I w as testing the security of my wifi network following this guide and I am using "RTL8191SU 802.11n WLAN Adapter" in my debian pc for this purpose.

iwconfig lists :

lo        no wireless extensions.

wlan0     unassociated  Nickname:"rtl_wifi"
          Mode:Managed  Access Point: Not-Associated   Sensitivity:0/0  
          Retry:off   RTS thr:off   Fragment thr:off
          Power Management:off
          Link Quality:0  Signal level:0  Noise level:0
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:0   Missed beacon:0

eth0      no wireless extensions.

when I try to put it into monitor mode by issuing , sudo iwconfig wlan mode monitor , i get :

Error for wireless request "Set Mode" (8B06) :
    SET failed on device wlan ; Operation not permitted.

and when I issue , sudo airmon-ng start wlan0:

Found 4 processes that could cause trouble.
If airodump-ng, aireplay-ng or airtun-ng stops working after
a short period of time, you may want to kill (some of) them!

PID Name
1794    avahi-daemon
1795    avahi-daemon
2174    NetworkManager
2858    wpa_supplicant

Interface   Chipset     Driver

I have'nt installed any drivers for the same and yet I can connect to my wifi access point and surf Internet without any trouble using this device but couldn't complete the above procedure.

What may be the reason for the same ? And How to fix it ?


Monitor mode is not hugely common. You need driver and hardware support, and there's very few use cases for them, Wikidevi seems to suggest that monitor mode is unsupported for your specific adaptor, and if so, you can't switch to monitor mode

In theory if it did support it iwconfig wlan0 mode monitor SHOULD work, so its not your command, its probably the hardware

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