Re: [Qemu-devel] Qemu-KVM VETH

[Tim Epkes]

Stefan,

The problem aI face is that a bridge in the middle ( using taps) breaks LLDP ( L2 discovery protocol) and should break ISIS as well.  Sockets aren't bad, but if for some reason I take the listener VM down and bring back up, then I have to bring down all connector VMs, which chains on itself if there is a lot of connectivity defined.  When UDP was available that wasn't an issue. 

Anyway, that is how I came to VETHs.  I am aware that you can use UDP multicast, but performance past one hop is extremely poor ( resulting in 3 of 5 pings to be lost.  Thanks

Tim

On Friday, September 20, 2013, Stefan Hajnoczi wrote:

On Thu, Sep 19, 2013 at 05:31:01PM -0400, Tim Epkes wrote:
> Any plans to provide VETH support for Qemu-KVM.  It is a great pt-pt tie
> when when connecting to KVM's on the same machine.  I have multiple reasons
> for doing so (one is educational).  Thanks

QEMU already supports -netdev tap (if you want to use the host Linux
networking stack) and -netdev socket (if you just want point-to-point
tunneling).

The veth driver isn't suitable for QEMU's use case.  QEMU is a userspace
process that wants to inject/extract Ethernet frames.  That's exactly
what the tun (tap) driver does.  veth is useful for containers where you
want a Linux network interface that is handled by the host network stack.

Two solutions for point-to-point:

1. Run two guests with -netdev tap.  Put the interfaces on a software
   bridge (see brctl(8)).  Or you could also use IP forwarding instead
   of a bridge if you like.

2. Run two guests with -netdev socket.  They send Ethernet frames
   directly to each other.

See the qemu man page for configuration details.

Stefan