# $OpenLDAP$ # Copyright 2007 The OpenLDAP Foundation, All Rights Reserved. # COPYING RESTRICTIONS APPLY, see COPYRIGHT. H1: Overlays Overlays are software components that provide hooks to functions analogous to those provided by backends, which can be stacked on top of the backend calls and as callbacks on top of backend responses to alter their behavior. Overlays may be compiled statically into {{slapd}}, or when module support is enabled, they may be dynamically loaded. Most of the overlays are only allowed to be configured on individual databases, but some may also be configured globally. Essentially, they represent a means to: * customize the behavior of existing backends without changing the backend code and without requiring one to write a new custom backend with complete functionality * write functionality of general usefulness that can be applied to different backend types Overlays are usually documented by separate specific man pages in section 5; the naming convention is > slapo- All distributed core overlays have a man page. Feel free to contribute to any, if you think there is anything missing in describing the behavior of the component and the implications of all the related configuration directives. Official overlays are located in > servers/slapd/overlays/ That directory also contains the file slapover.txt, which describes the rationale of the overlay implementation, and may serve as guideline for the development of custom overlays. Contribware overlays are located in > contrib/slapd-modules// along with other types of run-time loadable components; they are officially distributed, but not maintained by the project. They can be stacked on the frontend as well; this means that they can be executed after a request is parsed and validated, but right before the appropriate database is selected. The main purpose is to affect operations regardless of the database they will be handled by, and, in some cases, to influence the selection of the database by massaging the request DN. All the current overlays in 2.4 are listed and described in detail in the following sections. H2: Access Logging H3: Overview This overlay can record accesses to a given backend database on another database. This allows all of the activity on a given database to be reviewed using arbitrary LDAP queries, instead of just logging to local flat text files. Configuration options are available for selecting a subset of operation types to log, and to automatically prune older log records from the logging database. Log records are stored with audit schema to assure their readability whether viewed as LDIF or in raw form. It is also used for {{SECT:delta-syncrepl replication}} H3: Access Logging Configuration The following is a basic example that implements Access Logging: > database bdb > suffix dc=example,dc=com > ... > overlay accesslog > logdb cn=log > logops writes reads > logold (objectclass=person) > > database bdb > suffix cn=log > ... > index reqStart eq > access to * > by dn.base="cn=admin,dc=example,dc=com" read The following is an example used for {{SECT:delta-syncrepl replication}}: > database hdb > suffix cn=accesslog > directory /usr/local/var/openldap-accesslog > rootdn cn=accesslog > index default eq > index entryCSN,objectClass,reqEnd,reqResult,reqStart Accesslog overlay definitions for the primary db > database bdb > suffix dc=example,dc=com > ... > overlay accesslog > logdb cn=accesslog > logops writes > logsuccess TRUE > # scan the accesslog DB every day, and purge entries older than 7 days > logpurge 07+00:00 01+00:00 An example search result against {{B:cn=accesslog}} might look like: > [ghenry@suretec ghenry]# ldapsearch -x -b cn=accesslog > # extended LDIF > # > # LDAPv3 > # base with scope subtree > # filter: (objectclass=*) > # requesting: ALL > # > > # accesslog > dn: cn=accesslog > objectClass: auditContainer > cn: accesslog > > # 20080110163829.000004Z, accesslog > dn: reqStart=20080110163829.000004Z,cn=accesslog > objectClass: auditModify > reqStart: 20080110163829.000004Z > reqEnd: 20080110163829.000005Z > reqType: modify > reqSession: 196696 > reqAuthzID: cn=admin,dc=suretecsystems,dc=com > reqDN: uid=suretec-46022f8$,ou=Users,dc=suretecsystems,dc=com > reqResult: 0 > reqMod: sambaPwdCanChange:- ###CENSORED### > reqMod: sambaPwdCanChange:+ ###CENSORED### > reqMod: sambaNTPassword:- ###CENSORED### > reqMod: sambaNTPassword:+ ###CENSORED### > reqMod: sambaPwdLastSet:- ###CENSORED### > reqMod: sambaPwdLastSet:+ ###CENSORED### > reqMod: entryCSN:= 20080110163829.095157Z#000000#000#000000 > reqMod: modifiersName:= cn=admin,dc=suretecsystems,dc=com > reqMod: modifyTimestamp:= 20080110163829Z > > # search result > search: 2 > result: 0 Success > > # numResponses: 3 > # numEntries: 2 For more information, please see {{slapo-accesslog(5)}} and the {{SECT:delta-syncrepl replication}} section. H2: Audit Logging The Audit Logging overlay can be used to record all changes on a given backend database to a specified log file. H3: Overview If the need arises whereby changes need to be logged as standard LDIF, then the auditlog overlay {{B:slapo-auditlog (5)}} can be used. Full examples are available in the man page {{B:slapo-auditlog (5)}} H3: Audit Logging Configuration If the directory is running vi {{F:slapd.d}}, then the following LDIF could be used to add the overlay to the overlay list in {{B:cn=config}} and set what file the {{TERM:LDIF}} gets logged to (adjust to suit) > dn: cn=module{0},cn=config > changetype: modify > add: olcModuleLoad > olcModuleLoad: {2}auditlog.la > > dn: olcOverlay=auditlog,olcDatabase={1}hdb,cn=config > changetype: add > objectClass: olcOverlayConfig > objectClass: olcAuditLogConfig > olcOverlay: auditlog > olcAuditlogFile: /tmp/auditlog.ldif In this example for testing, we are logging changes to {{F:/tmp/auditlog.ldif}} A typical {{TERM:LDIF}} file created by {{B:slapo-auditlog (5)}} would look like: > # add 1196797576 dc=suretecsystems,dc=com cn=admin,dc=suretecsystems,dc=com > dn: dc=suretecsystems,dc=com > changetype: add > objectClass: dcObject > objectClass: organization > dc: suretecsystems > o: Suretec Systems Ltd. > structuralObjectClass: organization > entryUUID: 1606f8f8-f06e-1029-8289-f0cc9d81e81a > creatorsName: cn=admin,dc=suretecsystems,dc=com > modifiersName: cn=admin,dc=suretecsystems,dc=com > createTimestamp: 20051123130912Z > modifyTimestamp: 20051123130912Z > entryCSN: 20051123130912.000000Z#000001#000#000000 > auditContext: cn=accesslog > # end add 1196797576 > > # add 1196797577 dc=suretecsystems,dc=com cn=admin,dc=suretecsystems,dc=com > dn: ou=Groups,dc=suretecsystems,dc=com > changetype: add > objectClass: top > objectClass: organizationalUnit > ou: Groups > structuralObjectClass: organizationalUnit > entryUUID: 160aaa2a-f06e-1029-828a-f0cc9d81e81a > creatorsName: cn=admin,dc=suretecsystems,dc=com > modifiersName: cn=admin,dc=suretecsystems,dc=com > createTimestamp: 20051123130912Z > modifyTimestamp: 20051123130912Z > entryCSN: 20051123130912.000000Z#000002#000#000000 > # end add 1196797577 H2: Chaining H3: Overview The chain overlay provides basic chaining capability to the underlying database. What is chaining? It indicates the capability of a DSA to follow referrals on behalf of the client, so that distributed systems are viewed as a single virtual DSA by clients that are otherwise unable to "chase" (i.e. follow) referrals by themselves. The chain overlay is built on top of the ldap backend; it is compiled by default when {{B:--enable-ldap}}. H3: Chaining Configuration In order to demonstrate how this overlay works, we shall discuss a typical scenario which might be one master server and three Syncrepl slaves. On each replica, add this near the top of the file (global), before any database definitions: > overlay chain > chain-uri "ldap://ldapmaster.example.com" > chain-idassert-bind bindmethod="simple" > binddn="cn=Manager,dc=example,dc=com" > credentials="" > mode="self" > chain-tls start > chain-return-error TRUE Add this below your {{syncrepl}} statement: > updateref "ldap://ldapmaster.example.com/" The {{B:chain-tls}} statement enables TLS from the slave to the ldap master. The DITs are exactly the same between these machines, therefore whatever