slapo-dds - Man Page

Dynamic Directory Services overlay to slapd

The dds overlay to slapd(8) implements dynamic objects as per RFC 2589. The name dds stands for Dynamic Directory Services. It allows one to define dynamic objects, characterized by the dynamicObject objectClass.

Dynamic objects have a limited lifetime, determined by a time-to-live (TTL) that can be refreshed by means of a specific refresh extended operation. This operation allows one to set the Client Refresh Period (CRP), namely the period between refreshes that is required to preserve the dynamic object from expiration. The expiration time is computed by adding the requested TTL to the current time. When dynamic objects reach the end of their lifetime without being further refreshed, they are automatically deleted. There is no guarantee of immediate deletion, so clients should not count on it.

Dynamic objects can have subordinates, provided these also are dynamic objects. RFC 2589 does not specify what the behavior of a dynamic directory service should be when a dynamic object with (dynamic) subordinates expires. In this implementation, the lifetime of dynamic objects with subordinates is prolonged until all the dynamic subordinates expire.

This slapd.conf(5) directive adds the dds overlay to the current database:

overlay dds

The database must have a rootdn specified, otherwise, the dds overlay will not be able to delete expired objects. The dds overlay may be used with any backend that implements the add, modify, search, and delete operations. Since its use may result in many internal entry lookups, adds and deletes, it should be best used in conjunction with backends that have reasonably good write performances.

The config directives that are specific to the dds overlay are prefixed by dds-, to avoid potential conflicts with directives specific to the underlying database or to other stacked overlays.

dds-max-ttl <time>: Specifies the max TTL value. This is also the default TTL newly created dynamic objects receive, unless dds-default-ttl is set. When the client with a refresh extended operation requests a TTL higher than it, sizeLimitExceeded is returned. This value must be between 86400 (1 day, the default) and 31557600 (1 year plus 6 hours, as per RFC 2589).
dds-min-ttl <time>: Specifies the min TTL value; clients requesting a lower TTL by means of the refresh extended operation actually obtain this value as CRP. If set to 0 (the default), no lower limit is set.
dds-default-ttl <time>: Specifies the default TTL value that newly created dynamic objects get. If set to 0 (the default), the dds-max-ttl is used.
dds-interval <time>: Specifies the interval between expiration checks; defaults to 1 hour.
dds-tolerance <time>: Specifies an extra time that is added to the timer that actually wakes up the thread that will delete an expired dynamic object. So the nominal lifetime of the entry is that specified in the entryTtl attribute, but its lifetime will actually be entryTtl + tolerance. Note that there is no guarantee that the lifetime of a dynamic object will be exactly the requested TTL; due to implementation details, it may be longer, which is allowed by RFC 2589. By default, tolerance is 0.
dds-max-dynamicObjects <num>: Specifies the maximum number of dynamic objects that can simultaneously exist within a naming context. This allows one to limit the amount of resources (mostly in terms of run-queue size) that are used by dynamic objects. By default, no limit is set.
dds-state {TRUE|false}: Specifies if the Dynamic Directory Services feature is enabled or not. By default it is; however, a proxy does not need to keep track of dynamic objects itself, it only needs to inform the frontend that support for dynamic objects is available.

Access Control

The dds overlay restricts the refresh operation by requiring manage access to the entryTtl attribute (see slapd.access(5) for details about the manage access privilege). Since the entryTtl is an operational, NO-USER-MODIFICATION attribute, no direct write access to it is possible. So the dds overlay turns refresh extended operation into an internal modification to the value of the entryTtl attribute with the relax control set.

RFC 2589 recommends that anonymous clients should not be allowed to refresh a dynamic object. This can be implemented by appropriately crafting access control to obtain the desired effect.

Example: restrict refresh to authenticated clients

access to attrs=entryTtl
	by users manage
	by * read

Example: restrict refresh to the creator of the dynamic object

access to attrs=entryTtl
	by dnattr=creatorsName manage
	by * read

Another suggested usage of dynamic objects is to implement dynamic meetings; in this case, all the participants to the meeting are allowed to refresh the meeting object, but only the creator can delete it (otherwise it will be deleted when the TTL expires)

Example: assuming participant is a valid DN-valued attribute, allow users to start a meeting and to join it; restrict refresh to the participants; restrict delete to the creator

access to dn.base="cn=Meetings"
		attrs=children
	by users write

access to dn.onelevel="cn=Meetings"
		attrs=entry
	by dnattr=creatorsName write
	by * read

access to dn.onelevel="cn=Meetings"
		attrs=participant
	by dnattr=creatorsName write
	by users selfwrite
	by * read

access to dn.onelevel="cn=Meetings"
		attrs=entryTtl
	by dnattr=participant manage
	by * read

Replication

This implementation of RFC 2589 provides a restricted interpretation of how dynamic objects replicate. Only the provider takes care of handling dynamic object expiration, while consumers simply see the dynamic object as a plain object.

When replicating these objects, one needs to explicitly exclude the dynamicObject class and the entryTtl attribute. This implementation of RFC 2589 introduces a new operational attribute, entryExpireTimestamp, that contains the expiration timestamp. This must be excluded from replication as well.

The quick and dirty solution is to set schemacheck=off in the syncrepl configuration and, optionally, exclude the operational attributes from replication, using

syncrepl ...
	exattrs=entryTtl,entryExpireTimestamp

In any case the overlay must be either statically built in or run-time loaded by the consumer, so that it is aware of the entryExpireTimestamp operational attribute; however, it must not be configured in the shadow database. Currently, there is no means to remove the dynamicObject class from the entry; this may be seen as a feature, since it allows one to see the dynamic properties of the object.