MAC Address Filtering

The Media Access Control, or MAC, Address is one of the most universally present identity features in computer networking. Whether your computer uses Internet Protocol (the default and preferred protocol) or IPX/SPX or NetBEUI (possible alternates), as its Layer 3/4 transport, each networking device on your computer will have a MAC Address. Some devices will even have 2 MAC addresses, and here's where a problem starts. Besides the Universally Administered Address (UAA), which is assigned to a network device when it is assembled at the factory, some devices will be assigned a Locally Administered Address (LAA) by the network administrators, when a network is being setup.

Setting up an LAA is trivial in nature. The hard part is deciding what address to use. Once you decide that, just run the Network Adapter Settings Wizard. Depending upon the vendor, the ability to assign a LAA will be somewhere in the wizard. For 3Com, for instance, the Advanced tab will have a value "Network Address". TYpe in the LAA that you wish to use on the adapter in question, hit the OK buttons a couple times, and you're good to go.

If you change the MAC address of the WAN connection on your NAT router, you're setting a LAA there.

One of the most common security selections, when you setup a router, is the ability to filter by client MAC address, and permit network access to a select few addresses. Like hiding the SSID beacon, filtering by MAC address is just another form of security by obscurity. It's similar in effect to disabling DHCP, and manually issuing IP addresses to all computers.

An attacker who is interested in connecting to your WiFi network has only to learn the MAC address of a device on your network, and assign the observed address. As described above, assigning an address is a trivial exercise; and learning an address is the same. Learning an address is simply a prerequisite in interesting exercises such as a Man In The Middle attack, or WEP cracking.

The bottom line? MAC address filtering is probably the lamest form of WiFi security that you can try. It's easy to do, but easy to bypass too.

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