How to know the frequency of AP mode

[Rafał Miłecki]

On 20 November 2017 at 18:46, Grégoire Gentil <gregoire at gentil.com> wrote:
> My question is summarized here:
>
> https://stackoverflow.com/questions/47398018/hostapd-how-to-know-the-frequency-of-ap-mode
>
>
> "On Linux, I'm using hostapd in AP mode. In other words, the system is the
> hotspot.
>
> I would like to know what is the frequency of the Wi-Fi via c code (or a
> bash script that I could translate to c code).
>
> The following code which queries the kernel, only works in normal mode. It
> fails in AP mode.
>
> ioctl(net_sock, SIOCGIWFREQ, &wrq);
>
> In bash, I have tried:
>
> iwconfig
> iw dev
>
> but it doesn't report the frequency of the Wi-Fi. Same problem: it only
> reports in normal mode when the system associates to a router but nothing in
> AP mode.
>
> How can I query hostapd to know the frequency hostapd is using or is
> configured to use?"

Use nl80211, send NL80211_CMD_GET_INTERFACE request and check for the
NL80211_ATTR_WIPHY_FREQ. This will work as long as your Linux wireless
driver implements get_channel cfg80211 callback.

In Linux source it's handled by:
NL80211_CMD_GET_INTERFACE
nl80211_get_interface / nl80211_dump_interface
nl80211_send_iface
rdev_get_channel
nl80211_send_chandef

When using "iw" tool, just try
iw dev wlan0 info
(replace "wlan0" if needed)