Doc/pxelinux

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This document is partially outdated. Please see PXELINUX.
The content of doc/pxelinux.txt (release 3.72):

Contents

PXELINUX

A bootloader for Linux using the PXE network booting protocol
Copyright 1994-2008 H. Peter Anvin - All Rights Reserved

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.




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. 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.

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.


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/

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...

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. 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".


ALTERNATE TFTP SERVERS

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.


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:

  • 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.
  • 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.)

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