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Re: Set hostname of guest during installation time

On 5/5/2020 8:29 AM, Michal Privoznik wrote:
On 5/5/20 8:11 AM, john doe wrote:
On 3/30/2020 4:10 PM, Michal Prívozník wrote:
On 30. 3. 2020 15:26, john doe wrote:
On 3/30/2020 1:20 PM, Andrea Bolognani wrote:
On Mon, 2020-03-30 at 12:04 +0200, Michal Prívozník wrote:
On 27. 3. 2020 20:49, john doe wrote:
If my understanding is correct, using 'try06' or 'debian' should
do the
same thing?

Yes, that is the idea. However, these plugins were not introduced
at the
same time. I think that especially Debian has delayed libvirt_guest
plugin. You can check if both plugins exist:

ls /usr/lib64/libnss_libvirt*


Note that paths are different in Debian-based distros, so the correct
ones in this case are actually


I have upgraded that Buster host to Bullseye, so I have now a 6...
I have also the above two modules.

I'm still not able to use 'libvirt_guest' though:

--- Guest output (try06):

root try06:~# cat /etc/hostname

--- Host output (host):

root host:# grep hosts: /etc/ns*
hosts: libvirt_guest
root host:# virsh net-dhcp-leases default
  Expiry Time           MAC address         Protocol   IP address
    Hostname   Client ID or DUID

  2020-03-30 16:03:41   52:54:00:d4:e6:f0   ipv4
   -          -

root host:# getent hosts try06; echo $?

What am I missing?

Hold on. 'libvirt_guest' NSS plugin is supposed to translate libvirt
names to IP addresses. For instance:

   virsh start myDomain; ssh myDomain

The 'libvirt' NSS plugin is supposed to translate hostnames as sent by
guests to IP addresses. These two can be viewed as the following:

libvirt_guest: virsh domifaddr --source lease $dom
       libvirt: virsh net-dhcp-leases $net | grep $hostname

And this is where it gets interesting. In your previous e-mails,
net-dhcp-leases (aka 'libvirt' plugin) worked because the guest did send
hostname when doing DHCP. Now it doesn't (see '-' under 'Hostname' in
the output). Therefore, the 'libvirt' NSS plugin won't work.

And 'libvirt_guest' translates names of guests as seen by libvirt, i.e.
names you pass to virsh commands. These are unrelated to '/etc/hostname'
within guest. But hopefully, you haven't renamed your guest since then
and it is still named 'try06', that is virsh list --all produces 'try06'
in the list. If this is all true, can you share the output of:

   strace $(which getent) hosts try06

Thanks to the friendly feedback/support from Michal Privoznik and Andrea
Bolognani, I managed to get it working with the below command:

virt-install --name=try06 --pxe --os-variant=debian10 --network

What I think I mist while trying to implement the recommendation given
in here is that, if you change the network by doing 'virsh edit default'
the name of the interface in the guest might also change.

I don't understand why libvirt_guest won't work if a network bridge is

Because interface type='bridge' and type='network' are not the same. The
libvirt_guest plugin needs a list of MAC addresses because it tries to
find a match across libvirt domain name -> list of domain's MAC
addresses -> list of leases. And whenever a domain is started, all its
interfaces with type='network' will notify the corresponding network and
the code that handles libvirt networks will dump the MAC address into a
file for the NSS plugin to use. But, if plain type='bridge' is used then
all that is done is that the TAP is plugged into the bridge (which
ensures the connectivity), but does not notify the network which in turn
means that the MAC dumping code is not run and hence the NSS plugin
won't find a match.

I don't understand why the network can not be notified, could it be a
feature request?

Is there a reason why you are using interface type='bridge' with the
default network?

I'm just starting with libvirt, so I could be missing something.

Yes, I don't need libvirt touching iptables at all.

In other words, is there a way to be able to use libvirt_guest without
having libvirt interacting with iptables.

Also, from (1):

"virsh net-dhcp-leases $network
where $network iterates through
all running network..."

If I understand correctly, the below should list all running network:

$ virsh net-dhcp-leases $network
error: command 'net-dhcp-leases' requires <network> option

This lists DHCP leases for given network. To list all running networks
you can use 'virsh net-list'.

I would suggest rephrasing the above to something along the lines of:

"virsh net-dhcp-leases $network, where '$network' is to be supstituted
by the desired network (E.G, 'default') or use virsh net-list to list
all available network."

In my case, I substituted '$network' by 'default'.

It doesn't look like you did, otherwise you would either get leases or
an error that there is no network named "default".

# virsh net-dhcp-leases
error: command 'net-dhcp-leases' requires <network> option
# virsh net-dhcp-leases ""
error: Failed to get option 'network': Option argument is empty

# virsh net-dhcp-leases default
  Expiry Time           MAC address         Protocol   IP address
  Hostname   Client ID or DUID

  2020-05-05 09:01:11   52:54:00:a4:6f:91   ipv4
  fedora     01:52:54:00:a4:6f:91

Thanks for this.

John Doe

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