slapd-asyncmeta - asynchronous metadirectory backend to slapd


The asyncmeta backend to slapd(8) performs basic LDAP proxying with respect to a set of remote LDAP servers, called "targets". The information contained in these servers can be presented as belonging to a single Directory Information Tree (DIT).

A good knowledge of the functionality of the slapd-meta(5) backend is recommended. This backend has been designed as an asynchronous version of the meta backend. Unlike meta , the operation handling threads are no longer pending on the response from the remote server, thus decreasing the number of threads necessary to handle the same load. While asyncmeta maintains the functionality of meta and has a largely similar codebase, some changes in operation and some new configuration directives have been added. Some configuration options, such as conn-pool-max , conn-ttl , single-conn , and use-temporary-conn have been removed, as they are no longer relevant.

New connection handling:

Unlike meta, which caches bound connections, the asyncmeta works with a configured maximum number of connections per target. For each request redirected to a target, a different connection is selected. Each connection has a queue, to which the request is added before it is sent to the remote server, and is removed after the last response for that request is received.
For each new request, a new connection is chosen using round-robin scheduling.


Due to implementation specifics, there is no guarantee that any of the existing OpenLDAP overlays will work with asyncmeta backend.

Refer to slapd-meta(5) for configuration examples.

These slapd.conf options apply to the ASYNCMETA backend database. That is, they must follow a "database asyncmeta" line and come before any subsequent "backend" or "database" lines. Other database options are described in the slapd.conf(5) manual page.

Target configuration starts with the "uri" directive. All the configuration directives that are not specific to targets should be defined first for clarity, including those that are common to all backends. They are:

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 constraint that at most one must succeed. This directive can also be used when processing targets to mark a specific target as default.
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 is required, in the format illustrated for the idle-timeout directive.
This directive allows one to select the behavior in case an error is returned by one target during a search. The default, continue, consists in continuing the operation, trying to return as much data as possible. If the value is set to stop, the search is terminated as soon as an error is returned by one target, and the error is immediately propagated to the client. If the value is set to report, the search is continued to the end but, in case at least one target returned an error code, the first non-success error code is returned.
Specify the number of consecutive timed out requests, after which the connection will be considered faulty and dropped.
The maximum number of pending requests stored in a connection's queue. The default is 128. When this number is exceeded, LDAP_BUSY will be returned to the client.
The maximum number of connections per target. Unlike slapd-meta(5), no new connections will be created once this number is reached. The default value is 255.
If yes, do not return search reference responses. By default, they are returned unless request is LDAPv2. If set before any target specification, it affects all targets, unless overridden by any per-target directive.
If yes, return success instead of searching if a filter is undefined or contains undefined portions. By default, the search is propagated after replacing undefined portions with (!(objectClass=*)), which corresponds to the empty result set. If set before any target specification, it affects all targets, unless overridden by any per-target directive.
This directive indicates what protocol version must be used to contact the remote server. If set to 0 (the default), the proxy uses the same protocol version used by the client, otherwise the requested protocol is used. The proxy returns unwillingToPerform if an operation that is incompatible with the requested protocol is attempted. If set before any target specification, it affects all targets, unless overridden by any per-target directive.
This directive, when set to yes, causes the authentication to the remote servers with the pseudo-root identity (the identity defined in each idassert-bind directive) to be deferred until actually needed by subsequent operations. Otherwise, all binds as the rootdn are propagated to the targets.
Turns on quarantine of URIs that returned LDAP_UNAVAILABLE, so that an attempt to reconnect only occurs at given intervals instead of any time a client requests an operation. The pattern is: retry only after at least interval seconds elapsed since last attempt, for exactly num times; then use the next pattern. If num for the last pattern is "+", it retries forever; otherwise, no more retries occur. This directive must appear before any target specification; it affects all targets with the same pattern.
If this option is given, the client's bind credentials are remembered for rebinds, when trying to re-establish a broken connection, or when chasing a referral, if chase-referrals is set to yes.
Adds session tracking control for all requests. The client's IP and hostname, and the identity associated to each request, if known, are sent to the remote server for informational purposes. This directive is incompatible with setting protocol-version to 2. If set before any target specification, it affects all targets, unless overridden by any per-target directive.

Target specification starts with a "uri" directive:

Identical to meta. See slapd-meta(5) for details.
DN which is used to query the target server for acl checking, as in the LDAP backend; it is supposed to 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. The acl-authcDN identity is by no means implicitly used by the proxy when the client connects anonymously.
Password used with the acl-authcDN above.
This directive defines the timeout, in microseconds, used when polling for response after an asynchronous bind connection. See slapd-meta(5) for details.
enable/disable automatic referral chasing, which is delegated to the underlying libldap, with rebinding eventually performed if the rebind-as-user directive is used. The default is to chase referrals. If set before any target specification, it affects all targets, unless overridden by any per-target directive.
This feature allows one to use RFC 2696 Paged Results control when performing search operations with a specific target, irrespective of the client's request. See slapd-meta(5) for details.
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 marks target <target> as the default one, starting from 1. Target <target> must be defined.
This directive allows specifying a regex(5) pattern to indicate what search filter terms are actually served by a target.

