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I'm having a strange issue with kvm and bridged networking on several ubuntu servers running ubuntu 16.04. Bridged networking works fine on one of my servers, but doesn't work on my other three servers. I believe they are all configured the same and correctly.

On the machine that is working, my /etc/network/interfaces file has:

auto eno2s0
iface eno2s0 inet manual

auto br0
iface br0 inet dhcp
    bridge_ports enp2s0
    bridge_stp off
    bridge_fd 0
    bridge_maxwait 0

On the machines that don't work, the only difference is the bridge_ports line, because the real ethernet device name is different on each server.

When I boot each machine, the OS correctly gets an IP from dhcp, and configures the network on br0, however when I try to boot a VM via virt-manager or virsh, the network acts like it's not connected, basically hanging for 5 min during boot until it times out.

The interface in the virsh xml looks like the following on each machine definition with the mac address different for each one of course.

<interface type='bridge'>
  <mac address='52:54:00:12:4d:ec'/>
  <source bridge='br0'/>
  <model type='virtio'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>

I'm really at a loss to explain why the 3 servers aren't working when the one is, all three have bridge-utils installed, and the same qemu/libvirt packages installed. I haven't messed with any of the libvirt configuration files in /etc on any of the machines, so they are all default values. I've tried a number of different things to fix this problem after googling for answers, but nothing seems to affect it.

Any idea what's going on? Thanks for any help.

  • Actually, I figured out the problem. Apparently installing docker changed the firewall default on forwarding from allow to deny, which broke dhcp on bridged network devices . – Steven Jorgensen May 4 '18 at 3:40
  • Had exactly the same problem, caused by installing docker on the VM host and your comment is the only thing I could find that mentioned docker messing with the firewall rules. Maybe that comment should be turned into an answer? – glennr Jun 26 '18 at 22:57
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I think

bridge_ports enp2s0 

should be

bridge_ports eno2s0

to match the interface above

  • It's a typo, the correct interface name is enp2s0. – Michael Hampton Apr 28 '18 at 21:20
  • @MichaelHampton is correct, that was a typo. I discovered the issue was firewall related, see my comment above. – Steven Jorgensen May 4 '18 at 3:41

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