Doc/syslinux

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This document is partially outdated. Please see SYSLINUX and Config.
The content of doc/syslinux.txt (release 3.72):

Contents

SYSLINUX

A suite of bootloaders for Linux
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.


SYSLINUX now has a home page at http://syslinux.zytor.com/


The SYSLINUX suite contains the following boot loaders ("derivatives"), for their respective boot media:

SYSLINUX - MS-DOS/Windows FAT filesystem
PXELINUX - PXE network booting
ISOLINUX - ISO9660 CD-ROM
EXTLINUX - Linux ext2/ext3 filesystem

For historical reasons, some of the sections in this document applies to the FAT loader only; see pxelinux.txt, isolinux.txt and extlinux.txt for what differs in these versions.

Help with cleaning up the docs would be greatly appreciated.


Options

These are the options common to all versions of Syslinux:

-s Safe, slow, stupid; uses simpler code that boots better
-f Force installing

These are only in the Windows version:

-m Mbr; install a bootable MBR sector to the beginning of the drive.
-a Active; marks the partition used active (=bootable)


CREATING A BOOTABLE LINUX FLOPPY

In order to create a bootable Linux floppy using SYSLINUX, prepare a normal MS-DOS formatted floppy. Copy one or more Linux kernel files to it, then execute the DOS command:

syslinux [-sfma][-d directory] a:

(or whichever drive letter is appropriate; the [] meaning optional.)

Use "syslinux.com" (in the dos subdirectory of the distribution) for plain DOS (MS-DOS, DR-DOS, PC-DOS, FreeDOS...) or Win9x/ME.

Use "syslinux.exe" (in the win32 subdirectory of the distribution) for WinNT/2000/XP.

Under Linux, execute the command:

syslinux [-sf][-d directory][-o offset] /dev/fd0

(or, again, whichever device is the correct one.)

This will alter the boot sector on the disk and copy a file named LDLINUX.SYS into its root directory (or a subdirectory, if the -d option is specified.)

The -s option, if given, will install a "safe, slow and stupid" version of SYSLINUX. This version may work on some very buggy BIOSes on which SYSLINUX would otherwise fail. If you find a machine on which the -s option is required to make it boot reliably, please send as much info about your machine as you can, and include the failure mode.

The -o option is used with a disk image file and specifies the byte offset of the filesystem image in the file.

For the DOS and Windows installers, the -m and -a options can be used on hard drives to write a Master Boot Record (MBR), and to mark the specific partition active.

On boot time, by default, the kernel will be loaded from the image named LINUX on the boot floppy. This default can be changed, see the section on the SYSLINUX config file.

If the Shift or Alt keys are held down during boot, or the Caps or Scroll locks are set, SYSLINUX will display a LILO-style "boot:" prompt. The user can then type a kernel file name followed by any kernel parameters. The SYSLINUX loader does not need to know about the kernel file in advance; all that is required is that it is a file located in the root directory on the disk.

There are two versions of the Linux installer; one in the "mtools" directory which requires no special privilege (other than write permission to the device where you are installing) but requires the mtools program suite to be available, and one in the "unix" directory which requires root privilege.


CONFIGURATION FILE

All the configurable defaults in SYSLINUX can be changed by putting a file called "syslinux.cfg" in the root directory of the boot disk.

This is a text file in either UNIX or DOS format, containing one or more of the following items (case is insensitive for keywords; uppercase is used here to indicate that a word should be typed verbatim):

Starting with version 3.35, the configuration file can also be in either the /boot/syslinux or /syslinux directories (searched in that order.) If that is the case, then all filenames are assumed to be relative to that same directory, unless preceded with a slash or backslash.

All options here applies to PXELINUX, ISOLINUX and EXTLINUX as well as SYSLINUX unless otherwise noted. See the respective .txt files.

# comment

A comment line. The whitespace after the hash mark is mandatory.

INCLUDE filename

Inserts the contents of another file at this point in the configuration file. Files can currently be nested up to 16 levels deep, but it is not guaranteed that more than 8 levels will be supported in the future.

DEFAULT kernel options...

