Creating a Kerberos configuration file

The Kerberos configuration file contains client configuration information, including the locations of Key Distribution Centers (KDCs) for the realms of interest, defaults for the current Kerberos realm and mappings of host names onto Kerberos realms.

Before you begin

About this task

Kerberos configuration settings, the Kerberos key distribution center (KDC) name, and realm settings for both Simple and Protected GSS-API Negotiation Mechanism (SPNEGO) Web authentication and Kerberos authentication are provided in the Kerberos configuration file or through the java.security.krb5.kdc and java.security.krb5.realm Java™ virtual machine system properties.

If you don't have a Kerberos configuration file (krb5.ini or krb5.conf), you must first create a Kerberos configuration file and place it in every instance in a cell using a wsadmin command. The following information lists the default Kerberos configuration files and their locations:
  • The default location is /etc/krb5/krb5.conf.

Use the wsadmin command to create a Kerberos configuration file for WebSphere® Application Server:

Procedure

  1. Start the command-line utility by running the wsadmin command from the app_server_root/bin directory.
  2. At the wsadmin prompt, enter the following command:
    $AdminTask help createKrbConfigFile 
    Table 1. createKrbConfigFile command parameters.

    This table lists the parameters you can use with the createKrbConfigFile command:

    Option Description
    <krbPath> This parameter is required. It provides the fully qualified file system location of the Kerberos configuration (krb5.ini or krb5.conf) file.
    <realm> This parameter is required. It provides the Kerberos realm name. The value of this attribute is used by SPNEGO to form the Kerberos service principal name for each of the hosts specified with the com.ibm.ws.security.spnego.SPN<id>.hostName property.
    <kdcHost> This parameter is required. It provides the host name of the Kerberos Key Distribution Center (KDC).
    <kdcPort> This parameter is optional. It provides the port number of the Kerberos Key Distribution Center. If this port is omitted, it defaults to 88.
    <dns> This parameter is required. It is a list of default domain name services DNS, separated by a pipe character, that is used to produce a fully qualified host name. The first one in the list is the default domain name service.
    <keytabPath> This parameter is required. It provides the file system location of the Kerberos keytab path and file name.
    <encryption> This parameter is optional. It identifies the list of supported encryption types, separated by a pipe character. The default value is des-cbc-md5.
    The following encryption types are supported:
    • des-cbc-md5
    • des-cbc-crc
    • des3-cbc-sha1
    • rc4-hmac
    • arcfour-hmac
    • arcfour-hmac-md5
    • aes128-cts-hmac-sha1-96
    • aes256-cts-hmac-sha1-96
    Attention: Not all of the KDC solutions available today support all of the previously listed encryption types. Before you choose an encryption type, ensure that your KDC supports the encryption type that you want to use by consulting your Kerberos Administrator's and User's Guide.

    Ensure you have a common encryption type for the Kerberos configuration file, the Kerberos keytab file, the Kerberos service principal name and the Kerberos client. For example, if the Kerberos client uses the RC4-HMAC encryption type, the target server must also support the RC4-HMAC encryption type and the Kerberos configuration file must list RC4-HMAC first in default_tgt_enctypes and default_tkt_enctypes.

    The following is an example of the createKrbConfigFile command:
    $AdminTask createKrbConfigFile {-krbPath c:/winnt/krb5.ini 
                                    -realm WSSEC.AUSTIN.IBM.COM 
                                    -kdcHost host1.austin.ibm.com 
                                    -dns austin.ibm.com|raleigh.ibm.com 
                                    -keytabPath c:/winnt/krb5.keytab}
    Note: WebSphere variables can also be used to specify krbPath and krbKeytab location paths for the createKrbConfigFile command.
    This example creates the c:/winnt/krb5.ini file as follows:
    [libdefaults]
    	default_realm = WSSEC.AUSTIN.IBM.COM
    	default_keytab_name = FILE:c:\winnt\krb5.keytab
    	default_tkt_enctypes = rc4-hmac des-cbc-md5
    	default_tgs_enctypes = rc4-hmac des-cbc-md5
    	forwardable  = true
    	renewable  = true
    	noaddresses = true
    	clockskew  = 300
    [realms]
    	WSSEC.AUSTIN.IBM.COM = {
    		kdc = host1.austin.ibm.com:88
    		default_domain = austin.ibm.com
    	}
    [domain_realm]
    	.austin.ibm.com = WSSEC.AUSTIN.IBM.COM
    	.raleigh.ibm.com = WSSEC.AUSTIN.IBM.COM
    

    The createKrbConfigFile command creates a simple Kerberos configuration file. You might have to edit this file, as needed, to specify a TCP or UDP preference or when you have a cross or trusted realm environment.

