PXE network booting

This page described the installation of a Linux OS on a computer by means of PXE (Preboot Execution Environment) booting. See also pages on:

• PXE_and_UEFI network booting.

• The section Automated_network_installation_with_pxeconfig below.

SYSLINUX and PXELINUX

PXE network booting from Linux servers uses the SYSLINUX and PXELINUX utilities (see also this Wikipedia article). Read the documentation on these pages to get an understanding of the process. The SYSLINUX source code has additional very useful documentation which we have copied here: syslinux.doc and pxelinux.doc.

There is a SYSLINUX mailing list as well as a SYSLINUX Wiki.

SYSLINUX documentation

• README.menu
• pxelinux.doc
• syslinux.doc

Boot process summary

When a client computer performs a PXE network boot, the Linux DHCP server assigns the client an IP-address and further information including a TFTP-server address (DHCP next-server option) and a boot image file name pxelinux.0 (DHCP filename option). The client retrieves the file pxelinux.0 from the TFTP server and executes it.

The pxelinux.0 PXELINUX boot image then attempts to download configurations files from the TFTP server in the boot process explained here. To summarize: The PXE/network client will use TFTP to download a PXELINUX configuration file from the server’s /tftpboot/pxelinux.cfg/ directory whose name is usually either:

1. the client’s hexadecimally encoded IP-address (such as 0A018219), or

2. the file named default.

With newer versions of SYSLINUX it is also possible to PXE-boot into the SYSLINUX menu systems where many booting options can be configured. This is a very flexible way to boot, for example, diskette images with BIOS upgrades, hardware testers, or SystemImager installation, etc.

Installing the SYSLINUX tools

Although your Linux machine may already have some of the SYSLINUX tools installed, it is recommended that you get the latest version of SYSLINUX.

Unpack the tar-ball and copy the following SYSLINUX files to the /tftpboot directory on the DHCP/TFTP server:

tar xzvf syslinux-4.02.tar.gz
cd syslinux-4.02
cp core/pxelinux.0 memdisk/memdisk com32/modules/chain.c32 com32/menu/menu.c32 /tftpboot/

(version 4.02 is used in this example). Additional com32/*/*.c32 modules might be needed if further features from SYSLINUX will be used.

For the older SYSLINUX version 3.x the locations of files differ a bit, so do:

tar xzvf syslinux-3.51.tar.gz
cd syslinux-3.51
cp pxelinux.0 memdisk/memdisk com32/modules/*.c32 /tftpboot/

If for some reason you must rebuild SYSLINUX, first verify that you have the nasm package installed:

rpm -q nasm

Then build SYSLINUX in the top-level directory by doing simply make. See also the file distrib.doc for further details.

The default file

You must also create a default PXE boot file /tftpboot/pxelinux.cfg/default instructing the node how to boot in case there is no hexadecimally encoded IP-address file. Probably the most sensible default boot method is local hard disk which is configured as follows:

We assume that you have installed the SYSLINUX tools as shown above, in particular the chain.c32 tool. Then create the file named default containing these lines:

default harddisk
label harddisk
  kernel chain.c32
  append hd0

For comparison, in many places you will find the following recipe for the default file:

default harddisk
label harddisk
  localboot 0

This recipe can be error-prone and actually means boot from the next device in the BIOS boot order, rather than booting from the hard disk as you would be led to believe. For more information read this article from the SYSLINUX mailing list (look at the bottom of the article).

Etherboot/gPXE

Something to consider for the future is the Etherboot/gPXE project which permits a larger set of boot media, includign also HTTP-servers. For PXE-booting into gPXE see PXE chainloading.

Linux and Windows deployment (WDS)

A Microsoft Windows Deployment Service WDS takes over PXE booting of any PXE clients it has configured, thus the clients will ignore the Linux PXE boot server.

For a solution, see Linux and Windows deployment:

• This covers the setup and deployment of a PXE boot solution consisting of 2 pxe servers and one dhcp server. The 2 PXE servers are linux and windows - the former running pxelinux and tftp and the latter one running WDS (Windows Deployment Services), with a linux server providing DHCP services.

See also Peaceful Coexistence: WDS and Linux PXE Servers:

• As it turns out, thanks to the lesser-known pxechain utility, it is possible to seamlessly jump from one PXE host to another. With a few tweaks to your WDS server, you can continue to use it for Windows OS installs and bounce over to a Linux host for Linux, ESXi, or rescue-CD purposes.

