AP doesn't work at 5 GHz after upgrading hostapd from 2.10 to 2.11

[Jouni Malinen]

On Fri, Nov 08, 2024 at 02:09:32PM +0200, me at beroal.in.ua wrote:
> Hi. I set up my Wi-Fi USB dongle EDUP AC1200 2.4 GHz / 5 GHz (the Realtek
> RTL8812BU chipset inside, driver
> https://aur.archlinux.org/packages/rtl88x2bu-dkms-git/ ) as a Wi-Fi access
> point in the 5 GHz band. After upgrading hostapd from version 2.10 to
> version 2.11 the AP stopped working. I see the AP in WiFiAnalyzer, but with
> a narrower channel (20 MHz instead of 80 MHz). The first smartphone sees the
> AP, but can't connect. The second smartphone see the AP, says that it uses
> WEP (which isn't correct), and asks for a password. It shouldn't ask for a
> password because it remembers the password. I can't enter my password anyway
> probably because WEP restricts its length too much. When I switch to the old
> version of hostapd, all smartphones connect without asking for a password. I
> can make the 2.11 version of hostapd work in the 2.4 GHz band though.

Would you happen to have means for capturing a full Beacon frame from
that AP? I'd like to see what exactly it ends up including, i.e.,
whether this truly is just because of the DSSS Parameter Set element
being removed or whether there could be something else involved with
this.

> I bisected to the commit |8056b79ff1e5d746a82c2ea12011e69e99c11d3d |.
> Applying the patch below solves the problem. My `hw_mode` is `a` as you can
> see in my hostapd configuration file below.

That commit ('Add DSSS Parameter Set element only for 2.4 GHz') removed
the DSSS Parameter Set element from all other bands than 2.4 GHz which
makes this match what the IEEE 802.11 standard has defined for over 20
years. It is interesting if there are numerous deployed STAs that do not
handle that..

The history on this change in the standard is such that there was
actually a period between IEEE Std 802.11a-1999 and IEEE Std
802.11g-2003 amendments when the standard could have been interpreted to
include the DSSS Parameter Set element (called the DS Parameter Set
information element at the time) in Beacon frames and Probe Response
frames since IEEE Std 802.11a-1999 did not define any changes to the
Beacon frame format and IEEE Std 802.11-1999 did not limit this element
to be included only on the 2.4 GHz band. As such, there could be
theoretically some 20 year old IEEE 802.11a STAs that would be compliant
with the standard at the time they were manufactured but that would have
been unable to accept Beacon frames from APs that follow the newer (but
still over 20 years old..) version of the standard.

Taken into account this change in hostapd is only from a year ago, I
guess one could justify reverting it if it does indeed result in
interoperability issues. I would just want to make sure that this is
indeed the sole reason for the issues and nothing else (e.g., the driver
changing something else in Beacon frames based on this) could be at play
here.

-- 
Jouni Malinen                                            PGP id EFC895FA