O'Reilly logo

SSH, The Secure Shell: The Definitive Guide, 2nd Edition by Robert G. Byrnes, Richard E. Silverman, Daniel J. Barrett

Stay ahead with the world's most comprehensive technology and business learning platform.

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, tutorials, and more.

Start Free Trial

No credit card required

X Forwarding

Now that you’ve seen general TCP port forwarding, we move to a new topic: forwarding of X protocol connections. X is a popular window system for Unix workstations, and one of its best features is its transparency. Using X, you can run remote X applications that open their windows on your local display (and vice versa, running local applications on remote displays). Unfortunately, the inter-machine communication is insecure and wide open to snoopers. But there’s good news: SSH X forwarding makes the communication secure by tunneling the X protocol.

X forwarding also addresses some firewall-related difficulties. Suppose you’re a system administrator with a set of exposed production machines on the other side of a firewall from you. You log into one of these machines using SSH, and want to run a graphical performance-monitoring tool, such as Solaris’s perfmon, that uses the X Window System. You can’t, though, because to do that, the external machine needs to make a TCP connection back to the internal machine you started on, and the firewall blocks it (as it should, since X is quite insecure). X forwarding solves this problem, permitting X protocol connections to pass through the firewall, securely tunneled via SSH.

Our discussion begins with a brief overview, then explains the details of X forwarding. In addition to explaining how to use X forwarding, we also expose the internals of X authentication and how it interacts with SSH, as well as other technical topics.

9.4.1 The ...

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, interactive tutorials, and more.

Start Free Trial

No credit card required