How is the hidden interface bridge100 working? This interface bridge100 has been introduced with
Mavericks and exists on Yosemite.
(On Mountain Lion the same kind of mechanism was
taking place with the bridge0 associated with the visible interface: Thunderbolt Bridge).

This interface is related to the starting of Internet Sharing.
When Internet Sharing is off:

$ ifconfig bridge100
ifconfig: interface bridge100 does not exist
$


When Internet Sharing is on:

$ ifconfig bridge 100
bridge100: flags=8863&lt;UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICASRT&gt; mtu1500
        options=3&lt;RXCSUM,TXCSUM&gt;
        ether ••My_Ethernet_@MAC••
[...]


But this interface doesn't appear within System Preferences &gt; Network.



Where is the correct documentation about this hidden interface?

How to make this interface appear within the System Preferences &gt; Network list of interfaces?

How is this interface configuration managed?
(This is isn't a correct answer, but just a starting point.)

Not mandatory

This bridge interface is not at all necessary. Here is the way to inactivate it:


Stop Internet Sharing through the GUI.
Wait one full minute for all the processes to terminate their house keeping:

ps ax | egrep '[ /](PID|Int)'

modify the Internet Sharing plist:

cd /System/Library/LaunchDaemons
/usr/bin/sudo -s
# make a quick backup
cp com.apple.NetworkSharing.plist com.apple.NetworkSharing.plist~orig
vi com.apple.NetworkSharing.plist


within the ProgramArguments array, insert the 2 new arguments -B (disable bridge interface) and -v (verbose output), so that this array definition will look like:

&lt;array&gt;
    &lt;string&gt;/usr/libexec/InternetSharing&lt;/string&gt;
    &lt;string&gt;-B&lt;/string&gt;
    &lt;string&gt;-v&lt;/string&gt;
&lt;/array&gt;


save this new file and exit vi
start Internet Sharing through the GUI,
do a verification ifconfig -a.


Now Internet Sharing is directly working on en1 and all necessary processes are started correctly: pfctl, wifid, natpmpd, bootpd.