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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsA software KVM that cannot connect is usually failing in one of four places: the two computers are running incompatible software, the client is pointed at the wrong server, the network cannot reach the server, or a firewall/VPN is filtering the application. First identify the server and client, then test the server locally before debugging the path between computers. Only after the connection is established should you troubleshoot screen layout or pointer movement.
What a software KVM is actually connecting
A software KVM shares keyboard and mouse commands over a network. It is not a hardware KVM and does not switch or transmit video. One computer runs the server (also called the primary or host); it shares its input devices. The other runs the client (secondary); it receives those commands and applies them to its local session.
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Deskflow documents this arrangement as a TCP connection between a server and one or more clients. That network dependency explains why a perfectly functioning keyboard and mouse can still fail to control the other computer: the application must be running, the client must know where the server is, and security software must allow the traffic.
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Run this diagnostic sequence in order
Work from the simplest explanation to the most specific. Change one thing at a time and record what happens; that creates a useful boundary between a server problem, a network problem and an input-layout problem.
- Record the application and version. On both PCs, write down the exact product name, version, operating system and whether the machine is configured as server or client. Do not assume that two similarly named releases can connect.
- Confirm the same product is installed on both machines. A server from one implementation generally cannot accept a client from another. Install the current compatible release for the product you selected, and restart both applications after updating.
- Verify the intended sharing direction. The computer with the keyboard and mouse you want to share must be the server. The other computer must be configured as the client. A reversed role can look like a network failure because each side is waiting for the other to initiate.
- Put both computers on the same reachable network. Use the same home or office LAN over Ethernet or Wi-Fi. Check that neither PC has moved to a guest wireless network, an isolated SSID, a different VLAN or a mobile hotspot that blocks device-to-device traffic.
- Check that the server application is running and listening. Open the server interface and confirm it reports a ready, listening or sharing state. If the application cannot start its server locally, fix that before testing the second PC.
- Test the server locally when the product supports it. Barrier’s troubleshooting guidance recommends proving that the server accepts a local connection before testing across the network. A local test isolates the server process from routers, Wi-Fi isolation and firewalls between machines.
- Find the server’s current address. Use the address or hostname shown by the operating system or the product. If the client uses a saved address, compare it with the server’s current address; DHCP may have changed it after a reboot. Enter the server hostname or IP exactly as the product expects.
- Test basic reachability from the client. On Windows, run
ping SERVER_NAMEin Command Prompt or PowerShell. On macOS or Linux, runping -c 4 SERVER_NAME. A failed ping does not prove the KVM port is blocked—many networks suppress ICMP—but it is a useful sign that name resolution, routing or isolation needs attention. If your product documents a port test, use that product-specific port with the operating system’s network-test tool; there is no universal software-KVM port that is safe to substitute. - Check application permissions. Allow the specific KVM application through the operating system firewall and endpoint-security software on the network profile actually in use. Permit only the product and networks you need; do not create a broad permanent rule just because a test succeeded after disabling protection.
- Use a VPN as a controlled diagnostic. Temporarily disconnect the VPN on one computer and retry. If the connection works only with the VPN off, its routing or filtering is involved. Re-enable the VPN and create the narrow, product-specific split-tunnel or firewall exception required by your organization instead of leaving security disabled.
- Check the screen arrangement after the status says connected. Configure the client screen next to the correct edge of the server screen. Move the pointer toward that edge. If the pointer remains locked to one PC, check whether Scroll Lock is engaged; Synergy identifies Scroll Lock as a control that can prevent crossing.
- Capture the result. Note whether the failure is discovery, refusal, timeout, immediate disconnection or input that will not cross. Save the exact error or log line, the application versions, operating systems, roles and the result of the reachability test before changing more settings.
Identify the product, version and roles before changing anything
Version details determine which instructions apply. Symless Support’s current basic checklist was checked on October 3, 2026, for Synergy 1 version 1.21.4 and Synergy 3 version 3.7.2. That guidance covers Windows, macOS and Linux, while individual firewall and permission behavior still varies by operating system. Record whether each computer is running Synergy 1 or Synergy 3 rather than writing only “Synergy.”
