- DHCP Overview
- Configuring the Cisco IOS DHCP Server
- DHCP Server MIB
- Configuring the DHCP Server On-Demand Address Pool Manager
- DHCP Server RADIUS Proxy
- Configuring the Cisco IOS DHCP Relay Agent
- Configuring the Cisco IOS DHCP Client
- DHCP Option 82 Configurable Circuit ID and Remote ID
- Configuring DHCP Services for Accounting and Security
- Configuring DHCP Enhancements for Edge Session Management
- ISSU and SSO--DHCP High Availability Features
- DHCP Option 82 Support for Routed Bridge Encapsulation
- DHCPv6 Bulk-Lease Query
- DHCPv6 Relay and Server - MPLS VPN Support
- DHCPv6 Relay Source Configuration
- IPv6 Access Services: DHCPv6 Relay Agent
- DHCPv6 Server Stateless Autoconfiguration
- IPv6 Access Services: Stateless DHCPv6
- DHCPv6 Repackaging
- IPv6 Access Services: DHCPv6 Prefix Delegation
- DHCPv6 Guard
- Finding Feature Information
- Prerequisites for Configuring DHCP Services for Accounting and Security
- Information About DHCP Services for Accounting and Security
- How to Configure DHCP Services for Accounting and Security
- Configuring AAA and RADIUS for DHCP Accounting
- Configuring DHCP Accounting
- Verifying DHCP Accounting
- Securing ARP Table Entries to DHCP Leases
- Configuring DHCP Authorized ARP
- Configuring a DHCP Lease Limit to Globally Control the Number of Subscribers
- Configuring a DHCP Lease Limit to Control the Number of Subscribers on an Interface
Configuring DHCP Services for Accounting and Security
Cisco IOS 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
- Prerequisites for Configuring DHCP Services for Accounting and Security
- Information About DHCP Services for Accounting and Security
- How to Configure DHCP Services for Accounting and Security
- Configuration Examples for DHCP Services for Accounting and Security
- Additional References
- Technical Assistance
- Feature Information for 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.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
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
- Security Vulnerabilities in Public Wireless LANs
- DHCP Services for Security and Accounting Overview
- DHCP Lease Limits
DHCP Operation in Public Wireless LANs
The configuration of DHCP in a 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 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 hackers or unauthorized clients from gaining illegal entry to the network by spoofing authorized DHCP leases.
Three other features have been designed and implemented to address the security concerns in PWLANs. The first feature secures ARP table entries to DHCP leases in the DHCP database. The secure ARP functionality prevents IP spoofing by synchronizing the database of the DHCP server with the ARP table to avoid address hijacking. Secure ARP 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 second feature is DHCP authorized ARP. This functionality provides a complete solution by addressing the need for DHCP to explicitly know when a user logs out. Before the introduction of DHCP authorized ARP, there was no mechanism to inform the DHCP server if a user had left the system ungracefully, which could result in excessive billing for a customer that had logged out but without the system detecting it. To prevent this problem, DHCP authorized ARP sends periodic ARP messages on a per-minute basis to determine if a user is still logged in. Only authorized users can respond to the ARP request. ARP responses from unauthorized users are blocked at the DHCP server, providing an extra level of security.
In addition, DHCP authorized ARP disables dynamic ARP learning on an interface. The address mapping can be installed only by the authorized component specified by the arp authorized interface configuration command. DHCP is the only authorized component allowed to install ARP entries.
The third feature is ARP Auto-logoff, 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 frequently 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 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
- Configuring DHCP Accounting
- Verifying DHCP Accounting
- Securing ARP Table Entries to DHCP Leases
- Configuring DHCP Authorized ARP
- Configuring a DHCP Lease Limit to Globally Control the Number of Subscribers
- Configuring a DHCP Lease Limit to Control the Number of Subscribers on an Interface
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. |
DETAILED STEPS
Troubleshooting Tips
To monitor and troubleshoot the configuration of RADIUS accounting, use the following command:
debug radius accounting
Configuring DHCP Accounting
Perform this task to configure DHCP accounting.
