Can a Router Detect the MAC Address of Nearby Unconnected Devices?

When it comes to detecting the MAC addresses of nearby devices that have not connected to a router, the answer is not straightforward. Understanding the nuances and specifics of how routers and devices communicate in a wireless network environment can help clarify this.

Understanding MAC Addresses in Router Detection

MAC (Media Access Control) addresses are unique identifiers assigned to network interface controllers (NIC) for communication within a local network. Routers use these addresses to identify and manage devices on their network. However, the question of whether a router can obtain the MAC addresses of nearby devices that have not connected to it is more complex.

Active Connections

A router can certainly see the MAC addresses of devices that are actively connected to its network. This is because devices communicate with the router, allowing it to log and categorize their MAC addresses.

Probe Requests

Devices often send out probe requests when they are searching for Wi-Fi networks. These requests can be monitored by nearby routers, and this can potentially capture the MAC addresses of devices in the vicinity. However, this is limited to devices that are actively looking for connections, which is not a guarantee for devices that are not yet connected.

Passive Scanning

In some cases, routers can perform passive scanning, where they listen for any wireless traffic around them. This includes MAC addresses from devices that are communicating with other networks. However, it won’t detect devices that are completely inactive or turned off since they won’t be sending any data.

Privacy Considerations

Many modern devices come with built-in privacy features that randomize their MAC addresses when searching for networks. This further limits a router's ability to identify such devices. Additionally, devices may use techniques like Wi-Fi MAC randomization, which further complicates the detection process.

Practical Applications and Examples

Several real-world examples can illustrate how these principles play out. For instance, Mearki CMX Analytics relies on the transmission of probe requests by 802.11 wireless clients. These devices send probe requests at least once a minute, which contain the client's MAC address. Any listening device (including other Wi-Fi routers) can collect this information.

If a router analyzes any incoming radio traffic, it can potentially detect and log the MAC addresses of devices. This is particularly true for devices that are actively sending out probe requests. However, this detection is not guaranteed for devices that are not currently engaged in network activity.

Additionally, connecting to a device with TCP will cause the router to populate its ARP (Address Resolution Protocol) table with the MAC address. This also allows the router to detect the MAC address through broadcast ARP replies, such as Gratuitous ARPs (GARPs), even if the device has never connected to the router.

Final Summary

In summary, a router can certainly capture the MAC addresses of devices that are actively seeking connections or communicating. However, it cannot obtain the MAC addresses of devices that are not engaged in any network activity. Active device connections and probe requests provide the router with the necessary information, while privacy features and passive scanning methods create significant barriers for detection in the absence of active network activity.

Keywords: Router, MAC Address Detection, Wi-Fi Probe Requests