| Implementation | What the documentation establishes | How to use that fact |
|---|---|---|
| Synergy 1 (v1.21.4) and Synergy 3 (v3.7.2) | The current checklist covers the named releases on Windows, macOS and Linux; it calls out same-product setup, current versions, same network, firewall/VPN checks, screen adjacency and Scroll Lock. | Use the checklist for setup and input symptoms, but apply firewall and system-permission steps for the installed operating system. |
| Deskflow | Project documentation describes a TCP server sharing input commands with clients. | Confirm which machine is the server, which is the client, and that the client can reach the server over the network path required by the installed release. |
| Barrier | The troubleshooting wiki recommends testing the server locally before testing a remote client. | Use the staged local-then-remote method to separate a server-process fault from a network-path fault. The wiki is older community documentation, so verify release-specific commands and settings against your installed version. |
Do not mix a Barrier configuration example with a Synergy release, or infer a Deskflow port from a different implementation. If a guide does not identify your exact version, treat its interface labels and commands as diagnostic concepts rather than guaranteed menu paths.
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Use the same product and a compatible release
Install the same KVM implementation on both PCs and bring both to a current, compatible release. A partially updated pair can fail before any firewall rule is evaluated. After updating, quit and relaunch both ends so an old background process is not still listening.
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Make sure the server is sharing the intended devices
On the server, select the option that shares its keyboard and mouse. On the client, select the option that receives input. If the application has separate “server,” “host,” “primary,” “client” or “secondary” labels, write down the mapping; do not rely on which PC you consider your main computer.
Check LAN placement
Synergy’s installation guidance describes Ethernet or Wi-Fi on the same network. “Same network” means more than having internet access at the same time: the devices must be allowed to communicate with one another. Guest Wi-Fi, client isolation, VLAN boundaries and some corporate wireless policies intentionally prevent peer-to-peer traffic. Move both PCs temporarily to the same trusted LAN, or ask the network administrator whether peer traffic is blocked.
Refresh addresses after network changes
A server’s IP address can change after reconnecting to Wi-Fi or restarting the router. Compare the address currently assigned to the server with the address stored in the client. If a hostname resolves to an old address, test the current IP and then correct the product’s saved server entry or establish a reliable internal name through your network administrator.
Separate a broken server from an unreachable server
Testing in stages prevents wasted effort. If the server cannot accept a local connection, the client’s network is not yet the main problem. Look for a stopped service, an invalid screen configuration, a second process already using the required listener, or a permission prompt that was denied.
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If a local connection works but a remote client cannot connect, focus on the path between machines: the target hostname or IP, network isolation, firewall policy, VPN routing and the application’s listening interface. A successful ping only shows that some traffic reaches the host; it does not prove that the KVM application’s own TCP listener is reachable.
When the client reports “connection refused,” the host may be reachable while no KVM server is listening on the expected interface or port. A timeout more often indicates filtering, routing or an incorrect address, although the exact meaning depends on the product and operating system. Use the application’s own log wording and release documentation rather than treating either message as conclusive.
Check the address, name resolution and server selection
Discovery and connection are separate stages. Some products can display nearby servers automatically; others require a manually entered hostname or IP. If the server does not appear, first verify that the server is running and that both computers are on a network where discovery is permitted. If discovery finds a server but connection fails, inspect the selected address and any manually entered port.
- Confirm there is no typo, extra space or stale hostname in the client’s server field.
- Compare the hostname result with the server’s current local address; test the IP only as a diagnostic, because a DHCP address may change again.
- Check whether the client is using an old saved server after the server was reinstalled or renamed.
- On managed networks, ask whether multicast or broadcast discovery is blocked even though direct host-to-host traffic is allowed.
Do not publish or apply a universal port number from an unrelated KVM guide. Product and version determine the listener, and the supplied documentation does not establish one port shared by Synergy, Deskflow and Barrier.
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Resolve firewall, antivirus and VPN interference safely
Allow the application, not an entire computer
Synergy’s checklist advises checking that its application is allowed by antivirus or firewall software. On Windows, review the firewall’s “Allow an app through Windows Defender Firewall” list and ensure the KVM program is permitted on the private network profile you actually use. On macOS, review the application firewall settings in System Settings and approve the signed application when prompted. Linux firewall commands and graphical paths vary by distribution and firewall manager; use the rule format documented for your installed product and distribution.
Apply the permission on the computer that accepts connections—the server—and, when the client makes outbound connections subject to local policy, on the client as well. If security software identifies the KVM process by executable path, an update may create a new path that needs approval. Prefer a rule limited to the application and trusted LAN over an “allow everything” rule.
