Cisco 1000 Series Connected Grid Routers Security Software Configuration Guide

TACACS+ Advantages

TACACS+ has the following advantages over RADIUS authentication:

• Provides independent AAA facilities. For example, the Cisco CG-OS router can authorize access without authenticating.
• Uses the TCP transport protocol to send data between the AAA client and server, making reliable transfers with a connection-oriented protocol.
• Encrypts the entire protocol payload between the Cisco CG-OS router and the AAA server to ensure higher data confidentiality. The RADIUS protocol only encrypts passwords.

TACACS+ Operation for User Login

When a user attempts a Password Authentication Protocol (PAP) login to a Cisco CG-OS router using TACACS+, the following actions occur:

1. When the Cisco CG-OS router establishes a connection, it contacts the TACACS+ daemon to obtain the username and password.

Note TACACS+ allows an arbitrary conversation between the daemon and the user until the daemon receives enough information to authenticate the user. This action is usually done by prompting for a username and password combination, but may include prompts for other items, such as the maiden name of your mother.

2. The Cisco CG-OS router will eventually receive one of the following responses from the TACACS+ daemon:

a. ACCEPT—User authentication succeeds and service begins. When the Cisco CG-OS router requires user authorization, authorization begins.

b. REJECT—User authentication fails. The TACACS+ daemon either denies further access to the user or prompts the user to retry the login sequence.

c. ERROR—An error occurs at some time during authentication either at the daemon or in the network connection between the daemon and the Cisco CG-OS router. When the
Cisco CG-OS router receives an ERROR response, the Cisco CG-OS router tries to use an alternative method for authenticating the user.

After authentication, the user also undergoes an additional authorization phase when authorization is enabled on the Cisco CG-OS router. Users must first successfully complete TACACS+ authentication before proceeding to TACACS+ authorization.

3. If TACACS+ authorization is required, the Cisco CG-OS router again contacts the TACACS+ daemon and it returns an ACCEPT or REJECT authorization response. An ACCEPT response contains attributes that are used to direct the EXEC or NETWORK session for that user and determines the services that the user can access.

Services include the following:

• Telnet, rlogin, Point-to-Point Protocol (PPP), Serial Line Internet Protocol (SLIP), or EXEC services
• Connection parameters, including the host or client IP address (IPv4 or IPv6), access list, and user timeouts

Default TACACS+ Server Encryption Type and Secret Key

You must configure the TACACS+ secret key to authenticate the Cisco CG-OS router to the TACACS+ server. A secret key is a secret text string shared between the Cisco CG-OS router and the TACACS+ server host. The length of the key is restricted to 63 characters and can include any printable ASCII characters (white spaces are not allowed). You can configure a global secret key for all TACACS+ server configurations on the Cisco CG-OS router to use.

You can override the global secret key assignment by explicitly using the key option when configuring an individual TACACS+ server.

TACACS+ Server Monitoring

An unresponsive TACACS+ server can delay the processing of AAA requests.

A Cisco CG-OS router periodically monitors a TACACS+ server to check whether it is responding (or alive) to save time in processing AAA requests. The router marks an unresponsive TACACS+ server as dead and does not continue to send AAA requests to that dead TACACS+ server.

A Cisco CG-OS router periodically monitors dead TACACS+ servers and brings them to the alive state once they are responding. Continual monitoring ensures that a TACACS+ server is in a working state before it receives real AAA requests. Whenever an TACACS+ server changes to the dead or alive state, a Simple Network Management Protocol (SNMP) trap is generated and the Cisco CG-OS router displays an error message that a failure is taking place before it can impact performance. (See Figure 3-1.)

Figure 3-1 TACACS+ Server States

Note The Cisco CG-OS router initiates TACACS+ server monitoring by sending a test authentication request to the TACACS+ server.

Vendor-Specific Attributes

The Internet Engineering Task Force (IETF) draft standard specifies a method for communicating vendor-specific attributes (VSAs) between the network access server and the TACACS+ server. The IETF uses attribute 26. VSAs allow vendors to support their own extended attributes that are not suitable for general use.

This section includes the following topics:

• Cisco VSA Format
• Cisco TACACS+ Privilege Levels

Cisco VSA Format

The Cisco TACACS+ implementation supports one vendor-specific option using the format recommended in the IETF specification. The Cisco vendor ID is 9, and the supported option is vendor type 1, which is named cisco-av-pair. The value is a string with the following format:

The protocol is a Cisco attribute for a particular type of authorization, separator is an equal sign (=) for mandatory attributes, and an asterisk ( * ) indicates optional attributes.

When you use TACACS+ servers for authentication on a Cisco CG-OS router, the TACACS+ protocol directs the TACACS+ server to return user attributes, such as authorization information, along with authentication results. This authorization information is specified through VSAs.

The Cisco CG-OS software supports the following VSA protocol options:

• Shell—Protocol used in access-accept packets to provide user profile information.
• Accounting—Protocol used in accounting-request packets. If a value contains any white spaces, enclose the value within double quotation marks.

