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2-3 months ago I followed this article https://brooks.sh/2017/12/22/configuring-kvm-on-clear-linux/ to setup bridge network for kvm/qemu guest ubuntu. My Host os is ClearLinux. However the guest OS network stopped working 1 week ago, couldn't get IP address from dhcp. I couldn't recall I did anything special other than follow ClearLinux update.

My case is quite similar to this one Configuring a bridge interface with systemd-networkd but the solution there doesn't suite me.

My configs

/etc/systemd/network/br0.netdev

[NetDev]
Name=br0
Kind=bridge

/etc/systemd/network/br0.network

[Match]
Name=br0

[Network]
DHCP=yes

/etc/systemd/network/80-dhcp.network

[Match]
Name=enp12s0

[Network]
Bridge=br0

/etc/sysctl.d/90-bridge.conf

net.bridge.bridge-nf-call-arptables = 0
net.bridge.bridge-nf-call-ip6tables = 0
net.bridge.bridge-nf-call-iptables = 0

$ brctl show

bridge name bridge id       STP enabled interfaces
br0     8000.fedc02dff363   no      enp12s0

networkctl

IDX LINK             TYPE               OPERATIONAL SETUP     
  1 lo               loopback           carrier     unmanaged 
  2 enp12s0          ether              degraded    configured
  3 br0              bridge             routable    configured

3 links listed.

$ systemctl status systemd-networkd

● systemd-networkd.service - Network Service
   Loaded: loaded (/usr/lib/systemd/system/systemd-networkd.service; disabled; vendor preset: disabled)
   Active: active (running) since Sun 2019-01-27 17:30:17 AEDT; 19min ago
     Docs: man:systemd-networkd.service(8)
 Main PID: 504 (systemd-network)
   Status: "Processing requests..."
    Tasks: 1 (limit: 4915)
   Memory: 976.0K
   CGroup: /system.slice/systemd-networkd.service
           └─504 /usr/lib/systemd/systemd-networkd

Jan 27 17:30:18 clr systemd-networkd[504]: enp12s0: Gained carrier
Jan 27 17:30:18 clr systemd-networkd[504]: enp12s0: Configured
Jan 27 17:30:18 clr systemd-networkd[504]: br0: Gained carrier
Jan 27 17:30:19 clr systemd-networkd[504]: enp12s0: Gained IPv6LL
Jan 27 17:30:20 clr systemd-networkd[504]: br0: Gained IPv6LL
Jan 27 17:30:21 clr systemd-networkd[504]: br0: DHCPv4 address 192.168.2.107/24 via 192.168.2.1
Jan 27 17:30:32 clr systemd-networkd[504]: br0: Configured
Jan 27 17:31:06 clr systemd-networkd[504]: vnet0: Gained carrier
Jan 27 17:31:07 clr systemd-networkd[504]: vnet0: Gained IPv6LL
Jan 27 17:32:17 clr systemd-networkd[504]: vnet0: Lost carrier

in my vm.xml, the network interface part is

<interface type='bridge'>
  <mac address='00:16:3e:11:22:33'/>
  <source bridge='br0'/>
  <model type='virtio'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>

on host ip link show, result is

$ ip link show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: enp12s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master br0 state UP mode DEFAULT group default qlen 1000
    link/ether 40:8d:5c:11:22:33 brd ff:ff:ff:ff:ff:ff
3: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default qlen 1000
    link/ether fe:dc:02:df:f3:63 brd ff:ff:ff:ff:ff:ff
5: vnet0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq master br0 state UNKNOWN mode DEFAULT group default qlen 1000
    link/ether fe:16:3e:04:22:5b brd ff:ff:ff:ff:ff:ff

in the guest os, ethernet interface is en3, link status is also "BROADCAST,MULTICAST,UP,LOWER_UP".

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After another week, the issue is gone. I should conclude this is a on-the-fly bug in ClearLinux introduced during the past three weeks.

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