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No incoming traffic on Hyper-V Adapter inside VM

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HackXIt

I have a very annoying problem with my Hyper-V setup and it's bugging me quite a bit, because I can't figure out why.

My Hyper-V Host is a Windows Server 2019 and has three network adapters. Adapter 1 & 2 are physical and have static configurations.

Adapter 3 has been configured as a Hyper-V extensible Switch. (External) This adapter is connected to a trunk port on a cisco switch, where multiple vlans have been tagged. The host-adapter uses VLAN 777 and can connect just fine.

I've created a new VM, where I've installed another instance of Windows Server 2019. This VM uses the same vSwitch but configured to the VLAN 666.

Inside the VM, I looked at the status information of the adapter:

  • X number of packets since device was started were sent out
  • 0 packets were received

At first I suspected some weird firewall to block the connection.

For sanity, I disabled ALL firewall rules/settings/profiles, because this host is inside a local network anyways. I did this for the Host and the VM.

Nothing changed, still no packets incoming.

I checked the switch that it's connected to - It showed the static MAC-Adress of the VM, confirming that it somehow registered it. However, appearantly it can't receive any packets, which makes DHCP or any communication impossible.

  • What else could be blocking this connection?
  • What logs could I activate or look at to figure this out?

There aren't any blocking rules or anything on the switch. The host confirms this, because the virtual Host-Adapter works just fine.

It's just inside the VM.

Some technical notes about the VM:

  • It was setup using ISO 17763.737.190906-2324.rs5_release_svc_refresh_SERVERESSENTIALS_OEM_x64FRE_de-de_1
  • It is set to Generation 2
  • Additional Features and checkbox inside the VM configuration of the Network adapter are all unchecked.

Update - What I have checked so far:

  • Eventlog in VM and on Host (However, I did not know what to look for specifically!)
  • Used an Adapter mirror to look at the connection inside Wireshark (Adapter from VM as Source with new Host-Adapter as Destination) -> Didn't really lead to anything but I did notice that I can't see ICMP pings going out, weirdly enough
  • Firewall settings on Host and VM -> On both systems those have been turned OFF on ALL profiles to troubleshoot this without further possible disturbances
  • Activated logging of firewall for dropped packets and checked those, nothing there.
  • Triple-Checked Hyper-V Settings, there are no special features activated, regarding the network connection.
  • Change the configuration on the Switch to be an access port for the vlan instead of a Trunk -> No difference
  • Use a dynamic MAC Adress for the Adapter instead of a Static one -> No difference
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HackXIt

I figured it out and the answer is quite sad.

On the physical NIC that was used for the vSwitch, there was a VLAN configured on the hardware-driver. This didn't get deleted upon creation of the vSwitch and it somehow managed to block all traffic in/out, since nobody really could communicate, when the hardware itself changed the VLAN ID.

How did I find out? I plugged the NIC closer to the switch and noticed it's not getting traffic on the host as well. After numerous attempts to ping the host NIC, I've come to the conclusion that the fault must be on that machine's network card. I switched network cards and it worked.

After checking the adapter hardware settings again, I noticed the VLAN ID that was hardcoded by Intel ProSet Adapter Configuration Utility.

So yeah... Cleared that, now everything works.

Taught myself a stupidity lesson.

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