The Cisco CG-OS software supports the following attributes:

• roles—Lists all the roles to which the user belongs. The value field is a string that lists the role names delimited by white space. For example, if the user belongs to roles network-operator and vdc-admin, the value field would be “network-operator vdc-admin.” This subattribute, which the TACACS+ server sends in the VSA portion of the Access-Accept frames, can only be used with the shell protocol value. The following examples show the roles attribute as supported by Cisco ACS:

Note When you specify a VSA as shell:roles*“network-operator vdc-admin”, this VSA is flagged as an optional attribute and other Cisco devices ignore this attribute.

• accountinginfo—Stores accounting information in addition to the attributes covered by the standard TACACS+ accounting protocol. This attribute is sent only in the VSA portion of the Account-Request frames from the TACACS+ client on the Cisco CG-OS router. It can be used only with the accounting protocol data units (PDUs).

Cisco TACACS+ Privilege Levels

TACACS+ servers support privilege levels for specifying the permissions that users have when logging onto a Cisco CG-OS router. For the maximum privilege level 15, the Cisco CG-OS software applies the network-admin role in the default VDC or the vdc-admin role for non-default VDCs. All other privilege levels are translated to the vdc-operator role. For more information on user roles, see Chapter7, “Configuring User Accounts and RBAC”

Note When you specify a user role in the cisco-av-pair, that takes precedence over the privilege level.

Note Although references to a default VDC might be seen in CLI displays, the Cisco CG-OS router does not support the configuration of more than one VDC. The Cisco CG-OS router only supports a default VDC.

TACACS+ Server Configuration Process

To configure TACACS+ servers, follow these steps:

Step 2 Establish the TACACS+ server connections to the Cisco CG-OS router. (See Configuring TACACS+ Server Hosts.)

Step 3 Configure the secret keys for the TACACS+ servers. (See Configuring Global TACACS+ Keys and Configuring a Key for a Specific TACACS+ Server.)

Step 4 When needed, configure TACACS+ server groups with subsets of the TACACS+ servers for AAA authentication methods. (See Configuring TACACS+ Server Groups and Configuring AAA.)

Step 5 When needed, configure any of the following optional parameters:

• Dead-time interval (See Configuring the Dead-Time Interval.)
• TACACS+ server specification allowed at user login (See Configuring the Global TACACS+ Timeout Interval.)
• Timeout interval (See Configuring the Global TACACS+ Timeout Interval.)
• TCP port (See Configuring TCP Ports.)

Step 6 When needed, configure periodic TACACS+ server monitoring. (See Configuring Periodic TACACS+ Server Monitoring.)

Enabling TACACS+

By default, the TACACS+ feature is disabled on the Cisco CG-OS router. You must enable the TACACS+ feature to access the configuration and verification commands for authentication.

BEFORE YOU BEGIN

No prerequisites.

DETAILED STEPS

EXAMPLE

This example shows how to enable the TACACS+ feature on the Cisco CG-OS router before configuring commands that support authentication.

Configuring TACACS+ Server Hosts

To access a remote TACACS+ server, you must configure the IP address or the hostname for the TACACS+ server on the Cisco CG-OS router. You can configure up to 64 TACACS+ servers.

Note By default, when you configure a TACACS+ server IP address or hostname on the Cisco CG-OS router, the TACACS+ server is added to the default TACACS+ server group. You can also add the TACACS+ server to another TACACS+ server group. For information about creating TACACS+ server groups, see Configuring TACACS+ Server Groups).

BEFORE YOU BEGIN

Enable TACACS+. (See Enabling TACACS+.)

Obtain the IPv4 or IPv6 addresses or the hostnames for the remote TACACS+ servers.

DETAILED STEPS

EXAMPLE

This example shows how to configure an IP address for the TACACS+ server on the
Cisco CG-OS router.

Configuring Global TACACS+ Keys

You can configure secret TACACS+ keys at the global level for all servers used by the Cisco CG-OS router. A secret key is a secret text string shared between the Cisco CG-OS router and the TACACS+ server hosts.

BEFORE YOU BEGIN

Enable TACACS+. (See Enabling TACACS+.)

Obtain the secret key values for the remote TACACS+ servers.

DETAILED STEPS

EXAMPLE

This example shows how to configure a global TACACS+ key for the Cisco CG-OS router.

Configuring a Key for a Specific TACACS+ Server

You can configure secret keys for a TACACS+ server. A secret key is a shared secret text string between the Cisco CG-OS router and the TACACS+ server host to allow secure communication.

BEFORE YOU BEGIN

Enable TACACS+. (See Enabling TACACS+.)

Obtain the secret key values for the remote TACACS+ servers.

