The Global Catalog
The Global Catalog (often abbreviated as GC) is a mechanism to access information about objects from all domains in an Active Directory Forest on a simple way. Typically, each domain controller stores only the objects of its own domain in its domain partition. If you need for example a list of all users in the entire forest, or you want to search an object in the entire forest according to certain criteria, normally you would have to perform separately for each domain.
To avoid this expense, you can add the function of the global catalog to any domain controller (for the first installed domain controller of an Active Directory forest this is done automatically). These are the changes regarding such a domain controllers:
- The domain controller stores information about all objects of all domains of the entire forest.
- Not the complete set of attributes for these objects are stored, but only the most important.
- The Global catalog servers replicate the data with all other GCs in the forest. For this the Global Catalog function increases replication load on the regarding server.
- You can query the Global Catalog over LDAP, just use the special TCP port 3268 (or 3269 for LDAP over SSL).
- All requests to the Global Catalog are Read Only.
- On the other hand, a Global Catalog server works as a normal domain controller - so it still contains the complete data of its own domain, the Configuration partition and schema. Here you could perform write operations through the normal LDAP port.
- There can be any number of Global Catalog servers in the forest. In many environments where the network load by AD replication can be neglected, each domain controller is conured as a Global Catalog.
- For each Global Catalog, an SRV record is created in DNS. So you can identify the Global Catalog servers in an Active Directory environment with DNS queries S queries. However, you could also read this information from the configuration partition in AD itself.
The following topics are available acording to scripting with the Global Catalog: