Application Working Group L. Howard
INTERNET-DRAFT Independent Consultant
Expires in six months from 12 September 1997
Intended Category: Experimental
An Approach for Using LDAP as a Network Information Service
<draft-howard-nis-schema-00.txt>
Status of this Memo
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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 entries so
that they may be resolved with the Lightweight Directory Access
Protocol [1]. 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 [13]) 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 [12] 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(3c) 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 only 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 usually not tightly integrated with
the operating system, information such as users needs 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.)
"In general, it is advantageous for different network
applications and services to refer to the directory for
user account information, rather than each service keeping
its own collection of user account records, which requires
the network administrator to separately create or destroy
user entities, passwords, etc., in many different systems
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each time a user joins or leaves the organization." [4]
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. While the schema is by no means
deemed to be authoritative, it is considered desirable to have a
single, open schema rather than the proliferation of multiple
proprietary schema. This document is one step towards such a schema.
2. General Issues
2.1. Terminology
In this document, the term 'NIS-related entities' is used rather
loosely to refer to those entities (described in the previous
section) which are typically represented in the Network Information
Service. (NIS was previously known as Yellow Pages, or YP.) It should
not be inferred from this that deploying LDAP for resolving such
entities (nisObject excluded) requires NIS to be used, as a gateway
or otherwise. The host and network classes are generically
applicable, and may be implemented on operating systems other than
the UNIX system that wish to use LDAP to resolve these entities.
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 term 'principal' is used to
distinguish accounts that may be used for authentication from those
that are not.
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 support extensible schema and
multiple naming contexts.
The phrase 'resolving an entity' or 'resolution of entities' refers
to enumerating NIS-related entities of a given type, or matching them
against a given search criterion. One or more entities are returned
as a result of successful 'resolutions' (a 'match' operation will
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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 rooted at 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 reader is referred to [2] for the BFN for attribute type
definitions.
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
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Additionally, the attributes defined in [2], [9] and [16] are
imported.
2.3. Object classes
The reader is referred to [2] for the BFN for object class
definition.
The following object classes are defined in this document:
posixAccount
shadowAccount
posixGroup
ipService
ipProtocol
oncRpc
ipHost
ipNetwork
nisNetgroup
automount
nisObject
Additionally, the classes defined in [2] and [9] are imported.
2.4. Syntax definitions
The following syntax definition [2] is used in representing NIS
netgroup triples.
( nisSchema.0.0 NAME 'nisNetgroupTripleSyntax'
DESC 'NIS netgroup triple' )
Values in this syntax are encoded according to the following BNF:
nisnetgrouptriple = "(" hostname "," username "," domainname ")"
hostname = "" / "-" / keystring
username = "" / "-" / keystring
domainname = "" / "-" / keystring
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'
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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, including 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
SUBSTRINGS caseExactIA5SubstringsMatch
SYNTAX 'IA5String{128}' )
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( 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 in dotted decimal notation, eg. 192.168.1.1'
EQUALITY caseIgnoreIA5Match
SYNTAX 'IA5String{128}' )
( nisSchema.1.20 NAME 'ipNetworkNumber'
DESC 'IP address in dotted decimal notation, eg. 192.168'
EQUALITY caseIgnoreIA5Match
SYNTAX 'IA5String{128}' )
( nisSchema.1.21 NAME 'ipNetmaskNumber'
DESC 'IP address in dotted decimal notation, 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}' )
( nisSchema.1.23 NAME 'bootParameter'
DESC 'rpc.bootparamd parameter; informal syntax is key=value'
EQUALITY caseExactIA5Match
SYNTAX 'IA5String' )
( nisSchema.1.24 NAME 'bootFile'
DESC 'name of the boot image, which may be used by bootpd.
Alternatively, this may specified as a value of
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bootParameter.'
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 definitions under the OID 2.5.6 are imported. 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 have been proposed [4]; these issues
will not be considered in detail here.)
