Configuring DHCP Services for Accounting and Security

Cisco IOS XE software supports several capabilities that enhance DHCP security, reliability, and accounting in Public Wireless LANs (PWLANs). This functionality can also be used in other network implementations. This module describes the concepts and tasks needed to configure DHCP services for accounting and security.

Finding Feature Information

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Prerequisites for Configuring DHCP Services for Accounting and Security

Before you configure DHCP services for accounting and security, you should understand the concepts documented in the “DHCP Overview” module.

Information About DHCP Services for Accounting and Security

DHCP Operation in Public Wireless LANs

The configuration of DHCP in a public wireless LAN (PWLAN) simplifies the configuration of wireless clients and reduces the overhead necessary to maintain the network. DHCP clients are leased IP addresses by the DHCP server and then authenticated by the Service Selection Gateway (SSG), which allows the clients to access network services. The DHCP server and client exchange DHCP messages for IP address assignments. When a DHCP server assigns an IP address to a client, a DHCP binding is created. The IP address is leased to the client until the client explicitly releases the IP address and disconnects from the network. If the client disconnects without releasing the address, the server terminates the lease after the lease time is over. In either case, the DHCP server removes the binding and the IP address is returned to the pool.

Security Vulnerabilities in Public Wireless LANs

As more people start using PWLANs, security becomes an important concern. Most implementations of PWLANs rely on DHCP for users to obtain an IP address while in a hot spot (such as a coffee shop, airport terminal, hotel, and so on) and use this IP address provided by the DHCP server throughout their session.

IP spoofing is a common technique used by hackers to spoof IP addresses. For example, customer A obtains an IP address from DHCP and has already been authenticated to use the PWLAN, but a hacker spoofs the IP address of customer A and uses this IP address to send and receive traffic. Customer A will still be billed for the service even though he or she is not using the service.

Address Resolution Protocol (ARP) table entries are dynamic by design. Request and reply ARP packets are sent and received by all the networking devices in a network. In a DHCP network, the DHCP server stores the leased IP address to the MAC address or the client-identifier of the client in the DHCP binding. But as ARP entries are learned dynamically, an unauthorized client can spoof the IP address given by the DHCP server and start using that IP address. The MAC address of this unauthorized client will replace the MAC address of the authorized client in the ARP table allowing the unauthorized client to freely use the spoofed IP address.

DHCP Services for Security and Accounting Overview

DHCP security and accounting features have been designed and implemented to address the security concerns in PWLANs but also can be used in other network implementations.

DHCP accounting provides authentication, authorization, and accounting (AAA) and Remote Authentication Dial-In User Service (RADIUS) support for DHCP. The AAA and RADIUS support improves security by sending secure START and STOP accounting messages. The configuration of DHCP accounting adds a layer of security that allows DHCP lease assignment and termination to be triggered for the appropriate RADIUS START and STOP accounting records so that the session state is properly maintained by upstream devices, such as an SSG. This additional security can help to prevent unauthorized clients or hackers from gaining illegal entry to the network by spoofing authorized DHCP leases.

The DHCP Secured IP Address Assignment feature prevents IP spoofing by synchronizing the database of the DHCP server with the ARP table to avoid address hijacking. This secure ARP functionality adds an entry to the ARP table for a client when an address is allocated that can be deleted by the DHCP server only when a binding expires.

The third feature is ARP autologoff, which adds finer control for probing when authorized users log out. The arp probe interval command specifies when to start a probe (the timeout), how frequent a peer is probed (the interval), and the maximum number of retries (the count).

DHCP Lease Limits

You can control the number of subscribers globally or on a per-interface basis by configuring a DHCP lease limit. This functionality allows an Internet service provider (ISP) to limit the number of leases available to clients per household or connection.

How to Configure DHCP Services for Accounting and Security

Configuring AAA and RADIUS for DHCP Accounting

RADIUS provides the accounting capability for the transmission of secure START and STOP messages. AAA and RADIUS are enabled prior to the configuration of DHCP accounting but can also be enabled to secure an insecure DHCP network. The configuration steps in this section are required for configuring DHCP accounting in a new or existing network.

RADIUS Accounting Attributes

DHCP accounting introduces the attributes shown in the table below. These attributes are processed directly by the RADIUS server when DHCP accounting is enabled. These attributes can be monitored in the output of the debug radius command. The output will show the status of the DHCP leases and specific configuration details about the client. The accounting keyword can be used with the debug radius command to filter the output and display only DHCP accounting messages.

