If a software KVM cannot find the other computer, troubleshoot it as a chain of prerequisites rather than as one mysterious fault. Confirm that both machines are on the same reachable local network, running compatible software, assigned the correct server/primary and client roles, and using the right computer name or security key. Then check firewall permissions, product-specific ports, discovery, and trust prompts.
A computer that appears in the app is not necessarily ready to share input. Screen layout, client names, Scroll Lock, and the selected primary computer determine whether the keyboard and mouse actually cross from one display to the next.
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First, identify which stage is failing
“Cannot detect” can describe several different failures. Separating them prevents you from changing unrelated settings.
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- The computer appears but will not connect: check the host name or IP address, security key, server/client assignment, ports, and trust prompts.
- It connects but the pointer will not cross: inspect the configured screen arrangement, client names, primary/server role, and Scroll Lock.
- The connection repeatedly times out: treat it as a reachability, credential, or firewall problem rather than a display-layout problem.
A software KVM normally shares a physical keyboard and mouse over the local network. It does not automatically transmit the other computer’s screen. Synergy’s architecture documentation describes keyboard and mouse sharing, not video-KVM functionality.
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1. Confirm the product, versions, and roles
Use the same product on every computer
Install the same product and, where the product requires it, the same major version on each participating machine. Synergy’s current troubleshooting guidance covers Synergy 3 version 3.7.2 and Synergy 1 version 1.21.4 as checked on October 3, 2026. Instructions and port requirements differ between Synergy 1, Synergy 3, Barrier, and Mouse Without Borders, so do not apply one product’s settings to another.
Assign the keyboard-and-mouse computer correctly
In Synergy 1, the computer with the physical keyboard and mouse is the server; every other machine is a client. Synergy 3 calls the sharing computer the primary. If the roles are reversed, a peer may be visible while input sharing still fails.
Mouse Without Borders uses a different setup: generate a security key on the first computer, then enter that key and the first computer’s host name on the second computer. The host name and key must match exactly.
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2. Prove that the computers can reach each other
Check the local network
Both computers must be connected to the same reachable local network, either by Wi-Fi or Ethernet. Guest Wi-Fi, client-isolation settings, separate VLANs, and some corporate networks can prevent devices from talking even when they have internet access. Confirm the network name and connection status on both machines before changing KVM settings.
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Test by name and by address
Use the peer’s computer name or IP address to test basic reachability. Barrier’s troubleshooting guidance gives these examples:
ping <hostname or IP>Test-Connection -ComputerName <hostname or IP>Run the first command in Windows Command Prompt or the second in PowerShell, replacing the placeholder with the server or primary computer’s actual name or address. A failed test points to a network path or name-resolution problem, although a blocked ping alone does not prove that the KVM connection itself is broken; some networks and firewalls suppress ICMP replies.
Prefer a stable identity
For Synergy 1, Symless recommends entering the server’s computer name rather than relying on an address assigned by the router. A router can change a DHCP-assigned IP address after a reconnect or lease renewal. If the name does not resolve, test the current IP address temporarily to distinguish name-resolution trouble from a wider connection failure.
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A wired Ethernet connection can be a useful diagnostic when Wi-Fi is unstable. It will not correct a wrong server name, a mismatched key, or a blocked port, and it is not required when the wireless network is configured correctly.
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3. Check firewall permissions and ports
Allow the application on the local network
Permit the KVM application, and its background service when the product uses one, through the host firewall on the computers that need to communicate. Windows Synergy installers normally create the required rules. macOS, Linux, and third-party security firewalls may require an explicit allow rule.
Mouse Without Borders exposes an Add a firewall rule setting. Use that product control instead of creating broad, unrelated exceptions. Apply the narrowest rule that permits local-network communication, and do not expose these services to the public internet.
Use the ports for the product you actually installed
| Product or service | Port and protocol | Purpose |
|---|---|---|
| Synergy Core (Synergy 1 and Synergy 3) | TCP 24800 | Main Synergy 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 |
| Mouse Without Borders | Not stated in the cited guidance | Use its firewall-rule control and product documentation rather than Synergy port numbers |
| Barrier | Not stated in the cited guidance | Validate reachability, the running processes, and the firewall for the Barrier installation |
Do not assume that opening TCP 24800 will make every software KVM work. Synergy 3’s discovery and synchronization use additional ports, while other products use their own mechanisms.
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4. Handle discovery separately from manual connection
When Synergy 3 shows only one computer
Synergy 3 discovers peers on the local network using multicast DNS by default. Multicast traffic can be filtered by a router, access point, VPN, or firewall even when ordinary unicast traffic works. If auto-discovery fails, open Manage computers and choose Manually add computer. Enter the peer’s correct name or address, then retry the connection.
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When Synergy 1 cannot place a client
In the Synergy 1 server configuration, add each client in the screen layout grid. The configured client name must match the actual Synergy name reported by that computer. A client with a spelling, capitalization, or punctuation mismatch can be running and reachable yet still be rejected by the server.
