In today's show we will talk about installing synergy so that you can control the keyboard and mouse of another computer securely over ssh.

Install synergy on both computers as root

# dnf install synergy
# apt install synergy

The main pc is pc_middle and it is the one with the keyboard and mouse we intend to use for all the computers.

The only other pc in this configuration is, one on the right which we call pc_right

On pc_middle create a configuration file. I put it in ~/etc/synergy-work.conf

section: screens
    pc_middle:
    pc_right:
end

section: links
pc_middle:
    right = pc_right
pc_right:
    left  = pc_middle
end

On pc_middle add entry to ~/.ssh/config to allow portforwarding back, for the synergy port 24800

Host pc_right
    Hostname 192.168.0.150
    RemoteForward 127.0.0.1:24800 127.0.0.1:24800 # send back from the client to me

On pc_middle run synergy server in the foreground with debug enabled

[user@pc_middle ~]$ synergys --debug DEBUG --no-daemon --server --address 127.0.0.1 --config ~/etc/synergy-work.conf --name pc_middle --log /tmp/synergy-work.conf.log
[2022-12-03T16:29:05] DEBUG: opening configuration "~/etc/synergy-work.conf"
[2022-12-03T16:29:05] DEBUG: configuration read successfully
[2022-12-03T16:29:05] DEBUG: XOpenDisplay(":0")
[2022-12-03T16:29:05] DEBUG: xscreensaver window: 0x00c00001
[2022-12-03T16:29:05] DEBUG: screen shape: 0,0 5760x2160 (xinerama)
[2022-12-03T16:29:05] DEBUG: window is 0x05e00004
[2022-12-03T16:29:05] DEBUG: adopting new buffer
[2022-12-03T16:29:05] DEBUG: opened display
[2022-12-03T16:29:05] WARNING: LANGUAGE_DEBUG Poll result 0
[2022-12-03T16:29:05] DEBUG: registered hotkey ScrollLock (id=ef14 mask=0000) as id=1
[2022-12-03T16:29:05] NOTE: started server, waiting for clients
[2022-12-03T16:29:05] DEBUG: event queue is ready
[2022-12-03T16:29:05] DEBUG: add pending events to buffer
[2022-12-03T16:29:05] DEBUG: screen "pc_middle" shape changed

On pc_middle you can check that it's running

[user@pc_middle ~]$ netstat -anp | grep 24800
tcp        0      0 127.0.0.1:24800     0.0.0.0:*               LISTEN      90859/synergys

On pc_middle you can connect to pc_right

[user@pc_middle ~]$ ssh pc_right
user@pc_right:~$

On pc_right (either on its own keyboard, or via ssh session from pc_middle), check that port 24800 is listening

user@pc_right:~$ netstat -anp | grep 24800
tcp        0      0 127.0.0.1:24800         0.0.0.0:*               LISTEN

On the other keyboard that is connected to the pc_right (see note below †)

user@pc_right:~$ synergyc --debug INFO --no-daemon --name pc_right 127.0.0.1
[2022-12-03T16:38:59] NOTE: started client
        /build/synergy-3N7yN5/synergy-1.8.8-stable+dfsg.1/src/lib/synergy/ClientApp.cpp,404
[2022-12-03T16:38:59] NOTE: connecting to '127.0.0.1': 127.0.0.1:24800
        /build/synergy-3N7yN5/synergy-1.8.8-stable+dfsg.1/src/lib/client/Client.cpp,146
[2022-12-03T16:38:59] NOTE: connected to server
        /build/synergy-3N7yN5/synergy-1.8.8-stable+dfsg.1/src/lib/synergy/ClientApp.cpp,294

Back on pc_middle, you should see the the log that you have connected

[2022-12-03T16:40:15] DEBUG: Opening new socket: 18FC73A0
[2022-12-03T16:40:15] NOTE: accepted client connection
[2022-12-03T16:40:16] DEBUG: received client "pc_right" info shape=0,0 5760x2160 at 2787,1371
[2022-12-03T16:40:16] NOTE: client "pc_right" has connected

† Note: If you tried to run the client synergyc over the ssh connection on pc_middle it will connect, but the mouse will never move to the other screen.

Now from the pc_middle, you should be able to move the mouse over to the pc_right screen.

Now using the keyboard and mouse on the pc_middle, you should now be able to move the mouse and type on the pc_right screen.

The logs on the pc_middle, should show you information about switching from one computer to the other.

