A suite of bootloaders for Linux
Copyright (C) 1994-2005 H. Peter Anvin
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.doc, isolinux.doc and extlinux.doc for what differs in these versions.
Help with cleaning up the docs would be greatly appreciated.
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.
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; upper case 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 .doc files:
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.
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.
The IPAPPEND option is available only on PXELINUX. The flag_val is an OR of the following options:
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.doc.)
This allows an initrd program to determine from which interface the system booted.
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 2.20, LABEL statements are compressed internally, therefore the maximum number of LABEL statements depends on their complexity. Typical is around 600. SYSLINUX will print an error message if the internal memory for labels is overrun.
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.)
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.
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.
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.
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.
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
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.
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
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.
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.
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.
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.doc contains the documentation for this program.
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.
Prints the message on the screen.
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.
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.
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 indicies.
Color indicies 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.
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
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.
This section applies to SYSLINUX only, not to PXELINUX or ISOLINUX. See isolinux.doc 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.
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
Boot Linux. Copy the DOS boot sector from the disk into a file:
dd if=/dev/fd0 of=dos.bss bs=512 count=1
Run SYSLINUX on the disk:
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
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
Unmount the disk (if applicable.):
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:
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
Copy the DOS boot sector from the disk into a file. The file must have extension .bss:
copybs a: a:dos.bss
Run SYSLINUX on the disk:
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:
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.doc for more information on COMBOOT and COM32 programs.
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 installed automatically sets the readonly/hidden/system attributes on LDLINUX.SYS.
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:
Use the -s (safe, slow and stupid) option to SYSLINUX;
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.doc.
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:
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.
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:
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.
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:
Please DO NOT send HTML messages or attachments to the mailing list (including multipart/alternative or similar.) All such messages will be bounced.