Referral Chasing

A referral is the mechanism that a directory server uses to direct a client to another server when it does not contain sufficient information about the object(s) requested by a query.

In a one-level or subtree search (see Search Scope), referrals are returned for known, immediately subordinate domain, schema or configuration containers only (that is, child domains that are direct descendants).

In a directory, not all information is available on a single server, rather, it is distributed over several different servers across the network. If the servers share the knowledge of the information that other servers can provide, they can provide referrals to a client when a requested query cannot be resolved on the originating server. For example, when a client asks Server A to query a user object (U), then A can suggest that the client continue the search on Server B if U does not reside on A, but is known to be on B. The client has the choice to pursue the referral or not. Search referrals free the client from possessing previous knowledge of the capability of each server. But the client must specify the type of referrals a server should make.

To enable referral chasing, use the ADS_SEARCHPREF_CHASE_REFERRALS preference with values from the ADS_CHASE_REFERRALS_ENUM enumeration. A client can choose any of the following four types of referral chasing:

• Never. The server should not generate a referral to a client even though it knows that another server holds the requested information. This is the default setting.
• External. The server should generate referrals if the request can be resolved on another server of a different directory tree. For example, a client queries OU=Sales, DC=Microsoft, DC=COM on the msft01 server on the Microsoft.com domain. However, the object does not belong to msft01, but is known to be on the arc01 server on the Microsoft.com domain. Thus, msft01 will refer arc01 to the client.
• Subordinate. The server should generate referrals if the request can be resolved on a server whose name forms a contiguous path from the originating server. The search scope must be at the subtree level. For example, Server A contains objects in DC=Sales, DC=Microsoft, DC=Com. Server B contains objects in DC=Seattle, DC=Sales, DC=Microsoft, DC=Com. Notice that the name of Server B forms a contiguous path from Server A. When a client contacts Server A, requests a sub-tree search on DC=Sales, DC=Microsoft, DC=Com, and specifies the referral to be the Subordinate type, the following event occurs:
  • Server A returns all objects that it knows within its scope.
  • Server A informs the client that objects in DC=Seattle, DC=Sales, DC=Microsoft, DC=COM can be found on Server B.

  The client can choose to contact Server B. If so, the following event occurs:

  • Server B responds with the requested objects.
  • If Server B detects other servers on the contiguous naming path, the process continues.
• Always. The server generates referrals if the search can be resolved based on either the external type or the subordinate type.

For more information, see Referrals.