ipNetworkNumber'
DESC 'IP network as a dotted decimal, eg. 192.168'
EQUALITY caseIgnoreIA5Match
SYNTAX 'IA5String{128}' )
( nisSchema.1.21 NAME 'ipNetmaskNumber'
DESC 'IP netmask as a dotted decimal, eg. 255.255.255.0'
EQUALITY caseIgnoreIA5Match
SYNTAX 'IA5String{128}' )
( nisSchema.1.22 NAME 'macAddress'
DESC 'MAC address in colon-separated hex notation, for
example 0:0:92:90:ee:e2'
EQUALITY caseIgnoreIA5Match
SYNTAX 'IA5String{128}' )
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( 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.1.25 NAME 'automountInformation'
DESC 'An entry in an automount map'
EQUALITY caseExactIA5Match
SUBSTRINGS caseExactIA5SubstringsMatch
SYNTAX 'IA5String' )
( nisSchema.1.26 NAME 'nisMapName'
EQUALITY caseExactIA5Match
SUBSTRINGS caseExactIA5SubstringsMatch
SYNTAX 'IA5String{1024}' SINGLE-VALUE )
( nisSchema.1.27 NAME 'nisMapEntry'
EQUALITY caseExactIA5Match
SUBSTRINGS caseExactIA5SubstringsMatch
SYNTAX 'IA5String{1024}' SINGLE-VALUE )
4. Class definitions
This section contains class definitions which must be implemented by
DUAs supporting the schema.
The rfc822MailGroup object class may 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 ) )
( 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 ) )
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( 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 STRUCTURAL
DESC 'Abstraction of a host. See section 5.4.'
MUST ( ipHostNumber $ associatedDomain )
MAY ( macAddress $ bootParameter $ bootFile $ dc $
l $ description $ manager $ serialNumber ) )
( nisSchema.2.7 NAME 'ipNetwork' SUP top
STRUCTURAL
DESC 'Abstraction of a network. See section 5.4.'
MUST ( ipNetworkNumber $ associatedDomain )
MAY ( ipNetmaskNumber $ l $ description $ manager $ dc ) )
( 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.2.9 NAME 'automount' SUP top STRUCTURAL
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DESC 'Abstraction of an automount map; each entry in the map
is represented by a value of the automountInformation
attribute. The map name is given by the cn attribute.
Each value of the automountInformation attribute
constitutes a mount entry.'
MUST cn
MAY ( automountInformation $ description ) )
( nisSchema.2.10 NAME 'nisObject' SUP top STRUCTURAL
DESC 'Abstraction of a generic NIS map or entry.'
MUST nisMapName
MAY ( cn $ nisMapEntry $ description ) )
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.
There exists no standard mechanism, other than NIS, for resolving
automount and nisObject entries. The former may be supported by the
automounter itself; both classes should be supported by an LDAP to
NIS gateway. DUAs implementing this schema are not required to
support all the classes defined herein.
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. It is
anticipated that DNS SRV records [RFC2052] may be used for this.
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
[LDAPFILT] which may be used to satisfy the function call is included
alongside each function name. Parameters are denoted by %s and %d for
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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)
(associatedDomain=%s))
gethostbyaddr() (&(objectClass=ipHost)(ipHostNumber=%s))
gethostent() (objectClass=ipHost)
getnetbyname() (&(objectClass=ipNetwork)
(associatedDomain=%s))
getnetbyaddr() (&(objectClass=ipNetwork)
(ipNetworkNumber=%s))
getnetent() (objectClass=ipNetwork)
setnetgrent() (&(objectClass=nisNetgroup)(cn=%s))
getaliasbyname() (&(objectClass=rfc822MailGroup)(cn=%s))
getaliasent() (objectClass=rfc822MailGroup)
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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 be denied the opportunity to
authenticate. 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 the above 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:
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userPassword: {crypt}X5/DBrWPOQQaI
A future standard may use LDAP v3 attribute descriptions to represent
hashed userPasswords, as noted below. This schema should 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 means for representing DNS [RFC1034] domains in LDAP
distinguished names described in [RFC1279] and [LDAPDOMAINS] are
incorporated into this schema. However, the ipHostNumber and
ipNetworkNumber attributes are defined in preference to dNSRecord, 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.
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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].
The trailing zeros in a network address must be omitted. CIDR-style
network addresses (eg. 192.168.1/24) may be used.
The associatedDomain attribute must contain the host or network's
canonical, qualified domain name, and any other names by which the
host or network is known. This should include the host's or network's
non qualified name. When interrogated the directory for ipHosts, the
associatedDomain attribute must be used in the search filter.
The use of the dc attribute to distinguish hosts is recommended, but
not required. If used, it must contain the host's non-qualified name.
It has no role in interrogation.
If an entry of class ipHost or ipNetwork belongs to a naming context
containing relative distinguished names (RDNs) [LDAPDN] of attribute
type dc (domainComponent), the distinguished name (DN) may be used to
select which value of associatedDomain is the canonical host name.
Such a DN is mapped to a domain name by concatenating each RDN value
with a period ('.'). For example, an entry of class ipHost with a DN
of dc=foo, dc=bar, dc=edu or dc=foo, dc=bar, dc=edu, o=Internet may
select the domain foo.bar.edu from the values of associatedDomain.
The mapping must terminate at the first relative distinguished name
which is not a domainComponent. This mapping must not be applied if
the entry's RDN type is not dc. Instead, associatedDomain alone must
determine the hostname.