( nisSchema.2.0 NAME 'posixAccount' SUP top STRUCTURAL
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 ) )
( nisSchema.2.2 NAME 'posixGroup' SUP top STRUCTURAL
DESC 'Abstraction of a group of posixAccounts.'
MUST ( cn $ gidNumber )
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MAY ( userPassword $ memberUid $ description ) )
( nisSchema.2.3 NAME 'ipService' SUP top STRUCTURAL
DESC 'Abstraction an Internet Protocol service. Maps an IP
port and protocol (eg. 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) [12]
Remote Procedure Call (RPC) binding. 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. The schema defined in [3] is used
to denote the canonical hostname, by mapping the
distinguished name into a DNS domain name.
The associatedDomain attribute is used for interrogating
the DIT, and as such must contain values for the host's
canonical name and its aliases.'
MUST ( dc $ ipHostNumber $ associatedDomain )
MAY ( macAddress $ bootParameter $ bootFile $
l $ description $ manager $ serialNumber ) )
( nisSchema.2.7 NAME 'ipNetwork' SUP top
STRUCTURAL
DESC 'Abstraction of a network.'
MUST ( dc $ ipNetworkNumber $ associatedDomain )
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.2.9 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. Resolution methods
The ideal 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 code to commercial C libraries
and applications is rarely available to the end-user, it is
acceptable to emulate a supported nameservice (such as NIS) and
modify the resolution code to use LDAP. (This is also an appropriate
opportunity to perform caching of entries across client address
spaces.) In the case of NIS, reference implementations are widely
available and the client-server 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. However, an implementation which claims to conform to
this specification is not required to support these classes. (To
mandate otherwise would exclude implementations integrated with the C
library.)
Some operating systems and C libraries support end-user extensible
resolvers using dynamically loadable libraries and a nameservice
"switch". Others allow end-user defined symbols to be substituted at
runtime. Regardless, the means by which the operating system is
directed to use LDAP is implementation dependent, as is the means by
which the DUA locates LDAP servers. (It is anticipated that the
Dynamic Host Configuration Protocol (DHCP) may be used for the latter
[16].)
5.2. Affected resolver calls
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The following entry points are found in the C libraries of most UNIX
and POSIX compliant systems. An LDAP search filter [5] which may be
used to satisfy the function call is included alongside each function
name, with printf(3s) format notation used to denote the function
parameter(s), if any. Generally, those functions in section 3n of the
UNIX system's manual pages refer to TCP/IP entries, and those in
section 3c refer to the remainder. Long lines are broken with the '\'
character.
getpwnam(3c) (&(objectClass=posixAccount)(uid=%s))
getpwuid(3c) (&(objectClass=posixAccount)\
(uidNumber=%d))
getpwent(3c) (objectClass=posixAccount)
getspnam(3c) (&(objectClass=shadowAccount)(uid=%s))
getspent(3c) (objectclass=shadowAccount)
getgrnam(3c) (&(objectClass=posixGroup)(cn=%s))
getgrgid(3c) (&(objectClass=posixGroup)\
(gidNumber=%d))
getgrent(3c) (objectClass=posixGroup)
getservbyname(3n) (&(objectClass=ipService)\
(cn=%s)(ipServiceProtocol=%s))
getservbyport(3n) (&(objectClass=ipService)\
(ipServicePort=%d)\
(ipServiceProtocol=%s))
getservent(3n) (objectClass=ipService)
getrpcbyname(3n) (&(objectClass=oncRpc)(cn=%s))
getrpcbynumber(3n) (&(objectClass=oncRpc)(oncRpcNumber=%d))
getrpcent(3n) (objectClass=oncRpc)
getprotobyname(3n) (&(objectClass=ipProtocol)(cn=%s))
getprotobynumber(3n) (&(objectClass=ipProtocol)\
(ipProtocolNumber=%d))
getprotoent(3n) (objectClass=ipProtocol)
gethostbyname(3n) (&(objectClass=ipHost)\
(associatedDomain=%s))
gethostbyaddr(3n) (&(objectClass=ipHost)(ipHostNumber=%s))
gethostent(3n) (objectClass=ipHost)
getnetbyname(3n) (&(objectClass=ipNetwork)\
(associatedDomain=%s))
getnetbyaddr(3n) (&(objectClass=ipNetwork)\
(ipNetworkNumber=%s))
getnetent(3n) (objectClass=ipNetwork)
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setnetgrent(3n) (&(objectClass=nisNetgroup)(cn=%s))
getaliasbyname(3n) (&(objectClass=rfc822MailGroup)(cn=%s))
getaliasent(3n) (objectClass=rfc822MailGroup)
5.3. Interpreting user and group entries
User and group resolution is initiated by the functions prefixed by
getpw and getgr respectively. A user's login name is denoted by the
value of the uid attribute (which will typically be used as a
relative distinguished name); a group's name is denoted by a value of
the cn attribute.
