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When running VPN software, you may occasionally get error messages like, "The specified port is already in use" or "The specified port is already open." What do these errors mean, and how can you fix them?
What causes the 'specified port is already in use' error?
The error and the message it generates occur when more than one application on your computer attempts to open a network connection that uses a nonsharable resource. A nonsharable resource can manage only one process or request at a time, like a cellular modem, for example. A modem can only handle one connection at a time, and when one application is using it, other applications are prevented from using it at the same time.
Another example of a nonsharable resource is a network port used by VPN software. In this case, the VPN software opens a network port through which all network communications are encrypted and forwarded to a remote VPN concentrator located in an organization's data center. Any application that opens the local network port needed by the VPN will cause the conflict.
What causes the 'port already open' message and how to fix it
Sleeping computer with locked resource
A common cause of the "port already open" error occurs when a computer automatically goes to sleep to conserve power after a period of inactivity. The transition to sleep followed by reawakening causes the connection to drop. But the computer's OS doesn't release the lock it created on the nonsharable resource. The network application, upon attempting to reestablish the connection, encounters the locked resource, causing the "port already open" error message. Rebooting the computer clears the locked resource, and the network connection can be reestablished.
Software bugs can also cause the error. Applications should release resource locks when they stop running, but an application that encounters a failure condition may not always gracefully handle the situation and leave a network resource locked. Computers with COM ports, typically used with modems, can sometimes work around the issue by changing COM ports.
Another application uses a VPN's network port
Another cause, though less frequent, is when another application also uses the network port that the VPN software is using. This was the case with a VPN software problem as described on the Cisco Meraki forum -- "Windows 10 VPN error: The modem (or other connecting device) is already in use." The solution in this case was to edit the Windows registry to prevent the other application from using the network port reserved for the VPN software. The linked articles above describe a step of using a Netstat command prompt to find the application creating the conflict.
Windows 10's increased security functionality seems to have increased the frequency of the error. Many users report the error started happening when they updated to the newer version of Windows. Note: This is not a valid reason to skip computer OS updates or avoid patches.
SSH port forwarding
Linux, Unix and macOS are not exempt from the problem, but the messages are slightly different. They have the same cause: a nonsharable resource being used by another application. Again, the netstat tool can discover the other application attempting to connect.
The most frequent source of problems for non-Windows OSes is due to using Secure Socket Shell (SSH) port forwarding. When the SSH connection dies, an immediate attempt to use port forwarding may report a message: "Address already in use." This occurs because TCP must wait for the final handshake that closes the network connection, called TIME_WAIT (see Request for Comments 793). Waiting a few minutes will enable the application to reuse the network ports in question.
The basic cause of these errors is the same: A nonsharable resource is locked by another application or another instance of the same application. The confusing element is that the details can vary. Is it a COM port or Linux /dev device? Or is it due to network port utilization from VPN software or SSH port forwarding? Identifying the type of situation can help narrow the search for an answer.
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