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What technique is a security attack that depletes the pool of IP addresses available for legitimate hosts?

• reconnaissance attack
• DHCP spoofing
• DHCP snooping
• DHCP starvation

The correct answer is DHCP starvation.

DHCP Starvation: An In-Depth Analysis

Introduction to DHCP

The Dynamic Host Configuration Protocol (DHCP) is a network management protocol used on IP networks where a DHCP server automatically assigns an IP address and other network configuration parameters to each device on the network, enabling them to communicate with other IP networks. This allows devices (such as computers, phones, or IoT devices) to join a network without needing to manually assign an IP address, which is essential for scalability and ease of management. DHCP plays a crucial role in most modern networking environments.

What is DHCP Starvation?

DHCP starvation is a type of Denial of Service (DoS) attack that targets the DHCP server by depleting its pool of IP addresses. The attack aims to exhaust the available IP addresses in the DHCP scope so that legitimate devices cannot obtain a valid IP address, thus preventing them from accessing the network.

The attack is typically performed by an attacker or malicious device repeatedly sending forged DHCP requests with randomized MAC addresses. Since DHCP servers lease IP addresses based on MAC addresses, each request appears to come from a different client. As a result, the DHCP server leases out multiple IP addresses, believing they are going to different devices, until it runs out of available IPs. When legitimate devices try to obtain an IP address, the server has none left to assign, effectively denying network access to those devices.

How DHCP Starvation Works

The typical sequence of events during a DHCP starvation attack follows these steps:

1. Initial Phase:
   • A malicious device is connected to the network, or the attack is launched remotely if the attacker has access to the local area network (LAN).
   • The attacker begins by sending out a flood of DHCP requests. Each request has a unique (but likely spoofed) MAC address. The attacker uses tools like Yersinia or DHCPig to automate the process of generating these spoofed requests.
2. Server Response:
   • The DHCP server responds to each DHCP request by assigning an IP address to the MAC address specified in the request.
   • Every time the DHCP server receives a new request, it allocates a new IP address from its available pool, as the server perceives that a new device has joined the network.
3. Address Pool Depletion:
   • Eventually, the server’s available IP addresses are all leased out, and the IP pool is depleted. Legitimate devices that try to join the network and request an IP address are denied service because the DHCP server can no longer assign them an IP address.
4. Denial of Service:
   • Legitimate users are now unable to access the network. Since they cannot obtain an IP address, they are effectively cut off from using network resources such as file servers, internet access, or any internal services.

This attack can significantly disrupt network operations, especially in environments that rely heavily on DHCP for device connectivity, such as enterprise networks, campus environments, and even home networks.

Why DHCP Starvation is Effective

DHCP starvation is effective because the attacker does not need to break encryption or perform complex tasks. The attack exploits the basic functionality of how DHCP assigns IP addresses, making it easy to execute, especially for attackers with physical access to the network or compromised devices inside the network.

The scalability of DHCP also becomes a vulnerability, as servers designed to assign thousands of addresses can be overwhelmed by relatively simple scripts that send hundreds or thousands of requests per second. The attack can be carried out stealthily over time or in a matter of minutes, depending on the size of the address pool and the resources available to the attacker.

Consequences of DHCP Starvation

1. Network Downtime:
   • DHCP starvation can bring down entire networks by preventing legitimate devices from obtaining IP addresses. In environments where most or all devices rely on DHCP for network access, the attack can lead to widespread outages.
2. Business Disruption:
   • For enterprises, this attack can cause significant business disruption. Employees may be unable to access critical systems, services, or the internet. The resulting downtime can lead to loss of productivity, missed deadlines, and potentially lost revenue.
3. Increased Security Risks:
   • In some cases, the attacker might follow the DHCP starvation attack with a DHCP spoofing attack. After exhausting the IP pool, the attacker sets up their own rogue DHCP server to assign malicious configuration settings (e.g., assigning the attacker’s machine as the default gateway or DNS server). This allows the attacker to intercept network traffic (man-in-the-middle attack), redirect traffic to malicious sites, or perform further attacks such as data exfiltration.
4. Recovery Challenges:
   • Recovering from a DHCP starvation attack can be challenging, especially in large networks. Administrators may need to manually release IP addresses or restart DHCP services, which could involve downtime for legitimate users.

Defenses Against DHCP Starvation

Given the potential severity of a DHCP starvation attack, it’s essential for organizations to implement defenses to mitigate the risks:

1. DHCP Snooping:
   • One of the most effective countermeasures against DHCP starvation is enabling DHCP snooping on network switches. DHCP snooping acts as a firewall between untrusted hosts and the DHCP server. It ensures that only legitimate DHCP messages (from trusted ports or interfaces) can reach the DHCP server.
   • DHCP snooping maintains a database of IP-to-MAC address bindings and monitors the network for abnormal activity, such as the rapid issuance of multiple DHCP requests. If an attack is detected, the switch can block the offending device.
2. Rate Limiting:
   • Network administrators can configure rate limiting on network interfaces, limiting the number of DHCP requests that any single port can generate in a given time frame. This prevents an attacker from flooding the server with requests.
3. Port Security:
   • Configuring port security on network switches allows administrators to limit the number of MAC addresses that can be learned on a specific port. If a device tries to send DHCP requests from multiple spoofed MAC addresses, port security can block that port.
4. IP Address Allocation Management:
   • Administrators can partition the DHCP address pool into smaller subnets or reserve specific IP ranges for critical devices. This ensures that essential services (e.g., servers, routers) can continue to operate even during an attack.
5. Monitoring and Alerts:
   • Continuous monitoring of the network for unusual patterns of DHCP requests or unusually high address allocation rates can help detect and respond to DHCP starvation attacks in real-time.
6. Strong Network Access Controls:
   • Implementing Network Access Control (NAC) or using IEEE 802.1X authentication can prevent unauthorized devices from even attempting DHCP requests. Only authenticated devices are allowed onto the network, reducing the likelihood of a DHCP starvation attack.

Conclusion

DHCP starvation is a potent and disruptive attack that targets the availability of network resources by depleting the DHCP server’s pool of available IP addresses. This prevents legitimate devices from obtaining IP addresses and effectively disconnects them from the network. While the attack is relatively simple to execute, its consequences can be severe, leading to network outages, business disruptions, and further security vulnerabilities if not properly addressed.

Fortunately, organizations can implement a range of defenses, including DHCP snooping, port security, rate limiting, and monitoring, to mitigate the risks and protect their networks from such attacks. By understanding how DHCP starvation works and employing robust security practices, network administrators can help safeguard their environments against this and other related attacks.