How do you diagnose your Internet dropping out under Ubuntu 16.04?

I have a Dell 5520 laptop, and it’s Internet (both wired and wireless) randomly stops working for seemingly no reason. I don’t think it’s my router, as no other device on my network exhibits this problem. Also, simply turning my wifi off and on, or unplugging and then re-plugging the ethernet cable immediately fixes it, so it seems like it’s some local bug with DHCP or the networking driver.

As you can imagine, this is quite frustrating, and breaks a lot of automated systems that need Internet access.

How do I diagnose this? Running cat /var/log/syslog |grep -i network finds:

Nov 30 08:15:47 localhost dhclient[3646]: receive_packet failed on enxd481d72559eb: Network is down Nov 30 08:15:47 localhost NetworkManager[29467]: <info>  [1512047747.9144] device (enxd481d72559eb): state change: activated -> unmanaged (reason 'removed') [100 10 36] Nov 30 08:15:47 localhost NetworkManager[29467]: <info>  [1512047747.9360] dhcp4 (enxd481d72559eb): canceled DHCP transaction, DHCP client pid 3646 Nov 30 08:15:47 localhost NetworkManager[29467]: <info>  [1512047747.9360] dhcp4 (enxd481d72559eb): state changed bound -> done Nov 30 08:15:47 localhost NetworkManager[29467]: <info>  [1512047747.9367] dns-mgr: Writing DNS information to /sbin/resolvconf Nov 30 08:15:47 localhost NetworkManager[29467]: <info>  [1512047747.9567] manager: NetworkManager state is now CONNECTED_LOCAL Nov 30 08:15:48 localhost systemd[1]: Starting Network Manager Script Dispatcher Service... Nov 30 08:15:48 localhost systemd[1]: Started Network Manager Script Dispatcher Service. Nov 30 08:15:48 localhost NetworkManager[29467]: <error> [1512047748.7027] platform-linux: do-change-link[23]: failure changing link: failure 19 (No such device) Nov 30 08:15:48 localhost NetworkManager[29467]: <warn>  [1512047748.7030] device (enxd481d72559eb): failed to disable userspace IPv6LL address handling Nov 30 08:15:48 localhost NetworkManager[29467]: <info>  [1512047748.7033] devices removed (path: /sys/devices/pci0000:00/0000:00:1d.6/0000:05:00.0/0000:06:02.0/0000:3d:00.0/usb4/4-1/4-1:1.0/net/enxd481d72559eb, iface: enxd481d72559eb) Nov 30 08:16:03 localhost NetworkManager[29467]: <warn>  [1512047763.7603] device (eth0): failed to find device 24 'eth0' with udev Nov 30 08:16:03 localhost NetworkManager[29467]: <info>  [1512047763.7620] manager: (eth0): new Ethernet device (/org/freedesktop/NetworkManager/Devices/8) 

Clearly, that shows there’s some sort of network problem occurring, but I’m not sure what. How do I figure out what’s going on here?