Table 1 RADIUS Accounting Attributes

Attribute

Description

Calling-Station-ID

The output from this attribute displays the MAC address of the client.

Framed-IP-Address

The output from this attribute displays the IP address that is leased to the client.

Acct-Terminate-Cause

The output from this attribute displays the message “session-timeout” if a client does not explicitly disconnect.

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    aaa new-model

    4.    aaa group server radius group-name

    5.    server ip-address auth-port port-number acct-port port-number

    6.    exit

    7.    aaa accounting {system | network | exec | connection | commands level} {default | list-name} {start-stop | stop-only | none} [broadcast] group group-name

    8.    aaa session-id {common | unique}

    9.    ip radius source-interface type number [vrf vrf-name]

    10.    radius-server host {hostname | ip-address} [auth-port port-number] [acct-port port-number]

    11.    radius-server retransmit number-of-retries


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example:
    Router> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.

     
    Step 2 configure terminal


    Example:
    Router# configure terminal
     

    Enters global configuration mode.

     
    Step 3 aaa new-model


    Example:
    Router(config)# aaa new-model
     

    Enables the AAA access control model.

    • DHCP accounting functions only in the access control model.

    Note   

    TACACS and extended TACACS commands are not available after this command is configured and are not supported by DHCP accounting.

     
    Step 4 aaa group server radius group-name


    Example:
    Router(config)# aaa group server radius RGROUP-1
     

    Creates a server group for AAA or TACACS+ services and enters server group RADIUS configuration mode.

    • The server group is created in this step so that accounting services can be applied.

     
    Step 5 server ip-address auth-port port-number acct-port port-number


    Example:
    Router(config-sg-radius)# server 10.0.0.1 auth-port 1645 acct-port 1646
     

    Specifies the servers that are members of the server group that was created in Step 4.

    • You must open port numbers for authorization and accounting. 1645 is the default port number for authorization, and 1646 is the default port number for accounting. The range of port numbers that can be specified is from 0 to 65535.

    • The values entered for the auth-port port-numberand acct-port port-number keywords and arguments must match the port numbers that will be configured in Step 10.

     
    Step 6 exit


    Example:
    Router(config-sg-radius)# exit
     

    Exits server group RADIUS configuration mode and enters global configuration mode.

     
    Step 7 aaa accounting {system | network | exec | connection | commands level} {default | list-name} {start-stop | stop-only | none} [broadcast] group group-name


    Example:
    Router(config)# aaa accounting network RADIUS-GROUP1 start-stop group RGROUP-1
     

    Configures RADIUS accounting for the specified server group.

    • The RADIUS accounting server is specified in the first list-name argument (RADIUS-GROUP1), and the target server group is specified in the second group-nameargument (RGROUP-1).

    • This command enables start and stop accounting for DHCP accounting. The start-stop keyword enables the transmission of both START and STOP accounting messages. The stop-only keyword will enable the generation and verification of STOP accounting messages only.

     
    Step 8 aaa session-id {common | unique}


    Example:
    Router(config)# aaa session-id common
     

    Specifies whether the same session ID will be used for each AAA accounting service type within a call or whether a different session ID will be assigned to each accounting service type.

     
    Step 9 ip radius source-interface type number [vrf vrf-name]


    Example:
    Router(config)# ip radius source-interface Ethernet 0
     

    Forces RADIUS to use the IP address of the specified interface for all outgoing RADIUS packets.

     
    Step 10 radius-server host {hostname | ip-address} [auth-port port-number] [acct-port port-number]


    Example:
    Router(config)# radius-server host 10.1.1.1 auth-port 1645 acct-port 1646
     

    Specifies the RADIUS server host.

    • The values entered for the auth-port port-numberand acct-port port-number keywords and arguments must match the port numbers that were configured in Step 5.

     
    Step 11 radius-server retransmit number-of-retries


    Example:
    Router(config)# radius-server retransmit 3
     

    Specifies the number of times that Cisco IOS software will look for RADIUS server hosts.

     

    Troubleshooting Tips

    You can use the debug ip dhcp server packet and debug ip server events commands to troubleshoot the DHCP lease limit.

    Configuring DHCP Accounting

    Perform this task to configure DHCP accounting.

