[syslinux] Confusion on lpxelinux vs. gpxelinux vs. ipxe vs gpxe.

[Doug Scoular]

Hi All,
I've been trying to understand how to use pxechn.c32 to chain a local
pxelinux menu item to a remote server which has it's own pxelinux hierarchy
served via TFTP and HTTP.

We have no control over DHCP next-server and filename fields so I wanted to
exploit the "prefix" -p option that pxechn.c32 accepts.

I spent a long time hitting my head against a brick wall until I recompiled
with various DEBUG macros set... my pxechn stanza looked like this:

  # Test Doug's chaining...
  LABEL chain018
          MENU LABEL ^Doug's REMOTE lpxelinux.
          KERNEL pxechn.c32
          APPEND
http://wwwin-kickstart.remote.com/tftpboot/dscoular/syslinux/lpxelinux.0 -p
http://wwwin-kickstart.remote.com/tftpboot/dscoular/syslinux

With DEBUG set I could see that my local lpxelinux.0 would successfully
load pxechn.c32, then pxechn.c32 would load the remote lpxelinux.0 via
HTTP... but then it would fail to find it's needed ldlinux.c32.

It took a wireshark packet dump to discover that the pxechn "prefix" -p
option *MUST* be given a trailing slash (ouch):

    HTTP/1.1 404 Not Found
    The requested URL /tftpboot/dscoular/syslinuxldlinux.c32 was not found
on this server.

Adding the trailing slash solved our issues and we were able to load the
remote lpxelinux.0 and have it use our remote .../syslinux/pxelinux.cfg.
Hurray!

However, I'm left feeling a bit confused about all the Network Boot
Programs mentioned in the syslinux documentation. I'd like to try and
summarise my understanding here and have someone in the know correct any
mistaken impressions I have (no doubt quite a few).

So for syslinux 6.03 we have the following syslinux config compatible
Network Boot Programs which can load com32 modules:

1. pxelinux.0 - a syslinux Network Boot program that only understands TFTP
not HTTP or FTP, etc.
2. lpxelinux.0 - a syslinux Network Boot program linked against the
light-weight TCP/IP (lwIP) stack. Offering fast HTTP, FTP, etc.
3. gpxelinux.0 - a syslinux Network Boot program linked against another
Network Boot Program's implementation of HTTP (gpxe). Possibly problematic
and slow on VMs.

We also have a gpxecmd.c32 which doesn't seem to be documented in the
"modules" section of the syslinux wiki.

As separate projects we appear to have:

1. gpxe - a mothballed Network Boot Program but with a possibly problematic
and slow HTTP implementation.
2. ipxe - an active fork of gpxe but still with the possibly problematic
and slow HTTP implementation (?)

I presume that for pxechn.c32 to use HTTP, then it has to have been loaded
either by gpxelinux.0 or, preferably lpxelinux.0 with it's shiny new lwIP
stack or is pxechn independent loading it's own lwIP stack through ldlinux
or another library module ?

I see mentions of both iPXE and gPXE in the comments in the source.

I note that if I try to use pxechn.c32 to load gpxelinux.0 it seems to
ignore the "prefix" -p argument and for older versions my test VM seems to
freeze.

I further note that when I successfully pxechn to a remote lpxelinux.0 it
doesn't seem to be quite as verbose as pxelinux.0 once was about the search
enumerations it makes in it's sibling pxelinux.cfg directory, which I miss:

/mybootdir/pxelinux.cfg/b8945908-d6a6-41a9-611d-74a6ab80b83d
/mybootdir/pxelinux.cfg/01-88-99-aa-bb-cc-dd
/mybootdir/pxelinux.cfg/C0A8025B
/mybootdir/pxelinux.cfg/C0A8025
/mybootdir/pxelinux.cfg/C0A802
/mybootdir/pxelinux.cfg/C0A80
/mybootdir/pxelinux.cfg/C0A8
/mybootdir/pxelinux.cfg/C0A
/mybootdir/pxelinux.cfg/C0
/mybootdir/pxelinux.cfg/C
/mybootdir/pxelinux.cfg/default

Any thoughts and/or detailed clarification of how all the various bits of
the puzzle fit hugely appreciated. Sorry to ask so many questions...

Cheers,

Doug