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 sign (=) and a right hand side (the left hand side tag must be separated from the equal sign with some whitespace). Subsections have a { as the first non-whitespace character after the equal sign. 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
                     kdc = 130.237.237.17
                     kdc = [2001:6b0:1:ea::100]:88
             }
     [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 DNS SRV-record for your realm, or your Kerberos server has DNS CNAME kerberos.my.realm, you can omit the realms section too.

If you want to use a different configuration file then the default you can point a file with the enviroment variable KRB5_CONFIG.

     env KRB5_CONFIG=$HOME/etc/krb5.conf kinit user@REALM

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

The database library will look for the database in the directory dbdir, so you should probably create that directory. Make sure the directory has 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:

If you want to generate a random master key you can use the --random-key flag to kstash. This will make sure you have a good key on which attackers can't do a dictionary attack.

If you have a master key, make sure you make a backup of your master key file; without it backups of the database are of no use.

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 maximum 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 set up 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 ...

Modifying the database
======================

All modifications of principals are done with with kadmin.

A principal has several attributes and lifetimes associated with it.

Principals are added, renamed, modified, and deleted with the kadmin commands add, rename, modify, delete. Both interactive editing and command line flags can be used (use --help to list the available options).

There are different kinds of types for the fields in the database; attributes, absolute time times and relative times.

Attributes
----------

When doing interactive editing, attributes are listed with ?.

The attributes are given in a comma (,) separated list. Attributes are removed from the list by prefixing them with -.

     kadmin> modify me
     Max ticket life [1 day]:
     Max renewable life [1 week]:
     Principal expiration time [never]:
     Password expiration time [never]:
     Attributes [disallow-renewable]: requires-pre-auth,-disallow-renewable
     kadmin> get me
                 Principal: me@MY.REALM
     [...]
                Attributes: requires-pre-auth

Absolute times
--------------

The format for absolute times are any of the following:

     never
     now
     YYYY-mm-dd
     YYYY-mm-dd HH:MM:SS

Relative times
--------------

The format for relative times are any of the following combined:

     N year
     M month
     O day
     P hour
     Q minute
     R second

Checking the setup
==================

There are two tools that can check the consistency of the Kerberos configuration file and the Kerberos database.

The Kerberos configuration file is checked using verify_krb5_conf. The tool checks for common errors, but commonly there are several uncommon configuration entries that are never added to the tool and thus generates ``unknown entry'' warnings. This is usually nothing to worry about.

The database check is built into the kadmin tool. It will check for common configuration error that will cause problems later. Common check are for existence and flags on important principals. The database check by run by the following command :

     kadmin -l check REALM.EXAMPLE.ORG

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 add (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
     kadmin> exit
     # 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 these services are turned off by default. Kerberos 4 is always supported by the KDC, but the Kerberos 4 client support also depends on Kerberos 4 support having been included at compile-time, using --with-krb4=dir.

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 AFS.

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 supports single DES. You should only enable Kerberos 4 support if you have needs 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 has some extra features over plain Kerberos 4, but like Kerberos 4, only uses single DES.

You should only enable Kaserver support if you have needs 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 file has the following syntax:

     principal       [priv1,priv2,...]       [glob-pattern]

The matching is from top to bottom for matching principals (and if given, glob-pattern).  When there is a match, the access rights of that line are applied.

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 correspond to the different commands in kadmin.

If a GLOB-PATTERN is given on a line, it restricts the access rights for the principal to only apply for subjects that match the pattern.  The patterns 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.  If your realm is not setup to use DNS, you might also need to add a kpasswd_server entry to the realm configuration in /etc/krb5.conf on client machines:

     [realms]
             MY.REALM = {
                     kdc = my.kdc my.slave.kdc
                     kpasswd_server = my.kdc
             }

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 (although pre-authentication provides some defence against off-line attacks). To ensure that the users choose good passwords, you can enable password quality controls in kpasswdd and kadmind. The controls themselves are done in a shared library or an external program that is used by kpasswdd.  To configure in these controls, add lines similar to the following to your /etc/krb5.conf:

     [password_quality]
     	policies = external-check builtin:minimum-length modulename:policyname
     	external_program = /bin/false
     	policy_libraries = LIBRARY1.SO LIBRARY2.SO

In [password_quality]policies the module name is optional if the policy name is unique in all modules (members of policy_libraries).  All built-in policies can be qualified with a module name of builtin to unambiguously specify the built-in policy and not a policy by the same name from a loaded module.

