[syslinux] Chainloading from one PXELINUX to another (with some iPXE in the mix)?

Jeffrey Hutzelman jhutz at cmu.edu
Wed Jun 8 14:04:03 PDT 2011

On Wed, 2011-06-08 at 10:29 -0700, Mike Sollanych wrote:

> This sounds great, but pxechain.com looks to be poorly documented. 
> I can only find a smattering of mailing lists posts about how to use it, and 
> a very short section on the SYSLINUX wiki.

I admit there's not a whole lot of documentation, but there's also not
really much to document.

> My attempts thus far:
> - At the boot prompt, "pxechain.com [FOG SERVER FQDN]::pxelinux.0"
> - In the config file, a label with:
> 	COMBOOT pxechain.com
> 	APPEND [FOG SERVER FQDN]::pxelinux.0 
> Same thing either way, I guess.

Yup, that's what should happen.  Assuming, of course, that you don't
actually have brackets in your config file.

> This will load the PXELINUX from the FOG server, which is still 3.86 compared
> to the 4.xx running on the master server. *However*, it still tries to load its
> config file from the master server, using the precedence order (mac->ip->default).
> That won't work. I guess I need to change the TFTP prefix more, somehow, in order 
> to get it to load config files from the other server as well as its PXELINUX image?

PXECHAIN should do that for you; in fact, that's its main purpose.
It works by updating the PXE stack's cached DHCP reply, which works with
the Intel reference stack and with every PXE stack I've seen on real
hardware, but might _not_ work with software stacks like gpxe/ipxe.  I
thought someone had fixed that problem, but since I don't use those
stacks, I haven't particularly paid attention.

> I don't know if 
> this method will be reliable across a wide gamut of hardware.

We've had it work on pretty much every PXE-capable box we've seen in the
last few years.  But, again, without first loading an alternate PXE

> Seeing as how I've had luck with that CONFIG directive (config.c32) at loading
> the default file, it makes sense (to me anyway) that a modification of that
> module to support loading PXELINUX config files using the same precedence
> as the initial load would do the trick quite nicely.

I don't believe there's currently a way to do that.  It's possible
there's something new, though; I haven't looked at recent code in a

-- Jeff

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