Changing Your MAC Address In Window XP/Vista, Linux And Mac OS X : MAC spoofing

First let me explain a few things about MAC addresses. MAC stands for Media Access Control and in a sense the MAC address is a computer's true name on a LAN. An Ethernet MAC address is a six byte number, usually expressed as a twelve digit hexadecimal number (Example: 1AB4C234AB1F).

IPs are translated to MAC address by a protocol called ARP (Address Resolution Protocol). Let's say a computer with and IP of 192.168.1.1 wants to send information to another computer on the LAN that has an IP of 192.168.1.2 . First 192.168.1.1 will send out a broadcast to all stations on the LAN asking who has the IP 192.168.1.2. Then the box that has 192.168.1.2 will respond to 192.168.1.1 with it's MAC address which is cached in 192.168.1.1's ARP table for later use. To put this in Socratic Dialog form (with just a touch of Stallone):

Host 1 (192.168.1.1): Yo everyone on the LAN (FF:FF:FF:FF:FF:FF), who has the IP 192.168.1.2? My MAC is DE:AD:BE:EF:CA:FE so you can respond back to me.

Host 2 (192.168.1.2): Hello DE:AD:BE:EF:CA:FE, I have IP 192.168.1.2 and my MAC address is 12:34:56:78:90:12 so you can send your IP packets to me.

 You can see the ARP table of a box by dropping out to a command prompt and typing "arp –a" in Windows or just "arp" in Linux. ARP can also work the other way by a host on the LAN sending its MAC address to another machine on the LAN for preemptive caching unless the host is configured to not accept un-requested ARP replies.

 A person might want to change the MAC address of a NIC for many reasons:

1. To get past MAC address filtering on a router. Valid MAC addresses can be found by sniffing them and then the deviant user could assume the MAC of a valid host. Having two hosts on the same network can cause some network stability problems, but much of the time it's workable. This is one of the reasons why MIC Address filtering on a wireless router is pointless. An attacker can just sniff the MAC address out of the air while in monitor mode and set his WiFi NIC to use it. Interestingly, a lot of hotels use MAC filtering in their "pay to surf" schemes, so this method can be an instant in for cheap skate road warriors. 
2. Sniffing other connections on the network. By assuming another host's MAC as their own they may receive packets not meant for them. However, ARP poisoning is generally a better method than MAC spoofing to accomplish this task.
3. So as to keep their burned in MAC address out of IDS and security logs, thus keeping deviant behavior from being connected to their hardware. For example, two of the main things a DHCP server logs when it leases an IP to a client is the MAC address and host name. If you have a wireless router look around on it's web interface for where it logs this info. Luckily there are tools to randomize this information (MadMACs).
4. To pull off a denial of service attack, for instance assuming the MAC of the gateway to a sub net might cause traffic problems. Also, a lot of WiFi routers will lock up if a client tries to connect with the same MAC as the router's BSSID.

BSD

1) Bring down the interface: "ifconfig xl0 down"
2) Enter new MAC address: "ifconfig xl0 link 00:00:00:AA:AA:AA"
3) Bring up the interface: "ifconfig xl0 up"

Linux

1) Bring down the interface: "ifconfig eth0 down"
2) Enter new MAC address: "ifconfig eth0 hw ether 00:00:00:AA:AA:AA"
3) Bring up the interface: "ifconfig eth0 up"

Windows 2000/XP
Method 1:

This is depending on the type of Network Interface Card (NIC) you have. If you have a card that doesn't support Clone MAC address, then you have to go to second method.

a) Go to Start->Settings->Control Panel and double click on Network and Dial-up Connections.
b) Right click on the NIC you want to change the MAC address and click on properties.
c) Under "General" tab, click on the "Configure" button
d) Click on "Advanced" tab
e) Under "Property section", you should see an item called "Network Address" or "Locally Administered Address", click on it.
f) On the right side, under "Value", type in the New MAC address you want to assign to your NIC. Usually this value is entered without the "-" between the MAC address numbers.
g) Goto command prompt and type in "ipconfig /all" or "net config rdr" to verify the changes. If the changes are not materialized, then use the second method.
h) If successful, reboot your system.

Method 2:

This should work on all Windows 2000/XP systems
a) Go to Start -> Run, type "regedt32" to start registry editor. Do not use "Regedit".
b) Go to "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\ Control\Class\{4D36E972-E325-11CE-BFC1-08002BE10318}". Double click on it to expand the tree. The subkeys are 4-digit numbers, which represent particular network adapters. You should see it starts with 0000, then 0001, 0002, 0003 and so on.
c) Find the interface you want by searching for the proper "DriverDesc" key.
d) Edit, or add, the string key "NetworkAddress" (has the data type "REG_SZ") to contain the new MAC address.
e) Disable then re-enable the network interface that you changed (or reboot the system).

Method 3:

Use the program Etherchange from http://ntsecurity.nu/toolbox/etherchange/
Windows 9x
Use the same method as Windows 2000/XP except for the registry key location is "HKEY_LOCAL_MACHINE\System\ CurrentControlSet\Services\Class\Net" and you must reboot your system.