[syslinux] Support DHCP sname field in PXELINUX

[Dyks, Axel (XL)]

Chris Adams wrote:
> Once upon a time, H. Peter Anvin <hpa at zytor.com> said:
>> I mean, the right thing to do is probably, in order:
>>
>> 1. Use siaddr
>> 2. Use sname
>> 3. Use option 54.
> 
> I've done some more digging, and it appears that the PXE BIOS in the
> computers I'm testing with do not look at the sname field.  They do look
> at option 54 ("server-identifier" in Juniper JUNOS).  It would probably
> be more "correct" then to duplicate that and use (siaddr, option 54) and
> ignore sname (since that is what it looks like the PXE BIOS does).

Seems like a (at least) pointwise convergence ...

I've attached two versions of a "next-server-fallback" patch.
A "verbose" one that makes "pxelinux talk about the reason for falling back",
and a "silent" one. The later is -- of course -- much smaller and
does not require to increase LATEBSS_START in "layout.inc".

Axel

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