Boot ISO or img from http

[mikygee]

Hello,

I have files provided by a vendor that are to be burnt on a cd or used for a usb key
distrib.iso
usbfatboot.img
The underlying software is a package Freebsd

I'm able to get the menu from ipxe and be able to HTTP GET the files, so no technical problems here

But I don't understand what to use
:5.0.4.P5a
initrd http://100.64.1.100/distrib.P5a.iso
chain http://100.64.1.100/distrib.P5a.iso ||

:6.0.2.P1a
initrd http://100.64.1.100/usbfatboot-amd64-6.0.2.P1a.img
chain http://100.64.1.100/usbfatboot-amd64-6.0.2.P1a.img ||

:6.0.2.P5b
imgfetch http://100.64.1.100/usbfatboot-amd64-6.0.2.P5b.img
boot memdisk iso raw ||
goto MENU


For the ISO I get the message
Could not boot: Exec format error (http://ipxe.org/2e008001)
I read that page
For the first img I have the same message

For the second img I have memdisk no such file or directory

At boot ipxe displays DNS HTTP iSCSI TFTP AoE ELF MBOOT PXE bzImage Menu PXEXT
I downloaded undionly.kpxe

Does anybody has any hint on how to make something to work