Modifying operating channel in AP mode

Lara Deek laradeek at cs.ucsb.edu
Tue Feb 15 22:41:37 EST 2011


Hello,

A device in master mode running hostapd has its operating channel set
in hostapd.conf prematurely. While hostapd is running, I want to
modify the operating channel, however, I am getting unusual behavior.

First, I bring the master interface down with "sudo ifconfig wlan1
down", then change channels with "sudo iw dev wlan1 set freq 5180
HT20", followed by "sudo ifconfig wlan1 up". "iwconfig" shows that the
frequency does not in fact change, but a separate device in monitor
mode does in fact receive probe responses from the AP at the new
frequency. The AP's beacon transmission is, however, terminated, even
on its previous operating channel. Furthermore, devices in managed
mode can no longer connect to the AP. Then, if I issue an "sudo iw dev
wlan1 set freq 5180 HT20" on the AP device again, I do not get
"command failed: Device or resource busy (-16)", but "iwconfig" shows
that the channel is updated to the new channel. Normal AP activity
ceases from modifying channels when running hostapd.

If I return to the channel set in hostapd.conf, devices can connect to
the AP, "iw dev wlan1 link" shows the connection, but the AP ceases
beacon activity, and there is an unusually high rate of probe activity
between the client and AP.

I would appreciate any insight on how to address this issue. To change
channels, must I just restart hostapd with a new "channel" value in
hostapd.conf every time?

Regards,
Lara


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