Chapter 10.

Kerberos

Kerberos is a network authentication protocol created by MIT which uses secret key cryptography  

obviating the need to send passwords over the network. By authenticating using Kerberos, unautho 

rized users trying to intercept passwords on the network are effectively thwarted.

10.1. Advantages of Kerberos

Most conventional network systems use password based authentication schemes. When a user au 

thenticates to a network server, he must supply a user name and password for each service requiring

authentication. This information is sent over the network, and the server verifies their identity using

this information.

However, the transmission of authentication information for many services is in plain text. An attacker

with access to the network and a packet analyzer, also known as a packet sniffer, can intercept any

passwords sent in this manner.

The primary design goal of Kerberos is to eliminate the transmission across the network of authenti 

cation information. The proper use of Kerberos effectively eliminates the threat packet sniffers would

otherwise pose on a network.

10.2. Kerberos Disadvantages

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

various 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 at

the following URL: http://www.nrl.navy.mil/CCS/people/kenh/kerberos faq.html.

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 10.5.

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 size or frequency

that krb libraries must be called. For other 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 using trusted hosts 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 physical access to any of the hosts, especially the one that issues tickets used

for authentication, 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 any passwords transferred to a service which does not use Kerberos for authentication

run the risk of being captured by packet sniffers. Thus, your network gains no benefit from the

use of Kerberos. To secure your network with Kerberos, you must either kerberize all applications

which send plain text passwords or do not use those applications on your network at all.