To create secure communication among its various components, HDP uses Kerberos. Kerberos is a third party authentication mechanism, in which users and services that users wish to access rely on a third party - the Kerberos server - to authenticate each to the other. This mechanism also can be used to encrypt all traffic between the user and the service, although this has a significant performance impact. The Kerberos server itself is known as the Key Distribution Center, or KDC. At a high level, it has three parts:
A database of the users and services (known as principals) that it knows about and their respective Kerberos passwords
An authentication server (AS) which performs the initial authentication and issues a Ticket Granting Ticket (TGT)
A Ticket Granting Server (TGS) that issues subsequent service tickets based on the initial TGT.
A user principal requests authentication from the AS. The AS returns a TGT that is encrypted using the user principal's Kerberos password, which is known only to the user principal and the AS. The user principal decrypts the TGT locally using its Kerberos password, and from that point forward, until the ticket expires, the user principal can use the TGT to get service tickets from the TGS.
Because a service principal cannot provide a password each time to decrypt the TGT, it uses a special file, called a keytab, which contains its authentication credentials.
The service tickets are what allow the principal to access various services. The set of hosts, users, and services over which the Kerberos server has control is called a realm.
![[Note]](../common/images/admon/note.png) | Note |
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Because Kerberos is a time-sensitive protocol, all hosts in the realm must be time-synchronized, for example, by using the Network Time Protocol (NTP). If the local system time of a client differs from that of the KDC by as little as 5 minutes (the default), the client will not be able to authenticate. |
