Setting up a realm
******************

A realm is an administrative domain.  The name of a Kerberos realm is usually the Internet domain name in uppercase.  Call your realm the same as your Internet domain name if you do not have strong reasons for not doing so.  It will make life easier for you and everyone else.

Configuration file
==================

To setup a realm you will first have to create a configuration file: /etc/krb5.conf. The krb5.conf file can contain many configuration options, some of which are described here.

There is a sample krb5.conf supplied with the distribution.

The configuration file is a hierarchical structure consisting of sections, each containing a list of bindings (either variable assignments or subsections). A section starts with [section-name].  A binding consists of a left hand side, an equal (=) and a right hand side (the left hand side tag must be separated from the equal with some whitespace.) Subsections has a { as the first non-whitespace character after the equal. All other bindings are treated as variable assignments. The value of a variable extends to the end of the line.

     [section1]
             a-subsection = {
                     var = value1
                     other-var = value with {}
                     sub-sub-section = {
                             var = 123
                     }
             }
             var = some other value
     [section2]
             var = yet another value

In this manual, names of sections and bindings will be given as strings separated by slashes (/). The other-var variable will thus be section1/a-subsection/other-var.

For in-depth information about the contents of the config file, refer to the krb5.conf manual page. Some of the more important sections are briefly described here.

The libdefaults section contains a list of library configuration parameters, such as the default realm and the timeout for kdc responses. The realms section contains information about specific realms, such as where they hide their KDC. This section serves the same purpose as the Kerberos 4 krb.conf file, but can contain more information. Finally the domain_realm section contains a list of mappings from domains to realms, equivalent to the Kerberos 4 krb.realms file.

To continue with the realm setup, you will have to create a config file, with contents similar to the following.

     [libdefaults]
             default_realm = MY.REALM
     [realms]
             MY.REALM = {
                     kdc = my.kdc
             }
     [domain_realm]
             .my.domain = MY.REALM

If you use a realm name equal to your domain name, you can omit the libdefaults, and domain_realm, sections. If you have a SRV-record for your realm, or your kerberos server has CNAME called kerberos.my.realm, you can omit the realms section too.

Creating the database
=====================

The database library will look for the database in /var/heimdal, so you should probably create that directory.

The keys of all the principals are stored in the database.  If you choose to, these can be encrypted with a master key.  You do not have to remember this key (or password), but just to enter it once and it will be stored in a file (/var/heimdal/m-key).  If you want to have a master key, run kstash to create this master key:

     # kstash
     Master key:
     Verifying password - Master key:

To initialise the database use the kadmin program, with the -l option (to enable local database mode). First issue a init MY.REALM command. This will create the database and insert default principals for that realm. You can have more than one realm in one database, so init does not destroy any old database.

Before creating the database, init will ask you some questions about max ticket lifetimes.

After creating the database you should probably add yourself to it. You do this with the add command. It takes as argument the name of a principal. The principal should contain a realm, so if you haven't setup a default realm, you will need to explicitly include the realm.

     # kadmin -l
     kadmin> init MY.REALM
     Realm max ticket life [unlimited]:
     Realm max renewable ticket life [unlimited]:
     kadmin> add me
     Max ticket life [unlimited]:
     Max renewable life [unlimited]:
     Attributes []:
     Password:
     Verifying password - Password:

Now start the KDC and try getting a ticket.

     # kdc &
     # kinit me
     me@MY.REALMS's Password:
     # klist
     Credentials cache: /tmp/krb5cc_0
             Principal: me@MY.REALM

       Issued           Expires          Principal
     Aug 25 07:25:55  Aug 25 17:25:55  krbtgt/MY.REALM@MY.REALM

If you are curious you can use the dump command to list all the entries in the database.  It should look something similar to the following example (note that the entries here are truncated for typographical reasons):

     kadmin> dump
     me@MY.REALM 1:0:1:0b01d3cb7c293b57:-:0:7:8aec316b9d1629e3baf8 ...
     kadmin/admin@MY.REALM 1:0:1:e5c8a2675b37a443:-:0:7:cb913ebf85 ...
     krbtgt/MY.REALM@MY.REALM 1:0:1:52b53b61c875ce16:-:0:7:c8943be ...
     kadmin/changepw@MY.REALM 1:0:1:f48c8af2b340e9fb:-:0:7:e3e6088 ...

keytabs
=======

To extract a service ticket from the database and put it in a keytab you need to first create the principal in the database with ank (using the --random flag to get a random password) and then extract it with ext_keytab.

     kadmin> add --random host/my.host.name
     Max ticket life [unlimited]:
     Max renewable life [unlimited]:
     Attributes []:
     kadmin> ext host/my.host.name
     # ktutil list
     Version  Type             Principal
          1   des-cbc-md5      host/my.host.name@MY.REALM
          1   des-cbc-md4      host/my.host.name@MY.REALM
          1   des-cbc-crc      host/my.host.name@MY.REALM
          1   des3-cbc-sha1    host/my.host.name@MY.REALM

Remote administration
=====================

The administration server, kadmind, is started by inetd and you should add a line similar to the one below to your /etc/inetd.conf.

     kerberos-adm stream     tcp     nowait  root /usr/heimdal/libexec/kadmind kadmind

You might need to add kerberos-adm to your /etc/services as 749/tcp.

Access to the admin server is controlled by an acl-file, (default /var/heimdal/kadmind.acl.) The lines in the access file, has the following syntax:

     principal       [priv1,priv2,...]

The privileges you can assign to a principal are: add, change-password (or cpw for short), delete, get, list, and modify, or the special privilege all. All of these roughly corresponds to the different commands in kadmin.

Password changing
=================

To allow users to change their passwords, you should run kpasswdd. It is not run from inetd.

You might need to add kpasswd to your /etc/services as 464/udp.

Password quality assurance
--------------------------

It is important that users have good passwords, both to make it harder to guess them and to avoid off-line attacks (pre-authentication provides some defense against off-line attacks).  To ensure that the users choose good passwords, you can enable password quality controls in kpasswdd.  The controls themselves are done in a shared library that is used by kpasswdd.  To configure in these controls, add lines similar to the following to your /etc/krb5.conf:

     [password_quality]
             check_library = LIBRARY
             check_function = FUNCTION

The function FUNCTION in the shared library LIBRARY will be called for proposed new passwords.  The function should be declared as:

     const char *
     function(krb5_context context, krb5_principal principal, krb5_data *pwd);

The function should verify that PWD is a good password for PRINCIPAL and if so return NULL.  If it is deemed to be of low quality, it should return a string explaining why that password should not be used.

Code for a password quality checking function that uses the cracklib library can be found in kpasswd/sample_password_check.c in the source code distribution.  It requires the cracklib library built with the patch available at ftp://ftp.pdc.kth.se/pub/krb/src/cracklib.patch.

If no password quality checking function is configured, it is only verified that it is at least six characters of length.

Testing clients and servers
===========================

Now you should be able to run all the clients and servers.  Refer to the appropriate man pages for information on how to use them.