X Protocol Forwarding
The X Window System provides support for graphical display on UNIX systems. X protocol forwarding provides a way to secure the communication between X clients and remote X servers. X forwarding is enabled by default. X forwarding works as follows:
Working with X11 Settings
The client setting ForwardX11 enables or disables X11 forwarding. (The default is yes.) The client setting TrustX11Applications specifies whether the X server treats forwarded X11 client applications as trusted. (The default is no.)
Under some conditions, the configuration of these settings may affect the launch speed of X client applications. This happens when more than two systems are involved. For example:
System1 runs an X server and the Secure Shell client.
When a user makes an ssh connection from System1 to System2 with X11Forwarding=yes (the default) and TrustX11Applications=no (the default), there is no delay in starting X applications.
If the user makes a subsequent ssh connection from the new shell to System3 with X11Forwarding=yes (the default) and TrustX11Applications=no (the default), there will be a long delay (as much as 6 seconds) after the user authenticates during which X applications started from System3 will not be displayed to the X server running on System1. This delay is added by the xauth application as it tries to communicate with the X server and register a new cookie. In order to avoid this delay and run the X applications from System3, set TrustX11Applications=yes for the second connection.
Note: Setting TrustX11Applications=yes for the second connection does not create any additional security risk to the X server running on System1. This is because the xauth application registers into the existing cookie created on System2 by the initial X11 forwarding (done from System1), for which TrustX11Applications=no.