What is ARP Poisoning?

Introduction

The Address Resolution Protocol (ARP) is a widely used protocol that enables devices within a LAN to communicate by linking their IP addresses with their MAC addresses. However, this protocol has a flaw as it cannot verify the authenticity of received ARP messages. This vulnerability exposes it to a type of cyberattack called ARP poisoning or ARP spoofing.

In an ARP poisoning attack, a malicious individual sends falsified ARP messages into a network to disrupt, redirect, or monitor network traffic. By doing this, the attacker can associate their MAC address with the IP address of another device on the network, such as a router, server, or computer. The attacker’s primary purpose is to deceive devices by sending data packets to them instead of reaching their intended destination. In this blog, we will discuss ARP poisoning in detail, different ARP poisoning attacks, its detection, and its different prevention methods.

To understand how ARP poisoning works, we first have to understand how ARP operates. Below, we have discussed the overview of ARP for better understanding.

What is ARP?

ARP, or Address Routing Protocol, is a protocol that works on the data link layer of the OSI model. When we talk about its purpose, it allows devices within a Local Area Network to find and keep track of the connection between their IP addresses and MAC addresses.

When a device needs to communicate with another device, it must obtain the MAC address of that device. This is achieved through a two-step process:

1. The sending device broadcasts an ARP request packet to all devices on the LAN. The packet includes the senders’ IP and MAC addresses as well as the IP address of the desired recipient.
2. The target device responds by sending an ARP reply packet to the sender. This packet contains the target’s IP and MAC addresses.

Once received, the sender updates its ARP cache, which acts as a table storing mappings between IP addresses and MAC addresses of devices on the LAN. With this information, the sender can then directly send data packets to the target using its MAC address.

However, this process has a flaw; it lacks authentication for received ARP messages. Any device on the LAN can respond to an ARP request. Further, it can send an ARP reply without verifying if it is the intended recipient or not. This situation allows an intruder to manipulate the LAN potentially by sending ARP messages to devices, causing their ARP caches to be filled with incorrect mappings.

Now that we have a basic understanding of ARP and how it operates. Let’s now discuss the ARP poisoning.

What is ARP Poisoning?

ARP poisoning is a technique that allows an attacker to manipulate the ARP cache of a device. The ARP cache is a table that stores the mappings between IP addresses and MAC addresses of devices on the same network. The ARP cache is updated whenever a device sends or receives an ARP request or reply. An attacker can poison the ARP cache by sending spoofed ARP messages claiming to have another device’s MAC address on the network.

ARP poisoning can have a serious impact in terms of network security and performance. With ARP poisoning, one can:

• Hijack network traffic as well as intercepts sensitive information such as passwords, credit card details, or confidential documents.
• Deny service to legitimate users by blocking their packets.
• Launch other attacks such as phishing, man-in-the-middle, malware injection, or DNS spoofing.

Therefore, studying ARP poisoning and learning how to detect and prevent it is important.

Let’s now discuss how an ARP poisoning attack works.

How does an ARP poisoning attack work?

Firstly, the attacker monitors the network traffic in order to identify the IP and MAC addresses of the two devices that are communicating with each other. These devices can be a user’s computer and a router.

• The attacker then sends an unsolicited ARP reply packet to the user’s computer. The purpose is to claim that it has the router’s IP address and, in return, provides its own MAC address.
• After this, the user’s computer updates the ARP cache with false mapping and begins the process of sending its data packets to the attacker’s MAC address instead of the router’s MAC address.
• The attacker also sends an unsolicited ARP reply packet to the router. This again claims that it has the IP address of the user’s computer and, after that, provides its own MAC address.
• The router also updates its ARPO cache with the false mapping and starts the process of sending the packets to the attacker’s MAC address instead of the user’s MAC address.
• Once the attacker is done with the above steps, the attacker is now in a position where it places itself in the middle of the communication between the user’s computer and the router. Now, the attacker can easily intercept, modify, or drop data packets that pass through it.

This is how ARP Poisoning works.

Types of ARP Poisoning Attacks

After manipulating the ARP cache of devices on the local area network (LAN), attackers can easily carry out various types of attacks. Below, we have discussed the most common attacks.

Traffic Interception

Attackers can intercept and examine data packets passing through their devices. This allows them to steal information like passwords, credit card numbers, or confidential documents. Additionally, they can redirect the traffic to websites or servers designed for phishing or distributing malware.

