slapd-meta(5)
SLAPD-META(5) FILE FORMATS SLAPD-META(5)
NAME
slapd-meta - metadirectory backend
SYNOPSIS
ETCDIR/slapd.conf
DESCRIPTION
The meta backend to slapd(8) performs basic LDAP proxying
with respect to a set of remote LDAP servers, called "tar-
gets". The information contained in these servers can be
presented as belonging to a single Directory Information
Tree (DIT).
A basic knowledge of the functionality of the slapd-ldap(5)
backend is recommended. This backend has been designed as
an enhancement of the ldap backend. The two backends share
many features (actually they also share portions of code).
While the ldap backend is intended to proxy operations
directed to a single server, the meta backend is mainly
intended for proxying of multiple servers and possibly nam-
ing context masquerading. These features, although useful
in many scenarios, may result in excessive overhead for some
applications, so its use should be carefully considered. In
the examples section, some typical scenarios will be dis-
cussed.
EXAMPLES
There are examples in various places in this document, as
well as in the slapd/back-meta/data/ directory in the
OpenLDAP source tree.
CONFIGURATION
These slapd.conf options apply to the META backend database.
That is, they must follow a "database meta" line and come
before any subsequent "backend" or "database" lines. Other
database options are described in the slapd.conf(5) manual
page.
Note: as with the ldap backend, operational attributes
related to entry creation/modification should not be used,
as they would be passed to the target servers, generating an
error. Moreover, it makes little sense to use such attri-
butes in proxying, as the proxy server doesn't actually
store data, so it should have no knowledge of such attri-
butes. While code to strip the modification attributes has
been put in place (and #ifdef'd), it implies unmotivated
overhead. So it is strongly recommended to set
lastmod off
for every ldap and meta backend.
SPECIAL CONFIGURATION DIRECTIVES
Target configuration starts with the "uri" directive. All
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the configuration directives that are not specific to tar-
gets should be defined first for clarity, including those
that are common to all backends. They are:
default-target none
This directive forces the backend to reject all those
operations that must resolve to a single target in case
none or multiple targets are selected. They include:
add, delete, modify, modrdn; compare is not included,
as well as bind since, as they don't alter entries, in
case of multiple matches an attempt is made to perform
the operation on any candidate target, with the con-
straint that at most one must succeed. This directive
can also be used when processing targets to mark a
specific target as default.
dncache-ttl {forever|disabled|<ttl>}
This directive sets the time-to-live of the DN cache.
This caches the target that holds a given DN to speed
up target selection in case multiple targets would
result from an uncached search; forever means cache
never expires; disabled means no DN caching; otherwise
a valid ( > 0 ) ttl in seconds is required.
TARGET SPECIFICATION
Target specification starts with a "uri" directive:
uri <protocol>://[<host>[:<port>]]/<naming context>
The "server" directive that was allowed in the LDAP
backend (although deprecated) has been discarded in the
Meta backend. The <protocol> part can be anything
ldap_initialize(3) accepts ({ldap|ldaps|ldapi} and
variants); <host> and <port> may be omitted, defaulting
to whatever is set in /etc/ldap.conf. The <naming con-
text> part is mandatory. It must end with one of the
naming contexts defined for the backend, e.g.:
suffix "dc=foo,dc=com"
uri "ldap://x.foo.com/dc=x,dc=foo,dc=com"
The <naming context> part doesn't need to be unique across
the targets; it may also match one of the values of the
"suffix" directive. Multiple URIs may be defined in a sin-
gle argument. The URIs must be separated by TABs (e.g.
