Cisco Nexus 1000V Security Configuration Guide, 4.2(1)SV2(1.1)
Configuring TACACS+
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Contents

Configuring TACACS+

This chapter contains the following sections:

Information About TACACS+

The TACACS+ security protocol provides centralized validation of users who are attempting to gain access to a device. TACACS+ services are maintained in a database on a TACACS+ daemon that is 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.

TACACS+ Operation for User Login

The following sequence of events take place when you attempt to log in to a TACACS+ server using the 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 might include prompts for additional information, such as your mother’s maiden name.
  2. The TACACS+ daemon provides one of the following responses:
    1. ACCEPT—User authentication succeeds and service begins. If user authorization is needed, authorization begins.
    2. REJECT—User authentication failed. The TACACS+ daemon either denies further access to the user or prompts the user to retry the login sequence.
    3. 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 an 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 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. The 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, with authentication results. This authorization information is specified through VSAs.

The following VSA protocol options are supported:

  • 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 attributes are other supported:

  • roles—Lists all the roles to which the user belongs. The value consists of a string that lists 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+

  • 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.

Guidelines and Limitations for TACACS+

  • You can configure a maximum of 64 TACACS+ servers.
  • The logging level for TACACS + must be set to 5.

Default Settings for TACACS+

Parameters

Default

TACACS+

Disabled

Dead timer interval

0 minutes

Timeout interval

5 seconds

Idle timer interval

0 minutes

Periodic server monitoring username

test

Periodic server monitoring password

test

Configuring TACACS+

The following flowchart guides you through the TACACS+ configuration process.


Note


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


Figure 2. Configuring TACACS+ Flowchart



Figure 3. Configuring TACACS+ Flowchart (continued)



Figure 4. Configuring TACACS+ Flowchart (continued)



Enabling or Disabling TACACS+

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.


Before You Begin

Log in to the CLI in EXEC mode.

Procedure
      Command or Action Purpose
    Step 1 switch# configure terminal 

    Enters global configuration mode.

     
    Step 2 switch(config)# [no] tacacs+ enable 

    Enables or disables TACACS+.

     
    Step 3 switch(config)# exit 

    Exits global configuration mode and returns to EXEC mode.

     
    Step 4 switch(config)# copy running-config startup-config  

    Copies the running configuration to the startup configuration.

     

    This example shows how to enable TACACS+:

    switch# configure terminal
    switch(config)# tacacs+ enable
    switch(config)# exit
    switch# copy running-config startup-config
    

    Configuring Shared Keys

    By default, no global key is configured.

    You can configure the following:

    • The global key or a secret text string that is shared between the Cisco Nexus 1000V and all TACACS+ server hosts
    • The key or secret text string that is shared between the Cisco Nexus 1000V and a single TACACS+ server host
    Before You Begin
    • Log in to the CLI in EXEC mode.
    • Enable TACACS+ for authentication.
    • Know the key for the TACACS+ server host(s).
    Procedure
        Command or Action Purpose
      Step 1 switch# configure terminal 

      Enters global configuration mode.

      Do one of the following:
      • To configure a global key for all TACACS+ server hosts, continue to the next step.
      • To configure a key for a single TACACS+ server host, go to Step 3.
       
      Step 2 switch(config)# tacacs-server key [ 0 | 7 ] global_key  
      Designates the global key shared between the Cisco Nexus 1000V and the TACACS+ server hosts.
      • 0—Specifies a clear text string (key) to follow. This is the default.
      • 7—Specifies an encrypted string (key) to follow.
      • global_key— String of up to 63 characters. By default, no global key is configured.

      Go to Step 4.

       
      Step 3 switch(config)# tacacs-server host { ipv4-address | host-name} key [0 | 7] shared_key 

      Designates the key shared between the Cisco Nexus 1000V and this specific TACACS+ server host.

      0—Specifies a clear text string (key) to follow. This is the default.

      7—Specifies an encrypted string (key) to follow.

      global key—String of up to 63 characters.

      This shared key is used instead of the global shared key.

       
      Step 4 switch(config)# exit 

      Exits global configuration mode and returns to EXEC mode.

       
      Step 5 switch# show tacacs-server  (Optional)

      Displays the TACACS+ server configuration.

      Note   

      The global shared key is saved in encrypted form in the running configuration. To display the key, use the show running-config command.

       
      Step 6 switch(config)# copy running-config startup-config   (Optional)

      Copies the running configuration to the startup configuration.

