dhcp option 150

[David W. Hankins]

On Tue, Aug 25, 2009 at 12:23:56PM +1000, Glenn Satchell wrote:
> You could try configuring the client to ask for the option, this may
> involve some coding to do something sensible with the value. For
> ISC dhclient, try man dhclient.conf and man dhclient
> 
> >         Vendor-Class Option 60, length 32: 
> >"PXEClient:Arch:00000:UNDI:002001"

PXE is in firmware.  there's basically no configuring it, because if
you do configure it, then you've kind of lost much of the benefit of
dynamic configuration...while you're sitting there you may as well
give it the rest of its config.

it also has no way of knowing what software the server is going to
give it to load.  so it can't know what options to request and cache
for the booting image.

one network boot os, WinCE, works around this by performing a new
DHCPREQUEST once the WinCE image starts running.  however, it fails
to identify itself consistently to the PXE image, because while PXE
does not include a dhcp-client-identifier option, WinCE does include
a dhcp-client-identifier option that is equal to the chaddr contents.

an rfc2131 conforming DHCP server will attempt to give this newly
and differently identified client a unique address.  the client,
however, cannot change its address as in this stage of booting, PXE
still governs the network interface and treats the new allocation as
a fatal error.  it reboots.

so, we should probably get used to over-riding PRL's to match the
needs of the supplied image.


network bootloaders are finnicky creatures and you must rely heavily
on the vendors' documentation to determine what they require.  the
problem is, this documentation generally doesn't exist ("buy our DHCP
server"), is out dated, or is written in a lexicon that misuses
common DHCP terms and definitions the author believes they have
invented.

-- 
David W. Hankins	"If you don't do it right the first time,
Software Engineer		     you'll just have to do it again."
Internet Systems Consortium, Inc.		-- Jack T. Hankins
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