The built-in policies are

  * external-check

     Executes the program specified by [password_quality]external_program.

     A number of key/value pairs are passed as input to the program, one per line, ending with the string end.  The key/value lines are of the form

          principal: PRINCIPAL
          new-password: PASSWORD

     where PASSWORD is the password to check for the previous PRINCIPAL.

     If the external application approves the password, it should return APPROVED on standard out and exit with exit code 0.  If it doesn't approve the password, an one line error message explaining the problem should be returned on standard error and the application should exit with exit code 0.  In case of a fatal error, the application should, if possible, print an error message on standard error and exit with a non-zero error code.

  * minimum-length

     The minimum length password quality check reads the configuration file stanza [password_quality]min_length and requires the password to be at least this length.

  * character-class

     The character-class password quality check reads the configuration file stanza [password_quality]min_classes. The policy requires the password to have characters from at least that many character classes. Default value if not given is 3.

     The four different characters classes are, uppercase, lowercase, number, special characters.

If you want to write your own shared object to check password policies, see the manual page kadm5_pwcheck,3.

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 that the cracklib library be built with the patch available at ftp://ftp.pdc.kth.se/pub/krb/src/cracklib.patch.

A sample policy external program is included in lib/kadm5/check-cracklib.pl.

If no password quality checking function is configured, the only check performed is that the password is at least six characters long.

To check the password policy settings, use the command verify-password-quality in kadmin program. The password verification is only performed locally, on the client.  It may be convenient to set the environment variable KRB5_CONFIG to point to a test version of krb5.conf while you're testing the [password_quality] stanza that way.

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 must have the same database so that they present the same service to 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 hpropd, 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 hands-on 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 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 assigning 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 version than the oldest one in the log, the whole database has to be sent.

Protocol-wise, all the slaves connect 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.  There is also a keep-alive protocol that makes sure all slaves are up and running.

In addition on listening on the network to get connection from new slaves, the ipropd-master also listens on a status unix socket. kadmind and kpasswdd both open that socket when a transation is done and written a notification to the socket. That cause ipropd-master to check for new version in the log file. As a fallback in case a notification is lost by the unix socket, the log file is checked after 30 seconds of no event.

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 those 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`

     slave# /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 domain 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 &

To manage the iprop log file you should use the iprop-log command. With it you can dump, truncate and replay the logfile.

Encryption types and salting
============================

The encryption types that the KDC is going to assign by default is possible to change. Since the keys used for user authentication is salted the encryption types are described together with the salt strings.

Salting is used to make it harder to pre-calculate all possible keys. Using a salt increases the search space to make it almost impossible to pre-calculate 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-cbc-crc, arcfour-hmac-md5, aes256-cts-hmac-sha1-96), 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 include

  * v4 (or des:pw-salt:)

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

  * v5 (or pw-salt)

     pw-salt uses the default salt for each encryption type is specified for. If the encryption type etype isn't given, all default encryption will be used.

  * afs3-salt

     afs3-salt is the salt that is used with Transarc kaserver. It's the cell name appended to the password.

Credential cache server - KCM
=============================

When KCM running is easy for users to switch between different kerberos principals using kswitch or built in support in application, like OpenSSH's GSSAPIClientIdentity.

Other advantages are that there is the long term credentials are not written to disk and on reboot the credential is removed when kcm process stopps running.

Configure the system startup script to start the kcm process, /usr/heimdal/libexec/kcm and then configure the system to use kcm in krb5.conf.

     [libdefaults]
     	default_cc_type = KCM

Now when you run kinit it doesn't overwrite your existing credentials but rather just add them to the set of credentials. klist -l lists the credentials and the star marks the default credential.

     $ kinit lha@KTH.SE
     lha@KTH.SE's Password:
     $ klist -l
       Name         Cache name               Expires
     lha@KTH.SE   0                        Nov 22 23:09:40   *
     lha@SU.SE    Initial default ccache   Nov 22 14:14:24

When switching between credentials you can use kswitch.

     $ kswitch -i
          Principal
     1    lha@KTH.SE
     2    lha@SU.SE
     Select number: 2

After switching, a new set of credentials are used as default.

     $ klist -l
       Name         Cache name               Expires
     lha@SU.SE    Initial default ccache   Nov 22 14:14:24   *
     lha@KTH.SE   0                        Nov 22 23:09:40

Som applications, like openssh with Simon Wilkinsons patch applied, support specifiying that credential to use.  The example below will login to the host computer.kth.se using lha@KTH.SE (not the current default credential).

