Cisco Nexus 1000V Security Configuration Guide, Release 4.2(1) SV1(4)

Configuring TACACS+

This chapter describes how to configure the Terminal Access Controller Access Control System Plus (TACACS+) protocol.

This chapter includes the following sections:

•Information About TACACS+

•Prerequisites for TACACS+

•Guidelines and Limitations

•Default Settings

•Configuring TACACS+

•Displaying Statistics for a TACACS+ Host

•Example TACACS+ Configuration

•Additional References

•Feature History for TACACS+

Information About TACACS+

You can use TACACS+ to provide centralized validation of users attempting to gain access to a device. TACACS+ services are maintained in a database on a TACACS+ daemon running, typically, on a UNIX or Windows NT workstation. You must have access to and must configure a TACACS+ server before the configured TACACS+ features on your device are available.

The TACACS+ security protocol provides centralized validation of users attempting to gain access to a device. TACACS+ services are maintained in a database on a TACACS+ daemon running, typically, on a UNIX or Windows NT workstation. You must have access to and must configure a TACACS+ server before the configured TACACS+ features on your device are available.

TACACS+ provides for separate authentication, authorization, and accounting services. The TACACS+ daemon provides each service independently. Each service can be tied into its own database to take advantage of other services available on that server or on the network, depending on the capabilities of the daemon.

The TACACS+ client/server protocol uses TCP (TCP port 49) for transport requirements. Centralized authentication is provided using the TACACS+ protocol.

This section includes the following topics:

•TACACS+ Operation for User Login

•Default TACACS+ Server Encryption Type and Preshared Key

•TACACS+ Server Monitoring

•Vendor-Specific Attributes

TACACS+ Operation for User Login

THe following sequence of events take place when you attempt to login to a TACACS+ server using Password Authentication Protocol (PAP):

1. When a connection is established, the TACACS+ daemon is contacted 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 additional information, such as mother's maiden name.

2. The TACACS+ daemon provides one of the following responses:

a. ACCEPT—User authentication succeeds and service begins. If user authorization is needed, authorization begins.

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

c. ERROR—An error occurred at some time during authentication either at the daemon or in the network connection. If an ERROR response is received, the device tries to use an alternative method for authenticating the user.

If further authorization is required after authentication, the user also undergoes an additional authorization phase. Users must first successfully complete TACACS+ authentication before proceeding to TACACS+ authorization.

3. If TACACS+ authorization is required, the TACACS+ daemon is contacted 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, access list, and user timeouts

Default TACACS+ Server Encryption Type and Preshared Key

You must configure the TACACS+ preshared key to authenticate to the TACACS+ server. A preshared key is a secret text string shared between the device 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 preshared secret key for all TACACS+ server configurations.

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

TACACS+ Server Monitoring

Unresponsive TACACS+ servers are marked as dead and are not sent AAA requests. Dead TACACS+ servers are periodically monitored and brought back alive once they respond. This process confirms that a TACACS+ server is in a working state before real AAA requests are sent its way. The following figure shows how a TACACS+ server state change generates a Simple Network Management Protocol (SNMP) trap and an error message showing the failure before it impacts performance.

Figure 6-1 TACACS+ Server States

Note The monitoring interval for alive servers and dead servers are different and can be configured by the user. The TACACS+ server monitoring is performed 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.

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:

protocol : attribute separator value *

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

When you use TACACS+ servers for authentication, 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 following are supported 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, you should enclose the value within double quotation marks.

The following are other supported attributes:

•roles—Lists all the roles to which the user belongs. The value consists of a string listing the role names delimited by white space. 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.

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

Prerequisites for TACACS+

TACACS+ has the following prerequisites:

•Obtain the IP addresses or hostnames for the TACACS+ servers.

•Obtain the preshared keys from the TACACS+ servers, if any.

•Ensure that the Cisco Nexus 1000V is configured as a TACACS+ client of the AAA servers.

•You have already configured AAA, including remote TACACS+ authentication using the following procedures:

–Configuring a Login Authentication Method

–Configuring AAA

Guidelines and Limitations

TACACS+ has the following guidelines and limitations:

•You can configure a maximum of 64 TACACS+ servers.

•The logging level for TACACS + must be set to 5.

Default Settings

The following table lists the TACACS+ defaults.

