14.7. KerberosIV

Napisał Mark Murray. Na podstawie pracy Mark Dapoz.

Kerberos to sieciowy system/protokół umożliwiający uwierzytelnianie się użytkownikom dzięki usługom udostępnianym przez bezpieczny serwer. Kerberos zwiększa bezpieczeństwo i poziom kontroli nad usługami takimi jak rlogin, zdalne kopiowanie, kopiowanie w obrębie systemu.

Poniższe instrukcje można zastosować jako przewodnik po konfiguracji Kerberosa w takiej postaci, w jakiej dostępny jest on w systemie FreeBSD. Powinniśmy jednak zajrzeć również do stron podręcznika aby mieć kompletny obraz systemu.

14.7.1. Instalacja KerberosIV

Kerberos jest opcjonalnym komponentem FreeBSD. Najprostszym sposobem instalacji jest wybranie pozycji krb4 lub krb5 podczas instalacji systemu za pomocą programu sysinstall. Zainstaluje to odpowiednio “eBones” (KerberosIV) lub “Heimdal” (Kerberos5). Załączono je do dystrybucji systemu, ponieważ tworzone są poza USA/Kanadą i były dostępne w czasie restrykcji co do eksportu kodu kryptograficznego poza granice USA.

Możliwe jest również zastosowanie implementacji MIT Kerberosa, dostępnej w kolekcji portów jako security/krb5.

14.7.2. Tworzenie początkowej bazy danych

Operację tę przeprowadza się tylko na serwerze Kerberosa. Na początek upewnijmy się, że nie posiadamy starszych baz danych. Przejdźmy do katalogu /etc/kerberosIV i sprawdźmy, czy znajdują się w nim tylko poniższe pliki:

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

Jeśli znajdują się tu dodatkowe pliki (np. principal.* lub master_key), musimy wpierw użyć polecenia kdb_destroy, by skasować starą bazę Kerberosa - lub jeśli akurat usługa ta nie pracuje, po prostu usunąć dodatkowe pliki.

Następnie powinniśmy zmodyfikować pliki krb.conf i krb.realms, by określić otoczenie (ang. realm) Kerberosa. W tym przypadku będzie to EXAMPLE.COM, a serwerem grunt.example.com. Edytujemy lub tworzymy plik krb.conf:

# cat krb.conf
EXAMPLE.COM
EXAMPLE.COM grunt.example.com 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 host's name means that host also provides an administrative database server. For further explanation of these terms, please consult the Kerberos manual pages.

Now we have to add grunt.example.com to the EXAMPLE.COM realm and also add an entry to put all hosts in the .example.com domain in the EXAMPLE.COM realm. The krb.realms file would be updated as follows:

# cat krb.realms
grunt.example.com EXAMPLE.COM
.example.com EXAMPLE.COM
.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 ]: EXAMPLE.COM
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.

14.7.3. 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(1), rlogin(1) and rsh(1).

Now let us 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

14.7.4. 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 servers can pick it up. Use the mv(1) 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

14.7.5. Populating the Database

We now have to add some user entries into the database. First let us 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

14.7.6. 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 automatically 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: EXAMPLE.COM
# 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.example.com)
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@EXAMPLE.COM

  Issued           Expires          Principal
Apr 30 11:23:22  Apr 30 19:23:22  krbtgt.EXAMPLE.COM@EXAMPLE.COM

Now try changing the password using passwd(1) to check if the kpasswd daemon can get authorization to the Kerberos database:

% passwd
realm EXAMPLE.COM
Old password for jane:
New Password for jane:
Verifying password
New Password for jane:
Password changed.

14.7.7. Adding su Privileges

Kerberos allows us to give each user who needs root privileges their own separate su(1) password. We could now add an ID which is authorized to su(1) 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.example.com)
Kerberos Initialization for "jane.root"
Password:

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

# cat /root/.klogin
jane.root@EXAMPLE.COM

Now try doing the su(1):

% su
Password:

and take a look at what tokens we have:

# klist
Ticket file:    /tmp/tkt_root_245
Principal:      jane.root@EXAMPLE.COM

  Issued           Expires          Principal
May  2 20:43:12  May  3 04:43:12  krbtgt.EXAMPLE.COM@EXAMPLE.COM

14.7.8. 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(1) to root if the necessary entries are in the .klogin file in root's home directory:

# cat /root/.klogin
jane.root@EXAMPLE.COM

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

% cat ~/.klogin
jane@EXAMPLE.COM
jack@EXAMPLE.COM

This allows anyone in the EXAMPLE.COM realm who has authenticated themselves as jane or jack (via kinit, see above) to access to jane's account or files on this system (grunt) via rlogin(1), rsh(1) or rcp(1).

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

% kinit
MIT Project Athena (grunt.example.com)
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.example.com)
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

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