kb4571756 Breaks hyper V Synthetic Ethernet Port

Roger Fraser 41 Reputation points
2020-09-09T08:34:36.007+00:00

After installing kb4571756 update none of my installed Hyper-V instances launch they all throw exceptions across all OS Windows & Ubuntu :

failed to restore virtual machine state.
failed to start worker process: Class not registered (0x80040154).
could not initialize.
Synthetic Ethernet Port.

I tried to set to no connection but it still throws the error .If I remove the ethernet adapters they will start .After I uninstalled the update then of the Hyper-V instances all worked again as expected .

Is there a reason or security update that causes this or is it a issue with the update ? The only notes in the update refer to virtual Graphics adapters not ethernet .

Windows 10
Windows 10
A Microsoft operating system that runs on personal computers and tablets.
10,560 questions
Hyper-V
Hyper-V
A Windows technology providing a hypervisor-based virtualization solution enabling customers to consolidate workloads onto a single server.
2,530 questions

3 answers

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  1. Abhishek Saigal 26 Reputation points
    2020-09-16T10:43:46.8+00:00

    Get a copy of \windows\System32\VmSynthNic.dll from a Windows 10 2004 or higher build (10.0.19041 N/A Build 19041, probably before kb4571756 update )

    My file version that i picked is 10.0.19041.1 (WinBuild.160101.0800)

    Copy it to the affected machine at same location i.e., \windows\System32\

    from admin prompt, run below

    regsvr32 c:\windows\system32\VmSynthNic.dll

    you should see a prompt "DllRegisterServer in VmSynthNic.dll succeeded."

    Now try starting your VM or container

    5 people found this answer helpful.
  2. Paul Dash 31 Reputation points
    2020-09-13T19:43:02.453+00:00

    Helder, thanks for sending feedback to Microsoft.

    Work-around, provided you have a Generation 1 virtual machine, is to remove all Network Adapters and add Legacy Network Adapters.

    2 people found this answer helpful.
  3. Xiaowei He 9,866 Reputation points
    2020-09-10T06:27:22+00:00

    Hi,

    1. When the update is installed, please check if the issue remains if we add back the ethernet adapter after the VM starts.
    2. Please check if you are running anti-virus application, if yes, please uninstall anti-virus application.

    Below is a similar case for your reference:

    https://social.technet.microsoft.com/Forums/ie/en-US/41daddd9-b2ad-4ec2-ab83-0b72858f15ce/error-with-synthetic-ethernet-port?forum=winserverhyperv

    Besides, I didn't find related information about the known issue on KB4571756 with Synthetic Ethernet Port.

    Thanks for your time!
    Best Regards,
    Anne

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    1 person found this answer helpful.