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 configuration 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 configuration file, with contents similar to the following.

     [libdefaults]
             default_realm = MY.REALM
     [realms]
             MY.REALM = {
                     kdc = my.kdc my.slave.kdc
                     kdc = my.third.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 the directory /var/heimdal, so you should probably create that directory. Make sure the directory have restrictive permissions.

     # mkdir /var/heimdal

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-key flag to get a random key) and then extract it with ext_keytab.

     kadmin> add --random-key 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

Serving Kerberos 4/524/kaserver
===============================

Heimdal can be configured to support 524, Kerberos 4 or kaserver. All theses services are default turned off. Kerberos 4 support also depends on if Kerberos 4 support is compiled in with Heimdal.

524
---

524 is a service that allows the KDC to convert Kerberos 5 tickets to Kerberos 4 tickets for backward compatibility. See also Using 2b tokens with AFS in see Things in search for a better place.

524 can be turned on by adding this to the configuration file

     [kdc]
     	enable-524 = yes

Kerberos 4
----------

Kerberos 4 is the predecessor to to Kerberos 5. It only support single DES. You should only enable Kerberos 4 support if you have a need for for compatibility with an installed base of Kerberos 4 clients/servers.

Kerberos 4 can be turned on by adding this to the configuration file

     [kdc]
     	enable-kerberos4 = yes

kaserver
--------

Kaserver is a Kerberos 4 that is used in AFS, the protocol have some features over plain Kerberos 4, but like Kerberos 4 only use single DES too.

You should only enable Kerberos 4 support if you have a need for for compatibility with an installed base of AFS machines.

Kaserver can be turned on by adding this to the configuration file

     [kdc]
     	enable-kaserver = yes

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

The administration server, kadmind, can be started by inetd (which isn't recommended) or run as a normal daemon. If you want to start it from inetd 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 administration 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,...]       [glob-pattern]

The matching is from top to bottom for matching principal (and if given, glob-pattern).  When there is a match, the rights of that lines are used.

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.

If a GLOB-PATTERN is given on a line, it restricts the right for the principal to only apply for the subjects that match the pattern. The patters are of the same type as those used in shell globbing, see fnmatch(3).

In the example below lha/admin can change every principal in the database. jimmy/admin can only modify principals that belong to the realm E.KTH.SE. mille/admin is working at the help desk, so he should only be able to change the passwords for single component principals (ordinary users). He will not be able to change any /admin principal.

     lha/admin@E.KTH.SE	all
     jimmy/admin@E.KTH.SE	all		*@E.KTH.SE
     jimmy/admin@E.KTH.SE	all		*/*@E.KTH.SE
     mille/admin@E.KTH.SE	change-password	*@E.KTH.SE

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 lib/kadm5/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.

Slave servers, Incremental propagation, Testing clients and servers, Setting up a realm
=======================================================================================

It is desirable to have at least one backup (slave) server in case the master server fails. It is possible to have any number of such slave servers but more than three usually doesn't buy much more redundancy.

All Kerberos servers for a realm shall have the same database so that they present the same service to all the users.  The hprop program, running on the master, will propagate the database to the slaves, running hpropd processes.

Every slave needs a database directory, the master key (if it was used for the database) and a keytab with the principal hprop/HOSTNAME.  Add the principal with the ktutil command and start propd, as follows:

     slave# ktutil get -p foo/admin hprop/`hostname`
     slave# mkdir /var/heimdal
     slave# hpropd

The master will use the principal kadmin/hprop to authenticate to the slaves.  This principal should be added when running kadmin -l init but if you do not have it in your database for whatever reason, please add it with kadmin -l add.

Then run hprop on the master:

     master# hprop slave

This was just an on-hands example to make sure that everything was working properly.  Doing it manually is of course the wrong way and to automate this you will want to start hpropd from inetd on the slave(s) and regularly run hprop on the master to regularly propagate the database. Starting the propagation once an hour from cron is probably a good idea.

