PXE Boot and extlinux.conf

The pxe commands provide a near subset of the functionality provided by the PXELINUX boot loader. This allows U-Boot based systems to be controlled remotely using the same PXE based techniques that many non U-Boot based servers use.

The sysboot command and Extlinux boot method use the same file format as PXE boot for extlinux.conf.

Commands

pxe get

Syntax: pxe get

follows PXELINUX’s rules for retrieving configuration files from a tftp server, and supports a subset of PXELINUX’s config file syntax. It requires certain environment variables to be set, see the Environment section below.

File Paths

pxe get repeatedly tries to download config files until it either successfully downloads one or runs out of paths to try. The order and contents of paths it tries mirrors exactly that of PXELINUX - you can read in more detail about it at:

http://syslinux.zytor.com/wiki/index.php/Doc/pxelinux

pxe boot

Syntax: pxe boot [pxefile_addr_r]

Interprets a pxe file stored in memory.

pxefile_addr_r is an optional argument giving the location of the pxe file. The file must be terminated with a NUL byte.

There are some environment variables that may need to be set, depending on conditions, see the Environment section below.

sysboot

Syntax: sysboot [-p] <interface> <dev[:part]> <ext2|fat|any> [addr] [filename]

Load and boot an extlinux.conf file from a local filesystem. Paths in the extlinux.conf file (kernel, initrd, FDT and overlays) will be interpreted within that filesystem.

Example:

sysboot mmc 0.0:2 any ${pxefile_addr_r} /boot/extlinux.conf

Environment

pxefile_addr_r: Should be set to a location in RAM large enough to hold pxe files while they’re being processed. Up to 16 config files may be held in memory at once. The exact number and size of the files varies with how the system is being used. A typical config file is a few hundred bytes long. Required for pxe get, for pxe boot if the optional argument pxefile_addr_r is not supplied.
bootfile: Typically set in the DHCP response handler, required for pxe get. For pxe boot, this path is used to generate the base directory that all other paths to files retrieved by pxe boot will use. If no bootfile is specified, paths used in pxe files will be used as is.
serverip: Typically set in the DHCP response handler, this is the IP address of the tftp server from which other files will be retrieved. Required for pxe get.
kernel_addr_r, initrd_addr_r: Locations in RAM to store the kernel (or FIT image) and initrd. These locations will be passed to the bootm command to boot the kernel. These environment variables are required to be set.
fdt_addr_r: Location in RAM to store the retrieved fdt blob. Retrieval is possible if fdt label is defined in pxe file and fdt_addr_r is set. If retrieval is possible, fdt_addr_r will be passed to bootm command to boot the kernel.
fdt_addr: Location of a fdt blob. fdt_addr will be passed to bootm command if it is set and fdt_addr_r is not passed to bootm command.
fdtoverlay_addr_r: Location in RAM to temporarily store fdt overlay(s) before applying them to the fdt blob stored at fdt_addr_r. Required to use the fdtoverlays command in the PXE file.
extension_overlay_addr: Location in RAM to temporarily store extension fdt overlay(s) before applying them to the fdt blob stored at fdt_addr_r.
pxe_label_override: Override label to be used, if exists, instead of the default label. This will allow consumers to choose a pxe label at runtime instead of having to prompt the user. If pxe_label_override is set but does not exist in the pxe menu, pxe would fallback to the default label if given, and no failure is returned but rather a warning message.
ethaddr: This is the standard MAC address for the ethernet adapter in use. pxe get uses it to look for a configuration file specific to a system’s MAC address.
pxeuuid: This is a UUID in standard form using lower case hexadecimal digits, for example, 550e8400-e29b-41d4-a716-446655440000. pxe get uses it to look for a configuration file based on the system’s UUID.

pxe file format

The pxe file format is nearly a subset of the PXELINUX file format; see http://syslinux.zytor.com/wiki/index.php/PXELINUX. It’s composed of one line commands - global commands, and commands specific to labels. Lines beginning with # are treated as comments. White space between and at the beginning of lines is ignored.

The size of pxe files and the number of labels is only limited by the amount of RAM available to U-Boot. Memory for labels is dynamically allocated as they’re parsed, and memory for pxe files is statically allocated, and its location is given by the pxefile_addr_r environment variable. The pxe code is not aware of the size of the pxefile memory and will outgrow it if pxe files are too large.

Supported global commands

Unrecognized commands are ignored.

default <label>: The label named here is treated as the default and is the first label ‘pxe boot’ attempts to boot.
fallback <label>: The label named here is treated as a fallback option that may be attempted should it be detected that booting of the default has failed to complete, for example via U-Boot’s boot count limit functionality.
menu title <string>: Sets a title for the menu of labels being displayed.
menu include <path>: Use tftp to retrieve the pxe file at <path>, which is then immediately parsed as if the start of its contents were the next line in the current file. nesting of include up to 16 files deep is supported.
prompt <flag>: If 1, always prompt the user to enter a label to boot from. If 0, only prompt the user if timeout expires.
timeout <num>: Wait for user input for <num>/10 seconds before auto-booting a node.
label <name>: Begin a label definition. Labels continue until a command not recognized as a label command is seen, or EOF is reached.

