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openldap/doc/guide/admin/overlays.sdf
Gavin Henry 7bb6eddf06 Wee chaining update.
2007-10-18 22:46:07 +00:00

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# $OpenLDAP$
# 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-<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
> 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 more.
H3: Dynamic List Configuration
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/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
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.
H3: Referential Integrity Configuration
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
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