     $ ssh \
        -o GSSAPIAuthentication=yes \
        -o GSSAPIKeyExchange=yes \
        -o GSSAPIClientIdentity=lha@KTH.SE \
        computer.kth.se

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

Suppose you reside 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 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 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
==============

Under some circumstances, you may not wish to set up direct cross-realm trust with every realm to which you wish to authenticate or from which you wish to accept authentications. Kerberos supports multi-hop cross-realm trust where a client principal in realm A authenticates to a service in realm C through a realm B with which both A and C have cross-realm trust relationships. In this situation, A and C need not set up cross-realm principals between each other.

If you want to use cross-realm authentication through an intermediate realm, it must be explicitly allowed by either the KDCs for the realm to which the client is authenticating (in this case, realm C), or the server receiving the request. This is done in krb5.conf in the [capaths] section.

In addition, the client in realm A need to be configured to know how to reach realm C via realm B. This can be done either on the client or via KDC configuration in the KDC for realm A.

Allowing cross-realm transits
-----------------------------

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, since there is no way to know if know if one of the transited-realms changed the order of the list. For the authentication to be accepted by the final destination realm, all of the transited realms must be listed as trusted in the [capaths] configuration, either in the KDC for the destination realm or on the server receiving the authentication.

The syntax for [capaths] section is:

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

In the following example, the realm STACKEN.KTH.SE only has direct cross-realm set up with KTH.SE.  KTH.SE has direct cross-realm set up with STACKEN.KTH.SE and SU.SE. DSV.SU.SE only has direct cross-realm set up with SU.SE. The goal is to allow principals in the DSV.SU.SE or SU.SE realms to authenticate to services in STACKEN.KTH.SE.  This is done with the following [capaths] entry on either the server accepting authentication or on the KDC for STACKEN.KTH.SE.

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

The first entry allows cross-realm authentication from clients in SU.SE transiting through KTH.SE to STACKEN.KTH.SE.  The second entry allows cross-realm authentication from clients in DSV.SU.SE transiting through both SU.SE and KTH.SE to STACKEN.KTH.SE.

Be careful of which realm goes where; it's easy to put realms in the wrong place.  The block is tagged with the client realm (the realm of the principal authenticating), and the realm before the equal sign is the final destination realm: the realm to which the client is authenticating.  After the equal sign go all the realms that the client transits through.

The order of the PERMITTED-CROSS-REALMS is not important when doing transit cross realm verification.

Configuring client cross-realm transits
---------------------------------------

The [capaths] section is also used for another purpose: to tell clients which realm to transit through to reach a realm with which their local realm does not have cross-realm trust.  This can be done by either putting a [capaths] entry in the configuration of the client or by putting the entry in the configuration of the KDC for the client's local realm.  In the latter case, the KDC will then hand back a referral to the client when the client requests a cross-realm ticket to the destination realm, telling the client to try to go through an intermediate realm.

For client configuration, the order of PERMITTED-CROSS-REALMS is significant, since only the first realm in this section (after the equal sign) is used by the client.

For example, again consider the [capaths] entry above for the case of a client in the SU.SE realm, and assume that the client or the SU.SE KDC has that [capaths] entry.  If the client attempts to authenticate to a service in the STACKEN.KTH.SE realm, that entry says to first authenticate cross-realm to the KTH.SE realm (the first realm listed in the PERMITTED-CROSS-REALMS section), and then from there to STACKEN.KTH.SE.

Each entry in [capaths] can only give the next hop, since only the first realm in PERMITTED-CROSS-REALMS is used.  If, for instance, a client in DSV.SU.SE had a [capaths] configuration as above but without the first block for SU.SE, they would not be able to reach STACKEN.KTH.SE.  They would get as far as SU.SE based on the DSV.SU.SE entry in [capaths] and then attempt to go directly from there to STACKEN.KTH.SE and get stuck (unless, of course, the SU.SE KDC had the additional entry required to tell the client to go through KTH.SE).

Active Directory forest example
-------------------------------

One common place where a [capaths] configuration is desirable is with Windows Active Directory forests.  One common Active Directory configuration is to have one top-level Active Directory realm but then divide systems, services, and users into child realms (perhaps based on organizational unit).  One generally establishes cross-realm trust only with the top-level realm, and then uses transit policy to permit authentications to and from the child realms.

