The Authentication Server and the Ticket Granting Server
The Authentication Server performs one function: receive a request containing the username of the client requesting authentication, and return an encrypted Ticket Granting Ticket for that user. Then, the client can use this Ticket Granting Ticket (the TGT) to request further tickets for other services.
When a client has no Kerberos tickets cached, or tickets that have expired, then the client must communicate with the Authentication Server to get an initial ticket to the Ticket Granting Server. This typically happens at the beginning of each day, as most Kerberos implementations default to 8-10 hour ticket lifetimes.
The first message sent to the Kerberos KDC from the client is the Authentication Server Request message, also known as an AS_REQ message. This message is sent in plain text, containing the identity of the client, the client’s local time, and the Ticket Granting Server’s principal name. By convention, in Kerberos 4, the Ticket Granting Server’s principal name is krbtgt . In addition, the Ticket Granting Server’s principal always has an instance component. The instance component is the realm for which the ticket granting server can issue further tickets. Every realm contains at least one Ticket Granting Server, a TGS with an instance of the realm itself. For example, the Ticket Granting Server for the WEDGIE.ORG realm is krbtgt.WEDGIE.ORG@WEDGIE.ORG.
A special case occurs when two or more realms trust each other’s ...