Kerberos is a network authentication protocol created by MIT, and uses symmetric-key cryptography[] to authenticate users to network services, which means passwords are never actually sent over the network.
Consequently, when users authenticate to network services using Kerberos, unauthorized users attempting to gather passwords by monitoring network traffic are effectively thwarted.
3.1.1. How Kerberos Works
Most conventional network services use password-based authentication schemes, where a user supplies a password to access a given network server. However, 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.
With simple, password-based authentication, a network that is connected to the Internet cannot be assumed to be secure. Any attacker who gains access to the network can use a simple packet analyzer, or packet sniffer, to intercept usernames and passwords, compromising user accounts and, therefore, the integrity of the entire security infrastructure.
Kerberos eliminates the transmission of unencrypted passwords across the network and removes the potential threat of an attacker sniffing the network.
Rather than authenticating each user to each network service separately as with simple password authentication, Kerberos uses symmetric encryption and a trusted third party (a key distribution center or KDC) to authenticate users to a suite of network services. The computers managed by that KDC and any secondary KDCs constitute a realm.
When a user authenticates to the KDC, the KDC sends a set of credentials (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.
As shown in
Figure 3.1, “Kerberos Authentication, in Steps”, each user is identified to the KDC with a unique identity, called a
principal. When a user on a Kerberos-aware network logs into his workstation, his principal is sent to the KDC as part of a request for a
ticket-getting ticket (or TGT) from the authentication server. This request can be sent by the login program so that it is transparent to the user or can be sent manually by a user through 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, encrypts it using the user's key, and sends the TGT 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 ticket (or credentials) sent by the KDC are stored in a local file, the credentials cache, which can be checked by Kerberos-aware services.
After authentication, servers can check an unencrypted list of recognized principals and their keys rather than checking kinit; this is kept in a keytab.
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 ticket-granting server (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 (including telnet and FTP) is highly discouraged. Other encrypted protocols, such as SSH or SSL-secured services, is preferred to unencrypted services, bit this is still not ideal.
Kerberos relies on being able to resolve machine names and on accurate timestamps to issue and expire tickets. Thus, Kerberos requires both adequate clock synchronization and a working domain name service (DNS) to function correctly.
Approximate clock synchronization between the machines on the network can be set up using a service such as ntpd, which is documented in /usr/share/doc/ntp-version-number/html/index.html.
Both DNS entries and hosts on the network must be properly configured, which is covered in the Kerberos documentation in /usr/share/doc/krb5-server-version-number.
