Ubuntu

Find all files in a directory /path/to/directory and its sub-directories recursively:

find /path/to/directory/ -size 0 -type f

or

find /path/to/directory/ -size 0 -type f -print

Extend:

1. Change permission of all files:

find /<path_to_desired_directory> -type f -print0 | xargs -0 chmod 0755

2. Change permission of all folder:

find /<path_to_desired_directory> -type d -print0 | xargs -0 chmod 0644

When I searched a solution for connecting through VPN with L2TP/IPSec on Ubuntu (I used Ubuntu 18.04 at this time), I found this article and kept it in my note.

Just as a followup to the great information you provided, thought I'd give people a complete set of steps I took to get this working on Ubuntu GNOME 17.04: