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Boot Loader for Linux


syslinux.cfg - *Syslinux* configuration file


Configuration for the boot behavior and user experience of Syslinux boot loaders, the format of display files and the boot prompt behavior.
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.


SYSLINUX (before 4.00) used the configuration filename of syslinux.cfg. EXTLINUX (merged into SYSLINUX as of 4.00) used the filename extlinux.conf. Both default to searching for the config file in the installed directory (containing ldlinux.sys/extlinux.sys). As of 4.00, SYSLINUX will search for extlinux.conf then syslinux.cfg in each directory before falling back to the next directory.
As of 3.35, SYSLINUX also searches /boot/syslinux, /syslinux and /.
ISOLINUX (before 4.02) used the configuration filename of isolinux.cfg, searching /boot/isolinux (starting 2.00), then /isolinux and /. As of 4.02, ISOLINUX will search for isolinux.cfg then syslinux.cfg in /boot/isolinux before searching for the same files in /isolinux, /boot/syslinux, /syslinux, and /.


# comment
A line comment. As of version 3.10, the space between the # and the comment is no longer required.
MENU any string
(3.00+) A directive for the simple menu system, treated as a comment outside the menu. See menu.txt.
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 (which often references a LABEL). If Syslinux boots automatically, it will act just as if the entries after DEFAULT had been typed in at the boot: prompt. Multiple uses will result in an override.
If no configuration file is present, or no DEFAULT or UI entry is present in the config file, an error message is displayed and the boot: prompt is shown (3.85+).
UI module options...
Selects a specific user interface module (typically menu.c32 or vesamenu.c32). The command-line interface treats this as a directive that overrides the DEFAULT directive to load this module instead at startup, for an empty command line and at timeout and PROMPT directive to not prompt (but these directives may have effects on other configuration parsers). Multiple uses will result in an override.
LABEL mylabel
Begin a new LABEL clause. If mylabel is entered as the kernel to boot, Syslinux should instead boot "image" (specified by a directive from KERNEL-LIKE DIRECTIVES) with any specified DUAL-PURPOSE DIRECTIVES being used instead of the global instance.
mylabel must be unique. Currently the first instance is used but may result in an error or undesired behavior. mylabel ends at the first character that is not a non-white-space printable character and should be restricted to non-white-space typeable characters. Prior to version 3.32, this would transformed to a DOS compatible format of 8.3 with a restricted character set. A LABEL clause must contain exactly 1 of the KERNEL-LIKE DIRECTIVES and may contain 1 each of the LABEL-ONLY DIRECTIVES or DUAL-PURPOSE DIRECTIVES.
Within a LABEL, using multiple KERNEL-LIKE DIRECTIVES or reuse of LABEL-ONLY DIRECTIVES or DUAL-PURPOSE DIRECTIVES will result in an override. Otherwise, multiple instances of the same directive will result in the last being effective.