       

      This example shows how to configure a shared key:

      switch# configure terminal
      switch(config)# tacacs-server key 0 QsEFtkI#
      switch(config)# exit
      switch# show tacacs-server
      Global TACACS+ shared secret:********
      timeout value:5
      deadtime value:0
      total number of servers:1
      
      following TACACS+ servers are configured:
              10.10.2.2:
                      available on port:49
      switch# copy running-config startup-config
      

      Configuring a TACACS+ Server Host

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

      Before You Begin
      • Log in to the CLI in EXEC mode.
      • Enable TACACS+ for authentication.
      • Configure the shared key.
      • Know the IP addresses or the hostnames for the remote TACACS+ server hosts.
      Procedure
          Command or Action Purpose
        Step 1 switch# configure terminal 

        Enters global configuration mode.

         
        Step 2 switch(config)# tacacs-server host {ipv4-address | host-name} 

        Configures the server IP address or hostname as a TACACS+ server host.

         
        Step 3 switch(config)# exit 

        Exits global configuration mode and returns to EXEC mode.

         
        Step 4 switch(config)# show tacacs-server  (Optional)

        Displays the TACACS+ server configuration.

         
        Step 5 switch(confgi)# copy running-config startup-config   (Optional)

        Copies the running configuration to the startup configuration

         

        This example shows how to configure a TACACS+ server host:

        switch# configure terminal
        switch(config)# tacacs-server host 10.10.2.2
        switch(config)# exit
        switch# show tacacs-server 
        timeout value:5
        deadtime value:0
        total number of servers:1
        
        following TACACS+ servers are configured:
                10.10.2.2:
                        available on port:49
        switch# copy running-config startup-config
        

        Configuring a TACACS+ Server Group

        You can configure a TACACS+ server group whose member servers share authentication functions.

        After you configure the TACACS+ server group, the server members are tried in the same order in which you configured them.

        A TACACS+ server group can provide a failover if 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 failovers for each other in this same way.

        Before You Begin
        • Log in to the CLI in EXEC mode.
        • Know that all servers added to a TACACS+ server group use the TACACS+ protocol.
        • Configure the preshared keys.
        • Enable TACACS+ for authentication.
        Procedure
            Command or Action Purpose
          Step 1 switch# configure terminal 

          Enters global configuration mode.

           
          Step 2 switch(config)# aaa group server tacacs+ group-name 

          Creates a TACACS+ server group with the specified name and places you into the TACACS+ configuration mode for that group.

           
          Step 3 switch(config-tacacs+)# server {ipv4-address | host-name} 

          Configures the TACACS+ server hostname or IP address as a member of the TACACS+ server group.

          If the specified TACACS+ server is not found, configure it using the tacacs-server host command and retry this command.

           
          Step 4 switch(config-tacacs+)# deadtime minutes  (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.

           
          Step 5 switch(config-tacacs+)# use-vrf vrf-name  (Optional)

          Specifies the virtual routing and forwarding instance (VRF) to use to contact this server group

           
          Step 6 switch(config-tacacs+)# source-interface {interface-type} {interface-number}  (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 switch(config-tacacs+)# show tacacs-server groups  (Optional)

          Displays the TACACS+ server group configuration

           
          Step 8 switch(config-tacacs+)# copy running-config startup-config  (Optional)

          Copies the running configuration to the startup configuration.

           

          This example shows how to configure a TACACS+ server group:

          switch# config terminal
          switch(config)# aaa group server tacacs+ TacServer
          switch(config-tacacs+)# server 10.10.2.2
          switch(config-tacacs+)# deadtime 30
          switch(config-tacacs+)# use-vrf management
          switch(config-tacacs+)# source-interface mgmt0
          switch(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
          switch# copy running-config startup-config
          

          Enabling TACACS+ Server-Directed Requests

          You can designate which TACACS+ server to send an authentication request to. This process is called a directed request.

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

          Note


          User-specified logins are supported only for Telnet sessions.


          Before You Begin
          • Log in to the CLI in EXEC mode.
          • Enable TACACS+ for authentication.
          Procedure
              Command or Action Purpose
            Step 1 switch# configure terminal 

            Enters global configuration mode.

             
            Step 2 switch(config)# tacacs-server directed-request 

            Enables use of directed requests for specifying the TACACS+ server to send an authentication request to when logging in. The default is disabled.

