/etc/xl2tpd/l2tp-secrets is in xl2tpd 1.3.6+dfsg-1.
This file is owned by root:root, with mode 0o600.
The actual contents of the file can be viewed below.
1 2 3 4 5 | # Secrets for authenticating l2tp tunnels
# us them secret
# * marko blah2
# zeus marko blah
# * * interop
|