How to Bridge VirtualBox VM Over OpenVPN TAP Adapter on Windows for Penetration Testing


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When setting up a VirtualBox VM (running Backtrack 4) with bridged networking over an OpenVPN TAP connection on Windows 7, the host successfully obtains a DHCP address from the university's cybersecurity club VPN, but the VM fails to acquire an IP. This creates a roadblock for conducting penetration testing in the sandboxed environment.

The university's IT policies strictly prohibit active scanning tools (nmap, Nessus) and attacks (ARP spoofing) on the main network. Bridging the VM through the VPN would:

  • Isolate all testing traffic to the sandbox LAN
  • Prevent accidental leaks to the production network
  • Maintain host machine compliance

The core issue appears to be OpenVPN filtering packets from unrecognized MAC addresses. Here's what's happening at the network level:


Host Machine:
1. OpenVPN TAP adapter established (e.g., "tap0")
2. DHCP lease obtained (e.g., 192.168.1.100)
3. Bridge created between physical NIC and tap0

VirtualBox VM:
1. Configured for bridged networking
2. Attempts DHCP via tap0 bridge
3. Requests are dropped by OpenVPN server

Here's how to properly configure the bridge:

1. Verify TAP Adapter Installation

First ensure the TAP driver is properly installed:


# In command prompt:
C:\> tapinstall.exe list
# Should show "TAP-Windows Adapter V9"

2. Create the Network Bridge

Manual bridge creation is more reliable than VirtualBox's automatic bridging:


:: PowerShell commands
PS> New-NetSwitchTeam -Name "VPNBridge" -TeamMembers "Ethernet","TAP-Windows Adapter"
PS> Set-NetAdapter "VPNBridge" -MacAddress "DEADBEEF1234"  # Optional MAC spoofing

3. VirtualBox Network Configuration

Configure the VM to use the bridged adapter:


VBoxManage modifyvm "BacktrackVM" --nic1 bridged
VBoxManage modifyvm "BacktrackVM" --bridgeadapter1 "VPNBridge"

If bridging still fails, check these aspects:


# Check DHCP server logs for dropped requests
# Verify MAC address filtering isn't enabled on OpenVPN server
# Test with static IP assignment first

When working with penetration testing tools through a VPN bridge:

  • Always confirm you're targeting sandbox systems
  • Monitor network traffic with Wireshark
  • Consider using --redirect-gateway in OpenVPN config

When setting up a bridged connection between VirtualBox and OpenVPN's TAP adapter on Windows, you'll encounter layer 2 networking complexities that don't exist with standard TUN interfaces. The core issue stems from how Windows handles TAP adapter bridging and VirtualBox's network filtering.

The most common symptoms include:

  • Host gets IP via DHCP but VM shows "No DHCP offers received"
  • ARP requests from VM never reach the VPN gateway
  • Packet capture shows traffic being dropped at the TAP interface

First, verify your OpenVPN configuration contains these essential settings:

dev tap
dev-node MyTAP
server-bridge
client-to-client

Then configure VirtualBox networking with these commands:

VBoxManage modifyvm "Backtrack 4" --nic1 bridged
VBoxManage modifyvm "Backtrack 4" --bridgeadapter1 "OpenVPN TAP-Windows Adapter V9"
VBoxManage modifyvm "Backtrack 4" --macaddress1 auto

Critical Windows-specific adjustments:

  1. Open Network Connections (ncpa.cpl)
  2. Select both your physical NIC and TAP adapter
  3. Right-click and choose "Bridge Connections"
  4. Configure the new bridge to obtain IP automatically

Run these diagnostic commands in your VM:

# Check interface status
ip link show dev eth0

# Test DHCP
dhclient -v eth0

# Verify routing
ip route show

To prevent accidental leakage of attack traffic:

# Add firewall rules in the VM
iptables -A OUTPUT -o eth0 -d 192.168.1.0/24 -j ACCEPT
iptables -A OUTPUT -o eth0 -j DROP

If MAC addresses aren't propagating correctly:

# On the VM:
arp -s 192.168.1.1 00:FF:AB:CD:12:34

# On Windows host:
netsh interface ipv4 set interface "TAP-Windows" forwarding=enabled