[PATCH] Add support for pxeboot

[Richard W.M. Jones]

The attached patch adds support for turning a liveCD ISO image into a 
network-bootable PXE image.

Myself and Chris Lalancette looked into this in some detail and came up 
with two different ways to do this.  This is the simpler of the two 
methods and results in the least invasive changes to livecd-creator -- 
just a single line patch to mayflower and a 100-or-so-line extra script 
called 'livecd-iso-to-pxeboot' which works analogously to 
'livecd-iso-to-disk'.

The way it works is to bundle the complete ISO image inside the initrd. 
  The kernel and (bloated) initrd are downloaded using PXE in the normal 
way, and the init script finds and loopback-mounts the ISO image and 
booting continues as normal.

One possible con of this approach is whether or not it's a good idea to 
download a very large (min 200 MB) initrd image over TFTP.  There are 
two reasons we can think of why you *wouldn't* want to do this:

(1) "traditional" TFTP is limited to 32 MB max file size, although all 
modern TFTP implementations supersede that with a 4 GB limit (RFC 2348).

(2) TFTP uses its own transport layer which might not be able to 
back-off as nicely as TCP on busy networks.

However in my testing there was no problem at all using TFTP on large 
files over an ethernet LAN.

The other approach which we looked at and partially implemented was to 
modify the normal initrd/init so that it could do a separate request to 
download the ISO.  It could possibly do this over another protocol such 
as HTTP (although in Chris's tests he just used a separate TFTP 
connection).  The implementation of this is considerably more complicated:

(a) The initrd has to carry around dhclient, network scripts, and all 
the network programs like '/sbin/ip'.

(b) The initrd has to be able to find the TFTP server again, which 
basically involves making another DHCP query.  In Chris's original 
implementation he also added specific DHCP options to carry the TFTP 
server information since it wasn't clear how to get this out of stock 
dhclient.

(c) After booting, because of (b), the network is already set up, which 
is not how a live CD expects it to be.  Therefore the live CD itself 
would need to be changed if it does any network configuration.

All in all, I would say that the attached patch is a simpler and better 
approach.

Rich.

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