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# This contribution is derived from OpenLDAP Software.
# All of the modifications to OpenLDAP Software represented in this contribution
# were developed by Andrew Findlay <andrew.findlay@skills-1st.co.uk>.
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# Copyright 2008 Andrew Findlay
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H1: Limits

H2: Introduction

It is usually desirable to limit the server resources that can be
consumed by each LDAP client. OpenLDAP provides two sets of limits:
a size limit, which can restrict the {{number}} of entries that a
client can retrieve in a single operation, and a time limit
which restricts the length of time that an operation may continue.
Both types of limit can be given different values depending on who
initiated the operation.

H2: Soft and Hard limits

The server administrator can specify both {{soft limits}} and
{{hard limits}}. Soft limits can be thought of as being the
default limit value. Hard limits cannot be exceeded by ordinary
LDAP users.

LDAP clients can specify their own
size and time limits when issuing search operations.
This feature has been present since the earliest version of X.500.

If the client specifies a limit then the lower of the requested value
and the {{hard limit}} will become the limit for the operation.

If the client does not specify a limit then the server applies the
{{soft limit}}.

Soft and Hard limits are often referred to together as {{administrative
limits}}. Thus, if an LDAP client requests a search that would return
more results than the limits allow it will get an {{adminLimitExceeded}}
error. Note that the server will usually return some results even if
the limit has been exceeded: this feature is useful to clients that
just want to check for the existence of some entries without needing
to see them all.

The {{rootdn}} is not subject to any limits.

H2: Global Limits

Limits specified in the global part of the server configuration act
as defaults which are used if no database has more specific limits set.

In a {{slapd.conf}}(5) configuration the keywords are {{EX:sizelimit}} and
{{EX:timelimit}}. When using the {{slapd config}} backend, the corresponding
attributes are {{EX:olcSizeLimit}} and {{EX:olcTimeLimit}}. The syntax of
these values are the same in both cases.

The simple form sets both soft and hard limits to the same value:

>   sizelimit {<integer>|unlimited}
>   timelimit {<integer>|unlimited}

The default sizelimit is 500 entries and the default timelimit is
3600 seconds.

An extended form allows soft and hard limits to be set separately:

>   sizelimit size[.{soft|hard}]=<integer> [...]
>   timelimit time[.{soft|hard}]=<integer> [...]

Thus, to set a soft sizelimit of 10 entries and a hard limit of 75 entries:

E:  sizelimit size.soft=10 size.hard=75

H3: Special Size Limits

There are other forms of size limits in addition to the soft and hard limits.
Note that when using the simple {{sizelimit}} form, none of these special
limits are changed.

H4: Unchecked Limits

The {{unchecked}} keyword sets a limit on how many entries the server
will examine after doing index lookups but before evaluating filter
matches. If the set of candidates exceeds this limit, the search is aborted.
The purpose is to avoid causing excessive workload on {{slapd}}
if a filter uses attributes that are not properly indexed, and can
be critical for very large directories.

>   sizelimit size.unchecked={<integer>|unlimited|disabled}

The default is unlimited. The {{disabled}} setting prevents a search
from being performed at all. This may be useful in the per-database
limits described below, to disallow searches for a specific set of users.

H4: Paged Results Limits

If the LDAP client adds the {{pagedResultsControl}} to the search operation,
the hard size limit is used by default, because the request for a specific
page size is considered an explicit request for a limitation on the number
of entries to be returned. However, the size limit applies to the total
count of entries returned within the search, and not to a single page.

Additional size limits may be enforced for paged searches.

The {{EX:size.pr}} limit controls the maximum page size:

>   sizelimit size.pr={<integer>|noEstimate|unlimited}

{{EX:<integer>}} is the maximum page size if no explicit size is set.
{{EX:noEstimate}} has no effect in the current implementation as the
server does not return an estimate of the result size anyway.
{{EX:unlimited}} indicates that no limit is applied to the maximum
page size.

The {{EX:size.prtotal}} limit controls the total number of entries
that can be returned by a paged search. By default the limit is the
same as the normal {{EX:size.hard}} limit.

>   size.prtotal={<integer>|unlimited|disabled}

{{EX:unlimited}} removes the limit on the number of entries that can be
returned by a paged search.
{{EX:disabled}} can be used to selectively disable paged result searches.

