3. The Procedure

3.1. Obtain a Kerberos cross-realm Principal

  • The Kerberos cross-realm principal (of the form krbtgt/ADFQDN where ADFQDN is the fully qualified domain name of your Active Directory domain in upper case, for example 'krbtgt/OUCS.OX.AC.UK') can be requested via email to sysdev@it.ox.ac.uk. Be sure to specify the Kerberos principal you are requesting, and specify the /itss principals that should be granted administration rights over the requested principal.

3.2. Set a password for the Principal

When Sysdev create your Kerberos cross-realm service principal, they will also assign administration rights over it to the requested /itss principal(s) (of the form oxfordusername/itss) to enable you to set a password on the principal.

  • Use the kadmin tool with the /itss principal on a Linux/UNIX workstation that is suitably secure, as in the following example (since linux.ox.ac.uk is a shared service it is not recommended for this). The example uses a fictitious principal unit9999/itss, with a cross-realm principal for the fictitious 'Jordan College'. We tell kadmin the name of the kerberos admin server in the OX.AC.UK realm to connect to with the -s option. Keyboard input is underlined in red.
Setting the password and encryption type(s) for the principal

The above screenshot shows the /itss user authenticating, setting the password (and the encryption types) for the cross-realm principal using the cpw command, and then checking the current settings on the cross-realm principal using the getprinc command.

The -e switch on cpw defines the encryption type. RC4 is supported in Windows Server 2003 Service Pack 2 and later (and ALL your DCs will need to be at this level or above for RC4 to work). Since AES support was added in Windows Server 2008 it has become the preferred encryption type for the latest versions of Windows Server.

The list of encryption types in the image, for the convenience of copy/paste, is:

"aes256-cts-hmac-sha1-96:normal aes128-cts-hmac-sha1-96:normal rc4-hmac:normal"

If you have an existing AD-MIT cross-realm trust running on Windows Server 2003 (SP2 or later) and you are migrating your domain controllers to Windows Server 2008 R2, it is important you reset the password and the encryption types as above, and then remove and add the realm trusts again as in section 3.3. Configure Domain Controllers prior to introducing the 2008 R2 DCs. This is to ensure that the key encryption types remain compatible.

NOTE: Make sure that you choose a very strong password, and keep it secure! You'll only need it to create the other side of the trust, so you're not going to need to use it very often. You should also ensure that it conforms to the strength requirements of your Windows system — this may mean three out of four categories of character (lower and upper case, numbers, and special characters).

3.3. Configure Domain Controllers

  • For Windows Server 2003 only: you will need to install the Kerberos tools from the Windows Support Tools installer (suptools.msi). This can be found in the \Support\tools folder on the Windows Server CD (these tools are included in the standard installation on Windows Server 2008 and later).
  • Configure the domain controller so that it knows about the Kerberos KDC servers. From a command prompt, use ksetup /addkdc ... to add the OX.AC.UK KDCs (invoking ksetup without arguments shows the current settings), as in the following example:

    Add OX.AC.UK KDCs to each domain controller

    The KDCs can be added in any order. Make sure you run the /addkdc commands on all your domain controllers.

  • Next we need to set up the realm trust:

    On your domain controller, run Active Directory Domains and Trusts. Right-click on your domain name, select Properties, and then select the tab labelled Trusts.

    Click on New Trust to open the New Trust wizard. Advance beyond the initial screen by clicking Next.

    When prompted for the Trust Name enter 'OX.AC.UK'.

    New Trust Wizard: Trust Name

    Click on Next, and ensure that the trust type is set to Realm trust.

    New Trust Wizard: Trust Type

    Click on Next. The next screen asks whether the trust should be transitive or non-transitive. If in any doubt choose Transitive as this is generally only likely to be important if you have more than one domain in your Active Directory forest - in which case transitive trust would also probably be the best option.

    New Trust Wizard: Trust Transitivity

    Click on Next. Now you need to choose the direction for the trust. You should set this to be One-way: outgoing.

    New Trust Wizard: Trust Direction

    Click on Next. When prompted for the trust password, enter the password you set on the principal for your domain (i.e. the password from section 3.2. Set a password for the Principal).

    New Trust Wizard: Trust Password

    Click on Next. You will then be shown the Trust Selections Screen, where you can review your choices. NB this screen is the last point at which the Back button is enabled.

    New Trust Wizard: Trust Selections Complete

    Click Next to advance to the New Trust Wizard completion screen. Click Finish to complete the process, and you should see the new realm trust show up in the top panel under Domains trusted by this domain (outgoing trusts).

    AD Domains and Trusts Properties: Trusts Tab

    Click OK to close the Properties window.

  • For Windows Server 2003 (SP2+) only: you will also need to run the 'ktpass' command, as in the following example:

    Set trust encryption type to rc4 for Windows Server 2003 SP2+ domain controller

    This is to tell the DC to use RC4 encryption rather than DES encryption for all dealings with the external realm OX.AC.UK. 'ktpass' is included with the Support Tools, but make sure you use the version from the CD appropriate to your O/S and service pack level, or download it from the Microsoft site. Only the Windows Server 2003 versions of 'ktpass.exe' support the switches above.

