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
Not all distributed overlays have a man page yet. Feel free to contribute one, if you think you well understood the behavior of the component and the implications of all the related configuration directives.
Official overlays are located in
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
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.
This overlay can record accesses to a given backend database on another database.
This overlay records changes on a given backend database to an LDIF log file.
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 --enable-ldap.
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="<secret>" mode="self" chain-tls start chain-return-error TRUE updateref "ldap://ldapmaster.example.com/"
The 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: conn=11 fd=31 ACCEPT from IP=220.127.116.11:45181 (IP=18.104.22.168:389) Sep 6 09:27:25 slave1 slapd: conn=11 op=0 STARTTLS Sep 6 09:27:25 slave1 slapd: conn=11 op=0 RESULT oid= err=0 text= Sep 6 09:27:25 slave1 slapd: conn=11 fd=31 TLS established tls_ssf=256 ssf=256 Sep 6 09:27:28 slave1 slapd: conn=11 op=1 BIND dn="uid=user1,ou=people,dc=example,dc=com" method=128 Sep 6 09:27:28 slave1 slapd: 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: conn=11 op=1 RESULT tag=97 err=0 text= Sep 6 09:27:28 slave1 slapd: conn=11 op=2 MOD dn="uid=user1,ou=People,dc=example,dc=com" Sep 6 09:27:28 slave1 slapd: conn=11 op=2 MOD attr=mail Sep 6 09:27:28 slave1 slapd: conn=11 op=2 RESULT tag=103 err=0 text= Sep 6 09:27:28 slave1 slapd: conn=11 op=3 UNBIND Sep 6 09:27:28 slave1 slapd: conn=11 fd=31 closed Sep 6 09:27:28 slave1 slapd: syncrepl_entry: LDAP_RES_SEARCH_ENTRY(LDAP_SYNC_MODIFY) Sep 6 09:27:28 slave1 slapd: syncrepl_entry: be_search (0) Sep 6 09:27:28 slave1 slapd: syncrepl_entry: uid=user1,ou=People,dc=example,dc=com Sep 6 09:27:28 slave1 slapd: syncrepl_entry: be_modify (0)
And on the master you will see this:
Sep 6 09:23:57 ldapmaster slapd: conn=55902 op=3 PROXYAUTHZ dn="uid=user1,ou=people,dc=example,dc=com" Sep 6 09:23:57 ldapmaster slapd: conn=55902 op=3 MOD dn="uid=user1,ou=People,dc=example,dc=com" Sep 6 09:23:57 ldapmaster slapd: conn=55902 op=3 MOD attr=mail Sep 6 09:23:57 ldapmaster slapd: 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.
This overlay enforces a regular expression constraint on all values of specified attributes. It is used to enforce a more rigorous syntax when the underlying attribute syntax is too general.
This overlay supports dynamic objects, which have a limited life after which they expire and are automatically deleted.
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 Dynamic Lists overlay.
This overlay allows expansion of dynamic groups and more.
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. (shoesize>=9) is contained in (shoesize>=8) and (sn=Richardson) is contained in (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 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 (http://www.openldap.org/pub/kapurva/proxycaching.pdf) provides design and implementation details.
The cache configuration specific directives described below must appear after a overlay proxycache directive within a "database meta" or database ldap section of the server's slapd.conf(5) file.
proxyCache <DB> <maxentries> <nattrsets> <entrylimit> <period>
This directive enables proxy caching and sets general cache parameters. The <DB> parameter specifies which underlying database is to be used to hold cached entries. It should be set to bdb or hdb. The <maxentries> parameter specifies the total number of entries which may be held in the cache. The <nattrsets> parameter specifies the total number of attribute sets (as specified by the proxyAttrSet directive) that may be defined. The <entrylimit> parameter specifies the maximum number of entries in a cacheable query. The <period> specifies the consistency check period (in seconds). In each period, queries with expired TTLs are removed.
proxyAttrset <index> <attrs...>
Used to associate a set of attributes to an index. Each attribute set is associated with an index number from 0 to <numattrsets>-1. These indices are used by the proxyTemplate directive to define cacheable templates.
proxyTemplate <prototype_string> <attrset_index> <TTL>
Specifies a cacheable template and the "time to live" (in sec) <TTL> for queries belonging to the template. A template is described by its prototype filter string and set of required attributes identified by <attrset_index>.
An example slapd.conf(5) database section for a caching server which proxies for the "dc=example,dc=com" subtree held at server ldap.example.com.
database ldap suffix "dc=example,dc=com" rootdn "dc=example,dc=com" uri ldap://ldap.example.com/dc=example%2cdc=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
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: mail postaladdress telephonenumber are cached for the following proxyTemplates.
Filter: (&(sn=Richard*)(givenName=jack)) Attrs: mail telephoneNumber
is cacheable, because it matches the template (&(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 "&" )
This overlay provides a variety of password control mechanisms, e.g. password aging, password reuse and duplication control, mandatory password resets, etc.
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.
This overlay is useful to test the behavior of clients when server-generated erroneous and/or unusual responses occur.
It performs basic DN/data rewrite and objectClass/attributeType mapping.
This overlay implements the provider-side support for syncrepl replication, including persistent search functionality
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.
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.
This overlay can be used to enforce a specific order for the values of an attribute when it is returned in a search.