    DHCP accounting is enabled with the accounting command. This command configures DHCP to operate with AAA and RADIUS to enable secure START and STOP accounting messages. This configuration adds a layer of security that allows DHCP lease assignment and termination to be triggered for the appropriate RADIUS START and STOP accounting records so that the session state is properly maintained by upstream devices, such as the SSG.

    DHCP accounting is configured on a per-client or per-lease basis. Separate DHCP accounting processes can be configured on a per-pool basis.

    Before You Begin

    You must configure an SSG for client authentication. AAA and RADIUS must be enabled before DHCP accounting will operate.


    Note

    The following restrictions apply to DHCP accounting:

    • DHCP accounting can be configured only for DHCP network pools in which bindings are created automatically and destroyed upon lease termination or when the client sends a DHCPRELEASE message.

    • DHCP bindings are destroyed when the clear ip dhcp binding or no service dhcp commands are entered, which also triggers an accounting STOP message. You should exercise caution when entering these commands if a pool is configured with DHCP accounting, as these commands will clear active leases.

    SUMMARY STEPS

      1.    enable

      2.    configure terminal

      3.    ip dhcp pool pool-name

      4.    accounting method-list-name


    DETAILED STEPS
       Command or ActionPurpose
      Step 1 enable


      Example: 
      Device> enable
       

      Enables privileged EXEC mode.

      • Enter your password if prompted.

       
      Step 2 configure terminal


      Example: 
      Device# configure terminal
       

      Enters global configuration mode.

       
      Step 3 ip dhcp pool pool-name


      Example: 
      Device(config)# ip dhcp pool WIRELESS-POOL
       

      Configures a DHCP address pool and enters DHCP pool configuration mode.

       
      Step 4 accounting method-list-name


      Example: 
      Device(dhcp-config)# accounting RADIUS-GROUP1
       

      Enables DHCP accounting if the specified server group is configured to run RADIUS accounting.

      • The example configures DHCP accounting START and STOP messages to be sent if RADIUS-GROUP1 is configured as a start-stop group. STOP messages will only be sent if RADIUS-GROUP1 is configured as a stop-only group. See Step 7 in the "Configuring AAA and RADIUS for DHCP Accounting" configuration task table for more details.

       

      Verifying DHCP Accounting

      Perform this task to verify the DHCP accounting configuration.

      The debug radius, debug ip dhcp server events, debug aaa accounting, debug aaa id commands do not need to be issued together or in the same session as there are differences in the information that is provided. These commands, however, can be used to display DHCP accounting start and stop events, AAA accounting messages, and information about AAA and DHCP hosts and clients. See the "RADIUS Accounting Attributes" section of this module for a list of AAA attributes that have been introduced by DHCP accounting. The show running-config | begin dhcp command can be used to display the local DHCP configuration including the configuration of DHCP accounting.

      SUMMARY STEPS

        1.    enable

        2.    debug radius accounting

        3.    debug ip dhcp server events

        4.    debug aaa accounting

        5.    debug aaa id

        6.    show running-config | begin dhcp


      DETAILED STEPS
         Command or ActionPurpose
        Step 1 enable


        Example: 
        Device> enable
         

        Enables higher privilege levels, such as privileged EXEC mode.

        • Enter your password if prompted.

         
        Step 2 debug radius accounting


        Example: 
        Device# debug radius accounting
         

        Displays RADIUS events on the console of the device.

        • These events provide information about RADIUS processes. DHCP accounting information can be filtered with the accounting keyword. START and STOP accounting messages will be displayed in the output.

         
        Step 3 debug ip dhcp server events


        Example: 
        Device# debug ip dhcp server events
         

        Displays DHCP IP address assignments, DHCP lease expirations, and DHCP database changes.

         
        Step 4 debug aaa accounting


        Example: 
        Device# debug aaa accounting
         

        Displays AAA accounting events.

        • START and STOP accounting messages will be displayed in the output.

         
        Step 5 debug aaa id


        Example: 
        Device# debug aaa id
         

        Displays AAA events as they relate to unique AAA session IDs.

         
        Step 6 show running-config | begin dhcp


        Example: 
        Device# show running-config | begin dhcp
         

        The show running-config command is used to display the local configuration of the device. The sample output is filtered with the begin keyword to start displaying output at the DHCP section of the running configuration.

         

        Securing ARP Table Entries to DHCP Leases

        Perform this task to secure ARP table entries to DHCP leases in the DHCP database.

