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✏️ Editing: pxelinux.txt

PXELINUX

A bootloader for Linux using the PXE network booting protocol

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.

This documentation file is slightly out of date; please check the NEWS
file for changes.

----------------------------------------------------------------------

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 Etherboot (http://www.etherboot.org/).
Etherboot 5.4 or later can also be used to create a PXE-compliant boot
PROM for many network cards.

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

PXELINUX operates in many ways like SYSLINUX.  If you are not familiar
with SYSLINUX, read syslinux.txt 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.txt 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 the client UUID, if
  one is provided by the PXE stack (note, some BIOSes don't have a
  valid UUID, and you might end up with something like all 1's.)  This is
  in the standard UUID format using lower case hexadecimal digits, e.g.
  b8945908-d6a6-41a9-611d-74a6ab80b83d.

Next, it will search for the config file using the hardware type
  (using its ARP type code) and address, all in lower case hexadecimal
  with dash separators; for example, for an Ethernet (ARP type 1)
  with address 88:99:AA:BB:CC:DD it would search for the filename
  01-88-99-aa-bb-cc-dd.

Next, 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, if the boot file name is /mybootdir/pxelinux.0, the
  UUID is b8945908-d6a6-41a9-611d-74a6ab80b83d, the Ethernet MAC
  address is 88:99:AA:BB:CC:DD and the IP address 192.0.2.91, it will
  try:

/mybootdir/pxelinux.cfg/b8945908-d6a6-41a9-611d-74a6ab80b83d
	/mybootdir/pxelinux.cfg/01-88-99-aa-bb-cc-dd
	/mybootdir/pxelinux.cfg/C000025B
	/mybootdir/pxelinux.cfg/C000025
	/mybootdir/pxelinux.cfg/C00002
	/mybootdir/pxelinux.cfg/C0000
	/mybootdir/pxelinux.cfg/C000
	/mybootdir/pxelinux.cfg/C00
	/mybootdir/pxelinux.cfg/C0
	/mybootdir/pxelinux.cfg/C
	/mybootdir/pxelinux.cfg/default

... in that order.

Note that all filename references are relative to the directory
pxelinux.0 lives in.  PXELINUX generally requires that filenames
(including any relative path) are 127 characters or shorter in length.

Starting in release 3.20, PXELINUX will no longer apply a built-in
default if it cannot find any configuration file at all; instead it
will reboot after the timeout interval has expired.  This keeps a
machine from getting stuck indefinitely due to a boot server failure.

Starting in release 3.50, PXELINUX displays network information at
the boot prompt pressing <Ctrl-N>.

PXELINUX does not support MTFTP, and I have no plans of doing so, as
MTFTP is inherently broken for files more than 65535 packets (about
92 MB) in size.  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.

++++ HTTP AND FTP DOWNLOADS ++++

Since version 5.10, native pxelinux.0 can support HTTP and FTP
transfers, greatly increasing load speed and allowing for standard
HTTP scripts to present PXELINUX's configuration file.  To use http or
ftp, use standard URL syntax as filename; use the DHCP options below
to transmit a suitable URL prefix to the client, or use the
"pxelinux-options" tool provided in the utils directory to program it
directly into the pxelinux.0 file.

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

For best results, use 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/

If your boot server is running Windows (and you can't fix that), try
tftpd32 by Philippe Jounin (you need version 2.11 or later; previous
versions had a bug which made it incompatible with PXELINUX):

http://tftpd32.jounin.net/

++++ 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...

# 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...

# 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.  See
RFC 5071 for some additional information about these options.

Option 208	pxelinux.magic
	- Earlier versions of PXELINUX required this to be set to
	  F1:00:74:7E (241.0.116.126) for PXELINUX to
	  recognize any special DHCP options whatsoever.  As of
	  PXELINUX 3.55, this option is deprecated and is no longer
	  required.

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;

NOTE: In earlier versions of PXELINUX, this would only work as a
    "site-option-space".  Since PXELINUX 2.07, this will work both as a
    "site-option-space" (unencapsulated) and as a "vendor-option-space"
    (type 43 encapsulated.)  This may avoid messing with the
    dhcp-parameter-request-list, as detailed below.

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 (specified in hexadecimal)
		option dhcp-parameter-request-list = concat(option dhcp-parameter-request-list,d0,d1,d2,d3);
	}
	# 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 "option dhcp-parameter-request-list" statement forces the DHCP
server to send the PXELINUX-specific options, even though they are not
explicitly requested.  Since the DHCP request is done before PXELINUX
is loaded, the PXE client won't know to request them.

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 (specified in hexadecimal)
		option dhcp-parameter-request-list = concat(option dhcp-parameter-request-list,d0,d1,d2,d3);
	}
	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".

++++ HARDCODED OPTIONS ++++

Since version 3.83, the program "pxelinux-options" can be used to
hard-code DHCP options into the pxelinux.0 image file; this is
sometimes useful when the DHCP server is under different
administrative control.

++++ ALTERNATE TFTP SERVERS AND URL SYNTAX ++++

PXELINUX supports the following special pathname conventions:

::filename

Suppresses the common filename prefix, i.e. passes the string
	"filename" unmodified to the server.

IP address::filename		(e.g. 192.0.2.1::filename)

Suppresses the common filename prefix, *and* sends a request
	to an alternate TFTP server.  Instead of an IP address, a
	DNS name can be used.  It will be assumed to be fully
	qualified if it contains dots; otherwise the local domain as
	reported by the DHCP server (option 15) will be added.

:: was chosen because it is unlikely to conflict with operating system
usage.  However, if you happen to have an environment for which the
special treatment of :: is a problem, please contact the Syslinux
mailing list.

Since version 4.00, PXELINUX also supports standard URL syntax.

++++ 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.

Lots of PXE stacks, especially old ones, have various problems of
varying degrees of severity.  Please see:

http://syslinux.zytor.com/hardware.php

... for a list of currently known hardware problems, with workarounds
if known.

++++ KEEPING THE PXE STACK AROUND ++++

Normally, PXELINUX will unload the PXE and UNDI stacks before invoking
the kernel.  In special circumstances (for example, when using MEMDISK
to boot an operating system with an UNDI network driver) it might be
desirable to keep the PXE stack in memory.  If the option "keeppxe"
is given on the kernel command line, PXELINUX will keep the PXE and
UNDI stacks in memory.  (If you don't know what this means, you
probably don't need it.)

++++ PROBLEMS WITH YOUR PXE STACK ++++

There are a number of extremely broken PXE stacks in the field.  The
gPXE project (formerly known as Etherboot) provides an open-source PXE
stack that works with a number of cards, and which can be loaded from
a CD-ROM, USB key, or floppy if desired.

Information on gPXE is available from:

http://www.etherboot.org/

... and ready-to-use ROM or disk images from:

http://www.rom-o-matic.net/

Some cards, like may systems with the SiS 900, has a PXE stack which
works just barely well enough to load a single file, but doesn't
handle the more advanced items required by PXELINUX.  If so, it is
possible to use the built-in PXE stack to load gPXE, which can then
load PXELINUX.  See:

http://www.etherboot.org/wiki/pxechaining

++++ CURRENTLY KNOWN PROBLEMS ++++

The following problems are known with PXELINUX, so far:

+ The error recovery routine doesn't work quite right.  For right now,
  it just does a hard reset - seems good enough.
+ We should probably call the UDP receive function in the keyboard
  entry loop, so that we answer ARP requests.
+ Boot sectors/disk images are not supported yet.

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

MR SKB