[syslinux] pxelinux magic options 208

[Geert Stappers]

Op 20101108 om 11:39 schreef Daniel Feenberg:
> On Sun, 7 Nov 2010, Gene Cumm wrote:
>>> On Wed, 3 Nov 2010, Jeffrey Hutzelman wrote:
>>>> --On Wednesday, November 03, 2010 06:18:52 PM -0400 Daniel Feenberg
>>>>
>>> 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?

a DHCP client asks mostly for an IP address in BOOTP way
However an DHCP client can ask for more parameters
DHCP servers tend to provide only for ask parameters.
DHCP clients don't tend to ask for the 0xD0..0xD3 (208..211) parameters.


The lines
       if exists dhcp-parameter-request-list {
               # Always send the PXELINUX options (specified in # hexadecimal)
               option dhcp-parameter-request-list = concat(option dhcp-parameter-request-list,d0,d1,d2,d3);
       }
can read as
       if DHCP client ask for more as just an IP-address
       then provide what was asked for PLUS add parameters 0xD0..0xD3



The advice 'try a packet capture' should be read as "Do capture network
packets with a tool like 'tcpdump' or 'Wireshark'"



Groeten
Geert Stappers