DETAILED STEPS

EXAMPLE

This example shows how to configure a key for a specific TACACS+ server.

router(config)# copy running-config startup-config

Configuring TACACS+ Server Groups

You can specify one or more remote AAA servers to authenticate users using server groups. All members of a group must use the TACACS+ protocol. The Cisco CG-OS router attempts access to the servers in the same order in which you configure them.

You can configure these server groups at any time but they only take effect when you apply them to an AAA service. For information on AAA services, see Remote AAA Services.

BEFORE YOU BEGIN

Enable TACACS+. (See Enabling TACACS+.)

DETAILED STEPS

EXAMPLE

This example shows how to configure a TACACS+ server group for authentication.

router(config)# aaa group server tacacs+ TacServer

Configuring the Global Source Interface for TACACS+ Server Groups

You can configure a global source interface for TACACS+ server groups to use when accessing TACACS+ servers. To configure a different source interface for a specific TACACS+ server group, see Configuring TACACS+ Server Groups. By default, the Cisco CG-OS software uses any available interface.

BEFORE YOU BEGIN

Enable TACACS+. (See Enabling TACACS+.)

DETAILED STEPS

EXAMPLE

This example shows how to configure a global source interface for TACACS+ server groups.

Configuring the Global TACACS+ Timeout Interval

You can set a global timeout interval for all TACACS+ servers. The timeout interval determines how long the Cisco CG-OS router waits for responses from TACACS+ servers before declaring a timeout failure.

BEFORE YOU BEGIN

Enable TACACS+. (See Enabling TACACS+.)

DETAILED STEPS

EXAMPLE

This example shows how to configure a global TACACS+ timeout interval for the Cisco CG-OS router.

Configuring the Timeout Interval for a Server

The timeout interval determines how long the Cisco CG-OS router waits for responses from a TACACS+ server before declaring a timeout failure.

BEFORE YOU BEGIN

Enable TACACS+. (See Enabling TACACS+.)

DETAILED STEPS

EXAMPLE

This example shows how to configure a timeout for a TACACS+ server.

Configuring TCP Ports

You can configure another TCP port for the TACACS+ servers if there are conflicts with another application. By default, the Cisco CG-OS router uses port 49 for all TACACS+ requests.

BEFORE YOU BEGIN

Enable TACACS+. (See Enabling TACACS+.)

DETAILED STEPS

EXAMPLE

Configuring Periodic TACACS+ Server Monitoring

You can monitor the availability of TACACS+ servers. The monitoring parameters include the username and password to use for the server and an idle timer. The idle timer specifies the interval in which a TACACS+ server receives no requests before the Cisco CG-OS router sends out a test packet. You can configure this option to test servers periodically, or you can run a one-time only test.

Note To protect network security, Cisco recommends that you use a username that is not the same as an existing username in the TACACS+ database.

Note The default idle timer value is 0 minutes. When the idle time interval is 0 minutes, the Cisco CG-OS router does not perform periodic TACACS+ server monitoring.

BEFORE YOU BEGIN

Enable TACACS+. (See Enabling TACACS+.)

DETAILED STEPS

EXAMPLE

This example shows how to configure a dead time interval to monitor the availability of a TACACS+ server and how to configure a unique monitoring username and password.

Configuring the Dead-Time Interval

You can configure a global dead-time interval for all TACACS+ servers. The dead-time interval specifies the time that the Cisco CG-OS router waits, after declaring a TACACS+ server is dead, before sending out a test packet to determine if the server is now alive.

Note When the dead timer interval is 0 minutes, TACACS+ servers are not marked as dead even if they are not responding. You can configure the dead-timer per server group. (See Configuring TACACS+ Server Groups.)

BEFORE YOU BEGIN

Enable TACACS+. (See Enabling TACACS+.)

DETAILED STEPS

EXAMPLE

This example shows how to deliver a global deadtime interval that applies to all TACACS+ servers.

Enabling ASCII Authentication

You can enable ASCII authentication on the TACACS+ server.

Note Only TACACS+ servers support ASCII authentication.

BEFORE YOU BEGIN

Enable TACACS+. (See Enabling TACACS+.)

DETAILED STEPS

EXAMPLE

This example shows how to enable ASCII authentication on the TACACS+ server.

Manually Monitoring TACACS+ Servers or Groups

You can manually issue a test message to a TACACS+ server or to a server group.

BEFORE YOU BEGIN

Enable TACACS+. (See Enabling TACACS+.)

DETAILED STEPS

Enter one or both of the commands below as applicable.

EXAMPLE

This example shows how to configure a command to send a manual test of a TACACS+ server.

This example shows how to configure a command to send a manual test of a TACACS+ server group.

Disabling TACACS+

You can disable TACACS+.

BEFORE YOU BEGIN

Ensure that an alternate TACACS+ server resource is available for those devices that are authenticated by the server before it is taken out of service.

DETAILED STEPS

EXAMPLE

This example shows how to disable a TACACS+ server.

BEFORE YOU BEGIN

Enable TACACS+. (See Enabling TACACS+.)