Sets the default command line. If SYSLINUX boots automatically, it will act just as if the entries after DEFAULT had been typed in at the "boot:" prompt.
If no configuration file is present, or no DEFAULT entry is present in the config file, the default is "linux auto".
NOTE: Earlier versions of SYSLINUX used to automatically append the string "auto" to whatever the user specified using the DEFAULT command. As of version 1.54, this is no longer true, as it caused problems when using a shell as a substitute for "init." You may want to include this option manually.

APPEND options...

Add one or more options to the kernel command line. These are added both for automatic and manual boots. The options are added at the very beginning of the kernel command line, usually permitting explicitly entered kernel options to override them. This is the equivalent of the LILO "append" option.

IPAPPEND flag_val [PXELINUX only]

The IPAPPEND option is available only on PXELINUX. The flag_val is an OR of the following
1: indicates that an option of the following format should be generated and added to the kernel command line:
ip=<client-ip>:<boot-server-ip>:<gw-ip>:<netmask>
... based on the input from the DHCP/BOOTP or PXE boot server.
THE USE OF THIS OPTION IS NOT RECOMMENDED. If you have to use it, it is probably an indication that your network configuration is broken. Using just "ip=dhcp" on the kernel command line is a preferrable option, or, better yet, run dhcpcd/dhclient, from an initrd if necessary.
2: indicates that an option of the following format should be generated and added to the kernel command line:
BOOTIF=<hardware-address-of-boot-interface>
... in dash-separated hexadecimal with leading hardware type
(same as for the configuration file; see pxelinux.txt.)
This allows an initrd program to determine from which interface the system booted.

LABEL label

KERNEL image
APPEND options...
IPAPPEND flag_val [PXELINUX only]
Indicates that if "label" is entered as the kernel to boot, SYSLINUX should instead boot "image", and the specified APPEND and IPAPPEND options should be used instead of the ones specified in the global section of the file (before the first LABEL command.) The default for "image" is the same as "label", and if no APPEND is given the default is to use the global entry (if any).
Starting with version 3.62, the number of LABEL statements is virtually unlimited.
Note that LILO uses the syntax:
        image = mykernel
          label = mylabel
          append = "myoptions"
       ... whereas SYSLINUX uses the syntax:
        label mylabel
          kernel mykernel
          append myoptions
Note: The "kernel" doesn't have to be a Linux kernel; it can be a boot sector or a COMBOOT file (see below.)
Since version 3.32 label names are no longer mangled into DOS format (for SYSLINUX.)
The following commands are available after a LABEL statement:
LINUX image - Linux kernel image (default)
BOOT image - Bootstrap program (.bs, .bin)
BSS image - BSS image (.bss)
PXE image - PXE Network Bootstrap Program (.0)
FDIMAGE image - Floppy disk image (.img)
COMBOOT image - COMBOOT program (.com, .cbt)
COM32 image - COM32 program (.c32)
CONFIG image - New configuration file
Using one of these keywords instead of KERNEL forces the filetype, regardless of the filename.
CONFIG means restart the boot loader using a different configuration file.
APPEND -
Append nothing. APPEND with a single hyphen as argument in a LABEL section can be used to override a global APPEND.
LOCALBOOT type [ISOLINUX, PXELINUX]
On PXELINUX, specifying "LOCALBOOT 0" instead of a "KERNEL" option means invoking this particular label will cause a local disk boot instead of booting a kernel.
The argument 0 means perform a normal boot. The argument 4 will perform a local boot with the Universal Network Driver Interface (UNDI) driver still resident in memory. Finally, the argument 5 will perform a local boot with the entire PXE stack, including the UNDI driver, still resident in memory. All other values are undefined. If you don't know what the UNDI or PXE stacks are, don't worry -- you don't want them, just specify 0.
On ISOLINUX, the "type" specifies the local drive number to boot from; 0x00 is the primary floppy drive and 0x80 is the primary hard drive. The special value -1 causes ISOLINUX to report failure to the BIOS, which, on recent BIOSes, should mean that the next boot device in the boot sequence should be activated.
INITRD initrd_file
Starting with version 3.71, an initrd can be specified in a separate statement (INITRD) instead of as part of the APPEND statement; this functionally appends "initrd=initrd_file" to the kernel command line.