             
            Step 3 switch(config)# exit 

            Exits global configuration mode and returns to EXEC mode.

             
            Step 4 switch(config)# show tacacs-server directed-request   (Optional)

            Displays the TACACS+ directed request configuration.

             
            Step 5 switch(config)# copy running-config startup-config  

            Copies the running configuration to the startup configuration

             

            This example shows how to enable a TACACS+ server-directed request:

            switch# config terminal
            switch(config)# tacacs-server directed-request
            switch(config)# exit
            switch# show tacacs-server directed-request
            enabled
            switch# copy running-config startup-config

            Setting the TACACS+ Global Timeout Interval

            You can set the interval in seconds that the Cisco Nexus 1000V waits for a response from any TACACS+ server before declaring a timeout.

            The timeout specified for an individual TACACS+ server overrides the global timeout interval. To set the timeout for an individual server.

            Before You Begin
            • Log in to the CLI in EXEC mode.
            • Enable TACACS+ for authentication.
            Procedure
                Command or Action Purpose
              Step 1 switch# configure terminal 

              Enters global configuration mode.

               
              Step 2 switch(config)# tacacs-server timeout seconds 

              Specifies the interval in seconds that the Cisco Nexus 1000V waits for a response from a server. The default timeout interval is 5 seconds. The range is from 1 to 60 seconds.

               
              Step 3 switch(confi)# exit 

              Exits global configuration mode and returns to EXEC mode.

               
              Step 4 switch(config)# show tacacs-server  (Optional)

              Displays the TACACS+ server configuration.

               
              Step 5 switch(confi)# copy running-config startup-config   (Optional)

              Copies the running configuration to the startup configuration

               

              This example shows how to set a TACACS+ timeout interval:

              switch# configure terminal
              switch(config)# tacacs-server timeout 10 
              switch(config)# exit 
              switch# n1000v# show tacacs-server
              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
              switch# copy running-config startup-config

              Setting a Timeout Interval for an Individual TACACS+ Host

              You can 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.

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

              Before You Begin
              • Log in to the CLI in EXEC mode.
              • Enable TACACS+ for authentication.
              • Configure the TACACS+ server.
              Procedure
                  Command or Action Purpose
                Step 1 switch# configure terminal 

                Enters global configuration mode.

                 
                Step 2 switch(config)# tacacs-server host {ipv4-address | host-name} timeout seconds 

                Specifies the timeout interval for a specific server. The default is the global timeout interval.

                 
                Step 3 switch(config)# exit 

                Exits global configuration mode and returns to EXEC mode.

                 
                Step 4 switch(config)# show tacacs-server  (Optional)

                Displays the TACACS+ server configuration.

                 
                Step 5 switch(config)# copy running-config startup-config   (Optional)

                Copies the running configuration to the startup configuration

                 

                This example shows how to set a timeout interval for an individual TACACS+ host:

                switch# config terminal
                switch(config)#  tacacs-server host 10.10.2.2 timeout 10 
                switch(config)# exit
                switch# n1000v# show tacacs-server
                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
                                timeout:10
                switch# copy running-config startup-config
                

                Configuring the TCP Port for a TACACS+ Host

                You can configure a TCP port other than port 49 (the default for TACACS+ requests).

                Before You Begin
                • Log in to the CLI in EXEC mode.
                • Enable TACACS+ for authentication.
                • Configure the TACACS+ server
                Procedure
                    Command or Action Purpose
                  Step 1 switch# configure terminal 

                  Enters global configuration mode.

                   
                  Step 2 switch(config)# tacacs-server host {ipv4-address | host-name} port tcp-port 

                  Specifies the TCP port to use.

                  The port range is from 1 to 65535.

                  The default is 49.

                   
                  Step 3 switch(config)# exit 

                  Exits global configuration mode and returns to EXEC mode.

                   
                  Step 4 switch(config)# show tacacs-server  (Optional)

                  Displays the TACACS+ server configuration.

                   
                  Step 5 switch(config)# copy running-config startup-config   (Optional)

                  Copies the running configuration to the startup configuration

                   

                  This example shows how to configure the TCP port for a TACACS+ host:

                  switch# configure terminal
                  switch(config)# tacacs-server host 10.10.2.2 port 2
                  switch(config)# exit
                  switch# show tacacs-server
                  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
                  switch# copy running-config startup-config

                  Configuring Monitoring for a TACACS+ Host

                  You should know the following information:

                  • 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.
                  Before You Begin
                  • Log in to the CLI in EXEC mode.
                  • Enable TACACS+ for authentication.
                  • Configure the TACACS+ server.
                  Procedure
                      Command or Action Purpose
                    Step 1 switch# configure terminal 

                    Enters global configuration mode.

