Cisco Nexus 6000 Series NX-OS Security Configuration Guide, Release 6.x
Configuring Authentication, Authorization, and Accounting
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Configuring Authentication, Authorization, and Accounting

Contents

Configuring Authentication, Authorization, and Accounting

This chapter contains the following sections:

Information About AAA

AAA Security Services

The authentication, authorization, and accounting (AAA) features allows you to verify the identity of, grant access to, and track the actions of users who manage Cisco Nexus devices. The Cisco Nexus device supports Remote Access Dial-In User Service (RADIUS) or Terminal Access Controller Access Control device Plus (TACACS+) protocols.

Based on the user ID and password that you provide, the switches perform local authentication or authorization using the local database or remote authentication or authorization using one or more AAA servers. A preshared secret key provides security for communication between the switch and AAA servers. You can configure a common secret key for all AAA servers or for only a specific AAA server.

AAA security provides the following services:

  • Authentication—Identifies users, including login and password dialog, challenge and response, messaging support, and, encryption depending on the security protocol that you select.
  • Authorization—Provides access control. Authorization to access a Cisco Nexus device is provided by attributes that are downloaded from AAA servers. Remote security servers, such as RADIUS and TACACS+, authorize users for specific rights by associating attribute-value (AV) pairs, which define those rights with the appropriate user.
  • Accounting—Provides the method for collecting information, logging the information locally, and sending the information to the AAA server for billing, auditing, and reporting.

Note


The Cisco NX-OS software supports authentication, authorization, and accounting independently. For example, you can configure authentication and authorization without configuring accounting.


Benefits of Using AAA

AAA provides the following benefits:

  • Increased flexibility and control of access configuration
  • Scalability
  • Standardized authentication methods, such as RADIUS and TACACS+
  • Multiple backup devices

Remote AAA Services

Remote AAA services provided through RADIUS and TACACS+ protocols have the following advantages over local AAA services:

  • User password lists for each switch in the fabric are easier to manage.
  • AAA servers are already deployed widely across enterprises and can be easily used for AAA services.
  • The accounting log for all switches in the fabric can be centrally managed.
  • User attributes for each switch in the fabric are easier to manage than using the local databases on the switches.

AAA Server Groups

You can specify remote AAA servers for authentication, authorization, and accounting using server groups. A server group is a set of remote AAA servers that implement the same AAA protocol. A server group provides for failover servers if a remote AAA server fails to respond. If the first remote server in the group fails to respond, the next remote server in the group is tried until one of the servers sends a response. If all the AAA servers in the server group fail to respond, that server group option is considered a failure. If required, you can specify multiple server groups. If a switch encounters errors from the servers in the first group, it tries the servers in the next server group.

AAA Service Configuration Options

On Cisco Nexus devices, you can have separate AAA configurations for the following services:

  • User Telnet or Secure Shell (SSH) login authentication
  • Console login authentication
  • User management session accounting

The following table lists the CLI commands for each AAA service configuration option.

Table 1 AAA Service Configuration Commands

AAA Service Configuration Option

Related Command

Telnet or SSH login

aaa authentication login default

Console login

aaa authentication login console

User session accounting

aaa accounting default

You can specify the following authentication methods for the AAA services:

  • RADIUS server groups—Uses the global pool of RADIUS servers for authentication.
  • Specified server groups—Uses specified RADIUS or TACACS+ server groups for authentication.
  • Local—Uses the local username or password database for authentication.
  • None—Uses only the username.

Note


If the method is for all RADIUS servers, instead of a specific server group, the Cisco Nexus devices choose the RADIUS server from the global pool of configured RADIUS servers in the order of configuration. Servers from this global pool are the servers that can be selectively configured in a RADIUS server group on the Cisco Nexus devices.


The following table describes the AAA authentication methods that you can configure for the AAA services.

Table 2  AAA Authentication Methods for AAA Services

AAA Service

AAA Methods

Console login authentication

Server groups, local, and none

User login authentication

Server groups, local, and none

User management session accounting

Server groups and local


Note


For console login authentication, user login authentication, and user management session accounting, the Cisco Nexus devices try each option in the order specified. The local option is the default method when other configured options fail.


Authentication and Authorization Process for User Logins

The authentication and authorization process for user login is as occurs:

  • When you log in to the required Cisco Nexus device, you can use the Telnet, SSH, Fabric Manager or Device Manager, or console login options.
  • When you have configured the AAA server groups using the server group authentication method, the Cisco Nexus device sends an authentication request to the first AAA server in the group as follows: If the AAA server fails to respond, then the next AAA server is tried and so on until the remote server responds to the authentication request. If all AAA servers in the server group fail to respond, the servers in the next server group are tried. If all configured methods fail, the local database is used for authentication.
  • If a Cisco Nexus device successfully authenticates you through a remote AAA server, the following conditions apply: If the AAA server protocol is RADIUS, user roles specified in the cisco-av-pair attribute are downloaded with an authentication response. If the AAA server protocol is TACACS+, another request is sent to the same server to get the user roles specified as custom attributes for the shell.
  • If your username and password are successfully authenticated locally, the Cisco Nexus device logs you in and assigns you the roles configured in the local database.

