MAC address spoofing

From ArchWiki

This article gives several methods to spoof a Media Access Control (MAC) address.

Manually

There are two methods for spoofing a MAC address: installing and configuring either iproute2 or macchanger. Both of them are outlined below.

iproute2

First, you can check your current MAC address with the command: 

# ip link show interface

where interface is the name of your network interface.

The section that interests us at the moment is the one that has "link/ether" followed by a 6-byte number. It will probably look something like this: 

link/ether 00:1d:98:5a:d1:3a

The first step to spoofing the MAC address is to bring the network interface down. It can be accomplished with the command: 

# ip link set dev interface down

Next, we actually spoof our MAC. Any hexadecimal value will do, but some networks may be configured to refuse to assign IP addresses to a client whose MAC does not match up with any of known vendors. Therefore, unless you control the network(s) you are connecting to, use MAC prefix of any real vendor (basically, the first three bytes), and use random values for next three bytes. For more information please read Wikipedia:Organizationally unique identifier.

To change the MAC, we need to run the command: 

# ip link set dev interface address XX:XX:XX:XX:XX:XX

Where any 6-byte value will suffice for XX:XX:XX:XX:XX:XX.

The final step is to bring the network interface back up. This can be accomplished by running the command: 

# ip link set dev interface up

If you want to verify that your MAC has been spoofed, simply run ip link show interface again and check the value for 'link/ether'. If it worked, 'link/ether' should be whatever address you decided to change it to.

macchanger

Another method uses macchanger (a.k.a., the GNU MAC Changer). It provides a variety of features such as changing the address to match a certain vendor or completely randomizing it.

Install the package macchanger.

The spoofing is done on per-interface basis, specify network interface name as interface in each of the following commands.

The MAC address can be spoofed with a fully random address: 

# macchanger -r interface

To randomize only device-specific bytes of current MAC address (that is, so that if the MAC address was checked it would still register as being from the same vendor), you would run the command: 

# macchanger -e interface

To change the MAC address to a specific value, you would run: 

# macchanger --mac=XX:XX:XX:XX:XX:XX interface

Where XX:XX:XX:XX:XX:XX is the MAC you wish to change to.

Finally, to return the MAC address to its original, permanent hardware value: 

# macchanger -p interface
Note: A device cannot be in use (connected in any way or with its interface up) while the MAC address is being changed.

Automatically

systemd-udevd

udev allows you to perform MAC address spoofing by creating systemd.link(5) files or udev rules.

systemd.link

To set a static spoofed MAC address: 

/etc/systemd/network/01-mac.link
[Match]
PermanentMACAddress=original MAC

[Link]
MACAddress=spoofed MAC

To randomize the MAC address on every boot, set MACAddressPolicy=random instead of MACAddress=spoofed MAC.

udev rule

Use address attribute to match the correct device by its original MAC address and change it using the ip command: 

/etc/udev/rules.d/81-mac-spoof.rules
ACTION=="add", SUBSYSTEM=="net", ATTR{address}=="original MAC", RUN+="/usr/bin/ip link set dev $name address spoofed MAC"
Note: udev only accepts MAC addresses in lowercase.

systemd unit

Creating unit

Below you find two examples of systemd units to change a MAC address at boot, one sets a static MAC using ip and one uses macchanger to assign a random MAC address. The systemd network-pre.target is used to ensure the MAC is changed before a network manager like Netctl or NetworkManager, systemd-networkd or dhcpcd service starts.

iproute2

systemd unit setting a predefined MAC address: 

/etc/systemd/system/macspoof@.service
[Unit]
Description=MAC Address Change %I
Wants=network-pre.target
Before=network-pre.target
BindsTo=sys-subsystem-net-devices-%i.device
After=sys-subsystem-net-devices-%i.device

[Service]
Type=oneshot
ExecStart=/usr/bin/ip link set dev %i address 36:aa:88:c8:75:3a
ExecStart=/usr/bin/ip link set dev %i up

[Install]
WantedBy=multi-user.target
macchanger

systemd unit setting a random address while preserving the original NIC vendor bytes. Ensure that macchanger is installed: 

/etc/systemd/system/macspoof@.service
[Unit]
Description=macchanger on %I
Wants=network-pre.target
Before=network-pre.target
BindsTo=sys-subsystem-net-devices-%i.device
After=sys-subsystem-net-devices-%i.device

[Service]
ExecStart=/usr/bin/macchanger -e %I
Type=oneshot

[Install]
WantedBy=multi-user.target

A full random address can be set using the -r option, see #macchanger.

Enabling service

Append the desired network interface to the service name (e.g. eth0) and enable the service (e.g. macspoof@eth0.service).

Reboot, or stop and start the prerequisite and requisite services in the proper order. If you are in control of your network, verify that the spoofed MAC has been picked up by your router by examining the static, or DHCP address tables within the router.

netctl interfaces

You can use a netctl hook to run a command each time a netctl profile is re-/started for a specific network interface. Replace interface accordingly: 

/etc/netctl/interfaces/interface
#!/usr/bin/env sh
/usr/bin/macchanger -r interface

Make the script executable.

Source: akendo.eu (archive)

NetworkManager

See NetworkManager#Configuring MAC address randomization.

wpa_supplicant

wpa_supplicant can use random MAC address for each ESS connection(AP) (see [1] for details).

Add this to your configuration: 

/etc/wpa_supplicant/wpa_supplicant-wlan0.conf
mac_addr=1
preassoc_mac_addr=1
gas_rand_mac_addr=1

iwd

To randomize the MAC address when iwd starts (see iwd.config(5) for details): 

/etc/iwd/main.conf
[General]
AddressRandomization=once
AddressRandomizationRange=nic

Specifying AddressRandomizationRange enables control over which part of the address is randomized. If set to nic, only the NIC specific octets (last three octets) are randomized. The permanent mac address of the network interface is used for the initial 3 octets. If set to full, all six octets of the address are randomized.

Troubleshooting

Connection to DHCPv4 network fails

If you cannot connect to a DHCPv4 network and you are using dhcpcd, which is the default for NetworkManager, you might need to modify the dhcpcd configuration to obtain a lease.

See also