[SOLVED] DHCP replies not received by QEMU Deb9 guest during graphical install

Hey guys/gals,
I am brand new to this forum, and am still new to asking for help on forum platforms such as this. So any and all advice is appreciated.

The core problem I'm facing is that DHCP reply messages are not being received by the graphical installer that is setting up a Debian 9.5 QEMU guest VM over a bridge interface.
Here are some of the high level details....

Topology
Each node is connected with 1Gbps ethernet (so no wireless interfaces)

[router]---[switch1]---[switch2]---[host]

Host

Runs Ubuntu 16.04.5
1x 1Gbps NIC configured with dhcp for management activities (em1)
1x 1Gbps NIC configured as a bridge with a static IP address (br0 / eth1 slave)
Bridge connectivity tested successfully using SSH & ping from different host in same LAN network.
I am explicitly disabling spanning-tree-protocol in the interfaces file,
and brctl show command correctly displays my bridge with eth1 as the only interface.

Install Command
virt-install --name={guest-name} --vcpus=2 --memory=4096 --network bridge=br0
--cdrom={.iso-img-path} --disk size=20,path={diskimg-path} --os-variant=debian8
--graphics vnc,password={pass},listen=0.0.0.0 --noautoconsole

Observed behavior and troubleshooting so far
-> Debian 9 graphical installer reports a failure to autoconfigure the guests network interface for DHCP over the host bridge interface
-> command 'sudo tcpdump -i br0 | grep -i dhcp' (output below) only shows the requests being broadcasted, and no replies.
10:08:57.833669 IP 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from 52:54:00:8a:9e:69 (oui Unknown), length 300

I have a technical background but am unfamiliar with the low level details and how to troubleshoot DHCP properly.
Like I said before I am new to asking for help on these forums, any and all advice is appreciated.

Thanks!

Couple of ideas:

Firewall issue?

Do you have the routing setup correctly for the bridge?

Is libvirt starting dnsmasq? i.e. Who is providing dhcp?

Apologies for not seeing the previous reply.

So the next thing I tried was to use the MAC address in the DHCP Discover packets, and configure a static IP on my router GUI. This succeeded as it allowed me to get past the network configuration part of the installer, but it failed when I reached the point where I needed to configure the package manager by connecting to a mirror server.

Now ARP resolution IS something I am familiar with, and I found my ARP packets being sent out the bridge were not getting a reply, similar to the DHCP discover packets. When I inspected the packets from pinging the bridge interface from another host on the LAN I found the wrong MAC address in the source field for the reply. It turns out the Linux kernel treats an IP address as a "system object" in a loosely coupled system, so having multiple interfaces connected to the same network (192.168.1.0/24) won't guarantee which physical interface gets used for handling packets pertaining to a specific IP.

To resolve this, I just added every physical interface in the system to the bridge interface definition and everything is working now after reboot. Not my ideal setup, but it works, and DHCP works as expected.

Further info: https://lwn.net/Articles/45373/

Thanks for the update and solution.