PXELINUX

                               PXELINUX

    A bootloader for Linux using the PXE network booting protocol

		Copyright (C) 1994-2001 H. Peter Anvin

This program is provided under the terms of the GNU General Public
License, version 2 or, at your option, any later version.  There is no
warranty, neither expressed nor implied, to the function of this
program.  Please see the included file COPYING for details.

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PXELINUX is a SYSLINUX derivative, for booting Linux off a network
server, using a network ROM conforming to the Intel PXE (Pre-Execution
Environment) specification.  PXELINUX is *not* a program that is
intended to be flashed or burned into a PROM on the network card; if
you want that, check out NILO (http://www.nilo.org/).  NILO can also
be used to create a PXE-compliant boot PROM for most network cards
which have drivers for Linux or FreeBSD.


    ++++ HOW TO CONFIGURE PXELINUX ++++

PXELINUX operates in many ways like SYSLINUX.  If you are not familiar
with SYSLINUX, read syslinux.doc first, since this documentation only
explains the differences.

On the TFTP server, create the directory "/tftpboot", and copy the
following files to it:

	pxelinux.0		- from the SYSLINUX distribution

	any kernel or initrd images you want to boot

Finally, create the directory "/tftpboot/pxelinux.cfg".  The
configuration file (equivalent of syslinux.cfg -- see syslinux.doc for
the options here) will live in this directory.  Because more than one
system may be booted from the same server, the configuration file name
depends on the IP address of the booting machine.  PXELINUX will
search for its config file on the boot server in the following way:

  First, it will search for the config file using its own IP address
  in upper case hexadecimal, e.g. 192.0.2.91 -> C000025B
  (you can use the included progam "gethostip" to compute the
  hexadecimal IP address for any host.)

  If that file is not found, it will remove one hex digit and try
  again.  Ultimately, it will try looking for a file named "default"
  (in lower case).

  As an example, for 192.0.2.91, it will try C000025B, C000025,
  C00002, C0000, C000, C00, C0, C, and default, in that order.

(See also the section on special DHCP options.)

It should be noted that all filename references are relative to the
directory pxelinux.0 lives in (usually /tftpboot).  PXELINUX
generally requires that filenames (including any relative path) are 63
characters or shorter in length.

PXELINUX does not support MTFTP, and I have no immediate plans of
doing so.  It is of course possible to use MTFTP for the initial boot,
if you have such a setup.  MTFTP server setup is beyond the scope of
this document.


    ++++ SETTING UP THE TFTP SERVER ++++

PXELINUX currently requires that the boot server has a TFTP server
which supports the "tsize" TFTP option (RFC 1784/RFC 2349).  The
"tftp-hpa" TFTP server, which support options, is available at:

	http://www.kernel.org/pub/software/network/tftp/
	ftp://www.kernel.org/pub/software/network/tftp/

... and on any kernel.org mirror (see http://www.kernel.org/mirrors/).

Another TFTP server which supports this is atftp by Jean-Pierre
Lefebvre:

	ftp://ftp.mamalinux.com/pub/atftp/

Unfortunately, the Intel LANDesk Service Agent II prior to version
0.99h (PXE PDK V2.4) seems to have a host of serious bugs.  Among
other things, it requests the TFTP "blksize" option, but will be
mortally confused if this option is actually accepted by the server!
There are three possible workarounds for this bug:

1. Use a TFTP server with doesn't support "blksize".

   Unfortunately, PXELINUX requires the "tsize" option to be
   supported, and it is very unusual for TFTP servers to implement one
   and not the other.  Both the tftp-hpa and atftp TFTP servers (see
   above) therefore can be configured to disable the blksize option.
   Use "-r blksize" for tftp-hpa, "--no-blksize" for atftp.

2. Use MTFTP for the initial bootstrap.  You need an MTFTP server with
   the appropriate DHCP setup to do this.

3. Use appropriate DHCP options that the client will attempt MTFTP
   before trying conventional TFTP.  It seems that the client will not
   request the "blksize" option if it has tried MTFTP and failed.
   These DHCP options are beyond the scope of this document.

