Error for wireless request "Set Mode"

Issue: When doing iwconfig wlan0 mode master I get the following error: 

root@OpenWrt:~# iwconfig wlan0 mode master
Error for wireless request "Set Mode" (8B06) :
    SET failed on device wlan0 ; Invalid argument.

Root cause + solution:

The mac80211 subsystem moves all aspects of master mode (a.k.a. AP mode) into user space, thus it depends on hostapd (also read Hostapd) to

  • handle authenticating clients,
  • set encryption keys,
  • establishing key rotation policy,
  • handle other aspects of the wireless infrastructure.

Due to this, the old method of issuing iwconfig <wireless interface> mode master no longer works. Userspace programs like hostapd now use netlink (the nl80211 driver) to create a master mode interface for your traffic and a monitor mode interface for receiving and transmitting management frames.

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