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When a software KVM cannot find a second computer, troubleshoot it as a chain of dependencies: both machines must be on a reachable local network, running compatible software, using the correct host identity and roles, and allowed through the required firewall ports. Only after those checks should you investigate discovery, trust prompts, or screen layout.
What “cannot detect” can mean
A software KVM shares a physical keyboard and mouse over the network. It normally does not transmit the other computer’s video output, so this problem is different from a monitor or HDMI fault. “Cannot detect” may describe several distinct states:
- The second computer never appears in the product’s computer list.
- The computer appears, but a connection attempt times out.
- The connection is rejected because a host name, security key, or trust fingerprint is wrong.
- The computers connect, but the pointer and keyboard do not cross to the other screen.
Use the sequence below. Do not jump straight to port changes or reinstallations: a wrong role or an unreachable network makes every later setting fail.
1. Confirm the product, version, and computer roles
Use the same software family
Install the same product and compatible version on every participating computer. Do not mix Synergy 1 and Synergy 3 components and expect them to configure identically. Symless’s troubleshooting guidance checked Synergy 3 version 3.7.2 and Synergy 1 version 1.21.4 on October 3, 2026; labels can differ in other releases and operating systems.
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Assign the sharing roles
The computer with the physical keyboard and mouse must be the sharing side. In Synergy 1 this is the server; the other machines are clients. In Synergy 3, select the intended primary computer. A client cannot discover a useful peer if the only running instance is configured as another client.
For Microsoft PowerToys Mouse Without Borders, create the security key on the first computer. On the second computer, enter that key together with the first computer’s host name. A missing or mismatched key produces a credential failure rather than a network-discovery problem.
2. Prove that the computers can reach each other
Check the local network first
Connect both computers to the same reachable local network, using Ethernet or Wi-Fi. Guest Wi-Fi, client-isolation settings, separate VLANs, and some VPN configurations can place two machines on networks that look similar but cannot communicate. A wired Ethernet connection is a useful diagnostic for unreliable Wi-Fi, but it cannot correct a wrong host name, role, or firewall rule.
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Find the sharing computer’s host name or local IP address, then test it from the other computer. On Windows Command Prompt, run:
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ping <hostname or IP>In PowerShell, Barrier’s troubleshooting guidance uses:
Test-Connection -ComputerName <hostname or IP>A successful response shows that basic network routing and, when using a name, name resolution are working. A failed ping points to a network path or host-resolution issue, although ping failure alone does not prove that the KVM application is the cause; some networks block ICMP while allowing application traffic.
If the host name fails but the local IP works, correct name resolution or use the verified address temporarily. Synergy 1 documentation recommends the server’s computer name because a router-assigned IP address can change. If the address changes later, the client may appear to have stopped detecting the server even though the network is healthy.
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Allow the application on the correct network
On Windows, review Windows Security > Firewall & network protection > Allow an app through firewall and allow the KVM application on the private network profile when appropriate. Check the host and client sides; a rule on only one computer is not enough. Windows Synergy installers normally create their rules, while macOS, Linux, and third-party firewall products may require an explicit allowance. Mouse Without Borders provides an Add a firewall rule setting.
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Prefer a scoped rule for the named application or documented local ports. Do not expose these ports to the public internet; the documented design is local-network communication.
Use the port that matches the product
| Product or service | Port and protocol | Purpose |
|---|---|---|
| Synergy Core (Synergy 1 and Synergy 3) | TCP 24800 | Main keyboard-and-mouse connection |
| Synergy 3 | TCP 24802 | Settings synchronization |
| Synergy 3 | TCP 24804 | Auto-discovery |
| Synergy 3 local service | TCP 24803 on 127.0.0.1 | Local-only communication; it does not require a network firewall opening |
These port assignments come from Symless’s current port guidance. Do not apply Synergy’s ports to Mouse Without Borders or Barrier automatically; each product has its own implementation and firewall behavior.
Interpret a port symptom
If the computers respond to ping but the KVM still times out, an application firewall rule or blocked product port is a likely next check. If the peer appears through discovery but the connection is refused, inspect the main connection port and the server/client role. If Synergy 3 cannot list the peer at all, also check TCP 24804 and multicast discovery conditions.
4. Separate discovery from manual connection
Synergy 3: add the computer manually
Synergy 3 discovers peers on the local network using multicast DNS by default. Discovery can fail even when direct communication works. If the application shows only one computer, open Manage computers > Manually add computer and enter the other computer’s verified host name or local address. This bypasses the discovery announcement; it does not bypass a firewall or an incorrect role.
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Synergy 1: configure the client explicitly
In Synergy 1, add each client in the server’s configuration grid. The configured client name must exactly match the name reported by the Synergy client, including spelling and punctuation. A computer can be online and reachable yet rejected when those identities differ.
