Set hostname of guest during installation time

[Michal Privoznik]

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*
>>>>>
>>>>> /usr/lib64/libnss_libvirt_guest.so.2
>>>>> /usr/lib64/libnss_libvirt.so.2
>>>>
>>>> Note that paths are different in Debian-based distros, so the correct
>>>> ones in this case are actually
>>>>
>>>>    /lib/x86_64-linux-gnu/libnss_libvirt.so.2
>>>>    /lib/x86_64-linux-gnu/libnss_libvirt_guest.so.2
>>>>
>>>
>>> I have upgraded that Buster host to Bullseye, so I have now a 6... 
>>> version.
>>> I have also the above two modules.
>>>
>>> I'm still not able to use 'libvirt_guest' though:
>>>
>>> --- Guest output (try06):
>>>
>>> root at try06:~# cat /etc/hostname
>>> try06
>>>
>>> --- Host output (host):
>>>
>>> root at host:# grep hosts: /etc/ns*
>>> hosts: libvirt_guest
>>> root at 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       
>>> 192.168.122.137/24
>>>    -          -
>>>
>>>
>>> root at host:# getent hosts try06; echo $?
>>> 2
>>>
>>>
>>> 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
> network=default
> 
> 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 
> used?

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.

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

> 
> 
> 
> 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'.

> 
> 
> 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       192.168.122.3/24 
  fedora     01:52:54:00:a4:6f:91


Michal