static IP allocations in dynamic ranges -why not?
dhcp1 at thehobsons.co.uk
Thu Jun 3 08:49:36 UTC 2010
Dorsey, Chris wrote:
>What is the technical reason why dhcpd cannot mix static addresses
>within dynamic ranges?
I think a better question might be to ask "what is the design
decision behind the restriction".
The issue comes up from time to time, and often from people migrating
from a Microsoft server. To a certain extent, it's just a different
design philosophy - Microsoft chose to use a system where you define
a range and then exclude addresses from it*, ISC decided to let you
just define the range you want to use.
* Complicated by the fact that on early versions (ie NT), once you'd
defined a range you could not alter it - I've no idea if this is
still the case. As a result of that restriction, it because common
practice to define a range covering the entire subnet and then
exclude the bits you didn't want to use for dynamic leases.
When defining the interface, the ISC team took the approach that it's
a technical tool used by network admins who really should know their
own network. On this basis, the interface assumes the admin knows
what he is doing (ie won't use an address twice - dynamically and
also for a fixed address host), but conversely doesn't constrain the
more complex setups behind all the safety features.
Anyone who has worked with both will recognise the difference.
The ISC server offers a rich configuration environment, but is
technically more complex to configure.
The Microsoft server provides a relatively easy to use GUI, but
significantly restricts the capabilities to those supported by the
As to the way fixed addresses don't get automatically excluded, well
it may help to step back quite a few years. In computing terms, a lot
of years ! Unfortunately, the ISC page on the history of their DHCP
server doesnt' actually cover it's history - only going back as far
as version 3 which it states went alpha in March 1999. Prior to that,
version 2 had been around for "quite some time", and I don't have any
idea at all when version 1 first appeared. I see that RFC1541 goes
back to October 1993 - when I was first finding out about this
complicated thing called TCP/IP when I got dial-up access to this new
fangled Internet thingy.
The (IIRC, it's at work so I can't look it up) first chapter of Ralph
Droms and Ted Lemon's book "The DHCP Handbook" gives an overview of
network configuration techniques when DHCP was first conceived. Back
then, you either manually configured stuff, or you used reverse ARP
(only gives an address, you still have to manually configure
everything else), or you used BOOTP (still limited in config
abilities, and address assignment was still static).
The ISC set out to build not just a production usable piece of
software, but a reference implementation of the standards. I'd say
the fact that it's almost the de-facto standard DCHP server on Linux
distributions suggests they got something right ! Applying a bit of
speculation, I'd think that their priorities were on getting the
dynamic allocation/handling side of things right - and that's harder
than you might thing once you start considering some of the obscure
corner cases that can crop up. So it's easy to see why they made
fixed-address host assignments something of an ugly duckling - as
others have said, these go through a different and much simplified
Although it's caused various issues over the years, there has never
been a compelling reason to justify the effort involved in going back
and changing this.
As an aside, some of the current limitations/problems stem from
adherence to the standards - so it's really that the standards were
'wrong'. That's easy to say with hindsight, you try predicting what
computing will do in the next 17 years ! A good example of that is
the difficulty faced by people wanting to use Option 82 (relay agent
link identifier) as the key for address allocation - ie allocate an
address to <whatever> is attached to a specific port on a piece of
network gear. A proposal requiring a sponsor is to add a config
option to override the default (and standards compliant) selection of
database key - that would allow an admin to use Option 82 as the key
and may fix a lot of issues for a lot of people.
17 years ago, switched networks were the exception - ethernet was
still commonly that chunk of 1/2" yellow "hosepipe" draped around the
building - or it's cheaper cousin the thinner version with more
limited length restriction. The upstart twisted pair cabling was only
just appearing on the scene, and was struggling with the cost of the
hubs (not switches which were even more costly) it required.
Getting back to your other question ...
You may well want to look into Reserved Leases. So new that they
aren't documented and don't have a user interface to configure them !
A reserved lease is just like any other lease except for two details :
1) It includes the statement (I think) "reserved;" in the lease - and
editing the lease file is the only way to set/unset this.
2) Once set, the address will be bound to that client and never
re-assigned to another client.
But being otherwise a standard lease, it goes through the normal
lease lifecycle - including Dynamic DNS updates if configured.
That means, if you want to toggle a client onto a fixed address, you
can do it by setting it's lease to reserved. And when you no longer
want it to be fixed, you can unset that.
In due course, it's been expressed that they'd like to have a user
friendly way of administering this - but at the moment development
effort is going into IPv6 support.
I hope that answers your question - effectively it comes down to "yes
it's ugly in places, and with the benefit of 17 years of hindsight it
might have been done differently".
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