Debian – starting hostapd fails — ACS: Unable to collect survey data

access-pointdebianhostapdwlan

I'm on a jessie on an embedded device, trying to set the WLAN into AP mode. When I try

# /usr/sbin/hostapd -P /run/hostapd.wlan0.pid /etc/hostapd/hostapd.conf
Configuration file: /etc/hostapd/hostapd.conf
ACS: Automatic channel selection started, this may take a bit
wlan0: interface state UNINITIALIZED->ACS
wlan0: ACS-STARTED 
ACS: Unable to collect survey data
ACS: All study options have failed
Interface initialization failed
wlan0: interface state ACS->DISABLED
wlan0: AP-DISABLED 
ACS: Possibly channel configuration is invalid, please report this along with your config file.
ACS: Failed to start
wlan0: AP-DISABLED 
hostapd_free_hapd_data: Interface wlan0 wasn't started
wlan0: interface state DISABLED->DISABLED
hostapd_free_hapd_data: Interface wlan0 wasn't started

My config file is:

# cat /etc/hostapd/hostapd.conf 
interface=wlan0
driver=nl80211
channel=acs
hw_mode=g
wmm_enabled=0
max_num_sta=1
ssid=foobarfoo

The error message can be found in a web search, but the solutions don't fit for me: I don't use ǹetwork-manager and the nl80211 driver works on a different device (handmade buildroot system) and hostapd version is v2.3 (not the buggy v2.1)

Any idea how to narrow the problem further?

Edit: With channel=1 it runs, so it's definitely an ACS problem, but what can I try to find out the root of the problem?

Best Answer

FWIW I'm currently troubleshooting what I believe is a similar problem. I am using the built-in wireless interface on a RPi4 (2GB RAM) with hostapd + dnsmasq. It seems to work fine when I specify a channel manually, but when I try to auto-select an optimal channel it fails (somewhat silently). If I enable lots of debug logging I can see the following output, which suggests to me that there's something broken between the code that's measuring the survey data and the part that connects it to hostapd.

In lieu of a better solution, I wrote a NodeJS script to select a channel number and posted it here.

Config synopsis 
# cat /etc/hostapd/hostapd.conf
# This is the name of the WiFi interface we configured above
interface=wlan0

# (per RPi4 access-point instructions)
driver=nl80211

# This is the name of the network
ssid=MySSID

# Use 802.11g (2.4GHz)
hw_mode=g

# Channel number (IEEE 802.11)
# (default: 0, i.e., not set)
# Please note that some drivers do not use this value from hostapd and the
# channel will need to be configured separately with iwconfig.
#
# If CONFIG_ACS build option is enabled, the channel can be selected
# automatically at run time by setting channel=acs_survey or channel=0, both of
# which will enable the ACS survey based algorithm.
#
# Works fine when we set the channel manually
#channel=4
channel=acs_survey

