Start Disconnected [exclusive]: Virtual Device Serial0 Will

Troubleshooting VMware: "Virtual device serial0 will start disconnected"

If you’ve recently powered on a VMware Virtual Machine (VM) only to be greeted by the message "Virtual device serial0 will start disconnected," you aren’t alone. While this warning might seem alarming, it’s usually a minor configuration mismatch rather than a sign of a failing system.

Here is a deep dive into why this happens and how you can fix it. Why Does This Message Appear?

In the world of virtualization, "Serial0" refers to a virtual serial (COM) port assigned to your VM. This error occurs because the VM is configured to look for a physical or virtual serial port on the host machine that either doesn't exist or is currently in use by another application. Common scenarios include: virtual device serial0 will start disconnected

Legacy Templates: You are using a VM template that originally had a serial port mapped for debugging.

Hardware Migration: You moved the VM from a host that had a physical COM port to one that doesn't.

Disconnected Peripherals: The VM is set to connect to a named pipe or a physical port that is disconnected. How to Fix the "Serial0" Warning libvirt XML check: virsh dumpxml vmname | sed

Depending on whether you actually need the serial port, there are two main ways to handle this. Method 1: Remove the Serial Port (Recommended)

Most modern users and operating systems do not require a serial port. Removing it is the cleanest way to stop the error message. Shut down the Virtual Machine.

Right-click the VM in your library and select Edit Settings. Locate Serial Port 1 (or Serial0) in the hardware list. Click the Remove or "X" icon. Click OK and power the VM back on. Scenario 2: "I See It Every Time I Boot

Example quick checks and commands

Scenario 2: "I See It Every Time I Boot. Is My VM Broken?"

No. Many pre-built Linux appliances include a virtual serial port for legacy console access. The message is cosmetic.

How to "fix" it (if you want the port to be connected)

If you need serial0 active at boot:

  1. Connect to a real or emulated endpoint – e.g., a named pipe, TCP server, or physical COM port.
  2. In VMware: Set the serial port to "Use socket (named pipe)" and ensure the other application connects before powering on.
  3. In GNS3/EVE-NG: Manually connect serial0 to another device's serial interface.
  4. In QEMU: Use a valid backend like -serial stdio or -serial tcp::4444,server=on.

Troubleshooting Steps

  1. Review VM Configuration: Check the VM settings through the virtualization software's interface or command-line tools to ensure the serial port is enabled and properly configured.
  2. Connect the Serial Device: If the serial port is supposed to connect to a physical device, ensure the device is properly connected to the host and accessible.
  3. Check Host Resources: Verify that the host has sufficient resources to support the VM and its devices.
  4. Consult Documentation: Refer to the virtualization platform's documentation for specific instructions on configuring virtual serial ports.

Executive summary

The message "virtual device serial0 will start disconnected" is a benign informational log from virtualization software (commonly QEMU/KVM, VirtualBox, or libvirt) indicating that a virtual serial device (named serial0) is configured but will not be connected to a host endpoint at VM start. It does not by itself indicate an error but explains why a guest serial port is inactive; troubleshooting depends on whether this was intentional.

Common Scenarios Where You See This Message

You will typically encounter this message in three specific environments:

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