frequent network collapse possibly due to bridging

Hakan E. Duran ehakanduran at
Tue Jan 25 01:29:06 UTC 2022

Thank you for the reply. I truly appreciate them.

I used virt-manager to set up my VMs and manage them on a daily basis.
After reading your responses I used the commands below to gather some

$sudo virsh net-list --all
 Name      State      Autostart   Persistent
 default   inactive   no          yes

$sudo virsh net-info default
Name:           default
UUID:           some-number
Active:         no
Persistent:     yes
Autostart:      no
Bridge:         virbr0

I thought it was interesting that the default network was not marked to
restart at boot and changed that:

$sudo virsh net-autostart default
Network default marked as autostarted

Then due to its inactive status, I thought it would be a good idea to
start it with:

$sudo virsh net-start default
Network default started

Of note, even though the default network was marked as inactive as
above, it was working. In other words, I was able to reach the VMs,
which are part of that network, even before the `virsh net-start
default` command. Nothing seemed to break with the command either, and
everyting still seemed to work afterwards.

$sudo virsh net-info default
Name:           default
UUID:           some-number
Active:         yes
Persistent:     yes
Autostart:      yes
Bridge:         virbr0

I would really appreciate if you can confirm that this is the desired
state for my network for the purposes I discussed previously. I
apologize if I am oversimplifying things here, it is because of my lack
of in-depth understanding the appropriate set up.



On 22/01/24 05:30PM, Laine Stump wrote:
> On 1/24/22 4:35 AM, Martin Kletzander wrote:
> > On Fri, Jan 21, 2022 at 08:42:58AM -0600, Hakan E. Duran wrote:
> > > Hi,
> > >
> > > I would like some help to troubleshoot the problem I have been having
> > > lately with my VM host, which contains 5 VMs, one of which is for
> > > pi-hole, unbound services. It has been a relatively common occurrence in
> > > the last few weeks for me to find that the host machine has lost its
> > > network when I get back home from work. Restoring the VM/VMs do not fix
> > > the problem, the host needs to be restarted for a fix, otherwise there
> > > is both loss of name resolution, as well as an internet connection; I
> > > cannot ping even IPs such as Since I use the pi-hole VM as
> > > the DNS
> > > server for my LAN, this means that my whole LAN gets disconnected from
> > > internet, until the host machine is rebooted. The host machine has a
> > > little complicated network setup: the two gigabit connections are bonded
> > > and bridged to the VMs; however this set up has been serving me so well
> > > for several years now. The problem, on the other hand, appeared a few
> > > weeks ago. This doesn't happen every day but often enough to be annoying
> > > and disruptive for my family.
> > >
> >
> > Always good to check what has changed those weeks ago, but I understand
> > it is difficult to find out what you were updating and where.
> >
> > > My question is, how can I troubleshoot this problem and figure out
> > > whether it is truly due to network bridging somehow collapsing or not? I
> > > tried to find some log files but all I could find were the
> > > /var/log/libvirt/qemu/$VM files, and the particular log file for the
> > > pi-hole
> > > VM reported the following lines; however, I am not sure if they are
> > > associated with a real crash or just due to shutting down and restarting
> > > the host (please excuse the word-wrapping):
> > >
> > > char device redirected to /dev/pts/2 (label charserial0)
> > > qxl_send_events: spice-server bug: guest stopped, ignoring
> > > 2022-01-20T23:41:17.012445Z qemu-system-x86_64: terminating on
> > > signal 15 from pid 1 (/sbin/init)
> >
> > Probably restarting the host as it got SIGTERM'd by init.  Maybe it was
> > restarted in a bad time and there is some inconsistency on the disk?
> > Using something like libvirt-guests which can manage your machines when
> > rebooting would be a good idea.
> >
> > > 2022-01-20 23:41:17.716+0000: shutting down, reason=crashed
> > > 2022-01-20 23:42:46.059+0000: starting up libvirt version: 7.10.0, qemu
> > > version: 6.2.0, kernel: 5.10.89-1-MANJARO, hostname: -redacted-
> > >
> > > Please excuse my ignorance but is there a way to restart the
> > > networking without rebooting the host machine? This will not solve my
> >
> > You can do:
> >
> > virsh net-destroy <network_name>
> > virsh net-start <network_name>
> >
> > but depending on what the network looks like, how it is set up etc. you
> > might need to restart some of the VMs or manually plug them in.
> The connection between any guest tap device and a host bridge device will be
> broken by virsh net-destroy, and not restored by virsh net-start (because
> the network driver has no good way of notifying the QEMU driver that it has
> restarted a network). This is something that's been on my "list of annoying
> things I should fix some day" for a very long time, but I've never been
> motivated enough to figure out a clean solution.
> In the meantime, if you destroy/start a network, you can get all the guest
> tap devices reconnected by restarting libvirtd:
>    systemctl restart libvirtd.service
> or if you're using split daemons:
>    systemctl restart virtqemud.service
> One of the things the QEMU driver does when it's initializing is to check
> where each guest tap device *should* be connected, compare that to where it
> *is* connected, and if those don't match then fix it.
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