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17.5. Configuring a Kerberos 5 Server

When you are setting up Kerberos, install the server first. If you need 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.

To configure a basic Kerberos server, follow these steps:

  1. Be sure that you have clock synchronization and DNS working on your server before configuring Kerberos 5. Pay particular attention to time synchronization between the Kerberos server and its various clients. If the server and client clocks are different by more than five minutes (this default amount is configurable in Kerberos 5), Kerberos clients will not be able to 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.

    You should set up a Network Time Protocol (NTP) compatible client/server network even if you are not using Kerberos. Red Hat Linux includes the ntp package for easy installation. See /usr/share/doc/ntp-<version-number>/index.htm for details on setting up Network Time Protocol servers and http://www.eecis.udel.edu/~ntp for additional information on NTP.

  2. Install the krb5-libs, krb5-server, and krb5-workstation packages on the dedicated machine which will run the KDC. This machine needs to be very secure — if possible, it should not run any services other than the KDC.

    If you would like to use a graphical user interface utility to administrate Kerberos, you should also install the gnome-kerberos package. It contains krb5, a GUI tool for managing tickets.

  3. Edit the /etc/krb5.conf and /var/kerberos/krb5kdc/kdc.conf configuration files to reflect your realm name and domain-to-realm mappings. A simple realm can be constructed by replacing instances of EXAMPLE.COM and example.com with your 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 your Kerberos server. By convention, all realm names are uppercase and all DNS hostnames and domain names are lowercase. For full details on the formats of these files, see their respective man pages.

  4. 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 will be used to store keys for your 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) will prompt the user for the master server password (which can be used to regenerate the key) every time it starts.

  5. 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 will be able to get by with a single line:

    */admin@EXAMPLE.COM  *

    Most users will be 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) will be able to wield full power over the realm's Kerberos database.

    Once kadmind is started on the server, any user will be able 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 will be able to modify the database in any way, except for changing their own passwords.

    NoteNote
     

    The kadmin utility communicates with the kadmind server over the network, and they use Kerberos to handle authentication. Of course, you need to create the first principal before you can connect 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"
  6. Start Kerberos using the following commands:

    /sbin/service krb5kdc start
    /sbin/service kadmin start
    /sbin/service krb524 start
  7. Add principals for your 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. See the kadmin man page for more information.

  8. Verify that your server will issue 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 your cache and use kdestroy to destroy the cache and the credentials it contains.

    NoteNote
     

    By default, kinit attempts to authenticate using the login user name of the account you used when you first logged into your system (not the Kerberos server). If that system user name does not correspond to a principal in the Kerberos database, you will receive an error message. If that happens, supply kinit with the name of your principal as an argument on the command line (kinit principal).

Once you have completed the steps listed above, the Kerberos server should be up and running. Next, we will set up a Kerberos client.

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