Configuring TACACS+

This section includes the following topics:

•Flow Chart:   Configuring TACACS+

•Configuring a TACACS+ Server Host

•Configuring a TACACS+ Server Host

•Configuring Shared Keys

•Configuring a TACACS+ Server Group

•Enabling TACACS+ Server Directed Requests

•Setting the TACACS+ Global Timeout Interval

•Setting a Timeout Interval for an Individual TACACS+ Host

•Configuring the TCP Port for a TACACS+ Host

•Configuring Monitoring for a TACACS+ Host

•Configuring the TACACS+ Global Dead-Time Interval

Note Be aware that the Cisco Nexus 1000V commands may differ from the Cisco IOS commands.

Use the following flow chart to configure TACACS+.

Flow Chart:   Configuring TACACS+

Flow Chart:   Configuring TACACS+ (Continued)

Flow Chart:   Configuring TACACS+ (Continued)

Enabling or Disabling TACACS+

Use this procedure to either enable or disable TACACS+.

BEFORE YOU BEGIN

Before beginning this procedure, you must know or do the following.

•You are logged in to the CLI in EXEC mode.

•By default, TACACS+ is disabled. You must explicitly enable the TACACS+ feature to access the configuration and verification commands that support TACACS+ authentication.

Caution When you disable TACACS+, all related configurations are automatically discarded.

SUMMARY STEPS

1. config t

2. [no] tacacs+ enable

3. exit

4. copy running-config startup-config

DETAILED STEPS

Configuring Shared Keys

Use this procedure to configure the following:

•The global key, or a secret text string shared between the Cisco Nexus 1000V and all TACACS+ server hosts

•The key, or secret text string shared between the Cisco Nexus 1000V and a single TACACS+ server host

BEFORE YOU BEGIN

Before beginning this procedure, you must know or do the following:

•You are logged in to the CLI in EXEC mode.

•You have already enabled TACACS+ for authentication.
See the "Enabling or Disabling TACACS+" procedure.

•You know the key for the TACACS+ server host(s).

•By default, no global key is configured.

SUMMARY STEPS

1. config t

2. tacacs-server key [0 | 7] global_key

3. exit

4. show tacacs-server

5. copy running-config startup-config

DETAILED STEPS

Configuring a TACACS+ Server Host

Use this procedure to configure a TACACS+ server as a TACACS+ host.

BEFORE YOU BEGIN

Before beginning this procedure, you must know or do the following:

•You are logged in to the CLI in EXEC mode.

•You have already enabled TACACS+ for authentication.
See the "Enabling or Disabling TACACS+" procedure.

•You have already configured the shared key, using the following:

"Configuring Shared Keys" procedure

•You know the IP addresses or the hostnames for the remote TACACS+ server hosts.

•All TACACS+ server hosts are added to the default TACACS+ server group.

SUMMARY STEPS

1. config t

2. tacacs-server host {ipv4-address | host-name}

3. exit

4. show tacacs-server

5. copy running-config startup-config

DETAILED STEPS

Configuring a TACACS+ Server Group

Use this procedure to configure a TACACS+ server group whose member servers share authentication functions.

BEFORE YOU BEGIN

Before beginning this procedure, you must know or do the following:

•You are logged in to the CLI in EXEC mode.

•All servers added to a TACACS+ server group must use the TACACS+ protocol.

•Once the TACACS+ server group is configured, the server members are tried in the same order in which you configured them.

•You have already enabled TACACS+ for authentication.
See the "Enabling or Disabling TACACS+" procedure.

•You have already configured the preshared keys, using the following:

"Configuring Shared Keys" procedure

•A TACACS+ server group can provide fail-over in case one server fails to respond. If the first server in the group fails, the next server in the group is tried until a server responds. Multiple server groups can provide fail-over for each other in this same way.