user bound to the slave will also exist on the master. If that DN does not have update privileges on the master, nothing will happen. You will need to restart the slave after these changes. Then, if you are using {{loglevel 256}}, you can monitor an {{ldapmodify}} on the slave and the master. Now start an {{ldapmodify}} on the slave and watch the logs. You should expect something like: > Sep 6 09:27:25 slave1 slapd[29274]: conn=11 fd=31 ACCEPT from IP=143.199.102.216:45181 (IP=143.199.102.216:389) > Sep 6 09:27:25 slave1 slapd[29274]: conn=11 op=0 STARTTLS > Sep 6 09:27:25 slave1 slapd[29274]: conn=11 op=0 RESULT oid= err=0 text= > Sep 6 09:27:25 slave1 slapd[29274]: conn=11 fd=31 TLS established tls_ssf=256 ssf=256 > Sep 6 09:27:28 slave1 slapd[29274]: conn=11 op=1 BIND dn="uid=user1,ou=people,dc=example,dc=com" method=128 > Sep 6 09:27:28 slave1 slapd[29274]: conn=11 op=1 BIND dn="uid=user1,ou=People,dc=example,dc=com" mech=SIMPLE ssf=0 > Sep 6 09:27:28 slave1 slapd[29274]: conn=11 op=1 RESULT tag=97 err=0 text= > Sep 6 09:27:28 slave1 slapd[29274]: conn=11 op=2 MOD dn="uid=user1,ou=People,dc=example,dc=com" > Sep 6 09:27:28 slave1 slapd[29274]: conn=11 op=2 MOD attr=mail > Sep 6 09:27:28 slave1 slapd[29274]: conn=11 op=2 RESULT tag=103 err=0 text= > Sep 6 09:27:28 slave1 slapd[29274]: conn=11 op=3 UNBIND > Sep 6 09:27:28 slave1 slapd[29274]: conn=11 fd=31 closed > Sep 6 09:27:28 slave1 slapd[29274]: syncrepl_entry: LDAP_RES_SEARCH_ENTRY(LDAP_SYNC_MODIFY) > Sep 6 09:27:28 slave1 slapd[29274]: syncrepl_entry: be_search (0) > Sep 6 09:27:28 slave1 slapd[29274]: syncrepl_entry: uid=user1,ou=People,dc=example,dc=com > Sep 6 09:27:28 slave1 slapd[29274]: syncrepl_entry: be_modify (0) And on the master you will see this: > Sep 6 09:23:57 ldapmaster slapd[2961]: conn=55902 op=3 PROXYAUTHZ dn="uid=user1,ou=people,dc=example,dc=com" > Sep 6 09:23:57 ldapmaster slapd[2961]: conn=55902 op=3 MOD dn="uid=user1,ou=People,dc=example,dc=com" > Sep 6 09:23:57 ldapmaster slapd[2961]: conn=55902 op=3 MOD attr=mail > Sep 6 09:23:57 ldapmaster slapd[2961]: conn=55902 op=3 RESULT tag=103 err=0 text= Note: You can clearly see the PROXYAUTHZ line on the master, indicating the proper identity assertion for the update on the master. Also note the slave immediately receiving the Syncrepl update from the master. H3: Handling Chaining Errors By default, if chaining fails, the original referral is returned to the client under the assumption that the client might want to try and follow the referral. With the following directive however, if the chaining fails at the provider side, the actual error is returned to the client. > chain-return-error TRUE H2: Constraints H3: Overview This overlay enforces a regular expression constraint on all values of specified attributes during an LDAP modify request that contains add or modify commands. It is used to enforce a more rigorous syntax when the underlying attribute syntax is too general. H3: Constraint Configuration Configuration via {{slapd.conf}}(5) would look like: > overlay constraint > constraint_attribute mail regex ^[:alnum:]+@mydomain.com$ > constraint_attribute title uri > ldap:///dc=catalog,dc=example,dc=com?title?sub?(objectClass=titleCatalog) A specification like the above would reject any {{mail}} attribute which did not look like {{@mydomain.com}}. It would also reject any title attribute whose values were not listed in the title attribute of any {{titleCatalog}} entries in the given scope. An example for use with {{cn=config}}: > dn: cn=module{0},cn=config > changetype: modify > add: olcModuleLoad > olcModuleLoad: {1}constraint.la > > dn: olcOverlay=constraint,olcDatabase={1}hdb,cn=config > changetype: add > objectClass: olcOverlayConfig > objectClass: olcConstraintConfig > olcOverlay: constraint > olcConstraintAttribute: mail regex ^[:alnum:]+@mydomain.com$ > olcConstraintAttribute: title uri ldap:///dc=catalog,dc=example,dc=com?title?sub?