Product: Synergy (all versions)
Operating system: Windows, macOS, and Linux
What is a port?
When your computer receives a message from another computer over the network (in our case the LAN, or, local area network), it needs to know which process is expecting the message. That's where ports come in. When a process first uses the network, it "binds" itself to a port number that is also included in the message it sends out so the other computer(s) know which port to send back to. Then, when your computer gets the response, it looks at the port number and passes the message on to the process that's bound to that port.
Port numbers
Synergy 1 uses port 24800.
Synergy 3 also uses port 24800, but adds two extra ports, 24802 and 24804.
How we use the ports
Port 24800 is used by Synergy Core, a common component used by both Synergy 1 and Synergy 3. The Core is the part of our app that does that actual keyboard and mouse sharing.
In Synergy 1, the Core process that uses port 24800 is called "synergy-server" or "synergy-client"("synergys" or synergyc" in versions older than 1.17.1).
In Synergy 3, the Core process is called "synergy-core".
Ports 24802 and 24804 are used by the Background Service in Synergy 3, which manages auto discovery, config synchronization, and launching of Synergy Core. The process name of the Background Service is "synergy-service".
Note: In case you're wondering, about 24801 and 24803, these are for local interprocess communication (IPC) and not used over the network so you won't need to add these to your firewall. Synergy 1 and Synergy 3 use port 24801 for communicating between the Daemon and Core on Windows. Synergy 3 uses 24803 for IPC between the GUI and the Background Service on all operating systems.
What you can do
Normally you won't need to do anything because the setup process for both Synergy 1 and Synergy 3, ensures that the correct ports are open with your operating system's native firewall.
However, if you're using a 3rd party firewall (one that you or your company installed or setup), you may need to manually add either the correct Synergy process or port numbers to allow traffic through your firewall to Synergy's processes. Please check the documentation for your firewall if you're not sure how.
How to change the port
In Synergy 1, you can change the port number for the Core process. You may want to do this, for example, if you have a port conflict (e.g. some other app is using 24800 for some reason). This can be done in the Settings.
In Synergy 3, you can change the Core port number (right now this is under the Miscellaneous menu but this is likely to change in the near future). Unfortunately, at this time you can't change the Background Service port number (this is a planned feature).