[syslinux] pxelinux magic options 208

[Daniel Feenberg]

On Sun, 7 Nov 2010, Gene Cumm wrote:

> On Sun, Nov 7, 2010 at 16:11, Daniel Feenberg <feenberg at nber.org> wrote:
>> On Wed, 3 Nov 2010, Jeffrey Hutzelman wrote:
>>> --On Wednesday, November 03, 2010 06:18:52 PM -0400 Daniel Feenberg
>>> <feenberg at nber.org> wrote:
>>>> I have 2 groups of machines, and within each group the pxelinux menu
>>>> configuration is standard. It would be great to have 2 configuration
>>>> files rather than one for each machine.
>>>>
>> ...
>>>
>> Any idea what I am missing?
>
> http://syslinux.zytor.com/wiki/index.php/PXELINUX#Can_I_send_information_to_PXELINUX_via_special_options_in_the_DHCP_response.3F
>
> Look at the part about dhcp-parameter-request-list and the code
> section just above that point on the wiki page for some information
> about specifying it in vendor space and forcibly returning that
> option.

I have looked at those lines in the example and am baffled. The example 
lines are:

   if exists dhcp-parameter-request-list (

Do I define need to dhcp-paramteter-request-list somewhere else? If not, 
what difference does this statement make? Or is this something the dhcp 
client sends to the server? Then the next line is:

   option dhcp-parameter-request-list = concat(option dhcp-parameter-request-list,d0,d1,d2,d3);}

OK, I get it that d0 is hex for 208, etc. But are you saying I need this 
statement? Why is this the only option statement with an = sign? Is that 
correct or a typo?  In any case, I did add that exact statement, and it 
had no effect on the client.

I am still under the impression from an RFC and two messages in this 
mailing list that the "option pxelinux.magic" statement should be omitted, 
although the wiki says it must always be included.

What about the other statement that the wiki says must be included, 
"site-option-space"? Is that required/allowed/deprecated/forbidden with a 
recent ISC dhcp?

I am also confused by your mention of vendor-space options. That looks 
much simpler, but:

   Does it possibly work with ISC version 3+?

   Do I still need to include pxelinux.magic option?

   Do I still need to include the statements defining 208-211 (from the
   first box)?

   Does the option space statement from that box change in the case of
   vendor options?

When I specify vendor-option-space "pxelinux" I get the following error:

   vendor-option-space "pxelinux"
                        ^
   /etc/dhcpd.conf line 121: expecting a parameter or declaration
   vendor-option-space "pxelinux";

Does it require some preparation? A change to the option space statement 
or other statements from the box declaring 208-211?

>
> Also, try a packet capture.  More than likely, you'll see that your
> server is not returning option 209 as most DHCP servers don't return
> options that are not requested.

Do you mean that if I add the dhcp-parameter-request-list, you think the 
configfile option will be transmitted, or do you mean that most servers do 
not support what I want to do? What about vendor-space?

The pxelinux extensions have apparently been accepted as IETF standard - 
would one of you who have used them please post an example that will work 
with a recent ISC dhcp?

Daniel Feenberg
NBER

>
> -- 
> -Gene
>
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