Use of any of the DUAL-PURPOSE DIRECTIVES as GLOBAL DIRECTIVES is discouraged if there will be any non-Linux images loaded as ALL images will get these, including those manually entered at the boot: prompt.
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.
Use of the parameter initrd= supports multiple filenames separated by commas (ie initrd=initrd_file1,initrd_file2) within a single instance. This is mostly useful for initramfs, which can be composed of multiple separate cpio or cpio.gz archives.
Note: all initrd files except the last one are zero-padded to a 4K page boundary. This should not affect initramfs.
Note: Only the last effective initrd= parameter is used for loading initrd files.
Append nothing. APPEND with a single hyphen as argument in a LABEL section can be used to override a global APPEND.
SYSAPPEND bitmask, IPAPPEND bitmask
(SYSAPPEND: 5.10+; IPAPPEND: PXELINUX only) The SYSAPPEND option was introduced in Syslinux 5.10; it is an enhancement of a previous option IPAPPEND which was only available on PXELINUX. bitmask is interpreted as decimal format unless prefixed with "0x" for hexadecimal or "0" (zero) for octal. The bitmask is an OR (sum) of the following integer options:
An option of the following format should be generated, based on the input from the DHCP/BOOTP or PXE boot server and added to the kernel command line(see note below; empty for non-PXELINUX variants):
The use of option 1 is no substitute for running a DHCP client in the booted system and should instead only be used to seed the client for a request. Without regular renewals, the lease acquired by the PXE BIOS will expire, making the IP address available for reuse by the DHCP server.
An option of the following format should be generated, in dash-separated hexadecimal with leading hardware type (same as for the configuration file; see pxelinux.txt.) and added to the kernel command line, allowing an initrd program to determine from which interface the system booted(empty for non-PXELINUX variants):
An option of the following format should be generated, in lower case hexadecimal in the format normally used for UUIDs (same as for the configuration file; see pxelinux.txt.) and added to the kernel command line:
SYSUUID=<system uuid>
(5.10+) indicate the CPU family and certain particularly significant CPU feature bits:
The <family> is a single digit from 3 (i386) to 6 (i686 or higher.) The following CPU features are currently reported; additional flags may be added in the future:
P       Physical Address Extension (PAE)
V       Intel Virtualization Technology (VT/VMX)
T       Intel Trusted Exection Technology (TXT/SMX)
X       Execution Disable (XD/NX)
L       Long Mode (x86-64)
S       AMD SMX virtualization
(5.10+) The following strings are derived from DMI/SMBIOS information if available:
Bit     String          Significance
0x00010 SYSVENDOR=      System vendor name
0x00020 SYSPRODUCT=     System product name
0x00040 SYSVERSION=     System version
0x00080 SYSSERIAL=      System serial number
0x00100 SYSSKU=         System SKU
0x00200 SYSFAMILY=      System family
0x00400 MBVENDOR=       Motherboard vendor name
0x00800 MBPRODUCT=      Motherboard product name
0x01000 MBVERSION=      Motherboard version
0x02000 MBSERIAL=       Motherboard serial number
0x04000 MBASSET=        Motherboard asset tag
0x08000 BIOSVENDOR=     BIOS vendor name
0x10000 BIOSVERSION=    BIOS version
0x20000 SYSFF=          System form factor
If these strings contain white-space characters, they are replaced with underscores (_).
The system form factor value is a number defined in the SMBIOS specification, available at As of version 2.7.1 of the specification, the following values are defined:
 1     Other
 2     Unknown
 3     Desktop
 4     Low profile desktop
 5     Pizza box
 6     Mini tower
 7     Tower
 8     Portble
 9     Laptop
10     Notebook
11     Handheld
12     Docking station
13     All-in-one
14     Subnotebook
15     Space-saving
16     Lunch box
17     Main server chassis
18     Expansion chassis
19     Subchassis
20     Bus expansion chassis
21     Peripheral chassis
22     RAID chassis
23     Rack mount chasss
24     Sealed-case PC
25     Multi-system chassis
26     Compact PCI
27     Advanced TCI
28     Blade
29     Blade enclosure
(6.03+) Append a file system UUID string. For EXT2/3/4, this is the typical file system UUID. For FAT12/16/32, this is the 32-bit file system serial number (ie DA1A-0B2E).


KERNEL image
Load a kernel-like file image with automatic filetype detection based on file extension, listed under the non-auto-detecting directives, defaulting to LINUX.
LINUX is used as an example] LINUX image:: Load image as a Linux-like kernel. MEMDISK is an example of a non-Linux kernel loaded in a Linux-like fashion.
BOOT image
(ISOLINUX only: .bin; SYSLINUX only: .bs) Load a boot sector. .bin is a "CD boot sector" and .bs is a regular disk boot sector.
BSS image
(SYSLINUX only: .bss) Load a BSS image, a .bs image with the DOS superblock patched in.
(.com, .cbt; Removed as of 5.00) Load a Syslinux COMBOOT image. .com images may also be runnable from DOS while .cbt images are not. See also comboot.txt
COM32 image
(.c32) Load a Syslinux COM32 (32-bit COMBOOT) image. See also comboot.txt
CONFIG image
Load a new configuration file. The configuration file is read, the working directory is changed (if specified via an APPEND), then the configuration file is parsed.
(Removed as of 4.05, added 1.65; ISOLINUX only: .img) Load a disk image.
(PXELINUX 1.53+; ISOLINUX ??3.10+; SYSLINUX 3.70+)Attempt a different local boot method. The special value -1 causes the boot loader to report failure to the BIOS, which, on recent BIOSes, should mean that the next boot device in the boot sequence should be activated. Values other than those documented may produce undesired results.
On PXELINUX, type 0 means perform a normal boot. type 4 will perform a local boot with the Universal Network Driver Interface (UNDI) driver still resident in memory. Finally, type 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\(cqt know what the UNDI or PXE stacks are, don\(cqt worry — you don\(cqt want them, just specify 0.
On ISOLINUX/SYSLINUX, the type specifies the local drive number to boot from; 0x00 is the primary floppy drive and 0x80 is the primary hard drive.
PXE image
(PXELINUX only: .0) Load a PXE NBP (Network Boot Program) image. The PXE protocol does not provide any means for specifiying or using a command line or initrd.