        When the update arp command is used, ARP table entries and their corresponding DHCP leases are secured automatically for all new leases and DHCP bindings. However, existing active leases are not secured. These leases are still insecure until they are renewed. When the lease is renewed, it is treated as a new lease and will be secured automatically. If this command is disabled on the DHCP server, all existing secured ARP table entries will automatically change to dynamic ARP entries.

        SUMMARY STEPS

          1.    enable

          2.    configure terminal

          3.    ip dhcp pool pool -name

          4.    update arp

          5.    renew deny unknown


        DETAILED STEPS
           Command or ActionPurpose
          Step 1 enable


          Example: 
          Device> enable
           

          Enables privileged EXEC mode.

          • Enter your password if prompted.

           
          Step 2 configure terminal


          Example: 
          Device# configure terminal
           

          Enters global configuration mode.

           
          Step 3 ip dhcp pool pool -name


          Example: 
          Device(config)# ip dhcp pool WIRELESS-POOL
           

          Configures a DHCP address pool and enters DHCP pool configuration mode.

           
          Step 4 update arp


          Example: 
          Device(dhcp-config)# update arp
           

          Secures insecure ARP table entries to the corresponding DHCP leases.

          • Existing active DHCP leases will not be secured until they are renewed. Using the no update arp command will change secured ARP table entries back to dynamic ARP table entries.

           
          Step 5 renew deny unknown


          Example: 
          Device(dhcp-config)# renew deny unknown
           

          (Optional) Configures the renewal policy for unknown clients.

           

          Troubleshooting Tips

          Use the following command to debug any errors that you may encounter when you configure DHCP to automatically generate a unique ID:

          • debug ip dhcp server packets

          Configuring DHCP Authorized ARP

          Perform this task to configure DHCP authorized ARP, which disables dynamic ARP learning on an interface.

          DHCP authorized ARP has a limitation in supporting accurate one-minute billing. DHCP authorized ARP probes for authorized users once or twice, 30 seconds apart. In a busy network the possibility of missing reply packets increases, which can cause a premature logoff. If you need a more accurate and finer control for probing of the authorized user, configure the arp probe interval command. This command specifies when to start a probe, the interval between unsuccessful probes, and the maximum number of retries before triggering an automatic logoff.


          Note

          If both static and authorized ARP are installing the same ARP entry, static configuration overrides authorized ARP. You can install a static ARP entry by using the arp global configuration command. You can only remove a nondynamic ARP entry by the same method in which it was installed.

          The ARP timeout period should not be set to less than 30 seconds. The feature is designed to send out an ARP message every 30 seconds, beginning 90 seconds before the ARP timeout period specified by the arp timeoutcommand. This behavior allows probing for the client at least three times before giving up on the client. If the ARP timeout is set to 60 seconds, an ARP message is sent twice, and if it is set to 30 seconds, an ARP message is sent once. An ARP timeout period set to less than 30 seconds can yield unpredictable results.

          >
          SUMMARY STEPS

            1.    enable

            2.    configure terminal

            3.    interface type number

            4.    ip address ip-address mask

            5.    arp authorized

            6.    arp timeout seconds

            7.    arp probe interval seconds count number

            8.    end

            9.    show arp


          DETAILED STEPS
             Command or ActionPurpose
            Step 1 enable


            Example:
            Router> enable
             

            Enables privileged EXEC mode.

            • Enter your password if prompted.

             
            Step 2 configure terminal


            Example:
            Router# configure terminal
             

            Enters global configuration mode.

             
            Step 3 interface type number


            Example:
            Router(config)# interface ethernet 1
             

            Configures an interface type and enters interface configuration mode.

             
            Step 4 ip address ip-address mask


            Example:
            Router(config-if)# ip address 209.165.200.224 209.165.200.224
             

            Sets a primary IP address for an interface.

             
            Step 5 arp authorized


            Example:
            Router(config-if)# arp authorized
             

            Disables dynamic ARP learning on an interface.

            • The IP address to MAC address mapping can be installed only by the authorized subsystem.

             
            Step 6 arp timeout seconds


            Example:
            Router(config-if)# arp timeout 60
             

            Configures how long an entry remains in the ARP cache.

             
            Step 7 arp probe interval seconds count number


            Example:
            Router(config-if)# arp probe interval 5 count 30
             

            (Optional) Specifies an interval, in seconds, and number of probe retries.

            • seconds --Interval, in seconds, after which the next probe will be sent to see if a peer is present. The range is from 1 to 10.

