Creating a Monitor mode Interface:
Let’s now set our wireless adapter into monitor mode.
Follow these instructions to get started:
1. Boot Kali with your adapter connected. Once you are within the console, enter iwconfig to confirm that your card has been detected and the driver has been loaded properly.
2.Use the ifconfig wlan1 up command to bring the card up (where wlan1 is your adapter). Verify whether the card is up by running ifconfig wlan1.
You should see the word UP in the second line of the output as shown in the following screenshot:
3. To put our card into monitor mode, we will use the airmon-ng utility that is available by default on Kali. First run airmon-ng command to verify whether it detects the available cards. You should see the wlan0 interface listed in the output:
4. Now enter airmon-ng start wlan1 command to create a monitor mode interface corresponding to the wlan0 device. This new monitor mode interface will be named mon0. (You can verify if it has been created by running airmon-ng without arguments again).
5. Also, running ifconfig mon0 should now display a new interface called mon0.
What just happened?
We have successfully created a monitor mode interface called mon0. This interface will be used to sniff wireless packets off the air. This interface has been created for our wireless adapter.
Creating multiple Monitor mode Interfaces:
It is possible to create multiple monitor mode interfaces using the same physical card. Use the airmon-ng utility to see how you can do this. Awesome! We have a monitor mode interface just waiting to read some packets off the air. So let’s get started. In the next exercise, we will use Wireshark to sniff packets off the air using the mon0 monitor mode interface we just created.
Source : Penetration Testing with Kali Linux