IMPLICIT flag_val

If flag_val is 0, do not load a kernel image unless it has been explicitly named in a LABEL statement. The default is 1.

ALLOWOPTIONS flag_val

If flag_val is 0, the user is not allowed to specify any arguments on the kernel command line. The only options recognized are those specified in an APPEND statement. The default is 1.

TIMEOUT timeout

Indicates how long to wait at the boot: prompt until booting automatically, in units of 1/10 s. The timeout is cancelled as soon as the user types anything on the keyboard, the assumption being that the user will complete the command line already begun. A timeout of zero will disable the timeout completely, this is also the default.

TOTALTIMEOUT timeout

Indicates how long to wait until booting automatically, in units of 1/10 s. This timeout is not cancelled by user input, and can thus be used to deal with serial port glitches or "the user walked away" type situations. A timeout of zero will disable the timeout completely, this is also the default.
Both TIMEOUT and TOTALTIMEOUT can be used together, for example:
		# Wait 5 seconds unless the user types something, but
		# always boot after 15 minutes.
		TIMEOUT 50
		TOTALTIMEOUT 9000

ONTIMEOUT kernel options...

Sets the command line invoked on a timeout. Normally this is the same thing as invoked by "DEFAULT". If this is specified, then "DEFAULT" is used only if the user presses <Enter> to boot.

ONERROR kernel options...

If a kernel image is not found (either due to it not existing, or because IMPLICIT is set), run the specified command. The faulty command line is appended to the specified options, so if the ONERROR directive reads as:
ONERROR xyzzy plugh
... and the command line as entered by the user is:
foo bar baz
... SYSLINUX will execute the following as if entered by the user:
xyzzy plugh foo bar baz

SERIAL port [[baudrate] flowcontrol]

Enables a serial port to act as the console. "port" is a number (0 = /dev/ttyS0 = COM1, etc.) or an I/O port address (e.g. 0x3F8); if "baudrate" is omitted, the baud rate defaults to 9600 bps. The serial parameters are hardcoded to be 8 bits, no parity, 1 stop bit.
"flowcontrol" is a combination of the following bits:
0x001 - Assert DTR
0x002 - Assert RTS
0x010 - Wait for CTS assertion
0x020 - Wait for DSR assertion
0x040 - Wait for RI assertion
0x080 - Wait for DCD assertion
0x100 - Ignore input unless CTS asserted
0x200 - Ignore input unless DSR asserted
0x400 - Ignore input unless RI asserted
0x800 - Ignore input unless DCD asserted
All other bits are reserved.
Typical values are:
    0 - No flow control (default)
0x303 - Null modem cable detect
0x013 - RTS/CTS flow control
0x813 - RTS/CTS flow control, modem input
0x023 - DTR/DSR flow control
0x083 - DTR/DCD flow control
For the SERIAL directive to be guaranteed to work properly, it should be the first directive in the configuration file.
NOTE: "port" values from 0 to 3 means the first four serial ports detected by the BIOS. They may or may not correspond to the legacy port values 0x3F8, 0x2F8, 0x3E8, 0x2E8.

CONSOLE flag_val

If flag_val is 0, disable output to the normal video console.
If flag_val is 1, enable output to the video console (this is the default.)
Some BIOSes try to forward this to the serial console and sometimes make a total mess thereof, so this option lets you disable the video console on these systems.

FONT filename

Load a font in .psf format before displaying any output (except the copyright line, which is output as ldlinux.sys itself is loaded.) SYSLINUX only loads the font onto the video card; if the .psf file contains a Unicode table it is ignored. This only works on EGA and VGA cards; hopefully it should do nothing on others.

KBDMAP keymap

Install a simple keyboard map. The keyboard remapper used is very simplistic (it simply remaps the keycodes received from the BIOS, which means that only the key combinations relevant in the default layout -- usually U.S. English -- can be mapped) but should at least help people with AZERTY keyboard layout and the locations of = and , (two special characters used heavily on the Linux kernel command line.)
The included program keytab-lilo.pl from the LILO distribution can be used to create such keymaps. The file keytab-lilo.txt contains the documentation for this program.

