Does hostapd and support background scan for DFS ?

Xue Liu xue.liu at
Mon Feb 13 07:57:47 PST 2017

Hello Zefir,

Thanks a lot for your explanation. It is what I want to know.

On 13.02.2017 16:52, Zefir Kurtisi wrote:
> 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
> Hi,
> 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.
> Cheers,
> Zefir
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