            • number --Number of probe retries. If there is no reply after the count has been reached, the peer has logged off. The range is from 1 to 60.

            Note   

            You must use the no form of the command to stop the probing process.

             
            Step 8 end


            Example:
            Router(config-if)# end
             

            Exits interface configuration mode and returns to privileged EXEC mode.

             
            Step 9 show arp


            Example:
            Router# show arp
             

            (Optional) Displays the entries in the ARP table.

             

            Configuring a DHCP Lease Limit to Globally Control the Number of Subscribers

            Perform this task to globally control the number of DHCP leases allowed for clients behind an ATM Routed Bridged Encapsulation (RBE) unnumbered interface or serial unnumbered interface.

            This feature allows an ISP to globally limit the number of leases available to clients per household or connection.

            If this feature is enabled on a Cisco IOS DHCP relay agent connected to clients through unnumbered interfaces, the relay agent keeps information about the DHCP leases offered to the clients per subinterface. When a DHCPACK message is forwarded to the client, the relay agent increments the number of leases offered to clients on that subinterface. If a new DHCP client tries to obtain an IP address and the number of leases has already reached the configured lease limit, DHCP messages from the client will be dropped and will not be forwarded to the DHCP server.

            If this feature is enabled on the Cisco IOS DHCP server directly connected to clients through unnumbered interfaces, the server allocates addresses and increments the number of leases per subinterface. If a new client tries to obtain an IP address, the server will not offer an IP address if the number of leases on the subinterface has already reached the configured lease limit.


            Note

            This feature is not supported on numbered interfaces. The lease limit can be applied only to ATM with RBE unnumbered interfaces or serial unnumbered interfaces.

            >
            SUMMARY STEPS

              1.    enable

              2.    configure terminal

              3.    ip dhcp limit lease log

              4.    ip dhcp limit lease per interface lease-limit

              5.    end

              6.    show ip dhcp limit lease [type number]


            DETAILED STEPS
               Command or ActionPurpose
              Step 1 enable


              Example:
              Router> enable
               

              Enables privileged EXEC mode.

              • Enter your password if prompted.

               
              Step 2 configure terminal


              Example:
              Router# configure terminal
               

              Enters global configuration mode.

               
              Step 3 ip dhcp limit lease log


              Example:
              Router(config)# ip dhcp limit lease log
               

              (Optional) Enables DHCP lease violation logging when a DHCP lease limit threshold is exceeded.

              • If this command is configured, any lease limit violations will display in the output of the show ip dhcp limit lease command.

               
              Step 4 ip dhcp limit lease per interface lease-limit


              Example:
              Router(config)# ip dhcp limit lease per interface 2
               

              Limits the number of leases offered to DHCP clients behind an ATM RBE unnumbered or serial unnumbered interface.

               
              Step 5 end


              Example:
              Router(config)# end
               

              Exits global configuration mode and returns to privileged EXEC mode.

               
              Step 6 show ip dhcp limit lease [type number]


              Example:
              Router# show ip dhcp limit lease
               

              (Optional) Displays the number of times the lease limit threshold has been violated.

              • You can use the clear ip dhcp limit lease privileged EXEC command to manually clear the stored lease violation entries.

               

              Troubleshooting Tips

              To verify the configuration, you can use the debug dhcp detail EXEC command to display the DHCP packets that were sent and received. To display the server side of the DHCP interaction, use the debug ip dhcp server packets command.

              Configuring a DHCP Lease Limit to Control the Number of Subscribers on an Interface

              Perform this task to limit the number of DHCP leases allowed on an interface.

              This feature allows an ISP to limit the number of leases available to clients per household or connection on an interface.

              If this feature is enabled on the Cisco IOS XE DHCP server directly connected to clients through unnumbered interfaces, the server allocates addresses and increments the number of leases per subinterface. If a new client tries to obtain an IP address, the server will not offer an IP address if the number of leases on the subinterface has already reached the configured lease limit.


              Note

              This feature is not supported on numbered interfaces. The lease limit can be applied only to ATM with RBE unnumbered interfaces or serial unnumbered interfaces.

              SUMMARY STEPS

                1.    enable

                2.    configure terminal

                3.    ip dhcp limit lease log

                4.    interface type number

                5.    ip dhcp limit lease lease-limit

                6.    end

                7.    show ip dhcp limit lease [type number]

                8.    show ip dhcp server statistics [type number]


              DETAILED STEPS
                 Command or ActionPurpose
                Step 1 enable


                Example: 
                Device> enable
                 

                Enables privileged EXEC mode.