For example, suppose an organization has a Heimdal realm EXAMPLE.COM, a Windows Active Directory realm WIN.EXAMPLE.COM, and then child Active Directory realms ENGR.WIN.EXAMPLE.COM and SALES.WIN.EXAMPLE.COM.  The goal is to allow users in any of these realms to authenticate to services in any of these realms.  The EXAMPLE.COM KDC (and possibly client) configuration should therefore contain a [capaths] section as follows:

     [capaths]
     	ENGR.WIN.EXAMPLE.COM = {
     		EXAMPLE.COM = WIN.EXAMPLE.COM
     	}
     	SALES.WIN.EXAMPLE.COM = {
     		EXAMPLE.COM = WIN.EXAMPLE.COM
     	}
     	EXAMPLE.COM = {
     		ENGR.WIN.EXAMPLE.COM = WIN.EXAMPLE.COM
     		SALES.WIN.EXAMPLE.COM = WIN.EXAMPLE.COM
     	}

The first two blocks allow clients in the ENGR.WIN.EXAMPLE.COM and SALES.WIN.EXAMPLE.COM realms to authenticate to services in the EXAMPLE.COM realm.  The third block tells the client (or tells the KDC to tell the client via referrals) to transit through WIN.EXAMPLE.COM to reach these realms.  Both sides of the configuration are needed for bi-directional transited cross-realm authentication.

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

Using DNS to find KDC
---------------------

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

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

The downside of using DNS is 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.

An 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)).

Using DNS to map hostname to Kerberos realm
-------------------------------------------

Heimdal also supports a way to lookup a realm from a hostname. This to minimise configuration needed on clients. Using this has the drawback that clients can be redirected by an attacker to realms within the same cross realm trust and made to believe they are talking to the right server (since Kerberos authentication will succeed).

An example configuration that informs clients that for the realms it.example.com and srv.example.com, they should use the realm EXAMPLE.COM:

     $ORIGIN example.com.
     _kerberos.it		TXT     "EXAMPLE.COM"
     _kerberos.srv		TXT     "EXAMPLE.COM"

How clients locate Kerberos KDCs
================================

clients use the following order to search for Kerberos KDC locations:

 1. plugins

 2. configuration files

      1. $KRB5_CONFIG (if not setuid)

      2. ~/Library/Preferences/edu.mit.Kerberos (on Mac OS X only)

      3. ~/Library/Preferences/com.apple.Kerberos.plist (on Mac OS X only)

      4. /Library/Preferences/edu.mit.Kerberos (on Mac OS X only)

      5. /Library/Preferences/com.apple.Kerberos.plist (on Mac OS X only)

      6. /etc/krb5.conf

 3. dns

Using LDAP to store the database
================================

This document describes how to install the LDAP backend for Heimdal. Note that before attempting to configure such an installation, you should be aware of the implications of storing private information (such as users' keys) in a directory service primarily designed for public information. Nonetheless, with a suitable authorisation policy, it is possible to set this up in a secure fashion. A knowledge of LDAP, Kerberos, and C is necessary to install this backend. The HDB schema was devised by Leif Johansson.

This assumes, OpenLDAP 2.3 or later.

Requirements:

  * A current release of Heimdal, configured with --with-openldap=/usr/local (adjust according to where you have installed OpenLDAP).

     You can verify that you manage to configure LDAP support by running kdc --builtin-hdb, and checking that ldap: is one entry in the list.

     Its also possible to configure the ldap backend as a shared module, see option --hdb-openldap-module to configure.

  * Configure OpenLDAP with --enable-local to enable the local transport.

  * Add the hdb schema to the LDAP server, it's included in the source-tree in lib/hdb/hdb.schema. Example from slapd.conf:

          include /usr/local/etc/openldap/schema/hdb.schema

  * Configure the LDAP server ACLs to accept writes from clients over the local transport. For example:

          access to *
                  by dn.exact="uid=heimdal,dc=services,dc=example,dc=com" write
                  ...

          authz-regexp "gidNumber=.*\\\+uidNumber=0,cn=peercred,cn=external,cn=auth''
          	"uid=heimdal,dc=services,dc=example,dc=com"

     The sasl-regexp is for mapping between the SASL/EXTERNAL and a user in a tree.  The user that the key is mapped to should be have a krb5Principal aux object with krb5PrincipalName set so that the ``creator'' and ``modifier'' is right in kadmin.