DISPLAY filename

Displays the indicated file on the screen at boot time (before the boot: prompt, if displayed). Please see the section below on DISPLAY files.
NOTE: If the file is missing, this option is simply ignored.

SAY message

Prints the message on the screen.

PROMPT flag_val

If flag_val is 0, display the boot: prompt only if the Shift or Alt key is pressed, or Caps Lock or Scroll lock is set (this is the default). If flag_val is 1, always display the boot: prompt.

NOESCAPE flag_val

If flag_val is set to 1, ignore the Shift/Alt/Caps Lock/Scroll Lock escapes. Use this (together with PROMPT 0) to force the default boot alternative.

NOCOMPLETE flag_val

If flag_val is set to 1, the Tab key does not display labels at the boot: prompt.

F1 filename
F2 filename

...etc...

F9 filename
F10 filename
F11 filename
F11 filename

Displays the indicated file on the screen when a function key is pressed at the boot: prompt. This can be used to implement pre-boot online help (presumably for the kernel command line options.) Please see the section below on DISPLAY files.
When using the serial console, press <Ctrl-F><digit> to get to the help screens, e.g. <Ctrl-F><2> to get to the F2 screen.
For F10-F12, hit <Ctrl-F><A>, <Ctrl-F>B, <Ctrl-F>C'. For compatiblity with earlier versions, F10 can also be entered as <Ctrl-F>0.

Blank lines are ignored.

Note that the configuration file is not completely decoded. Syntax different from the one described above may still work correctly in this version of SYSLINUX, but may break in a future one.

DISPLAY FILE FORMAT

DISPLAY and function-key help files are text files in either DOS or UNIX format (with or without <CR>). In addition, the following special codes are interpreted:

< FF >
< FF > = <Ctrl-L> = ASCII 12

Clear the screen, home the cursor. Note that the screen is filled with the current display color.

< SI ><bg><fg>
< SI > = <Ctrl-O> = ASCII 15

Set the display colors to the specified background and foreground colors, where <bg> and <fg> are hex digits, corresponding to the standard PC display attributes:
        0 = black               8 = dark grey
        1 = dark blue           9 = bright blue
        2 = dark green          a = bright green
        3 = dark cyan           b = bright cyan
        4 = dark red            c = bright red
        5 = dark purple         d = bright purple
        6 = brown               e = yellow
        7 = light grey          f = white
Picking a bright color (8-f) for the background results in the corresponding dark color (0-7), with the foreground flashing.
Colors are not visible over the serial console.

< CAN >filename<newline>
< CAN > = <Ctrl-X> = ASCII 24

If a VGA display is present, enter graphics mode and display the graphic included in the specified file. The file format is an ad hoc format called LSS16; the included Perl program "ppmtolss16" can be used to produce these images. This Perl program also includes the file format specification.
The image is displayed in 640x480 16-color mode. Once in graphics mode, the display attributes (set by <SI> code sequences) work slightly differently: the background color is ignored, and the foreground colors are the 16 colors specified in the image file. For that reason, ppmtolss16 allows you to specify that certain colors should be assigned to specific color indices.
Color indices 0 and 7, in particular, should be chosen with care: 0 is the background color, and 7 is the color used for the text printed by SYSLINUX itself.

< EM >
< EM > = <Ctrl-Y> = ASCII 25

If we are currently in graphics mode, return to text mode.

< DLE > .. < ETB >
<Ctrl-P>..<Ctrl-W> = ASCII 16-23

These codes can be used to select which modes to print a certain part of the message file in. Each of these control characters select a specific set of modes (text screen, graphics screen, serial port) for which the output is actually displayed:
	Character			Text	Graph	Serial
	------------------------------------------------------
	<DLE> = <Ctrl-P> = ASCII 16	No	No	No
	<DC1> = <Ctrl-Q> = ASCII 17	Yes	No	No
	<DC2> = <Ctrl-R> = ASCII 18	No	Yes	No
	<DC3> = <Ctrl-S> = ASCII 19	Yes	Yes	No
	<DC4> = <Ctrl-T> = ASCII 20	No	No	Yes
	<NAK> = <Ctrl-U> = ASCII 21	Yes	No	Yes
	<SYN> = <Ctrl-V> = ASCII 22	No	Yes	Yes
	<ETB> = <Ctrl-W> = ASCII 23	Yes	Yes	Yes
For example:
	<DC1>Text mode<DC2>Graphics mode<DC4>Serial port<ETB>
... will actually print out which mode the console is in!

