Host configuration

Jeffrey Hutzelman jhutz at
Tue Feb 12 18:35:31 UTC 2008

--On Monday, February 11, 2008 05:42:40 PM -0500 Frank Sweetser 
<fs at WPI.EDU> wrote:

> michael at wrote:
>> Todd Snyder wrote:
>>> Good day,
>>> I realize this perhaps isn't the ideal spot for this question, but I
>>> know of no other ideal spot, so I'll try.
>>> There is much power in the use of DHCP - being able to centralize and
>>> change key network configs dynamically is ideal.  However, that
>>> functionality is lost the minute you want to use static ip addresses.
>>> Now, I realize you could assign a static IP on the dhcp server, so a
>>> server always gets the same one, but that isn't ideal for a data centre.
>> Why isn't this ideal for a data center?
>> Many of us have very large data centers that are managed via DHCP
>> exactly as you described without issues.
> Well, at a minimum, it's quite difficult to manage your DHCP server
> infrastructure that way =)

Sure; you want to manually configure at least a couple of DHCP servers, if 
not all of them.  For a particularly large network, it is likely the case 
that even most DHCP servers should also be DHCP clients; the number of 
machines that actually need static configuration should be very small.

> Personally, my preference would be to use static IP configuration, and
> then leave all of the other buttons and knobs up to a more flexible
> configuration management system

How about "most of the other buttons and knobs".  IP addresses, 
nameservers, and other things that depend on what network segment you 
happen to be attached to are better managed via DHCP.  It's hard to use 
puppet or a GPO to change a machine's default router at exactly the time 
the machine moves to a new subnet.

I can't imagine running a facility of any noticeable size with static 
configuration.  Actually, I can, but only because we used to do so, many 
years ago.  Of course, that was in the days when every machine at Carnegie 
Mellon had the same netmask and default router (world's largest flat 
bridged Ethernet, and proud of it!).  I can't imagine transitioning from 
that to the network architecture we have today without dynamic 

These days, we use static configuration only for DHCP and DNS servers, 
machines too old and broken to run DHCP, and a handful of services that we 
want to be able to boot without waiting for DHCP, solely to reduce the 
startup time after a cold shutdown.

> There's nothing quite so much fun as creating a
> circular service dependency between two servers, where neither one may be
> turned on until the other is fully booted...

Nonsense.  The fun part is discovering such a dependency during a cold 
start and figuring out how to manually resolve it, all without any of the 
normal infrastructure available.  :-)

-- Jeffrey T. Hutzelman (N3NHS) <jhutz+ at>
   Carnegie Mellon University - Pittsburgh, PA

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