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Manual Chapter: Kerberos Authentication with End-User Logons
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Kerberos Authentication with End-User Logons

About basic authentication and Kerberos end-user logon

Access Policy Manager® (APM®) provides an alternative to a form-based login authentication method. This alternative method uses a browser login box that is triggered by an HTTP 401 response to collect credentials. A SPNEGO/Kerberos or basic authentication challenge can generate a HTTP 401 response.

This option is useful when a user is already logged in to the local domain and you want to avoid submitting an APM HTTP form for collecting user credentials. The browser automatically submits credentials to the server and bypasses the login box to collect the credentials again.

Note: Because SPNEGO/Kerberos is a request-based authentication feature, the authentication process is different from other authentication methods, which run at session creation time. SPNEGO/Kerberos authentication can occur at any time during the session.

The benefits of this feature include:

  • Provides flexible login mechanism instead of restricting you to use only the form-based login method.
  • Eliminates the need for domain users to explicitly type login information again to log in to Access Policy Manager.
  • Eliminates the need for user password transmission with Kerberos method.
Important: Administrators should not turn off the KeepAlive setting on the web server because turning that setting off might interfere with Kerberos authentication.

How does end-user logon work?

To retrieve user credentials for end-user logon, you can use basic authentication or SPEGNO/Kerberos methods or both.

Basic authentication
Use this method to retrieve user credentials (user name and password) from a browser. You can think of this method as a replacement for form-based authentication used by the standard login screen. If you use basic authentication, the system populates the user name and password session variables, which can then be used by any other authentication actions, such as Active Directory or RADIUS.
SPNEGO/Kerberos
Use this method to retrieve user credentials through SPNEGO/Kerberos authentication header. With the Kerberos method, the client system must first join a domain and a Kerberos action must follow. The Kerberos action does not run immediately; it runs only when clients request SPNEGO/Kerberos authentication. By default, Kerberos authentication runs not only on the first request, but also on subsequent requests where authentication is needed, such as for new connections. Access Policy Manager® ( APM®) validates the request by confirming that a valid ticket is present.
Note: You can disable Kerberos per request-based authentication in the Kerberos authentication access policy item configuration in APM. If you disable it, authentication occurs while the access policy runs and subsequent authentications do not occur. In that case, end-user logon does not occur.
Note: You can achieve multi-domain support for Kerberos authentication through multiple virtual servers. Each virtual server must have its own access policy and its own Kerberos configuration.

Both methods require that an HTTP 401 Response action item be configured in the access policy and that the authentication method be specified in the action item. In cases where both methods are selected, the browser determines which method to perform based on whether the system has joined a domain. The HTTP 401 Response action has two default branches to indicate whether basic authentication or Kerberos method is performed.

How SPNEGO/Kerberos end-user logon works

How SPNEGO/Kerberos end-user logon works

The end-user logon works with events happening in this order:

  • The client becomes a member and connects to the domain.
  • The client connects to a virtual server on the BIG-IP® system.
  • The access policy runs and issues a 401 HTTP request action.
  • If Kerberos is present, the browser forwards the Kerberos ticket along with the request when it receives the 401 HTTP request.
  • Access Policy Manager validates the Kerberos ticket after the request is received and determines whether or not to permit the request.

About Kerberos authentication requirements

To configure Kerberos authentication, you must meet specific configuration requirements as described here.

Virtual server
The virtual server IP address and host name are necessary to configure DNS.
DNS configuration
Make sure you have the zone file and PTR record for the virtual server IP address. For example:
testbed.lab.companynet 10.10.4.100
Browser configuration
Configure the browser to use Kerberos. Typically, Internet Explorer is already configured for Kerberos; however, you might need to configure it for trusted sites. To use Firefox, you must configure it for negotiate authentication.

Task summary for configuring end-user login support

To set up this configuration, perform the procedures in the task list.

