[PATCH 0/2] Include lease time option into DHCP settings

[Laine Stump]

On 4/17/20 12:00 PM, Daniel P. Berrangé wrote:
> 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


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