     Another option is to create an admins group and add the dn to that group.

     Since Heimdal talks to the LDAP server over a UNIX domain socket, and uses external sasl authentication, it's not possible to require security layer quality (ssf in cyrus-sasl lingo). So that requirement has to be turned off in OpenLDAP slapd configuration file slapd.conf.

          sasl-secprops minssf=0

  * Start slapd with the local listener (as well as the default TCP/IP listener on port 389) as follows:

              slapd -h "ldapi:/// ldap:///"

     Note: These is a bug in slapd where it appears to corrupt the krb5Key binary attribute on shutdown. This may be related to our use of the V3 schema definition syntax instead of the old UMich-style, V2 syntax.

  * You should specify the distinguished name under which your principals will be stored in krb5.conf. Also you need to enter the path to the kadmin acl file:

          [kdc]
                  database = {
                          dbname = ldap:ou=KerberosPrincipals,dc=example,dc=com
                          hdb-ldap-structural-object = inetOrgPerson
                          acl_file = /path/to/kadmind.acl
                          mkey_file = /path/to/mkey
                  }

     mkey_file can be excluded if you feel that you trust your ldap directory to have the raw keys inside it.  The hdb-ldap-structural-object is not necessary if you do not need Samba comatibility.

  * Once you have built Heimdal and started the LDAP server, run kadmin (as usual) to initialise the database. Note that the instructions for stashing a master key are as per any Heimdal installation.

          kdc# kadmin -l
          kadmin> init EXAMPLE.COM
          Realm max ticket life [unlimited]:
          Realm max renewable ticket life [unlimited]:
          kadmin> add lukeh
          Max ticket life [1 day]:
          Max renewable life [1 week]:
          Principal expiration time [never]:
          Password expiration time [never]:
          Attributes []:
          lukeh@EXAMPLE.COM's Password:
          Verifying password - lukeh@EXAMPLE.COM's Password:
          kadmin> exit

     Verify that the principal database has indeed been stored in the directory with the following command:

          kdc# ldapsearch -L -h localhost -D cn=manager \
           -w secret -b ou=KerberosPrincipals,dc=example,dc=com \
           'objectclass=krb5KDCEntry'

  * Now consider adding indexes to the database to speed up the access, at least theses should be added to slapd.conf.

          index	objectClass		eq
          index	cn			eq,sub,pres
          index	uid			eq,sub,pres
          index	displayName		eq,sub,pres
          index	krb5PrincipalName	eq

smbk5pwd overlay
----------------

The smbk5pwd overlay, updates the krb5Key and krb5KeyVersionNumber appropriately when it receives an LDAP Password change Extended Operation:

http://www.openldap.org/devel/cvsweb.cgi/contrib/slapd-modules/smbk5pwd/README?hideattic=1&sortbydate=0

Troubleshooting guide
---------------------

https://sec.miljovern.no/bin/view/Info/TroubleshootingGuide

Using Samba LDAP password database
----------------------------------

The Samba domain and the Kerberos realm can have different names since arcfour's string to key functions principal/realm independent.  So now will be your first and only chance name your Kerberos realm without needing to deal with old configuration files.

First, you should set up Samba and get that working with LDAP backend.

Now you can proceed as in see Using LDAP to store the database. Heimdal will pick up the Samba LDAP entries if they are in the same search space as the Kerberos entries.

Providing Kerberos credentials to servers and programs
======================================================

Some services require Kerberos credentials when they start to make connections to other services or need to use them when they have started.

The easiest way to get tickets for a service is to store the key in a keytab. Both ktutil get and kadmin ext can be used to get a keytab. ktutil get is better in that way it changes the key/password for the user. This is also the problem with ktutil. If ktutil is used for the same service principal on several hosts, they keytab will only be useful on the last host. In that case, run the extract command on one host and then securely copy the keytab around to all other hosts that need it.

     host# ktutil -k /etc/krb5-service.keytab \
           get -p lha/admin@EXAMPLE.ORG service-principal@EXAMPLE.ORG
     lha/admin@EXAMPLE.ORG's Password:

To get a Kerberos credential file for the service, use kinit in the --keytab mode. This will not ask for a password but instead fetch the key from the keytab.

     service@host$ kinit --cache=/var/run/service_krb5_cache \
                    --keytab=/etc/krb5-service.keytab \
            service-principal@EXAMPLE.ORG