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
attributes embedded in the GECOS field, such as a user's telephone
number, to be returned to the client without overloading the cn
attribute.)
An entry of class posixAccount or shadowAccount without a
userPassword attribute must be denied the opportunity to
authenticate. For example, the client may be returned a non-matchable
password such as "*" by the DUA.
A user which is a member of a posixGroup which has no userPassword
attribute must not be allowed to authenticate themself as a member of
that group, unless the user's gidNumber attribute implies a user has
the same group ID (in which case the operating system may determine
this implicitly).
userPassword values must be represented by following BNF syntax:
passwordvalue = schemeprefix encryptedpassword
schemeprefix = "{" scheme "}"
scheme = "crypt" / "md5" / "sha" / altscheme
altscheme = keystring
encryptedpassword = encrypted password
(where the encrypted password consists of a plaintext key encrypted
using appropriate encoding algorithm; for example, crypt(3) with a
two-character random salt for "crypt")
userPassword values which do not adhere to the BNF above must not be
used for authentication. (The DUA must iterate through the values of
the attribute until a value matching the above BNF is found.) Only if
encryptedPassword is an empty string does the user have no password.
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DUAs are not required to consider encryption schemes which the client
will not recognise; in many cases, it may be sufficient to consider
only "crypt".
A future document may use LDAP v3 attribute descriptions to represent
hashed userPasswords, as in the following:
attributetype = attributename sep attributeoption
attributename = "userPassword"
sep = ";"
attributeoption = schemeclass "-" scheme
schemeclass = "hash" / altschemeclass
scheme = "crypt" / "md5" / "sha" / altscheme
altschemeclass = keystring
altscheme = keystring
For example, consider the attribute:
userPassword;hash-crypt: X5/DBrWPOQQaI
which would otherwise be represented as
userPassword: {crypt}X5/DBrWPOQQaI.
A DUA may make use of the attributes in the shadowAccount class to
provide shadow password service (getspnam(3c) and getspent(3c)). In
such cases, the DUA must not make use of the userPassword attribute
for getpwnam(3c) 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 [6] domains in LDAP distinguished
names described in [3] and [9] is used in part to represent TCP/IP
hosts and networks in LDAP.
Note the use of the ipHostNumber attribute instead of the dNSRecord
attribute. The rationale is that, in order to minimize the
responsibility placed on the DUA, attribute values ought to directly
contain the information they seek to represent. This contrasts with,
for example, a dNSRecord value which expresses a complete DNS
resource record including time to live and class data. It is
considered that this information is extraneous to using LDAP as a
direct means to resolve hosts and networks. Additionally, it is
considered more appropriate for an entity, and all its aliases, to be
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represented by a single entry in the DIT, which is not always
possible when a DNS resource record is mapped directly to an LDAP
entry.
