Overview

Kerberos is a network authentication protocol based on symmetric key cryptography. It is a client-server based model that provides mutual authentication. It requires a trusted third party called the Key Distribution Center (KDC), which in turn consists of an Authentication Server (AS) and Ticket Granting Server (TGS). A Kerberos "realm" consists of Kerberos principals and services that are registered within a Kerberos server.

Windows Active Directory (AD), along with LDAP, also provides Kerberos services. Windows AD forest supports multiple domains and each of them can constitute of a Kerberos realm. The multiple domains may be divided based on business unit, geography, and other variables.

Advantages of multiple realms

There are several advantages to having multiple realms. They are:

• Data is distributed across different realms instead of a single database, so there will not be a single point of failure.
• It is easier to manage several small independent Kerberos realms than a single huge one.
• Having interoperability between different KDCs, like MIT and Windows, leads to easier integration of new Kerberos realms into an existing environment.
• It provides flexibility in terms of future add-ons or developments.
• It provides flexibility in that a user from a different realm can access services from another realm based on the trust relationship that resides between the realms.
• Users from different Kerberos realms in a particular AD forest can log in to a single AIX Kerberos client.

Scope, assumptions, and the environment

The scope of this article is to configure AIX as a Kerberos client to multiple Kerberos realms hosted on a Windows AD server. It is assumed that the reader of this article has a medium to high level of knowledge of the Kerberos protocol and Windows Active Directory.

The steps described in this article have been tested with Microsoft Windows 2003 AD server as Kerberos servers and IBM NAS 1.4.0.7 the as Kerberos client. The same steps should work with other versions of IBM NAS.

Configuration steps

Setting up Microsoft Windows Active Directory

For information on setting up Microsoft Active Directory on Microsoft Windows Server 2003, refer to the relevant Microsoft Windows documentation. Here we configured an Active Directory forest containing a parent and child domain (refer above diagram). The Active Directory Domain is also referred as the Microsoft Kerberos Realm running on Microsoft Active Directory.

• Parent Domain - IBM.COM, windows AD server: ibmad1.ibm.com
• Child Domain - SEC.IBM.COM, Windows AD server: ibmad2.sec.ibm.com
• AIX Kerberos client - indus28.in.ibm.com

Configuring the AIX Kerberos client

1. Make sure that the AIX client is configured with a FQDN (fully qualified domain name).

   For example, the hostname should be "indus28.in.ibm.com".

2. Initially configure the AIX Kerberos client with child domain on the Windows AD server.

   Use the config.krb5 command to set up the Kerberos client on AIX.

   
   
   Syntax
   

   config.krb5 -C -d (domain name ) -r (realm name) -c (windows server) -s (windows server) Ex:config.krb5 -C -d in.ibm.com -r SEC.IBM.COM -c ibmad2.sec.ibm.com -s ibmad2.sec.ibm.com

   
   
3. Change the /etc/krb5/krb5.conf file to include the details of parent domain "IBM.COM" and the Windows-supported encryption types.

   [libdefaults] default_realm = SEC.IBM.COM default_keytab_name = FILE:/etc/krb5/krb5.keytab default_tkt_enctypes = des-cbc-md5 des-cbc-crc default_tgs_enctypes = des-cbc-md5 des-cbc-crc [realms] SEC.IBM.COM = { kdc = ibmad2.sec.ibm.com:88 admin_server = ibmad2.sec.ibm.com:749 default_domain = in.ibm.com } IBM.COM = { kdc = ibmad1.ibm.com:88 admin_server = ibmad1.ibm.com:749 default_domain = in.ibm.com } [domain_realm] .in.ibm.com = SEC.IBM.COM ibmad2.sec.ibm.com = SEC.IBM.COM ibmad1.ibm.com = SEC.IBM.COM [logging] kdc = FILE:/var/krb5/log/krb5kdc.log admin_server = FILE:/var/krb5/log/kadmin.log default = FILE:/var/krb5/log/krb5lib.log

   
   
4. Add the following KRB5A stanza in the /usr/lib/security/methods.cfg file:

   KRB5A: program = /usr/lib/security/KRB5A options = authonly KRB5Afiles: options = db=BUILTIN,auth=KRB5A Note: For AIX 6.1 TL02 onwards add the Kerberos stanza as below: KRB5: program = /usr/lib/security/KRB5 options = authonly,is_kadmind_compat=no KRB5files: options = db=BUILTIN,auth=KRB5

   
   
5. On the child domain "SEC.IBM.COM", create a user using the Active Directory users and computer snap in for the AIX machine.

   For example, in this case a username, indus28, is created on the AD as the Kerberos client is being configured on indus28.in.ibm.com. While creating a user, you will be asked for information such as password. Provide a valid password for the username.

   On the Active Directory server, do the following:

   Use the Active Directory Management tool to create a new user corresponding to the AIX Kerberos client machine indus28.in.ibm.com.

   • Select the Users folder.
   • Right-click and select New.
   • Select user.
   • Give the name indus28.
   