[2022-12-03T17:05:18] INFO: switch from "pc_middle" to "pc_right" at 0,225
[2022-12-03T17:05:18] INFO: leaving screen
[2022-12-03T17:05:18] WARNING: LANGUAGE_DEBUG Poll result 0
[2022-12-03T17:05:18] DEBUG: open clipboard 0
[2022-12-03T17:05:18] DEBUG: ICCCM fill clipboard 0
[2022-12-03T17:05:18] DEBUG:   available targets: text/plain (654), UTF8_STRING (445), STRING (31), TEXT (444)
[2022-12-03T17:05:18] DEBUG: added format 0 for target UTF8_STRING (445) (8 bytes)
[2022-12-03T17:05:18] DEBUG: close clipboard 0
[2022-12-03T17:05:18] INFO: screen "pc_middle" updated clipboard 0
[2022-12-03T17:05:18] DEBUG: open clipboard 1
[2022-12-03T17:05:18] DEBUG: ICCCM fill clipboard 1
[2022-12-03T17:05:18] DEBUG:   available targets: text/plain (654), UTF8_STRING (445), STRING (31), TEXT (444), text/html (653)
[2022-12-03T17:05:18] DEBUG: added format 1 for target text/html (653) (113 bytes)
[2022-12-03T17:05:18] DEBUG: added format 0 for target UTF8_STRING (445) (5 bytes)
[2022-12-03T17:05:18] DEBUG: close clipboard 1
[2022-12-03T17:05:18] INFO: screen "pc_middle" updated clipboard 1
[2022-12-03T17:05:18] DEBUG: sending clipboard 0 to "pc_right"
[2022-12-03T17:05:18] DEBUG: sent clipboard size=20
[2022-12-03T17:05:18] DEBUG: sending clipboard 1 to "pc_right"
[2022-12-03T17:05:18] DEBUG: sent clipboard size=138
[2022-12-03T17:05:19] INFO: switch from "pc_right" to "pc_middle" at 5757,583
[2022-12-03T17:05:19] INFO: entering screen
[2022-12-03T17:05:19] DEBUG: send xscreensaver command: 582 0 0

Back on pc_right you can close the client by holding Control and pressing C, or Ctrl+C for short.

The logs on the pc_middle, should show you that the client disconnected.

[2022-12-03T16:40:18] NOTE: client "pc_right" has disconnected
[2022-12-03T16:40:18] DEBUG: Closing socket: 18FC73A0

As we are running over ssh, there is no need to configure --enable-crypto but you can if you wish.

Now that everything is working correctly you can make it easier to start.

As we saw before (†) the client needs to be run from the physical X Session that you see on the second computer.

On pc_right create a new bash script file eg: nano ~/bin/start-synergy-client.bash

#!/bin/bash
killall synergyc
sleep 2
synergyc --name pc_right 127.0.0.1
exit 0

Still on pc_right allow the file to be executable chmod +x ~/bin/start-synergy-client.bash

Still on pc_right and in the session you wish to control, run start-synergy-client.bash

I find it easiest to just run this in a shell once I login on pc_right, but you could configure it to run automatically once you log in.

Back on pc_middle, create a new bash script file eg: nano ~/bin/start-synergy-server.bash

#!/bin/bash
server_name=synergys # may also be synergy-core
killall "${server_name}"
${server_name} --server --address 127.0.0.1 --config ~/etc/synergy-work.conf --name pc_middle --log /tmp/synergy-work.conf.log
setxkbmap -option "compose:ralt"
setxkbmap -option "ctrl:nocaps"
ssh pc_right

Still on pc_middle allow the file to be executable chmod +x ~/bin/start-synergy-server.bash

Still on pc_middle you can run the command start-synergy-server.bash and it will open a ssh shell to pc_right.

Over that connection pc_right can send back commands to the server.

A side note about the special address 127.0.0.1.

It's often referred to as loopback, home, or localhost and is usually defined in /etc/hosts

The address is used by programs running on a given computer to communicate with other programs running on the same computer.

https://en.wikipedia.org/wiki/Localhost

IPv4 network standards reserve the entire address block 127.0.0.0/8 (more than 16 million addresses) for loopback purposes.

If you are confused, then just think of it like you when your boss says "I'm going home now, you should also go home."

It's clear that they mean "I'm going to my home now, and you should also go to your home."

So the address 127.0.0.1 on pc_middle is only available on pc_middle, and equally the address 127.0.0.1 on pc_right is only available on pc_right.

The server is listening on its loopback address 127.0.0.1 on pc_middle, while the client is listening on its loopback address 127.0.0.1 on pc_right

It is the RemoteForward configuration that creates a ssh tunnel that is doing the heavy lifting.

RemoteForward 127.0.0.1:24800 127.0.0.1:24800

It tells the Remote (in this case pc_right ) to listen to the port 24800 its loopback address.

This is where the client on pc_right will be talking to.

The ssh connection will then Forward any packets back to the other side (in this case pc_middle )

And to send to the port 24800 its loopback address.

And on that address the server is listening.