This document redefines (although not to the extent of excluding the
existing definition) the ipNetwork class defined in [3], for naming
consistency with ipHost. The ipNetworkNumber attribute is also used
in the siteContact object class [14]. (The trailing zeros in a
network address should be omitted.) CIDR-style network addresses (eg.
192.168.1/24) can be used but this is not required.
If an entry of class ipHost or ipNetwork belongs to a naming context
denoted by relative distinguished names (RDNs) [10] of attribute type
dc (domainComponent), then the distinguished name (DN) is transformed
into a domain name system (DNS) suffix 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 is parsed into the host
name foo.bar.edu. If the naming context is does not contain 'dc'
values, a non-qualified host name is returned. For organizations
which wish to use existing X.500 container classes to form their
context (ie. organization and organizationalUnit) the RDN values of
unrequired type are skipped by the DUA in determining the domain
name. As such, a DN of dc=foo, dc=bar, dc=edu, o=Ace Industry, c=US
may be parsed as foo.bar.edu. As this may be considered a naming
violation, this document does not specifically endorse this.
Hosts with IPv6 addresses should be written in their "preferred" form
as defined in section 2.2.1 of [15], such that all components of the
address are indicated and leading zeros are omitted. This is to
provide 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=aceindustry, dc=com
cn: domain
cn: nameserver
objectClass: top
objectClass: ipService
ipServicePort: 53
ipServiceProtocol: tcp
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ipServiceProtocol: udp
This entry would map to the following two (2) services entities:
domain 53/tcp nameserver
domain 53/udp nameserver
While the above two entities could have been equally represented as
separate LDAP entities, with different distinguished names (such as
cn=domain+ipServiceProtocol=tcp, ... and
cn=domain+ipServiceProtocol=udp, ...) it is considered that
representing them as a single entry is more convenient. (If a service
is represented in multiple protocol domains with different respective
ports, then multiple entries are mandatory, with multivalued RDNs
being used to distinguish between them.)
Entries of class automount inherently represent more than one entity:
each value of the automountInformation attribute is a NIS record.
With the exception of userPassword values, which must be parsed
according to the BNF considered in section 5.2, any empty values
(those that consist of a zero length string) are returned by the DUA
to the client. The client may not make sense of them, but this
situation is no different to parsing files which contain empty
fields. (By contrast, the DUA must reject any entries which do not
conform to the schema, ie. are missing certain mandatory attributes.
Non-conforming entries should be ignored while enumerating entries;
whether the enumeration is terminated at such an entry is
implementation dependent, although it is strongly suggested that the
offending entry be treated as if it were not present.)
The nisObject object class is provided 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. The nisObject class may be useful were one to use LDAP to
query a NIS server, although it is anticipated that the converse will
be more common. (Where the nisObject class is used, the nisMapName
attribute may establish part of the DN, to assist the DUA in locating
entries belonging to a particular map.)
Entries which inherit also from the cacheObject object class (and
thus contain the 'ttl' attribute) may be used by DUAs to perform
cache validation. [17]
5.6. Canonicalizing entries with multi-valued naming attributes
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For entities such as services, protocols, and RPCs, where there may
be one or more aliases, the respective entry's relative distinguished
name is 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'.
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 based 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 entire
distinguished name is considered, as per section 5.4.) This fact aids
the DUA in determining the canonical name of an entity: it can simply
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 with conforming entries that belong to
additional object classes, it is possible that the entity's canonical
name cannot be deduced from the RDN. In this situation, the DUA must
choose one of the non-distinguished values to represent the entity's
canonical name. Because the directory server guarantees no ordering
of attribute values, attempting to distinguish an entry in a
deterministic fashion may require the DUA to maintain a mapping
between entries' DNs and their canonical names as considered by the
DUA. This document does not require this, nor does it advocate that
such situations be resolved by mapping one DIT entry into multiple
entities.
