What is mon.wlan0?

Question

I used hostapd to create a wifi hotspot on Linux, but when it created a interface named "mon.wlan0", what is it used for ? Should I assign it a ip address ?

Now I have "wlan0" and "mon.wlan0", I only give "wlan0" a ip address and it works fine.

Solution

mon.wlan0 is the monitor interface. It is used by tools such as Wireshark to capture all traffic seen by the NIC, including packets for a different SSID (wireless network) or not addressed to the machine. You shouldn't assign it an IP address or use it anywhere, just leave it.