18.5. Configuring a Kerberos 5 Server
When setting up Kerberos, install the server first. If it is necessary to set up slave servers, the details of setting up relationships between master and slave servers are covered in the Kerberos 5 Installation Guide located in the /usr/share/doc/krb5-server-<version-number> directory (replace <version-number> with the version number of the krb5-server package installed on the system).
To configure a basic Kerberos server, follow these steps:
Be sure that clock synchronization and DNS are functioning on all client and server machines before configuring Kerberos 5. Pay particular attention to time synchronization between the Kerberos server and its clients. If the server and client clocks are different by more than five minutes (this default amount is configurable in Kerberos 5), Kerberos clients can not authenticate to the server. This clock synchronization is necessary to prevent an attacker from using an old Kerberos ticket to masquerade as a valid user.
It is advisable to set up a Network Time Protocol (NTP) compatible client/server network even if Kerberos is not being used. Red Hat Enterprise Linux includes the ntp package for this purpose. Refer to /usr/share/doc/ntp-<version-number>/index.htm for details about how to set up Network Time Protocol servers and http://www.eecis.udel.edu/~ntp for additional information about NTP.
Install the krb5-libs, krb5-server, and krb5-workstation packages on the dedicated machine which runs the KDC. This machine needs to be very secure — if possible, it should not run any services other than the KDC.
If a graphical user interface is required to administrate Kerberos, install the gnome-kerberos package. It contains krb5, a GUI tool for managing tickets.
Edit the /etc/krb5.conf and /var/kerberos/krb5kdc/kdc.conf configuration files to reflect the realm name and domain-to-realm mappings. A simple realm can be constructed by replacing instances of EXAMPLE.COM and example.com with the correct domain name — being certain to keep uppercase and lowercase names in the correct format — and by changing the KDC from kerberos.example.com to the name of the Kerberos server. By convention, all realm names are uppercase and all DNS hostnames and domain names are lowercase. For full details about the formats of these files, refer to their respective man pages.
Create the database using the kdb5_util utility from a shell prompt:
/usr/kerberos/sbin/kdb5_util create -s
The create command creates the database that used to store keys for the Kerberos realm. The -s switch forces creation of a stash file in which the master server key is stored. If no stash file is present from which to read the key, the Kerberos server (krb5kdc) prompts the user for the master server password (which can be used to regenerate the key) every time it starts.
Edit the /var/kerberos/krb5kdc/kadm5.acl file. This file is used by kadmind to determine which principals have administrative access to the Kerberos database and their level of access. Most organizations can get by with a single line:
*/admin@EXAMPLE.COM *
Most users are represented in the database by a single principal (with a NULL, or empty, instance, such as joe@EXAMPLE.COM). With this configuration, users with a second principal with an instance of admin (for example, joe/admin@EXAMPLE.COM) are able to wield full power over the realm's Kerberos database.
Once kadmind is started on the server, any user can to access its services by running kadmin on any of the clients or servers in the realm. However, only users listed in the kadm5.acl file can modify the database in any way, except for changing their own passwords.
Note The kadmin utility communicates with the kadmind server over the network, and uses Kerberos to handle authentication. For this reason, the first principal must already exist before connecting to the server over the network to administer it. Create the first principal with the kadmin.local command, which is specifically designed to be used on the same host as the KDC and does not use Kerberos for authentication.
Type the following kadmin.local command at the KDC terminal to create the first principal:
/usr/kerberos/sbin/kadmin.local -q "addprinc username/admin"
Start Kerberos using the following commands:
/sbin/service krb5kdc start /sbin/service kadmin start /sbin/service krb524 start
Add principals for the users using the addprinc command with kadmin. kadmin and kadmin.local are command line interfaces to the KDC. As such, many commands are available after launching the kadmin program. Refer to the kadmin man page for more information.
Verify that the KDC is issuing tickets. First, run kinit to obtain a ticket and store it in a credential cache file. Next, use klist to view the list of credentials in the cache and use kdestroy to destroy the cache and the credentials it contains.
Note By default, kinit attempts to authenticate using the login user name of the account used when logging into the system (not the Kerberos server). If that user name does not correspond to a principal in the Kerberos database, kinit issues an error message. If that happens, supply kinit with the name of the correct principal as an argument on the command line (kinit <principal>).
Once these steps are completed, the Kerberos server should be up and running.