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Chapter 19. Kerberos
System security and integrity within a network can be unwieldy. It can occupy the time of several administrators just to keep track of what services are being run on a network and the manner in which these services are used. Moreover, authenticating users to network services can prove dangerous when the method used by the protocol is inherently insecure, as evidenced by the transfer of unencrypted passwords over a network under the FTP and Telnet protocols. Kerberos is a way to eliminate the need for protocols that allow unsafe methods of authentication, thereby enhancing overall network security.
19.1. What is Kerberos?
Kerberos, a network authentication protocol created by MIT, uses symmetric-key cryptography[5] 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.
19.1.1. Advantages of Kerberos
Most conventional network services use password-based authentication schemes. Such schemes require a user to authenticate to a given network server by supplying their username 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. Any attacker who gains access to the network 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.
19.1.2. 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/passwdor/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 online Kerberos FAQ: - Kerberos has only partial compatibility with the Pluggable Authentication Modules (PAM) system used by most Red Hat Enterprise Linux servers. For more information about this issue, refer to Section 19.4, “Kerberos and PAM”.
- Kerberos assumes that each user is trusted but is using an untrusted host on an untrusted network. Its primary goal is to prevent unencrypted 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.
- For an application to use Kerberos, its source must be modified to make the appropriate calls into the Kerberos libraries. Applications modified in this way are considered to be kerberized. 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 is an all or nothing solution. Once Kerberos is used on the network, any unencrypted passwords transferred to a non-kerberized service is at risk. Thus, the 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.
[5]
A system where both the client and the server share a common key that is used to encrypt and decrypt network communication.
