Thunderbolt docking (WD15) Ethernet card with Dell XPS'13 Ubuntu

The WD15 was not tested/validated by Dell in any Linux. Check the WD15 drivers page in this document, "How to Use and Troubleshoot Dell Docking Station WD15". There is no driver listing for the Linux operating system. Only Windows 7/8.1/10. This, "WD15 (USB Type-C) Information, Compatibility and Specifications" page also only shows Windows 7/8.1/10.

Thunderbolt 3  unlike 2 or 1 has hardware level security.

https://christian.kellner.me/2017/12/14/introducing-bolt-thunderbolt-3-security-levels-for-gnulinux/

Thunderbolt 3 — supports different security levels:

• none: No security. The behavior is identical to previous Thunderbolt versions.
• dponly: No PCIe tunnels are created at all, but DisplayPort tunnels are allowed and will work.
• user: Connected devices must be authorized by the user. Only then will the PCIe tunnels be activated.
• secure: Basically the same as user mode, but additionally a key will be written to the device the first time the device is connected. This key will then be used to verify the identity of the connected device.

The active security level can normally be selected prior boot via a BIOS option, but it is interesting to note that in the future the none option is likely to go away. This of course means connected thunderbolt devices wont work at all unless they are authorized by the user from the running operating system.  Bolt is supposed to address this in linux.

sudo apt install bolt