Kerberos

10.6. Kerberos

Contributed by Mark Murray (based on contribution by ).

Kerberos is a network add-on system/protocol that allows users to authenticate themselves through the services of a secure server. Services such as remote login, remote copy, secure inter-system file copying and other high-risk tasks are made considerably safer and more controllable.

The following instructions can be used as a guide on how to set up Kerberos as distributed for FreeBSD. However, you should refer to the relevant manual pages for a complete description.

In FreeBSD, the Kerberos is not that from the original 4.4BSD-Lite, distribution, but eBones, which had been previously ported to FreeBSD 1.1.5.1, and was sourced from outside the USA/Canada, and was thus available to system owners outside those countries during the era of restrictive export controls on cryptographic code from the USA.

10.6.1. Creating the initial database

This is done on the Kerberos server only. First make sure that you do not have any old Kerberos databases around. You should change to the directory /etc/kerberosIV and check that only the following files are present:

    # cd /etc/kerberosIV
    # ls
    README		krb.conf        krb.realms

If any additional files (such as principal.* or master_key) exist, then use the kdb_destroy command to destroy the old Kerberos database, of if Kerberos is not running, simply delete the extra files.

You should now edit the krb.conf and krb.realms files to define your Kerberos realm. In this case the realm will be GRONDAR.ZA and the server is grunt.grondar.za. We edit or create the krb.conf file:

    # cat krb.conf
    GRONDAR.ZA
    GRONDAR.ZA grunt.grondar.za admin server
    CS.BERKELEY.EDU okeeffe.berkeley.edu
    ATHENA.MIT.EDU kerberos.mit.edu
    ATHENA.MIT.EDU kerberos-1.mit.edu
    ATHENA.MIT.EDU kerberos-2.mit.edu
    ATHENA.MIT.EDU kerberos-3.mit.edu
    LCS.MIT.EDU kerberos.lcs.mit.edu
    TELECOM.MIT.EDU bitsy.mit.edu
    ARC.NASA.GOV trident.arc.nasa.gov

In this case, the other realms do not need to be there. They are here as an example of how a machine may be made aware of multiple realms. You may wish to not include them for simplicity.

The first line names the realm in which this system works. The other lines contain realm/host entries. The first item on a line is a realm, and the second is a host in that realm that is acting as a "key distribution center". The words admin server following a hosts name means that host also provides an administrative database server. For further explanation of these terms, please consult the Kerberos man pages.

Now we have to add grunt.grondar.za to the GRONDAR.ZA realm and also add an entry to put all hosts in the .grondar.za domain in the GRONDAR.ZA realm. The krb.realms file would be updated as follows:

    # cat krb.realms
    grunt.grondar.za GRONDAR.ZA
    .grondar.za GRONDAR.ZA
    .berkeley.edu CS.BERKELEY.EDU
    .MIT.EDU ATHENA.MIT.EDU
    .mit.edu ATHENA.MIT.EDU

Again, the other realms do not need to be there. They are here as an example of how a machine may be made aware of multiple realms. You may wish to remove them to simplify things.

The first line puts the specific system into the named realm. The rest of the lines show how to default systems of a particular subdomain to a named realm.

Now we are ready to create the database. This only needs to run on the Kerberos server (or Key Distribution Center). Issue the kdb_init command to do this:

    # kdb_init
    Realm name [default  ATHENA.MIT.EDU ]: GRONDAR.ZA
    You will be prompted for the database Master Password.
    It is important that you NOT FORGET this password.
    		
    Enter Kerberos master key: 

Now we have to save the key so that servers on the local machine can pick it up. Use the kstash command to do this.

    # kstash
    	      
    Enter Kerberos master key:
    
    Current Kerberos master key version is 1.
    
    Master key entered. BEWARE!

This saves the encrypted master password in /etc/kerberosIV/master_key.

10.6.2. Making it all run

Two principals need to be added to the database for each system that will be secured with Kerberos. Their names are kpasswd and rcmd These two principals are made for each system, with the instance being the name of the individual system.

These daemons, kpasswd and rcmd allow other systems to change Kerberos passwords and run commands like rcp, rlogin and rsh.