Task list

Joining a Kerberos user account to a domain

To use Kerberos authentication, you need the client joined and connected to a domain and you need a keytab file.
  1. Create a surrogate user in the domain.
    In this example, the hostname of the virtual server on the BIG-IP® system is testbed.lab.companynet and the user name is john.
    setspn -U -A HTTP/testbed.lab.companynet john
  2. Map the user account to the service account and generate a keytab file for the service.
    You can use the ktpass utility to do this. In this example, LAB.COMPANYNET specifies the Kerberos authentication realm.
    c:>ktpass -princ HTTP/testbed.lab.companynet.com@LAB.COMPANYNET -mapuser john@LAB.COMPANYNET -crypto rc4-hmac-nt -ptype KRB5_NT_SRV_HST -pass password -out c:\temp\john.keytab

Configuring an AAA server for Kerberos authentication

Configure a Kerberos AAA server so that you can add it to a Kerberos authentication action in an access policy.
  1. On the Main tab, click Access Policy > AAA Servers > Kerberos .
    The Kerberos Servers list screen opens.
  2. Click Create.
    The New Server properties screen opens.
  3. In the Name field, type a unique name for the authentication server.
  4. In the Auth Realm field, type a Kerberos authentication realm name (administrative name), such as LAB.COMANYNET.
    Type the realm name all uppercase; it is case-sensitive.
  5. In the Service Name field, type a service name; for example, HTTP.
  6. In the Keytab File area, click Choose File to locate and upload the keytab file.
    A keytab file contains Kerberos encryption keys (these are derived from the Kerberos password).
  7. Click Finished.
    The new server displays on the list.

Creating an access profile

You create an access profile to provide the access policy configuration for a virtual server that establishes a secured session.
  1. On the Main tab, click Access Policy > Access Profiles .
    The Access Profiles List screen opens.
  2. Click Create.
    The New Profile screen opens.
  3. In the Name field, type a name for the access profile.
    Note: An access profile name must be unique among all access profile and any per-request policy names.
  4. From the Profile Type list, select one these options:
    • LTM-APM: Select for a web access management configuration.
    • SSL-VPN: Select to configure network access, portal access, or application access. (Most access policy items are available for this type.)
    • ALL: Select to support LTM-APM and SSL-VPN access types.
    • SSO: Select to configure matching virtual servers for Single Sign-On (SSO).
      Note: No access policy is associated with this type of access profile
    • RDG-RAP: Select to validate connections to hosts behind APM when APM acts as a gateway for RDP clients.
    • SWG - Explicit: Select to configure access using Secure Web Gateway explicit forward proxy.
    • SWG - Transparent: Select to configure access using Secure Web Gateway transparent forward proxy.
    • System Authentication: Select to configure administrator access to the BIG-IP® system (when using APM as a pluggable authentication module).
    • Identity Service: Used internally to provide identity service for a supported integration. Only APM creates this type of profile.
      Note: You can edit Identity Service profile properties.
    Note: Depending on licensing, you might not see all of these profile types.
    Additional settings display.
  5. In the Language Settings area, add and remove accepted languages, and set the default language.
    A browser uses the highest priority accepted language. If no browser language matches the accepted languages list, the browser uses the default language.
  6. Click Finished.
The access profile displays in the Access Profiles List. Default-log-setting is assigned to the access profile.

Verifying log settings for the access profile

Confirm that the correct log settings are selected for the access profile to ensure that events are logged as you intend.
Note: Log settings are configured in the Access Policy Event Logs area of the product. They enable and disable logging for access system and URL request filtering events. Log settings also specify log publishers that send log messages to specified destinations.
  1. On the Main tab, click Access Policy > Access Profiles .
    The Access Profiles List screen opens.
  2. Click the name of the access profile that you want to edit.
    The properties screen opens.
  3. On the menu bar, click Logs.
    The access profile log settings display.
  4. Move log settings between the Available and Selected lists.
    You can assign up to three log settings that enable access system logging to an access profile. You can assign additional log settings to an access profile provided that they enable logging for URl request logging only.
    Note: Logging is disabled when the Selected list is empty.
  5. Click Update.
An access profile is in effect when it is assigned to a virtual server.

Configuring an access policy for end-user logon support

To use basic authentication in addition to Kerberos authentication, you need an AAA server configured for the authentication agent that you plan to use.
Configure an access policy like this one to handle basic and SPEGNO/Kerberos authentication challenges without submitting an Access Policy Manager® HTTP form to collect user credentials.
  1. On the Main tab, click Access Policy > Access Profiles .
    The Access Profiles List screen opens.
  2. In the Access Policy column, click the Edit link for the access profile you want to configure.
    The visual policy editor opens the access policy in a separate screen.
  3. Click the (+) icon anywhere in the access policy to add a new action item.
    Note: Only an applicable subset of access policy items is available for selection in the visual policy editor for any access profile type.
    A popup screen opens, listing predefined actions on tabs such as General Purpose, Authentication, and so on.
  4. Under General Purpose, select HTTP 401 Response, and click Add item.
    A properties screen opens.
  5. In the 401 Response Setting area from the HTTP Auth Level list, select basic+negotiate, and click Save.
    The properties screen closes. The visual policy editor displays the HTTP 401 Response item with 3 branches: Basic, Negotiate, and fallback.
  6. To perform basic authentication, add an authentication server agent on the Basic branch.
  7. To use the Kerberos authentication method:
    1. Add the Kerberos Auth agent on the Negotiate branch.
      After you add the Kerberos Auth item, a properties popup screen displays.
    2. On the properties screen for the AAA Server setting, select the Kerberos AAA server.
    3. Click Save.
      The properties screen closes and the visual policy editor displays.
  8. Complete the access policy:
    1. Add any additional access policy items you require.
    2. Change the ending from Deny to Allow on any access policy branch on which you want to grant access.
  9. Click Apply Access Policy.
To apply this access policy to network traffic, add the access profile to a virtual server.
Note: To ensure that logging is configured to meet your requirements, verify the log settings for the access profile.

Access policy example for end-user login

This is an example of an access policy with all the associated elements needed to successfully support the end-user login feature. Notice that separate branches are created automatically to support using either basic authentication or Kerberos method to retrieve user credentials.

Note: For basic authentication, the user name and password validation occurs at the session creation time. After the access policy completes, the session cookie is used to validate the session.
Note: By default, Kerberos runs not only at the access policy run time but also at any time in the session.
401 Response followed by AD auth (basic) and Kerberos auth (negotiate)

Example access policy for end-user login

Example of branch rule for HTTP 401 Response

Example properties for an HTTP 401 response action

Example of branch rule for Kerberos

Example properties for a Kerberos Auth action on the Negotiate branch

Kerberos authentication troubleshooting tips

You might choose to verify Kerberos authentication configurations in some instances. Use these troubleshooting tips to help resolve any issues you might encounter.

Verify the keytab file

From the command line, use the klist command as shown in this example.

Important: The command must be typed on one line.
klist -ke WRFILE:/config/filestore/files_d/Common_d/kerberos_keytab_file_d/\:Common\:SUN-SPNEGO-APM106_key_file_2

The output for the example contains information like this.

                     
Keytab name:
FILE:/config/filestore/files_d/Common_d/kerberos_keytab_file_d/:Common:SUN-SPNEGO-APM106_key_file_2
KVNO Principal
3    HTTP/apm106.labt.companynet.com@labt.companynet.com(arcfour-hmac)
                  

Verify Kerberos delegation

From the command line, use the kinit command, as shown in this example.
kinit HTTP/apm106.labt.companynet.com@labt.companynet.com
You are prompted for a password and should receive a ticket (no output, no error).

Verify ticket

From the command line, type klist . Here is sample output: /etc/krb5.conf

Capture a TCP dump

Make sure the client sends the ticket to the BIG-IP® system; this verifies that the client setup is successful.

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