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When assigning physical NICs to a VM in bridged mode, it seems you can't choose "dial up" adapters. Specifically, if you've created a PPPoE connection in your Windows (2008) host, because you have a plain ADSL modem (or router internally bridged to act like one), then you can't give it to the VM - the "WAN Miniport (PPPoE)" adapter (as it appears in Windows) doesn't appear in the NIC selection combobox in the VBox UI. Yet this seems a reasonable thing to want to do - you just want the VM to see, and have total control of, a virtual NIC that's connected to the Internet.

Is this by design? Is it because of a reasonable technical limitation? Is there a way around it (other than creating the PPPoE link inside the VM)?

No, you can't bridge to a PPPoE interface. Bridging works by adding a filter driver which cooperates with the NIC driver in the host OS to allow the virtual machine to share the physical NIC. A PPPoE interface does not have this sort of driver software.

Your only option is to do what you have to do to share a PPPoE connection to another physical machine - ie use ICS in the host to share the connection.

Ok cheers,

Can't use ICS as I don't want NAT behind NAT in the virtual world, and don't want the host active on the external network at all.

Might switch to Linux as a host. I've never set up a PPPoE adapter on Linux, but it's networking is generally far more flexible than Windows, so maybe that'll work. Or I might have to go half bridge mode on the ADSL router.

Thanks again.

You gave the answer in your first post already, you can't assign a virtual adapter as bridged, it must be physical. And with PPPoE connections, you already get an IP address on that interface. It's part of the connection that you make.

Additionally, you can't run a router in 'half bridge' mode. It's either the PPPoE client itself, or it creates a bridge which deals with the connection itself. When the computer does a DHCP request, the router establishes the PPPoE connection and sends it to the PC transparently.
There is no way that you can have the PC and a VM use the same PPPoE connection at the same time as if you have two physical computers hooked up to it, as you're only allowed to use 1 IP address. The only way to do it, is to use NAT. You don't have NAT with PPPoE connections by default, so you won't have a NAT behind a NAT as you think you would have when using ICS.

Thanks for the reply, but that's not how I understand it. If you put the ADSL router into half bridge mode, as many do, it transparently handles the PPPoE session and bridges traffic over that session to the physically attached device (LAN router or PC). Specifically (I imagine) to the port on which it received the DHCP request. Subject to certain limitations (*) that allows the attached device to function as if it had a direct connection to the internet, receiving a public IP through DHCP, and having no knowledge of nor involvement in PPPoE.

If the device is a PC running vbox, the NIC through which it is attached to the (now transparent) ADSL router is just like any other NIC, and should be able to be bridged to a VM, allowing the VM to behave like it owns the physical NIC and has a direct connection to the WAN (internet). Which is what I want to achieve. I used to do that all the time when I had cable, before I switched ISPs to an ADSL one.

And I understand it's quite common to use more than one physical connection to the ADSL router, for a specific purpose - many ADSL routers (in bridged or half bridged modes) still do routing, and that allows you to make a second physical connection between the ADSL router and internal device (PC or LAN router) to access its internal management pages. You typically want to do that to retain the ability to see your ADSL link status, for example, but you can't typically do that over your primary device-ADSL router connection any more, because the latter is now blindly and transparently bridging the former's traffic to the internet (over its internal PPPoE session). So it's a common technique to put a second NIC in your PC (or use a second port on your LAN router), on a different subnet (the one on which the ADSL routers statically configured LAN IP address resides), and attach it to another port on your (bridged or half bridged) ADSL router for that purpose.

(*) which can be deal breaking. One is that some LAN routers can't handle being DHCP-ed an IP address which is on a different subnet to the default gateway. Another is that this doesn't support multiple IPs, if you have that from your ISP. A third, and more typically encountered, problem is connection interruptions when your ISP changes your IP, but you can work around that using short DHCP lease times between the ADSL router and PC (or LAN router). For these reasons, some consider using half bridged mode on the ADSL router to be the least desirable solution. it probably depends on your specific hardware and your ISP.

Also, TBH, I still don't see why, in principle, a PPPoE "logical network adapter" on the PC, as you have when running the ADSL router in full bridged mode, couldn't be directly bridged to something else (such as a vbox virtual NIC). It's still a network port that has an IP address and TCP/IP binding that can be filtered, after all. It's my experience that Windows' way of doing networking creates the limitations, which Linux doesn't suffer from. I haven't looked at it yet, but I'll be genuinely surprised if Linux doesn't let me create a PPPoE "logical NIC" and bridge it to something else. We'll see!

I've stated it before, the PPPoE connection that Windows makes is NOT a physical interface. By that nature, it only exists as long as the connection exists. VB would die horribly if that connection suddenly drops. The same thing will happen on Linux and you can't select logical interfaces there either. If you have an IPv6 tunnel or similar, you can't select them for bridged either.

And as I've stated before, I know it's not a physical interface. And it does exist all the time - it sits there in your list of network connections even when not connected. Just like a physical NIC, if you disconnected it, eg. by pulling the cable out (and I'd be massively surprised if that caused VB to "die horribly").

I think we'll have to agree to disagree on this. I've still heard no plausible reason why a logical network connection (as opposed to a physical one) can't be filtered and bridged, in principle, even if the method of doing so might be somewhat different.

Even if the PPPoE interface exists all the time and it looks like a normal interface it still is a virtual interface which doesn't have all the characteristics/features in the Windows networking subsystem needed for bridged networking.