'\t'), and the additional URIs must have no <naming context>
part. This causes the underlying library to contact the
first server of the list that responds.
default-target [<target>]
The "default-target" directive can also be used during
target specification. With no arguments it marks the
current target as the default. The optional number
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marks target <target> as the default one, starting from
1. Target <target> must be defined.
binddn <administrative DN for access control purposes>
This directive, as in the LDAP backend, allows to
define the DN that is used to query the target server
for acl checking; it should have read access on the
target server to attributes used on the proxy for acl
checking. There is no risk of giving away such values;
they are only used to check permissions.
bindpw <password for access control purposes>
This directive sets the password for acl checking in
conjunction with the above mentioned "binddn" direc-
tive.
rebind-as-user
If this option is given, the client's bind credentials
are remembered for rebinds when chasing referrals.
pseudorootdn <substitute DN in case of rootdn bind>
This directive, if present, sets the DN that will be
substituted to the bind DN if a bind with the backend's
"rootdn" succeeds. The true "rootdn" of the target
server ought not be used; an arbitrary administrative
DN should used instead.
pseudorootpw <substitute password in case of rootdn bind>
This directive sets the credential that will be used in
case a bind with the backend's "rootdn" succeeds, and
the bind is propagated to the target using the "pseu-
dorootdn" DN.
Note: cleartext credentials must be supplied here; as a
consequence, using the pseudorootdn/pseudorootpw directives
is inherently unsafe.
rewrite* ...
The rewrite options are described in the "REWRITING"
section.
suffixmassage <virtual naming context> <real naming context>
All the directives starting with "rewrite" refer to the
rewrite engine that has been added to slapd. The "suf-
fixmassage" directive was introduced in the LDAP back-
end to allow suffix massaging while proxying. It has
been obsoleted by the rewriting tools. However, both
for backward compatibility and for ease of configura-
tion when simple suffix massage is required, it has
been preserved. It wraps the basic rewriting instruc-
tions that perform suffix massaging.
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Note: this also fixes a flaw in suffix massaging, which
operated on (case insensitive) DNs instead of normalized
DNs, so "dc=foo, dc=com" would not match "dc=foo,dc=com".
See the "REWRITING" section.
map {attribute|objectclass} [<local name>|*] {<foreign name>|*}
This maps object classes and attributes as in the LDAP
backend. See slapd-ldap(5).
SCENARIOS
A powerful (and in some sense dangerous) rewrite engine has
been added to both the LDAP and Meta backends. While the
former can gain limited beneficial effects from rewriting
stuff, the latter can become an amazingly powerful tool.
Consider a couple of scenarios first.
1) Two directory servers share two levels of naming context;
say "dc=a,dc=foo,dc=com" and "dc=b,dc=foo,dc=com". Then, an
unambiguous Meta database can be configured as:
database meta
suffix "dc=foo,dc=com"
uri "ldap://a.foo.com/dc=a,dc=foo,dc=com"
uri "ldap://b.foo.com/dc=b,dc=foo,dc=com"
Operations directed to a specific target can be easily
resolved because there are no ambiguities. The only opera-
tion that may resolve to multiple targets is a search with
base "dc=foo,dc=com" and scope at least "one", which results
in spawning two searches to the targets.
2a) Two directory servers don't share any portion of naming
context, but they'd present as a single DIT [Caveat: unique-
ness of (massaged) entries among the two servers is assumed;
integrity checks risk to incur in excessive overhead and
have not been implemented]. Say we have "dc=bar,dc=org" and
"o=Foo,c=US", and we'd like them to appear as branches of
"dc=foo,dc=com", say "dc=a,dc=foo,dc=com" and
"dc=b,dc=foo,dc=com". Then we need to configure our Meta
backend as:
database meta
suffix "dc=foo,dc=com"
uri "ldap://a.bar.com/dc=a,dc=foo,dc=com"
suffixmassage "dc=a,dc=foo,dc=com" "dc=bar,dc=org"
uri "ldap://b.foo.com/dc=b,dc=foo,dc=com"
suffixmassage "dc=b,dc=foo,dc=com" "o=Foo,c=US"
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Again, operations can be resolved without ambiguity,
although some rewriting is required. Notice that the vir-
tual naming context of each target is a branch of the
database's naming context; it is rewritten back and forth
when operations are performed towards the target servers.
What "back and forth" means will be clarified later.
When a search with base "dc=foo,dc=com" is attempted, if the
scope is "base" it fails with "no such object"; in fact, the
common root of the two targets (prior to massaging) does not
exist. If the scope is "one", both targets are contacted
with the base replaced by each target's base; the scope is
derated to "base". In general, a scope "one" search is
honored, and the scope is derated, only when the incoming
base is at most one level lower of a target's naming context
(prior to massaging).
