otbr can't create wpan0

[goossensbas]

Describe the bug A clear and concise description of what the bug is.

otbr cannot create the wpan0 interface and crashes on start

To Reproduce Information to reproduce the behavior, including:

1. Git commit id:
   commit 04bbe756a6a31261409bb0815cb1b2dfe6d7e310

2. IEEE 802.15.4 hardware platform
   nrf52840Dongle

3. Build steps

$ git clone https://github.com/openthread/ot-br-posix --depth 1
$ cd ot-br-posix
$ sudo apt-get install avahi-daemon avahi-discover avahi-utils
$ ./script/bootstrap WEB_GUI=1 OTBR_MDNS="avahi"
$ export NAT64=1 NAT64_SERVICE=openthread
$ INFRA_IF_NAME=X2 WEB_GUI=1 USE_AVAHI=1 ./script/setup

4. Network topology
   hardware: Wago Edge PC with network plugged into X2

Expected behavior A clear and concise description of what you expected to happen.
OTBR creates the wpan0 interface

Console/log output If applicable, add console/log output to help explain your problem.

Jan 04 08:34:43 edgebas systemd[1]: Starting otbr-agent.service - OpenThread Border Router Agent...
░░ Subject: A start job for unit otbr-agent.service has begun execution
░░ Defined-By: systemd
░░ Support: https://www.debian.org/support
░░
░░ A start job for unit otbr-agent.service has begun execution.
░░
░░ The job identifier is 83224.
Jan 04 08:34:47 edgebas systemd[1]: Started otbr-agent.service - OpenThread Border Router Agent.
░░ Subject: A start job for unit otbr-agent.service has finished successfully
░░ Defined-By: systemd
░░ Support: https://www.debian.org/support
░░
░░ A start job for unit otbr-agent.service has finished successfully.
░░
░░ The job identifier is 83224.
Jan 04 08:34:47 edgebas otbr-agent[32994]: [NOTE]-AGENT---: Backbone interface: X2
Jan 04 08:34:47 edgebas otbr-agent[32994]: [NOTE]-AGENT---: Running 0.3.0-04bbe75
Jan 04 08:34:47 edgebas otbr-agent[32994]: [NOTE]-AGENT---: Thread version: 1.4.0
Jan 04 08:34:47 edgebas otbr-agent[32994]: [NOTE]-AGENT---: Thread interface: wpan0
Jan 04 08:34:47 edgebas otbr-agent[32994]: [NOTE]-AGENT---: Radio URL: spinel+hdlc+uart:///dev/ttyACM0
Jan 04 08:34:47 edgebas otbr-agent[32994]: [NOTE]-AGENT---: Radio URL: trel://X2
Jan 04 08:34:47 edgebas otbr-agent[32994]: [NOTE]-ILS-----: Infra link selected: X2
Jan 04 08:34:47 edgebas otbr-agent[32994]: [INFO]-RCP_HOS-: OpenThread log level changed to 4
Jan 04 08:34:47 edgebas otbr-agent[32994]: 49d.18:14:48.083 [I] P-SpinelDrive-: co-processor reset: RESET_POWER_ON
Jan 04 08:34:47 edgebas otbr-agent[32994]: 49d.18:14:48.083 [C] P-SpinelDrive-: Software reset co-processor successfully
Jan 04 08:34:47 edgebas otbr-agent[32994]: [INFO]-BA------: Ephemeral Key is: enabled during initialization
Jan 04 08:34:47 edgebas otbr-agent[32994]: 49d.18:14:48.091 [C] Platform------: platformConfigureTunDevice() at netif.cpp:2022: No such file or directory
Jan 04 08:34:47 edgebas systemd[1]: otbr-agent.service: Main process exited, code=exited, status=5/NOTINSTALLED
░░ Subject: Unit process exited
░░ Defined-By: systemd
░░ Support: https://www.debian.org/support
░░
░░ An ExecStart= process belonging to unit otbr-agent.service has exited.
░░
░░ The process' exit code is 'exited' and its exit status is 5.
Jan 04 08:34:47 edgebas systemd[1]: otbr-agent.service: Failed with result 'exit-code'.
░░ Subject: Unit failed
░░ Defined-By: systemd
░░ Support: https://www.debian.org/support
░░
░░ The unit otbr-agent.service has entered the 'failed' state with result 'exit-code'.

Additional context Add any other context about the problem here.

the otbr-agent worked before. I reinstalled a newer OS on the edge PC, and now i get the above errors.

[jwhui]

The error indicates that otbr-agent is not able to open a tun interface:

openthread/src/posix/platform/netif.cpp

Lines 2021 to 2022 in 54f6c27

	sTunFd = open(OPENTHREAD_POSIX_TUN_DEVICE, O_RDWR | O_CLOEXEC | O_NONBLOCK);
	VerifyOrDie(sTunFd >= 0, OT_EXIT_ERROR_ERRNO);

I don't know the specifics of your OS, but I would make sure it supports opening tun interfaces with the following path:

openthread/src/posix/platform/netif.cpp

Line 180 in 54f6c27

#define OPENTHREAD_POSIX_TUN_DEVICE "/dev/net/tun"