Re: [libvirt-users] connecting host and guest vm using a dummy nic

Sent: Wednesday, May 02, 2018 at 5:56 PM From: "Laine Stump" <laine(a)redhat.com&gt; To: libvirt-users(a)redhat.com Cc: daggs <daggs(a)gmx.com&gt; Subject: Re: [libvirt-users] connecting host and guest vm using a dummy nic On 05/01/2018 12:12 PM, daggs wrote: > > I want to see if I understood you correctly, I add: > <interface type='ethernet'> > <model type='e1000'/> > </interface> > to the vm's xml, this will result in a new nic in the vm which and in the host which I can use by the host to request dhcp from the router within the vm? > Yes. As long as the emulated device in the guest is listening for DHCP on that interface. Also, note that (as long as your libvirt is at least version 2.1.0) if you don't want to bother doing dhcp on the host and guest, you can assign just a static IP address for the host side of that interface in the libvirt XML like this: <interface type='ethernet'> <source> <ip address='192.168.254.2'/> </source> ... You can even specify routes to be added automatically to the host when that guest is started, e.g.: <interface type='ethernet'> <source> <ip address='192.168.254.2'/> <route family='ipv4' address='0.0.0.0' prefix='0' gateway='192.168.254.1' metric='1'/> </source> ... (this assumes that on the guest side, the OS has configured the emulated ethernet device to have address 192.168.254.1). This can be useful if you want the host to use one path for traffic when the guest isn't running, but switch when the guest *is* running. (In this case, if you can set the default route on the host to have a metric of 2 or higher, then the default route that goes through the guest would be preferred when present). (NB: if you read the documentation for configuring interfaces at https://libvirt.org/formatdomain.html#ipconfig you'll see references on how to configure the IP address of the *guest* in libvirt's xml. Note that this *does not work* for qemu, but only for lxc (and possibly xen, I don't recall right now). This is a limitation of the way that qemu/kvm works (the fact that all guest-side IP info is configured in the guest OS, and libvirt has no reasonable to get to it).)