Denial of Service (DoS) attacks

By blocking or discarding data packets that traverse their device, attackers can disrupt user’s access to network resources and services such as email, web pages, or file servers. This can lead to network downtime, performance issues, or even data loss.

Man in the middle attacks

Attackers gain the capability to modify or tamper with data packets flowing through their devices. Consequently, they can manipulate the content or behavior of network communication by altering website text or images, injecting code or commands, and even impersonating users or entities.

Now, the question that arises is how one can detect ARP poisoning.

How to Detect ARP Poisoning?

ARP poisoning is hard to detect using antivirus software or firewalls. However, there are some methods that can help to identify and monitor ARP poisoning on a network. Some of these are:

Passive Detection

With the help of passive detection, one can monitor the ARP traffic on the network and look for any inconsistencies in the ARP messages or cache entries. Let’s take an example for better understanding. Let’s take a device that receives multiple ARP replies from different MAC addresses for the same IP address.

It is also possible that the device receives an ARP reply without sending an ARP request. Both situations indicate an ARP poisoning attempt. Passive detection can be either done manually or using tools such as Wireshark or tcpdumb, or it can be done automatically using tools such as XArp or Arpwatch.

One can also compare their ARP caches with other devices on the network via CMD by typing arp -a. Below we have shown the image for better understanding.

If the table contains two separate IP addresses with the same MAC address, an ARP attack happens.

Active Detection

With the help of Active Detection, one can easily detect ARP poisoning by sending falsified ARP packets to the network and observing responses. For example, let’s take a device that sends an ARP request for an IP address that does not exist on the network, and it receives an ARP reply from another device, it simply indicates that the device is an attacker. Active detection can be done via using tools such as arp-scan or arping.

After detecting ARP poisoning, one has to act to prevent it. Below, we have explained how one can prevent ARP poisoning.

How to Prevent ARP Poisoning?

ARP poisoning results in network security threats as well as leads to an effect the performance. Hence, taking preventive measures to protect the network from such attacks is crucial. Some of the preventive measures one can take are:

Static ARP Entries

With the help of this method, one can manually configure the devices on the network with static ARP entries that further map their IP addresses to their MAC addresses. With the help of this, the devices no longer need to use ARP to discover each other’s MAC addresses and also do not rely on dynamic ARP cache updates.

This helps in preventing the attacker from poisoning the ARP cache with false mappings. Apart from all the merits this technique has, there are also some demerits. Some of the demerits include scalability issues, maintenance overhead, and lack of flexibility.

Dynamic Host Configuration Protocol (DHCP) Snooping

One can also prevent ARP poisoning through DHCP snooping. In this method, one uses a switch feature that assists in filtering DHCP messages on the network and also builds a database of IP-to-MAC mappings for each port on the switch. This way, the switch can validate any ARP messages that match these mappings and further discard any ARP messages that do not match these mappings. With the help of this technique, one can prevent an attacker from sending fake ARP messages to other devices on the network. However, this method requires a DHCP server and also a switch that supports DHCP snooping.

Network Segmentation

Using network segmentation or VLANs, one can isolate different parts of your network and limit the scope of ARP poisoning attacks.

This is the ways how one can prevent ARP Poisoning.

Frequently Asked Questions

Q1. What is ARP poisoning a method for?

ARP is a type of cyberattack for intercepting network traffic by sending falsified messages that associate the attacker’s MAC address with the IP address of a target device.

Q2. How is ARP poisoning prevented?

ARP poisoning can be prevented by various means. Some of these are:

• Static ARP entries
• DHCP Snooping
• Network Segmentation of VLANs

Q3. Is ARP poisoning a threat?

Yes, ARP poisoning is a threat to network security.

Q4. How do ARP poisoning and DNS poisoning attacks work?

ARP poisoning and DNS poisoning are network attacks that spoof IP or domain names to redirect traffic or steal data.

Conclusion

ARP poisoning is a type of cyberattack that can compromise network security and performance. By understanding how it works, the different types of attacks it can lead to, how one can detect it, and how to prevent it, one can protect their network from ARP poisoning and, in return, ensure network reliability and integrity.

Address Resolution Protocol is an integral part of CCNA Course. However, if you want to learn more about ARP Poisoning, you should opt for CCNP ENARSI Training.