3.4. Map user principals to names

  • Next we map OX.AC.UK principals to AD users

    Start Active Directory Users and Computers as an Administrative user and turn on the Advanced Features view (in the View menu).

    Pick a username, right-click and select Name Mappings...

    Select the Kerberos Names tab.

    Name Mappings: Kerberos Names Tab

    Add a valid Kerberos principal in the form 'oxfordusername@OX.AC.UK' (note case).

    Name Mappings: Add Kerberos Name

    It's helpful, but not essential, for the Active Directory username and the oxfordusername to match. For example, if they don't match and the user authenticates as (example SSO user) abcd0123 but is authorised to use the Active Directory resources that (example AD user) wxyz9876 has access to, scripts using %username% will use wxyz9876 and this could cause problems if a user has a home directory called abcd0123 with an automatic mapping.

    Name Mappings: Populated Kerberos Names Tab

    Repeat for other usernames. If you have a lot of users, you're going to want to look at scripting this.

3.5. Domain Member Workstation (XP/Vista/7) and Domain Member Server Configuration

NOTE: Windows OS 'Home' editions do not have the capability to join a domain, so cannot be configured for AD cross-realm SSO login. The Windows OS editions listed below exclude all 'Home' editions.

  • ksetup

    First you will need to install ksetup.exe on the Domain Client if it's not already installed.

    • Windows XP: ksetup.exe should be installed from the Windows Support Tools package for Windows XP.
    • Windows Vista: ksetup.exe is not provided for Windows Vista, so either use the Windows XP executable or change the registry directly (see below).
    • Windows 7: ksetup.exe is included in the default Windows 7 installation.

    If ksetup.exe is available on the client (or you have installed it), run the same ksetup commands that were run on the server:

    Add OX.AC.UK KDCs to each domain client

    If ksetup.exe is unavailable on the client, use the direct registry method:

    • Open the registry editor to the path 'HKLM\System\CurrentControlSet\Control\LSA\Kerberos\Domains'
    • Create a new key and name it 'OX.AC.UK'
    • Create a new REG_MULTI_SZ entry, named 'KdcNames' inside the new 'OX.AC.UK' key
    • Add the four KDC values to 'KdcNames': 'kdc0.ox.ac.uk', 'kdc1.ox.ac.uk', 'kdc2.ox.ac.uk', 'kdc3.ox.ac.uk'

    For larger numbers of users we would generally recommend that the above workstation configuration is set up using Group Policy.

    Domain member servers do not need the addition of the OX.AC.UK KDCs using ksetup.exe unless users in the OX.AC.UK realm are permitted to log into it directly.

    Ideally the above settings would be established using Group Policy for workstations.

  • Domain Group Policies

    When a user logs in using their OX.AC.UK credentials, the system will not apply roaming profiles or group policy objects (GPOs) associated with that user in the local domain, unless the Allow Cross-Forest User Policy and Roaming User Profiles policy is applied to the workstation they are logging into. The path to this policy in the Group Policy Management Editor is Computer Configuration\Policies\Administrative Templates\System\Group Policy.

  • Additional configuration required for Windows 7 clients

    Microsoft deprecated the use of DES ciphers from Windows 7 (and Windows Server 2008 R2) onwards, so your Windows 7 clients will require DES ciphers to be re-enabled by setting the Network Security: Configure encryption types allowed for Kerberos policy (under Computer Configuration\Policies\Windows Settings\Security Settings\Local Policies\Security Options) to include all of the following:

    1. des-cbc-crc
    2. des-cbc-md5
    3. rc4-hmac
    4. rc4-hmac-exp
    5. aes128-cts-hmac-sha1-96
    6. aes256-cts-hmac-sha1-96

    Ideally the above encryption type settings (and optionally the default domain as OX.AC.UK) would be established using a GPO for workstations. The optional Assign a default domain for logon policy is under Computer Configuration\Policies\Administrative Templates\System\Logon.

    DES ciphers are only necessary to accommodate the OX.AC.UK master TGT, and would not be used for service tickets and/or their session keys; this is a requirement due to another stronger master TGT cipher not being accommodated by Windows OSes. We are planning to add extra ciphers to the master TGT to obviate the need to enable DES ciphers.

    NOTE: This cipher change would also be required for Windows Server 2008 R2 domain servers if you were allowing users in the OX.AC.UK realm to log onto those servers directly.

  • Password changes

    SSO Password changes using Ctrl+Alt+Del will not work, so SSO users should be configured to disallow the password changing option using the Remove Change Password Group Policy. The path to this policy in the Group Policy Management Editor is User Configuration\Policies\Administrative Templates\System\Ctrl+Alt+Del Options. If SSO passwords need changing they should be changed using the Webauth Change Password web page instead.

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