Does hostapd and support background scan for DFS ?

Zefir Kurtisi zefir.kurtisi at
Mon Feb 13 07:52:26 PST 2017

On 02/13/2017 02:48 PM, Xue Liu wrote:
> Hello everyone,
> I have a question about DFS functions implementation in hostapd. I know that
> hostapd will start DFS every time when it start, after that is DFS still
> periodically activated in background ?
> Thanks.
> Xue Liu

DFS is handled mostly in kernel as part of cfg/mac80211. The DFS state of each
channel is kept there and can be retrieved with e.g. 'iw phy0 channels'.

hostapd itself does not actively modify these states but reacts on the state
changes notified over nl80211 events. When you start operating on a usable DFS
channel, no TX happens over the CAC time (typically 60s). If no radar was seen,
channel becomes available and starts TXing. While it does, it continuously scans
for radar, i.e. there is nothing hostapd needs to trigger but
In-Service-Monitoring is mandatory to operate on a DFS channel. Once a radar is
detected, hostapd handles switching to a different channel via CSA - which if not
already in available state will perform a CAC.


More information about the Hostap mailing list