The following figure shows a flowchart of the authentication and authorization process.

Figure 1. Authentication and Authorization Flow for User Login


Note


"No more server groups left" means that there is no response from any server in all server groups.

"No more servers left" means that there is no response from any server within this server group.


Prerequisites for Remote AAA

Remote AAA servers have the following prerequisites:

  • At least one RADIUS or TACACS+ server must be IP reachable.
  • The Cisco Nexus device is configured as a client of the AAA servers.
  • The preshared secret key is configured on the Cisco Nexus device and on the remote AAA servers.
  • The remote server responds to AAA requests from the Cisco Nexus device.

Guidelines and Limitations for AAA

The Cisco Nexus devices do not support all numeric usernames, whether created with TACACS+ or RADIUS, or created locally. If an all numeric username exists on an AAA server and is entered during a login, the Cisco Nexus device still logs in the user.


Caution


You should not create user accounts with usernames that are all numeric.


Configuring AAA

Configuring Console Login Authentication Methods

The authentication methods include the following:

  • Global pool of RADIUS servers
  • Named subset of RADIUS or TACACS+ servers
  • Local database on the Cisco Nexus device.
  • Username only ( none )

The default method is local.


Note


The group radius and group server-name forms of the aaa authentication command are used for a set of previously defined RADIUS servers. Use the radius server-host command to configure the host servers. Use the aaa group server radius command to create a named group of servers.


Before you configure console login authentication methods, configure RADIUS or TACACS+ server groups as needed.

Procedure
      Command or Action Purpose
    Step 1 switch# configure terminal
     

    Enters configuration mode.

     
    Step 2 switch(config)# aaa authentication login console {group group-list [none] | local | none}
     

    Configures login authentication methods for the console.

    The group-list argument consists of a space-delimited list of group names. The group names are the following:

    • radius —Uses the global pool of RADIUS servers for authentication.
    • named-group —Uses a named subset of TACACS+ or RADIUS servers for authentication.

    The local method uses the local database for authentication. The none method uses the username only.

    The default console login method is local, which is used when no methods are configured or when all of the configured methods fail to respond.

     
    Step 3 switch(config)# exit
     

    Exits configuration mode.

     
    Step 4 switch# show aaa authentication
     
    (Optional)

    Displays the configuration of the console login authentication methods.

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

    Copies the running configuration to the startup configuration.

     

    The following example shows how to configure authentication methods for the console login:

    switch# configure terminal
    switch(config)# aaa authentication login console group radius
    switch(config)# exit
    switch# show aaa authentication
    switch# copy running-config startup-config

    Configuring Default Login Authentication Methods

    The default method is local.

    Before you configure default login authentication methods, configure RADIUS or TACACS+ server groups as needed. To configure default login authentication methods, perform this task:

    Procedure
        Command or Action Purpose
      Step 1 switch# configure terminal
       

      Enters configuration mode.

       
      Step 2 switch(config)# aaa authentication login default {group group-list [none] | local | none}
       

      Configures the default authentication methods.

      The group-list argument consists of a space-delimited list of group names. The group names are the following:

      • radius —Uses the global pool of RADIUS servers for authentication.
      • named-group —Uses a named subset of TACACS+ or RADIUS servers for authentication.

      The local method uses the local database for authentication. The none method uses the username only.

      The default login method is local , which is used when no methods are configured or when all of the configured methods do not respond.

       
      Step 3 switch(config)# exit
       

      Exits configuration mode.

       
      Step 4 switch# show aaa authentication
       
      (Optional)

      Displays the configuration of the default login authentication methods.

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

      Copies the running configuration to the startup configuration.

       

      Enabling Login Authentication Failure Messages

      When you log in, the login is processed by the local user database if the remote AAA servers do not respond. If you have enabled the displaying of login failure messages, the following message is displayed:

      Remote AAA servers unreachable; local authentication done.
      Remote AAA servers unreachable; local authentication failed.
      Procedure
          Command or Action Purpose
        Step 1 switch# configure terminal
         

        Enters configuration mode.

         
        Step 2 switch(config)# aaa authentication login error-enable
         

        Enables login authentication failure messages. The default is disabled.

         
        Step 3 switch(config)# exit
         

        Exits configuration mode.

         
        Step 4 switch# show aaa authentication
         
        (Optional)

        Displays the login failure message configuration.

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

        Copies the running configuration to the startup configuration.