Incremental propagation
=======================

There is also a newer and still somewhat experimental mechanism for doing incremental propagation in Heimdal.  Instead of sending the whole database regularly, it sends the changes as they happen on the master to the slaves.  The master keeps track of all the changes by assigned a version number to every change to the database.  The slaves know which was the latest version they saw and in this way it can be determined if they are in sync or not.  A log of all the changes is kept on the master and when a slave is at an older versioner than the oldest one in the log, the whole database has to be sent.

Protocol-wise, all the slaves connects to the master and as a greeting tell it the latest version that they have (IHAVE message).  The master then responds by sending all the changes between that version and the current version at the master (a series of FORYOU messages) or the whole database in a TELLYOUEVERYTHING message.

Configuring incremental propagation
-----------------------------------

The program that runs on the master is ipropd-master and all clients run ipropd-slave.

Create the file /var/heimdal/slaves on the master containing all the slaves that the database should be propagated to.  Each line contains the full name of the principal (for example iprop/hemligare.foo.se@FOO.SE).

You should already have iprop/tcp defined as 2121, in your /etc/services.  Otherwise, or if you need to use a different port for some peculiar reason, you can use the --port option.  This is useful when you have multiple realms to distribute from one server.

Then you need to create these principals that you added in the configuration file.  Create one iprop/hostname for the master and for every slave.

     master# /usr/heimdal/sbin/ktutil get iprop/`hostname`

The next step is to start the ipropd-master process on the master server.  The ipropd-master listens on the UNIX-socket /var/heimdal/signal to know when changes have been made to the database so they can be propagated to the slaves.  There is also a safety feature of testing the version number regularly (every 30 seconds) to see if it has been modified by some means that do not raise this signal.  Then, start ipropd-slave on all the slaves:

     master# /usr/heimdal/libexec/ipropd-master &
     slave#  /usr/heimdal/libexec/ipropd-slave master &

Salting
=======

Salting is used to make it harder to precalculate all possible keys. Using a salt increases the search space to make it almost impossible to precalculate all keys. Salting is the process of mixing a public string (the salt) with the password, then sending it through an encryption-type specific string-to-key function that will output the fixed size encryption key.

In Kerberos 5 the salt is determined by the encryption-type, except in some special cases.

In des there is the Kerberos 4 salt (none at all) or the afs-salt (using the cell (realm in afs-lingo)).

In arcfour (the encryption type that Microsoft Windows 2000 uses) there is no salt. This is to be compatible with NTLM keys in Windows NT 4.

[kadmin]default_keys in krb5.conf controls what salting to use,

The syntax of [kadmin]default_keys is [etype:]salt-type[:salt-string]. etype is the encryption type (des, des3, arcfour), salt-type is the type of salt (pw-salt or afs3-salt), and the salt-string is the string that will be used as salt (remember that if the salt is appended/prepended, the empty salt "" is the same thing as no salt at all).

Common types of salting includes

  * v4 (or des:pw-salt:)

     The Kerberos 4 salting is using no salt att all. Reason there is colon that the end or the salt string is that it makes the salt the empty string (same as no salt).

  * v5 (or pw-salt)

     pw-salt means all regular encryption-types that is regular

  * afs3-salt

     afs3-salt is the salting that is used with Transarc kaserver. Its the cell appended to the password.

Cross realm
===========

Suppose you are residing in the realm MY.REALM, how do you authenticate to a server in OTHER.REALM? Having valid tickets in MY.REALM allows you to communicate with kerberised services in that realm. However, the computer in the other realm does not have a secret key shared with the Kerberos server in your realm.

It is possible to add a share keys between two realms that trust each other. When a client program, such as telnet or ssh, finds that the other computer is in a different realm, it will try to get a ticket granting ticket for that other realm, but from the local Kerberos server. With that ticket granting ticket, it will then obtain service tickets from the Kerberos server in the other realm.

For a two way trust between MY.REALM and OTHER.REALM add the following principals to each realm. The principals should be krbtgt/OTHER.REALM@MY.REALM and krbtgt/MY.REALM@OTHER.REALM in MY.REALM, and krbtgt/MY.REALM@OTHER.REALM and krbtgt/OTHER.REALM@MY.REALMin OTHER.REALM.