   
   
   
6. Use the ktpass tool to generate the keytab file on the Windows machine.

   ktpass -princ host/indus28.in.ibm.com@SEC.IBM.COM -mapuser indus28 -pass 123 -out indus28.keytab -ptype KRB5_NT_PRINCIPAL Ktpass: -princ user@realm , The user will be the aix client box, so in our case it will be indus28. in.ibm.com, this has to be full domain name -mapuser : The user name will be indus28 -pass provide the password or * if you want it to prompt for password -out , the name of output keytab file.

   
   

   Securely transfer the indus28.keytab file to the AIX Kerberos client indus28.in.ibm.com.

7. Similarly, create user "indus28_ibm" on the parent domain, IBM.COM (machine: ibmad1.ibm.com).
8. Run the ktpass command in the domain IBM.COM:

   ktpass.exe -princ host/indus28.in.ibm.com@IBM.COM -mapuser indus28_ibm -pass 123 -out indus28_ibm.keytab -ptype KRB5_NT_PRINCIPAL Note: The map user is indus28_ibm and the keytab is indus28_ibm.keytab and the princ name is again host/indus28.in.ibm,com

   
   

   Securely transfer the indus28_ibm.keytab file to the AIX Kerberos client.

9. On the AIX Kerberos client, use the ktutil function to read the keytab file, which has been copied from the Windows server, and write it to default keytab file /etc/krb5/krb5.keytab.

   #/usr/krb/sbin/ktutil ktutil: rkt /usr/krb/sbin/indus28.keytab ktutil: wkt /etc/krb5/krb5.keytab ktutil: q # /usr/krb5/bin/klist -ke Keytab name: FILE:/etc/krb5/krb5.keytab KVNO Principal ---- --------- 18 host/indus28.in.ibm.com@SEC.IBM.COM (ArcFour with HMAC/md5) #/usr/krb5/sbin/ktutil ktutil: rkt /usr/krb/sbin/indus28_ibm.keytab ktutil: wkt /etc/krb5/krb5.keytab ktutil: q #/usr/krb5/bin/klist -ke Keytab name: FILE:/etc/krb5/krb5.keytab KVNO Principal ---- --------- 18 host/indus28.in.ibm.com@SEC.IBM.COM (ArcFour with HMAC/md5) 4 host/indus28.in.ibm.com@IBM.COM (ArcFour with HMAC/md5)

   
   
10. Create a user foo on domain IBM.COM and foo1 on domain SEC.IBM.COM. Also create them locally on the AIX Kerberos client.

    #mkuser -R KRB5Afiles registry= KRB5Afiles SYSTEM= KRB5Afiles foo #mkuser -R KRB5Afiles registry= KRB5Afiles SYSTEM= KRB5Afiles foo1

    
    Make sure the lsuser command lists all the relevant user attributes.
11. Run kinit for both the users to make sure you are able to authenticate them using Kerberos and can get tickets from the Kerberos realm they belong to.

    # /usr/krb5/bin/kinit foo1 Password for foo1@SEC.IBM.COM: # /usr/krb5/bin/klist Ticket cache: FILE:/var/krb5/security/creds/krb5cc_0 Default principal: foo1@SEC.IBM.COM Valid starting Expires Service principal 02/27/08 15:29:37 02/28/08 01:29:40 krbtgt/SEC.IBM.COM@SEC.IBM.COM Renew until 02/28/08 15:29:37 # /usr/krb5/bin/kinit foo@IBM.COM Password for foo@IBM.COM: # /usr/krb5/bin/klist Ticket cache: FILE:/var/krb5/security/creds/krb5cc_0 Default principal: foo@IBM.COM Valid starting Expires Service principal 02/27/08 15:29:49 02/28/08 01:29:49 krbtgt/IBM.COM@IBM.COM Renew until 02/28/08 15:29:49

    
    
12. Log in as both the users from the AIX Kerberos client.

Resources

Learn

• "Configuring AIX 5L for Kerberos Based Authentication Using Windows Kerberos Service.This white paper describes how to use Kerberos as an alternative authentication mechanism to AIX using Windows 2000/2003 server Kerberos Service.
  
  
• In the Resources on the IBM Network Authentication Service and related technologies for AIX
  
  
• Browse the technology bookstore for books on these and other technical topics.
  
  

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About the authors

Shruthi Sharma works as a Development support specialist for AIX security. She has three years experience and has gained knowledge in components of AIX security and AIX system administration. She also has experience of working with customers and handling critical situations. You can reach her at shrutsha@in.ibm.com.

Uma M. Chandolu works as a Development Support Specialist on AIX. He is currently the team lead of the AIX Security Development Support team at IBM Bangalore.He has three years of extensive hands-on experience in AIX environments, and demonstrated expertise in AIX system administration and other subsystems. He has experience interfacing with customers and handling customer-critical situations. You can reach him at uchandol@in.ibm.com.

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Table of contents

• Overview
• Advantages of multiple realms
• Scope, assumptions, and the environment
• Configuration steps
• Resources
• About the authors
• Comments

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