< SUB >
< SUB > = <Ctrl-Z> = ASCII 26

End of file (DOS convention).

< BEL >
< BEL > = <Ctrl-G> = ASCII 7

Beep the speaker.


COMMAND LINE KEYSTROKES

The command line prompt supports the following keystrokes:

<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


COMBOOT IMAGES AND OTHER OPERATING SYSTEMS

This version of SYSLINUX supports chain loading of other operating systems (such as MS-DOS and its derivatives, including Windows 95/98), as well as COMBOOT-style standalone executables (a subset of DOS .COM files; see separate section below.)

Chain loading requires the boot sector of the foreign operating system to be stored in a file in the root directory of the filesystem. Because neither Linux kernels, boot sector images, nor COMBOOT files have reliable magic numbers, SYSLINUX will look at the file extension. The following extensions are recognized (case insensitive):

  none or other	Linux kernel image
  .0			PXE bootstrap program (NBP) [PXELINUX only]
  .bin			"CD boot sector" [ISOLINUX only]
  .bs			Boot sector [SYSLINUX only]
  .bss			Boot sector, DOS superblock will be patched in [SYSLINUX only]
  .c32			COM32 image (32-bit COMBOOT)
  .cbt			COMBOOT image (not runnable from DOS)
  .com			COMBOOT image (runnable from DOS)
  .img			Disk image [ISOLINUX only]

For filenames given on the command line, SYSLINUX will search for the file by adding extensions in the order listed above if the plain filename is not found. Filenames in KERNEL statements must be fully qualified.

If this is specified with one of the keywords LINUX, BOOT, BSS, FDIMAGE, COMBOOT, COM32, or CONFIG instead of KERNEL, the filetype is considered to be the one specified regardless of the filename.


BOOTING DOS (OR OTHER SIMILAR OPERATING SYSTEMS)

This section applies to SYSLINUX only, not to PXELINUX or ISOLINUX. See isolinux.txt for an equivalent procedure for ISOLINUX.

This is the recommended procedure for creating a SYSLINUX disk that can boot either DOS or Linux. This example assumes the drive is A: in DOS and /dev/fd0 in Linux; for other drives, substitute the appropriate drive designator.

Linux procedure

1.

Make a DOS bootable disk. This can be done either by specifying the /s option when formatting the disk in DOS, or by running the DOS command SYS (this can be done under DOSEMU if DOSEMU has direct device access to the relevant drive):
format a: /s
or
sys a:

2.

Boot Linux. Copy the DOS boot sector from the disk into a file:
dd if=/dev/fd0 of=dos.bss bs=512 count=1

3.

Run SYSLINUX on the disk:
syslinux /dev/fd0

4.

Mount the disk and copy the DOS boot sector file to it. The file must have extension .bss:
mount -t msdos /dev/fd0 /mnt
cp dos.bss /mnt

5.

Copy the Linux kernel image(s), initrd(s), etc to the disk, and create/edit syslinux.cfg and help files if desired:
cp vmlinux /mnt
cp initrd.gz /mnt

6.

Unmount the disk (if applicable.)
umount /mnt

DOS/Windows procedure

To make this installation in DOS only, you need the utility copybs.com (included with SYSLINUX) as well as the syslinux.com installer. If you are on an WinNT-based system (WinNT, Win2k, WinXP or later), use syslinux.exe instead.

1.

Make a DOS bootable disk. This can be done either by specifying the /s option when formatting the disk in DOS, or by running the DOS command SYS:
format a: /s
or
sys a:

2.

Copy the DOS boot sector from the disk into a file. The file must have extension .bss:
copybs a: a:dos.bss

3.

