Chapter 17. Kerberos

Kerberos is a network authentication protocol created by MIT which uses symmetric key cryptography to authenticate users to network services — eliminating the need to send passwords over the network. When users authenticate to network services using Kerberos, unauthorized users attempting to gather passwords by monitoring network traffic are effectively thwarted.

17.1. Advantages of Kerberos

Most conventional network systems use password-based authentication schemes. Such schemes require a user to authenticate to a given network server by supplying their user name and password. Unfortunately, the transmission of authentication information for many services is unencrypted. For such a scheme to be secure, the network has to be inaccessible to outsiders, and all computers and users on the network must be trusted and trustworthy.

Even if this is the case, once a network is connected to the Internet, it can no longer be assumed that the network is secure. An attacker who gains access can use a simple packet analyzer, also known as a packet sniffer, to intercept usernames and passwords sent in this manner, compromising user accounts and the integrity of the entire security infrastructure.

The primary design goal of Kerberos is to eliminate the transmission of unencrypted passwords across the network. If used properly, Kerberos effectively eliminates the threat packet sniffers would otherwise pose on a network.

17.1.1. Disadvantages of Kerberos

Although Kerberos removes a common and severe security threat, it may be difficult to implement for a variety of reasons:

  • Migrating user passwords from a standard UNIX password database, such as /etc/passwd or /etc/shadow, to a Kerberos password database can be tedious, as there is no automated mechanism to perform this task. For more information, refer to question number 2.23 in the Kerberos FAQ online at the following URL:

  • Kerberos has only partial compatibility with the Pluggable Authentication Modules (PAM) system used by most servers running Red Hat Linux. For more information on this issue, see Section 17.4 Kerberos and PAM.

  • For an application to use Kerberos, its source must be modified to make the appropriate calls into the Kerberos libraries. For some applications, this can be quite problematic due to the size of the application or its design. For other incompatible applications, changes must be made to the way in which the server and client side communicate. Again, this may require extensive programming. Closed-source applications that do not have Kerberos support by default are often the most problematic.

  • Kerberos assumes that you are a trusted user using an untrusted host on an untrusted network. Its primary goal is to prevent plain text passwords from being sent across that network. However, if anyone other than the proper user has access to the one host that issues tickets used for authentication — called the key distribution center (KDC) — the entire Kerberos authentication system is at risk of being compromised.

  • Kerberos is an all or nothing solution. If you decide to use Kerberos on your network, you must remember that any passwords transferred to a service which does not use Kerberos for authentication are at risk of being captured by packet sniffers. Thus, your network gains no benefit from the use of Kerberos. To secure a network with Kerberos, one must either use kerberized versions of all client/server applications which send unencrypted passwords or not use any such client/server applications at all.