suggestion to improve dhcpd behavior regarding default setting of max-lease-time statement

Thu Apr 24 15:51:54 UTC 2008

Hi Eduardo,

Thanks for your second response.

You prevously wrote:
> IMHO a change in the manpage is welcome but we should not change the code
> behavior, which is not a good pratice unless really necessary (imagine the
> huge user base having to adopt to new rules, it's wiser to teach new admins
> on how it works).

Many administrators are not aware of how dhcpd currently works regarding the default 'max-lease-time' setting of 86400 seconds, because it was never documented, and I think that even when we document it properly, many administrators may not really appreciate that they suddenly must configure a 'max-lease-time' in case they configure a 'default-lease-time' greater than 86400 seconds.

Nevertheless, my suggestion for dhcpd will not make any difference to configuration of administrators, who were already aware of this behavior and configured a 'max-lease-time' equal to or greater than the 'default-lease-time'. My suggestion is not to set the 'max-lease-time' in case it is configured, but to change the default value of 'max-lease-time' in case it is not configured, so that we can avoid a 'hidden' configuration conflict. With 'hidden' I mean that de current hardcoded default setting for 'max-lease-time' of 86400 seconds is not shown anywhere.

Thank you and kind regards,

Fons
>>> "Eduardo Fleury" <efleury at gmail.com> 4/24/2008 5:31 PM >>>
Hi Fons,
Yes, I understand that. But that's because actually there isn't such thing
as 'max-lease-time' not being configured at all. It must have some value,
and it defaults to 86400 in case the admin omits that info.
But you're right, I think that should be documented as well. Like, two
additions to dhcpd.conf(5):

1) In 'max-lease-time' description, let users know it will assume the
default value of 86400 in case it's omited.
2) In 'default-lease-time' description, let users know 'max-lease-time' has
priority over this setting.

Best regards,
Eduardo

On Thu, Apr 24, 2008 at 12:22 PM, Fons de Jongh <fdjongh at novell.com> wrote:

> Hi Eduardo
>
> Thank you for your response.
> Please note that it is not just a matter of having configured a
> 'default-lease-time' greater than the 'max-lease-time'. I agree that an
> administrator should understand that in such case the 'max-lease-time'
> overrules the 'default-lease-time'. No extra documentation necessary for
> that case.
>
> However, the problem is that if an administrator only configures a
> 'default-lease-time' (so no 'max-lease-time' is configured at all), and he
> sets the 'default-lease-time' to a value greater than 86400 seconds, dhcpd
> only returns 86400 seconds with option 51.
>
> Thank you and kind regards,
>
> Fons
>
>
> >>> "Eduardo Fleury" <efleury at gmail.com> 4/24/2008 5:13 PM >>>
> Hi,
> I understand that setting a default-lease-time higher than the
> max-lease-time is a configuration error, one should not do that.
>
> I do agree however that a sysadmin who isn't very familiar with dhcp may be
> induced to make this error due to lack of information in the man-page, thus
> I suggest a note is added to the default-lease-time explanation in
> dhcpd.conf(5).
>
> Actually default-lease-time is just a hint for dhcpd in case the client
> didn't provide a desired expiration time, whereas max-lease-time has a
> larger scope which is to enforce expiration times remain between well
> defined boundaries.
>
> default lease > max lease is supposed to have the same behavior of a client
> asking for a lease > max lease, thus the situation you explained is
> expected.
>
> IMHO a change in the manpage is welcome but we should not change the code
> behavior, which is not a good pratice unless really necessary (imagine the
> huge user base having to adopt to new rules, it's wiser to teach new admins
> on how it works).
>
> Best regards,
> --
> Eduardo M. Fleury
> Researcher at INdT - Instituto Nokia de Tecnologia
> http://eduardofleury.com
>
> On Thu, Apr 24, 2008 at 11:45 AM, Fons de Jongh <fdjongh at novell.com>
> wrote:
>
> > Dear DHCP Hackers,
> >
> > My name is Fons de Jongh and I work at Novell as a technical support
> > engineer. I understood from S Kalyanasundaram, who is a developer at
> Novell,
> > that you can verify my suggestion for dhcpd, and may-be approve it in
> case
> > it makes sense.
> >
> > Okay, here I start...
> >
> > I have installed dhcp-server-3.0.3-23.38 on SuSE Linux Enterprise Server
> > 10.
> >
> > One can configure a 'default-lease-time' and a 'max-lease-time' statement
> > with the 'global parameters' of a DHCP server or with any declaration. As
> > desired, dhcpd will use the configured statements of a declaration that
> is
> > most specific to the IP address to be offered/allocated (yiaddr) for the
> > setting of the IP Address Lease Time option (option 51) in DHCPOFFER and
> > DHCPACK reply messages.
> >
> > Apparently the default value of 'max-lease-time' is 86400 seconds (1
> day).
> > However, this is not mentioned with the description of the
> 'max-lease-time'
> > statement in the manual page of dhcpd.conf. So dhcpd administrators
> cannot
> > be aware of this hardcoded limit.
> >
> > The consequential behavior of dhcpd is that if one does not configure a
> > 'max-lease-time' statement, but only a 'default-lease-time' statement
> with a
> > value greater than 86400 seconds, dhcpd will return an IP Address Lease
> Time
> > of 86400 seconds with option 51 in DHCPOFFER and DHCPACK messages,
> > regardless to the higher setting of the 'default-lease-time' statement.
> >
> > Hence, if one configures a 'default-lease-time' statement with a value
> > greater than 86400 seconds, one must also configure a 'max-lease-time'
> > statement with the same value as 'default-lease-time' or higher, in order
> to
> > make dhcpd return the desired configuration of 'default-lease-time' with
> > option 51 in DHCPOFFER and DHCPACK messages. This condition is not
> described
> > in the manual page of dhcpd.conf, so dhcpd administrators do not know
> they
> > should set a 'max-lease-time' greater than or equal to the
> > 'default-lease-time' if they configure a 'default-lease-time' greater
> than
> > 86400 seconds.
> >
> > My suggestion is to change the default setting of 'max-lease-time' to a
> > variable value instead of the current hardcoded constant 86400. I suggest
> to
> > make the default value of 'max-lease-time' equal to the value of
> > 'default-lease-time', so that if one configures the 'default-lease-time'
> > only, the 'max-lease-time' will never limit the 'default-lease-time'.
> >
> > An alternate suggestion to handle the same configuration conflict is to
> > make the default setting of 'max-lease-time' 0xFFFFFFFF (infinite), but
> that
> > may result into undesired behavior when clients request for an infinite
> > lease time per DHCP option 51 in a DHCPREQUEST message.
> >
> > Please let me know if you agree with my suggestion to make the default
> > value of 'max-lease-time' equal to the value of 'default-lease-time' or
> if
> > you have comments or questions.
> >
> > Thank you and kind regards,
> >
> > Fons de Jongh
> > Novell Technical Services
> >
> >
>
>
>
>
>
>

-- 
Eduardo M. Fleury
http://eduardofleury.com