Now let's add these entries:

    # kdb_edit
    Opening database...
    
    Enter Kerberos master key:
    
    Current Kerberos master key version is 1.
    
    Master key entered.  BEWARE!
    Previous or default values are in [brackets] ,
    enter return to leave the same, or new value.
    
    Principal name: passwd
    Instance: grunt
    
    <Not found>, Create [y] ? y
    
    Principal: passwd, Instance: grunt, kdc_key_ver: 1
    New Password:                    <---- enter RANDOM here
    Verifying password
    
    New Password: <---- enter RANDOM here
    
    Random password [y] ? y
    
    Principal's new key version = 1
    Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ?
    Max ticket lifetime (*5 minutes) [ 255 ] ?
    Attributes [ 0 ] ?
    Edit O.K.
    Principal name: rcmd
    Instance: grunt
    
    <Not found>, Create [y] ?
    
    Principal: rcmd, Instance: grunt, kdc_key_ver: 1
    New Password:		<---- enter RANDOM here
    Verifying password
    
    New Password:           <---- enter RANDOM here
    
    Random password [y] ?
    
    Principal's new key version = 1
    Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ?
    Max ticket lifetime (*5 minutes) [ 255 ] ?
    Attributes [ 0 ] ?
    Edit O.K.
    Principal name:         <---- null entry here will cause an exit

10.6.3. Creating the server file

We now have to extract all the instances which define the services on each machine. For this we use the ext_srvtab command. This will create a file which must be copied or moved by secure means to each Kerberos client's /etc/kerberosIV directory. This file must be present on each server and client, and is crucial to the operation of Kerberos.

    # ext_srvtab grunt
    Enter Kerberos master key:
    		
    Current Kerberos master key version is 1.
    
    Master key entered. BEWARE!
    Generating 'grunt-new-srvtab'....

Now, this command only generates a temporary file which must be renamed to srvtab so that all the server can pick it up. Use the mv command to move it into place on the original system:

    # mv grunt-new-srvtab srvtab

If the file is for a client system, and the network is not deemed safe, then copy the client-new-srvtab to removable media and transport it by secure physical means. Be sure to rename it to srvtab in the client's /etc/kerberosIV directory, and make sure it is mode 600:

    # mv grumble-new-srvtab srvtab
    # chmod 600 srvtab

10.6.4. Populating the database

We now have to add some user entries into the database. First let's create an entry for the user jane. Use the kdb_edit command to do this:

    # kdb_edit
    Opening database...
    
    Enter Kerberos master key:
    
    Current Kerberos master key version is 1.
    
    Master key entered.  BEWARE!
    Previous or default values are in [brackets] ,
    enter return to leave the same, or new value.
    
    Principal name: jane
    Instance:
    
    <Not found>, Create [y] ? y
    
    Principal: jane, Instance: , kdc_key_ver: 1
    New Password:                <---- enter a secure password here
    Verifying password
    
    New Password:                <---- re-enter the password here
    Principal's new key version = 1
    Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ?
    Max ticket lifetime (*5 minutes) [ 255 ] ?
    Attributes [ 0 ] ?
    Edit O.K.
    Principal name:		   <---- null entry here will cause an exit

10.6.5. Testing it all out

First we have to start the Kerberos daemons. NOTE that if you have correctly edited your /etc/rc.conf then this will happen automatically when you reboot. This is only necessary on the Kerberos server. Kerberos clients will automagically get what they need from the /etc/kerberosIV directory.

    # kerberos &
    Kerberos server starting
    Sleep forever on error
    Log file is /var/log/kerberos.log
    Current Kerberos master key version is 1.
    
    Master key entered. BEWARE!
    
    Current Kerberos master key version is 1
    Local realm: GRONDAR.ZA
    # kadmind -n &
    KADM Server KADM0.0A initializing
    Please do not use 'kill -9' to kill this job, use a
    regular kill instead
    
    Current Kerberos master key version is 1.
    
    Master key entered.  BEWARE!