6. Implementation focus
A NIS daemon which uses LDAP instead of local files has been
developed which supports the schema defined in this document. A set
of extensions to a particular implementation of the Mach operating
system has also been developed, which sidesteps NIS and uses LDAP
directly.
Work is underway to develop a freely available (under the GNU General
Library Public License) reference implementation of the C library
resolution code that supports LDAP using the draft schema. The code
will be compatible with the Free Software Foundation's GNU C library
and other C libraries which support the Name Service Switch (NSS) or
Information Retrieval Service (IRS).
The alias lookup functions referred to in section 5.2 are presently
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available only in the GNU C library, and (albeit with different
names) in the C library of one commercial UNIX system vendor. It is
anticipated that the mail transport agent (MTA) will typically
consult LDAP or NIS directly instead of using the C library; however,
support for the suggested library calls is encouraged.
The author has made available a freely distributable set of Perl
scripts for parsing configuration files such as /etc/passwd and
/etc/hosts and generating LDIF data suitable for preparing an LDIF
database, as well as a set of Java classes for generating flat files
from the DIT.
7. Security considerations
The entirety of related security considerations are outside the scope
of this document. However, it should be noted that making passwords
encrypted with a widely understood one way function (such as
crypt(3)) available to non-privileged users is potentially 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 may consider using
Kerberos or another authentication service for logins. A variation on
this is to authenticate to an LDAP server by binding over an
encrypted connection (such as SSL [8]).
Alternatively, the encrypted password could be made available only to
a subset of privileged DUAs, which would provide 'shadow' password
service to client applications.
Because the schema represents operating system-level entities, access
to these entities should be granted on a discretionary basis. (That
said, there is little point in restricting access to data which will
be republished without restriction, eg. by a NIS server.) 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 root 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.)
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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
[1] M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access
Protocol (Version 3)", INTERNET-DRAFT <draft-ietf-asid-ldapv3-
protocol-06.txt>, June 1997.
[2] M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access
Protocol: Standard and Pilot Attribute Definitions", INTERNET-
DRAFT <draft-ietf-asid-ldapv3-attributes-06.txt>, June 1997.
[3] S. Kille, "X.500 and Domains", RFC 1279, November 1991.
[4] H. Lachman, "LDAP-based Routing of SMTP Messages: Approach Used
by Netscape", INTERNET-DRAFT <draft-ietf-asid-email-routing-ns-
00.txt>, March 1997.
[5] T. Howes, "A String Representation of LDAP Search Filters",
INTERNET-DRAFT <draft-ietf-asid-ldapv3-filter-00.txt>, March
1997. See also [10].
[6] P. Mockapetris, "Domain names - concepts and facilities", RFC
1034, November 1987.
[7] "Information Processing Systems - Open Systems Interconnection -
The Directory: Overview of Concepts, Models and Service",
ISO/IEC JTC 1/SC21, International Standard 9594-1, 1988.
[8] A. O. Freier, P. Karlton, P. Kocher, "The SSL Protocol, Version
3.0", INTERNET-DRAFT <draft-ietf-tls-ssl-version3-00.txt>
November 1996.
[9] S. Kille, M. Wahl, "An Approach for Using Domains in LDAP
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Distinguished
Names", INTERNET-DRAFT <draft-ietf-asid-ldap-domains-00.txt>,
July 1996.
[10] S. Kille, "A String Representation of Distinguished Names", RFC
1779, March 1995.
[11] G. Good, "The LDAP Data Interchange Format (LDIF)", INTERNET-
DRAFT <draft-ietf-asid-ldif-00.txt>, November 1996.
[12] Sun Microsystems, Inc., "RPC: Remote Procedure Call: Protocol
Specification Version 2", RFC 1057, June 1988.
[13] ISO/IEC 9945-1:1990, Information Technology - Portable Operating
Systems Interface (POSIX) - Part 1: Systems Application
Programming Interface (API) [C Language]
[14] M. T. Rose, "The Little Black Book: Mail Bonding with OSI
Directory Services", ISBN 0-13-683210-5, Prentice-Hall, Inc.,
1992.