# Enable 802.11n (doesn't seem to affect whether or not ACS works)
ieee80211n=1
#ieee80211n=0

# Disable Wifi MultiMedia
wmm_enabled=0

# Accept all MAC addresses
macaddr_acl=0

# Use WPA authentication
auth_algs=1

# Require clients to know the network name
ignore_broadcast_ssid=0

# Allow WPA1 or WPA2
wpa=3

# Use a pre-shared key
wpa_key_mgmt=WPA-PSK

# The network passphrase
wpa_passphrase=mypassphrase

# Default to (weak) TKIP for WPA1
wpa_pairwise=TKIP CCMP

# Use AES, instead of TKIP for RSN/WPA2
rsn_pairwise=CCMP


# hostapd event logger configuration
#
# Two output method: syslog and stdout (only usable if not forking to background).
#
# Module bitfield (ORed bitfield of modules that will be logged; -1 = all modules):
# bit 0 (1) = IEEE 802.11
# bit 1 (2) = IEEE 802.1X
# bit 2 (4) = RADIUS
# bit 3 (8) = WPA
# bit 4 (16) = driver interface
# bit 5 (32) = IAPP
# bit 6 (64) = MLME
#
# Levels (minimum value for logged events):
#  0 = verbose debugging
#  1 = debugging
#  2 = informational messages
#  3 = notification
#  4 = warning
#
logger_syslog=-1
logger_syslog_level=0
logger_stdout=-1
logger_stdout_level=0
Output 
# hostapd -dd /etc/hostapd/hostapd.conf
random: getrandom() support available
Configuration file: /etc/hostapd/hostapd.conf
nl80211: Using driver-based roaming
nl80211: TDLS supported
nl80211: Supported cipher 00-0f-ac:1
nl80211: Supported cipher 00-0f-ac:5
nl80211: Supported cipher 00-0f-ac:2
nl80211: Supported cipher 00-0f-ac:4
nl80211: Supported cipher 00-0f-ac:6
nl80211: Using driver-based off-channel TX
nl80211: Supported vendor command: vendor_id=0x1018 subcmd=1
nl80211: Use separate P2P group interface (driver advertised support)
nl80211: Enable multi-channel concurrent (driver advertised support)
nl80211: use P2P_DEVICE support
nl80211: interface wlan0 in phy phy0
nl80211: Set mode ifindex 4 iftype 3 (AP)
nl80211: Setup AP(wlan0) - device_ap_sme=1 use_monitor=0
nl80211: Subscribe to mgmt frames with AP handle 0x5567d637a0 (device SME)
nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x5567d637a0 match=04
nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x5567d637a0 match=0501
nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x5567d637a0 match=0503
nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x5567d637a0 match=0504
nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x5567d637a0 match=06
nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x5567d637a0 match=08
nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x5567d637a0 match=09
nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x5567d637a0 match=0a
nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x5567d637a0 match=11
nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x5567d637a0 match=12
nl80211: Register frame type=0xd0 (WLAN_FC_STYPE_ACTION) nl_handle=0x5567d637a0 match=7f
nl80211: Enable Probe Request reporting nl_preq=0x5567d631d0
nl80211: Register frame type=0x40 (WLAN_FC_STYPE_PROBE_REQ) nl_handle=0x5567d631d0 match=
rfkill: initial event: idx=0 type=1 op=0 soft=0 hard=0
nl80211: Add own interface ifindex 4 (ifidx_reason -1)
nl80211: if_indices[16]: 4(-1)
phy: phy0
BSS count 1, BSSID mask 00:00:00:00:00:00 (0 bits)
nl80211: Regulatory information - country=00
nl80211: 2402-2472 @ 40 MHz 20 mBm
nl80211: 2457-2482 @ 20 MHz 20 mBm (no IR)
nl80211: 2474-2494 @ 20 MHz 20 mBm (no OFDM) (no IR)
nl80211: 5170-5250 @ 80 MHz 20 mBm (no IR)
nl80211: 5250-5330 @ 80 MHz 20 mBm (DFS) (no IR)
nl80211: 5490-5730 @ 160 MHz 20 mBm (DFS) (no IR)
nl80211: 5735-5835 @ 80 MHz 20 mBm (no IR)
nl80211: 57240-63720 @ 2160 MHz 0 mBm
nl80211: Added 802.11b mode based on 802.11g information
nl80211: Mode IEEE 802.11g: 2412 2417 2422 2427 2432 2437 2442 2447 2452 2457 2462 2467 2472 2484
nl80211: Mode IEEE 802.11a: 5170 5180 5190 5200 5210 5220 5230 5240 5260 5280 5300 5320 5500 5520 5540 5560 5580 5600 5620 5640 5660 5680 5700 5720 5745 5765 5785 5805 5825
nl80211: Mode IEEE 802.11b: 2412 2417 2422 2427 2432 2437 2442 2447 2452 2457 2462 2467 2472 2484
Allowed channel: mode=1 chan=1 freq=2412 MHz max_tx_power=20 dBm