                • Enter your password if prompted.

                 
                Step 2 configure terminal


                Example: 
                Device# configure terminal
                 

                Enters global configuration mode.

                 
                Step 3 ip dhcp limit lease log


                Example: 
                Device(config)# ip dhcp limit lease log
                 

                (Optional) Enables DHCP lease violation logging when a DHCP lease limit threshold is exceeded.

                • If this command is configured, any lease limit violations will display in the output of the show ip dhcp limit lease command.

                 
                Step 4 interface type number


                Example: 
                Device(config)# interface Serial0/0/0
                 

                Enters interface configuration mode.

                 
                Step 5 ip dhcp limit lease lease-limit


                Example: 
                Device(config-if)# ip dhcp limit lease 6
                 

                Limits the number of leases offered to DHCP clients per interface.

                • The interface configuration will override any global setting specified by the ip dhcp limit lease per interface global configuration command.

                 
                Step 6 end


                Example: 
                Device(config-if)# end
                 

                Exits the configuration mode and returns to privileged EXEC mode.

                 
                Step 7 show ip dhcp limit lease [type number]


                Example: 
                Device# show ip dhcp limit lease Serial0/0/0
                 

                (Optional) Displays the number of times the lease limit threshold has been violated.

                • You can use the clear ip dhcp limit lease privileged EXEC command to manually clear the stored lease violation entries.

                 
                Step 8 show ip dhcp server statistics [type number]


                Example: 
                Device# show ip dhcp server statistics Serial 0/0/0
                 

                (Optional) Displays DHCP server statistics.

                 

                Troubleshooting Tips

                Use the debug ip dhcp server class command to display the class matching results.

                Configuration Examples for DHCP Services for Accounting and Security

                Example Configuring AAA and RADIUS for DHCP Accounting

                The following example shows how to configure AAA and RADIUS for DHCP accounting: 

                aaa new-model
aaa group server radius RGROUP-1
 server 10.1.1.1 auth-port 1645 acct-port 1646
 exit
aaa accounting network RADIUS-GROUP1 start-stop group RGROUP-1
aaa session-id common
ip radius source-interface Ethernet 0
radius-server host 10.1.1.1 auth-port 1645 acct-port 1646
radius-server retransmit 3
exit

                Example Configuring DHCP Accounting

                DHCP accounting is configured on a per-client or per-lease basis. Separate DHCP accounting processes can be configured on a per-pool basis. The following example shows how to configure DHCP accounting START and STOP messages to be sent if RADIUS-GROUP1 is configured as a start-stop group:

                ip dhcp pool WIRELESS-POOL
 accounting RADIUS-GROUP1
 exit

                Example Verifying DHCP Accounting

                DHCP accounting is enabled after both RADIUS and AAA for DHCP are configured. DHCP START and STOP accounting generation information can be monitored with the debug radius accounting and debug ip dhcp server eventscommands. See the "RADIUS Accounting Attributes" task for a list of AAA attributes that have been introduced by DHCP accounting.

                The following is sample output from the debug radius accountingcommand. The output shows the DHCP lease session ID, the MAC address, and the IP address of the client interface. 

                00:00:53: RADIUS: Pick NAS IP for uid=2 tableid=0 cfg_addr=10.0.18.3 best_addr=0.0.0.0
00:00:53: RADIUS(00000002): sending
00:00:53: RADIUS(00000002): Send to unknown id 21645/1 10.1.1.1 :1646, Accounting-Request, len 76
00:00:53: RADIUS: authenticator C6 FE EA B2 1F 9A 85 A2 - 9A 5B 09 B5 36 B5 B9 27
00:00:53: RADIUS: Acct-Session-Id [44] 10 "00000002"
00:00:53: RADIUS: Framed-IP-Address [8] 6 10.0.0.10
00:00:53: RADIUS: Calling-Station-Id [31] 16 "00000c59df76"
00:00:53: RADIUS: Acct-Status-Type [40] 6 Start [1]
00:00:53: RADIUS: Service-Type [6] 6 Framed [2]
00:00:53: RADIUS: NAS-IP-Address [4] 6 10.0.18.3
00:00:53: RADIUS: Acct-Delay-Time [41] 6 0

                The following is sample output from the debug ip dhcp server eventscommand. The output was generated on a DHCP server and shows an exchange of DHCP messages between the client and server to negotiate a DHCP lease. The acknowledgment that confirms to the DHCP server that the client has accepted the assigned IP address triggers the accounting START message. It is shown in the last line of the following output: 

