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nixos/wireless: use udev to wait for interfaces
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I may have finally found a clean solution to the issues[1][2][3] with
the automatic discovery of wireless network interfaces.

[1]: NixOS#101963
[2]: NixOS#23196
[3]: NixOS#125917 (comment)

Currently the start script fails right away if no interface is available
by the time it's running, possibly leaving the system without network.
This happens when running a little early in the boot. A solution is to
instead wait for at least one interface to appear before scanning the
/sys/class/net/ directory. This is done here by listening for the right
udev events (from the net/wlan subsystem) using the `udevadm monitor`
command and grep to match its output.

This methods guarantees the availability of at least one interface to
wpa_supplicant, but won't add additional interfaces once it has started.
However, if the current interface is lost, say unplugged, the service is
automatically stopped and will be restarted as soon as a one (not
necessarily the same) is detected. It would be possible make this fully
dynamic by running another service that continously listen for udev
events and manages the main wpa_supplicant daemon, but this is probably
overkill.

I tested the following cases:

  - one interface, starting at boot, w/o predictable naming scheme
  - two interfaces, starting at boot (intel wireless and a usb adapter),
    w/o predictable naming scheme
  - one interface after the system booted, w/o predictable naming scheme
  - two interfaces after the system booted, w/o predictable naming scheme
  - unplugging and plugging back the current interface
rnhmjoj committed Aug 11, 2021
1 parent dc54724 commit 99e8af5
Showing 1 changed file with 26 additions and 25 deletions.
51 changes: 26 additions & 25 deletions nixos/modules/services/networking/wpa_supplicant.nix
Original file line number Diff line number Diff line change
@@ -42,11 +42,6 @@ in {
description = ''
The interfaces <command>wpa_supplicant</command> will use. If empty, it will
automatically use all wireless interfaces.
<warning><para>
The automatic discovery of interfaces does not work reliably on boot:
it may fail and leave the system without network. When possible, specify
a known interface name.
</para></warning>
'';
};

@@ -230,14 +225,6 @@ in {
message = ''options networking.wireless."${name}".{psk,pskRaw,auth} are mutually exclusive'';
});

warnings =
optional (cfg.interfaces == [] && config.systemd.services.wpa_supplicant.wantedBy != [])
''
No network interfaces for wpa_supplicant have been configured: the service
may randomly fail to start at boot. You should specify at least one using the option
networking.wireless.interfaces.
'';

environment.systemPackages = [ package ];

services.dbus.packages = [ package ];
@@ -257,31 +244,45 @@ in {
wantedBy = [ "multi-user.target" ];
stopIfChanged = false;

path = [ package ];
path = [ package pkgs.udev ];

script = let
configStr = if cfg.allowAuxiliaryImperativeNetworks
then "-c /etc/wpa_supplicant.conf -I ${configFile}"
else "-c ${configFile}";
in ''
if [ -f /etc/wpa_supplicant.conf -a "/etc/wpa_supplicant.conf" != "${configFile}" ]
then echo >&2 "<3>/etc/wpa_supplicant.conf present but ignored. Generated ${configFile} is used instead."
if [ -f /etc/wpa_supplicant.conf -a "/etc/wpa_supplicant.conf" != "${configFile}" ]; then
echo >&2 "<3>/etc/wpa_supplicant.conf present but ignored. Generated ${configFile} is used instead."
fi
iface_args="-s -u -D${cfg.driver} ${configStr}"
${if ifaces == [] then ''
for i in $(cd /sys/class/net && echo *); do
DEVTYPE=
UEVENT_PATH=/sys/class/net/$i/uevent
if [ -e "$UEVENT_PATH" ]; then
source "$UEVENT_PATH"
if [ "$DEVTYPE" = "wlan" -o -e /sys/class/net/$i/wireless ]; then
args+="''${args:+ -N} -i$i $iface_args"
fi
fi
# detect interfaces automatically
# check if there are no wireless interface
if ! find -H /sys/class/net/* -name wireless | grep -q .; then
# if so, wait until one appears
echo "Waiting for wireless interfaces"
grep -q '^ACTION=add' < <(stdbuf -oL -- udevadm monitor -s net/wlan -pu)
# Note: the above line has been carefully written:
# 1. The process substitution avoids udevadm hanging (after grep has quit)
# until it tries to write to the pipe again. Not even pipefail works here.
# 2. stdbuf is needed because udevadm output is buffered by default and grep
# may hang until more udev events enter the pipe.
fi
# add any interface found to the daemon arguments
for name in $(find -H /sys/class/net/* -name wireless | cut -d/ -f 5); do
echo "Adding interface $name"
args+="''${args:+ -N} -i$name $iface_args"
done
'' else ''
# add known interfaces to the daemon arguments
args="${concatMapStringsSep " -N " (i: "-i${i} $iface_args") ifaces}"
''}
# finally start daemon
exec wpa_supplicant $args
'';
};

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