DHCP accounting is enabled with the accountingDHCP 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.
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:
|
DETAILED STEPS
| Command or Action | Purpose | |
|---|---|---|
|
|
Example: Router> enable |
Enables privileged EXEC mode.
|
|
|
Example: Router# configure terminal |
Enters global configuration mode. |
|
|
Example: Router(config)# ip dhcp pool WIRELESS-POOL |
Configures a DHCP address pool and enters DHCP pool configuration mode. |
|
|
Example: Router(config-dhcp)# accounting RADIUS-GROUP1 |
Enables DHCP accounting if the specified server group is configured to run RADIUS accounting.
|
Verifying DHCP Accounting
Perform this task to verify the DHCP accounting configuration.
The debug radius, debug radius accounting, debug ip dhcp server events, debug aaa accounting, and debug aaa idcommands need not be issued together or in the same session because 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.
DETAILED STEPS
| Command or Action | Purpose | |
|---|---|---|
|
|
Example: Router> enable |
Enables privileged EXEC mode. |
|
|
Example: Router# debug radius accounting |
Displays RADIUS events on the console of the router. |
|
|
Example: Router# debug ip dhcp server events |
Displays DHCP IP address assignments, DHCP lease expirations, and DHCP database changes. |
|
|
Example: Router# debug aaa accounting |
Displays AAA accounting events. |
|
|
Example: Router# debug aaa id |
Displays AAA events as they relate to unique AAA session IDs. |
|
|
Example: Router# show running-config | begin dhcp |
Displays the local configuration of the router. |
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.
DETAILED STEPS
| Command or Action | Purpose | |
|---|---|---|
|
|
Example: Router> enable |
Enables privileged EXEC mode. |
|
|
Example: Router# configure terminal |
Enters global configuration mode. |
|
|
Example: Router(config)# ip dhcp pool WIRELESS-POOL |
Configures a DHCP address pool and enters DHCP pool configuration mode. |
|
|
Example: Router(config-dhcp)# update arp |
Secures insecure ARP table entries to the corresponding DHCP leases. |
|
|
Example: Router(config-dhcp)# renew deny unknown |
(Optional) Configures the renewal policy for unknown clients. |
Troubleshooting Tips
In some usage scenarios, such as a wireless hot spot, 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 awaken 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 awakens, 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 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. > |
DETAILED STEPS
| Command or Action | Purpose | |||
|---|---|---|---|---|
|
|
Example: Router> enable |
Enables privileged EXEC mode.
|
||
|
|
Example: Router# configure terminal |
Enters global configuration mode. |
||
|
|
Example: Router(config)# interface ethernet 1 |
Configures an interface type and enters interface configuration mode. |
||
|
|
Example: Router(config-if)# ip address 209.165.200.224 209.165.200.224 |
Sets a primary IP address for an interface. |
||
|
|
Example: Router(config-if)# arp authorized |
Disables dynamic ARP learning on an interface.
|
||
|
|
Example: Router(config-if)# arp timeout 60 |
Configures how long an entry remains in the ARP cache. |
||
|
|
Example: Router(config-if)# arp probe interval 5 count 30 |
(Optional) Specifies an interval, in seconds, and number of probe retries.
|
||
|
|
Example: Router(config-if)# end |
Exits interface configuration mode and returns to privileged EXEC mode. |
||
|
|
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. > |
DETAILED STEPS
| Command or Action | Purpose | |
|---|---|---|
|
|
Example: Router> enable |
Enables privileged EXEC mode.
|
|
|
Example: Router# configure terminal |
Enters global configuration mode. |
|
|
Example: Router(config)# ip dhcp limit lease log |
(Optional) Enables DHCP lease violation logging when a DHCP lease limit threshold is exceeded.
|
|
|
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. |
|
|
Example: Router(config)# end |
Exits global configuration mode and returns to privileged EXEC mode. |
|
|
Example: Router# show ip dhcp limit lease |
(Optional) Displays the number of times the lease limit threshold has been violated.
|
Troubleshooting Tips
You can use the debug ip dhcp server packet and debug ip server events commands to troubleshoot the DHCP lease limit.
