Set-Up X11 Forwarding Over Ssh

How to forward X over SSH to run graphics applications remotely?

X11 forwarding needs to be enabled on both the client side and the server side.

On the client side, the -X (capital X) option to ssh enables X11 forwarding, and you can make this the default (for all connections or for a specific connection) with ForwardX11 yes in ~/.ssh/config.

On the server side, X11Forwarding yes must be specified in /etc/ssh/sshd_config. Note that the default is no forwarding (some distributions turn it on in their default /etc/ssh/sshd_config), and that the user cannot override this setting.

The xauth program must be installed on the server side. If there are any X11 programs there, it's very likely that xauth will be there. In the unlikely case xauth was installed in a nonstandard location, it can be called through ~/.ssh/rc (on the server!).

Note that you do not need to set any environment variables on the server. DISPLAY and XAUTHORITY will automatically be set to their proper values. If you run ssh and DISPLAY is not set, it means ssh is not forwarding the X11 connection.

To confirm that ssh is forwarding X11, check for a line containing Requesting X11 forwarding in the output of ssh -v -X. Note that the server won't reply either way, a security precaution of hiding details from potential attackers.

forwarding x11 by SSH.NET c# library

I found the solution:

 SSHInstance = new SshClient(HostOrIP, Port, Username, Password);
 SSHInstance.Connect();
 streamSSH = SSHInstance.CreateShellStream(ShellID, 1, 0, 0, 0, 2000000);

 ForwardedPortRemote forwardedPortRemote = new ForwardedPortRemote(6000 +X11ServerDisplay, HostOrIP, 6000 + X11LocalDisplay);
 SSHInstance.AddForwardedPort(forwardedPortRemote);
 forwardedPortRemote.Start();

SSH compression for X11 forwarding

X11 graphics take up a lot of bandwidth. If your remote host is some distance away (i.e. not on the LAN), then you'll probably suffer sluggishness in your exported X11 applications.

I'm not sure about SSH compression. Performance may depend on other factors, such as CPU performance. From the ssh man page:

     -C      Requests compression of all data (including stdin, stdout,
             stderr, and data for forwarded X11 and TCP connections).  The
             compression algorithm is the same used by gzip(1), and the
             “level” can be controlled by the CompressionLevel option for pro‐
             tocol version 1.  Compression is desirable on modem lines and
             other slow connections, but will only slow down things on fast
             networks.  The default value can be set on a host-by-host basis
             in the configuration files; see the Compression option.

Here are some other workarounds you can use to make things faster:

• Instead of interacting with the GUI using X11 forwarding, consider something else that has better optimization/compression, such as VNC or NX/FreeNX.
• Use the terminal version of emacs instead of the GUI version.

SSH to NetBSD 8.2 guest with X11 gives X11 forwarding request failed on channel 0

X11 forwarding is disabled by default both in the ssh client and in the sshd server. The setting that controls it in the client is called ForwardX11, and the setting in the server is called X11Forwarding.

When connecting from the Ubuntu machine to the NetBSD machine using ssh, the following configuration files are being used:

• On the ssh client (Ubuntu machine)
  
  • ~/.ssh/config
  • /etc/ssh/ssh_config
• On the sshd server (NetBSD machine)
  
  • /etc/ssh/sshd_config

With the ssh(1) client, however, you could just use the -X switch instead of editing any of the client configuration files. In fact, I would highly recommend using the -X switch over enabling X11 forwarding for all hosts (i.e. in a Host * block like I'm showing below).

If you do want to edit the ssh client configuration, the ssh_config(5) settings are the ones you already noted:

Host *
  ForwardX11 yes
  XauthLocation /usr/bin/xauth

The default for XauthLocation is already /usr/bin/xauth on Ubuntu, so there should be no need to specify it, unless you have another xauth program installed in another location and you want to use it with ssh.

The sshd(8) server on the NetBSD machine also has to allow X11 forwarding. These sshd_config(5) settings are the ones you still need to change:

X11Forwarding yes
XAuthLocation /usr/X11R6/bin/xauth

Again, the default for XAuthLocation is already /usr/X11R6/bin/xauth on NetBSD 8.2, so there should be no need to specify it.

After changing the sshd_config settings, you also need to restart the sshd service (as the root user):

service sshd restart

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