Promiscuous mode on Hyper-V vswitch - Wiresharking from within a VM



Hyper-V host (2012) with an 'external'  vswitch configured against a physical NIC.

VM guest (various OS) work fine and can communicate fine. no issue with normal networking.

OK, so..... running Wireshark on the VM Guest can only pick up broadcast traffic etc, as it's 'behind'' a virtual switch that may be filtering out all the traffic seen by the NIC.
(Yes, The physical switch port is correctly set to mirror, confirmed by running Wireshark on a physical client.)

Anyone have any idea how to make a VM able to see all traffic, as if the NIC was assigned to it directly?

IE, the VM guest NIC should see all traffic flowing via the physical NIC.
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SteveConnect With a Mentor Author Commented:
The answer appears to be that it is not possible to monitor traffic from the physical LAN from within a VM.

Some good general info on vswitches that's worth remembering, but doesn't achieve what I needed.
Zephyr ICTCloud ArchitectCommented:
SteveAuthor Commented:
yes, I've seen that before but it doesn't cover my query.
I want to monitor traffic on the network in general, NOT just within the hyper-v environment.

That guide sets a VM as the source and another VM as the destination and would only apply if you could set the Physical NIC as the source, which it doesn't mention.
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Zephyr ICTCloud ArchitectCommented:
Hmmm I see what you mean now ... The only thing coming immediately to mind now is to use passthrough for the NIC to your VM, obviously this would make the NIC unavailable for the other VMs...
SteveAuthor Commented:
No problem with that. I could dedicate a NIC to it if necessary. Do you know how to do that?
Philip ElderTechnical Architect - HA/Compute/StorageCommented:
If the vSwitch is set to External, whether shared by the host or not, any VM attached to that vSwitch should be able to "see" all inbound and outbound traffic on the network.

There are settings that pass through or accelerate certain guest activities on the physical NIC (Intel VT-x, VMQ, SR-IOV, ETC) but essentially the vSwitch is a passive entity.

Perhaps there is a firewall setup of some sort causing the "filtering" that you are seeing?

In my experience the vSwitch does not interfere in any way.

SteveAuthor Commented:
Hi Philip,

I see what you're trying to say, but it is still a 'switch'. This means the VM only sees traffic that the switch chooses to send to it, not everything.

We would be able to see everything if it were more of a vHub instead of a vSwitch, but a switch does 'filter' out traffic by design. That's standard layer 2 stuff.

Managed switches often have a port-mirror facility to get around this layer 2 filtering, but it's not looking like that's possible on a vSwitch.
Philip ElderConnect With a Mentor Technical Architect - HA/Compute/StorageCommented:

TN: vSwitch Layer 2:

But, more to the point:
As we can see in this TN article there are a series of PowerShell commands that can be used to manage the vSwitch ACL setup for a VM.

That leads to: (Introducing the Hyper-V Extensible Switch)
This post talks about plugging into the switch and another way to manage it: Hyper-V Manager.

Perhaps what we are setting is that this neat new feature set is the victim of poor documentation? :)

SteveAuthor Commented:
Thanks Philip.
There's some interesting stuff there, and well worth a look, but I'm afraid it doesn't appear cover what I need.

So far everything in there is designed to allow traffic control (and even monitoring) between VMs as the switch element of the system still decides which traffic to send to the VM in the normal way.
There doesn't appear to be anything relating to allowing all traffic coming from the physical NIC to reach a VM even when it isn't intended for the VM, which currently makes a VM unsuitable for Wiresharking.
SteveAuthor Commented:
Some useful info from Philip but nothing to solve the original question.
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