In Kerberos 5 the trust can be one configured to be one way. So that users from MY.REALM can authenticate to services in OTHER.REALM, but not the opposite. In the example above, the krbtgt/MY.REALM@OTHER.REALM then should be removed.

The two principals must have the same key, key version number, and the same set of encryption types. Remember to transfer the two keys in a safe manner.

     vr$ klist
     Credentials cache: FILE:/tmp/krb5cc_913.console
             Principal: lha@E.KTH.SE

       Issued           Expires          Principal
     May  3 13:55:52  May  3 23:55:54  krbtgt/E.KTH.SE@E.KTH.SE

     vr$ telnet -l lha hummel.it.su.se
     Trying 2001:6b0:5:1095:250:fcff:fe24:dbf...
     Connected to hummel.it.su.se.
     Escape character is '^]'.
     Waiting for encryption to be negotiated...
     [ Trying mutual KERBEROS5 (host/hummel.it.su.se@SU.SE)... ]
     [ Kerberos V5 accepts you as ``lha@E.KTH.SE'' ]
     Encryption negotiated.
     Last login: Sat May  3 14:11:47 from vr.l.nxs.se
     hummel$ exit

     vr$ klist
     Credentials cache: FILE:/tmp/krb5cc_913.console
             Principal: lha@E.KTH.SE

       Issued           Expires          Principal
     May  3 13:55:52  May  3 23:55:54  krbtgt/E.KTH.SE@E.KTH.SE
     May  3 13:55:56  May  3 23:55:54  krbtgt/SU.SE@E.KTH.SE
     May  3 14:10:54  May  3 23:55:54  host/hummel.it.su.se@SU.SE

Transit policy
==============

If you want to use cross realm authentication through an intermediate realm it must be explicitly allowed by either the KDCs or the server receiving the request. This is done in krb5.conf in the [capaths] section.

When the ticket transits through a realm to another realm, the destination realm adds its peer to the "transited-realms" field in the ticket. The field is unordered, this is since there is no way to know if know if one of the transited-realms changed the order of the list.

The syntax for [capaths] section:

     [capaths]
             CLIENT-REALM = {
                     SERVER-REALM = PERMITTED-CROSS-REALMS ...
             }

The realm STACKEN.KTH.SE allows clients from SU.SE and DSV.SU.SE to cross in. Since STACKEN.KTH.SE only have direct cross realm with KTH.SE, and DSV.SU.SE only have direct cross realm with SU.SE they need to use both SU.SE and KTH.SE as transit realms.

     [capaths]
     	SU.SE = {
                         STACKEN.KTH.SE = KTH.SE
     	}
     	DSV.SU.SE = {
                         STACKEN.KTH.SE = SU.SE KTH.SE
     	}

Setting up DNS
==============

If there is information about where to find the KDC or kadmind for a realm in the krb5.conf for a realm, that information will be preferred and DNS will not be queried.

Heimdal will try to use DNS to find the KDCs for a realm. First it will try to find SRV resource record (RR) for the realm. If no SRV RRs are found, it will fall back to looking for a A RR for a machine named kerberos.REALM, and then kerberos-1.REALM, etc

Adding this information to DNS makes the client have less configuration (in the common case, no configuration) and allows the system administrator to change the number of KDCs and on what machines they are running without caring about clients.

The backside of using DNS that the client might be fooled to use the wrong server if someone fakes DNS replies/data, but storing the IP addresses of the KDC on all the clients makes it very hard to change the infrastructure.

Example of the configuration for the realm EXAMPLE.COM,

     $ORIGIN example.com.
     _kerberos._tcp          SRV     10 1 88 kerberos.example.com.
     _kerberos._udp          SRV     10 1 88 kerberos.example.com.
     _kerberos._tcp          SRV     10 1 88 kerberos-1.example.com.
     _kerberos._udp          SRV     10 1 88 kerberos-1.example.com.
     _kpasswd._udp           SRV     10 1 464 kerberos.example.com.
     _kerberos-adm._tcp	SRV	10 1 749 kerberos.example.com.

More information about DNS SRV resource records can be found in RFC-2782 (A DNS RR for specifying the location of services (DNS SRV)).