This is a pair of scripts which facilitate VNC desktop sharing using
x11vnc via an ssh tunnel, allowing sharing through firewalls.

So far this is only an experiment - and tested on Linux only.

Suppose the user wishing to share her desktop is called Alice.  Bob
wants to view Alice's desktop.  Alice and Bob both have accounts on a
server they can connect to via ssh (alice@gateway.host and
bob@gateway.host).  Both local and remote port forwarding is enabled
on that server.

That server must also have a index directory created, say
/etc/vnc-gateway, and populated with files named for each user who may
want to share their desktop.  Each file should contain a unique port
number for a user for connections to their desktop.

e.g. on gateway.host:

  gateway.host$ mkdir /etc/vnc-gateway
  gateway.host$ echo 5910 > /etc/vncgateway/alice
  gateway.host$ echo 5911 > /etc/vncgateway/charlie   # etc.

Alice must have x11vnc installed.  Bob must have vncviewer installed.
Both put the showdesktop and viewdesktop scripts somewhere they can
execute them.  This example assumes they're in their shell paths.

Alice creates a ~/vnc-gateway file on her desktop machine like this:

  GATEWAY_LOGIN=alice
  GATEWAY_HOST=gateway.host
  GATEWAY_PORT_DIR=/etc/vnc-gateway
  X11VNC_OPTS='-viewonly -shared' # grant no control; allow many visitors

Bob creates a similar one on his:

  GATEWAY_LOGIN=bob
  GATEWAY_HOST=gateway.host
  GATEWAY_PORT_DIR=/etc/vnc-gateway
  X11VNC_OPTS='-viewonly -shared' # grant no control; allow many visitors

Alice then runs showdesktop:

  alice$ showdesktop 

She will be prompted for her password twice, unless she has set up ssh
shared key authentication.  x11vnc will run.

Bob can then connect to Alice's desktop like this:

  bob$ viewdesktop alice

If Charlie sets up a ~/.vnc-gateway too, and runs showdesktop, bob can
view his desktop like this:

  bob$ viewdesktop charlie

Charlie can also view Alice's desktop (simultaneously with Bob, since
Alice defined the flag -shared in X11VNC_OPTS):

  charlie$ viewdesktop alice

Howver because Alice also defined the -viewonly flag, neither Charlie
or Bob can only watch what Alice does, and not do anything on her
desktop themselves.