Unable to configure bridge networking on Ubuntu 21.04 host (Netplan) / MacOS guest

[Adam Kessel]

I'm running Ubuntu 21.04 host (Netplan networking) and MacOS guest (from 
macOS-Simple-KVM). Attempting to get bridge networking to work either 
from qemu CLI or via virt-manager. While NAT works fine, I have not had 
any luck with bridge networking -- the guest cannot connect to the 
Internet and is unable to get an IP address via DHCP.

I've followed the various tutorials exactly, e.g. 
https://fabianlee.org/2019/04/01/kvm-creating-a-bridged-network-with-netplan-on-ubuntu-bionic/ 
, 
https://cplatform99.blogspot.com/2019/03/how-to-configure-linux-kvm-bridge-on.html 
, 
https://levelup.gitconnected.com/how-to-setup-bridge-networking-with-kvm-on-ubuntu-20-04-9c560b3e3991 
, 
https://www.answertopia.com/ubuntu/creating-an-ubuntu-kvm-networked-bridge-interface/

I've tried setting both the Ethernet interface and the bridge interface 
to DHCP and with statically configured IP addresses. I've also tried a 
LAN-valid static IP address on the guest as well as DHCP. No combination 
of these settings gets me through to the network from the guest. I've 
tried going through the virt-manager GUI and also manually crafting qemu 
command-line switches. I've tried tun/tap as well as just connecting 
directly to br0. I've tried disabling my firewall. I do have IP 
Forwarding configured on the host.

My host has a very simple setup -- just one network card, IP address set 
by DHCP.

Can anyone point me to a dead-simple, failproof/foolproof method for 
enabling bridge networking with this setup? Is there something that 
changed with Ubuntu 21.04 (either with networking or the latest qemu) 
such that these older tutorials aren't working? Any suggestions for how 
to troubleshoot?