SYSLINUX Menu systems

With newer versions of SYSLINUX it is possible to PXE-boot into the SYSLINUX menu systems where many booting options can be configured. This is a very flexible way to boot, for example, diskette images with BIOS upgrades, hardware testers, or SystemImager installation, etc.

Please consult the README.menu from the SYSLINUX source.

One must first install SYSLINUX files to /tftpboot on the DHCP/TFTP server as shown in Installing the SYSLINUX tools.

Secondly, for each client machine that should use the SYSLINUX menu systems a hexadecimally encoded IP-address file must be created in /tftpboot/pxelinux.cfg/, pointing to the menu configuration file. This can conveniently be done with the pxeconfig command discussed below.

An example default.menu SYSLINUX menu file in /tftpboot/pxelinux.cfg/ is:

DEFAULT menu.c32
PROMPT 0

MENU TITLE Menu from TFTP server

label AlmaLinux8.8 minimal-x86_64
      menu label Clean AlmaLinux-8.8-x86_64, minimal install
      kernel AlmaLinux-8.8-x86_64/vmlinuz
      append load_ramdisk=1 initrd=AlmaLinux-8.8-x86_64/initrd.img network inst.ks=nfs:<some-IP-address>:/u/kickstart/ks-almalinux-8.8-minimal-x86_64.cfg vga=792

label harddisk
      menu label Boot from local harddisk
      kernel chain.c32
      append hd0

This configuration will display a menu with 4 items, each performing a different task as described in the menu label lines.

Password protection of PXELINUX menu items

It is possible to password protect a PXELINUX menu item in recent versions of PXELINUX, see http://www.syslinux.org/wiki/index.php/Menu#MENU_PASSWD. For example, a menu item may have a line:

menu passwd <password-hash>

To generate the MD5 or SHA1 password hash, make sure you have a recent version of syslinux, or download the code from http://www.kernel.org/pub/linux/utils/boot/syslinux/. Locate the scripts sha1pass and md5pass (subdirectory utils/ in the source). Also, install this prerequisite:

yum install perl-Crypt-PasswdMD5

Then you can execute sha1pass or md5pass to generate password hashes.

Alternatively, you can use the command /sbin/grub-md5-crypt (MD5 passwords only), or find some web-based tools.

Hardware Detection Tool (HDT)

SYSLINUX version 4 contains a HDT - Hardware Detection Tool. HDT (stands for Hardware Detection Tool) is a Syslinux com32 module that displays low-level information for any x86 compatible system. It provides both a command line interface and a semi-graphical menu mode for browsing.

To enable HDT install the hdt.c32 module from the SYSLINUX source:

cd syslinux-4.02
cp com32/hdt/hdt.c32 /tftpboot/

Then add a PXE menu section to the default.menu file:

label hdt
      menu label HDT - Hardware Detection Tool
      COM32 hdt.c32

Boot images from Ultimatebootcd.com

If you would like a very complete set of tools to boot from the PXE menus, you may want to take a look at the Ultimate Boot CD project. You can download an ISO image for burning your own CD.

While you could boot the Ultimate Boot CD directly, you could also extract the utilities from the CD and put them into your PXE menus. This is easier than downloading the tools yourself from very many different places.

You can copy the Ultimate Boot CD tools from the CD, or from the ISO image if you mount it first onto /mnt:

root# mount -o loop /some-path/ubcd503.iso /mnt

You can very simply enable the complete Ultimate Boot CD tools in a PXE environment by copying the entire CD structure to /tftpboot/ on your TFTP server:

root# cp -rp /mnt/ubcd /tftpboot/

and add this configuration to the SYSLINUX menu file:

label UBCMenu
      menu label Ultimate Boot CD menu
      kernel menu.c32
      append ubcd/menus/syslinux/main.cfg

Alternatively, with SYSLINUX version 4 and above you can PXE-boot the 300MB UBC ISO-image directly (loading the TFTP server much more). The loading of ISO CD images with SYSLINUX is described in http://syslinux.zytor.com/wiki/index.php/MEMDISK#ISO_images. The SYSLINUX menu file could have a section like:

label UBCMenu-iso
      menu label Ultimate Boot CD (300 MB ISO file)
      linux memdisk
      initrd ubcd503.iso
      append iso

Commandline key strokes

The command line prompt supports the following keystrokes (see syslinux.doc):