Finally, if the scope is "sub" the incoming base is replaced
by each target's unmassaged naming context, and the scope is
not altered.
2b) Consider the above reported scenario with the two
servers sharing the same naming context:
database meta
suffix "dc=foo,dc=com"
uri "ldap://a.bar.com/dc=foo,dc=com"
suffixmassage "dc=foo,dc=com" "dc=bar,dc=org"
uri "ldap://b.foo.com/dc=foo,dc=com"
suffixmassage "dc=foo,dc=com" "o=Foo,c=US"
All the previous considerations hold, except that now there
is no way to unambiguously resolve a DN. In this case, all
the operations that require an unambiguous target selection
will fail unless the DN is already cached or a default tar-
get has been set. Practical configurations may result as a
combination of all the above scenarios.
ACLs
Note on ACLs: at present you may add whatever ACL rule you
desire to to the Meta (and LDAP) backends. However, the
meaning of an ACL on a proxy may require some considera-
tions. Two philosophies may be considered:
a) the remote server dictates the permissions; the proxy
simply passes back what it gets from the remote server.
b) the remote server unveils "everything"; the proxy is
responsible for protecting data from unauthorized access.
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Of course the latter sounds unreasonable, but it is not. It
is possible to imagine scenarios in which a remote host dis-
closes data that can be considered "public" inside an
intranet, and a proxy that connects it to the internet may
impose additional constraints. To this purpose, the proxy
should be able to comply with all the ACL matching criteria
that the server supports. This has been achieved with
regard to all the criteria supported by slapd except a spe-
cial subtle case (please drop me a note if you can find
other exceptions: <ando@openldap.org>). The rule
access to dn="<dn>" attr=<attr>
by dnattr=<dnattr> read
by * none
cannot be matched iff the attribute that is being requested,
<attr>, is NOT <dnattr>, and the attribute that determines
membership, <dnattr>, has not been requested (e.g. in a
search)
In fact this ACL is resolved by slapd using the portion of
entry it retrieved from the remote server without requiring
any further intervention of the backend, so, if the <dnattr>
attribute has not been fetched, the match cannot be assessed
because the attribute is not present, not because no value
matches the requirement!
Note on ACLs and attribute mapping: ACLs are applied to the
mapped attributes; for instance, if the attribute locally
known as "foo" is mapped to "bar" on a remote server, then
local ACLs apply to attribute "foo" and are totally unaware
of its remote name. The remote server will check permis-
sions for "bar", and the local server will possibly enforce
additional restrictions to "foo".
REWRITING
A string is rewritten according to a set of rules, called a
`rewrite context'. The rules are based on Regular Expres-
sions (POSIX regex) with substring matching; basic variable
substitution and map resolution of substrings is allowed by
specific mechanisms detailed in the following. The behavior
of pattern matching/substitution can be altered by a set of
flags.
The underlying concept is to build a lightweight rewrite
module for the slapd server (initially dedicated to the LDAP
backend).
Passes
An incoming string is matched agains a set of rules. Rules
are made of a regex match pattern, a substitution pattern
and a set of actions, described by a set of flags. In case
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of match a string rewriting is performed according to the
substitution pattern that allows to refer to substrings
matched in the incoming string. The actions, if any, are
finally performed. The substitution pattern allows map
resolution of substrings. A map is a generic object that
maps a substitution pattern to a value. The flags are
divided in "Pattern matching Flags" and "Action Flags"; the
former alter the regex match pattern behaviorm while the
latter alter the action that is taken after substitution.
Pattern Matching Flags
`C' honors case in matching (default is case insensitive)
`R' use POSIX Basic Regular Expressions (default is
Extended)
`M{n}'
allow no more than n recursive passes for a specific
rule; does not alter the max total count of passes, so
it can only enforce a stricter limit for a specific
rule.
Action Flags
`:' apply the rule once only (default is recursive)
`@' stop applying rules in case of match; the current rule
is still applied recursively; combine with `:' to apply
the current rule only once and then stop.