Supported label commands

Labels end when a command not recognized as a label command is reached, or EOF.

menu default

set this label as the default label to boot; this is the same behavior as the global default command but specified in a different way

kernel <path>

If this label is chosen, use tftp to retrieve the kernel (or FIT image) at <path>. it will be stored at the address indicated in the kernel_addr_r environment variable, and that address will be passed to bootm to boot this kernel. For FIT image, the configuration specification can be appended to the file name, with the format:

<path>#<conf>[#<extra-conf[#...]]

It will be passed to bootm with that address (see: doc/uImage.FIT/command_syntax_extensions.txt). It is useful for overlay selection in pxe file (see U-Boot FDT Overlay FIT usage).

fdtoverlays <path> [...]

If this label is chosen, use tftp to retrieve the DT overlay(s) at <path>. It will be temporarily stored at the address indicated in the fdtoverlay_addr_r environment variable, and then applied in the load order to the fdt blob stored at the address indicated in the fdt_addr_r environment variable.

devicetree-overlay <path> [...]

if this label is chosen, use tftp to retrieve the DT overlay(s) at <path>. It will be temporarily stored at the address indicated in the fdtoverlay_addr_r environment variable, and then applied in the load order to the fdt blob stored at the address indicated in the fdt_addr_r environment variable. Alias for fdtoverlays.

kaslrseed

set this label to request random number from hwrng as kaslr seed.

append <string>

Use <string> as the kernel command line when booting this label. Environment variable references like ${var} are substituted before boot.

initrd <path>

If this label is chosen, use tftp to retrieve the initrd at <path>. it will be stored at the address indicated in the initrd_addr_r environment variable, and that address will be passed to bootm. For FIT image, the initrd can be provided with the same value than kernel, including configuration:

<path>#<conf>[#<extra-conf[#...]]

In this case, kernel_addr_r is passed to bootm.

fdt <path>

If this label is chosen, use tftp to retrieve the fdt blob at <path>. It will be stored at the address indicated in the fdt_addr_r environment variable, and that address will be passed to bootm. For FIT image, the device tree can be provided with the same value than kernel, including configuration:

<path>#<conf>[#<extra-conf[#...]]

In this case, kernel_addr_r is passed to bootm.

devicetree <path>

If this label is chosen, use tftp to retrieve the fdt blob at <path>. it will be stored at the address indicated in the fdt_addr_r environment variable, and that address will be passed to bootm. Alias for fdt.

fdtdir <path>

If this label is chosen, use tftp to retrieve a fdt blob relative to <path>. If the fdtfile environment variable is set, <path>/<fdtfile> is retrieved. Otherwise, the filename is generated from the soc and board environment variables, i.e. <path>/<soc>-<board>.dtb is retrieved. If the fdt command is specified, fdtdir is ignored.

localboot <flag>

Run the command defined by localcmd in the environment. <flag> is ignored and is only here to match the syntax of PXELINUX config files.

Example

Here’s a couple of example files to show how this works.

/tftpboot/pxelinux.cfg/menus/base.menu

menu title Linux selections
# This is the default label
label install
 menu label Default Install Image
 kernel kernels/install.bin
 append console=ttyAMA0,38400 debug earlyprintk
 initrd initrds/uzInitrdDebInstall
# Just another label
label linux-2.6.38
 kernel kernels/linux-2.6.38.bin
 append root=/dev/sdb1
# The locally installed kernel
label local
 menu label Locally installed kernel
 append root=/dev/sdb1
 localboot 1

/tftpboot/pxelinux.cfg/default

menu include pxelinux.cfg/menus/base.menu
timeout 500
default linux-2.6.38

When a pxe client retrieves and boots the default pxe file, pxe boot will wait for user input for 5 seconds before booting the linux-2.6.38 label, which will cause /tftpboot/kernels/linux-2.6.38.bin to be downloaded, and boot with the command line root=/dev/sdb1

Differences with PXELINUX

The biggest difference between U-Boot’s pxe and PXELINUX is that since U-Boot’s pxe support is written entirely in C, it can run on any platform with network support in U-Boot. Here are some other differences between PXELINUX and U-Boot’s pxe support.

U-Boot’s pxe does not support the PXELINUX DHCP option codes specified in RFC 5071, but could be extended to do so.
when U-Boot’s pxe fails to boot, it will return control to U-Boot, allowing another command to run, other U-Boot command, instead of resetting the machine like PXELINUX.
U-Boot’s pxe doesn’t rely on or provide an UNDI/PXE stack in memory, it only uses U-Boot.
U-Boot’s pxe doesn’t provide the full menu implementation that PXELINUX does, only a simple text based menu using the commands described in this README. With PXELINUX, it’s possible to have a graphical boot menu, submenus, passwords, etc. U-Boot’s pxe could be extended to support a more robust menuing system like that of PXELINUX’s.
U-Boot’s pxe expects U-Boot uimg’s as kernels. Anything that would work with the ‘bootm’ command in U-Boot could work with the ‘pxe boot’ command.
U-Boot’s pxe only recognizes a single file on the initrd command line. It could be extended to support multiple.
in U-Boot’s pxe, the localboot command doesn’t necessarily cause a local disk boot - it will do whatever is defined in the ‘localcmd’ env variable. And since it doesn’t support a full UNDI/PXE stack, the type field is ignored.
the interactive prompt in U-Boot’s pxe only allows you to choose a label from the menu. If you want to boot something not listed, you can ctrl+c out of ‘pxe boot’ and use existing U-Boot commands to accomplish it.