About the howto:
1) This is sorely needed, so good one.
2) A few suggestions and additions from my own tormented experiences using XP as a host:
a) If you use vbox 1.3.8, you do not need the OpenVPN tap driver. If you open VirtualBox and click on settings->network, then type an interface name, eg: tap1, and then click 'Add a host interface' button, a compatible network adapter will be created for you. This seems easier than using the OpenVPN adapter.
(I did have to reboot after creating a tap adapter for the first time since it gave me two adapters, the tap I wanted as well as a new and broken, unneeded lan connection. After a reboot only the tap remained).
b) You forgot to show how to bridge the connection in XP, i.e, select your LAN connection, hold ctrl and then select the tap1 connection, then right click on tap1 connection and select 'bridge connection'.
c) For the XP bridge, one needs to check that your adapters have been forced into promiscuous mode, and you can do this by doing the following:
- Open a command prompt (Start, Run, cmd)
- type 'netsh bridge show adapter'. This will spew out a # of lines, one for the bridge and one for each tap you create and add to the bridge. Note the # of the adapter/s that are disabled.
- type 'netsh bridge set adapter # forcecompatmode=enable', where # is the disabled entry noted above, and do this for each disabled adapter.
- this change updates the registry and needs only be done once.
- type 'netsh bridge show adapter' again to be sure they're all enabled. If the bridge won't enable you may be OOL since not all NICs allow promiscuous mode.
- The above process may be needed for w2k as well. I dunno.
d) If you use a router as a DHCP server then an additional complexity may arise due to your NIC (Network Interface Card) exposing only a single MAC address to the router, despite it serving two or more connections.
On my router, a WRT54GL using DD-WRT software, I couldn't get the router to consistently assign a unique IP to the guest, so I sometimes got an IP conflict as soon as the guest started.
The way permanently around this conflict problem was to assign a static IP (outside the router dhcp serving range) to the bridge. Note, I did this via the TCP-IP properties tag for the bridge on the XP host. Not via the router's static ip serving service.
Oddly enough, additional guest do not have this problem, ie. they get their unique ip and even serve up their hostname.
Anyhow, I hope the above helps to fill out the XP host part of the howto