Long running services might need credentials longer then the expiration time of the tickets. kinit can run in a mode that refreshes the tickets before they expire. This is useful for services that write into AFS and other distributed file systems using Kerberos. To run the long running script, just append the program and arguments (if any) after the principal. kinit will stop refreshing credentials and remove the credentials when the script-to-start-service exits.

     service@host$ kinit --cache=/var/run/service_krb5_cache \
            --keytab=/etc/krb5-service.keytab \
            service-principal@EXAMPLE.ORG \
            script-to-start-service argument1 argument2

Setting up PK-INIT
==================

PK-INIT leverages an existing PKI (public key infrastructure), using certificates to get the initial ticket (usually the krbtgt ticket-granting ticket).

To use PK-INIT you must first have a PKI. If you don't have one, it is time to create it. You should first read the whole chapter of the document to see the requirements imposed on the CA software.

A mapping between the PKI certificate and what principals that certificate is allowed to use must exist. There are several ways to do this. The administrator can use a configuration file, store the principal in the SubjectAltName extension of the certificate, or store the mapping in the principals entry in the kerberos database.

Certificates
============

This section documents the requirements on the KDC and client certificates and the format used in the id-pkinit-san OtherName extention.

KDC certificate
---------------

The certificate for the KDC has serveral requirements.

First, the certificate should have an Extended Key Usage (EKU) id-pkkdcekuoid (1.3.6.1.5.2.3.5) set. Second, there must be a subjectAltName otherName using OID id-pkinit-san (1.3.6.1.5.2.2) in the type field and a DER encoded KRB5PrincipalName that matches the name of the TGS of the target realm.  Also, if the certificate has a nameConstraints extention with a Generalname with dNSName or iPAdress, it must match the hostname or adress of the KDC.

The client is not required by the standard to check the server certificate for this information if the client has external information confirming which certificate the KDC is supposed to be using. However, adding this information to the KDC certificate removes the need to specially configure the client to recognize the KDC certificate.

Remember that if the client would accept any certificate as the KDC's certificate, the client could be fooled into trusting something that isn't a KDC and thus expose the user to giving away information (like a password or other private information) that it is supposed to keep secret.

Client certificate
------------------

The client certificate may need to have a EKU id-pkekuoid (1.3.6.1.5.2.3.4) set depending on the certifiate on the KDC.

It possible to store the principal (if allowed by the KDC) in the certificate and thus delegate responsibility to do the mapping between certificates and principals to the CA.

This behavior is controlled by KDC configuration option:

     [kdc]
     	pkinit_principal_in_certificate = yes

Using KRB5PrincipalName in id-pkinit-san
........................................

The OtherName extention in the GeneralName is used to do the mapping between certificate and principal.  For the KDC certificate, this stores the krbtgt principal name for that KDC.  For the client certificate, this stores the principal for which that certificate is allowed to get tickets.

The principal is stored in a SubjectAltName in the certificate using OtherName. The OID in the type is id-pkinit-san.

     id-pkinit-san OBJECT IDENTIFIER ::= { iso (1) org (3) dod (6)
     internet (1) security (5) kerberosv5 (2) 2 }

The data part of the OtherName is filled with the following DER encoded ASN.1 structure:

     KRB5PrincipalName ::= SEQUENCE {
     	realm [0] Realm,
     	principalName [1] PrincipalName
     }

where Realm and PrincipalName is defined by the Kerberos ASN.1 specification.

Naming certificate using hx509
==============================

hx509 is the X.509 software used in Heimdal to handle certificates. hx509 supports several different syntaxes for specifying certificate files or formats. Several formats may be used:  PEM, certificates embedded in PKCS#12 files, certificates embedded in PKCS#11 devices, and raw DER encoded certificates.

Those formats may be specified as follows:

DIR:

     DIR specifies a directory which contains certificates in the DER or PEM format.

     The main feature of DIR is that the directory is read on demand when iterating over certificates. This allows applications, in some situations, to avoid having to store all certificates in memory.  It's very useful for tests that iterate over large numbers of certificates.

     The syntax is:

          DIR:/path/to/der/files

FILE:

     FILE: specifies a file that contains a certificate or private key. The file can be either a PEM (openssl) file or a raw DER encoded certificate. If it's a PEM file, it can contain several keys and certificates and the code will try to match the private key and certificate together. Multiple files may be specified, separated by commas.

     It's useful to have one PEM file that contains all the trust anchors.

