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I upgraded a server from 18.04 to 20.04. I am using libvirt/kvm to host a number of VMs. Some of the VMs failed to reconnect to the network after the upgrade.

Two VMs in question run Ubuntu 18.04 and Centos 7 and both fail to start up networking after a reboot. Both VMs connect successfully after I run "sudo dhclient"

$ ping 1.1.1.1
connect: Network is unreachable

$ sudo dhclient
cmp: EOF on /tmp/tmp.ej3Phm6q0A which is empty

$ ping 1.1.1.1
PING 1.1.1.1 (1.1.1.1) 56(84) bytes of data.
64 bytes from 1.1.1.1: icmp_seq=1 ttl=56 time=17.7 ms

Output of "systemctl status network" on Centos VM:

$ sudo systemctl status network
● network.service - LSB: Bring up/down networking
   Loaded: loaded (/etc/rc.d/init.d/network; bad; vendor preset: disabled)
   Active: failed (Result: exit-code) since Thu 2021-02-04 08:48:35 UTC; 31min ago
     Docs: man:systemd-sysv-generator(8)
  Process: 2524 ExecStart=/etc/rc.d/init.d/network start (code=exited, status=1/FAILURE)

Feb 04 08:48:35 centosvm network[2524]: RTNETLINK answers: File exists
Feb 04 08:48:35 centosvm network[2524]: RTNETLINK answers: File exists
Feb 04 08:48:35 centosvm network[2524]: RTNETLINK answers: File exists
Feb 04 08:48:35 centosvm network[2524]: RTNETLINK answers: File exists
Feb 04 08:48:35 centosvm network[2524]: RTNETLINK answers: File exists
Feb 04 08:48:35 centosvm network[2524]: RTNETLINK answers: File exists
Feb 04 08:48:35 centosvm systemd[1]: network.service: control process exited, code=exited status=1
Feb 04 08:48:35 centosvm systemd[1]: Failed to start LSB: Bring up/down networking.
Feb 04 08:48:35 centosvm systemd[1]: Unit network.service entered failed state.
Feb 04 08:48:35 centosvm systemd[1]: network.service failed.

On the Ubuntu 18.04 VM:

$ sudo systemctl status -l systemd-networkd
● systemd-networkd.service - Network Service
   Loaded: loaded (/lib/systemd/system/systemd-networkd.service; enabled; vendor
   Active: active (running) since Tue 2021-02-02 11:23:50 UTC; 1 day 21h ago
     Docs: man:systemd-networkd.service(8)
 Main PID: 760 (systemd-network)
   Status: "Processing requests..."
    Tasks: 1 (limit: 4915)
   CGroup: /system.slice/systemd-networkd.service
           └─760 /lib/systemd/systemd-networkd

Feb 04 09:22:10 ******* systemd-networkd[760]: vethf3988ae: Lost carrier
Feb 04 09:22:10 ubuntuvm systemd-networkd[760]: vethf3988ae: Link DOWN
Feb 04 09:22:11 ubuntuvm systemd-networkd[760]: br-32f293377024: Lost carrier
Feb 04 09:23:10 ubuntuvm systemd-networkd[760]: vethf7837d1: Link UP
Feb 04 09:23:11 ubuntuvm systemd-networkd[760]: vethf7837d1: Gained carrier
Feb 04 09:23:11 ubuntuvm systemd-networkd[760]: br-32f293377024: Gained carrier
Feb 04 09:23:12 ubuntuvm systemd-networkd[760]: vethf7837d1: Gained IPv6LL
Feb 04 09:23:12 ubuntuvm systemd-networkd[760]: vethf7837d1: Lost carrier
Feb 04 09:23:12 ubuntuvm systemd-networkd[760]: vethf7837d1: Link DOWN
Feb 04 09:23:13 ubuntuvm systemd-networkd[760]: br-32f293377024: Lost carrier

On the Ubuntu 20.04 host:

$ brctl show
bridge name bridge id STP enabled interfaces
...snip...
br0 8000.0cc47a228130 no enp4s0f0
       vnet0
       vnet1
       vnet2
       vnet3
       vnet5
...snip...

Both VMs have interfaces set up to connect to bridge br0:

<interface type='bridge'>
  <mac address='52:**********:90'/>
  <source bridge='br0'/>
  <model type='virtio'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>

Why are my VMs not able to connect after the host upgrade from 18.04 -> 20.04 and how can I fix this?

2
  • It looks like your bridge is up. Check your IP addresses and routes. Feb 4 '21 at 14:14
  • The host bridge br0 is definitely up and some VMs do not have any issues with connecting to the DHCP server and getting a lease. The two servers above manage to connect when I manually sudo dhclient but only until the next reboot. If I understand correctly systemd has a built-in DHCP client so dhclient will not normally be run at startup.
    – Ries
    Feb 4 '21 at 19:27

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