[syslinux] Cannot chain to another PXE server on the same subnet

[Gene Cumm]

On Fri, Mar 7, 2014 at 6:01 PM, Jeffrey Hutzelman <jhutz at cmu.edu> wrote:
> On Fri, 2014-03-07 at 17:46 -0500, Gene Cumm wrote:
>> On Fri, Mar 7, 2014 at 4:00 PM, Jeffrey Hutzelman <jhutz at cmu.edu> wrote:
>> > On Fri, 2014-03-07 at 05:49 -0500, Gene Cumm wrote:
>>
>> >> 1) Thinking about the responses again, I'm absolutely surprised that
>> >> you can even boot PXELINUX.  I would have expected the response from
>> >> the Altiris server to override your attempts to block it.
>> >
>> > Nope.  The PXE spec explicitly requires that a PXE response from the
>> > "real" DHCP server be given precedence.
>>
>> So PXE from DHCP, PXE from PXE and file/sname from DHCP is the order
>> of precedence.
>
> Yup.  But it doesn't take much PXE to decide the DHCP server is
> providing it.  He's sending the PXEClient vendor class ID and a (zero)
> mtftp IP address.  If I wanted to be sure, I'd send discovery control
> and a 0.0 boot item, but I'm not that surprised it works without them.

01:04:e0:01:01:03:02:02:de:06:03:02:df:06:04:01:01:05:01:01:06:01:0b:08:70:aa:aa:01:0a:d7:90:3c:00:a0:01:0a:d7:90:3c:00:81:01:0a:d7:90:3c:00:a1:01:0a:d7:90:3c:00:a2:01:0a:d7:90:3c:00:a4:01:0a:d7:90:3c:00:a5:01:0a:d7:90:3c:00:a6:01:0a:d7:90:3c:00:a7:01:0a:d7:90:3c:00:a8:01:0a:d7:90:3c:00:a9:01:0a:d7:90:3c:00:aa:01:0a:d7:90:3c:00:ab:01:0a:d7:90:3c:00:ac:01:0a:d7:90:3c:00:83:01:0a:d7:90:3c:bb:bb:01:0a:d7:90:3c:47:04:00:00:00:00:e0:02:00:00:ff

> I _am_ mildly surprised your example didn't work.  I wonder if maybe we
> can, for the purpose of testing, trim down the overall size of the
> encapsulated options to fit in 255 bytes.  For example, it would
> probably be sufficient to remove most of the menu items, and perhaps all
> of the mtftp-related options, which are all optional if you are willing
> to do everything unicast.
>
> What will pxechn do if you use -o more than once with the same option
> number?

Override.  Due to how the dhcp pack/unpack functions work and the
underlying data structures, it'd have to be redesigned to handle this.

Interesting, 16 different vendor-specific servers are specified.
Vendor (option 43; PXE) sub-option 8 is a list of type (2B), count
(1B), IPv4 (4B * Count) (each is 7 bytes since count is only 1 in this
situation).

-- 
-Gene