Section (5) slapo-pcache
Name
slapo−pcache — proxy cache overlay to slapd
Synopsis
ETCDIR/slapd.conf
DESCRIPTION
The pcache
overlay to slapd(8) allows caching of
LDAP search requests (queries) in a local database. For an
incoming query, the proxy cache determines its corresponding
template
. If the
template was specified as cacheable using the pcacheTemplate
directive and
the request is contained in a cached request, it is answered
from the proxy cache. Otherwise, the search is performed as
usual and cacheable search results are saved in the cache for
use in future queries.
A template is defined by a filter string and an index
identifying a set of attributes. The template string for a query can be
obtained by removing assertion values from the RFC 4515
representation of its search filter. A query belongs to a
template if its template string and set of projected
attributes correspond to a cacheable template. Examples of
template strings are (mail=)
, (|(sn=)(cn=))
, (&(sn=)(givenName=))
.
The config directives that are specific to the pcache
overlay can be
prefixed by pcache−
, to avoid
conflicts with directives specific to the underlying database
or to other stacked overlays. This may be particularly useful
for those directives that refer to the backend used for local
storage. The following cache specific directives can be used
to configure the proxy cache:
- verlay pcache
-
This directive adds the proxy cache overlay to the current backend. The proxy cache overlay may be used with any backend but is intended for use with the ldap,
meta
, andsql
backends. Please note that the underlying backend must have a configuredrootdn.
- pcache <database> <max_entries> <numattrsets> <entry_limit> <cc_period>
-
The directive enables proxy caching in the current backend and sets general cache parameters. A <database> backend will be used internally to maintain the cached entries. The chosen database will need to be configured as well, as shown below. Cache replacement is invoked when the cache size grows to <max_entries> entries and continues till the cache size drops below this size. <numattrsets> should be equal to the number of following
pcacheAttrset
directives. Queries are cached only if they correspond to a cacheable template (specified by thepcacheTemplate
directive) and the number of entries returned is less than <entry_limit>. Consistency check is performed every <cc_period> duration (specified in secs). In each cycle queries with expired time to live(TTL
) are removed. A sample cache configuration is:
pcache mdb 10000 1 50 100
- pcacheAttrset <index> <attrs...>
-
Used to associate a set of attributes <attrs..> with an <index>. Each attribute set is associated with an integer from 0 to <numattrsets>−1. These indices are used by the
pcacheTemplate
directive to define cacheable templates. A set of attributes cannot be empty. A set of attributes can contain the special attributes * (all user attributes), + (all operational attributes) or both; in the latter case, any other attribute is redundant and should be avoided for clarity. A set of attributes can contain 1.1 as the only attribute; in this case, only the presence of the entries is cached. Attributes prefixed by undef: need not be present in the schema. - pcacheMaxQueries <queries>
-
Specify the maximum number of queries to cache. The default is 10000.
- pcacheValidate { TRUE | FALSE }
-
Check whether the results of a query being cached can actually be returned from the cache by the proxy DSA. When enabled, the entries being returned while caching the results of a query are checked to ensure consistency with the schema known to the proxy DSA. In case of failure, the query is not cached. By default, the check is off.
- pcacheOffline { TRUE | FALSE }
-
Set the cache to offline mode. While offline, the consistency checker will be stopped and no expirations will occur. This allows the cache contents to be used indefinitely while the proxy is cut off from network access to the remote DSA. The default is FALSE, i.e. consistency checks and expirations will be performed.
- pcachePersist { TRUE | FALSE }
-
Specify whether the cached queries should be saved across restarts of the caching proxy, to provide hot startup of the cache. Only non-expired queries are reloaded. The default is FALSE.
CAVEAT
: of course, the configuration of the proxy cache must not change across restarts; the pcache overlay does not perform any consistency checks in this sense. In detail, this option should be disabled unless the existingpcacheAttrset
andpcacheTemplate
directives are not changed neither in order nor in contents. If new sets and templates are added, or if other details of the pcache overlay configuration changed, this feature should not be affected. - pcacheTemplate <template_string> <attrset_index> <ttl> [<negttl> [<limitttl> [<ttr>]]]
-
Specifies a cacheable template and time to live <ttl> of queries belonging to the template. An optional <negttl> can be used to specify that negative results (i.e., queries that returned zero entries) should also be cached for the specified amount of time. Negative results are not cached by default (<negttl> set to 0). An optional <limitttl> can be used to specify that results hitting a sizelimit should also be cached for the specified amount of time. Results hitting a sizelimit are not cached by default (<limitttl> set to 0). An optional <ttr> time to refresh can be used to specify that cached entries should be automatically refreshed after a certain time. Entries will only be refreshed while they have not expired, so the <ttl> should be larger than the <ttr> for this option to be useful. Entries are not refreshed by default (<ttr> set to 0).
