48.6.3. How Kerberos Works
Kerberos differs from username/password authentication methods. Instead of authenticating each user to each network service, Kerberos uses symmetric encryption and a trusted third party (a KDC), to authenticate users to a suite of network services. When a user authenticates to the KDC, the KDC sends a ticket specific to that session back to the user's machine, and any Kerberos-aware services look for the ticket on the user's machine rather than requiring the user to authenticate using a password.
When a user on a Kerberos-aware network logs in to their workstation, their principal is sent to the KDC as part of a request for a TGT from the Authentication Server. This request can be sent by the log-in program so that it is transparent to the user, or can be sent by the
kinit program after the user logs in.
The KDC then checks for the principal in its database. If the principal is found, the KDC creates a TGT, which is encrypted using the user's key and returned to that user.
The login or
kinit program on the client 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 transmitted over the network.
The TGT is set to expire after a certain period of time (usually ten to twenty-four hours) and is 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. After the TGT has been issued, the user does not have to re-enter their password until the TGT expires or until they log out and log in 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.
The Kerberos system can be compromised if a user on the network authenticates against a non-Kerberos aware service by transmitting a password in plain text. The use of non-Kerberos aware services is highly discouraged. Such services include Telnet and FTP. The use of other encrypted protocols, such as SSH or SSL-secured services, however, is preferred, although not ideal.
This is only a broad overview of how Kerberos authentication works. Refer to Section 48.6.10, “Additional Resources” for links to more in-depth information.
Kerberos depends on the following network services to function correctly.
- Approximate clock synchronization between the machines on the network.A clock synchronization program should be set up for the network, such as
ntpd. Refer to
/usr/share/doc/ntp-<version-number>/index.htmlfor details on setting up Network Time Protocol servers (where <version-number> is the version number of the
ntppackage installed on your system).
- Domain Name Service (DNS).You should ensure that the DNS entries and hosts on the network are all properly configured. Refer to the Kerberos V5 System Administrator's Guide in
/usr/share/doc/krb5-server-<version-number>for more information (where <version-number> is the version number of the
krb5-serverpackage installed on your system).