Allowed channel: mode=1 chan=2 freq=2417 MHz max_tx_power=20 dBm
Allowed channel: mode=1 chan=3 freq=2422 MHz max_tx_power=20 dBm
Allowed channel: mode=1 chan=4 freq=2427 MHz max_tx_power=20 dBm
Allowed channel: mode=1 chan=5 freq=2432 MHz max_tx_power=20 dBm
Allowed channel: mode=1 chan=6 freq=2437 MHz max_tx_power=20 dBm
Allowed channel: mode=1 chan=7 freq=2442 MHz max_tx_power=20 dBm
Allowed channel: mode=1 chan=8 freq=2447 MHz max_tx_power=20 dBm
Allowed channel: mode=1 chan=9 freq=2452 MHz max_tx_power=20 dBm
Allowed channel: mode=1 chan=10 freq=2457 MHz max_tx_power=20 dBm
Allowed channel: mode=1 chan=11 freq=2462 MHz max_tx_power=20 dBm
Allowed channel: mode=1 chan=12 freq=2467 MHz max_tx_power=20 dBm
Allowed channel: mode=1 chan=13 freq=2472 MHz max_tx_power=20 dBm
Allowed channel: mode=1 chan=14 freq=2484 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=34 freq=5170 MHz max_tx_power=3 dBm
Allowed channel: mode=2 chan=36 freq=5180 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=38 freq=5190 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=40 freq=5200 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=42 freq=5210 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=44 freq=5220 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=46 freq=5230 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=48 freq=5240 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=52 freq=5260 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=56 freq=5280 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=60 freq=5300 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=64 freq=5320 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=100 freq=5500 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=104 freq=5520 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=108 freq=5540 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=112 freq=5560 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=116 freq=5580 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=120 freq=5600 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=124 freq=5620 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=128 freq=5640 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=132 freq=5660 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=136 freq=5680 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=140 freq=5700 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=144 freq=5720 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=149 freq=5745 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=153 freq=5765 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=157 freq=5785 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=161 freq=5805 MHz max_tx_power=20 dBm
Allowed channel: mode=2 chan=165 freq=5825 MHz max_tx_power=20 dBm
Allowed channel: mode=0 chan=1 freq=2412 MHz max_tx_power=20 dBm
Allowed channel: mode=0 chan=2 freq=2417 MHz max_tx_power=20 dBm
Allowed channel: mode=0 chan=3 freq=2422 MHz max_tx_power=20 dBm
Allowed channel: mode=0 chan=4 freq=2427 MHz max_tx_power=20 dBm
Allowed channel: mode=0 chan=5 freq=2432 MHz max_tx_power=20 dBm
Allowed channel: mode=0 chan=6 freq=2437 MHz max_tx_power=20 dBm
Allowed channel: mode=0 chan=7 freq=2442 MHz max_tx_power=20 dBm
Allowed channel: mode=0 chan=8 freq=2447 MHz max_tx_power=20 dBm
Allowed channel: mode=0 chan=9 freq=2452 MHz max_tx_power=20 dBm
Allowed channel: mode=0 chan=10 freq=2457 MHz max_tx_power=20 dBm
Allowed channel: mode=0 chan=11 freq=2462 MHz max_tx_power=20 dBm
Allowed channel: mode=0 chan=12 freq=2467 MHz max_tx_power=20 dBm
Allowed channel: mode=0 chan=13 freq=2472 MHz max_tx_power=20 dBm
Allowed channel: mode=0 chan=14 freq=2484 MHz max_tx_power=20 dBm
ACS: Automatic channel selection started, this may take a bit
ACS: Scanning 1 / 5
wlan0: nl80211: scan request
nl80211: Passive scan requested
nl80211: Scan frequency 2412 MHz
nl80211: Scan frequency 2417 MHz
nl80211: Scan frequency 2422 MHz
nl80211: Scan frequency 2427 MHz
nl80211: Scan frequency 2432 MHz
nl80211: Scan frequency 2437 MHz
nl80211: Scan frequency 2442 MHz
