Mumble

+1
This is exactly the kind of thing I'd want. I wouldn't have called the
feature request "Stealth mode" though leads to misunderstanding.

*What I meant is that if I am opening an open Mumble server for joint
*operation/conference, I'd like the unregistered user who land in the
lobby
*or root to not be able to see the people in the different channels until
he
*is dragged in one of them. Giving this way a relative (very relative)
*amount of secrecy until he gets in.

This is exactly what I want.

While I understand your concerns, I would say that it is to the admin to
decide based on the power you gave him. Eavesdropping via linked channel
(or not) is bound by the fact that it is possible via the ACL and because
the admin allowed it. For example, only people in my "directors" group can
link channels. So unless a director, nobody else can link channels.

In fact, all you are mentioning is based on how the admin (or manager) set
it up. If the admin fails to set it up correctly, there should be no
ethical problem on your side since it is his misuse of the power you gave
him.

What I meant is that if I am opening an open Mumble server for joint
operation/conference, I'd like the unregistered user who land in the lobby
or root to not be able to see the people in the different channels until he
is dragged in one of them. Giving this way a relative (very relative)
amount of secrecy until he gets in.

An ACL like ~out View deny (or something like that) would do the trick.
View being obviously on by default on a normal basis. When the server sends
the channel tree and information to a user, if a user is unregistered or
not part of a group having the view ACL, server doesn't send the client
list from that channel (or the entire list of clients)... or even the
channel name.

I think you misunderstood slicer. He only wanted to say that to do this
right the server mustn't reveal any information about the hidden client to
the other ones. This is very hard when you use features like whisper,
linked channels and so on. We could let the client hide them but this could
be circumvented by editing it.

In any case we are not sure if we would want to have such a feature as it
could be abused for stuff like secret eavesdropping.

I am not sure I understand this right...

Obviously, I wouldn't want to edit (or my users to edit their client) but
if I understand right, there would be an option or parameter somewhere to
restrict the server to send the users list of a channel?

There are two ways to do this;

You can "fake" it; the client still knows about all the other users, it
just doesn't show them.

You can do it properly; the server doesn't send "other client connected"
messages to the client until it can "see" those users.

The first is useless; anyone can edit the client and have it show all the
users anyway.
If you limit "seen players" to "players in my own channel", the second one
works as long as you don't ever use whispers or linked channels -- if
client A can't "see" client B, it will correctly reject all voice packets
claiming to be from client B as they are fake.

There's also the eavesdropping problem; this makes it exceedingly easy to
"hide" into a channel to eavesdrop on other people, and we're generally
opposed to such things.