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ARP Packet Format
Address Resolution Protocol (ARP) is a protocol used to map a network address (such as an IP address) to a physical address (such as a MAC address). This is necessary because while IP addresses are used to route packets through a network, they are not directly usable by the physical layer of a network. Instead, packets are sent using MAC addresses, which are unique identifiers assigned to network interfaces.
The ARP packet format is used to request and provide information about the mapping of IP addresses to MAC addresses. This article will provide an overview of the ARP packet format, including its structure, fields, and examples of how it is used.
Structure of an ARP Packet
An ARP packet is made up of several fields, each of which contains specific information about the packet. The main fields of an ARP packet are −
Hardware type − This field specifies the type of hardware used on the network, such as Ethernet.
Protocol type − This field specifies the type of protocol used on the network, such as IPv4.
Hardware address length − This field specifies the length of the hardware address, in bytes.
Protocol address length − This field specifies the length of the protocol address, in bytes.
Operation − This field specifies the type of ARP operation being performed, such as request or reply.
Sender hardware address − This field contains the hardware address of the sender of the packet.
Sender protocol address − This field contains the protocol address of the sender of the packet.
Target hardware address − This field contains the hardware address of the target of the packet.
Target protocol address − This field contains the protocol address of the target of the packet.
Examples of ARP Packets
ARP Request Packet
When a device wants to find the MAC address associated with a specific IP address, it sends an ARP request packet. The packet includes the sender's hardware and protocol addresses, as well as the target protocol address. The target hardware address is set to all zeroes, indicating that the device is requesting the information.
For example, if a device with the IP address 192.168.1.100 wants to find the MAC address associated with the IP address 192.168.1.200, it would send an ARP request packet with the following fields −
Hardware type − Ethernet (0x0001)
Protocol type − IPv4 (0x0800)
Hardware address length − 6 bytes
Protocol address length − 4 bytes
Operation − Request (0x0001)
Sender hardware address − 00:11:22:33:44:55 (the device's MAC address)
Sender protocol address − 192.168.1.100 (the device's IP address)
Target hardware address − 00:00:00:00:00:00 (requesting the MAC address)
Target protocol address − 192.168.1.200 (the IP address being requested)
ARP Reply Packet
When a device receives an ARP request packet, it checks to see if the target protocol address matches its own IP address. If it does, the device sends an ARP reply packet containing its own hardware and protocol addresses, as well as the sender's protocol address.
For example, if a device with the IP address 192.168.1.200 receives the ARP request packet described above, it would send an ARP reply packet with the following fields −
Hardware type − Ethernet (0x0001)
Protocol type − IPv4 (0x0800)
Hardware address length − 6 bytes
Protocol address length − 4 bytes
Operation − Reply (0x0002)
Sender hardware address − 66:77:88:99:AA:BB (the device's MAC address)
Sender protocol address − 192.168.1.200 (the device's IP address)
Target hardware address − 00:11:22:33:44:55 (the sender's MAC address)
Target protocol address − 192.168.1.100 (the sender's IP address)
In this example, the device with IP address 192.168.1.200 responds to the ARP request by providing its own MAC address (66:77:88:99:AA:BB) as the mapping for its IP address (192.168.1.200). The device also includes the sender's IP and MAC addresses in the packet so that the sender can update its ARP cache.
ARP Spoofing Attack
ARP spoofing is a type of attack in which a malicious device sends fake ARP packets to a network, mapping its own MAC address to the IP address of another device on the network. This allows the attacker to intercept traffic intended for the other device, potentially allowing them to steal sensitive information or launch other attacks.
For example, a malicious device may send an ARP reply packet with the following fields −
Hardware type − Ethernet (0x0001)
Protocol type − IPv4 (0x0800)
Hardware address length − 6 bytes
Protocol address length − 4 bytes
Operation − Reply (0x0002)
Sender hardware address − AA:BB:CC:DD:EE:FF (the attacker's MAC address)
Sender protocol address − 192.168.1.200 (the IP address of a legitimate device on the network)
Target hardware address − 00:11:22:33:44:55 (the sender's MAC address)
Target protocol address − 192.168.1.100 (the sender's IP address)
In this example, the attacker is sending a fake ARP reply packet that maps its own MAC address (AA:BB:CC:DD:EE:FF) to the IP address of a legitimate device on the network (192.168.1.200). This allows the attacker to intercept traffic intended for the legitimate device, potentially allowing them to steal sensitive information or launch other attacks.
Conclusion
The ARP packet format is a crucial component of the Address Resolution Protocol, allowing devices on a network to map IP addresses to MAC addresses. Understanding the structure and fields of ARP packets is important for understanding how ARP works and for identifying and defending against ARP spoofing attacks. By understanding the ARP packet format and its usage in different types of ARP packets, network administrators and security professionals can ensure the security and functionality of their networks.
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