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142 lines
5.1 KiB
Plaintext
142 lines
5.1 KiB
Plaintext
# $OpenLDAP$
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# Copyright 1999-2007 The OpenLDAP Foundation, All Rights Reserved.
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# COPYING RESTRICTIONS APPLY, see COPYRIGHT.
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H1: Constructing a Distributed Directory Service
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For many sites, running one or more {{slapd}}(8) that hold an
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entire subtree of data is sufficient. But often it is desirable
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to have one {{slapd}} refer to other directory services for a
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certain part of the tree (which may or may not be running {{slapd}}).
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!if 0
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{{slapd}} supports {{subordinate}}, {{immediate superior}},
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and {{superior}} knowledge information.
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!else
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{{slapd}} supports {{subordinate}} and {{superior}} knowledge information.
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Subordinate knowledge information is held in {{EX:referral}}
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objects ({{REF:RFC3296}}).
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!endif
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H2: Subordinate Knowledge Information
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Subordinate knowledge information may be provided to delegate
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a subtree.
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Subordinate knowledge information is maintained in the directory
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as a special {{referral}} object at the delegate point.
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The referral object acts as a delegation point, gluing two services
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together.
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This mechanism allows for hierarchical directory services to be
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constructed.
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A referral object has a structural object class of
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{{EX:referral}} and has the same {{TERM[expand]DN}} as the
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delegated subtree. Generally, the referral object will also
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provide the auxiliary object class {{EX:extensibleObject}}.
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This allows the entry to contain appropriate {{TERM[expand]RDN}}
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values. This is best demonstrated by example.
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If the server {{EX:a.example.net}} holds {{EX:dc=example,dc=net}}
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and wished to delegate the subtree {{EX:ou=subtree,dc=example,dc=net}}
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to another server {{EX:b.example.net}}, the following named referral
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object would be added to {{EX:a.example.net}}:
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> dn: dc=subtree,dc=example,dc=net
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> objectClass: referral
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> objectClass: extensibleObject
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> dc: subtree
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> ref: ldap://b.example.net/dc=subtree,dc=example,dc=net
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The server uses this information to generate referrals and
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search continuations to subordinate servers.
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For those familiar with {{TERM:X.500}}, a {{named referral}} object is
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similar to an X.500 knowledge reference held in a {{subr}}
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{{TERM:DSE}}.
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!if 0
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H2: Immediate Superior Knowledge Information
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Immediate superior knowledge information may be provided in the
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entry at the root of a delegated subtree. The knowledge information
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is contained with {{EX:ref}} operational attribute.
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Extending the example above, a {{ref}} attribute can be added
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to the entry {{EX:dc=subtree,dc=example,dc=net}} in server B indicating
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that A holds the immediate superior naming context.
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> dn: dc=subtree,dc=example,dc=net
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> changetype: modify
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> add: ref
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> ref: ldap://a.example.net/
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The server uses this information to generate referrals to
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management operations.
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For those familiar with {{TERM:X.500}}, this use of the {{EX:ref}}
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attribute is similar to an X.500 knowledge reference held in a
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{{immSupr}} {{TERM:DSE}}.
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!endif
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H2: Superior Knowledge Information
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Superior knowledge information may be specified using the {{EX:referral}}
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directive. The value is a list of {{TERM:URI}}s referring to
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superior directory services. For servers without immediate superiors,
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such as for {{EX:a.example.net}} in the example above, the server
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can be configured to use a directory service with {{global knowledge}},
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such as the {{OpenLDAP Root Service}}
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({{URL:http://www.openldap.org/faq/index.cgi?file=393}}).
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> referral ldap://root.openldap.org/
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However, as {{EX:a.example.net}} is the {{immediate superior}}
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to {{EX:b.example.net}}, {{b.example.net}} would be configured
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as follows:
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> referral ldap://a.example.net/
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The server uses this information to generate referrals for operations
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acting upon entries not within or subordinate to any of the naming
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contexts held by the server.
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For those familiar with {{TERM:X.500}}, this use of the {{EX:ref}}
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attribute is similar to an X.500 knowledge reference held in a
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{{Supr}} {{TERM:DSE}}.
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H2: The ManageDsaIT Control
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Adding, modifying, and deleting referral objects is generally done
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using {{ldapmodify}}(1) or similar tools which support the ManageDsaIT
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control. The ManageDsaIT control informs the server that you intend
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to manage the referral object as a regular entry. This keeps the
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server from sending a referral result for requests which interrogate
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or update referral objects.
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The ManageDsaIT control should not be specified when managing regular
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entries.
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The {{EX:-M}} option of {{ldapmodify}}(1) (and other tools) enables
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ManageDsaIT. For example:
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> ldapmodify -M -f referral.ldif -x -D "cn=Manager,dc=example,dc=net" -W
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or with {{ldapsearch}}(1):
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> ldapsearch -M -b "dc=example,dc=net" -x "(objectclass=referral)" '*' ref
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Note: the {{EX:ref}} attribute is operational and must be explicitly
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requested when desired in search results.
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Note: the use of referrals to construct a Distributed Directory Service is
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extremely clumsy and not well supported by common clients. If an existing
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installation has already been built using referrals, the use of the
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{{chain}} overlay to hide the referrals will greatly improve the usability
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of the Directory system. A better approach would be to use explicitly
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defined local and proxy databases in {{subordinate}} configurations to
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provide a seamless view of the Distributed Directory.
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