H2: Per-Database Limits

Each database can have its own set of limits that override the global
ones. The syntax is more flexible, and it allows different limits to
be applied to different entities. Note that an {{entity}} is different from
an {{entry}}: the term {{entity}} is used here to indicate the ID of the
person or process that has initiated the LDAP operation.

In a {{slapd.conf}}(5) configuration the keyword is {{EX:limits}}.
When using the {{slapd config}} backend, the corresponding
attribute is {{EX:olcLimits}}. The syntax of
the values is the same in both cases.

>   limits <selector> <limit> [<limit> [...]]

The {{limits}} clause can be specified multiple times to apply different
limits to different initiators. The server examines each clause in turn
until it finds one that matches the operation's initiator or base DN.
If no match is found, the global limits will be used.

H3: Specify who the limits apply to

The {{EX:<selector>}} part of the {{limits}} clause can take any of these values:

!block table; align=Center; coltags="EX,N"; \
    title="Table 9.1: Limits Entity Specifiers"
Specifier|Entities
*|All, including anonymous and authenticated users
anonymous|Anonymous (non-authenticated) users
users|Authenticated users
dn[.<type>][.<style>]=<pattern>]|Entry or entries within a scope that match <pattern>
group[/oc[/at]]=<pattern>|Members of a group
!endblock

Where

{{type}} can be one of self or this and

{{style}} can be one of exact, base, onelevel, subtree, children, regex, or anonymous

More information can be found in the {{slapd.conf}}(5) or {{slapd-config}}(5) manual
pages.

H3: Specify time limits

The syntax for time limits is 

E:   time[.{soft|hard}]=<integer>

where integer is the number of seconds slapd will spend
answering a search request.

If neither {{soft}} nor {{hard}} is specified, the value is used for both,
e.g.:

E:   limits anonymous time=27

The value {{unlimited}} may be used to remove the hard time limit entirely,
e.g.:

E:   limits dn.exact="cn=anyuser,dc=example,dc=org" time.hard=unlimited

H3: Specifying size limits

The syntax for size limit is 

E:   size[.{soft|hard}]=<integer>

where {{EX:<integer>}} is the maximum number of entries slapd will return
when answering a search request.

In addition to soft and hard limits, other limits are also available,
with the same meanings described for the global limits configuration above.


H2: Example Limit Configurations

H3: Simple Global Limits

This simple global configuration fragment applies size and time limits
to all searches by all users except {{rootdn}}. It limits searches to
50 results and sets an overall time limit of 10 seconds.

E:   sizelimit 50
E:   timelimit 10

H3: Global Hard and Soft Limits

It is sometimes useful to limit the size of result sets but to allow
clients to request a higher limit where needed. This can be achieved
by setting separate hard and soft limits.

E:   sizelimit size.soft=5 size.hard=100

To prevent clients from doing very inefficient non-indexed searches,
add the {{unchecked}} limit:

E:   sizelimit size.soft=5 size.hard=100 size.unchecked=100

H3: Giving specific users larger limits

Having set appropriate default limits in the global configuration,
you may want to give certain users the ability to retrieve larger
result sets. Here is a way to do that in the per-database configuration:

E:   limits dn.exact="cn=anyuser,dc=example,dc=org" size=100000
E:   limits dn.exact="cn=personnel,dc=example,dc=org" size=100000
E:   limits dn.exact="cn=dirsync,dc=example,dc=org" size=100000

It is generally best to avoid mentioning specific users in the server
configuration. A better way is to give the higher limits to a group:

E:   limits group/groupOfNames/member="cn=bigwigs,dc=example,dc=org" size=100000

H3: Limiting who can do paged searches

It may be required that certain applications need very large result sets that
they retrieve using paged searches, but that you do not want ordinary
LDAP users to use the pagedResults control. The {{pr}} and {{prtotal}}
limits can help:

E:   limits group/groupOfNames/member="cn=dirsync,dc=example,dc=org" size.prtotal=unlimited
E:   limits users size.soft=5 size.hard=100 size.prtotal=disabled
E:   limits anonymous size.soft=2 size.hard=5 size.prtotal=disabled

H2: Glued/Subordinate database configurations

When using subordinate databases, it is necessary for any limits that
are to be applied across the parent and its subordinates to be defined in both
the parent and its subordinates. Otherwise the settings on the subordinate databases
are not honored.

H2: Further Information

For further information please see {{slapd.conf}}(5), {{ldapsearch}}(1) and {{slapd.access}}(5)