Mouse Without Borders: use the key-and-name pair
Mouse Without Borders does not use Synergy’s server grid. Recheck the host name and security key on both computers, then refresh or reconnect from its device controls. Its InvalidKey and Timeout states are useful clues: the former points to credentials, while the latter usually warrants another reachability and firewall check.
5. Complete trust and security prompts
Some Synergy 1 Pro or business first connections display a TLS fingerprint on both computers. Compare the fingerprints and accept matching prompts. If the client does not trust the server fingerprint, it refuses the connection even when the address and port are correct.
Barrier also uses a trusted-server fingerprint. Its client will not proceed until the server identity is trusted. Treat an unexpected fingerprint as a security warning: verify that you are connecting to the intended computer instead of accepting blindly.
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6. If the peer is visible but input does not move
Verify the sharing side
Set the computer with the physical keyboard and mouse as the Synergy server or primary. A visible peer is not proof that input sharing is enabled; discovery and input routing are separate stages.
Fix the screen arrangement
Place each client in the layout next to the physical edge where you want the pointer to leave the current screen. If a laptop is physically to the left but configured to the right, the pointer may appear trapped even though the connection is healthy. Confirm that every client occupies an adjacent position rather than an isolated or overlapping one.
Check the keyboard lock state
Synergy’s troubleshooting checklist specifically calls out Scroll Lock. Turn Scroll Lock off and try crossing the screen edge again; this key state can keep the cursor on one computer.
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In Mouse Without Borders, use the device layout and refresh the connections after a link is lost. Recheck the layout before changing network settings, because a stale or incorrect arrangement can mimic a detection failure.
7. Use a controlled diagnostic reset
- Temporarily disconnect or disable VPN software if your organization permits it. Synergy’s troubleshooting guidance recommends this as a diagnostic because a VPN can alter routes, name resolution, or firewall profiles.
- Close and reopen the KVM application on every computer after changing roles, names, keys, or firewall permissions.
- Run the name-or-IP reachability test again, then retry the product’s connection.
- If the problem remains, restart all participating computers so background services, network profiles, and firewall rules reload.
- Undo experimental settings one at a time. Keep security controls enabled and retain only the narrowly scoped application or port allowance needed for the local connection.
A restart is a recovery step, not a substitute for correcting an unreachable network or mismatched identity. If the issue returns after every restart, record which stage fails: no peer listed, timeout, trust rejection, invalid key, or connected-without-input.
Product-specific differences to keep in mind
| Area | Synergy | Mouse Without Borders | Barrier |
|---|---|---|---|
| Operating systems | Documents Windows, macOS, and Linux; exact desktop and Wayland support varies. | Distributed as part of Windows PowerToys. | Platform and package support depend on the project build. |
| Configuration | Synergy 3 offers auto-discovery and manual addition; Synergy 1 uses a server/client configuration. | Uses a host name plus security key. | Uses a server/client arrangement and trusted-server identity. |
| Discovery and firewall | Documents Core port 24800 and Synergy 3 service ports 24802 and 24804. | Offers an “Add a firewall rule” control. | Its guidance emphasizes ping or Test-Connection and checking running processes. |
| Trust mechanism | TLS fingerprint prompts may appear on first Synergy 1 Pro or business connections. | Security-key validation; an invalid key is reported separately from a timeout. | Clients must trust the server fingerprint. |
| What is shared | Keyboard and mouse input; the architecture does not transmit screen contents. | Keyboard and mouse control between Windows computers. | Keyboard and mouse sharing, not automatically a video KVM. |
Use the instructions for the product actually installed. A port, menu, or discovery method documented for one product should not be treated as universal.
Common mistakes that waste time
- Testing only internet access: Both computers can browse the web while local peer traffic is blocked by guest-network isolation.
- Using a remembered IP forever: A router can assign a different local address; a stable computer name is preferable where the product supports it.
- Opening every firewall port: Broad rules increase exposure and hide the real cause. Allow the application or documented local port instead.
- Assuming visibility equals usability: Discovery, trust, and input routing are separate checks.
- Changing screen layout before connection: Layout cannot fix a timeout or invalid key; use it only after the peer connects.
- Disabling all security software: Use a temporary, scoped diagnostic rule rather than leaving antivirus or firewall protection off.
A short decision path
- No computer appears: confirm the same product/version, same reachable local network, host name resolution, firewall permission, and Synergy 3 discovery port; then manually add the peer.
- The computer appears but times out: test by name and IP, inspect the main application port, remove VPN routing as a diagnostic, and restart both applications.
- The connection is rejected: verify the Synergy client name, Mouse Without Borders security key, or trusted fingerprint.
- The connection works but the pointer stays put: set the correct server/primary, fix adjacent screen placement, turn off Scroll Lock, and refresh the device layout.
This sequence narrows the fault without treating every software KVM as the same product. Once network reachability, identity, firewall access, discovery or manual configuration, trust, and layout all agree, the second computer should be available for keyboard-and-mouse sharing.
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