                     
                    Step 2 switch(config)# tacacs-server host {ipv4-address | host-name } test {idle-timeminutes |password password [idle-time minutes] | username name [password password [idle-timeminutes] ] }  

                    Configures server monitoring.

                    The keywords and arguments are as follows:

                    • username—Specifies that the default is test.
                      Note   

                      To protect network security, we recommend that you assign a username that is not already in the TACACS+ database.

                    • password—Specifies that the default is test.
                    • idle-time—The default is 0 minutes. The valid range is from 0 to 1440 minutes
                      Note   

                      For periodic TACACS+ server monitoring, the idle timer value must be greater than 0.

                     
                    Step 3 switch(config)# tacacs-server dead-time minutes 

                    Specifies the duration of time in minutes before checking a TACACS+ server that was previously unresponsive. The default value is 0 minutes and the valid range is from 0 to 1440 minutes.

                     
                    Step 4 switch(config)# exit 

                    Exits global configuration mode and returns to EXEC mode.

                     
                    Step 5 switch(config)# show tacacs-server  (Optional)

                    Displays the TACACS+ server configuration.

                     
                    Step 6 switch(config)# copy running-config startup-config   (Optional)

                    Copies the running configuration to the startup configuration

                     

                    This example shows how to configure monitoring for a TACACS+ host:

                    switch# configure terminal
                    switch(config)# tacacs-server host 10.10.2.2 test username pvk2 password a3z9yjqz7 idle-time 3
                    switch(config)# tacacs-server dead-time 5
                    switch(config)# exit
                    switch# show tacacs-server 
                    Global TACACS+ shared secret:********
                    timeout value:10
                    deadtime value:5
                    total number of servers:1
                    
                    following TACACS+ servers are configured:
                            10.10.2.2:
                                    available on port:2
                                    timeout:10
                    switch# copy running-config startup-config

                    Configuring the TACACS+ Global Dead-Time Interval

                    You can configure the interval to wait before sending a test packet to a previously unresponsive server.

                    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 time per group.

                    Before You Begin
                    • Log in to the CLI in EXEC mode.
                    • Enable TACACS+ for authentication.
                    • Configure the TACACS+ server.
                    Procedure
                        Command or Action Purpose
                      Step 1 switch# configure terminal 

                      Enters global configuration mode.

                       
                      Step 2 switch(config)# tacacs-server deadtime minutes  

                      Configures the global dead-time interval. The default value is 0 minutes. The range is from 1 to 1440 minutes

                       
                      Step 3 switch(config)# exit 

                      Exits global configuration mode and returns to EXEC mode.

                       
                      Step 4 switch(config)# show tacacs-server  (Optional)

                      Displays the TACACS+ server configuration.

                       
                      Step 5 switch(config)# copy running-config startup-config   (Optional)

                      Copies the running configuration to the startup configuration

                       

                      This example shows how to configure the TACACS+ global dead-time interval:

                      switch# configure terminal
                      switch(config)#  tacacs-server deadtime 5 
                      switch(config)# exit
                      switch# show tacacs-server 
                      Global TACACS+ shared secret:********
                      timeout value:10
                      deadtime value:5
                      total number of servers:1
                      
                      following TACACS+ servers are configured:
                              10.10.2.2:
                                      available on port:2
                                      timeout:10
                      switch# copy running-config startup-config
                      

                      Displaying Statistics for a TACACS+ Host

                      Use the following command to display statistics for a TACACS+ host.

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

                      Configuration Example for TACACS+

                      This example shows a TACACS+ configuration:

                      switch# configure terminal
                      switch(config)# feature tacacs+
                      switch(config-tacacs+)# tacacs-server key 7 "ToIkLhPpG"
                      switch# (config-tacacs+)# tacacs-server host 10.10.2.2 key 7 "ShMoMhTl"
                      switch# (config-tacacs+)# aaa group server tacacs+ TacServer
                          server 10.10.2.2
                      

                      Feature History for TACACS+

                      This table only includes updates for those releases that have resulted in additions to the feature.

                      Feature Name

                      Releases

                      Feature Information

                      TACACS+

                      4.0(4)SV1(1)

                      This feature was introduced.