On Wed, Apr 15, 2020 at 01:25:37PM -0300, Julio Faracco wrote:

I resubmitted this series because our team needs to hack dnsmasq
settings to change lease time. This feature would be so important to
us to avoid workarounds.

It is based on Alberto's patch from 2017. But personally I don't like
this approach.
IMHO, it would be nice to have specific attributes to configure lease time.
For example:
<range ... leasetime="10m"/>
<host ... leasetime="20m"/>

It is generally considered bad practice to have an XML attribute
value which then has to be parsed again to extract information.

For this reason, libvirt will use two attrbutes, one of the
value and one for the units (with some sensible default
units if not specified), even though this is admittedly
more verbose.

I agree it would be useful to have lease time per-host, as well
as globally though, and IIRC one of the original versions of
this patch did support that.

The name of the original contributor that you (Julio) referenced in your cover letter sounded familiar, and I tried to find the original BZ for this when I saw your patches on the list, but I failed (bugzilla kept timing out on a *very* basic search term - this seems to happen to 80% of my queries these days...) But then it passed by in mail when Dan was cleaning up all the upstream libvirt BZes and moving them to gitlab :-), so just so everyone has all the background info:

  https://bugzilla.redhat.com/show_bug.cgi?id=913446

I also found the other similar patch from Nehal J Wani from around the same time:

  https://www.redhat.com/archives/libvir-list/2016-September/msg01063.html

Without talking about the specifics of either patch, my recollection of the discussion from that time was that both contributors were trying to use a leasetime setting to solve a problem that it wouldn't have solved - their issue was that if a guest was turned off during the time when its lease expired, then when the guest was subsequently restarted it would end up with a different IP address. Of course setting a longer lease expiry would only delay the problem, not eliminate it. Further discussion revealed that if libvirt would just set the "dhcp-authoritative" option in the dnsmasq config, then dnsmasq would attempt to reissue the same IP to a guest even if its lease had already expired - Martin Wilck made this change to libvirt in commit 4ac20b3ae4, which seemed to satisfy the people who had thought they needed to modify the dhcp lease time, and so neither of the lease time patches was pushed upstream.

The reason I bring up this old history is just as a cautionary tale that sometimes what you think you need isn't really what you actually need :-)

(Recently Cole added the dnsmasq private namespace to the <network> XML, but that is only useful to add an entire option line, and can't do what you need, which is adding an extra option to every dhcp-host and dhcp-range line; there unfortunately is no standalone dnsmasq option to specify a global lease time)

We could do one of 

  <ip address="192.168.122.1" netmask="255.255.255.0">
    <dhcp>
      <range start="192.168.122.2" end="192.168.122.254" lease="12" units="hours"/>
      <host id="0:1:0:1:18:aa:62:fe:0:16:3e:44:55:66" ip="2001:db8:ca2:2:3::2" lease="30" units="mins"/>
    </dhcp>
  </ip>

or

  <ip address="192.168.122.1" netmask="255.255.255.0">
    <dhcp>
      <range start="192.168.122.2" end="192.168.122.254" leasetime="12" leaseunits="hours"/>
      <host id="0:1:0:1:18:aa:62:fe:0:16:3e:44:55:66" ip="2001:db8:ca2:2:3::2" leasetime="30" leaseunits="mins"/>
    </dhcp>
  </ip>

or

  <ip address="192.168.122.1" netmask="255.255.255.0">
    <dhcp>
      <range start="192.168.122.2" end="192.168.122.254">
         <lease expiry="12" units="hours"/>
      </range>
      <host id="0:1:0:1:18:aa:62:fe:0:16:3e:44:55:66" ip="2001:db8:ca2:2:3::2">
         <lease expiry="30" units="mins"/>
      </host>
    </dhcp>
  </ip>

Nehal's patch had used this syntax:

 <leasetime units='hours'>12</leasetime>

based on (I guess) the syntax of libvirt's <memory> element:

 <memory unit='KiB'>524288</memory>

I don't have a preference for any of them, just thought I would point out existing usage in libvirt that has a value/units combination.

In all of these we can default to "minutes" if no units are given.

Regards,
Daniel