         

        Configuring AAA Command Authorization

        When a TACACS+ server authorization method is configured, you can authorize every command that a user executes with the TACACS+ server which includes all EXEC mode commands and all configuration mode commands.

        The authorization methods include the following:

        • Group—TACACS+ server group
        • Local—Local role-based authorization
        • None—No authorization is performed

        The default method is Local.


        Note


        There is no authorization on the console session.


        Before You Begin

        You must enable TACACS+ before configuring AAA command authorization.

        Procedure
            Command or Action Purpose
          Step 1 configure terminal


          Example:
          switch# configure terminal
          switch(config)#
           

          Enters global configuration mode.

           
          Step 2 aaa authorization {commands | config-commands} {default} {{[group group-name] | [ local]} | {[group group-name] | [ none]}}


          Example:
          switch(config)# aaa authorization config-commands default group tac1
          
          


          Example:
          switch# aaa authorization commands default group tac1
          
          
           

          Configures authorization parameters.

          Use the commands keyword to authorize EXEC mode commandes.

          Use the config-commands keyword to authorize configuration mode commands.

          Use the group, local, or none keywords to identify the authorization method.

           

          The following example shows how to authorize EXEC mode commands with TACACS+ server group tac1:

          switch# aaa authorization commands default group tac1
          
          

          The following example shows how to authorize configuration mode commands with TACACS+ server group tac1:

          switch(config)# aaa authorization config-commands default group tac1
          
          

          The following example shows how to authorize configuration mode commands with TACACS+ server group tac1:

          • If the server is reachable, the command is allowed or not allowed based on the server response.
          • If there is an error reaching the server, the command is authorized based on the user's local role.
          switch(config)# aaa authorization config-commands default group tac1 local
          
          

          The followng example shows how to authorize configuration mode commands with TACACS+ server group tac1:

          • If the server is reachable, the command is allowed or not allowed based on the server response.
          • If there is an error reaching the server, allow the command regardless of the local role.
          switch# aaa authorization commands default group tac1 none 
          
          

          The following example shows how to authorize EXEC mode commands regardless of the local role:

          switch# aaa authorization commands default none
          
          

          The following example shows how to authorize EXEC mode commands using the local role for authorization:

          switch# aaa authorization commands default local
          
          

          Enabling MSCHAP Authentication

          Microsoft Challenge Handshake Authentication Protocol (MSCHAP) is the Microsoft version of CHAP. You can use MSCHAP for user logins to a Cisco Nexus device through a remote authentication server (RADIUS or TACACS+).

          By default, the Cisco Nexus device uses Password Authentication Protocol (PAP) authentication between the switch and the remote server. If you enable MSCHAP, you must configure your RADIUS server to recognize the MSCHAP vendor-specific attributes (VSAs).

          The following table describes the RADIUS VSAs required for MSCHAP.

          Table 3  MSCHAP RADIUS VSAs

          Vendor-ID Number

          Vendor-Type Number

          VSA

          Description

          311

          11

          MSCHAP-Challenge

          Contains the challenge sent by an AAA server to an MSCHAP user. It can be used in both Access-Request and Access-Challenge packets.

          211

          11

          MSCHAP-Response

          Contains the response value provided by an MSCHAP user in response to the challenge. It is only used in Access-Request packets.

          Procedure
              Command or Action Purpose
            Step 1 switch# configure terminal
             

            Enters configuration mode.

             
            Step 2 switch(config)# aaa authentication login mschap enable
             

            Enables MS-CHAP authentication. The default is disabled.

             
            Step 3 switch(config)# exit
             

            Exits configuration mode.

             
            Step 4 switch# show aaa authentication login mschap
             
            (Optional)

            Displays the MS-CHAP configuration.

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

            Copies the running configuration to the startup configuration.

             
            Related Concepts

            Configuring AAA Accounting Default Methods

            The Cisco Nexus device supports TACACS+ and RADIUS methods for accounting. The switches report user activity to TACACS+ or RADIUS security servers in the form of accounting records. Each accounting record contains accounting attribute-value (AV) pairs and is stored on the AAA server.

            When you activate AAA accounting, the Cisco Nexus device reports these attributes as accounting records, which are then stored in an accounting log on the security server.

            You can create default method lists defining specific accounting methods, which include the following:.

            • RADIUS server group—Uses the global pool of RADIUS servers for accounting.
            • Specified server group—Uses a specified RADIUS or TACACS+ server group for accounting.
            • Local—Uses the local username or password database for accounting.

            Note


            If you have configured server groups and the server groups do not respond, by default, the local database is used for authentication.


            Before You Begin

            Before you configure AAA accounting default methods, configure RADIUS or TACACS+ server groups as needed.

            Procedure
                Command or Action Purpose
              Step 1 switch# configure terminal
               

              Enters configuration mode.

               
              Step 2 switch(config)# aaa accounting default {group group-list | local}
               

              Configures the default accounting method. One or more server group names can be specified in a space-separated list.