When manual setup succeeds but discovery does not
This result narrows the fault to discovery rather than basic connectivity. Keep the manual entry if it is reliable, then investigate multicast filtering, VPN routes, and local firewall rules. Discovery is a convenience; a successful manual connection demonstrates that the core path can work.
5. Verify names, keys, and trust prompts
Synergy identity and TLS fingerprints
Synergy 1 Pro or business installations may show a TLS fingerprint prompt the first time computers connect. Compare the fingerprint displayed on both computers and accept only matching values. If the client does not trust the server fingerprint, the connection is refused even when the machines can ping each other.
Mouse Without Borders credentials
Check both the host name and security key. Mouse Without Borders can display states such as InvalidKey and Timeout; the former points toward a credential mismatch, while the latter indicates that the peer did not complete a connection in time. Regenerate and re-enter the key only after confirming that the host name is correct.
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Barrier trust
Barrier’s guidance likewise requires the client to trust the server fingerprint. A trust failure is different from a missing route: resolve the fingerprint prompt or stored trust decision before changing network topology.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.6. If the peer is visible but input does not move
Check the screen arrangement
Configure the primary/server computer and place the client on the correct side of it. The pointer crosses only at an edge that is adjacent in the layout. If the second display is physically to the left but configured to the right, moving the pointer toward the real-world position will appear to do nothing.
Confirm the sharing role and client name
Ensure the computer with the physical keyboard and mouse is selected as the Synergy primary/server. In Synergy 1, verify again that every grid entry exactly matches the corresponding client name. A visible connection does not override an incorrect layout or identity.
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Synergy’s troubleshooting checklist calls out Scroll Lock. If Scroll Lock is enabled, the cursor can remain locked to one computer. Turn it off and test each configured screen edge again.
Refresh Mouse Without Borders
Use the device layout and refresh the connections when Mouse Without Borders reports that a link was lost. Confirm that the machines still have the intended positions after reconnecting.
7. Reduce variables and restart in a controlled order
- Temporarily test without a VPN, if your environment permits it. Synergy’s troubleshooting checklist specifically recommends checking VPN software because it can alter routes or block local discovery. Do not remove a required corporate VPN without authorization.
- Close and reopen the KVM application on every participating computer after changing names, roles, firewall rules, or discovery settings.
- Recheck the ping or PowerShell test and then retry the KVM connection using the confirmed host name or address.
- Restart the computers if the service remains stuck after configuration changes. This clears stale background processes and network state.
- Undo one change at a time if the problem began after a firewall, VPN, network, or version change. Record which setting restored the connection.
Do not disable every security control as a troubleshooting shortcut. Use a scoped application or port allowance on the local network, then remove temporary exceptions that are no longer needed.
Product-specific decision guide
| Need | Synergy 1 | Synergy 3 | Mouse Without Borders | Barrier |
|---|---|---|---|---|
| Operating systems documented here | Windows, macOS, Linux | Windows, macOS, Linux | Windows through PowerToys | Use the project’s platform guidance |
| Initial configuration | Server/client roles and a configured screen grid | Primary computer, auto-discovery or manual addition | Host name plus security key | Server/client setup and trusted fingerprint |
| Discovery approach | Configured clients | Multicast DNS by default; manual addition available | Key-and-host pairing | Validate server reachability and processes |
| Security check | TLS fingerprint prompt may appear | Product trust and local service rules | Security key; InvalidKey and Timeout states | Trusted server fingerprint |
| Important caution | Client names must match exactly | Auto-discovery ports differ from the main connection | Do not substitute Synergy ports | Use Barrier-specific troubleshooting |
What the symptoms usually mean
- Ping by IP fails: investigate Wi-Fi isolation, routing, Ethernet/Wi-Fi state, or a firewall before editing KVM layouts.
- Ping by IP works but the name fails: troubleshoot name resolution or use the confirmed address while you repair naming.
- Synergy 3 manual addition works but auto-discovery fails: multicast DNS or discovery traffic is being filtered.
- Mouse Without Borders says InvalidKey: recheck or regenerate the security key; this is not primarily a screen-layout issue.
- The peer is listed but the pointer stops at the edge: inspect primary/server selection, adjacency, client naming, and Scroll Lock.
- It connected before a VPN or firewall change: compare routes and scoped allow rules, then restart the affected service.
When to stop troubleshooting the KVM
If both computers can reach each other, the correct product and roles are selected, names or keys match, required local firewall rules are present, trust prompts are accepted, and the layout is correct, the remaining fault may be product- or platform-specific. Synergy’s guidance asks Linux users to check current desktop and Wayland support details because capabilities can vary by desktop environment. Capture the product name and version, operating systems, exact error state, ping results, and which manual or discovery step failed before seeking support. That information distinguishes a network-path problem from a client-service or compatibility problem.
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