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Userspace processes such as network daemons may wish to be informed when any AP interface is brought up on the system, for example to initiate a (re)configuration of IP settings or to start a DHCP server. Currently nl80211 does not broadcast any such event on its multicast groups, leaving userspace only two options: 1. the process must be the one that actually issued the NL80211_CMD_START_AP request, so that it can react on the response to that request; 2. the process must react to RTM_NEWLINK events indicating a change in carrier state, and may query for further information about the AP and react accordingly. Option (1) is robust, but it does not cover all scenarios. It is easy to imagine a situation where this is not the case (e.g. hostapd + systemd-networkd). Option (2) is not robust, because RTM_NEWLINK events may be silently discarded by the linkwatch logic (cf. linkwatch_fire_event()). Concretely, consider a scenario in which the carrier state flip-flops in the following way: ^ carrier state (high/low = carrier/no carrier) | | _______ _______ ... | | | | | ______| "foo" |____| "bar" (SSID in "quotes") | +-------A-------B----C---------> time If the time interval between (A) and (C) is less than 1 second, then linkwatch may emit only a single RTM_NEWLINK event indicating carrier gain. This is problematic because it is possible that the network configuration that should be applied is a function of the AP's properties such as SSID (cf. SSID= in systemd.network(5)). As illustrated in the above diagram, it may be that the AP with SSID "bar" ends up being configured as though it had SSID "foo". Address the above issue by having nl80211 emit an NL80211_CMD_START_AP message on the MLME nl80211 multicast group. This allows for arbitrary processes to be reliably informed. Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk> Link: https://lore.kernel.org/r/20230128125844.2407135-1-alvin@pqrs.dk Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.