DFS support in FreeBSD

DFS (Dynamic Frequency Selection) is a part of the 802.11 specification, designed to provide interoperability with other devices on the 5GHz frequency bands. The relevant specification is 802.11h.


DFS is made up of multiple components:

FreeBSD support

FreeBSD currently supports:

How to use

For station mode operation, DFS support is mostly implemented by net80211.

However, the ath(4) driver has received the most testing in STA DFS mode. iwn(4) also has specific code to handle STA mode DFS issues. For example, when channel changing to a passive channel (which requires CAC - clear access check) beacon frames aren't transmitted, so the STA should trigger a scan to find another suitable channel otherwise the channel will be unusable for the CAC timeout (60 seconds by default.)

For hostap operation, the only chipset which currently supports signaling radar events is mwl(4). The card firmware handles radar event detection. When a regulatory domain is configured with channels that have DFS requirements, the driver and net80211 layer will automatically handle this for you.

The ath(4) driver has all of the framework needed to trigger radar events, but there is no open source radar classification code available. For now, please contact Adrian Chadd <adrian@freebsd.org> if you're a vendor who wishes to ship products using FreeBSD (and you're willing to go through the required regulatory classification tests.)

For users who wish to run 5GHz AP devices, please avoid using the DFS channels.


There's no support for 802.11n DFS channel handling (CAC, NOL) although CSA will work fine. The support wouldn't be that difficult to add; there's just been no need for it until the ath(4) 11n TX aggregation code is complete and in -HEAD.

net80211 doesn't currently mark DFS channels as unavailable in hostap/IBSS modes when the driver doesn't support DFS. This will likely be resolved by FreeBSD-10.0 and possibly backported to FreeBSD-9.x to help improve regulatory compliance.

Future work

Please contact freebsd-wireless@freebsd.org if you'd like to work on any of the following.

WiFi/DFS (last edited 2018-04-05T22:47:47+0000 by MateuszPiotrowski)