                00:45:50:DHCPD:DHCPDISCOVER received from client
0063.6973.636f.2d30.3030.312e.3432.6339.2e65.6337.352d.4574.31 on
interface Ethernet0.
00:45:52:DHCPD:assigned IP address 10.10.10.16 to client
0063.6973.636f.2d30.3030.312e.3432.6339.2e65.6337.352d.4574.31.
00:45:52:DHCPD:Sending DHCPOFFER to client
0063.6973.636f.2d30.3030.312e.3432.6339.2e65.6337.352d.4574.31(10.10.10.16)
00:45:52:DHCPD:broadcasting BOOTREPLY to client 0001.42c9.ec75.
00:45:52:DHCPD:DHCPREQUEST received from client
0063.6973.636f.2d30.3030.312e.3432.6339.2e65.6337.352d.4574.31.
00:45:52:DHCPD:Sending DHCPACK to client
0063.6973.636f.2d30.3030.312e.3432.6339.2e65.6337.352d.4574.31
(10.10.10.16).
00:45:52:DHCPD:broadcasting BOOTREPLY to client 0001.42c9.ec75.
00:45:52:DHCPD:triggered Acct Start for 0001.42c9.ec75 (10.10.10.16).

                The following is sample output from the debug ip dhcp server eventscommand. The output was generated on a DHCP server and shows the receipt of an explicit release message from the DHCP client. The DHCP server triggers an accounting STOP message and then returns the IP address to the DHCP pool. Information about the accounting STOP message is shown in the third line of the following output: 

                00:46:26:DHCPD:DHCPRELEASE message received from client
0063.6973.636f.2d30.3030.312e.3432.6339.2e65.6337.352d.4574.31 (10.10.10.16)
00:46:26:DHCPD:triggered Acct Stop for (10.10.10.16).
00:46:26:DHCPD:returned 10.10.10.16 to address pool WIRELESS-POOL.

                Example Configuring DHCP Authorized ARP

                Router 1 is the DHCP server that assigns IP addresses to the routers that are seeking IP addresses, and Router 2 is the DHCP client configured to obtain its IP address through the DHCP server. Because the update arp DHCP pool configuration command is configured on Router 1, the router will install a secure ARP entry in its ARP table. The arp authorized command stops any dynamic ARP on that interface. Router 1 sends periodic ARPs to Router 2 to make sure that the client is still active. Router 2 responds with an ARP reply. Unauthorized clients cannot respond to these periodic ARPs. The unauthorized ARP responses are blocked at the DHCP server. The timer for the entry is refreshed on Router 1 upon receiving the response from the authorized client.

                See the figure below for a sample topology.

                Figure 1. Sample Topology for DHCP Authorized ARP

                Router 1 (DHCP Server)

                ip dhcp pool name1
 network 10.0.0.0 255.255.255.0
 lease 0 0 20
 update arp
!
interface Ethernet 0
 ip address 10.0.0.1 255.255.255.0
 half-duplex
 arp authorized
 arp timeout 60
! optional command to adjust the periodic ARP probes sent to the peer
 arp probe interval 5 count 15

                Router 2 (DHCP Client)

                interface Ethernet 0/0
 ip address dhcp
 half-duplex

                Example Verifying DHCP Authorized ARP

                The following is sample output from the show arp command on Router 1 (see the figure above): 

                Router1# show arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.0.0.3                0   0004.dd0c.ffcb  ARPA   Ethernet01
Internet  10.0.0.1                -   0004.dd0c.ff86  ARPA   Ethernet0

                The following is sample output from the show arp command on Router 2 (see the figure above): 

                Router2# show arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.0.0.3                -   0004.dd0c.ffcb  ARPA   Ethernet0/02
Internet  10.0.0.1                0   0004.dd0c.ff86  ARPA   Ethernet0/0

                Example Configuring a DHCP Lease Limit

                In the following example, if more than three clients try to obtain an IP address from ATM interface 4/0.1, the DHCPDISCOVER packets will not be forwarded to the DHCP server. If the DHCP server resides on the same router, DHCP will not reply to more than three clients.

