Systemd networking service fails because of unknown interface

Question

My problem is related to USB tethering and network interfaces.

When I run ifconfig -a, normally I see 3 interfaces - loopback, WiFi and Ethernet. I have defined gateway, netmask and broadcast for each one of them in /etc/networking/interfaces.

Sometimes I connect my phone to my machine for USB tethering and when running ifconfig -a I can see 1 more interface called enp0s20u2. This interface is shown only when the phone is connected and USB tethering is enabled in it. In order to bring up that interface, I've added the following lines in /etc/networking/interfaces:

# For USB tethering
auto enp0s20u2
iface enp0s20u2 inet dhcp
    address 192.168.42.56
    gateway 192.168.42.1
    netmask 255.255.255.0
    broadcast 192.168.42.255

This way, When I connect my phone, this interface is automatically brought up and configured.

The problem is that the systemd service can't start if the phone is not connected. The enp0s20u2 interface is unknown most of the time, Therefore systemctl fails to bring it up on startup as well as when I manually run systemctl restart networking.

Is there a way to assure this interface configuration won't make the service fail to start?

Answer 1

Replacing the auto directive with allow-hotplug takes care of this gracefully. The interface will no longer be brought up by ifup -a (and therefore not by the systemd networking service), but it will be brought up via udev when it is plugged in, or detected at boot.

# For USB tethering
allow-hotplug enp0s20u2
iface enp0s20u2 inet dhcp
    address 192.168.42.56
    gateway 192.168.42.1
    netmask 255.255.255.0
    broadcast 192.168.42.255

From man 5 interfaces:

Lines beginning with "allow-" are used to identify interfaces that should be brought up automatically by various subsytems. This may be done using a command such as "ifup --allow=hotplug eth0 eth1", which will only bring up eth0 or eth1 if it is listed in an "allow-hotplug" line. Note that "allow-auto" and "auto" are synonyms. (Interfaces marked "allow-hotplug" are brought up when udev detects them. This can either be during boot if the interface is already present, or at a later time, for example when plugging in a USB network card. Please note that this does not have anything to do with detecting a network cable being plugged in.)