IP Addressing: DHCP Configuration Guide, Cisco IOS XE Release 3S (Cisco ASR 1000)
Configuring DHCP Services for Accounting and Security
Configuring DHCP Services for Accounting and Security
Last Updated: December 13, 2012
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
Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.
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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.
How to Configure DHCP Services for Accounting and Security
Configuring AAA and RADIUS for DHCP Accounting
Perform this task to configure 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.
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.
To monitor and troubleshoot the configuration of RADIUS accounting, use the following command:
Configuring DHCP Accounting
Perform this task to configure DHCP accounting.
DHCP accounting is enabled with the accounting DHCP pool configuration 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 BeginSUMMARY STEPS
You must configure an SSG for client authentication. AAA and RADIUS must be enabled before DHCP accounting will operate.
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.
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.
2. configure terminal
3. ip dhcp pool pool -name
4. update arp
5. renew deny unknown
In some usage scenarios, such as a wireless hotspot, where both DHCP and secure ARP are configured, a connected client device might go to sleep or suspend for a period of time. If the suspended time period is greater than the secure ARP timeout (default of 91 seconds), but less than the DHCP lease time, the client can awake with a valid lease, but the secure ARP timeout has caused the lease binding to be removed because the client has been inactive. When the client awakes, the client still has a lease on the client side but is blocked from sending traffic. The client will try to renew its IP address but the DHCP server will ignore the request because the DHCP server has no lease for the client. The client must wait for the lease to expire before being able to recover and send traffic again.
To remedy this situation, use the renew deny unknown command in DHCP pool configuration mode. This command forces the DHCP server to reject renewal requests from clients if the requested address is present at the server but is not leased. The DHCP server sends a DHCPNAK denial message to the client, which forces the client back to its initial state. The client can then negotiate for a new lease immediately, instead of waiting for its old lease to expire.
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.
2. configure terminal
3. ip dhcp limit lease log
4. interface type number
5. ip dhcp limit lease lease-limit
7. show ip dhcp limit lease [type number]
8. show ip dhcp server statistics [type number]
Configuration Examples for DHCP Services for Accounting and Security
Configuring AAA and RADIUS for DHCP Accounting Example
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 GigabitEthernet0/0/0 radius-server host 10.1.1.1 auth-port 1645 acct-port 1646 radius-server retransmit 3 exit
Configuring DHCP Accounting Example
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
Verifying DHCP Accounting Example
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" section of this module 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  10 "00000002" 00:00:53: RADIUS: Framed-IP-Address  6 10.0.0.10 00:00:53: RADIUS: Calling-Station-Id  16 "00000c59df76" 00:00:53: RADIUS: Acct-Status-Type  6 Start  00:00:53: RADIUS: Service-Type  6 Framed  00:00:53: RADIUS: NAS-IP-Address  6 10.0.18.3 00:00:53: RADIUS: Acct-Delay-Time  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 GigabitEthernet0/0/0. 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.
Configuring a DHCP Lease Limit Examples
In the following example, 5 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 loopback0 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 loopback0 exit snmp-server enable traps dhcp interface
The following sections provide references related to configuring DHCP services for accounting and security.
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.
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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.
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