Kernel Debugging With Serial Port - Proxmox

This guide explains how to set up Serial Kernel Debugging between two Windows VMs on Proxmox. I use VM 104 as the Debugger and VM 112 as the Target which you can find in Proxmox.

Phase 1: Proxmox Hardware Setup

You must add a virtual serial “wire” to both VMs.

  1. Log into the Proxmox Web UI.
  2. For VM 104 (Debugger):
    • Go to Hardware -> Add -> Serial Port.
    • Ensure 0 is selected and it says “socket”. Click Add.
  3. For VM 112 (Target):
    • Go to Hardware -> Add -> Serial Port.
    • Ensure 0 is selected and it says “socket”. Click Add.
  4. Start both VMs now.

Phase 2: Create the “Virtual Cable” (Proxmox Host)

Even though both VMs have serial ports, they aren’t connected yet. We use the Proxmox Shell to bridge them.

  1. Open the Shell on your Proxmox Host (the physical server).
  2. Run this command to link the two virtual sockets: 
    socat UNIX-CONNECT:/var/run/qemu-server/104.serial0 UNIX-CONNECT:/var/run/qemu-server/112.serial0 &

    Note: This command must stay running in the background. If you reboot Proxmox, you must run this again.

Phase 3: Configure the TARGET VM (VM 112)

This tells Windows to send its internal kernel data out through the serial port.

  1. Log into VM 112.
  2. Open Command Prompt (Admin).
  3. Run the following commands: 
    # Enable kernel debugging
bcdedit /debug on

# Configure the serial settings (COM1, 115200 baud)
bcdedit /dbgsettings serial debugport:1 baudrate:115200
  4. Restart VM 112.

Phase 4: Configure the DEBUGGER VM (VM 104)

This is where you will watch the logs and control the other VM.

  1. Log into VM 104.
  2. Verify the Port:
    • Right-click the Start button -> Device Manager.
    • Expand Ports (COM & LPT).
    • Confirm you see “Communications Port (COM1)”. If it says COM2, remember that for the next step.
  3. Open WinDbg Preview (or WinDbg Classic).
  4. Go to File -> Start Debugging -> Attach to Kernel.
  5. Select the COM tab and enter:
    • Baud Rate: 115200
    • Port: COM1 (Match what you saw in Device Manager)
    • Check the box for Initial Break (optional, but helpful).
  6. Click OK.
  7. The console should show: Waiting to reconnect...

Phase 5: Testing the Connection

  1. While WinDbg is “Waiting” on VM 104, Restart VM 112 (the Target).
  2. As VM 112 boots, you should see text scrolling in the WinDbg window on VM 104.
  3. To pause the target machine and start debugging, click the Break button (or press Ctrl + Break).
  4. The Target VM (112) should completely freeze. This is normal; the kernel is now under your control.
  5. Type g and press Enter in the WinDbg command line to let the Target VM continue running.