INITRD initrd_file
(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. Like initrd=, this also supports multiple comma separated file names (see APPEND).


These are global directives that are of lesser importance, often affecting the user experience and not the boot process.
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.
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.
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. The timer is reset to 0 upon return from an unsuccessful attempt to boot or from a module. A timeout of zero (the default) will disable the timeout completely.
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 (the default) will disable the timeout completely.
Both TIMEOUT and TOTALTIMEOUT can be used together, for example:
# Wait 5 seconds unless the user types something, but
# always boot after 15 minutes.
ONTIMEOUT kernel options...
Sets the command line invoked on a timeout (which often references a LABEL). If not specified, UI (if used) or DEFAULT is used.
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
0x008 - Enable interrupts
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.
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.
Enabling interrupts (setting the 0x008 bit) may give better responsiveness without setting the NOHALT option, but could potentially cause problems with buggy BIOSes.
This option is "sticky" and is not automatically reset when loading a new configuration file with the CONFIG command.
NOHALT flag_val
If flag_val is 1, don\(cqt halt the processor while idle. Halting the processor while idle significantly reduces the power consumption, but can cause poor responsiveness to the serial console, especially when using scripts to drive the serial console, as opposed to human interaction.
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 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.
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.
If flag_val is set to 1, the Tab key does not display labels at the boot: prompt.
F1 filename, F2 filename, F3 filename, F4 filename, F5 filename, F6 filename, F7 filename, F8 filename, F9 filename, F10 filename, F11 filename, F12 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 compatibility with earlier versions, F10 can also be entered as <Ctrl-F>0.
PATH path
(5.00+) Specify a space-separated ( ; 5.00-5.10 was a colon :) list of directories to search when attempting to load modules. This directive is useful for specifying the directories containing the lib*.c32 library files as other modules may be dependent on these files, but may not reside in the same directory. Multiple instances will append additional paths.
(PXELINUX 5.10+) When downloading files over http, the SYSAPPEND strings are prepended with Syslinux and sent to the server as cookies. The cookies are URL-encoded; whitespace is not replaced with underscores.
This command limits the cookies send; 0 means no cookies. The default is -1, meaning send all cookies.
This option is "sticky" and is not automatically reset when loading a new configuration file with the CONFIG command.


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:
identical to #3
= <Ctrl-L> = ASCII 12
Clear the screen, home the cursor. Note that the screen is filled with the current display color.
= <Ctrl-L> = ASCII 12; Clear the screen, home the cursor. Note that the screen is filled with the current display color.
<FF> = <Ctrl-L> = ASCII 12
Clear the screen, home the cursor. Note that the screen is filled with the current display color.
<FF> = <Ctrl-L> = ASCII 12
Clear the screen, home the cursor. Note that the screen is filled with the current display color.
<SI> = <Ctrl-O> = ASCII 15
Set the display colors to the specified background and foreground colors, where <bg> and <fg> are the 2 hex digits representing 1 byte, 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> = <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> = <Ctrl-Y> = ASCII 25
If we are currently in graphics mode, return to text mode.
<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, the following will actually print out which mode the console is in:
<DC1>Text mode<DC2>Graphics mode<DC4>Serial port<ETB>
<SUB> = <Ctrl-Z> = ASCII 26
End of file (DOS convention).
<BEL> = <Ctrl-G> = ASCII 7
Beep the speaker.


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.


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.


Here are some sample config files:
# SERIAL 0 115200

LABEL linux LINUX vmlinuz APPEND initrd=initrd1.gz,initrd2.gz
LABEL m COM32 menu.c32
In this example, serial port use is disabled but can be enabled by uncommenting the first line and utilize serial port 0 at 115200 bps. If linux is typed on the command line, the kernel-like file vmlinuz is executed as a Linux kernel, initrd files initrd1.gz and initrd2.gz are loaded as initial ramdisk files (like cpio.gz files for initramfs). If m is typed on the command line, the COM32 module menu.c32 is executed to launch a menu system.


Several known bugs/common problems are listed at and known hardware compatibility issues are listed at with filename translation difficulty and early PXE stacks being some of the most common. Reporting of other encountered issues is welcome and appreciated.


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.


H. Peter Anvin
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