SUMMARY STEPS

1. config t

2. aaa group server tacacs+ group-name

3. server {ipv4-address | host-name}

4. deadtime minutes

5. use-vrf vrf-name

6. (Optional) source-interface {interface-type} {interface-number}

7. (Optional) show tacacs-server groups

8. (Optional) copy running-config startup-config

DETAILED STEPS

	Command	Purpose
Step 1	config t Example: n1000v# config t n1000v(config)#	Places you in the CLI Global Configuration mode.
Step 2	aaa group server tacacs+ group-name Example: n1000v(config)# aaa group server tacacs+ TacServer n1000v(config-tacacs+)#	Creates a TACACS+ server group with the specified name and places you into the TACACS+ configuration mode for that group.
Step 3	server {ipv4-address | host-name} Example: n1000v(config-tacacs+)# server 10.10.2.2 n1000v(config-tacacs+)#	Configures the TACACS+ server host-name or IP address as a member of the TACACS+ server group. Tip If the specified TACACS+ server is not found, configure it using the tacacs-server host command and retry this command.
Step 4	deadtime minutes Example: n1000v(config-tacacs+)# deadtime 30 n1000v(config-tacacs+)#	(Optional) Configures the monitoring dead time for this TACACS+ group. The default is 0 minutes. The range is from 0 through 1440. Note If the dead-time interval for a TACACS+ server group is greater than zero (0), that value takes precedence over the global dead-time value (see the "Configuring the TACACS+ Global Dead-Time Interval" procedure).
Step 5	use-vrf vrf-name Example: n1000v(config-tacacs+)# use-vrf management n1000v(config-tacacs+)#	(Optional) Specifies the virtual routing and forwarding instance (VRF) to use to contact this server group.
Step 6	source-interface {interface-type} {interface-number} Example: n1000v(config-tacacs+)# source-interface mgmt0 n1000v(config-tacacs+)#	(Optional) Specifies a source interface to be used to reach the TACACS+ server. •loopback = Virtual interface number from 0 to 1023 •mgmt = Management interface 0 •null = Null interface 0 •port-channel = Port channel number from 1 to 4096
Step 7	show tacacs-server groups	(Optional) Displays the TACACS+ server group configuration.
	Example: n1000v(config-tacacs+)# show tacacs-server groups total number of groups:1 following TACACS+ server groups are configured: group TacServer: server 10.10.2.2 on port 49 deadtime is 30 vrf is management n1000v(config-tacacs+)#
Step 8	copy running-config startup-config Example: n1000v(config-tacacs+)# copy running-config startup-config	(Optional) Copies these changes made in the running configuration to the startup configuration.

Example:

n1000v(config)# aaa group server tacacs+ TacServer

n1000v(config-tacacs+)# server 10.10.2.2

n1000v(config-tacacs+)# deadtime 30

n1000v(config-tacacs+)# use-vrf management

n1000v(config-tacacs+)# show tacacs-server groups

total number of groups:1

following TACACS+ server groups are configured:

group TacServer:

server 10.10.2.2 on port 49

deadtime is 30

vrf is management

n1000v(config-tacacs+)#

Enabling TACACS+ Server Directed Requests

Use this procedure to let users designate the TACACS+ server to send their authentication request to. This is called a directed-request.

BEFORE YOU BEGIN

Before beginning this procedure, you must know or do the following:

•You are logged in to the CLI in EXEC mode.

•You have already enabled TACACS+ for authentication.
See the "Enabling or Disabling TACACS+" procedure.

Note User-specified logins are only supported for Telnet sessions.

•When directed requests are enabled, the user can log in as username@vrfname:hostname, where vrfname is the VRF to use and hostname is the name of a configured TACACS+ server.

SUMMARY STEPS

1. config t

2. tacacs-server directed-request

3. exit

4. show tacacs-server directed-request

5. copy running-config startup-config

DETAILED STEPS

Setting the TACACS+ Global Timeout Interval

Use this procedure to set the interval in seconds that the Cisco Nexus 1000V waits for a response from any TACACS+ server before declaring a timeout.

BEFORE YOU BEGIN

Before beginning this procedure, you must know or do the following:

•You are logged in to the CLI in EXEC mode.

•You have already enabled TACACS+ for authentication.
See the "Enabling or Disabling TACACS+" procedure.

•The timeout specified for an individual TACACS+ server overrides the global timeout interval. To set the timeout for an individual server, see the "Setting a Timeout Interval for an Individual TACACS+ Host" procedure.

SUMMARY STEPS

1. config t

2. tacacs-server timeout seconds

3. exit

4. show tacacs-server

5. copy running-config startup-config

DETAILED STEPS

Global TACACS+ shared secret:********

timeout value:10

deadtime value:0

total number of servers:1

following TACACS+ servers are configured:

        10.10.2.2:

                available on port:49

n1000v#

Setting a Timeout Interval for an Individual TACACS+ Host

Use this procedure to set the interval in seconds that the Cisco Nexus 1000V waits for a response from a specific TACACS+ server before declaring a timeout. This setting is configured per TACACS+ host.

BEFORE YOU BEGIN

Before beginning this procedure, you must know or do the following:

•You are logged in to the CLI in EXEC mode.

•You have already enabled TACACS+ for authentication.
See the "Enabling or Disabling TACACS+" procedure.

•The timeout setting for an individual TACACS+ server overrides the global timeout interval.

