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When a software KVM cannot connect to the second computer, first separate the problem into three possibilities: the wrong server/client setup, no network path between the computers, or an application blocked by a firewall or VPN. Confirm the product and roles, install the same current version on both PCs, verify that both are on the same network, test whether the client can reach the server, and then check security software. If the connection succeeds but the pointer will not cross screens, fix the screen layout or release Scroll Lock.
What a software KVM connection actually requires
A software KVM shares keyboard and mouse input through a network connection. One computer runs the server (also called the primary or sharing computer); the other runs the client (the secondary or receiving computer). Deskflow documents this arrangement as a server sending input commands to clients over TCP. The software KVM does not carry or switch the other computer’s video signal as a hardware KVM does: each monitor remains connected to its own computer.
That architecture creates three separate failure points:
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- Reachability: the computers cannot find or contact one another over the network.
- Input transfer: a connection exists, but the layout or a lock setting prevents the pointer and keyboard from moving across.
Diagnose those layers in that order. Do not begin by opening random ports or permanently disabling security tools; the required port and permission depend on the product and release.
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1. Record the setup before changing it
Write down the details that identify the failure. This prevents trying instructions for a different product and gives you useful information if you need support.
- Application name and version on each computer.
- Operating system and edition on each computer.
- Which computer is configured as the server/primary and which is the client/secondary.
- Whether both computers use Wi-Fi, Ethernet, or different connections.
- The server hostname or IP address entered on the client.
- The exact symptom: the server is not discovered, the client reports a connection error, or the connection appears active but input does not cross.
- The exact error text and the time it occurred.
Synergy’s official basic troubleshooting page, checked on October 3, 2026, covers Synergy 1 version 1.21.4 and Synergy 3 version 3.7.2 on Windows, macOS and Linux. Its labels and behavior can differ from Deskflow, Barrier, or another implementation, so use the instructions that match your installed product.
2. Confirm the product, versions and roles
Install the same product on both computers
A server from one implementation is not automatically compatible with a client from another. Confirm that both computers run the same software family and a compatible release. If one copy is old, update both rather than updating only the computer that shows the error. Restart the applications after updating.
Choose one server and one client
Configure the computer with the keyboard and mouse you intend to share as the server. Configure the other computer as the client. Avoid changing both machines to server mode while troubleshooting; two servers do not create a usable sharing path.
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Check the intended sharing computer
In the server application, confirm that the local keyboard and mouse are selected for sharing and that the service is started or listening. In the client application, confirm that it is pointed at that server rather than an old computer name or address. If the product offers automatic discovery, use it only after confirming that the manually selected target is correct; discovery can fail even when a direct connection works.
3. Put both computers on a usable network
Synergy’s checklist says both computers should be on the same network, and its installation guidance allows Ethernet or Wi-Fi. Connect both systems to the same home, office, or other trusted network for the initial test. A guest wireless network, client-isolation setting, separate VLAN, or a VPN route can prevent one device from reaching the other even when both have internet access.
Check the connection on each computer
- Confirm that each computer has an active network connection and a current address.
- Make sure neither computer has switched to a different access point, hotspot, guest SSID, or isolated wireless network.
- If one computer is wired and the other is wireless, verify that the router or access point permits traffic between those segments.
- Temporarily disconnect unused network adapters so the application does not select an unintended address.
Do not infer that internet access proves local reachability. The relevant test is whether the client can reach the server on the local path used by the KVM application.
4. Test reachability before debugging the application
Test the server locally first
Barrier’s troubleshooting guidance recommends checking that the server accepts a local connection before testing from another computer. This separates a server-process problem from a network-path problem. On the server, verify that the KVM service is running and that the application reports itself ready to accept clients. If the product has a local test or loopback option, use that according to its documentation.
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Test from the client
From the client, test the server’s hostname and then its IP address. A basic ICMP test can show whether the name resolves and whether packets return:
ping server-hostnameOn Windows, run the command in Command Prompt or PowerShell. On macOS or Linux, run it in Terminal. Replace server-hostname with the actual server name; if name resolution fails, repeat the test with the server’s current IP address.
Interpret the result carefully:
- Hostname fails but IP succeeds: the client may be using an incorrect name or local name resolution is failing. Enter the correct address in the KVM client or repair name resolution.
- Both hostname and IP fail: investigate network isolation, the server’s network connection, routing, or a firewall before changing KVM settings.
- Ping succeeds but the KVM still fails: the computers can reach each other generally, but the application may be stopped, listening on a product-specific port, or blocked by a host firewall.
- Ping is blocked: some networks disable ICMP. Treat this as inconclusive and use the KVM’s own connection test or the product’s documented port test rather than assuming the application is unreachable.
There is no universal port number or firewall command for all software KVMs. Deskflow, Synergy, Barrier, and other products can use different defaults or settings, so obtain the port from the installed release’s own configuration or documentation before testing it.
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5. Check firewall and security permissions
Allow the application, not an arbitrary open port
Operating-system firewalls and endpoint security products can block either the server’s listening process or the client’s outbound connection. Open the firewall’s allowed-app or application-permission view on both computers and check that the specific KVM executable is permitted on the network profile you are using. If the product has separate server and client components, allow the component that actually runs on each machine.
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Apply the narrowest rule that works: the correct executable, the required network profile, and the product’s documented port or ports. Do not create a permanent “allow everything” rule, and do not copy a port number from a different KVM product.
