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xpra-ssh.md

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Access a remote server with ssh and xpra together

X forwarding by ssh

X forwarding allows you to run remote GUI application on your local machine. ssh X forwarding makes the communication secure by tunneling the X protocol. For example, after you type the following command on a termianl locally:

simon@local$ ssh -X remote  # ssh to remote machine with X forwarding enabled
simon@remote$ firefox       # Open Firefox Web Browser

a window of firefox browser opened on the remote will show on the local machine.

However, ssh X forwarding may be very slow because of the network latency. One solution would be to compress the communication via:

ssh -X -C -c blowfish-cbc,arcfour user@host

However, the cipher blowfish-cbc and arcfour are not available in OpenSSH anymore. See https://www.openssh.com/releasenotes.html.

Speed up X forwarding by xpra

xpra is a command line tool that can be used to complement ssh X forwarding. It is relatively latency-insensitive, thus can be used over worse links than standard X and faster.

xpra use a custom protocol to forward the display on the remote to the local machine. It also record the display for re-connecting from another local machine. From this point of view, xpra is similar to byobu, because you can connect the same display from everywhere by xpra, just as you can log into the same terminal session from everywhere by byobu.

Installation of xpra

To use xpra, you have to install it on both the remote and your local machine. For example, on Ubuntu:

# see https://askubuntu.com/questions/1203609/how-to-add-xpra-repository-list
# and https://xpra.org/beta/

# install https support for apt (which may be installed already):
sudo apt-get update
sudo apt-get install -y apt-transport-https

# add Xpra GPG key
wget -q https://xpra.org/gpg.asc -O- | sudo apt-key add -

# add Xpra repository
sudo add-apt-repository "deb https://xpra.org/ $(lsb_release -cs) main"

# install Xpra package
sudo apt-get update
sudo apt-get install -y xpra

See Repository Installation Instructions for Window Switch for instructions on other systems. Window Switch is a GUI front end for xpra. However, I think there are some bugs for Window Switch that makes it hard to use.

xpra usage examples

ml display rain

This example will show you how to

  1. Open a terminal session on the local machine (here we call it s1), type:

    simon@s1$ ssh test@remote         # ssh to the remote machine
test@remote$ xpra start :100      # Start a xpra session on a display numbered 100
test@remote$ export DISPLAY=:100  # Make sure subsequent GUIs show on the display 100
test@remote$ pip3 install mlhub   # Install mlhub -- The Machine Learning Hub
test@remote$ ml configure         # Configure mlhub
test@remote$ ml install rain      # Install the rain model in mlhub
test@remote$ ml configure rain    # Configure rain
test@remote$ ml display rain      # Display rain

    Nothing happens!? Wait util you finish the next step.

  2. Open another terminal session on the local machine (we call it s2), type:

    simon@s2$ xpra attach ssh:test@remote:100  # Attach the local display to the remote display 100

    then some rain-related windows will be popped up right away on the local machine, which should be shown on the remote machine. This is because you export your display on 100 and make xpra capture the display on 100, so when you attach via xpra on the display on 100, all visual things will show on the machine from which you start attaching.

  3. When you finish using the remote machine, just Ctrl+C on session s2 to dettach. Finally, make sure you stop xpra before you shutdown the remote machine by xpra stop :100 on session s1.

Open Pycharm

There are two ways to open Pycharm locally via xpra:

  1. The general way is to follow the steps in the previous section.

  2. A more easy way is directly starting Pycharm with one command:

    simon@local$ xpra start ssh:remote:100 --start=pycharm

    Then Pycharm will show on the local machine. However, there may be an issue via xpra, so this way may fail.

Reconnection

When you close the terminal where the above xpra attach are typed, all the GUI windows attached to the remote display will disappear. But you can still get them back again by typing xpra attach ssh:remote:100 in any machine, as long as the GUI windows are not closed by mouse click and the processes are still running on the remote machine.

Postscript

  • NoMachine is a GUI tool can be used in similar way. But I think it is not open source.

  • Actually, you don't need to specify a display number when start xpra, since xpra will use 0 by default if 0 is not used. And use the default display is better because you need not to stop it manually likexpra stop :100. So I put the following into ~/.bashrc:

    if ! xpra list | grep 'LIVE session at' > /dev/null; then xpra start; fi
export DISPLAY=:0

    Then I just need to xpra attach ssh:remote without providing the display number to get the GUIs and there is no need to stop xpra manually before I shutdown the remote machine.

  • Sometimes, xpra will not show the windows, then you can just re-attach. Though there are some issues with xpra, I still think it is not an bad alternative to X2Go.