Eric Schaefer

How to Spoof a (Valid) Random MAC Address on Bootup

Updated for Yosemite 10.10.4

I spend a lot of time at coffee shops, using their wi-fi networks. I’m not sure who is operating these things, or if they’re hacked, so I wanted to add a layer anonymity to my connection. Tons of routers record and store the MAC address of anyone connected. That makes me uneasy. Here’s how I randomized a new (and valid) MAC address on every reboot of my MacBook pro, running OS X 10.10.4 (Yosemite).

Create a Bash script

Open your terminal. Let’s create a folder and an empty script file in one swoop.

sudo mkdir /Scripts && sudo nano /Scripts/mac-random.sh

Nano will open up a new mac-random.sh file with blank contents. Paste the following in there:

#!/bin/bash

# disconnect the airport from any network
sudo /System/Library/PrivateFrameworks/Apple80211.framework/Resources/airport -z

# use OpenSSL to generate a random MAC address
mac=$(openssl rand -hex 1 | tr '[:lower:]' '[:upper:]' | xargs echo "obase=2;ibase=16;" | bc | cut -c1-6 | sed 's/$/00/' | xargs echo "obase=16;ibase=2;" | bc | sed "s/$/:$(openssl rand -hex 5 | sed 's/\(..\)/:/g; s/.$//' | tr '[:lower:]' '[:upper:]')/" )

# reassign the wifi adapter with the saved $mac variable above
# the wifi adapter in my case is "en0". yours may be different. run "ifconfig" to find yours.
sudo ifconfig en0 ether $mac
networksetup -detectnewhardware
networksetup -setairportpower airport off
networksetup -setairportpower airport on

Notice the elaborate chain of commands that we used with OpenSSL? Props to commenter osmium for that. This ensures that the least significant bit of the first byte is 0.

Save mac-random.sh with the new contents. Now we get need to reference this file on startup!

Load the Bash script on every bootup

But first let’s make sure it has all the right permissions.

sudo chown -R root:admin /Scripts/mac-random.sh
sudo chmod u=rwx /Scripts/mac-random.sh

The kosher way OS X loads boot items is with a launchd .plist file. Let’s create one which references the script.

cd /Library/LaunchDaemons && sudo nano com.superuser.macrandom.plist

Paste the following XML-style contents into the blank file:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <key>Label</key>
  <string>com.superuser.macrandom</string>
  <key>ProgramArguments</key>
  <array>
    <string>/bin/sh</string>
    <string>/Users/ericschaefer/Scripts/mac-random.sh</string>
  </array>
  <key>UserName</key>
  <string>root</string>
  <key>GroupName</key>
  <string>wheel</string>
  <key>RunAtLoad</key>
  <true/>
  <key>Debug</key>
  <true/>
  <key>KeepAlive</key>
  <false/>
</dict>
</plist>

Save and exit nano. Then change the permissions on this file that you just saved:

sudo chown -R root:wheel com.superuser.macrandom.plist

Then load the file into launchd:

sudo launchctl load com.superuser.macrandom.plist

Check if you’re good to go…

sudo launchctl list | grep macrandom

If you see the launch item, then this will load on boot!

Once you’ve rebooted, run ifconfig to check if your MAC address has changed for your wi-fi adapter (named en1 in my case).