`#' stop current operation if the rule matches, and issue
an `unwilling to perform' error.
`G{n}'
jump n rules back and forth (watch for loops!). Note
that `G{1}' is implicit in every rule.
`I' ignores errors in rule; this means, in case of error,
e.g. issued by a map, the error is treated as a missed
match. The `unwilling to perform' is not overridden.
`U{n}'
uses n as return code if the rule matches; the flag
does not alter the recursive behavior of the rule, so,
to have it performed only once, it must be used in com-
bination with `:', e.g. `:U{16}' returns the value
`16' after exactly one execution of the rule, if the
pattern matches. As a consequence, its behavior is
equivalent to `@', with the return code set to n; or,
in other words, `@' is equivalent to `U{0}'. By con-
vention, the freely available codes are above 16
included; the others are reserved.
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The ordering of the flags can be significant. For instance:
`IG{2}' means ignore errors and jump two lines ahead both in
case of match and in case of error, while `G{2}I' means
ignore errors, but jump two lines ahead only in case of
match.
More flags (mainly Action Flags) will be added as needed.
Pattern matching:
See regex(7).
Substitution Pattern Syntax:
Everything starting with `%' requires substitution;
the only obvious exception is `%%', which is left as is;
the basic substitution is `%d', where `d' is a digit; 0
means the whole string, while 1-9 is a submatch, as dis-
cussed in regex(7);
a `%' followed by a `{' invokes an advanced substitution.
The pattern is:
`%' `{' [ <op> ] <name> `(' <substitution> `)' `}'
where <name> must be a legal name for the map, i.e.
<name> ::= [a-z][a-z0-9]* (case insensitive)
<op> ::= `>' `|' `&' `&&' `*' `**' `$'
and <substitution> must be a legal substitution pattern,
with no limits on the nesting level.
The operators are:
> sub context invocation; <name> must be a legal, already
defined rewrite context name
| external command invocation; <name> must refer to a
legal, already defined command name (NOT IMPL.)
& variable assignment; <name> defines a variable in the
running operation structure which can be dereferenced
later; operator & assigns a variable in the rewrite
context scope; operator && assigns a variable that
scopes the entire session, e.g. its value can be dere-
fenced later by other rewrite contexts
* variable dereferencing; <name> must refer to a variable
that is defined and assigned for the running operation;
operator * dereferences a variable scoping the rewrite
context; operator ** dereferences a variable scoping
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the whole session, e.g. the value is passed across
rewrite contexts
$ parameter dereferencing; <name> must refer to an exist-
ing parameter; the idea is to make some run-time param-
eters set by the system available to the rewrite
engine, as the client host name, the bind DN if any,
constant parameters initialized at config time, and so
on; no parameter is currently set by either back-ldap
or back-meta, but constant parameters can be defined in
the configuration file by using the rewriteParam direc-
tive.
Substitution escaping has been delegated to the `%' symbol,
which is used instead of `\' in string substitution patterns
because `\' is already escaped by slapd's low level parsing
routines; as a consequence, regex(7) escaping requires two
`\' symbols, e.g. `.*\.foo\.bar' must be written as
`.*\\.foo\\.bar'.
Rewrite context:
A rewrite context is a set of rules which are applied in
sequence. The basic idea is to have an application initial-
ize a rewrite engine (think of Apache's mod_rewrite ...)
with a set of rewrite contexts; when string rewriting is
required, one invokes the appropriate rewrite context with
the input string and obtains the newly rewritten one if no
errors occur.
Each basic server operation is associated to a rewrite con-
text; they are divided in two main groups: client -> server
and server -> client rewriting.
client -> server:
(default) if defined and no specific context
is available
bindDN bind
searchBase search
searchFilter search
compareDN compare
compareAttrDN compare AVA
addDN add
addAttrDN add AVA
modifyDN modify
modifyAttrDN modify AVA
modrDN modrdn
newSuperiorDN modrdn
deleteDN delete
server -> client:
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searchResult search (only if defined; no default;
acts on DN and DN-syntax attributes
of search results)
searchAttrDN search AVA
matchedDN all ops (only if applicable)
Basic configuration syntax
rewriteEngine { on | off }
If `on', the requested rewriting is performed; if
`off', no rewriting takes place (an easy way to stop
rewriting without altering too much the configuration
file).
rewriteContext <context name> [ alias <aliased context name> ]
<Context name> is the name that identifies the context,
i.e. the name used by the application to refer to the
set of rules it contains. It is used also to reference
sub contexts in string rewriting. A context may aliase
another one. In this case the alias context contains
no rule, and any reference to it will result in access-
ing the aliased one.
