How to get udp packet when wifi not connect any AP or client

Installing and Using OpenWrt Network and Wireless Configuration
1

I want try to study how to get udp packet when wifi not connect any AP or client.
My platform is MTK7620a ,openwrt is 18.06

This is my wlan0 info and i had set it in monitor mode.

root@OpenWrt:/etc/config# iwinfo wlan0 info
wlan0     ESSID: unknown
          Access Point: 00:0A:52:04:43:9C
          Mode: Monitor  Channel: unknown (unknown)
          Tx-Power: 20 dBm  Link Quality: unknown/70
          Signal: unknown  Noise: unknown
          Bit Rate: unknown
          Encryption: unknown
          Type: nl80211  HW Mode(s): 802.11bgn
          Hardware: unknown [Generic MAC80211]
          TX power offset: unknown
          Frequency offset: unknown
          Supports VAPs: yes  PHY name: phy0

use tcpdump to try monitor wlan0 and use Epstouch (ESP 8266 wifi smart config app)

this is my command :
tcpdump -s0 -y ieee802_11_radio -i wlan0 | grep 11n

but i can't get any udp packet.

what I mistake ?

15:50:33.472689 2462 MHz 11n -70dBm signal antenna 1 130.0 Mb/s MCS 14 20 MHz short GI Data IV:b279 Pad 20 KeyID 0
15:50:33.547869 2462 MHz 11n -70dBm signal antenna 1 130.0 Mb/s MCS 14 20 MHz short GI Data IV:b27a Pad 20 KeyID 0
15:50:33.548629 2462 MHz 11n -70dBm signal antenna 1 115.6 Mb/s MCS 13 20 MHz short GI Data IV:b27a Pad 20 KeyID 0
15:50:33.554709 2462 MHz 11n -70dBm signal antenna 1 115.6 Mb/s MCS 13 20 MHz short GI Data IV:b27a Pad 20 KeyID 0
15:50:33.892450 2462 MHz 11n -70dBm signal antenna 1 130.0 Mb/s MCS 14 20 MHz short GI Data IV:b27b Pad 20 KeyID 0
15:50:33.957890 2462 MHz 11n -72dBm signal antenna 1 130.0 Mb/s MCS 14 20 MHz short GI Data IV:b27c Pad 20 KeyID 0
2

I've heard this one before, so it's not too shocking:

  • You don't have a Wired/Wireless connection (Layer 2) established; but
  • You want to receive UDP packets (i.e Layer 3 traffic)

Do you see your mistake now?

(moved to Installing and Using OpenWrt - Network and Wireless Configuration)

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