These systems also seem to break if PMTU discovery is enabled on the
TFTP server.  This has to be disabled in the kernel on the TFTP
server.  On Linux, you can disable PMTU discovery by setting:

	echo 1 > /proc/sys/net/ipv4/ip_no_pmtu_disc

Note that this may cause bad performance for other services on the machine.


    ++++ SETTING UP THE DHCP SERVER ++++

The PXE protocol uses a very complex set of extensions to DHCP or
BOOTP.  However, most PXE implementations -- this includes all Intel
ones version 0.99n and later -- seem to be able to boot in a
"conventional" DHCP/TFTP configuration.  Assuming you don't have to
support any very old or otherwise severely broken clients, this is
probably the best configuration unless you already have a PXE boot
server on your network.

A sample DHCP setup, using the "conventional TFTP" configuration,
would look something like the following, using ISC dhcp 2.0 dhcpd.conf
syntax:

        allow booting;
        allow bootp;

	# Standard configuration directives...

        option domain-name "<domain name>";
        option subnet-mask <subnet mask>;
        option broadcast-address <broadcast address>;
        option domain-name-servers <dns servers>;
        option routers <default router>;

	# Group the PXE bootable hosts together
	group {
		# PXE-specific configuration directives...
		next-server <TFTP server address>;
		filename "/tftpboot/pxelinux.0";

		# You need an entry like this for every host
		# unless you're using dynamic addresses
	        host <hostname> {
        	        hardware ethernet <ethernet address>;
               		fixed-address <hostname>;
        	}
	}

Note that if your particular TFTP daemon runs under chroot (tftp-hpa
will do this if you specify the -s (secure) option; this is highly
recommended), you almost certainly should not include the /tftpboot
prefix in the filename statement.

If this does not work for your configuration, you probably should set
up a "PXE boot server" on port 4011 of your TFTP server; a free PXE
boot server is available at:

	http://www.kano.org.uk/projects/pxe/

With such a boot server defined, your DHCP configuration should look
the same except for an "option dhcp-class-identifier" ("option
vendor-class-identifier" if you are using DHCP 3.0):

        allow booting;
        allow bootp;

	# Standard configuration directives...

        option domain-name "<domain name>";
        option subnet-mask <subnet mask>;
        option broadcast-address <broadcast address>;
        option domain-name-servers <dns servers>;
        option routers <default router>;

	# Group the PXE bootable hosts together
	group {
		# PXE-specific configuration directives...
	        option dhcp-class-identifier "PXEClient";
		next-server <pxe boot server address>;

		# You need an entry like this for every host
		# unless you're using dynamic addresses
	        host <hostname> {
        	        hardware ethernet <ethernet address>;
               		fixed-address <hostname>;
        	}
	}

Here, the boot file name is obtained from the PXE server.

If the "conventional TFTP" configuration doesn't work on your clients,
and setting up a PXE boot server is not an option, you can attempt the
following configuration.  It has been known to boot some
configurations correctly; however, there are no guarantees:

        allow booting;
        allow bootp;

	# Standard configuration directives...

        option domain-name "<domain name>";
        option subnet-mask <subnet mask>;
        option broadcast-address <broadcast address>;
        option domain-name-servers <dns servers>;
        option routers <default router>;

	# Group the PXE bootable hosts together
	group {
		# PXE-specific configuration directives...
	        option dhcp-class-identifier "PXEClient";
		option vendor-encapsulated-options 09:0f:80:00:0c:4e:65:74:77:6f:72:6b:20:62:6f:6f:74:0a:07:00:50:72:6f:6d:70:74:06:01:02:08:03:80:00:00:47:04:80:00:00:00:ff;
		next-server <TFTP server>;
		filename "/tftpboot/pxelinux.0";

		# You need an entry like this for every host
		# unless you're using dynamic addresses
	        host <hostname> {
        	        hardware ethernet <ethernet address>;
               		fixed-address <hostname>;
        	}
	}

Note that this *will not* boot some clients that *will* boot with the
"conventional TFTP" configuration; Intel Boot Client 3.0 and later are
known to fall into this category.


    ++++ SPECIAL DHCP OPTIONS ++++

PXELINUX (starting with version 1.62) supports the following
nonstandard DHCP options, which depending on your DHCP server you may
be able to use to customize the specific behaviour of PXELINUX:

Option 208	pxelinux.magic
	- Must be set to F1:00:74:7E (241.0.116.126) for PXELINUX to
	  recognize any special DHCP options whatsoever.