- pcacheBind <filter_template> <attrset_index> <ttr> <scope> <base>
-
Specifies a template for caching Simple Bind credentials based on an already defined
pcacheTemplate
. The <filter_template> is similar to a <template_string> except that it may have some values present. Its purpose is to allow the overlay to generate filters similar to what other applications do when they do a Search immediately before a Bind. E.g., if a client like nss_ldap is configured to search for a user with the filter (&(objectClass=posixAccount)(uid=<username>)) then the corresponding template (&(objectClass=posixAccount)(uid=)) should be used here. When converted to a regular template e.g. (&(objectClass=)(uid=)) this template and the <attrset_index> must match an already definedpcacheTemplate
clause. The time to refresh <ttr> determines the time interval after which the cached credentials may be refreshed. The first Bind request that occurs after that time will trigger the refresh attempt. Refreshes are not performed when the overlay is Offline. There is no time to live parameter for the Bind credentials; the credentials will expire according to thepcacheTemplate
ttl. The <scope> and <base> should match the search scope and base used by the authentication clients. The cached credentials are not stored in cleartext, they are hashed using the default password hash. By default Bind caching is not enabled. - pcachePosition { head | tail }
-
Specifies whether the response callback should be placed at the
tail
(the default) or at thehead
(actually, wherever the stacking sequence would make it appear) of the callback list. This affects how the overlay interacts with other overlays, since the proxycache overlay should be executed as early as possible (and thus configured as late as possible), to get a chance to return the cached results; however, if executed early at response, it would cache entries that may be later massaged by other databases and thus returnedafter
massaging the first time, andbefore
massaging when cached. - There are some constraints:
-
all values must be positive;
<entry_limit>
must be less than or equal to<max_entries>
;<numattrsets>
attribute sets SHOULD be defined by using the directivepcacheAttrset
;all attribute sets SHOULD be referenced by (at least) one
pcacheTemplate
directive;The following adds a template with filter string
(&(sn=)(givenName=))
and attributes mail, postaladdress, telephonenumber and a TTL of 1 hour.
pcacheAttrset 0 mail postaladdress telephonenumber pcacheTemplate (&(sn=)(givenName=)) 0 3600
Directives for configuring the underlying database must also be given, as shown here:
directory /var/tmp/cache cachesize 100
Any valid directives for the chosen database type may be
used. Indexing should be used as appropriate for the queries
being handled. In addition, an equality index on the
pcacheQueryid
attribute should be configured, to assist in the removal of
expired query data.
BACKWARD COMPATIBILITY
The configuration keywords have been renamed and the older form is deprecated. These older keywords are still recognized but may disappear in future releases.
proxycache
-
use pcache
proxyattrset
-
use pcacheAttrset
proxycachequeries
-
use pcacheMaxQueries
proxycheckcacheability
-
use pcacheValidate
proxysavequeries
-
use pcachePersist
proxytemplate
-
use pcacheTemplate
- response-callback
-
use pcachePosition
CAVEATS
Caching data is prone to inconsistencies because updates
on the remote server will not be reflected in the response of
the cache at least (and at most) for the duration of the
pcacheTemplate
TTL
. These inconsistencies can
be minimized by careful use of the TTR.
The remote server should expose the objectClass
attribute because
the underlying database that actually caches the entries may
need it for optimal local processing of the queries.
The proxy server should contain all the schema information required for caching. Significantly, it needs the schema of attributes used in the query templates. If the objectClass attribute is used in a query template, it needs the definition of the objectClasses of the entries it is supposed to cache. It is the responsibility of the proxy administrator to keep the proxy schema lined up with that of the proxied server.
Another potential (and subtle) inconsistency may occur
when data is retrieved with different identities and specific
per-identity access control is enforced by the remote server.
If data was retrieved with an identity that collected only
partial results because of access rules enforcement on the
remote server, other users with different access privileges
on the remote server will get different results from the
remote server and from the cache. If those users have higher
access privileges on the remote server, they will get from
the cache only a subset of the results they would get
directly from the remote server; but if they have lower
access privileges, they will get from the cache a superset of
the results they would get directly from the remote server.
Either occurrence may or may not be acceptable, based on the
security policy of the cache and of the remote server. It is
important to note that in this case the proxy is violating
the security of the remote server by disclosing to an
identity data that was collected by another identity. For
this reason, it is suggested that, when using back-ldap
, proxy caching be
used in conjunction with the identity assertion feature of
slapd-ldap(5) (see the
idassert−bind
and the idassert−authz
statements), so that remote server interrogation occurs with
a vanilla identity that has some relatively high search
and read
access privileges, and
the real access control is delegated to the proxy_zsingle_quotesz_s ACLs.
Beware that since only the cached fraction of the real datum
is available to the cache, it may not be possible to enforce
the same access rules that are defined on the remote server.
When security is a concern, cached proxy access must be
carefully tailored.