Looks like you're hitting a classic SMB connectivity snag between your Windows Hyper-V host and Ubuntu 16.04 guest with Vagrant. That Host is down error usually boils down to network issues, missing guest dependencies, or security software blocking traffic. Let's walk through the fixes step by step:

1. Install Required Tools on the Ubuntu Guest #

First, the error mentions mount.cifs failing—Ubuntu 16.04 doesn't come with the necessary SMB tools pre-installed. You'll need to get cifs-utils:

• If you can still SSH into the guest (use vagrant ssh, or connect directly via Hyper-V Manager), run:

  sudo apt-get update && sudo apt-get install -y cifs-utils
  

• After installing, try re-mounting with sudo mount -a or restart the VM with vagrant reload.

2. Verify SMB Service & Firewall on Windows Host #

SMB relies on the Lanman Server service running on your Windows machine, and firewall rules allowing traffic:

• Check if the service is running: Open PowerShell as admin and run:

  Get-Service LanmanServer
  

  If it's stopped, start it and set it to auto-start:

  Start-Service LanmanServer
  Set-Service LanmanServer -StartupType Automatic
  

• Check Windows Firewall: Ensure the File and Printer Sharing (SMB-In) rule is enabled for your active network profile (Public/Private). You can verify this in Windows Defender Firewall > Advanced Settings > Inbound Rules.

3. Fix Vagrant SMB Configuration #

Your Vagrantfile might be using auto-detected network settings that aren't reliable. Tweak the synced folder config:

• Specify your Windows host's actual external IP address (find it via ipconfig in Command Prompt) to avoid auto-detection failures:

  Vagrant.configure("2") do |config|
    config.vm.box = "bento/ubuntu-16.04"
    config.vm.provider "hyperv"
    config.vm.network "public_network"
    # Add smb_host with your Windows host IP, and explicit mount options
    config.vm.synced_folder ".", "/vagrant", type: "smb", 
      smb_host: "192.168.1.100", # Replace with your host's external IP
      mount_options: ["sec=ntlmssp", "uid=1000", "gid=1000"]
    config.vm.provider "hyperv" do |h|
      h.enable_virtualization_extensions = true
      h.linked_clone = true
    end
  end
  

• The sec=ntlmssp option replaces the older ntlm that might not be compatible with Ubuntu 16.04.

4. Double-Check Security Software #

You mentioned temporarily closing Avast, but security tools often leave background processes running:

• Fully disable Avast shields until restart (right-click the tray icon > Avast Shields Control > Disable until computer restart).
• Also, check Windows Defender Real-Time Protection—temporarily turn it off to test if it's blocking SMB traffic. If it works, add your VM's IP and the shared folder path to Defender's exclusions.

5. Validate Network Connectivity #

Ensure the guest can actually reach the host:

• SSH into the guest and ping your Windows host's external IP. If ping fails, your Hyper-V external switch is misconfigured:
  • Open Hyper-V Manager > Virtual Switch Manager, confirm the external switch is bound to your active physical network adapter.
  • Make sure the guest and host are on the same subnet (check guest IP with ip a in Ubuntu).

After trying these steps, run vagrant reload to restart the VM and re-apply the config. That should resolve the SMB mount error.

内容的提问来源于stack exchange，提问作者kvjava1