libvirt can't setup simple bridged network?

[Laine Stump]

On 8/15/22 10:11 AM, Ian Pilcher wrote:
> I feel like I'm taking crazy pills!  I'm reading the libvirt network XML
> format documentation[1], and I can't figure out how to create a simple
> bridged network - no NAT, no routing, no OVS, no  macvtap, etc.  I.e.,
> just a Linux bridge with a single physical interface attached.
> 
> None of the 3 scenarios listed for <forward mode='bridge'> describe the
> simple setup that I'm trying to create, so it looks like I'll need to
> create the bridge separately.  (It's not hard to do, it just seems like
> such a weird gap the in the functionality.)
> 
> 
> [1] https://libvirt.org/formatnetwork.html
> 

libvirt's virtual network driver historically only creates networks that 
don't touch (and potentially mess up) the existing host system network 
config. But attaching a physical host system ethernet to a bridge 
requires moving the ethernet device's IP config over to the bridge, so 
that was considered "out of scope" for libvirt's network driver.

Back in 2008-2009, libvirt added an "interface driver" whose purpose was 
to configure/reconfigure host system network interfaces to, for example, 
attach a host ethernet to a bridge device, or add a vlan interface based 
on a host ethernet (and then attach that vlan interface to a bridge). 
This was initially supported on Fedora/CentOS/RHEL platforms using a (at 
the time new) library called netcf. After several years of floundering, 
I proposed in 2020 that we essentially admit failure and deprecate the 
netcf library (and libvirt's use of it). I don't have the energy to 
rehash the entire list of reasons here, but my message proposing the 
deprecation and listing all the reasons, is here:

https://listman.redhat.com/archives/libvir-list/2020-December/212781.html

These days (and even before, for the most part) if you want a bridge 
attached to a host system ethernet, it's recommended that you set that 
up using whatever host system network config you're using (e.g., 
NetworkManager, systemd-networkd, ifcfg files, /etc/network/interfaces 
file), and then either define your guest interfaces with <interface 
type='bridge'>, or if you want to use <interface type='network'> 
andrefer to that with a libvirt network name, create a libvirt network 
with <forward mode='bridge'> (which expects that a bridge device will 
have already been created in the host system network config).