    In the [libdefaults] section, you can specify a TCP or UDP protocol preference. By default, the Java Kerberos configuration uses the UDP protocol. However, the Java Kerberos supports a TCP or a UDP protocol configuration using the udp_preference_limit parameter. If you need to use the TCP protocol, specify the udp_preference_limit parameter with a 1 value to always use the TCP protocol. For example:
    udp_preference_limit =1
    If you do not specify this parameter, the Java Kerberos library uses the TCP protocol only if the Kerberos ticket request using the UDP protocol fails and the KDC returns the KRB_ERR_RESPONSE_TOO_BIG error code.
    [Fix Pack 5 or later] Avoid trouble: When the application server receives a client request, the Kerberos configuration on the server might return a Connection reset, IOException, or Broken pipe exception if you use the TCP protocol and the KDC returns a bad packet. The application server makes three attempts to capture the correct Kerberos packet. If a correct Kerberos packet is returned as a result of one of the three attempts, the client request is successfully processed and you can ignore the exceptions. If the application server cannot obtain the correct Kerberos packet after three attempts, the client request fails. At this point, review the KDC, network, and application server configurations to determine the issue.gotcha
    The [domain_realm] section of the Kerberos configuration file is needed for a cross realm environment.
    • [domain_realm] - Provides a translation from a domain name or host name to a realm name. The tag name can be a host name or a domain name. Domain names are indicated by a prefix of a period ('.'). The value of the relation is the realm name for that particular host or domain. Host names and domain names should be in lowercase.
      If no translation entry applies, the host's realm is considered to be the host name's domain portion converted to uppercase. For example, the following [domain_realm] section maps tech.ibm.com into the TEST.AUSTIN.IBM.COM realm:
      [domain_realm]
      	.austin.ibm.com = WSSEC.AUSTIN.IBM.COM
      	.raleigh.ibm.com = WSSEC.AUSTIN.IBM.COM

      All other hosts in the austin.ibm.com and .raleigh.ibm.com domains map default to WSSEC.AUSTIN.IBM.COM.

      The following example contains more than one Kerberos realm name:
      [domain_realm]
             .ibm.com =AUSTIN.IBM.COM
             ibm.com =AUSTIN.IBM.COM
             tech.ibm.com =TEST.AUSTIN.IBM.COM
             .fubar.org =FUBAR.ORG

      All other hosts in the ibm.com® domain map by default to the AUSTIN.IBM.COM realm and all hosts in the fubar.org domain map by default to the FUBAR.ORG realm.

    Note the entries for the hosts, ibm.com and fubar.org. Without these entries, these hosts would map into the realms COM and ORG, respectively.

    For peer trust cross-realm authentication, see your Kerberos Administrator's and User's Guide for information about how to set up the trust cross-realm authentication on the KDC.

    You must add information about the foreign realm to the realms and domain_realm sections of the Kerberos configuration file. For example:
    [realms]
           AUSTIN.IBM.COM = {
                 kdc = kdc.austin.ibm.com:88
                 default_domain = austin.ibm.com
           }
           FUBAR.ORG = {
                 kdc = kdc.fubar.org:88
                 default_domain = fubar.org
           }
    [domain_realm]
           austin.ibm.com = AUSTIN.IBM.COM
           .austin.ibm.com = AUSTIN.IBM.COM
           fubar.org = FUBAR.ORG
           .fubar.org = FUBAR.ORG

    In a transitive trust, two realms trust each other if they trust the intermediate realms involved in granting a ticket. If each realm involved in granting the service ticket is present in the trust path, then the ticket is trusted. See your Kerberos Administrator's and User's Guide for information about how to configure transitive trust on the KDC.

    To set up transitive trust, separate realms with cross-realm authentication must be defined between a and b and between b and c, but not between a and c. With transitive trust, REALMA and REALMC can communicate with each other, but only by going through REALMB.
    REALMA	<->	REALMB	<->	REALMC
    A [capaths] stanza must be added to each krb5.conf file.
    The krb5.conf files on all machines must list all three realms in the [realms] stanza. REALMA must list itself and REALMB and REALMC; REALMB must list itself and REALMA and REALMC; REALMC must list itself and REALMA and REALMB. In the [domain_realm] stanza of the krb5.conf files list all three host names and realm names to be able to execute from REALMA to REALMC and from REALMC to REALMA.
    [capaths]
           REALMA.AUSTIN.IBM.COM = {
           		REALMB.AUSTIN.IBM.COM = .
           		REALMC.AUSTIN.IBM.COM = REALMB.AUSTIN.IBM.COM
                }
           REALMB.AUSTIN.IBM.COM = {
           		REALMC.AUSTIN.IBM.COM = .
           		REALMA.AUSTIN.IBM.COM = .
                }
           REALMC.AUSTIN.IBM.COM = {
           		REALMB.AUSTIN.IBM.COM = .
           		REALMA.AUSTIN.IBM.COM = REALMB.AUSTIN.IBM.COM
                }

    The krb5.conf file permission must be set to 644, which means that you can read and write the file. However, members of the group that the file belongs to and all other users can only read the file.

    Because the Kerberos configuration and keytab file are set by the java.security.krb5.conf and KRB5_KTNAME Java virtual machine (JVM) system properties respectively, if SPNEGO Web authentication and Kerberos authentication are both enabled you must use the same Kerberos configuration and keytab files for both.

Results

You have now created a Kerberos configuration file.



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Last updated: Oct 22, 2010 12:21:29 AM CDT
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