Introduction to Kerberos

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Kerberos is a network authentication protocol. It is designed to provide strong authentication for client/server applications by using secret-key (symmetric) cryptography. With this protocol, a client can prove its identity to a server (and vice versa) across an insecure network connection, and further encrypt all of their communications to assure privacy and data integrity as they go about their business.

Not all services and applications can use Kerberos, but for those that can, it brings the network environment one step closer to being Single Sign On (SSO).

Kerberos components

Kerberos is principally composed of three parties:

  • The client: Usually a user or application requesting access
  • The service: The network service or application the client wants to access
  • The Key Distribution Center: Manages the authentication on behalf of the client and service

Key Distribution Center server

The Key Distribution Center, or KDC server, is also composed of three main parts:

  • The Admin Server, which manages the database of all the users, computers, and services (defined as Kerberos principals)
  • The Authentication Server (AS)
  • The Ticket Granting Server (TGS)

If you are new to Kerberos there are a few terms that are good to understand before setting up a Kerberos server. Most of the terms will relate to things you may be familiar with in other environments:

  • Principal: any users, computers, and services provided by servers need to be defined as Kerberos Principals.

  • Instances: are a variation for service principals. For example, the principal for an NFS service will have an instance for the hostname of the server, like nfs/server.example.com@REALM. Similarly admin privileges on a principal use an instance of /admin, like john/admin@REALM, differentiating it from john@REALM. These variations fit nicely with ACLs.

  • Realms: the unique realm of control provided by the Kerberos installation. Think of it as the domain or group your hosts and users belong to. Convention dictates the realm should be in uppercase. By default, Ubuntu will use the DNS domain converted to uppercase (EXAMPLE.COM) as the realm.

  • Key Distribution Center: (KDC) consist of three parts: a database of all principals, the authentication server, and the ticket granting server. For each realm there must be at least one KDC.

  • Ticket Granting Ticket: issued by the Authentication Server (AS), the Ticket Granting Ticket (TGT) is encrypted in the user’s password which is known only to the user and the KDC. This is the starting point for a user to acquire additional tickets for the services being accessed.

  • Ticket Granting Server: (TGS) issues service tickets to clients upon request.

  • Tickets: confirm the identity of the two principals. One principal being a user and the other a service requested by the user. Tickets establish an encryption key used for secure communication during the authenticated session.

  • Keytab Files: contain encryption keys for a service or host extracted from the KDC principal database.

To put the pieces together, a Realm has at least one KDC, preferably more for redundancy, which contains a database of Principals. When a user principal logs into a workstation that is configured for Kerberos authentication, the KDC issues a Ticket Granting Ticket (TGT). If the user supplied credentials match, the user is authenticated and can then request tickets for Kerberized services from the Ticket Granting Server (TGS). The service tickets allow the user to authenticate to the service without entering another username and password.

Resources

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