]
rewriteRule <regex match pattern> <substitution pattern> [
Determines how a string can be rewritten if a pattern
is matched. Examples are reported below.
Additional configuration syntax:
rewriteMap <map name> <map type> [ <map attrs> ]
Allows to define a map that transforms substring
rewriting into something else. The map is referenced
inside the substitution pattern of a rule.
rewriteParam <param name> <param value>
Sets a value with global scope, that can be derefer-
enced by the command `%{$paramName}'.
rewriteMaxPasses <number of passes> [<number of
Sets the maximum number of total rewriting passes that
can be performed in a single rewrite operation (to
avoid loops). A safe default is set to 100; note that
reaching this limit is still treated as a success;
recursive invocation of rules is simply interrupted.
The count applies to the rewriting operation as a
whole, not to any single rule; an optional per-rule
limit can be set. This limit is overridden by setting
specific per-rule limits with the `M{n}' flag.
Configuration examples:
# set to `off' to disable rewriting
rewriteEngine on
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# Everything defined here goes into the `default' context.
# This rule changes the naming context of anything sent
# to `dc=home,dc=net' to `dc=OpenLDAP, dc=org'
rewriteRule "(.*)dc=home,[ ]?dc=net"
"%1dc=OpenLDAP, dc=org" ":"
# since a pretty/normalized DN does not include spaces
# after rdn separators, e.g. `,', this rule suffices:
rewriteRule "(.*)dc=home,dc=net"
"%1dc=OpenLDAP,dc=org" ":"
# Start a new context (ends input of the previous one).
# This rule adds blanks between DN parts if not present.
rewriteContext addBlanks
rewriteRule "(.*),([^ ].*)" "%1, %2"
# This one eats blanks
rewriteContext eatBlanks
rewriteRule "(.*),[ ](.*)" "%1,%2"
# Here control goes back to the default rewrite
# context; rules are appended to the existing ones.
# anything that gets here is piped into rule `addBlanks'
rewriteContext default
rewriteRule ".*" "%{>addBlanks(%0)}" ":"
# Rewrite the search base according to `default' rules.
rewriteContext searchBase alias default
# Search results with OpenLDAP DN are rewritten back with
# `dc=home,dc=net' naming context, with spaces eaten.
rewriteContext searchResult
rewriteRule "(.*[^ ]?)[ ]?dc=OpenLDAP,[ ]?dc=org"
"%{>eatBlanks(%1)}dc=home,dc=net" ":"
# Bind with email instead of full DN: we first need
# an ldap map that turns attributes into a DN (the
# argument used when invoking the map is appended to
# the URI and acts as the filter portion)
rewriteMap ldap attr2dn "ldap://host/dc=my,dc=org?dn?sub"
# Then we need to detect DN made up of a single email,
# e.g. `mail=someone@example.com'; note that the rule
# in case of match stops rewriting; in case of error,
# it is ignored. In case we are mapping virtual
# to real naming contexts, we also need to rewrite
# regular DNs, because the definition of a bindDn
# rewrite context overrides the default definition.
rewriteContext bindDn
rewriteRule "^mail=[^,]+@[^,]+$" "%{attr2dn(%0)}" ":@I"
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# This is a rather sophisticated example. It massages a
# search filter in case who performs the search has
# administrative privileges. First we need to keep
# track of the bind DN of the incoming request, which is
# stored in a variable called `binddn' with session scope,
# and left in place to allow regular binding:
rewriteContext bindDn
rewriteRule ".+" "%{&&binddn(%0)}%0" ":"
