Question about dynamic ARP inspection

[pelican]

Hey, I am currently studying for CCNA and I could not get my head around why DAI would need to inspect the destination MAC address to mitigate ARP attacks since the destination MAC address should always be the victim. Any help in understanding a scenario where an attack could take place by modifying the destination MAC would be greatly appreciated.

[Sheraz.Salim]

Understanding the scenario where an attack could take place by modifying the destination MAC address requires some context about Address Resolution Protocol (ARP) attacks and the purpose of Dynamic ARP Inspection (DAI).

In an ARP attack, the attacker sends falsified ARP messages on the local network, associating their own MAC address with the IP address of another device (typically the default gateway or another important network device). This type of attack is known as ARP spoofing or ARP poisoning.

Dynamic ARP Inspection (DAI) is a security feature implemented on some network devices, including Cisco switches, to mitigate ARP attacks. DAI inspects ARP packets on the network and verifies the validity of the ARP messages before updating the ARP cache on the switch.

Now, let's consider a scenario where the attacker modifies the destination MAC address in an ARP attack:

1. Attacker's MAC address: AA:AA:AA:AA:AA:AA
2. Victim's MAC address: BB:BB:BB:BB:BB:BB
3. Gateway's MAC address: CC:CC:CC:CC:CC:CC

Here's a step-by-step breakdown:

1. The attacker sends falsified ARP messages to the network, claiming that their MAC address (AA:AA:AA:AA:AA:AA) is associated with the IP address of the default gateway (e.g., 192.168.1.1). The destination MAC address in these ARP messages is set to the MAC address of the victim device (BB:BB:BB:BB:BB:BB).

2. When the victim device receives these malicious ARP messages, it updates its ARP cache with the attacker's MAC address (AA:AA:AA:AA:AA:AA) for the IP address of the gateway (e.g., 192.168.1.1). Now, whenever the victim wants to communicate with the gateway, it will send packets to the attacker's MAC address instead of the actual gateway.

3. At this point, the attacker can intercept and manipulate the traffic between the victim and the gateway. They can eavesdrop on the traffic, modify it, or even launch more sophisticated attacks like man-in-the-middle attacks.

By inspecting the destination MAC address in ARP packets, DAI can detect such ARP attacks. DAI compares the destination MAC address in the ARP packet with the actual MAC address of the device that originated the ARP request. If there is a mismatch, indicating an ARP spoofing attempt, DAI can take actions such as dropping the ARP packet or logging the event to mitigate the attack.

In summary, DAI inspects the destination MAC address in ARP packets to verify the consistency between the sender's MAC address and the destination MAC address. By doing so, it helps detect and prevent ARP spoofing attacks, protecting the integrity and security of the network.

For more reading

https://ipcisco.com/lesson/dynamic-arp-inspection/

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4500/12-2/25ew/configuration/guide/conf/dynarp.html

[pelican]

Thanks so much for the in-depth reply, just wanted to check that when the victim sends out traffic after the ARP cache has been poisoned(i.e. step 3), does DAI inspect the MAC addresses there? Because in steps 1 and 2, the destination MAC of the victim stays the same and if DAI were to inspect that, there wouldn't be a mismatch. At least based on my understanding

[Sheraz.Salim]

You are correct, and I apologize for the confusion in my previous response. DAI does not typically inspect the destination MAC address in the victim's outgoing traffic. The primary purpose of DAI is to inspect and validate ARP packets, specifically the source MAC address and IP address, to prevent ARP spoofing attacks.

Once the ARP cache of the victim device has been poisoned and it starts sending traffic, DAI's role is not to inspect the destination MAC address in that traffic. Instead, the destination MAC address in the victim's outgoing traffic will still be the MAC address of the attacker (as set during the ARP poisoning).

DAI's primary focus is on preventing the initial ARP spoofing attack by validating the source MAC address and IP address in the ARP packets. By comparing the source MAC address in the ARP packet with the actual MAC address of the device that originated the ARP request, DAI can detect the inconsistency and take appropriate actions.

In summary, while DAI does not directly inspect the destination MAC address in the victim's outgoing traffic, its main purpose is to validate the source MAC address in ARP packets to prevent ARP spoofing attacks from occurring in the first place.

[pelican]

Thanks again for the reply, do you happen to know why in the netacad resources provided by cisco themself, they state that the destination address can be inspected?

[MHM Cisco World]

Good Q, 
I will check my note