The Wi-Fi sniffer mode can be enabled by esp_wifi_set_promiscuous(). If the sniffer mode is enabled, the following packets can be dumped to the application:
802.11 Management frame.
802.11 Data frame, including MPDU, AMPDU, and AMSDU.
802.11 MIMO frame, for MIMO frame, the sniffer only dumps the length of the frame.
802.11 Control frame.
802.11 CRC error frame.
The following packets will NOT be dumped to the application:
Other 802.11 error frames.
For frames that the sniffer can dump, the application can additionally decide which specific type of packets can be filtered to the application by using esp_wifi_set_promiscuous_filter() and esp_wifi_set_promiscuous_ctrl_filter(). By default, it will filter all 802.11 data and management frames to the application. If you want to filter the 802.11 control frames, the filter parameter in esp_wifi_set_promiscuous_filter() should include WIFI_PROMIS_FILTER_MASK_CTRL type, and if you want to differentiate control frames further, then call esp_wifi_set_promiscuous_ctrl_filter().
The Wi-Fi sniffer mode can be enabled in the Wi-Fi mode of WIFI_MODE_NULL, WIFI_MODE_STA, WIFI_MODE_AP, or WIFI_MODE_APSTA. In other words, the sniffer mode is active when the station is connected to the AP, or when the AP has a Wi-Fi connection. Please note that the sniffer has a great impact on the throughput of the station or AP Wi-Fi connection. Generally, the sniffer should be enabled only if the station/AP Wi-Fi connection does not experience heavy traffic.
Another noteworthy issue about the sniffer is the callback wifi_promiscuous_cb_t. The callback will be called directly in the Wi-Fi driver task, so if the application has a lot of work to do for each filtered packet, the recommendation is to post an event to the application task in the callback and defer the real work to the application task.