In a search request, if the search filter matches the pattern the target is considered while fulfilling the request; otherwise the target is ignored. There may be multiple occurrences of the filter directive for each target.

if defined, selects what local identities are authorized to exploit the identity assertion feature. The string <authz-regexp> follows the rules defined for the authzFrom attribute. See slapd.conf(5), section related to authz-policy, for details on the syntax of this field.

idassert-bind bindmethod=none|simple|sasl [binddn=<simple DN>] [credentials=<simple password>] [saslmech=<SASL mech>] [secprops=<properties>] [realm=<realm>] [authcId=<authentication ID>] [authzId=<authorization ID>] [authz={native|proxyauthz}] [mode=<mode>] [flags=<flags>] [starttls=no|yes|critical] [tls_cert=<file>] [tls_key=<file>] [tls_cacert=<file>] [tls_cacertdir=<path>] [tls_reqcert=never|allow|try|demand] [tls_reqsan=never|allow|try|demand] [tls_cipher_suite=<ciphers>] [tls_ecname=<names>] [tls_protocol_min=<major>[.<minor>]] [tls_crlcheck=none|peer|all] Allows one to define the parameters of the authentication method that is internally used by the proxy to authorize connections that are authenticated by other databases. See slapd-meta(5) for details.

This directive causes a a persistent connection to be dropped after it has been idle for the specified time. The connection will be re-created the next time it is selected for use. A connection is considered idle if no attempts have been made by the backend to use it to send a request to the backend server. If there are still pending requests in its queue, the connection will be dropped after the last request one has either received a result or has timed out.


where <d>, <h>, <m> and <s> are respectively treated as days, hours, minutes and seconds. If set before any target specification, it affects all targets, unless overridden by any per-target directive.

The keepalive parameter sets the values of idle, probes, and interval used to check whether a socket is alive; idle is the number of seconds a connection needs to remain idle before TCP starts sending keepalive probes; probes is the maximum number of keepalive probes TCP should send before dropping the connection; interval is interval in seconds between individual keepalive probes. Only some systems support the customization of these values; the keepalive parameter is ignored otherwise, and system-wide settings are used.
If non-zero, corresponds to the TCP_USER_TIMEOUT set on the target connections, overriding the operating system setting. Only some systems support the customization of this parameter, it is ignored otherwise and system-wide settings are used.
This maps object classes and attributes as in the LDAP backend. See slapd-ldap(5).
Sets the network timeout value after which poll(2)/select(2) following a connect(2) returns in case of no activity while sending an operation to the remote target. The value is in milliseconds, and it can be specified as for idle-timeout. If set before any target specification, it affects all targets, unless overridden by any per-target directive.
This directive defines how many times forwarding an operation should be retried in case of temporary failure in contacting a target. The number of retries is per operation, so if a bind to the target is necessary first, the remaining number is decremented. If defined before any target specification, it applies to all targets (by default, 3 times); the global value can be overridden by redefinitions inside each target specification.
The rewrite options are identical to the meta backend. See the REWRITING section of slapd-meta(5).
This directive allows one to indicate what subtrees are actually served by a target. See slapd-meta(5) for details.
slapd-asyncmeta does not support the rewrite engine used by the LDAP and META backends. suffixmassage can be used to perform DN suffix rewriting, the same way as the obsoleted suffixmassage directive previously used by the LDAP backend.
enable if the remote server supports absolute filters (see RFC 4526 for details). If set to discover, support is detected by reading the remote server's root DSE. If set before any target specification, it affects all targets, unless overridden by any per-target directive.
This directive allows one to set per-operation timeouts. Operations can be

<op> ::= bind, add, delete, modrdn, modify, compare, search

By default, the timeout for all operations is 2 seconds.

See slapd-meta(5) for details.

B [starttls=no] [tls_cert=<file>] [tls_key=<file>] [tls_cacert=<file>] [tls_cacertdir=<path>] [tls_reqcert=never|allow|try|demand] [tls_reqsan=never|allow|try|demand] [tls_cipher_suite=<ciphers>] [tls_ecname=<names>] [tls_crlcheck=none|peer|all]
Specify TLS settings regular connections.

If the first parameter is not "none" then this configures the TLS settings to be used for regular connections. The StartTLS extended operation will be used when establishing the connection unless the URI directive protocol scheme is ldaps://. In that case this keyword may only be set to "ldaps" and the StartTLS operation will not be used.

With propagate, the proxy issues the StartTLS operation only if the original connection has a TLS layer set up. The try- prefix instructs the proxy to continue operations if the StartTLS operation failed; its use is not recommended.

The TLS settings default to the same as the main slapd TLS settings, except for tls_reqcert which defaults to "demand", tls_reqsan which defaults to "allow", and starttls which is overshadowed by the first keyword and thus ignored.

If set before any target specification, it affects all targets, unless overridden by any per-target directive.

See slapd-meta(5) for configuration scenarios.

ACL behavior is identical to meta. See slapd-meta(5).

The asyncmeta backend does not honor all ACL semantics as described in slapd.access(5). In general, access checking is delegated to the remote server(s). Only read (=r) access to the entry pseudo-attribute and to the other attribute values of the entries returned by the search operation is honored, which is performed by the frontend.

default slapd configuration file

slapd.conf(5), slapd-ldap(5), slapd-meta(5), slapo-pcache(5), slapd(8), regex(7), re_format(7).

Nadezhda Ivanova, based on back-meta by Pierangelo Masarati.

2023/07/31 OpenLDAP 2.6.6