When it comes to fuzzing low-level network protocols like IPv4, UDPv4, and TCPv4, precise control over packet construction and transmission is paramount. Penzzer, our advanced fuzzing platform, is designed to probe deeply into protocol implementations, uncovering subtle bugs and vulnerabilities. But to operate effectively at this depth, it requires an environment that allows unfiltered, low-level access to the network stack. This is where Hyper-V with PCIe/PCI passthrough (also known as Discrete Device Assignment or DDA) becomes essential.

Modern operating systems and virtualized environments impose strict controls on network traffic. These controls are critical for security and stability, but they can also interfere with fuzzing operations that intentionally craft malformed or non-standard packets. For example, trying to send a packet with an incorrect checksum, an invalid header field, or a truncated payload might be silently corrected, dropped, or logged by the host system before it ever reaches the target.

Hyper-V's PCIe/PCI passthrough allows a virtual machine (VM) to directly access a physical network interface card (NIC), bypassing the host's network stack entirely. This setup gives the fuzzing environment raw access to the hardware, enabling full control over every byte on the wire. This capability is critical for testing edge cases and non-compliant protocol behavior exactly the kind of scenarios where real-world vulnerabilities often lurk.

Without this configuration, Penzzer may not be able to generate or transmit malformed packets as intended. Instead, packets might be sanitized or blocked at various layers of the host OS or hypervisor, preventing them from ever reaching the device under test (DUT). This undermines the fidelity of the fuzzing process, leading to incomplete coverage and missed vulnerabilities.

By using Hyper-V with PCIe/PCI passthrough, Penzzer can:

• Craft and transmit malformed network packets without interference
• Maintain protocol ambiguity essential for testing unknown or proprietary implementations
• Accurately emulate attacker behavior at the packet level
• Ensure precise timing and ordering of packets, which can be critical for triggering race conditions or state machine bugs

‍

Enabling PCIe/PCI Passthrough in Hyper-V

To enable PCIe/PCI passthrough in Hyper-V, follow these steps:

1. Check your hardware compatibility:
   • Ensure that your CPU and motherboard support Intel VT-d or AMD-Vi.
   • Enable these features in your BIOS/UEFI.
2. Identify the device:
   • Use Get-PnpDevice or Get-PCIdevice in PowerShell to find the Device Instance Path of your NIC.
3. Dismount the device from the host:
4. Dismount-VMHostAssignableDevice -LocationPath "<Device_Instance_Path>"
5. Assign the device to the VM:
6. Add-VMAssignableDevice -LocationPath "<Device_Instance_Path>" -VMName "<VM_Name>"
7. Start the VM:
   • Ensure the VM is configured with secure boot disabled and using a Generation 2 configuration.

‍

Verifying PCIe/PCI Passthrough Configuration

To verify that PCI passthrough is enabled and functioning correctly:

• Inside the VM:
  • Check the device manager or use lspci (Linux) to see if the NIC is recognized.
  • Verify that no host-side network filters or drivers are active on the passthrough NIC.
• On the host:
  • The NIC should no longer appear as an active network device in the host OS.
  • Use PowerShell to confirm that the device is in use by the specified VM.