     The syntax is:

          FILE:certificate.pem,private-key.key,other-cert.pem,....

PKCS11:

     PKCS11: is used to handle smartcards via PKCS#11 drivers, such as soft-token, opensc, or muscle. The argument specifies a shared object that implements the PKCS#11 API. The default is to use all slots on the device/token.

     The syntax is:

          PKCS11:shared-object.so

PKCS12:

     PKCS12: is used to handle PKCS#12 files. PKCS#12 files commonly have the extension pfx or p12.

     The syntax is:

          PKCS12:/path/to/file.pfx

Configure the Kerberos software
===============================

First configure the client's trust anchors and what parameters to verify. See the subsections below for how to do that. Then, you can use kinit to get yourself tickets. For example:

     $ kinit -C FILE:$HOME/.certs/lha.crt,$HOME/.certs/lha.key lha@EXAMPLE.ORG
     Enter your private key passphrase:
     : lha@nutcracker ; klist
     Credentials cache: FILE:/tmp/krb5cc_19100a
             Principal: lha@EXAMPLE.ORG

       Issued           Expires          Principal
     Apr 20 02:08:08  Apr 20 12:08:08  krbtgt/EXAMPLE.ORG@EXAMPLE.ORG

Using PKCS#11 it can look like this instead:

     $ kinit -C PKCS11:/usr/heimdal/lib/hx509.so lha@EXAMPLE.ORG
     PIN code for SoftToken (slot):
     $ klist
     Credentials cache: API:4
             Principal: lha@EXAMPLE.ORG

       Issued           Expires          Principal
     Mar 26 23:40:10  Mar 27 09:40:10  krbtgt/EXAMPLE.ORG@EXAMPLE.ORG

TODO: Write about the KDC.

Configure the client
====================

     [appdefaults]
     	pkinit_anchors = FILE:/path/to/trust-anchors.pem

     [realms]
             EXAMPLE.COM = {
     		pkinit_require_eku = true
     		pkinit_require_krbtgt_otherName = true
     		pkinit_win2k = no
     		pkinit_win2k_require_binding = yes
     	}

Configure the KDC
=================

     [kdc]
     	enable-pkinit = yes
     	pkinit_identity = FILE:/secure/kdc.crt,/secure/kdc.key
     	pkinit_anchors = FILE:/path/to/trust-anchors.pem
     	pkinit_pool = PKCS12:/path/to/useful-intermediate-certs.pfx
     	pkinit_pool = FILE:/path/to/other-useful-intermediate-certs.pem
     	pkinit_allow_proxy_certificate = no
     	pkinit_win2k_require_binding = yes
     	pkinit_principal_in_certificate = no

Using pki-mapping file
----------------------

Note that the file name is space sensitive.

     # cat /var/heimdal/pki-mapping
     # comments starts with #
     lha@EXAMPLE.ORG:C=SE,O=Stockholm universitet,CN=Love,UID=lha
     lha@EXAMPLE.ORG:CN=Love,UID=lha

Using the Kerberos database
---------------------------

Use hxtool to create certificates
=================================

Generate certificates
---------------------

First, you need to generate a CA certificate. This example creates a CA certificate that will be valid for 10 years.

You need to change --subject in the command below to something appropriate for your site.

     hxtool issue-certificate \
         --self-signed \
         --issue-ca \
         --generate-key=rsa \
         --subject="CN=CA,DC=test,DC=h5l,DC=se" \
         --lifetime=10years \
         --certificate="FILE:ca.pem"

The KDC needs to have a certificate, so generate a certificate of the type ``pkinit-kdc'' and set the PK-INIT specifial SubjectAltName to the name of the krbtgt of the realm.

You need to change --subject and --pk-init-principal in the command below to something appropriate for your site.

     hxtool issue-certificate \
         --ca-certificate=FILE:ca.pem \
         --generate-key=rsa \
         --type="pkinit-kdc" \
         --pk-init-principal="krbtgt/TEST.H5L.SE@TEST.H5L.SE" \
         --subject="uid=kdc,DC=test,DC=h5l,DC=se" \
         --certificate="FILE:kdc.pem"

The users also needs to have certificates. For your first client, generate a certificate of type ``pkinit-client''. The client doesn't need to have the PK-INIT SubjectAltName set; you can have the Subject DN in the ACL file (pki-mapping) instead.

