How to put Wifi card in monitor mode?

Question

When I run tcpdump, I do so after having connected to an access point. Is there a way to run tcpdump without having connected already?

Answer 1

Use tcpdump -I. -I will put the chosen interface into monitor mode, and doesn't need to be connected to an access point.

The full command would be (if your Wi-Fi interface is en0):

sudo tcpdump -Ii en0

Answer 2

My solution requires a connection to an access point; however, no actual access point must exist.

Creating an Ad Hoc access point on OS X

1. From the Wi-Fi Status icon in the menu bar, choose Create Network.
2. Give your ad hoc network a name
3. Choose security type (not required) and click Create.

Now you can run tcpdump without connection to a valid access point.