Use VPN disconnection only to isolate the cause
A VPN can change the route used for the server’s private address or filter local-LAN traffic. Disconnect it briefly on one computer, retry, and reconnect immediately after the test. If that changes the result, consult the VPN or organization policy for a local-network allowance, split tunneling or an approved route. Do not leave a VPN or firewall disabled as the fix.
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Respect managed-device policy
Company endpoint security may prevent users from changing firewall rules. In that case, provide the administrator with the product name, executable, server address, connection direction and required network scope. Asking for a narrow exception is safer and easier to audit than requesting that all security controls be turned off.
When it says connected but the cursor will not cross
A connected status proves that the application session exists; it does not prove that the virtual desktop layout matches the physical arrangement. In the server’s screen-layout editor, place the client screen directly against the correct edge of the server screen. If the client is physically to the right, its virtual screen must touch the server’s right edge; a diagonal or separated placement can make an otherwise healthy connection appear inactive.
Move the pointer slowly through the exact shared edge. If it stops at the boundary on one computer, check Scroll Lock. Synergy specifically identifies Scroll Lock as a way to lock the pointer to one computer. Release it and test again. Also verify that the active display is the one represented in the layout when a computer has multiple monitors.
If keystrokes reach the other PC but the pointer does not, the issue is likely layout or pointer capture rather than network reachability. If neither pointer nor keyboard input transfers and the status has changed to disconnected, return to the server, firewall and VPN checks instead of editing the layout.
Use the symptom to choose the next branch
| Observed symptom | Most useful boundary test | Next area to inspect |
|---|---|---|
| The client cannot discover any server | Open the server application locally and verify both PCs are on the same reachable LAN. | Server startup, discovery restrictions, network isolation, hostname resolution. |
| The client finds a server but times out | Compare the saved server address with the server’s current address; perform a basic reachability test. | Wrong or stale address, VPN route, firewall filtering or blocked peer traffic. |
| The client reports refusal | Confirm that the server is listening and that the client is using the product’s documented listener settings. | Stopped server, wrong interface, wrong product/version or a release-specific port setting. |
| It connects briefly, then drops | Repeat with the VPN disconnected and inspect both applications’ exact log lines. | VPN or firewall session filtering, duplicate server processes, sleep/wake behavior or incompatible releases. |
| Status is connected but input stays on one PC | Move the pointer toward the configured shared edge and check Scroll Lock. | Screen adjacency, active-display selection and pointer-lock state. |
| Only one direction works | Verify which computer is server and which is client, then inspect the layout for the return edge. | Role reversal, incomplete screen arrangement or a product-specific permission limitation. |
Collect evidence when the first pass fails
Further diagnosis is much faster when the failure is described precisely. Record the exact application and version on each PC, operating systems, server/client roles, network type, server hostname and current IP, and whether the computers can reach one another. Include the exact error or log line, not a paraphrase such as “it just does not work.” State whether the failure occurs during discovery, the connection attempt or input transfer.
There is no single log path or port that applies to every software KVM. Use the installed product’s documentation to locate logs and enable only the diagnostic level needed, especially on a managed or shared computer. Redact usernames, public addresses and organization details before sharing logs. Older Barrier community instructions can still help you stage tests, but verify any command or setting against the release you have installed.
What not to do
- Do not open an arbitrary inbound port copied from another KVM product.
- Do not permanently disable the operating-system firewall, antivirus or VPN to make the symptom disappear.
- Do not assume internet access means two computers can communicate; guest and isolated networks commonly block peer traffic.
- Do not troubleshoot screen edges or Scroll Lock while the client is still failing to connect.
- Do not mix server and client builds from different products or assume a version-specific menu exists in another release.
- Do not treat a successful ping as proof that the application listener is available.
When a hardware KVM is a different answer
A hardware KVM would replace the software arrangement with a physical switch and cables, but it does not repair a software KVM connection. If your requirement includes switching video as well as USB input, or your network policy forbids peer-to-peer software connections, that is a design decision rather than a troubleshooting step. For a software KVM that should share keyboard and mouse over an allowed LAN, the sequence above identifies whether the failure is in roles, software compatibility, reachability, security policy or screen behavior.
A concise handoff for support
Send support one complete report: “Product and version on server: …; product and version on client: …; operating systems: …; server/client roles: …; same LAN or not: …; server local test: passed/failed; client reachability test: result; firewall/VPN changes: …; exact error or log line: …; connected but input does not cross: yes/no.” This format gives the next technician the boundaries needed to continue without asking you to repeat the same basic checks.
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