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 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. > |
DETAILED STEPS
| Command or Action | Purpose | |
|---|---|---|
|
|
Example: Router> enable |
Enables privileged EXEC mode.
|
|
|
Example: Router# configure terminal |
Enters global configuration mode. |
|
|
Example: Router(config)# ip dhcp limit lease log |
(Optional) Enables DHCP lease violation logging when a DHCP lease limit threshold is exceeded.
|
|
|
Example: Router(config)# interface Seria l0/0 |
Enters interface configuration mode. |
|
|
Example: Router(config-if)# ip dhcp limit lease 6 |
Limits the number of leases offered to DHCP clients per interface.
|
|
|
Example: Router(config-if)# end |
Exits interface configuration mode and returns to privileged EXEC mode. |
|
|
Example: Router# show ip dhcp limit lease Serial 0/0 |
(Optional) Displays the number of times the lease limit threshold has been violated.
|
|
|
Example: Router# show ip dhcp server statistics Serial0/0 |
(Optional) Displays DHCP server statistics.
|
Troubleshooting Tips
You can use the debug ip dhcp server packet and debug ip server events commands to troubleshoot the DHCP lease limit.
Configuration Examples for DHCP Services for Accounting and Security
- Example Configuring AAA and RADIUS for DHCP Accounting
- Example Configuring DHCP Accounting
- Example Verifying DHCP Accounting
- Example Configuring DHCP Authorized ARP
- Example Verifying DHCP Authorized ARP
- Example Configuring a DHCP Lease Limit
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 |
|---|---|
| ARP commands: complete command syntax, command modes, defaults, usage guidelines, and examples |
Cisco IOS IP Addressing Services Command Reference |
| DHCP commands: complete command syntax, command modes, defaults, usage guidelines, and examples |
Cisco IOS IP Addressing Services Command Reference |
| DHCP conceptual information |
"DHCP Overview" module in the Cisco IOS IP Addressing Configuration Guide |
| DHCP server configuration |
"Configuring the Cisco IOS DHCP Server" module in the Cisco IOS IP Addressing Configuration Guide |
| DHCP ODAP configuration |
"Configuring the DHCP Server On-Demand Address Pool Manager" module in the Cisco IOS IP Addressing Configuration Guide |
| DHCP client configuration |
"Configuring the Cisco IOS DHCP Client" module in the Cisco IOS IP Addressing Configuration Guide |
| DHCP relay agent configuration |
"Configuring the Cisco IOS DHCP Relay Agent" module in the Cisco IOS IP Addressing Configuration Guide |
| DHCP enhancements for edge-session management |
"Configuring DHCP Enhancements for Edge-Session Management" module in the Cisco IOS IP Addressing Configuration Guide |
| AAA and RADIUS configuration tasks |
Cisco IOS Security Configuration Guide |
| AAA and RADIUS commands: complete command syntax, command mode, defaults, usage guidelines, and examples |
Cisco IOS Security Command Reference |
Standards
| Standards |
Title |
|---|---|
| No new or modified standards are supported by this functionality. |
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MIBs
| MIBs |
MIBs Link |
|---|---|
| No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature. |
To locate and download MIBs for selected platforms, Cisco software releases, and feature sets, use Cisco MIB Locator found at the following URL: |
RFCs
| RFCs |
Title |
|---|---|
| No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature. |
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Technical Assistance
| Description |
Link |
|---|---|
| The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password. |
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 www.cisco.com/go/cfn. 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. |
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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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