SUMMARY STEPS

1. config t

2. tacacs-server host {ipv4-address | host-name} timeout seconds

3. exit

4. show tacacs-server

5. copy running-config startup-config

DETAILED STEPS

Configuring the TCP Port for a TACACS+ Host

Use this procedure to configure a TCP port other than port 49 (the default for TACACSS+ requests).

BEFORE YOU BEGIN

Before beginning this procedure, you must know or do the following:

•You are logged in to the CLI in EXEC mode.

•You have already enabled TACACS+ for authentication.
See the "Enabling or Disabling TACACS+" procedure.

•You have configured the TACACS+ server using the "Configuring a TACACS+ Server Host" procedure.

SUMMARY STEPS

1. config t

2. tacacs-server host {ipv4-address | host-name} port tcp-port

3. exit

4. show tacacs-server

5. copy running-config startup-config

DETAILED STEPS

Global TACACS+ shared secret:********

timeout value:10

deadtime value:0

total number of servers:1

following TACACS+ servers are configured:

        10.10.2.2:

                available on port:2

                timeout:10

n1000v# 

Configuring Monitoring for a TACACS+ Host

Use this procedure to configure periodic monitoring of a TACACS+ host.

BEFORE YOU BEGIN

Before starting this procedure, you must know or do the following:

•You are logged in to the CLI in EXEC mode.

•You have already enabled TACACS+ for authentication.
See the "Enabling or Disabling TACACS+" procedure.

•You have configured the TACACS+ server.
See the "Configuring a TACACS+ Server Host" procedure.

•The idle timer specifies how long a TACACS+ server should remain idle (receiving no requests) before sending it a test packet.

•The default idle timer value is 0 minutes. When the idle time interval is 0 minutes, periodic TACACS+ server monitoring is not done.

SUMMARY STEPS

1. config t

2. tacacs-server host {ipv4-address | host-name} test {idle-time minutes | password password [idle-time minutes] | username name [password password [idle-time minutes]]}

3. tacacs-server dead-time minutes

4. exit

5. show tacacs-server

6. copy running-config startup-config

DETAILED STEPS

Configuring the TACACS+ Global Dead-Time Interval

Use this procedure to configure the interval to wait before sending a test packet to a previously unresponsive server.

BEFORE YOU BEGIN

Before starting this procedure, you must know or do the following:

•You are logged in to the CLI in EXEC mode.

•You have already enabled TACACS+ for authentication.
See the "Enabling or Disabling TACACS+" procedure.

•You have configured the TACACS+ server.
See the "Configuring a TACACS+ Server Host" procedure.

•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 group (see the "Configuring a TACACS+ Server Group" procedure).

SUMMARY STEPS

1. config t

2. tacacs-server deadtime minutes

3. exit

4. show tacacs-server

5. copy running-config startup-config

DETAILED STEPS

Displaying Statistics for a TACACS+ Host

Use this procedure to display the statistics for TACACS+ host.

BEFORE YOU BEGIN

Before starting this procedure, you must know or do the following:

•You are logged in to the CLI in EXEC mode.

•You have already enabled TACACS+ for authentication.
See the "Enabling or Disabling TACACS+" procedure.

•You have configured the TACACS+ server.
See the "Configuring a TACACS+ Server Host" procedure.

SUMMARY STEPS

1. show tacacs-server statistics {hostname | ipv4-address}

DETAILED STEPS

Example:

n1000v# show tacacs-server statistics 10.10.1.1

Server is not monitored

Authentication Statistics

        failed transactions: 9

        sucessfull transactions: 2

        requests sent: 2

        requests timed out: 0

        responses with no matching requests: 0

        responses not processed: 0

        responses containing errors: 0

Authorization Statistics

        failed transactions: 1

        sucessfull transactions: 0

        requests sent: 0

        requests timed out: 0

        responses with no matching requests: 0

        responses not processed: 0

        responses containing errors: 0

Accounting Statistics

        failed transactions: 0

        sucessfull transactions: 0

        requests sent: 0

        requests timed out: 0

        responses with no matching requests: 0

        responses not processed: 0

        responses containing errors: 0

Example TACACS+ Configuration

The following example shows a TACACS+ configuration:

feature tacacs+

tacacs-server key 7 "ToIkLhPpG"

tacacs-server host 10.10.2.2 key 7 "ShMoMhTl"

aaa group server tacacs+ TacServer

    server 10.10.2.2

Feature History for TACACS+

This section provides the TACACS+ release history.

Additional References

For additional information related to implementing TACACS+, see the following sections:

•Related Documents

•Standards

Related Documents

Standards