Chain overlay to slapd
The chain overlay to slapd(8) allows automatic referral chasing. Any time a referral is returned (except for bind operations), it is chased by using an instance of the ldap backend. If operations are performed with an identity (i.e. after a bind), that identity can be asserted while chasing the referrals by means of the identity assertion feature of back-ldap (see slapd-ldap(5) for details), which is essentially based on the proxied authorization control [RFC 4370]. Referral chasing can be controlled by the client by issuing the chaining control (see draft-sermersheim-ldap-chaining for details.)
The config directives that are specific to the chain overlay are prefixed by chain-, to avoid potential conflicts with directives specific to the underlying database or to other stacked overlays.
There are very few chain overlay specific directives; however, directives related to the instances of the ldap backend that may be implicitly instantiated by the overlay may assume a special meaning when used in conjunction with this overlay. They are described in slapd-ldap(5), and they also need to be prefixed by chain-.
Note: this overlay is built into the ldap backend; it is not a separate module.
This directive adds the chain overlay to the current backend. The chain overlay may be used with any backend, but it is mainly intended for use with local storage backends that may return referrals. It is useless in conjunction with the slapd-ldap and slapd-meta backends because they already exploit the libldap specific referral chase feature. [Note: this may change in the future, as the ldap(5) and meta(5) backends might no longer chase referrals on their own.]
This directive instructs the chain overlay to cache connections to URIs parsed out of referrals that are not predefined, to be reused for later chaining. These URIs inherit the properties configured for the underlying slapd-ldap(5) before any occurrence of the chain-uri directive; basically, they are chained anonymously.
chain-chaining [resolve=<r>] [continuation=<c>] [critical]
This directive enables the chaining control (see draft-sermersheim-ldap-chaining for details) with the desired resolve and continuation behaviors and criticality. The resolve parameter refers to the behavior while discovering a resource, namely when accessing the object indicated by the request DN; the continuation parameter refers to the behavior while handling intermediate responses, which is mostly significant for the search operation, but may affect extended operations that return intermediate responses. The values r and c can be any of chainingPreferred, chainingRequired, referralsPreferred, referralsRequired. If the critical flag affects the control criticality if provided. [This control is experimental and its support may change in the future.]
In case a referral is returned during referral chasing, further chasing occurs at most <n> levels deep. Set to 1 (the default) to disable further referral chasing.
In case referral chasing fails, the real error is returned instead of the original referral. In case multiple referral URIs are present, only the first error is returned. This behavior may not be always appropriate nor desirable, since failures in referral chasing might be better resolved by the client (e.g. when caused by distributed authentication issues).
This directive instantiates a new underlying ldap database and instructs it about which URI to contact to chase referrals. As opposed to what stated in slapd-ldap(5), only one URI can appear after this directive; all subsequent slapd-ldap(5) directives prefixed by chain- refer to this specific instance of a remote server.
Directives for configuring the underlying ldap database may also be required, as shown in this example:
overlay chain chain-rebind-as-user FALSE chain-uri "ldap://ldap1.example.com" chain-rebind-as-user TRUE chain-idassert-bind bindmethod="simple" binddn="cn=Auth,dc=example,dc=com" credentials="secret" mode="self" chain-uri "ldap://ldap2.example.com" chain-idassert-bind bindmethod="simple" binddn="cn=Auth,dc=example,dc=com" credentials="secret" mode="none"
Any valid directives for the ldap database may be used; see slapd-ldap(5) for details. Multiple occurrences of the chain-uri directive may appear, to define multiple "trusted" URIs where operations with identity assertion are chained. All URIs not listed in the configuration are chained anonymously. All slapd-ldap(5) directives appearing before the first occurrence of chain-uri are inherited by all URIs, unless specifically overridden inside each URI configuration.
default slapd configuration file
Originally implemented by Howard Chu; extended by Pierangelo Masarati.