nl80211: Scan frequency 2447 MHz
nl80211: Scan frequency 2452 MHz
nl80211: Scan frequency 2457 MHz
nl80211: Scan frequency 2462 MHz
nl80211: Scan frequency 2467 MHz
nl80211: Scan frequency 2472 MHz
nl80211: Scan frequency 2484 MHz
Scan requested (ret=0) - scan timeout 10 seconds
wlan0: interface state UNINITIALIZED->ACS
wlan0: ACS-STARTED
Interface initialization will be completed in a callback (ACS)
ctrl_iface not configured!
nl80211: Event message available
nl80211: Drv Event 33 (NL80211_CMD_TRIGGER_SCAN) received for wlan0
wlan0: nl80211: Scan trigger
wlan0: Event SCAN_STARTED (46) received
Unknown event 46
RTM_NEWLINK: ifi_index=4 ifname=wlan0 wext ifi_family=0 ifi_flags=0x1003 ([UP])
nl80211: Event message available
nl80211: Drv Event 34 (NL80211_CMD_NEW_SCAN_RESULTS) received for wlan0
wlan0: nl80211: New scan results available
nl80211: Scan included frequencies: 2412 2417 2422 2427 2432 2437 2442 2447 2452 2457 2462 2467 2472 2484
wlan0: Event SCAN_RESULTS (3) received
ACS: Using survey based algorithm (acs_num_scans=5)
nl80211: Fetch survey data
wlan0: Event SURVEY (45) received
No survey data received
ACS: Scanning 2 / 5
wlan0: nl80211: scan request
nl80211: Passive scan requested
nl80211: Scan frequency 2412 MHz
nl80211: Scan frequency 2417 MHz
nl80211: Scan frequency 2422 MHz
nl80211: Scan frequency 2427 MHz
nl80211: Scan frequency 2432 MHz
nl80211: Scan frequency 2437 MHz
nl80211: Scan frequency 2442 MHz
nl80211: Scan frequency 2447 MHz
nl80211: Scan frequency 2452 MHz
nl80211: Scan frequency 2457 MHz
nl80211: Scan frequency 2462 MHz
nl80211: Scan frequency 2467 MHz
nl80211: Scan frequency 2472 MHz
nl80211: Scan frequency 2484 MHz
Scan requested (ret=0) - scan timeout 30 seconds
nl80211: Event message available
nl80211: Drv Event 33 (NL80211_CMD_TRIGGER_SCAN) received for wlan0
wlan0: nl80211: Scan trigger
wlan0: Event SCAN_STARTED (46) received
Unknown event 46
RTM_NEWLINK: ifi_index=4 ifname=wlan0 wext ifi_family=0 ifi_flags=0x1003 ([UP])
nl80211: Event message available
nl80211: Drv Event 34 (NL80211_CMD_NEW_SCAN_RESULTS) received for wlan0
wlan0: nl80211: New scan results available
nl80211: Scan included frequencies: 2412 2417 2422 2427 2432 2437 2442 2447 2452 2457 2462 2467 2472 2484
wlan0: Event SCAN_RESULTS (3) received
ACS: Using survey based algorithm (acs_num_scans=5)
nl80211: Fetch survey data
wlan0: Event SURVEY (45) received
No survey data received
ACS: Scanning 3 / 5
wlan0: nl80211: scan request
nl80211: Passive scan requested
nl80211: Scan frequency 2412 MHz
nl80211: Scan frequency 2417 MHz
nl80211: Scan frequency 2422 MHz
nl80211: Scan frequency 2427 MHz
nl80211: Scan frequency 2432 MHz
nl80211: Scan frequency 2437 MHz
nl80211: Scan frequency 2442 MHz
nl80211: Scan frequency 2447 MHz
nl80211: Scan frequency 2452 MHz
nl80211: Scan frequency 2457 MHz
nl80211: Scan frequency 2462 MHz
nl80211: Scan frequency 2467 MHz
nl80211: Scan frequency 2472 MHz
nl80211: Scan frequency 2484 MHz
Scan requested (ret=0) - scan timeout 30 seconds
nl80211: Event message available
nl80211: Drv Event 33 (NL80211_CMD_TRIGGER_SCAN) received for wlan0
wlan0: nl80211: Scan trigger
wlan0: Event SCAN_STARTED (46) received
Unknown event 46
RTM_NEWLINK: ifi_index=4 ifname=wlan0 wext ifi_family=0 ifi_flags=0x1003 ([UP])
nl80211: Event message available
nl80211: Drv Event 34 (NL80211_CMD_NEW_SCAN_RESULTS) received for wlan0
wlan0: nl80211: New scan results available
nl80211: Scan included frequencies: 2412 2417 2422 2427 2432 2437 2442 2447 2452 2457 2462 2467 2472 2484
wlan0: Event SCAN_RESULTS (3) received
ACS: Using survey based algorithm (acs_num_scans=5)
nl80211: Fetch survey data
wlan0: Event SURVEY (45) received
No survey data received
ACS: Scanning 4 / 5
wlan0: nl80211: scan request
nl80211: Passive scan requested
nl80211: Scan frequency 2412 MHz
nl80211: Scan frequency 2417 MHz
nl80211: Scan frequency 2422 MHz
nl80211: Scan frequency 2427 MHz
nl80211: Scan frequency 2432 MHz
nl80211: Scan frequency 2437 MHz
nl80211: Scan frequency 2442 MHz
nl80211: Scan frequency 2447 MHz
nl80211: Scan frequency 2452 MHz
nl80211: Scan frequency 2457 MHz