Use a short VPN diagnostic
Synergy recommends temporarily turning off VPN software as a diagnostic. A VPN can change routing, advertise a different address, or filter local traffic. Disable it briefly on the client and server, retry the connection, and immediately restore it after the test. If the KVM works only while the VPN is off, keep the VPN enabled and configure an appropriate split-tunnel, local-network access, or application exception according to that VPN’s controls.
Check security software logs
Antivirus, endpoint protection, and host firewalls often record a blocked application even when no pop-up appears. Look for an event containing the KVM executable, the server address, or a denied inbound connection. Approve only the identified application and direction. If the security product is managed by an organization, ask the administrator for a policy exception instead of disabling protection.
6. Verify the client’s target and server state
Use the server’s current address
DHCP can change a computer’s IP address after a reboot or lease renewal. Compare the address shown by the server’s network settings with the address stored in the client. Correct the client target if they differ. A hostname can be preferable when local name resolution is reliable; an IP address can be a useful diagnostic when name resolution is suspect.
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Restart in a controlled order
- Stop the client application.
- Stop and start the server application or service.
- Wait until the server reports that it is ready.
- Start the client and retry using the confirmed target.
- Record whether the error changes.
A changed error is useful evidence: “host not found” points to naming or discovery, while “connection refused” generally means the host answered but no permitted server process accepted the request. Use the exact wording from your product rather than assuming all applications use those labels.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.7. If it connects but the pointer will not cross
Fix the screen layout
A successful connection does not guarantee that the virtual desktop arrangement matches the physical monitors. In the server’s screen-layout editor, place the client screen directly beside the edge of the server screen where you expect the pointer to leave. If the client is physically to the left, the configured client tile must touch the server tile on the left; a diagonal gap or a different edge can make crossing appear broken. Save or apply the layout, then move the pointer slowly across the touching edge.
Check Scroll Lock
Synergy specifically identifies Scroll Lock as a cause of the pointer remaining on one computer. Press Scroll Lock once, or use the product’s documented toggle if the keyboard lacks a dedicated key, then test the edge again. Check the keyboard indicator when available so you know whether the lock state changed.
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- If the pointer crosses but keystrokes do not, inspect the client’s input permissions and focus behavior.
- If neither pointer nor keyboard crosses, recheck the server/client connection status and layout before changing display settings.
- If input reaches the other computer but appears delayed, test both machines on the same local network without the VPN and compare behavior; do not assume the monitor or video cable is part of the software KVM path.
8. Symptom-to-cause decision table
| Observed symptom | Most useful next check | What the result tells you |
|---|---|---|
| Client cannot discover the server | Enter the server’s current hostname or IP manually and test reachability | Separates discovery/name resolution from general network access |
| Client reports host unreachable | Check network membership, isolation, VPN state and firewall logs | Points to routing or filtering rather than screen layout |
| Client reaches host but connection is refused | Confirm the server process is running and obtain its product-specific port | Indicates the host answered but the expected service did not accept the connection |
| Connection appears established; pointer stays on one screen | Inspect adjacent screen edges and Scroll Lock | Indicates an input-routing or layout state rather than discovery |
| Works only with VPN disabled | Restore VPN and configure its local-network or application exception | Identifies VPN routing or filtering as the interference |
9. Common approaches that make troubleshooting worse
- Mixing products: installing one application on the server and another on the client can create an apparent network failure that no firewall change will fix.
- Assuming every product uses the same port: a rule for one implementation may be ineffective or unnecessarily expose another.
- Leaving the VPN or firewall disabled: this removes protection and hides the configuration that must eventually be fixed.
- Changing several variables at once: if you update, change the address, move the screens and disable security software simultaneously, you will not know which change mattered.
- Testing only internet access: both computers can browse normally while local client-to-server traffic is isolated.
- Treating a hardware-KVM symptom as a software-KVM fault: a software KVM does not switch video, so a blank or incorrect monitor requires separate display troubleshooting.
10. Collect the details needed for escalation
If the connection still fails, provide a compact diagnostic record instead of only saying that the other PC will not connect:
- Product and exact version on both computers.
- Operating systems and whether each machine is server or client.
- Network type (Ethernet, Wi-Fi, guest network, VLAN, or VPN in use).
- Server hostname and current IP address, with sensitive public addresses removed.
- Whether the server works locally and whether the client can reach its hostname and IP.
- Firewall or security events showing an allowed or blocked KVM process.
- The exact error or log line, including which computer produced it.
- Whether the failure is discovery, connection establishment, pointer movement, keyboard input, or only performance.
Do not assume there is one universal log location or port. Barrier’s troubleshooting wiki is older community documentation, so use its staged diagnostic logic but verify commands and paths against your installed release. For Synergy, Deskflow, or another product, follow that project’s current support instructions for collecting logs and identifying the listening port.
Quick Recap
11. A concise recovery order
- Identify the exact product and version on both computers.
- Install the same current compatible release and assign one server and one client.
- Confirm both systems are on the same usable network and the client targets the server’s current name or IP.
- Verify the server is running and test reachability from the client.
- Allow the application through the applicable firewall profile on both systems.
- Temporarily disable the VPN only to isolate routing or filtering, then restore it and make a narrow exception if necessary.
- If connected, align adjacent screen edges and check Scroll Lock.
- Record the exact remaining error and product-specific logs before seeking further help.
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