<Enter>         boot specified command line
<BackSpace>     erase one character
<Ctrl-U>        erase the whole line
<Ctrl-V>        display the current SYSLINUX version
<Ctrl-W>        erase one word
<Ctrl-X>        force text mode
<F1>..<F10>     help screens (if configured)
<Ctrl-F><digit> equivalent to F1..F10
<Ctrl-C>        interrupt boot in progress
<Esc>           interrupt boot in progress

Automated network installation with pxeconfig

You can automate the PXE/network booting process completely using the pxeconfig_toolkit written by Bas van der Vlies. The pxeconfig toolkit manipulates configuration files in the server’s /tftpboot/pxelinux.cfg/ directory, namely the client’s hexadecimally encoded IP-address, for example:

0A018219 => 10.1.130.25

Hint: Use gethostip from the syslinux package to convert hostnames and IP-addresses to hexadecimal, for example:

$ gethostip -f s001
s001.(domainname) 10.2.130.21 0A028215
$ gethostip -x s001
0A028215

Download the pxeconfig_toolkit and read the pxeconfig installation instructions.

Ansible setup for pxeconfig

For CentOS/RHEL 7 we have written an Ansible role to automatically install and start the pxeconfig service:

• https://github.com/OleHolmNielsen/ansible/tree/master/roles/pxeconfigd

Download the role files to your Ansible setup and use this role in your Ansible playbooks by:

- pxeconfigd

The pxeconfig command

To use pxeconfig you should create any number of PXELINUX configuration files named default.<something> which contain different PXELINUX commands that perform the desired actions, for example, BIOS updates, firmware updates, hardware diagnostics, or network installation.

Use the pxeconfig command to configure those client nodes that you wish to install (the remaining nodes will simply boot from their hard disk). An example is:

# pxeconfig n003
Which pxe config file must we use: ?
1 : default.memdisk_ibm_s50_bios
2 : default.memdisk_326m_bmc_fw118
3 : default.memtest86
4 : default.node_install.s50
5 : default.harddisk.BAK
6 : default.node_install.thul
7 : default.node_install.ibm326
Select a number: 7

The pxeconfig command creates soft-links in the /tftpboot/pxelinux.cfg/ directory named as the hexadecimally encoded IP-address of the clients, and these links will point to one of the files default.*. As designed, the PXE network booting process will download the file given by the hexadecimal IP-address, and hence network installation of the node will take place.

The hexls command

To list the soft links created, use the pxeconfig tool hexls and look for the IP-addresses and/or hostnames. A sample output is:

# hexls /tftpboot/pxelinux.cfg/ | sort
0A018103 => 10.1.129.3 => n003.dcsc.fysik.dtu.dk -> default.node_install.ibm326
0A028248 => 10.2.130.72 => t072.dcsc.fysik.dtu.dk -> default.node_install.s50
0A02826B => 10.2.130.107 => t107.dcsc.fysik.dtu.dk -> default.node_install.s50
0A02833D => 10.2.131.61 => u061.dcsc.fysik.dtu.dk -> default.node_install.s50
default
default.harddisk
default.harddisk.BAK
default.memdisk_326m_bmc_fw118
default.memdisk_ibm_s50_bios
default.memtest86
default.node_install.ibm326
default.node_install.s50
default.node_install.thul

The pxeconfigd daemon

The second part of the pxeconfig_toolkit is the pxeconfigd daemon. This tool is presumably only useful when doing network installation using https://www.ansible.com/ SystemImager.

The pxeconfigd daemon will remove the hexadecimally encoded IP-address soft-link on the server when contacted by the client node. In order for this to happen, you must go to the image server’s /var/lib/systemimager/scripts/post-install directory and create the file 30all.pxeconfig:

#!/bin/sh
# To be used with the pxeconfig tool.
# Remove the <hex_ipaddr> file from the pxelinux.cfg directory so the client will boot from disk.
# Get pxeconfig from ftp://ftp.surfsara.nl/pub/outgoing/pxeconfig.tar.gz
# Get the Systemimager variables
. /tmp/post-install/variables.txt
telnet $IMAGESERVER 6611
sleep 1
exit 0

When this script is executed on the node by SystemImager in the post-install phase, the telnet command connects to the pxeconfigd daemon on the image server, and this daemon will remove the hexadecimally encoded IP-address soft-link in /tftpboot/pxelinux.cfg/ corresponding to the client IP-address which did the telnet connection.