There are many alternatives to Synergy for Linux if you are looking for a replacement. The best open source Linux alternative is Barrier. If that doesn't suit you, our users have ranked more than 10 alternatives to Synergy and five of them are open source and available for Linux so hopefully you can find a suitable replacement. Other interesting open source Linux alternatives to Synergy are Input Leap, Deskflow, x2vnc (win2vnc) and Lan Mouse.
Barrier is a cross-platform software utility for sharing your mouse and keyboard between multiple computers on your desk. It was forked from Synergy version 1.9's codebase.
Input Leap is software that mimics the functionality of a KVM switch, which historically would allow you to use a single keyboard and mouse to control multiple computers by physically turning a dial on the box to switch the machine you're controlling at any given moment.
Deskflow is a free and open-source keyboard and mouse sharing app. Use the keyboard, mouse, or trackpad of one computer to control nearby computers, and work seamlessly between them. It's like a software KVM (but without the video). TLS encryption is enabled by default.
This program will let you use two screens on two different computers as if they were connected to the same computer.
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This is the only cross-platform solution I've found so far, and it's working a heck of a lot better than Synergy!