17.3. How Kerberos Works

Kerberos differs from other authentication methods. Instead of authenticating each user to each network service, Kerberos uses symmetric encryption and a trusted third party — known as the Key Distribution Center (KDC) — to authenticate users to a suite of network services. Once a user authenticates to the KDC, it sends a ticket specific to that session back the user's machine and any kerberized service will look for the ticket on the user's machine rather than asking the user to authenticate using a password.

When a user on a kerberized network logs in to their workstation, their principal is sent to the KDC in a request for a Ticket Granting Ticket (TGT) from the Ticket Granting Service (TGS). This request can be sent by the login program so that it is transparent to the user or can be sent by the kinit program after the user logs in.

The KDC checks for the principal in its database. If the principal is found, the KDC tell the TGS to create a TGT, which is encrypted using the user's key and returned to that user.

The login or kinit program on the client machine then decrypts the TGT using the user's key (which it computes from the user's password). The user's key is used only on the client machine and is not sent over the network.

The TGT is set to expire after a certain period of time (usually ten hours) and stored in the client machine's credentials cache. An expiration time is set so that a compromised TGT is of use to an attacker for only a short period of time. Once the TGT is issued, the user will not have to re-enter their password to the KDC until the TGT expires or they logout and login again.

Whenever the user needs access to a network service, the client software uses the TGT to request a new ticket for that specific service from the TGS. The service ticket is then used to authenticate the user to that service transparently.

	Warning
	The Kerberos system can be compromised any time any user on the network authenticates against a non-kerberized service by sending a password in plain text. Therefore use of non-kerberized services is discouraged. Such services include Telnet and FTP. Use of other encrypted protocols, such as SSH or SSL secured services, however, is acceptable, though not ideal.

This is only a broad overview of how Kerberos authentication on a network works, those seeking a more in-depth look at Kerberos authentication, should refer to Section 17.7 Additional Resources.

Note

Kerberos depends on certain network services to work correctly. First, Kerberos requires approximate clock synchronization between the machines on the network. Therefore, a clock synchronization program should be set up for the network, such as ntpd. For more on configuring ntpd, see /usr/share/doc/ntp-<version-number>/index.htm for details on setting up Network Time Protocol servers. Also, since certain aspects of Kerberos rely on the Domain Name Service (DNS), be sure that the DNS entries and hosts on the network are all properly configured. See the Kerberos V5 System Administrator's Guide, provided in PostScript and HTML formats in /usr/share/doc/krb5-server-<version-number> for more information.