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Screen Sharing on Mac OS X and how to hide the screen of the target computer

Controlling the screen of your desktop computer here in Biochemistry is a really efficient way to work from a remote location, but unlike Remote Desktop on MS-Windows, Apple's screen sharing client doesn't automatically hide the screen of the computer being controlled (target computer). This is a problem if you're handling sensitive information.

Initial setup of Mac Screen Sharing is described by Apple here

Two workarounds are possible, the first being to dim down the display of the target computer and the second involving creating a "dummy account" on the target computer and using a sequence of steps to leave the screen properly locked while you log into your normal user account.


Contact julian.jordan@bioch.ox.ac.uk for more details

Dimming the screen

This is a quick and simple way to hide the screen of your target computer, but it's not secure - anybody with access to a keyboard plugged into the target computer could simply turn the brightness back up.

In an "emergency", though, it may be useful and is better than nothing.

To dim down your screen, download this Applescript Application onto the target computer, unzip and store it somewhere easy to find, e.g. your Applications folder.

Then, when making a remote Screen Sharing connection simply double-click the Screen Mute application from your remote session and the target computer's display will go black.

PLEASE NOTE - If your target system doesn't respond to keyboard brightness control (i.e. it's not an iMac, laptop or Apple display), then the Screen Mute application will have no effect!


Dummy Account method - preparatory steps

Set up a dummy account on your target computer, typically calling it "Screen Lock". Then go to System Preferences > Sharing > Screen Sharing and click the [+] under the "Allow access for:" box. Add "Screen Lock" to the list of allowed users.

Download this Applescript Application which can be triggered remotely to properly lock your screen. Save it in your Applications folder.


Dummy Account method - log in with Screen Lock account only

Next you'll need to have your Screen Lock account logged in, but not your usual user account (yet). 

If your normal user account is already logged out of the target computer:

Enter this string into the Safari address bar, assuming the short account name "screenlock" and adjusting the hostname as appropriate:

vnc://screenlock@hostname.bioch.ox.ac.uk

Enter the password for the Screen Lock account and log in.

If your normal user account is still logged in to the target computer:

Enter this string into the Safari address bar, adjusting yourusername and hostname as appropriate:

vnc://yourusername@hostname.bioch.ox.ac.uk

Use the Apple Menu to log your normal user out and then follow the steps above to log in with the Screen Lock account.


Dummy Account method - launch the Screen Lock App

Now that you're remotely Screen Sharing to the Screen Lock account, launch the Screen Lock App installed in the last-but-one step. Close the Screen Sharing window containing the view of the remote computer using the red button at the top left.

The remote computer is now screen locked.


Dummy Account method - log in with your normal user account

Enter this string into the Safari address bar, adjusting yourusername and hostname as appropriate:

vnc://yourusername@hostname.bioch.ox.ac.uk

Because the Screen Lock account is already logged in, you will be prompted "Ask for permission to view the display / Log in as yourself?" - choose "Log in as yourself" and enter your password. The Mac will now run two user accounts simultaneously - the password-locked one on the target computer and your normal user account on the screen sharing session.


Dummy Account method - returning to the target computer

When you return to your target computer at a later date, simply click the "Switch User" option to list all logins and resume your normal user's session.



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