[Dnsmasq-discuss] Can not set lease time to less than 2m

[Albert ARIBAUD]

Bonjour Tomas,

Le Wed, 25 Feb 2015 08:24:25 +0100, Tomas Hozza <thozza at redhat.com> a
écrit :

> On 02/24/2015 05:26 PM, Jim Alles wrote:
> > You must have a very small network and very special requirements?
> It was for dhcp client testing purposes.
> > From the man pages for dhcp-range:
> >
> > Enable the DHCP server. Addresses will be given out from the range
> > <start-addr> to <end-addr> and from statically defined addresses given in
> > *dhcp-host *options.
> > If the lease time is given, then leases will be given for that length of
> > time. The lease time is in seconds, or minutes (eg 45m) or hours (eg 1h) or
> > "infinite". If not given, the default lease time is one hour.
> >
> > The minimum lease time is two minutes.
> Ahh, Thank you. I was looking for "2m" and didn't see this statement.
> > For IPv6 ranges, the lease time maybe "deprecated"; this sets the preferred
> > lifetime sent in a DHCP lease or router advertisement to zero, which causes
> > clients to use other addresses, if available, for new connections as a
> > prelude to renumbering.
> >
> > On Tue, Feb 24, 2015 at 10:11 AM, Tomas Hozza <thozza at redhat.com> wrote:
> >
> > > Hi.
> > >
> > > Even though I configure dnsmasq to git leases
> > > e.g. for 1 minute it always configures it self
> > > with 2m lease time. I was not able to get under
> > > 2m, however if the value is greater than 2m, dnsmasq
> > > configures itself correctly.
> > >
> > > using:
> > > ...
> > > dhcp-range=192.168.99.2,192.168.99.254,1m
> > > dhcp-range=fd00:dead:beef:55::100,fd00:dead:beef:55::1ff,1m
> > > ...
> > >
> > > gives:
> > > ...
> > > dnsmasq-dhcp: DHCP, IP range 192.168.99.2 -- 192.168.99.254, lease time 2m
> > > dnsmasq-dhcp: DHCPv6, IP range fd00:dead:beef:55::100 --
> > > fd00:dead:beef:55::1ff,
> > > lease time 2m
> > > ...
> > >
> > > Why it is so? Seems like a bug IMHO.
> > >
> > > Regards,
> > > --
> > > Tomas Hozza
> > > Software Engineer - EMEA ENG Developer Experience
> > >
> > > PGP: 1D9F3C2D
> > > Red Hat Inc.                               http://cz.redhat.com
> 
> Anyway, my question why is this so and why is the user hard-limited?
> Is there some technical reason behind the limit?

I guess part of the answer is that the DHCP RFC 2131, while actively
not defining a minimum lease time, does specify various timings no
smaller than 60 seconds (e.g. 3.2 item 4, or 4.4.5); with a lease time
of less than 2 minutes, these various timings could not be respected.

(Besides, the smaller the lease time, the bigger the bandwidth DHCP uses
for a reason (IP address change for a given host) that is not supposed
to happen just every minute. I, for one -- besides welcoming our insects
overloards -- would like to know what the use case can be for a lease
of less than 120 seconds.)

> Thanks!
> 
> Regards,

Amicalement,
-- 
Albert.