When a consumer receives a request to modify data, it does not forward the request to the server that contains the master replica. Instead, it returns to the client a list of the URLs of the masters that can satisfy the request. These URLs are called referrals.
The replication mechanism automatically configures consumers to return referrals for all known masters in the replication topology. However, you can also add your own referrals and overwrite the referrals set automatically by the server. The ability to control referrals helps enables you to perform the following tasks:
Point referrals to secure ports only
Point to a Directory Proxy Server instead for load balancing
Redirect to local servers only in the case of servers separated by a WAN
Limit referrals to a subset of masters in four-way multi-master topologies
Directory Proxy Server is able to follow referrals.