How can DHCP leases be secured to prevent unauthorized network access?

1 What are DHCP leases?

A DHCP lease is the duration of time that a DHCP server grants a client the right to use a specific IP address and configuration. The lease time is determined by the DHCP server and can vary from minutes to days. When a lease expires, the client must renew it or request a new one. A DHCP lease ensures that the IP addresses are dynamically allocated and reused, avoiding conflicts and waste.

3 How can DHCP leases be hijacked?

A DHCP hijacking attack is when an attacker takes over an existing DHCP lease from a legitimate client and uses it to access the network. For example, the attacker can monitor the network traffic and identify a client that has a valid DHCP lease. Then, the attacker can send a DHCP release message to the server, pretending to be the client, and free up the IP address. Next, the attacker can request a new DHCP lease with the same IP address and configuration as the client. This way, the attacker can bypass network security policies, access restricted resources, or impersonate the client.

4 How can DHCP leases be exhausted?

A DHCP exhaustion attack is when an attacker floods the DHCP server with fake or excessive DHCP requests, depleting the pool of available IP addresses and configuration parameters. For example, the attacker can use a tool or script to generate random MAC addresses and request DHCP leases for each one. The DHCP server will assign leases to each request until it runs out of resources. This way, the attacker can deny service to legitimate clients, prevent them from joining the network, or cause network instability.

5 How can DHCP leases be secured?

To secure your DHCP leases, you need to implement measures on both the DHCP server and the clients. On the server side, DHCP snooping can filter and validate messages from clients to prevent rogue DHCP servers. IP source guard can bind IP addresses and MAC addresses of each client, while dynamic ARP inspection verifies ARP requests and replies between clients. Rate limiting controls the number and frequency of DHCP requests from clients, while DHCP authentication requires clients to present a valid certificate or token. On the client side, DHCP client validation verifies the identity of the DHCP server, while DHCP secure update encrypts and authenticates DNS updates. Firewall rules can also restrict incoming and outgoing traffic from the DHCP server. All of these measures can help prevent IP spoofing, ARP poisoning, man-in-the-middle attacks, DHCP exhaustion attacks, unauthorized access, DNS hijacking or poisoning, network intrusion or data leakage.

6 How can you monitor and audit your DHCP leases?

Monitoring and auditing your DHCP leases requires the use of tools and logs that can track and report the DHCP activity on your network. For instance, a network analyzer or sniffer can be used to capture and analyze the DHCP packets and messages, while a DHCP server management tool can help view and manage the DHCP leases, reservations, scopes, and options. Moreover, a DHCP server log can record and review the DHCP events, errors, and statistics, while a DHCP client log can record and review the DHCP requests, responses, and configuration. All of these tools are essential for optimizing DHCP performance, troubleshooting issues, verifying functionality, and ensuring security.

7 Here’s what else to consider

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