libvirt users - Aug 2021 - Sharing dhcp leases between multiple host systems

hi,

assume i have multiple host systems which spin up virtual machines using
the vagrant/vagrant-libvirt provider. Both host systems have a defined
network (which has the same name on both hosts) which the first network
interface of the virtual machine is assigned to.

During boot of the virtual machine, the first network device is
configured via DHCP and vagrant uses the mac address table or libvirt
dhcp leases table to find out about the IP address that was assigned to
the virtual machine: From that point on, i can reach the virtual machine
locally on the host system.

This works nicely if the network is a libvirt NAT network, as the IP
addresses are unique on both host systems.

Now i want to change the situation and provide routed addresses, thus i
want to make sure that an IP that is assigned for a virtual machine on
host A is not re-used on host B to not have IP address conflicts.

What im searching for is the "libvirt" way to have a central lease
file
between multiple hosts for the same network (without having another
layer like OVS/OVN).

What i guess would work is:

 1) share /var/lib/libvirt/dnsmasq between both host systems, of course
 means the virtual bridge for the network has to have the same
 name on both systems.

 2) replace /usr/libexec/libvirt_leaseshelper with my own version, that
 stores the leases in an central place.

 3) a way that exists and i dont know about?

Option 2) sounds the best for me, but i currently dont see a way to
specify the dhcp-script used for a network on libvirt side .. any
opinions on this?

Using libvirt 7.x and alike from the centos 8 advanced virtualization
stream.

thanks,
    - michael

On 8/4/21 8:12 AM, Michael Ablassmeier wrote:
> hi,
> 
> assume i have multiple host systems which spin up virtual machines using
> the vagrant/vagrant-libvirt provider. Both host systems have a defined
> network (which has the same name on both hosts) which the first network
> interface of the virtual machine is assigned to.
> 
> During boot of the virtual machine, the first network device is
> configured via DHCP and vagrant uses the mac address table or libvirt
> dhcp leases table to find out about the IP address that was assigned to
> the virtual machine: From that point on, i can reach the virtual machine
> locally on the host system.
> 
> This works nicely if the network is a libvirt NAT network, as the IP
> addresses are unique on both host systems.
> 
> Now i want to change the situation and provide routed addresses, thus i
> want to make sure that an IP that is assigned for a virtual machine on
> host A is not re-used on host B to not have IP address conflicts.
If you are using routed networking rather than NAT, then the routed 
network on each host will have to use a different subnet anyway[*], so 
there is no chance of any conflict in IP addresses.

[*] while you *could* use the same subnet for the routed virtual network 
on both hosts, each host would then only be able to reach the guests on 
its own virtual network, and 3rd parties would need to point their 
routing tables at one host or the other for that single subnet, and so 
would only be able to reach the guests on one of the hosts, but not the 
other.
> 
> What im searching for is the "libvirt" way to have a central
lease file
> between multiple hosts for the same network (without having another
> layer like OVS/OVN).
> 
> What i guess would work is:
> 
>   1) share /var/lib/libvirt/dnsmasq between both host systems, of course
>   means the virtual bridge for the network has to have the same
>   name on both systems.
> 
>   2) replace /usr/libexec/libvirt_leaseshelper with my own version, that
>   stores the leases in an central place.
> 
>   3) a way that exists and i dont know about?
> 
> Option 2) sounds the best for me, but i currently dont see a way to
> specify the dhcp-script used for a network on libvirt side .. any
> opinions on this?
> 
> Using libvirt 7.x and alike from the centos 8 advanced virtualization
> stream.
> 
> thanks,
>      - michael
>