Kerberos Principals

A client in the Kerberos service is identified by its principal. A principal is a unique identity to which the KDC can assign tickets. A principal can be a user, such as joe, or a service, such as nfs or telnet.

By convention, a principal name is divided into three components: the primary, the instance, and the realm. A typical Kerberos principal would be, for example, joe/admin@ENG.EXAMPLE.COM. In this example:

• joe is the primary. The primary can be a user name, as shown here, or a service, such as nfs. The primary can also be the word host, which signifies that this principal is a service principal that is set up to provide various network services, ftp, rcp, rlogin, and so on.

• admin is the instance. An instance is optional in the case of user principals, but it is required for service principals. For example, if the user joe sometimes acts as a system administrator, he can use joe/admin to distinguish himself from his usual user identity. Likewise, if joe has accounts on two different hosts, he can use two principal names with different instances, for example, joe/denver.example.com and joe/boston.example.com. Notice that the Kerberos service treats joe and joe/admin as two completely different principals.

  In the case of a service principal, the instance is the fully qualified host name. bigmachine.eng.example.com is an example of such an instance. The primary/instance for this example might be ftp/bigmachine.eng.example.com or host/bigmachine.eng.example.com.

• ENG.EXAMPLE.COM is the Kerberos realm. Realms are discussed in Kerberos Realms.

The following are all valid principal names:

• joe

• joe/admin

• joe/admin@ENG.EXAMPLE.COM

• nfs/host.eng.example.com@ENG.EXAMPLE.COM

• host/eng.example.com@ENG.EXAMPLE.COM