host statement scope rules (ISC DHCP 3.0.5b1)

[Glenn Satchell]

>Date: Fri, 28 Jul 2006 10:56:37 +0200
>From: Phil Pennock <isc-dhcp-server at phil.globnix.org>
>To: dhcp-users at isc.org
>Subject: host statement scope rules (ISC DHCP 3.0.5b1)
>
>On 2006-07-27 at 15:30 -0700, David W. Hankins wrote:
>> ISC DHCP 3.0.5b1 is now available for download.  This release is a
>> maintenance release that seeks to correct bugs introduced in 3.0.4
>> or prior.
>
>> 			Changes since 3.0.4
>> 
>> - A warning that host statements declared within subnet or shared-network
>>   scopes are actually global has been added.
>
>Whilst the host statement is matched at a global level, aren't
>attributes such as lease time values inherited based upon syntactic
>scope rather than address/netblock matching rules?
> 
>We have generated configs which use host statements inside subnet
>scopes, knowing full well that the host statements are matched globally
>but wanting to be able to inherit values based upon which relay server
>sent data to us.
>
>If ISC's dhcpd is inheriting values based upon address matching, then
>can someone please state if this has always been the case and, if not,
>which version introduced it?
>
>Thanks,
>-- 
>VISTA: Viruses, Infections, Spyware, Trojans & Adware

No, because they are global in scope they do not inherit from being
defined within the subnet scope. However, the assigned address will
cause the lease to inherit defintions from the proper subnet scope.
Note they do not inherit from the *pool* if any, within a subnet
scope.

I believe this has been the behaviour since at least 3.0, and quite
possibly 2.0.

It works this way becaue it is quite valid to define something like
this with multiple fixed-addresses. The correct address is assigned
based on the subnet where the device is located (eg think of a roaming
laptop):

host foo {
  hardware ethernet aa:bb:cc:dd:ee:ff;
  fixed-address 192.168.1.5, 192.168.2.5, ... ;
}

and it will pick up the correct subnet values based on which ever
address is assigned.

regards,
-glenn