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I am running a centos 8 as virt-host with libvirt an kvm/qemu. Networking is set to bridge-mode with virbr0. Suddenly and over-night this stops working: The clients don't connect anymore, and if I try to connect the network in virtual-machine-manager I get "Bridge virbr0:Empty Bridge" and this even before starting any client.

[root@labpc1 log]# ifconfig enp5s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500 inet 192.168.2.8 netmask 255.255.255.0 broadcast 192.168.2.255 inet6 2003:c0:3713:3000:fa5b:356b:c6a6:18ff prefixlen 64 scopeid 0x0<global> inet6 fe80::9b26:4d1a:f92c:c21c prefixlen 64 scopeid 0x20<link> ether 34:97:f6:8e:c0:73 txqueuelen 1000 (Ethernet) RX packets 4788 bytes 482052 (470.7 KiB) RX errors 0 dropped 912 overruns 0 frame 0 TX packets 2205 bytes 617635 (603.1 KiB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536 inet 127.0.0.1 netmask 255.0.0.0 inet6 ::1 prefixlen 128 scopeid 0x10<host> loop txqueuelen 1000 (Local Loopback) RX packets 0 bytes 0 (0.0 B) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 0 bytes 0 (0.0 B) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 virbr0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500 inet 192.168.2.8 netmask 255.255.255.0 broadcast 192.168.2.255 ether de:a2:bd:89:f8:d4 txqueuelen 1000 (Ethernet) RX packets 0 bytes 0 (0.0 B) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 0 bytes 0 (0.0 B) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

There are messages in /var/log/messages:

grep virbr0 /var/log/messages Jan 9 19:24:30 labpc1 NetworkManager[1051]: <info> [1578594270.5088] ifcfg-rh: new connection /etc/sysconfig/network-scripts/ifcfg-virbr0 (a2d02297-f0a1-4f1e-a5c4-ebd5d5e62c89,"virbr0") Jan 9 19:24:30 labpc1 NetworkManager[1051]: <info> [1578594270.5092] ifcfg-rh: new connection /etc/sysconfig/network-scripts/ifcfg-virbr0_slave_1 (7b0e15aa-3c0c-44a6-802e-0a1af319a2e2,"virbr0 slave 1") Jan 9 19:24:31 labpc1 NetworkManager[1051]: <info> [1578594271.2427] manager: (virbr0): new Bridge device (/org/freedesktop/NetworkManager/Devices/3) Jan 9 19:24:31 labpc1 systemd-udevd[1340]: Could not generate persistent MAC address for virbr0: No such file or directory Jan 9 19:24:31 labpc1 kernel: IPv6: ADDRCONF(NETDEV_UP): virbr0: link is not ready Jan 9 19:24:31 labpc1 NetworkManager[1051]: <info> [1578594271.4417] device (virbr0): state change: unmanaged -> unavailable (reason 'managed', sys-iface-state: 'external') Jan 9 19:24:31 labpc1 NetworkManager[1051]: <info> [1578594271.4439] device (virbr0): state change: unavailable -> disconnected (reason 'none', sys-iface-state: 'managed') Jan 9 19:24:31 labpc1 NetworkManager[1051]: <info> [1578594271.4449] policy: auto-activating connection 'virbr0' (a2d02297-f0a1-4f1e-a5c4-ebd5d5e62c89) Jan 9 19:24:31 labpc1 NetworkManager[1051]: <info> [1578594271.4455] device (virbr0): Activation: starting connection 'virbr0' (a2d02297-f0a1-4f1e-a5c4-ebd5d5e62c89) Jan 9 19:24:31 labpc1 NetworkManager[1051]: <info> [1578594271.4456] device (virbr0): state change: disconnected -> prepare (reason 'none', sys-iface-state: 'managed') Jan 9 19:24:31 labpc1 NetworkManager[1051]: <info> [1578594271.4543] device (virbr0): state change: prepare -> config (reason 'none', sys-iface-state: 'managed') Jan 9 19:24:32 labpc1 NetworkManager[1051]: <info> [1578594272.7065] device (virbr0): state change: config -> ip-config (reason 'none', sys-iface-state: 'managed') Jan 9 19:24:32 labpc1 NetworkManager[1051]: <info> [1578594272.7077] device (virbr0): state change: ip-config -> ip-check (reason 'none', sys-iface-state: 'managed') Jan 9 19:24:32 labpc1 kernel: IPv6: ADDRCONF(NETDEV_UP): virbr0: link is not ready Jan 9 19:24:32 labpc1 avahi-daemon[932]: Joining mDNS multicast group on interface virbr0.IPv4 with address 192.168.2.8. Jan 9 19:24:32 labpc1 NetworkManager[1051]: <info> [1578594272.7083] device (virbr0): state change: ip-check -> secondaries (reason 'none', sys-iface-state: 'managed') Jan 9 19:24:32 labpc1 avahi-daemon[932]: New relevant interface virbr0.IPv4 for mDNS. Jan 9 19:24:32 labpc1 NetworkManager[1051]: <info> [1578594272.7084] device (virbr0): state change: secondaries -> activated (reason 'none', sys-iface-state: 'managed') Jan 9 19:24:32 labpc1 avahi-daemon[932]: Registering new address record for 192.168.2.8 on virbr0.IPv4. Jan 9 19:24:32 labpc1 NetworkManager[1051]: <info> [1578594272.8032] policy: set 'virbr0' (virbr0) as default for IPv4 routing and DNS Jan 9 19:24:32 labpc1 NetworkManager[1051]: <info> [1578594272.8863] device (virbr0): Activation: successful, device activated. Jan 9 19:24:32 labpc1 nm-dispatcher[1121]: req:3 'up' [virbr0]: new request (3 scripts) Jan 9 19:24:32 labpc1 nm-dispatcher[1121]: req:3 'up' [virbr0]: start running ordered scripts...

Most specifically I am missing a message like kernel: virbr0: port 3(vnet1) entered forwarding state

It used to work without any deliberate change ..

Any hint would be highly appreciated

Wolfgang

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    I can't solve the problem but I can confirm it. I have been struggling with this for a week. I was also able to produce this problem in Ubuntu as well. It seems to be a NetworkManager issue because if I create the network bridge under NetPlan instead of NM, the bridge works Commented Feb 6, 2020 at 20:05
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    I can confirm this. I finally disabled NetworkManager and use the depreciated networking-scripts. I did not have any problems with bridging since ... Commented Feb 10, 2020 at 7:57

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Reset to default
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after I nuked and paved a 5th time I did this:

nmcli con show nmcli con delete <all interfaces but the one i was connected to> nmcli con add ifname br0 type bridge con-name br0 nmcli con add type bridge-slave ifname eno1 master br0 nmcli con up br0

Then I cheated and added the static ips to the br0 with nmtui and fired up libvirt... viola... working

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