# A search filter containing `uid=' is rewritten only
# if an appropriate DN is bound.
# To do this, in the first rule the bound DN is
# dereferenced, while the filter is decomposed in a
# prefix, in the value of the `uid=<arg>' AVA, and
# in a suffix. A tag `<>' is appended to the DN.
# If the DN refers to an entry in the `ou=admin' subtree,
# the filter is rewritten OR-ing the `uid=<arg>' with
# `cn=<arg>'; otherwise it is left as is. This could be
# useful, for instance, to allow apache's auth_ldap-1.4
# module to authenticate users with both `uid' and
# `cn', but only if the request comes from a possible
# `cn=Web auth,ou=admin,dc=home,dc=net' user.
rewriteContext searchFilter
rewriteRule "(.*\\()uid=([a-z0-9_]+)(\\).*)"
"%{**binddn}<>%{&prefix(%1)}%{&arg(%2)}%{&suffix(%3)}"
":I"
rewriteRule "[^,]+,ou=admin,dc=home,dc=net"
"%{*prefix}|(uid=%{*arg})(cn=%{*arg})%{*suffix}" ":@I"
rewriteRule ".*<>" "%{*prefix}uid=%{*arg}%{*suffix}" ":"
# This example shows how to strip unwanted DN-valued
# attribute values from a search result; the first rule
# matches DN values below "ou=People,dc=example,dc=com";
# in case of match the rewriting exits successfully.
# The second rule matches everything else and causes
# the value to be rejected.
rewriteContext searchResult
rewriteRule ".*,ou=People,dc=example,dc=com" "%0" ":@"
rewriteRule ".*" "" "#"
LDAP Proxy resolution (a possible evolution of slapd-ldap(5)):
In case the rewritten DN is an LDAP URI, the operation is
initiated towards the host[:port] indicated in the uri, if
it does not refer to the local server. E.g.:
rewriteRule '^cn=root,.*' '%0' 'G{3}'
rewriteRule '^cn=[a-l].*' 'ldap://ldap1.my.org/%0' ':@'
rewriteRule '^cn=[m-z].*' 'ldap://ldap2.my.org/%0' ':@'
rewriteRule '.*' 'ldap://ldap3.my.org/%0' ':@'
(Rule 1 is simply there to illustrate the `G{n}' action; it
could have been written:
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rewriteRule '^cn=root,.*' 'ldap://ldap3.my.org/%0' ':@'
with the advantage of saving one rewrite pass ...)
PROXY CACHE OVERLAY
The proxy cache overlay allows caching of LDAP search
requests (queries) in a local database. For an incoming
query, the proxy cache determines its corresponding tem-
plate. If the template was specified as cacheable using the
proxytemplate directive and the request is contained in a
cached request, it is answered from the proxy cache. Other-
wise, 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 iden-
tifying a set of attributes. The template string for a query
can be obtained by removing assertion values from the RFC
2254 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 following cache specific directives can be used to con-
figure the proxy cache:
overlay proxycache
This directive adds the proxycache overlay to the
current backend. The proxycache overlay may be used
with any backend but is intended for use with the ldap
and meta backends.
proxycache <database> <max_entries> <numattrsets> <entry_limit>
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 follow-
ing proxyattrset directives. Queries are cached only if
they correspond to a cacheable template (specified by
the proxytemplate 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:
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proxycache bdb 10000 1 50 100
proxyattrset <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 proxytemplate directive to define cacheable
templates.
proxytemplate <template_string> <attrset_index> <ttl>
Specifies a cacheable template and "time to live" (in
sec) <ttl> of queries belonging to the template.
The following adds a template with filter string
(&sn=)(givenName=)) and attributes mail, postaladdress,
telephonenumber and a TTL of 1 hour.
proxyattrset 0 mail postaladdress telephonenumber
proxytemplate (&(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.
FILES
ETCDIR/slapd.conf
default slapd configuration file
SEE ALSO
slapd.conf(5), slapd-ldap(5), slapd(8), regex(7).
AUTHOR
Pierangelo Masarati, based on back-ldap by Howard Chu
OpenLDAP LDVERSION Last change: RELEASEDATE 14
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