The global catalog contains:
●
The
attributes that are most frequently used in queries, such as a user’s first
name, last name, and logon name.
●
The
information that is necessary to determine the location of any object in the
directory.
●
The
access permissions for each object and attribute that is stored in the global
catalog. If you search for an object that you do not have the appropriate
permissions to view, the object will not appear in the search results. Access
permissions ensure that users can find only objects to which they have been
assigned access.
A
global catalog server is a domain controller that, in addition to its full,
writable domain directory partition replica, also stores a partial, read-only
replica of all other domain directory partitions in the forest. Taking a user
object as an example, it would by default have many different attributes such
as first name, last name, phone number, and many more. The GC will by default
only store the most common of those attributes that would be used in search
operations (such as a user’s first and last names, or login name, for example).
The partial attributes that it has for that object would be enough to allow a
search for that object to be able to locate the full replica of the object in
active directory. This allows searches done against a local GC, and reduces
network traffic over the WAN in an attempt to locate objects somewhere else in
the network.
Domain
Controllers always contain the full attribute list for objects belonging to
their domain. If the Domain Controller
is also a GC, it will also contain a partial replica of objects from all other
domains in the forest.
Active
Directory uses DNS as the name resolution service to identify domains and
domain host computers during processes such as logging on to the network.
Similar
to the way a Windows NT 4.0 client will query WINS for a NetBIOS DOMAIN[1B]
record to locate a PDC, or a NetBIOS DOMAIN[1C] record for domain controllers,
a Windows 2000, 2003, or Windows XP client can query DNS to find a domain
controller by looking for SRV records.