# $OpenLDAP: pkg/openldap-guide/admin/overlays.sdf,v 1.8.2.5 2007/11/27 19:06:07 quanah Exp $
# 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-<overlay name>

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

>        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/<overlay name>/

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.


H3: Access Logging Configuration


H2: Audit Logging

This overlay records changes on a given backend database to an LDIF log
file.
   
   
H3: Overview


H3: Audit Logging Configuration


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 --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="<secret>" 
>                                   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. It is used to enforce a more rigorous
syntax when the underlying attribute syntax is too general.


H3: Constraint Configuration
   
   
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 <group-oc> <URL-ad> [member-ad]

The parameters to the {{F:dynlist-attrset}} directive have the following meaning:
* {{F:<group-oc>}}: specifies which object class triggers the subsequent LDAP search.
Whenever an entry with this object class is retrieved, the search is performed.
* {{F:<URL-ad>}}: 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 behaviour 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 behaviour 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 dymamic 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 <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
{{EX:bdb}} or {{EX: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 {{EX: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.

H4: Defining attribute sets

> 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.

H4: Specifying 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>.


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 <attribute [attribute ...]>
>       refint_nothing <string>

* {{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