(objectClass=titleCatalog) H2: Dynamic Directory Services H3: Overview This overlay supports dynamic objects, which have a limited life after which they expire and are automatically deleted. H3: Dynamic Directory Service Configuration H2: Dynamic Groups H3: Overview This overlay extends the Compare operation to detect members of a dynamic group. This overlay is now deprecated as all of its functions are available using the {{SECT:Dynamic Lists}} overlay. H3: Dynamic Group Configuration H2: Dynamic Lists H3: Overview This overlay allows expansion of dynamic groups and lists. Instead of having the group members or list attributes hard coded, this overlay allows us to define an LDAP search whose results will make up the group or list. H3: Dynamic List Configuration This module can behave both as a dynamic list and dynamic group, depending on the configuration. The syntax is as follows: > overlay dynlist > dynlist-attrset [member-ad] The parameters to the {{F:dynlist-attrset}} directive have the following meaning: * {{F:}}: specifies which object class triggers the subsequent LDAP search. Whenever an entry with this object class is retrieved, the search is performed. * {{F:}}: is the name of the attribute which holds the search URI. It has to be a subtype of {{F:labeledURI}}. The attributes and values present in the search result are added to the entry unless {{F:member-ad}} is used (see below). * {{F:member-ad}}: if present, changes the overlay behavior into a dynamic group. Instead of inserting the results of the search in the entry, the distinguished name of the results are added as values of this attribute. Here is an example which will allow us to have an email alias which automatically expands to all user's emails according to our LDAP filter: In {{slapd.conf}}(5): > overlay dynlist > dynlist-attrset nisMailAlias labeledURI This means that whenever an entry which has the {{F:nisMailAlias}} object class is retrieved, the search specified in the {{F:labeledURI}} attribute is performed. Let's say we have this entry in our directory: > cn=all,ou=aliases,dc=example,dc=com > cn: all > objectClass: nisMailAlias > labeledURI: ldap:///ou=People,dc=example,dc=com?mail?one?(objectClass=inetOrgPerson) If this entry is retrieved, the search specified in {{F:labeledURI}} will be performed and the results will be added to the entry just as if they have always been there. In this case, the search filter selects all entries directly under {{F:ou=People}} that have the {{F:inetOrgPerson}} object class and retrieves the {{F:mail}} attribute, if it exists. This is what gets added to the entry when we have two users under {{F:ou=People}} that match the filter: !import "allmail-en.png"; align="center"; title="Dynamic list for email aliases" FT[align="Center"] Figure X.Y: Dynamic List for all emails The configuration for a dynamic group is similar. Let's see an example which would automatically populate an {{F:allusers}} group with all the user accounts in the directory. In {{F:slapd.conf}}(5): > overlay dynlist > dynlist-attrset groupOfNames labeledURI member Let's apply it to the following entry: > cn=allusers,ou=group,dc=example,dc=com > cn: all > objectClass: groupOfNames > labeledURI: ldap:///ou=people,dc=example,dc=com??one?(objectClass=inetOrgPerson) The behavior is similar to the dynamic list configuration we had before: whenever an entry with the {{F:groupOfNames}} object class is retrieved, the search specified in the {{F:labeledURI}} attribute is performed. But this time, only the distinguished names of the results are added, and as values of the {{F:member}} attribute. This is what we get: !import "allusersgroup-en.png"; align="center"; title="Dynamic group for all users" FT[align="Center"] Figure X.Y: Dynamic Group for all users Note that a side effect of this scheme of dynamic groups is that the members need to be specified as full DNs. So, if you are planning in using this for {{F:posixGroup}}s, be sure to use RFC2307bis and some attribute which can hold distinguished names. The {{F:memberUid}} attribute used in the {{F:posixGroup}} object class can hold only names, not DNs, and is therefore not suitable for dynamic groups. H2: Reverse Group Membership Maintenance H3: Overview In some scenarios, it may be desirable for a client to be