You need to change --subject and --pk-init-principal in the command below to something appropriate for your site. You can omit --pk-init-principal if you're going to use the ACL file instead.

     hxtool issue-certificate \
         --ca-certificate=FILE:ca.pem \
         --generate-key=rsa \
         --type="pkinit-client" \
         --pk-init-principal="lha@TEST.H5L.SE" \
         --subject="uid=lha,DC=test,DC=h5l,DC=se" \
         --certificate="FILE:user.pem"

Validate the certificate
------------------------

hxtool also contains a tool that will validate certificates according to rules from the PKIX document. These checks are not complete, but they provide a good test of whether you got all of the basic bits right in your certificates.

     hxtool validate FILE:user.pem

Use OpenSSL to create certificates
==================================

This section tries to give the CA owners hints how to create certificates using OpenSSL (or CA software based on OpenSSL).

Using OpenSSL to create certificates with krb5PrincipalName
-----------------------------------------------------------

To make OpenSSL create certificates with krb5PrincipalName, use an openssl.cnf as described below. To see a complete example of creating client and KDC certificates, see the test-data generation script lib/hx509/data/gen-req.sh in the source-tree. The certicates it creates are used to test the PK-INIT functionality in tests/kdc/check-kdc.in.

To use this example you have to use OpenSSL 0.9.8a or later.

     [user_certificate]
     subjectAltName=otherName:1.3.6.1.5.2.2;SEQUENCE:princ_name

     [princ_name]
     realm = EXP:0, GeneralString:MY.REALM
     principal_name = EXP:1, SEQUENCE:principal_seq

     [principal_seq]
     name_type = EXP:0, INTEGER:1
     name_string = EXP:1, SEQUENCE:principals

     [principals]
     princ1 = GeneralString:userid

Command usage:

     openssl x509 -extensions user_certificate
     openssl ca -extensions user_certificate

Using PK-INIT with Windows
==========================

Client configration
-------------------

Clients using a Windows KDC with PK-INIT need configuration since windows uses pre-standard format and this can't be autodetected.

The pkinit_win2k_require_binding option requires the reply for the KDC to be of the new, secure, type that binds the request to reply. Before, clients could fake the reply from the KDC. To use this option you have to apply a fix from Microsoft.

     [realms]
             MY.MS.REALM = {
                     pkinit_win2k = yes
                     pkinit_win2k_require_binding = no
     	}

Certificates
------------

The client certificates need to have the extended keyusage ``Microsoft Smartcardlogin'' (openssl has the OID shortname msSmartcardLogin).

See Microsoft Knowledge Base Article - 281245 ``Guidelines for Enabling Smart Card Logon with Third-Party Certification Authorities'' for a more extensive description of how set setup an external CA so that it includes all the information required to make a Windows KDC happy.

Configure Windows 2000 CA
-------------------------

To enable Microsoft Smartcardlogin for certificates in your Windows 2000 CA, you want to look at Microsoft Knowledge Base Article - 313274 ``HOW TO: Configure a Certification Authority to Issue Smart Card Certificates in Windows''.

KDC maintainence

As part of the regular maintainence of the KDC you should:

  * Backup the KDC database

  * Update the master key

Backup the KDC database
-----------------------

hprop can dump the data with the keys encrypted or non encrypted, encrypt is default.

For backups you should probably use encrypted backups.

# hprop --stdout | gzip -9 > backup-$(date '+%Y%m%d-%H%M%S').gz

Using ISO style dates are good since they sort in a senable order which makes it easy to pick up the last backup or prune the first.

Update the master key
---------------------

The master key encrypts the users keys. You should change it when stronger encryption types are available or when changes in maintainence of the KDC happens (KDC admin is removed from ACL, removing/changing a KDC slave server, etc).

The reason to change the master key is so you have a point in time from where if an older version of the master key is compromised, all changes past that point is safe to keep.

Note that you don't need to change keys just to get new keys, modern encryption ciphers can encrypt large amounts of data before needing to change keys.

Make sure you have propper backups before upgrade the master key.

This example will dump a un-encrypted database, add a new master key to the list of master keys, and then import the dump into the database and re-encrypt it.

     # hprop --decrypt --stdout > kdc-backup
     # kadmin -l kstash --random
     # hpropd --stdin < kdc-backup

Debugging Kerberos problems
===========================

To debug Kerberos client and server problems you can enable debug traceing by adding the following to /etc/krb5,conf. Note that the trace logging is sparse at the moment, but will continue to improve.

     [logging]
             libkrb5 = 0-/SYSLOG: