Antivirus and Hyper-V (or: Why can’t I start my virtual machine?)

Antivirus and Hyper-V (or: Why can’t I start my virtual machine?)

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A little while ago our support team put together this KB article in response to a problem that a lot of people have been reporting.  Basically, what is happening is that users are having problems starting virtual machines after they install antivirus software in the management operating system.  The root cause of the problem is that a number of these programs monitor file access in a way that interferes with Hyper-V’s attempts to open virtual machine files.  If you see this problem – you have two options:

  1. Don’t install antivirus.  Now – before you choke on your coffee or get your pitch-forks - listen to me for a moment.  If you are running a server core configuration, or a full server configuration, and you have nothing running in the management operating system other than Hyper-V, and you do not have people logging in and browsing the web in the management partition, etc… Then you do not really need to have antivirus software installed as there is limited risk of a virus.

  2. Install antivirus and set up the following exclusions (most antivirus programs allow you to exclude specific directories, files and processes from scanning to help deal with issues such as these):
    • Default virtual machine configuration directory (Normally this is C:\ProgramData\Microsoft\Windows\Hyper-V)
    • Custom virtual machine configuration directories
    • Default virtual hard disk directory (Normally this is C:\Users\Public\Documents\Hyper-V\Virtual Hard Disks)
    • Custom virtual hard disk directories
    • Snapshot directories
    • Vmms.exe
    • Vmwp.exe

Then everything should be just fine.

Cheers,
Ben

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  • Hello Ben,

    yesterday evening we had a big problem with one of our Hyper-V guests as described in our blog. Did you have a problem with your virtual guests, or do you only want to warn for a potential problem? We had the situation, that we could start the affected guest only without any virtual network cable connected. When we connected the NIC, the guest couldn't start. We migrated all guests from the host to another server, there all server started without problems. We had some theories, including a problem with the antivirus-software. It seems that this was the right theorie. Another long night :)

  • Hi Ben,

    we had "big problems" with virusscan-software on a HyperV Cluster.

    We did the required exclusions, but still machines did not start up ... but why?

    After reading exactly through debug logs, it is clear, that for example "virusscan enterprise", which has an option (like other products) to scan network drives/shares, blocks some actions, while starting a virtual machine, because the there will be an "open process" through the network share to the other cluster nodes ... and beacause of this, the "local" exclusions do not work.

    So we disabled on HyperV Cluster-Servers "Network-Scanning" and these things, too:

    *.hive

    *.container

    *.blf

    Maybe this helps someone.

    Regards

    Daniel Capilla

  • 2. You actually paid for this "Antivirus live platinum" thinking you are getting a cheap antivirus (sorry if this what happened, more then that, chances to get a refund is minimal - after all it is a scam).

  • Hey Ben,

    We've got a system right now where we're providing access to virtual machines (but not the root partition) to a set of users.  A number of them want to upload info to these virtual machines (We don't currently provide remote desktop access) so we wanted to allow them to upload an attach an ISO.

    How important is virus protection on this uploaded ISO to the health of the management partition.  In short, we're uploading the Iso, storing it someplace on the host drive, and using WMI to attach it to the virtual machine.  But what is the risk of having an unchecked file sitting on the drive, even if it is only attached to a guest machine?

    Thanks,

    -Mezz

  • Thanks, very helpful post :-), but could you tell me did I need to install antivirus on the guest VM or not?, thanks in advance

  • Thanks for this post, I didn't even think of this when the snapshot merges kept failing.  Disabled AV and everything worked!

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