nl80211: Scan frequency 2462 MHz
nl80211: Scan frequency 2467 MHz
nl80211: Scan frequency 2472 MHz
nl80211: Scan frequency 2484 MHz
Scan requested (ret=0) - scan timeout 30 seconds
nl80211: Event message available
nl80211: Drv Event 33 (NL80211_CMD_TRIGGER_SCAN) received for wlan0
wlan0: nl80211: Scan trigger
wlan0: Event SCAN_STARTED (46) received
Unknown event 46
RTM_NEWLINK: ifi_index=4 ifname=wlan0 wext ifi_family=0 ifi_flags=0x1003 ([UP])
nl80211: Event message available
nl80211: Drv Event 34 (NL80211_CMD_NEW_SCAN_RESULTS) received for wlan0
wlan0: nl80211: New scan results available
nl80211: Scan included frequencies: 2412 2417 2422 2427 2432 2437 2442 2447 2452 2457 2462 2467 2472 2484
wlan0: Event SCAN_RESULTS (3) received
ACS: Using survey based algorithm (acs_num_scans=5)
nl80211: Fetch survey data
wlan0: Event SURVEY (45) received
No survey data received
ACS: Scanning 5 / 5
wlan0: nl80211: scan request
nl80211: Passive scan requested
nl80211: Scan frequency 2412 MHz
nl80211: Scan frequency 2417 MHz
nl80211: Scan frequency 2422 MHz
nl80211: Scan frequency 2427 MHz
nl80211: Scan frequency 2432 MHz
nl80211: Scan frequency 2437 MHz
nl80211: Scan frequency 2442 MHz
nl80211: Scan frequency 2447 MHz
nl80211: Scan frequency 2452 MHz
nl80211: Scan frequency 2457 MHz
nl80211: Scan frequency 2462 MHz
nl80211: Scan frequency 2467 MHz
nl80211: Scan frequency 2472 MHz
nl80211: Scan frequency 2484 MHz
Scan requested (ret=0) - scan timeout 30 seconds
nl80211: Event message available
nl80211: Drv Event 33 (NL80211_CMD_TRIGGER_SCAN) received for wlan0
wlan0: nl80211: Scan trigger
wlan0: Event SCAN_STARTED (46) received
Unknown event 46
nl80211: Event message available
nl80211: Drv Event 34 (NL80211_CMD_NEW_SCAN_RESULTS) received for wlan0
wlan0: nl80211: New scan results available
nl80211: Scan included frequencies: 2412 2417 2422 2427 2432 2437 2442 2447 2452 2457 2462 2467 2472 2484
wlan0: Event SCAN_RESULTS (3) received
ACS: Using survey based algorithm (acs_num_scans=5)
nl80211: Fetch survey data
wlan0: Event SURVEY (45) received
No survey data received
ACS: Trying survey-based ACS
ACS: Unable to collect survey data
ACS: All study options have failed
Interface initialization failed
wlan0: interface state ACS->DISABLED
wlan0: AP-DISABLED
ACS: Possibly channel configuration is invalid, please report this along with your config file.
ACS: Failed to start
wlan0: AP-DISABLED
wlan0: Deauthenticate all stations
nl80211: sta_remove -> DEL_STATION wlan0 ff:ff:ff:ff:ff:ff --> 0 (Success)
hostapd_free_hapd_data: Interface wlan0 wasn't started
hostapd_deinit_driver: driver=0x55587256e8 drv_priv=0x5567d5df30 -> hapd_deinit
nl80211: deinit ifname=wlan0 disabled_11b_rates=0
nl80211: Disable Probe Request reporting nl_preq=0x888888ddef5eb959
nl80211: Remove monitor interface: refcount=0
nl80211: Remove beacon (ifindex=4)
netlink: Operstate: ifindex=4 linkmode=0 (kernel-control), operstate=6 (IF_OPER_UP)
nl80211: Set mode ifindex 4 iftype 2 (STATION)
nl80211: Teardown AP(wlan0) - device_ap_sme=1 use_monitor=0
nl80211: Unsubscribe mgmt frames handle 0x888888ddef5ebf29 (AP teardown (dev SME))
hostapd_deinit_driver:bss[0]->drv_priv=0x5567d5df30
hostapd_cleanup_iface_partial(0x5567d5b980)
Interface wlan0 disabled
wlan0: interface state DISABLED->DISABLED
hostapd_interface_deinit_free(0x5567d5b980)
hostapd_interface_deinit_free: num_bss=1 conf->num_bss=1
hostapd_interface_deinit(0x5567d5b980)
wlan0: interface state DISABLED->DISABLED
hostapd_bss_deinit: deinit bss wlan0
wlan0: AP-DISABLED
hostapd_cleanup(hapd=0x5567d5ce80 (wlan0))
wlan0: CTRL-EVENT-TERMINATING
hostapd_free_hapd_data: Interface wlan0 wasn't started
hostapd_interface_deinit_free: driver=0x55587256e8 drv_priv=(nil) -> hapd_deinit
hostapd_interface_free(0x5567d5b980)
hostapd_interface_free: free hapd 0x5567d5ce80
hostapd_cleanup_iface(0x5567d5b980)
hostapd_cleanup_iface_partial(0x5567d5b980)
hostapd_cleanup_iface: free iface=0x5567d5b980

I'm able to use iwlist wlan0 scanning or wavemon to see other APs, etc.

See also:

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