Run SYSLINUX on the disk:
syslinux a:

4.

Copy the Linux kernel image(s), initrd(s), etc to the disk, and create/edit syslinux.cfg and help files if desired:
copy vmlinux a:
copy initrd.gz a:


COMBOOT EXECUTABLES

SYSLINUX supports simple standalone programs, using a file format similar to DOS ".com" files. A 32-bit version, called COM32, is also provided. A simple API provides access to a limited set of filesystem and console functions.

See the file comboot.txt for more information on COMBOOT and COM32 programs.


NOVICE PROTECTION

SYSLINUX will attempt to detect booting on a machine with too little memory, which means the Linux boot sequence cannot complete. If so, a message is displayed and the boot sequence aborted. Holding down the Ctrl key while booting disables this feature.

Any file that SYSLINUX uses can be marked hidden, system or readonly if so is convenient; SYSLINUX ignores all file attributes. The SYSLINUX installer automatically sets the readonly/hidden/system attributes on LDLINUX.SYS.


NOTES ON BOOTABLE CD-ROMS

SYSLINUX can be used to create bootdisk images for El Torito-compatible bootable CD-ROMs. However, it appears that many BIOSes are very buggy when it comes to booting CD-ROMs. Some users have reported that the following steps are helpful in making a CD-ROM that is bootable on the largest possible number of machines:

a) Use the -s (safe, slow and stupid) option to SYSLINUX;
b) Put the boot image as close to the beginning of the ISO 9660 filesystem as possible.

A CD-ROM is so much faster than a floppy that the -s option shouldn't matter from a speed perspective.

Of course, you probably want to use ISOLINUX instead. See isolinux.txt.


BOOTING FROM A FAT FILESYSTEM PARTITION ON A HARD DISK

SYSLINUX can boot from a FAT filesystem partition on a hard disk (including FAT32). The installation procedure is identical to the procedure for installing it on a floppy, and should work under either DOS or Linux. To boot from a partition, SYSLINUX needs to be launched from a Master Boot Record or another boot loader, just like DOS itself would.

Under DOS, you can install a standard simple MBR on the primary hard disk by running the command:

FDISK /MBR

Then use the FDISK command to mark the appropriate partition active.

A simple MBR, roughly on par with the one installed by DOS (but unencumbered), is included in the SYSLINUX distribution. To install it under Linux, simply type:

cat mbr.bin > /dev/XXX

... where /dev/XXX is the device you wish to install it on.

Under DOS or Win32, you can install the SYSLINUX MBR with the -m option to the SYSLINUX installer, and use the -a option to mark the current partition active:

syslinux -ma c:

Note that this will also install SYSLINUX on the specified partition.


HARDWARE INFORMATION

I have started to maintain a web page of hardware with known problems. There are, unfortunately, lots of broken hardware out there; especially early PXE stacks (for PXELINUX) have lots of problems.

A list of problems, and workarounds (if known), is maintained at:

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


BOOT LOADER IDS USED

The Linux boot protocol supports a "boot loader ID", a single byte where the upper nybble specifies a boot loader family (3 = SYSLINUX) and the lower nybble is version or, in the case of SYSLINUX, media:

0x31 (49) = SYSLINUX
0x32 (50) = PXELINUX
0x33 (51) = ISOLINUX
0x34 (52) = EXTLINUX

In recent versions of Linux, this ID is available as /proc/sys/kernel/bootloader_type.


BUG REPORTS

I would appreciate hearing of any problems you have with SYSLINUX. I would also like to hear from you if you have successfully used SYSLINUX, especially if you are using it for a distribution.

If you are reporting problems, please include all possible information about your system and your BIOS; the vast majority of all problems reported turn out to be BIOS or hardware bugs, and I need as much information as possible in order to diagnose the problems.

There is a mailing list for discussion among SYSLINUX users and for announcements of new and test versions. To join, or to browse the archive, go to:

http://www.zytor.com/mailman/listinfo/syslinux

Please DO NOT send HTML messages or attachments to the mailing list (including multipart/alternative or similar.) All such messages will be bounced.

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