Now we can try using the kinit command to get a ticket for the id jane that we created above:

    % kinit jane
    MIT Project Athena (grunt.grondar.za)
    Kerberos Initialization for "jane"
    Password: 

Try listing the tokens using klist to see if we really have them:

    % klist
    Ticket file:    /tmp/tkt245
    Principal:      jane@GRONDAR.ZA
    
      Issued           Expires          Principal
    Apr 30 11:23:22  Apr 30 19:23:22  krbtgt.GRONDAR.ZA@GRONDAR.ZA

Now try changing the password using passwd to check if the kpasswd daemon can get authorization to the Kerberos database:

    % passwd
    realm GRONDAR.ZA
    Old password for jane:
    New Password for jane:
    Verifying password
    New Password for jane:
    Password changed.

10.6.6. Adding su privileges

Kerberos allows us to give each user who needs root privileges their own separate supassword. We could now add an id which is authorized to su to root. This is controlled by having an instance of root associated with a principal. Using kdb_edit we can create the entry jane.root in the Kerberos database:

    # kdb_edit
    Opening database...
    
    Enter Kerberos master key:
    
    Current Kerberos master key version is 1.
    
    Master key entered.  BEWARE!
    Previous or default values are in [brackets] ,
    enter return to leave the same, or new value.
    
    Principal name: jane
    Instance: root
    
    <Not found>, Create [y] ? y
    
    Principal: jane, Instance: root, kdc_key_ver: 1
    New Password:                    <---- enter a SECURE password here
    Verifying password
    
    New Password:    	 	 <---- re-enter the password here
    
    Principal's new key version = 1
    Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ?
    Max ticket lifetime (*5 minutes) [ 255 ] ? 12 <--- Keep this short!
    Attributes [ 0 ] ?
    Edit O.K.
    Principal name:		         <---- null entry here will cause an exit

Now try getting tokens for it to make sure it works:

    # kinit jane.root
    MIT Project Athena (grunt.grondar.za)
    Kerberos Initialization for "jane.root"
    Password:

Now we need to add the user to root's .klogin file:

    # cat /root/.klogin
    jane.root@GRONDAR.ZA

Now try doing the su:

    % su
    Password:

and take a look at what tokens we have:

    # klist
    Ticket file:	/tmp/tkt_root_245
    Principal:      jane.root@GRONDAR.ZA
    
      Issued           Expires          Principal
    May  2 20:43:12  May  3 04:43:12  krbtgt.GRONDAR.ZA@GRONDAR.ZA

10.6.7. Using other commands

In an earlier example, we created a principal called jane with an instance root. This was based on a user with the same name as the principal, and this is a Kerberos default; that a <principal>.<instance> of the form <username>.root will allow that <username> to su to root if the necessary entries are in the .klogin file in root's home directory:

    # cat /root/.klogin
    jane.root@GRONDAR.ZA

Likewise, if a user has in their own home directory lines of the form:

    % cat ~/.klogin
    jane@GRONDAR.ZA
    jack@GRONDAR.ZA

This allows anyone in the GRONDAR.ZA realm who has authenticated themselves to jane or jack (via kinit, see above) access to rlogin to jane's account or files on this system (grunt) via rlogin, rsh or rcp.

For example, Jane now logs into another system, using Kerberos:

    % kinit
    MIT Project Athena (grunt.grondar.za)
    Password:
    % rlogin grunt
    Last login: Mon May  1 21:14:47 from grumble
    Copyright (c) 1980, 1983, 1986, 1988, 1990, 1991, 1993, 1994
            The Regents of the University of California.   All rights reserved.
    
    FreeBSD BUILT-19950429 (GR386) #0: Sat Apr 29 17:50:09 SAT 1995

Or Jack logs into Jane's account on the same machine (Jane having set up the .klogin file as above, and the person in charge of Kerberos having set up principal jack with a null instance:

    % kinit
    % rlogin grunt -l jane
    MIT Project Athena (grunt.grondar.za)
    Password:
    Last login: Mon May  1 21:16:55 from grumble
    Copyright (c) 1980, 1983, 1986, 1988, 1990, 1991, 1993, 1994
            The Regents of the University of California.   All rights reserved.
    FreeBSD BUILT-19950429 (GR386) #0: Sat Apr 29 17:50:09 SAT 1995