Configuring TACACS+

This chapter describes the Terminal Access Controller Access Control System Plus (TACACS+), a Cisco proprietary version of TACACS.

Note STP UplinkFast has also been added for this release, and it is documented in "Enabling STP UplinkFast" section.

TACACS+ provides the means to manage network security (authentication, authorization, and accounting [AAA]) from a server. This section describes how TACACS+ works and how you can configure it. For complete syntax and usage information for the commands described in this chapter, refer to the Cisco IOS Desktop Switching Command Reference or to the "Security" chapter of the Cisco IOS 11.3 Command Summary.

These sections describe how to configure TACACS+:

Understanding How TACACS+ Works

In large enterprise networks, the task of administering passwords on each device can be simplified by doing the user authentication centrally on a server. TACACS+ is an access-control protocol that allows a switch to authenticate all login attempts through a central authentication server. The network administrator configures the switch with the address of the TACACS+ server, and the switch and the server exchange messages to authenticate each user before allowing access to the management console.

TACACS+ consists of three services: authentication, authorization, and accounting. Authentication is the action of determining who the user is and whether he or she is allowed access to the switch. Authorization is the action of determining what the user is allowed to do on the system. Accounting is the action of collecting data related to resource usage.

Configuring TACACS+

The TACACS+ feature is disabled by default, and it can be enabled and configured only through the command-line interface (CLI). You can access the CLI through the console port or via Telnet. In order to prevent a lapse in security, TACACS+ cannot be configured through a network-management application. When enabled, TACACS+ can authenticate users accessing the switch through either the console or Telnet.

The following sections describe how to configure the primary features of AAA/TACACS+:

Enabling AAA/TACACS+

Use the aaa new-model command to enable AAA/TACACS+. Enter the following commands in global configuration mode:


Task	Command
Enable AAA/TACACS+.	aaa new-model

Enabling Authentication for Login

Using the aaa authentication login command and the following keywords, you create one or more lists of authentication methods that are tried at login. The lists are used with the login authentication line configuration command.

Enter the following command in global configuration mode to enable authentication for login:

Switch# aaa authentication login {default | list-name} method1 [...[method3]]

The keyword list-name is any character string used to name the list you are creating. The method keyword refers to the actual method the authentication algorithm tries, in the sequence entered. You can enter up to three methods:


Keyword	Description
line	Uses the line password for authentication.
local	Uses the local username database for authentication.
tacacs+	Uses TACACS+ authentication.

To create a default list that is used if no list is specified in the login authentication command, use the default argument followed by the methods you want used in default situations.

The additional methods of authentication are used only if the previous method returns an error, not if it fails. To specify that the authentication succeed even if all methods return an error, specify none as the final method in the command line.

Specifying TACACS+ Authorization for EXEC Access and Network Services

You can use the aaa authorization command with the tacacs+ keyword to set parameters that restrict a user's network access to Cisco IOS privilege mode (EXEC access) and to network services such as Serial Line Internet Protocol (SLIP), Point to Point Protocol (PPP) with Network Control Protocols (NCPs), and AppleTalk Remote Access (ARA).

The aaa authorization exec tacacs+ local command sets the following authorization parameters:

Use TACACS+ for EXEC access authorization if authentication was done using TACACS+.
Use the local database if authentication was not done using TACACS+.

Note Authorization is bypassed for authenticated users who log in through the CLI, even if authorization has been configured.

To specify TACACS+ authorization for EXEC access and network services, perform the following tasks in global configuration mode:


Task	Command
User TACACS+ authorization for all network-related service requests, including SLIP, PPP NCPs, and ARA protocol.	aaa authorization network tacacs+
User TACACS+ authorization to determine if the user is allowed EXEC access. This keyword might return user profile information (such as autocommand information).	aaa authorization exec tacacs+

Starting TACACS+ Accounting

You use the aaa accounting command with the tacacs+ keyword to turn on TACACS+ accounting for each Cisco IOS privilege level and for network services.

To use TACACS+ accounting to send a start-record accounting notice at the beginning of an EXEC process and a stop-record at the end, enter the following command in global configuration mode:

Switch# aaa accounting exec start-stop tacacs+

To use TACACS+ to account for all network-related service requests, including SLIP, PPP, and PPP NCPs, perform the following task in global configuration mode:

Switch# aaa accounting network start-stop tacacs+

Note This command is documented in the "Accounting and Billing Commands" chapter of the Security Command Reference.

Establishing the TACACS+ Server Host

Use the tacacs-server host command to specify the names of the IP host or hosts maintaining a AAA/TACACS+ server. On TACACS+ servers, you can configure the following additional options:

Period of time (in seconds) the switch attempts to contact the server before it times out.
Encryption key to encrypt and decrypt all traffic between the router and the daemon.
Number of attempts that a user can make when entering a command that is being authenticated by TACACS+.

You can use the tacacs-server retransmit command to change the number of times the system software searches the list of TACACS servers (the default is two) and the interval it waits for a reply (the default is 5 seconds).

Perform the following tasks in global configuration mode:


Task	Command
Step 1 Define a TACACS+ host. Entering the timeout and key parameters with this command overrides the global values that you can enter with the tacacs-server timeout (Step 3) and the tacacs-server key commands (Step 5).	tacacs-server host name [timeout integer] [key string]
Step 2 Enter the number of times the server searches the list of TACACS+ servers before stopping.	tacacs-server retransmit retries
Step 3 Set the interval the server waits for a TACACS+ server host to reply.	tacacs-server timeout seconds
Step 4 Set the number of login attempts that can be made on the line.	tacacs-server attempts count
Step 5 Define a set of encryption keys for all TACACS+ and communication between the access server and the TACACS daemon. (Repeat the command for each encryption key.)	tacacs-server key key
Step 6 Return to privileged EXEC mode.	exit
Step 7 Confirm the TACACS+ server information and display statistics in privileged EXEC mode.	show tacacs

Configuring a Switch for Local AAA Configuration

You can configure AAA to operate without a server by setting the switch to implement AAA in local mode. Authentication and authorization are then handled by the switch. No accounting is available in this configuration.

Perform the following tasks in global configuration mode:


Task	Command
Step 1 Enable AAA.	aaa new-model
Step 2 Set login authorization to default to local.	aaa authentication login default local
Step 3 User AAA authorization for all network-related service requests, including SLIP, PPP NCPs, and ARA protocol.	aaa authorization exec local
Step 4 User AAA authorization to determine if the user is allowed to run an EXEC shell.	aaa authorization network local
Step 5 Enter the local database. (Repeat the command for each user.)	username name password password privilege level (0 to 15)

Table of Contents

Configuring TACACS+