              The group-list argument consists of a space-delimited list of group names. The group names are the following:

              • radius —Uses the global pool of RADIUS servers for accounting.
              • named-group —Uses a named subset of TACACS+ or RADIUS servers for accounting.

              The local method uses the local database for accounting.

              The default method is local, which is used when no server groups are configured or when all the configured server group do not respond.

               
              Step 3 switch(config)# exit
               

              Exits configuration mode.

               
              Step 4 switch# show aaa accounting
               
              (Optional)

              Displays the configuration AAA accounting default methods.

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

              Copies the running configuration to the startup configuration.

               

              Using AAA Server VSAs

              VSAs

              You can use vendor-specific attributes (VSAs) to specify the Cisco Nexus device user roles and SNMPv3 parameters on AAA servers.

              The Internet Engineering Task Force (IETF) draft standard specifies a method for communicating VSAs between the network access server and the RADIUS server. The IETF uses attribute 26. VSAs allow vendors to support their own extended attributes that are not suitable for general use. The Cisco RADIUS implementation supports one vendor-specific option using the format recommended in the 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 seperator value *

              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 RADIUS servers for authentication on a Cisco Nexus device, the RADIUS protocol directs the RADIUS server to return user attributes, such as authorization information, with authentication results. This authorization information is specified through VSAs.

              VSA Format

              The following VSA protocol options are supported by the Cisco Nexus device:

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

              The following attributes are supported by the Cisco Nexus device:

              • roles—Lists all the roles assigned to the user. The value field is a string that stores the list of group names delimited by white space.
              • accountinginfo—Stores additional accounting information in addition to the attributes covered by a standard RADIUS accounting protocol. This attribute is sent only in the VSA portion of the Account-Request frames from the RADIUS client on the switch, and it can only be used with the accounting protocol-related PDUs.

              Specifying Switch User Roles and SNMPv3 Parameters on AAA Servers

              You can use the VSA cisco-av-pair on AAA servers to specify user role mapping for the Cisco Nexus device using this format:

              shell:roles="roleA roleB …"
              

              If you do not specify the role option in the cisco-av-pair attribute, the default user role is network-operator.


              Note


              For information on Cisco Unified Wireless Network TACACS+ configurations and to change the user roles, see Cisco Unified Wireless Network TACACS+ Configuration.


              You can also specify your SNMPv3 authentication and privacy protocol attributes as follows:

              shell:roles="roleA roleB..." snmpv3:auth=SHA priv=AES-128 
              

              The SNMPv3 authentication protocol options are SHA and MD5. The privacy protocol options are AES-128 and DES. If you do not specify these options in the cisco-av-pair attribute, MD5 and DES are the default authentication protocols.

              For additional information, see the Configuring User Accounts and RBAC chapter in the System Management Configuration Guide for your Cisco Nexus device.

              Monitoring and Clearing the Local AAA Accounting Log

              The Cisco Nexus device maintains a local log for the AAA accounting activity.

              Procedure
                  Command or Action Purpose
                Step 1 switch# show accounting log [size] [start-time year month day hh : mm : ss]
                 

                Displays the accounting log contents. By default, the command output contains up to 250,000 bytes of the accounting log. You can use the size argument to limit command output. The range is from 0 to 250000 bytes. You can also specify a start time for the log output.

                 
                Step 2 switch# clear accounting log
                 
                (Optional)

                Clears the accounting log contents.

                 

                Verifying the AAA Configuration

                To display AAA configuration information, perform one of the following tasks:

                Procedure
                    Command or Action Purpose
                  Step 1 show aaa accounting
                   

                  Displays AAA accounting configuration.

                   
                  Step 2 show aaa authentication [login {error-enable | mschap}]
                   

                  Displays AAA authentication information.

                   
                  Step 3 show aaa authorization
                   

                  Displays AAA authorization information.

                   
                  Step 4 show aaa groups
                   

                  Displays the AAA server group configuration.

                   
                  Step 5 show running-config aaa [all]
                   

                  Displays the AAA configuration in the running configuration.

                   
                  Step 6 show startup-config aaa
                   

                  Displays the AAA configuration in the startup configuration.

                   

                  Configuration Examples for AAA

                  The following example shows how to configure AAA:

                   switch(config)# aaa authentication login default group radius
                  
                   switch(config)# aaa authentication login console group radius
                  
                   switch(config)# aaa accounting default group radius
                  

                  Default AAA Settings

                  The following table lists the default settings for AAA parameters.

                  Table 4  Default AAA Parameters

                  Parameters

                  Default

                  Console authentication method

                  local

                  Default authentication method

                  local

                  Login authentication failure messages

                  Disabled

                  MSCHAP authentication

                  Disabled

                  Default accounting method

                  local

                  Accounting log display length

                  250 KB