I just test MNet Hub functionality on Moodle 2.7.7.
I've intall two instances of Moodle. 0n the first I turn on "Register all hosts (promiscuous mode)" (below, I name this Moodle "hub"), not on the second (below, I name this Moodle "client").
The message on the page "Manage peers" in Site administration said : "You can choose to register all hosts that try to connect to you automatically. This means that a record will appear in your hosts list for any MNet site that connects to you and requests your public key.". This when promiscuous mode is turn on.
This is comfirmed in the MoodelDocs page https://docs.moodle.org/22/en/MNet doc. I follow this doc for my tests.
In fact, in my test this is not OK, I observe that folowing.
After turn on the promiscuous mode on the Moodle "hub", I add the Moodle "hub" in the "Manage peers" page of the Moodle "client". This connecting is OK, but the Moodle "client" don't appears automatically in the hosts list of the Moodle "hub".
To the Moodle "client" appears in the hosts list of the Moodle "hub", the Moodle "client" must be added manually in the Moodle "hub". After this manual adding, all is OK : the services selected in All Hosts on the Moodle "hub" are actived for the Moodle "client".
I've a big doubt about this because my tests don't go in the direction that I undersand in the MoodleDocs and the text in the "Manage Peers" page of administration in Moodle.
I'm tacker of some informations about this.