able to determine which groups an entry is a member of, without performing an additional search. Examples of this are applications using the {{TERM:DIT}} for access control based on group authorization. The {{B:memberof}} overlay updates an attribute (by default {{B:memberOf}}) whenever changes occur to the membership attribute (by default {{B:member}}) of entries of the objectclass (by default {{B:groupOfNames}}) configured to trigger updates. Thus, it provides maintenance of the list of groups an entry is a member of, when usual maintenance of groups is done by modifying the members on the group entry. H3: Member Of Configuration The typical use of this overlay requires just enabling the overlay for a specific database. For example, with the following minimal slapd.conf: > include /usr/share/openldap/schema/core.schema > include /usr/share/openldap/schema/cosine.schema > modulepath /usr/lib/openldap > moduleload memberof.la > authz-regexp "gidNumber=0\\\+uidNumber=0,cn=peercred,cn=external,cn=auth" > "cn=Manager,dc=example,dc=com" > database bdb > suffix "dc=example,dc=com" > rootdn "cn=Manager,dc=example,dc=com" > rootpw secret > directory /var/lib/ldap2.4 > checkpoint 256 5 > index objectClass eq > index uid eq,sub > > overlay memberof adding the following ldif: > cat memberof.ldif > dn: dc=example,dc=com > objectclass: domain > dc: example > > dn: ou=Group,dc=example,dc=com > objectclass: organizationalUnit > ou: Group > > dn: ou=People,dc=example,dc=com > objectclass: organizationalUnit > ou: People > > dn: uid=test1,ou=People,dc=example,dc=com > objectclass: account > uid: test1 > > dn: cn=testgroup,ou=Group,dc=example,dc=com > objectclass: groupOfNames > cn: testgroup > member: uid=test1,ou=People,dc=example,dc=com Results in the following output from a search on the test1 user: > # ldapsearch -LL -Y EXTERNAL -H ldapi:/// "(uid=test1)" -b dc=example,dc=com memberOf > SASL/EXTERNAL authentication started > SASL username: gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth > SASL SSF: 0 > version: 1 > > dn: uid=test1,ou=People,dc=example,dc=com > memberOf: cn=testgroup,ou=Group,dc=example,dc=com Note that the {{B:memberOf}} attribute is an operational attribute, so it must be requested explicitly. H2: The Proxy Cache Engine {{TERM:LDAP}} servers typically hold one or more subtrees of a {{TERM:DIT}}. Replica (or shadow) servers hold shadow copies of entries held by one or more master servers. Changes are propagated from the master server to replica (slave) servers using LDAP Sync replication. An LDAP cache is a special type of replica which holds entries corresponding to search filters instead of subtrees. H3: Overview The proxy cache extension of slapd is designed to improve the responsiveness of the ldap and meta backends. It handles a search request (query) by first determining whether it is contained in any cached search filter. Contained requests are answered from the proxy cache's local database. Other requests are passed on to the underlying ldap or meta backend and processed as usual. E.g. {{EX:(shoesize>=9)}} is contained in {{EX:(shoesize>=8)}} and {{EX:(sn=Richardson)}} is contained in {{EX:(sn=Richards*)}} Correct matching rules and syntaxes are used while comparing assertions for query containment. To simplify the query containment problem, a list of cacheable "templates" (defined below) is specified at configuration time. A query is cached or answered only if it belongs to one of these templates. The entries corresponding to cached queries are stored in the proxy cache local database while its associated meta information (filter, scope, base, attributes) is stored in main memory. A template is a prototype for generating LDAP search requests. Templates are described by a prototype search filter and a list of attributes which are required in queries generated from the template. The representation for prototype filter is similar to {{REF:RFC4515}}, except that the assertion values are missing. Examples of prototype filters are: (sn=),(&(sn=)(givenname=)) which are instantiated by search filters (sn=Doe) and (&(sn=Doe)(givenname=John)) respectively. The cache replacement policy removes the least recently used (LRU) query and entries belonging to only that query. Queries are allowed a maximum time to live (TTL) in the cache thus providing weak consistency. A background task periodically checks the cache for expired queries and removes them. The Proxy Cache paper ({{URL:http://www.openldap.org/pub/kapurva/proxycaching.pdf}}) provides design and implementation details. H3: Proxy Cache Configuration The cache configuration specific directives described below must appear after a {{EX:overlay proxycache}} directive within a {{EX:"database meta"}} or {{EX:database ldap}} section of the server's {{slapd.conf}}(5) file. H4: Setting cache parameters > proxyCache This directive enables proxy caching and sets general cache parameters. The parameter specifies which underlying database is to be used to hold cached entries. It should be set to {{EX:bdb}} or {{EX:hdb}}. The parameter specifies the total number of entries which may be held in the cache. The parameter specifies the total number of attribute sets (as specified by the {{EX:proxyAttrSet}} directive) that may be defined. The parameter specifies the maximum number of entries in a cacheable query. The specifies the consistency check period (in seconds). In each period, queries with expired TTLs are removed. H4: Defining attribute sets > proxyAttrset Used to associate a set of attributes to an index. Each attribute set is associated with an index number from 0 to -1. These indices are used by the proxyTemplate directive to define cacheable templates. H4: Specifying cacheable templates > proxyTemplate Specifies a cacheable template and the "time to live" (in sec) for queries belonging to the template. A template is described by its prototype filter string and set of required attributes identified by . H4: Example An example {{slapd.conf}}(5) database section for a caching server which proxies for the {{EX:"dc=example,dc=com"}} subtree held at server {{EX:ldap.example.com}}. > database ldap > suffix "dc=example,dc=com" > rootdn "dc=example,dc=com" > uri ldap://ldap.example.com/ > overlay proxycache > proxycache bdb 100000 1 1000 100 > proxyAttrset 0 mail postaladdress telephonenumber > proxyTemplate (sn=) 0 3600 > proxyTemplate (&(sn=)(givenName=)) 0 3600 > proxyTemplate (&(departmentNumber=)(secretary=*)) 0 3600 > > cachesize 20 > directory ./testrun/db.2.a > index objectClass eq > index cn,sn,uid,mail pres,eq,sub H5: Cacheable Queries A LDAP search query is cacheable when its filter matches one of the templates as defined in the "proxyTemplate" statements and when it references only the attributes specified in the corresponding attribute set. In the example above the attribute set number 0 defines that only the attributes: {{EX:mail postaladdress telephonenumber}} are cached for the following proxyTemplates. H5: Examples: > Filter: (&(sn=Richard*)(givenName=jack)) > Attrs: mail telephoneNumber is cacheable, because it matches the template {{EX:(&(sn=)(givenName=))}} and its attributes are contained in proxyAttrset 0. > Filter: (&(sn=Richard*)(telephoneNumber)) > Attrs: givenName is not cacheable, because the filter does not match the template, nor is the attribute givenName stored in the cache > Filter: (|(sn=Richard*)(givenName=jack)) > Attrs: mail telephoneNumber is not cacheable, because the filter does not match the template ( logical OR "|" condition instead of logical AND "&" ) H2: Password Policies H3: Overview This overlay provides a variety of password control mechanisms, e.g. password aging, password reuse and duplication control, mandatory password resets, etc. H3: Password Policy Configuration H2: Referential Integrity H3: Overview This overlay can be used with a backend database such as slapd-bdb(5) to maintain the cohesiveness of a schema which utilizes reference attributes. Whenever a {{modrdn}} or {{delete}} is performed, that is, when an entry's DN is renamed or an entry is removed, the server will search the directory for references to this DN (in selected attributes: see below) and update them accordingly. If it was a {{delete}} operation, the reference is deleted. If it was a {{modrdn}} operation, then the reference is updated with the new DN. For example, a very common administration task is to maintain group membership lists, specially when users are removed from the directory. When an user account is deleted or renamed, all groups this user is a member of have to be updated. LDAP administrators usually have scripts for that. But we can use the {{F:refint}} overlay to automate this task. In this example, if the user is removed from the directory, the overlay will take care to remove the user from all the groups he/she was a member of. No more scripting for this. H3: Referential Integrity Configuration The configuration for this overlay is as follows: > overlay refint > refint_attributes > refint_nothing * {{F:refint_attributes}}: this parameter specifies a space separated list of attributes which will have the referential integrity maintained. When an entry is removed or has its DN renamed, the server will do an internal search for any of the {{F:refint_attributes}} that point to the affected DN and update them accordingly. IMPORTANT: the attributes listed here must have the {{F:distinguishedName}} syntax, that is, hold DNs as values. * {{F:refint_nothing}}: some times, while trying to maintain the referential integrity, the server has to remove the last attribute of its kind from an entry. This may be prohibited by the schema: for example, the {{F:groupOfNames}} object class requires at least one member. In these cases, the server will add the attribute value specified in {{F:refint_nothing}} to the entry. To illustrate this overlay, we will use the group membership scenario. In {{F:slapd.conf}}: > overlay refint > refint_attributes member > refint_nothing "cn=admin,dc=example,dc=com" This configuration tells the overlay to maintain the referential integrity of the {{F:member}} attribute. This attribute is used in the {{F:groupOfNames}} object class which always needs a member, so we add the {{F:refint_nothing}} directive to fill in the group with a standard member should all the members vanish. If we have the following group membership, the refint overlay will automatically remove {{F:john}} from the group if his entry is removed from the directory: !import "refint.png"; align="center"; title="Group membership" FT[align="Center"] Figure X.Y: Maintaining referential integrity in groups Notice that if we rename ({{F:modrdn}}) the {{F:john}} entry to, say, {{F:jsmith}}, the refint overlay will also rename the reference in the {{F:member}} attribute, so the group membership stays correct. If we removed all users from the directory who are a member of this group, then the end result would be a single member in the group: {{F:cn=admin,dc=example,dc=com}}. This is the {{F:refint_nothing}} parameter kicking into action so that the schema is not violated. H2: Return Code H3: Overview This overlay is useful to test the behavior of clients when server-generated erroneous and/or unusual responses occur. H3: Return Code Configuration H2: Rewrite/Remap H3: Overview It performs basic DN/data rewrite and objectClass/attributeType mapping. H3: Rewrite/Remap Configuration H2: Sync Provider H3: Overview This overlay implements the provider-side support for syncrepl replication, including persistent search functionality H3: Sync Provider Configuration H2: Translucent Proxy H3: Overview This overlay can be used with a backend database such as slapd-bdb (5) to create a "translucent proxy". Content of entries retrieved from a remote LDAP server can be partially overridden by the database. H3: Translucent Proxy Configuration H2: Attribute Uniqueness H3: Overview This overlay can be used with a backend database such as slapd-bdb (5) to enforce the uniqueness of some or all attributes within a subtree. H3: Attribute Uniqueness Configuration H2: Value Sorting H3: Overview This overlay can be used to enforce a specific order for the values of an attribute when it is returned in a search. H3: Value Sorting Configuration H2: Overlay Stacking H3: Overview H3: Example Scenarios H4: Samba