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Manual Chapter : TACACS Authentication and Accounting

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BIG-IP APM

  • 11.5.1
Manual Chapter
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About TACACS+ authentication and accounting

Access Policy Manager (APM) supports authenticating and authorizing the client against Terminal Access Controller Access Control System (TACACS+) servers. TACACS+ is a mechanism used to encrypt the entire body of the authentication packet. If you use TACACS+ authentication, user credentials are authenticated on a remote TACACS+ server. If you use the TACACS+ Accounting feature, the accounting service sends start and stop accounting records to the remote server.

APM supports TACACS+ authentication with the TACACS+ Auth access policy item and supports TACACS+ accounting with the TACACS+ Acct access policy item.

Important: APM must include a TACACS+ server configuration for every TACACS+ server that exists.

About AAA high availability

Using AAA high availability with Access Policy Manager (APM), you can configure multiple authentication servers to process requests, so that if one authentication server goes down or loses connectivity, the others can resume authentication requests, and new sessions can be established, as usual.

Note: Although new authentications fail if the BIG-IP system loses connectivity to the server, existing sessions are unaffected provided that they do not attempt to re-authenticate.

APM supports the following AAA servers for high availability: RADIUS, Active Directory, LDAP, CRLDP, and TACACS+. APM supports high availability by providing the option to create a pool of server connections when you configure the supported type of AAA server.

Note: If you use AAA with pools, such as RADIUS pools or Active Directory pools, APM assigns each pool member with a different number for the pool member's priority group value. Since APM does not support AAA load balancing, APM must define each pool member with a different priority group. The priority group number increases automatically with each created pool member.

Task summary for TACACS+ authentication and accounting

This task list includes all steps required to set up this configuration. If you are adding TACACS+ authentication or accounting to an existing access policy, you do not need to create another access profile and the access policy might already include a logon page.

Task list

Configuring a TACACS+ AAA server for authentication and authorization

  1. On the Main tab, click Access Policy > AAA Servers > TACACS+. The TACACS+ 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. For the Server Connection setting, select one of these options:
    • Select Use Pool to set up high availability for the AAA server.
    • Select Direct to set up the AAA server for standalone functionality.
  5. If you selected Use Pool, type a name in the Server Pool Name field. You create a pool of servers on this screen.
  6. Provide the addresses required for your server connection:
    • If you selected Direct, type an IP address in the Server Address field.
    • If you selected Use Pool, for each pool member you want to add, type an IP address and click Add.
      Note: When you configure a pool, you have the option to type the server address in route domain format: IPAddress%RouteDomain.
  7. If you selected Use Pool, you have the option to select a Server Pool Monitor to track the health of the server pool.
  8. In the Service Port field, type a TACACS+ service port or select one from the list. The default is 49.
  9. In the Secret field, type a secret key to use to encrypt and decrypt packets sent or received from the server, and then re-type the secret key in the Confirm Secret field.
  10. For the Service setting, select the name of the service for the user who is being authenticated to use. Identifying the service enables the TACACS+ server to behave differently for different types of authentication requests.
  11. Click Finished. The new server displays on the list.

Using TACACS+ authentication in an access policy

You configure an access policy with a TACACS+ Auth action to provide TACACS+ authentication as an authentication option for users trying to gain access.
  1. 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.
  2. Click the (+) icon anywhere in the access policy to add a new action item. A popup screen opens, listing predefined actions on tabs such as General Purpose, Authentication, and so on.
  3. On the Logon tab, select Logon Page and click the Add Item button. The Logon Page Agent properties screen opens.
  4. Make any changes that you require to the logon page properties and click Save. The properties screen closes and the visual policy editor displays.
  5. Click the (+) icon anywhere in the access policy to add a new action item. A popup screen opens, listing predefined actions on tabs such as General Purpose, Authentication, and so on.
  6. Select TACACS+ Auth, and click Add item. A properties popup screen opens.
  7. From the AAA Server list, select the TACACS+ server to use for authentication.
  8. Optional: Add any other branches and actions that you need to complete the access policy.
  9. Click Save. The properties screen closes and the visual policy editor displays.
  10. Click Apply Access Policy to save your configuration.
This creates an access policy that presents a user with a logon page, and then uses the input credentials to authenticate the user with an external TACACS+ server specified in the TACACS+ AAA server that you select.
To apply this access policy to network traffic, add the access profile to a virtual server.

Testing AAA high availability for supported authentication servers

To effectively test that high availability works for your authentication servers, you should have two servers that are accessible, where you can remove one of them from the network.
Note: High availability is supported for these authentication server types only: RADIUS, Active Directory, LDAP, CRLDP, and TACACS+.
If you configured a supported authentication server type to use a pool of connection servers, you can test the configuration using these steps.
  1. Begin a tcpdump on the Access Policy Manager, using a protocol analyzer, and scanning for packets destined for the specific port for your authentication server.
  2. Log in to the virtual server with both servers active.
  3. Using the tcpdump records, verify that the requests are being sent to the higher priority server.
  4. Log out of the virtual server.
  5. Disable the higher-priority server.
  6. Log in to the virtual server again.
  7. Verify that the request is being sent to the other server.
  8. Log out again, re-enabling the server, and try one more time to verify that the new requests are being sent to the high priority server.

Example access policy for TACACS+ authentication and accounting

This is an example of an access policy with all the associated elements needed to authenticate and authorize users with TACACS+ authentication. Note that the server used for authentication can be different from the server used for TACACS+ accounting service.

How TACACS+ worksHow TACACS Plus works

TACACS+ session variables for access policy rules

When the TACACS+ Auth (or TACACS+ Acct) access policy item runs, it populates session variables which are then available for use in access policy rules. The tables list the session variables for the TACACS+ access policy items and for a logon access policy item.

Session variables for TACACS+

Session Variable Description
session.tacasplus.last.acct.start_date; session.tacasplus.last.acct.start_time Provides TACACS+ accounting start time and date set by the accounting agent.
session.tacacsplus.last.acctresult Allows the accounting agent to set the available values to either of the following values:
  • 0: Failed
  • 1: Succeeds
session.tacacsplus.last.errmsgs Contains the error message string when the TACACS+ authentication or accounting fails.
session.tacacsplus.last.result Sets to 1 when authentication succeeds, or 0 when it fails.

Common session variables

Session Variable Description
session.logon.last.username Provides user credentials. The username string is stored after encrypting, using the system's client key.
session.logon.last.password Provides user credentials. The password string is stored after encrypting, using the system's client key.

TACACS+ authentication troubleshooting tips

You might run into problems with TACACS+ authentication in some instances. Follow these tips to try to resolve any issues you might encounter.

TACACS+ auth and query troubleshooting

Possible error messages Possible explanations and corrective actions
No AAA server associated with the agent Make sure that a valid TACACS+ server configuration is assigned to the agent (TACACS+ Auth or TACACS+ Acct) used in the access policy.
Failure to connect to TACACS+ server Make sure that the TACACS+ server is up and running and reachable from the BIG-IP system.
Login incorrect Supplied user credentials are not valid.
Invalid reply content, incorrect key Make sure that the shared encryption key configured on the TACACS+ server configuration matches with the key on the remote TACACS+ server.
Invalid AUTHEN/START packet from server Indicates either the wrong keys or that the authentication action (LOGIN) is not supported on the server.
Unacceptable authen method Indicates that the TACACS+ server does not support the authentication. Check the settings on the server.
Unexpected failure return/legal status value from authentication function/Permission error Caused by internal errors on the remote TACACS+ server. Check the logs on the remote TACACS+ server and also the configuration.
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