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 Status of this Memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months. Internet-Drafts may be updated, replaced, or made obsolete by other documents at any time. It is not appropriate to use Internet- Drafts as reference material or to cite them other than as a "working draft" or "work in progress". To learn the current status of any Internet-Draft, please check the 1id-abstracts.txt listing contained in the Internet-Drafts Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). 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. Howard [Page 1] Internet Draft NIS X.500 schema 12 September 1997 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 Howard [Page 2] Internet Draft NIS X.500 schema 12 September 1997 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 Howard [Page 3] Internet Draft NIS X.500 schema 12 September 1997 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 Howard [Page 4] Internet Draft NIS X.500 schema 12 September 1997 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' Howard [Page 5] Internet Draft NIS X.500 schema 12 September 1997 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}' ) Howard [Page 6] Internet Draft NIS X.500 schema 12 September 1997 ( 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 Howard [Page 7] Internet Draft NIS X.500 schema 12 September 1997 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 ) Howard [Page 8] Internet Draft NIS X.500 schema 12 September 1997 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 Howard [Page 9] Internet Draft NIS X.500 schema 12 September 1997 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 Howard [Page 10] Internet Draft NIS X.500 schema 12 September 1997 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) Howard [Page 11] Internet Draft NIS X.500 schema 12 September 1997 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. Howard [Page 12] Internet Draft NIS X.500 schema 12 September 1997 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 Howard [Page 13] Internet Draft NIS X.500 schema 12 September 1997 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 Howard [Page 14] Internet Draft NIS X.500 schema 12 September 1997 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 Howard [Page 15] Internet Draft NIS X.500 schema 12 September 1997 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 Howard [Page 16] Internet Draft NIS X.500 schema 12 September 1997 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.) Howard [Page 17] Internet Draft NIS X.500 schema 12 September 1997 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 , June 1997. [2] M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access Protocol: Standard and Pilot Attribute Definitions", INTERNET- DRAFT , 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 , March 1997. [5] T. Howes, "A String Representation of LDAP Search Filters", INTERNET-DRAFT , 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 November 1996. [9] S. Kille, M. Wahl, "An Approach for Using Domains in LDAP Howard [Page 18] Internet Draft NIS X.500 schema 12 September 1997 Distinguished Names", INTERNET-DRAFT , 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 , 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 , July 1997. [17] T. Howes, L. Howard, "A Simple Caching Scheme for LDAP and X.500 Directories", INTERNET-DRAFT , 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: Howard [Page 19] Internet Draft NIS X.500 schema 12 September 1997 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 Howard [Page 20] Internet Draft NIS X.500 schema 12 September 1997 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. Howard [Page 21]