What's the correct way to read a STATUS response from the hostapd control interface?

[Benjamin Berg]

Hi,

On Mon, 2025-08-04 at 17:59 -0700, Jeremy Fleischman wrote:
> I'm a user of the wifi-presence openwrt package. It talks to hostapd
> over the control socket to know when people connect/disconnect from a
> given access point.
> 
> My home networks recently grew to the point where the hostapd response
> to the STATUS message is larger than 1024 bytes, which causes
> wifi-presense to fail to parse the response. I filed a bug with
> details here: https://github.com/awilliams/wifi-presence/issues/30.
> 
> What I don't understand is how to fix this bug. wifi-presence can
> increase its buffer size, but it seems to me that a better
> implementation would loop and keep reading until it gets to the end of
> hostapd's response. However, I haven't been able to find a mechanism
> in the protocol that allows a client to know if it has read a full
> response. I'd expect protocols like this to have a "content length"
> header, or a special delimiter that indicates the end of a response.
> 
> I've read through the source code of the example program
> (hostapd/hostapd_cli.c), which hardcodes a buffer length of 4096 in
> _wpa_ctrl_command:
> 
> static int _wpa_ctrl_command(struct wpa_ctrl *ctrl, const char *cmd, int print)
> {
> char buf[4096];
> 
> What's the correct way to handle this?

It is a DGRAM socket (possibly UDP) and hostapd itself hardcodes 
a maximum reply length of 4096 bytes. So you can simply do the same at
least for now.

Some commands such as "BSS" in wpa_supplicant have an option to limit
the range and only return part of the result set. For "STATUS", I
suppose it is currently just assumed to be small enough so that it can
be returned in a single 4096 byte chunk.

Benjamin

> 
> Thanks,
> Jeremy
> 
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