mjhjmtu

I'm running a buildroot custom linux (kernel 4.9.87) on a custom board based on an Atmel Sama5d2.

I'm trying to configure the network on the eth0 interface using dhcp only at hotplug.

Originally, the interface was configured at boot using auto eth0 into /etc/network/interfaces

I changed it to allow-hotplug eth0 to have the following file

# cat /etc/network/interfaces 

auto lo
iface lo inet loopback

allow-hotplug eth0
iface eth0 inet dhcp

After this change, udhcpc is not run at boot and is not blocking linux boot for 30 seconds anymore if the cable is not plugged.

However, nothing happens when I plug or unplug the cable.

If I mannually run ifup -a I obtain an IP address on eth0 interface.

From this results, I don't think the problem comes from network configuration but more from handling low level events.

I've tried to monitor the plug/unplug of the cable using udevadm but I can't see any kernel event when I'm pluging and unpluging the cable (but I can see events from usb subsystem for example).

I have also written a udev rule to log events in case udevadm was not relaying this subsystem events.

# cat /etc/udev/rules.d/20-network.rules 
SUBSYSTEM=="net", ACTION=="add", RUN+="/test.sh add"
SUBSYSTEM=="net", ACTION=="remove" , RUN+="/test.sh remove" 

However, nothing is trigerred by this rule neither.

So right now I think I have an issue detecting network cable hotplug.

I could be a kernel configuration mistake as well as a hardware wiring mistake.

I don't really know how the plug/unplug event is detected at hardware level however I think that the event is sent to udev either directly by the driver itself or by one of the numerous kernel frameworks in which it's deployed.

Do you think I may be missing some kernel configuration ?

Do you think it's possible to trace the state cable at a lower level than using udev ? Maybe by directly interfacing with sysfs ?