man -k sharing
InternetSharing(8) - simple NAT/router configuration daemon
threads::shared(3pm) - Perl extension for sharing data structures between threads
man InternetSharing
man defaults
It appears the com.apple.nat.plist is not created until internet sharing is turned on, so you might want to turn it on through the GUI and examine the resulting plist file.
defaults read /Library/Preferences/SystemConfiguration/com.apple.nat
Note that the defaults command reads plist files, but fails if you specify the plist file extension ".plist"
Leave the file extension off when accessing a plist file with the defaults command.
Good luck.
There may be a simpler way, so examine the other SystemConfiguration/ plist files after enabling the internet sharing feature...