Ubuntu 16.04.3 LTS挂载CIFS报错112，如何强制默认版本为3.0？

Hey there, I’ve worked through almost identical issues with Veeam Linux Agent and CIFS shares on Windows Server before. That "Host is down" error is tricky because it often masks actual root causes like protocol mismatches or permission issues, not just a dead host. Let’s break down the steps to fix this:

1. Verify Basic Network Connectivity First #

Start with the basics to rule out obvious network issues:

• Ping the Windows Server’s IP address and hostname from your Ubuntu VM. If hostname ping fails, check your VM’s DNS settings or just use the IP address temporarily in your Veeam config to eliminate DNS as a variable.
• Test if SMB port 445 is open using telnet <Windows_Server_IP> 445 or nc -zv <Windows_Server_IP> 445. If this fails, your ESXi host, VM firewall, or Windows Server firewall is blocking the port—you’ll need to allow SMB traffic through.

2. Force a Compatible CIFS/SMB Protocol Version #

Windows Server 2016 defaults to SMB 3.0, but Ubuntu 16.04’s cifs-utils might try negotiating an older protocol that’s disabled on Windows. This is one of the most common fixes:

• First, test manually with a specific protocol version to confirm:

  mount -t cifs //<Windows_Server_IP>/your_share /mnt/test_mount -o username=Your_Windows_User,password=Your_Windows_Pass,vers=3.0
  

• If this works, head into your Veeam Linux Agent’s CIFS storage configuration. Look for the "Additional mount options" field and add vers=3.0 there. This tells Veeam to use SMB 3.0 when mounting the share.

3. Double-Check Windows Share Permissions #

Don’t overlook both share-level and NTFS permissions—they work together, and the stricter one wins:

• Ensure the Windows user you’re using has Read/Write permissions on the share itself (right-click share > Properties > Sharing > Permissions).
• Also verify the user has matching NTFS permissions on the underlying folder (right-click folder > Properties > Security).
• If using a domain user, make sure to format the username correctly in Veeam: DOMAIN\Username or Username@domain.com. For local Windows users, add domain=Your_Windows_Server_Name as an extra mount option if needed.

4. Dig Into Detailed Mount Logs #

The generic "Host is down" message doesn’t tell the whole story. Get verbose output from a manual mount to see what’s really happening:

mount -v -t cifs //<Windows_Server_IP>/your_share /mnt/test_mount -o username=Your_Windows_User,password=Your_Windows_Pass

Look for lines about protocol negotiation failures, permission denied errors, or invalid credentials—these often hide behind the "Host is down" message.

5. Validate ESXi Network Configuration #

Make sure your Ubuntu VM isn’t stuck in a disconnected network:

• Confirm the VM’s network adapter is connected to the correct port group in ESXi, and that port group has access to the Windows Server’s network.
• If you’re using NAT mode for the VM, ensure ESXi’s NAT rules allow outbound SMB traffic. Bridge mode is usually more reliable for direct network shares.

Once you’ve worked through these steps, the mount error should clear up, and Veeam should be able to back up to your Windows share without issues.

内容的提问来源于stack exchange，提问作者Steven4x4