Option 209	pxelinux.configfile
	- Specifies the PXELINUX configuration file name.

Option 210	pxelinux.pathprefix
	- Specifies the PXELINUX common path prefix, instead of
	  deriving it from the boot file name.  This almost certainly
	  needs to end in whatever character the TFTP server OS uses
	  as a pathname separator, e.g. slash (/) for Unix.

Option 211	pxelinux.reboottime
	- Specifies, in seconds, the time to wait before reboot in the
	  event of TFTP failure.  0 means wait "forever" (in reality,
	  it waits approximately 136 years.)

ISC dhcp 3.0 supports a rather nice syntax for specifying custom
options; you can use the following syntax in dhcpd.conf if you are
running this version of dhcpd:

	option space pxelinux;
	option pxelinux.magic      code 208 = string;
	option pxelinux.configfile code 209 = text;
	option pxelinux.pathprefix code 210 = text;
	option pxelinux.reboottime code 211 = unsigned integer 32;

Then, inside your PXELINUX-booting group or class (whereever you have
the PXELINUX-related options, such as the filename option), you can
add, for example:

	# Always include the following lines for all PXELINUX clients
	site-option-space "pxelinux";
	option pxelinux.magic f1:00:74:7e;
	if exists dhcp-parameter-request-list {
		# Always send the PXELINUX options
		append dhcp-parameter-request-list 208, 209, 210, 211;
	}
	# These lines should be customized to your setup
	option pxelinux.configfile "configs/common";
	option pxelinux.pathprefix "/tftpboot/pxelinux/files/";
	option pxelinux.reboottime 30;
	filename "/tftpboot/pxelinux/pxelinux.bin";

Note that the configfile is relative to the pathprefix: this will look
for a config file called /tftpboot/pxelinux/files/configs/common on
the TFTP server.

The "append dhcp-parameter-request-list" is an undocumented (as of
DHCP 3.0rc2pl1) option which forces the DHCP server to send the
PXELINUX-specific options, even though they are not explicitly
requested.

Using ISC dhcp 3.0 you can create a lot of these strings on the fly.
For example, to use the hexadecimal form of the hardware address as
the configuration file name, you could do something like:

	site-option-space "pxelinux";
	option pxelinux.magic f1:00:74:7e;
	if exists dhcp-parameter-request-list {
		# Always send the PXELINUX options
		append dhcp-parameter-request-list 208, 209, 210, 211;
	}
	option pxelinux.configfile =
		concat("pxelinux.cfg/", binary-to-ascii(16, 8, ":", hardware));
	filename "/tftpboot/pxelinux.bin";

If you used this from a client whose Ethernet address was
58:FA:84:CF:55:0E, this would look for a configuration file named
"/tftpboot/pxelinux.cfg/1:58:fa:84:cf:55:e".


    ++++ SOME NOTES ++++

If the boot fails, PXELINUX (unlike SYSLINUX) will not wait forever;
rather, if it has not received any input for approximately five
minutes after displaying an error message, it will reset the machine.
This allows an unattended machine to recover in case it had bad enough
luck of trying to boot at the same time the TFTP server goes down.


    ++++ CURRENTLY KNOWN PROBLEMS ++++

The following problems are known with PXELINUX, so far:

+ Requires a TFTP server which supports the "tsize" option.
+ The error recovery routine doesn't work quite right.  For right now,
  it just does a hard reset - seems good enough.
+ There may be funnies with memory management.  The PXE spec has no
  decent way of telling it to free up all memory and unchain any
  interrupts; it allows the base stack to be unloaded, but not the
  UNDI driver.
+ There seems to be a problem with sending ACK "storms"; a number of
  ACK packets fired off without the proper delay in between.  I
  suspect this is a PXE firmware problem, rather than a PXELINUX
  problem.
+ We should probably call the UDP receive function in the keyboard
  entry loop, so that we answer ARP requests.
+ Boot sectors don't work yet... they probably need auxilliary information
  (such as device) to work at all.

If you have additional problems, please contact the SYSLINUX mailing
list (see syslinux.doc for the address.)