Hosts with IPv6 addresses should 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
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objectClass: top
objectClass: ipService
ipServicePort: 53
ipServiceProtocol: tcp
ipServiceProtocol: udp
This entry maps 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.)
Entries of class automount inherently represent more than one entity:
each value of the automountInformation attribute may be a record in a
NIS database.
With the exception of userPassword values, which must be 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
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 services, protocols, and RPCs, where there may
be one or more aliases, the respective entry's relative distinguished
name should be used to form 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'.
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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. (For users, either the cn or uid attributes may be used
to canonicalize an entry. For hosts and networks, the distinguished
name may be considered per section 5.4.) name is considered, as per
section 5.4.) 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 and
generates LDIF output.
7. Security considerations
The entirety of related security considerations are outside the scope
of this document. It should be 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. It 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
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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. It may be wiser to
treat such values as corresponding to the "nobody" or "nogroup" user
and group, respectively.
8. Acknowledgements
Thanks to Leif Hedstrom of Netscape Communications Corporation,
Rosanna Lee of Sun Microsystems 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.
UNIX is a registered trademark of The Open Group.
9. References
[LDIF]G. Good, "The LDAP Data Interchange Format (LDIF)", INTERNET-
DRAFT <draft-ietf-asid-ldif-02.txt>, November 1996.
[LDAPATTRS]
M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access
Protocol: Standard and Pilot Attribute Definitions", INTERNET-
DRAFT <draft-ietf-asid-ldapv3-attributes-08.txt>, June 1997.
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[LDAPFILT]
T. Howes, "A String Representation of LDAP Search Filters",
INTERNET-DRAFT <draft-ietf-asid-ldapv3-filter-02.txt>, May 1997.
[LDAPDN]
M. Wahl, S. Kille, T. Howes, "Lightweight Directory Access
Protocol (v3): UTF-8 String Representation of Distinguished
Names", INTERNET-DRAFT <draft-ietf-asid-ldapv3-dn-04.txt>,
November 1997.
[LDAPDOMAINS]
S. Kille, M. Wahl, A. Grimstad, R. Huber, S. Sataluri, "An
Approach for Using Domains in LDAP Distinguished Names",
INTERNET-DRAFT <draft-ietf-asid-ldap-domains-02.txt>, September
1997.
[LDAPV3]
M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access
Protocol (Version 3)", INTERNET-DRAFT <draft-ietf-asid-ldapv3-
protocol-08.txt>, June 1997.
[RFC1034]
P. Mockapetris, "Domain names - concepts and facilities", RFC
1034, November 1987.
[RFC1057]
Sun Microsystems, Inc., "RPC: Remote Procedure Call: Protocol
Specification Version 2", RFC 1057, June 1988.
[RFC1279]
S. Kille, "X.500 and Domains", RFC 1279, November 1991.
[RFC1884]
R. Hinden, S. Deering, "IP Version 6 Addressing Architecture",
RFC 1884, December 1995.
[RFC2052]
A. Gulbrandsen, P. Vixie, "A DNS RR for specifying the location
of services (DNS SRV)", RFC 2052, October 1996.
[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
A. Example entries
The examples described in this section are provided to illustrate the
schema described in this draft. They are not meant to be exhaustive.
Entries are presented in LDIF notation [LDIF].
The following entry is an example of the posixAccount class:
dn: uid=lester, dc=aja, dc=com
objectClass: top
objectClass: account
objectClass: posixAccount
uid: lester
cn: Lester the Nightfly
userPassword: {crypt}X5/DBrWPOQQaI
gecos: Lester
loginShell: /bin/csh
uidNumber: 10
gidNumber: 10
homeDirectory: /home/lester
This corresponds the UNIX system password file entry:
lester:X5/DBrWPOQQaI:10:10:Lester:/home/lester:/bin/sh
The following entry is an example of the ipHost class:
dn: dc=peg, dc=aja, dc=com
objectClass: top
objectClass: ipHost
dc: peg
associatedDomain: peg
associatedDomain: peg.aja.com
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associatedDomain: www.aja.com
ipHostNumber: 10.0.0.1
macAddress: 0:0:92:90:ee:e2
bootParameter: bootfile=mach
bootParameter: root=fs:/nfsroot/peg
bootParameter: swap=fs:/nfsswap/peg
bootParameter: dump=fs:/nfsdump/peg
This entry represents the host canonically peg.aja.com, also known as
www.aja.com and peg. Note while associatedDomain values are used to
search for the entry, the distinguished name may be parsed to
determine the host's canonical name. The Ethernet address and four
boot parameters are also specified.
An example of the nisNetgroup class:
dn: cn=nightfly, dc=aja, dc=com
objectClass: top
objectClass: nisNetgroup
cn: nightfly
nisNetgroupTriple: (charlemagne,peg,dunes.aja.com)
nisNetgroupTriple: (lester,-,)
memberNisNetgroup: kamakiriad
This entry represents the netgroup nightfly, which contains two
triples (the user charlemagne, the host peg, and the domain
dunes.aja.com; and, the user lester, no host, and any domain) and one
netgroup (kamakiriad).
Finally, an example of the nisObject class:
dn: nisMapName=tracks, dc=dunes, dc=aja, dc=com
objectClass: top
objectClass: nisObject
nisMapName: tracks
dn: cn=Maxine, nisMapName=tracks, dc=dunes, dc=aja, dc=com
objectClass: top
objectClass: nisObject
cn: Maxine
nisMapName: tracks
nisMapEntry: Nightfly$4
This entry represents the NIS map tracks, and a single map entry.
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