[15] R. Hinden, S. Deering, "IP Version 6 Addressing Architecture",
RFC 1884, December 1995.
[16] L. Hedstrom, L. Howard, "DHCP Options for LDAP", INTERNET-DRAFT
<draft-hedstrom-dhcp-ldap-00.txt>, July 1997.
[17] T. Howes, L. Howard, "A Simple Caching Scheme for LDAP and X.500
Directories", INTERNET-DRAFT <draft-ietf-asid-ldap-cache-
01.txt>, July 1997.
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 an authoritative
reference. Entries are presented in LDIF notation [11].
The following entry is an example of the posixAccount class:
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dn: uid=lukeh, dc=aceindustry, dc=com
cn: Luke Howard
objectClass: top
objectClass: person
objectClass: posixAccount
sn: Howard
telephoneNumber: +61 3 9428 0788
uid: lukeh
userPassword: {crypt}X5/DBrWPOQQaI
gecos: Luke Howard
loginShell: /bin/csh
uidNumber: 10
gidNumber: 10
homeDirectory: /home/lukeh
This corresponds the UNIX system password file entry:
lukeh:X5/DBrWPOQQaI:10:10:Luke Howard:/home/lukeh:/bin/sh
Note that the userPassword value is parsed into a password suitable
for matching with crypt(3). Attributes such as telephoneNumber and sn
(which belong to classes other than posixAccount), are not used in
determining the corresponding password file entry but may be useful
to other LDAP clients. (In most cases, entries of class posixAccount
will also inherit from person or organizationalPerson.)
The following entry is an example of the ipHost class:
dn: dc=yoyo, dc=aceindustry, dc=com
dc: yoyo
objectClass: top
objectClass: ipHost
objectClass: domainRelatedObject
associatedDomain: yoyo.aceindustry.com
associatedDomain: www.aceindustry.com
ipHostNumber: 10.0.0.1
macAddress: 0:0:92:90:ee:e2
bootParameter: bootfile=mach
bootParameter: root=fs:/nfsroot/yoyo
bootParameter: swap=fs:/nfsswap/yoyo
bootParameter: dump=fs:/nfsdump/yoyo
This entry represents the host yoyo.aceindustry.com, also known as
www.aceindustry.com. Note that the associatedDomain values are used
in searching for the entry, but the distinguished name is parsed to
determine the host's canonical name. The MAC address, boot image, and
two boot parameters are also specified in this entry. The auxilary
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class domainRelatedObject is not mandatory. (Thus, the NIS maps
prefixed by 'hosts', 'ethers', and 'bootparams' could all be derived
from similar entries.)
An example of the nisNetgroup class:
dn: cn=nightfly, dc=aceindustry, dc=com
cn: nightfly
objectClass: top
objectClass: nisNetgroup
nisNetgroupTriple: (fagen,peg,dunes.aceindustry.com)
nisNetgroupTriple: (becker,-,)
memberNisNetgroup: kamakiriad
This entry represents the netgroup nightfly, which contains two
triples (the user fagen, the host peg, and the domain
dunes.aceindustry.com; and, the user becker, no host, and any domain)
and one netgroup (kamakiriad).
Finally, an example of the nisObject class:
dn: nisMapName=quote.byname, dc=dunes, dc=aceindustry, dc=com
objectClass: top
objectClass: nisObject
nisMapName: quote.byname
dn: cn=foobar, nisMapName=quote.byname, dc=dunes, dc=aceindustry, dc=com
objectClass: top
objectClass: nisObject
objectClass: cacheObject
ttl: 86400
cn: foobar
nisMapName: quote.byname
nisMapEntry: 75.00
This entry represents the NIS map quote.byname, and a constitutent
entry, with the key of foobar and a value of 75.00. The latter entry
has a time-to-live of 24 hours.
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