Tech Line
Virtual PC MAC Address Malfunction
Here's how to prevent duplicate Virtual PCs from using the same MAC address.
Chris: I have a 150 PCs running Windows XP across three
VLANs connecting to Active Directory on a Windows 2003 server. Each PC
runs Microsoft Virtual PC with a Windows NT 4.0 image connecting to a
domain on a Windows NT 4.0 Server. DHCP is running on the Windows 2003
server with a different subnet for each VLAN:
VLAN 1 = 10.143.47.0
VLAN 2 = 10.143.35.0
VLAN 3 = 10.143.34.0
DHCP has been set up with a range of 200 IP addresses in the pool for
each VLAN with an 8-hour lease. With the VPCs running, VLAN 1 is running
at about 70-percent capacity, VLAN 2 is running at around 50-percent capacity
and VLAN 3 running at 20 percent.
The problem: Some PCs are unable to obtain an IP address from the DHCP
server. We can fix this by recreating the VPC.vmc file or by editing it
and removing the MAC address, but then another Virtual PC will have the
same problem. We fix that and another will not connect; we fix that, then
the first won't be able to get an IP address and so on.
The XP host PCs never have any problems connecting. We don't have any
problems joining the NT VPCs to the domain when we first set them up.
— Andrei
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Andrei, your Virtual PC problem does have a small history. In fact, Microsoft
recognized the potential for the problem. In the Virtual PC help file,
you'll find this explanation from Microsoft:
If you create an image of a host operating system that includes Virtual
PC and virtual machines configuration files (.vmc files) and copy that
image to another computer, each virtual machine configuration file included
in the image contains a MAC address. The MAC address will not be reset
automatically when you place the image on a new physical computer. As
a result, the virtual machines that are copied onto the new computer will
have the same MAC addresses as the virtual machines on the computer that
was used to create the image.
The help file will further go on to state that a solution is to delete
the MAC address reference in the Virtual PC's associated .vmc file and
a new MAC address will be automatically generated the next time the Virtual
PC starts. In the .vmc file, you'll see a line that reads:
<ethernet_card_address type="bytes">0003FFxxxxxx</ethernet_card_address>
The MAC address referenced in the file will always exist between the
two ethernet_card_address tags. To cause Virtual PC to generate a new
MAC address the next time the VM starts, delete the MAC address referenced
in the file so that the line reads:
<ethernet_card_address type="bytes"></ethernet_card_address>
In fact, this is a recommended best practice when copying a single Virtual
PC image to several host systems.
As you already mentioned, you have attempted this procedure without any
luck. This is due to the behavior of the Virtual PC algorithm that generates
the MAC address. When a MAC address is created, Virtual PC will check
to ensure that the MAC address does not exist on the local LAN segment
before assigning it to the Virtual PC. However, when separated by three
VLANs, this default behavior will not work. This can cause the MAC address
conflicts and, thus, the intermittent problems with obtaining DHCP leases.
To solve this problem, you can take two approaches. Some of you are probably
thinking "Yeah, get rid of Virtual PC and replace it with VMware
Workstation!" But, that isn't one of the solutions I'm considering
here. The first solution is to statically assign MAC addresses in the
.vmc file of each Virtual PC VM. The Virtual PC MAC address prefix is
0003FF. So a workaround would be to statically assign new MAC addresses,
such as: 0003FF000001, 0003FF000002, 0003FF000003, 0003FF000004, 0003FF000005,
and so on. This approach would prevent MAC address conflicts altogether
and allow each VM to reliably obtain its own DHCP lease.
Another approach is to use the Virtual PC "Shared Networking (NAT)"
network binding, instead of directly binding each Virtual PC to the physical
network adapter of the host. This is configured by opening Virtual PC,
clicking on a Virtual Machine, and then clicking the Settings button.
From the Settings window, you then click Networking and select Shared
Network (NAT) from the drop-down menu on the right-side of the window.
With NAT configured, the Virtual PC would automatically receive a DHCP
address from the host system and would be assigned the same DNS server
that the host is configured to use. With this approach, having the host
systems obtain a DHCP address is all that's needed. The NATing provided
by Virtual PC would give each VM access to network resources, including
allowing them to join the domain.
With either of these approaches, you should be able to reliably connect
each Virtual PC to your LAN.