[syslinux] Chainload pxelinux from pxelinux and pass parameters or change root dir?

[Gene Cumm]

On Mon, Jun 16, 2014 at 10:23 AM, Alexander Perlis <aperlis at math.lsu.edu> wrote:
> When chainloading one pxelinux from another (e.g., chainloading from say
> pxelinux 4.07 to pxelinux 6.02), can one pass parameters or somehow control
> the root directory and/or server IP address?
>
> Here's the need: we have a loaded-up PXE server working great under 4.07,
> and I'm not prepared to just switch the entire setup to a newer version of
> pxelinux, too many folks depending on a working setup. However, if I could
> chainload to 6.02, then I could have a menu entry that allows us to go to
> 6.02 for testing purposes, or to deploy a particular thing that actually
> requires 6.02. Thus I would like to have a menu entry under pxelinux 4.07
> that says something like:
>
> LABEL chainloadtopxe6
> KERNEL /syslinux6/gpxelinux.0
> APPEND root=/syslinux6

The append is useless.  I'd also suggest the explicit KERNEL-like
directive PXE instead of KERNEL but alas it won't achieve your goals.
Chainloading to another PXELINUX in another directory won't adjust the
necessary data elements.

> Note the APPEND. The idea is that the gpxelinux.0 would then in turn fetch
> its *.c32 and pxelinux.cfg from the /syslinux6 subtree, so that those files
> can be different than the versions from pxelinux4.
>
> Is something like that possible? If not, how hard to add support for that?
> (That is, support for passing a parameter to a chainloaded pxelinux?) If
> totally impossible, is a possible workaround to dedicate another IP address,
> and do something like

Yes/no.  You need a different approach.  This is why pxechain.com was
written and is now superseded by pxechn.c32 as a more flexible COM32
solution.  The traditional COMBOOT APIs have been removed in later
versions like 6.02.  Initially I wrote pxechn.c32 to allow further
modifications of the DHCP data due to requirements by certain Nettwork
Boot Programs (specifically Windows Server 2008R2 Windows Deployment
Services.

> LABEL chainloadtopxe6
> KERNEL http://some.new.ip/gpxelinux.0
>
> Will that invocation of gpxelinux.0 then set the environment appropriately
> to pull the *.c32 and pxelinux.cfg from that new IP address?

No.

> I suppose my real quandary is my lack of understanding of how pxelinux.0 is
> executed: the DHCP server returns the IP and base image name, the ethernet
> card pulls down that image, so pxelinux.0 is just some binary code loaded
> into memory and executed, but how does that code then know the IP and path
> for pulling down additional components? That is, how does pxelinux.0 gain

By finding the DHCP data that can be found by 4 PXE-specific methods.

> access to the DHCP parameters that tell it the root path and host IP
> address? Do those get passed dynamically on the stack, or are they stored
> ahead of time at some hardcoded fixed memory locations, or how does it work?
> And can those values later be changed on a reinvocation of pxelinux.0?

See above.

-- 
-Gene