                ip dhcp limit lease per interface 3
!
interface loopback 0
 ip address 10.1.1.129 255.255.255.192
!
interface ATM 4/0.1
 no ip address
!
interface ATM 4/0.1 point-to-point
 ip helper-address 172.16.1.2
 ip unnumbered loopback 0
 atm route-bridged ip
  pvc 88/800
  encapsulation aal5snap

                In the following example, five DHCP clients are allowed to receive IP addresses. If a sixth client tries to obtain an IP address, the DHCPDISCOVER messages will not be forwarded to the DHCP server and a trap will be sent to the SNMP manager.

                ip dhcp limit lease log
!
ip dhcp pool pool1
 network 10.1.1.0 255.255.255.0
!
interface loopback 0
 ip address 10.1.1.1 255.255.255.0
!
interface serial 0/0.2 point-to-point
 ip dhcp limit lease 5
 ip unnumbered loopback 0
 exit
snmp-server enable traps dhcp interface

                Additional References

                Related Documents

                Related Topic

                Document Title

                Cisco IOS Commands

                Cisco IOS Master Command List, All Releases

                DHCP commands: complete command syntax, command mode, command history, defaults, usage guidelines, and examples

                Cisco IOS IP Addressing Services Command Reference

                DHCP conceptual information

                “DHCP Overview” module

                DHCP relay agent configuration

                “Configuring the Cisco IOS XE DHCP Relay Agent” module

                DHCP client configuration

                “Configuring the Cisco IOS XE DHCP Client” module

                DHCP On-Demand Address Pool Manager

                “Configuring the DHCP On-Demand Address Pool Manager” module

                Standards and RFCs

                Standard/RFC

                Title

                RFC 951

                Bootstrap Protocol (BOOTP)

                RFC 1542

                Clarifications and Extensions for the Bootstrap Protocol

                RFC 2131

                Dynamic Host Configuration Protocol

                RFC 2132

                DHCP Options and BOOTP Vendor Extensions

                Technical Assistance

                Description

                Link

                The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies.

                To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds.

                Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.

                http:/​/​www.cisco.com/​techsupport

                Technical Assistance

                Description

                Link

                The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies.

                To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds.

                Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.

                http:/​/​www.cisco.com/​techsupport

                Feature Information for DHCP Services for Accounting and Security

                The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

                Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to . An account on Cisco.com is not required.
                Table 2 Feature Information for DHCP Services for Accounting and Security

                Feature Name

                Releases

                Feature Information

                DHCP per Interface Lease Limit and Statistics

                12.2(33)SRC

                This feature limits the number of DHCP leases offered to DHCP clients on an interface. DHCP server statistics reporting was enhanced to display interface-level statistics.

                The following commands were introduced or modified by this feature: clear ip dhcp limit lease, ip dhcp limit lease, ip dhcp limit lease log, show ip dhcp limit lease, show ip dhcp server statistics.

                DHCP Lease Limit per ATM RBE Unnumbered Interface

                12.2(28)SB 12.3(2)T 15.1(1)S

                This feature limits the number of DHCP leases per subinterface offered to DHCP clients connected from an ATM RBE unnumbered interface or serial unnumbered interface of the DHCP server or DHCP relay agent.

                The following command was introduced by this feature: ip dhcp limit lease per interface.

                ARP Auto-logoff

                12.3(14)T

                The ARP Auto-logoff feature enhances DHCP authorized ARP by providing finer control and probing of authorized clients to detect a logoff.

                The following command was introduced by this feature: arp probe interval.

                DHCP Authorized ARP

                12.2(33)SRC 12.3(4)T

                DHCP authorized ARP enhances the DHCP and ARP components of the Cisco IOS software to limit the leasing of IP addresses to mobile users to authorized users. This feature enhances security in PWLANs by blocking ARP responses from unauthorized users at the DHCP server.

                The following command was introduced by this feature: arp authorized.

                DHCP Accounting

                12.2(15)T 12.2(28)SB 12.2(33)SRB

                DHCP accounting introduces AAA and RADIUS support for DHCP configuration.

                The following command was introduced by this feature: accounting.

                DHCP Secured IP Address Assignment

                12.2(15)T 12.2(28)SB 12.2(33)SRC

                DHCP secure IP address assignment provides the capability to secure ARP table entries to DHCP leases in the DHCP database. This feature secures and synchronizes the MAC address of the client to the DHCP binding, preventing hackers or unauthorized clients from spoofing the DHCP server and taking over a DHCP lease of an authorized client.

                The following commands were introduced or modified by this feature: show ip dhcp server statistics, update arp.