Syslinux

Syslinux is a collection of boot loaders capable of booting from hard drives, CDs, and over the network via PXE. It supports the FAT, ext2, ext3, ext4, and Btrfs file systems.

Note: Since Syslinux 4, Extlinux and Syslinux are the same thing.

Note: Syslinux UEFI support is currently present only in upstream git repo and is of alpha quality. See UEFI_Bootloaders#SYSLINUX for more info.

Syslinux Boot Process

1. Load MBR. At boot, the computer loads the MBR (/usr/lib/syslinux/mbr.bin).
2. Search active partition. The MBR looks for the partition that is marked as active (boot flag).
3. Execute volume boot record. Once found, the volume boot record (VBR) will be executed. In the case of ext2/3/4 and FAT12/16/32, the starting sector of ldlinux.sys is hard-coded into the VBR.
4. Execute ldlinux.sys. The VBR will execute (ldlinux.sys). Therefore, if the location of ldlinux.sys changes, Syslinux will no longer boot. (In the case of Btrfs, the above method will not work since files move around resulting in the sector location of ldlinux.sys changing. Therefore, the entire Syslinux code needs to be stored outside the filesystem. The code is stored in the sectors following the VBR.)
5. Search configuration file. Once Syslinux is fully loaded, it looks for a configuration file, either extlinux.conf or syslinux.cfg.
6. Load configuration. If one is found, the configuration file is loaded. If no configuration file is found, you will be given a Syslinux prompt.

Installation

Install syslinux from the official repositories.

Automatic Install

The syslinux-install_update script will install Syslinux, copy/symlink *.c32 modules to /boot/syslinux, set the boot flag and install the boot code in the MBR. It can handle MBR and GPT disks along with software RAID.

1. If you use a separate boot partition make sure that it is mounted. Check with lsblk; if you don't see a /boot mountpoint, mount it before you go any further.

2. Run syslinux-install_update with flags: -i (install the files), -a (mark the partition active with the boot flag), -m (install the MBR boot code):

# syslinux-install_update -i -a -m

3. Edit /boot/syslinux/syslinux.cfg.

Note: For this to work with GPT, the gptfdisk package is needed as the backend for setting the boot flag.

Manual Install

Note: If you are unsure of which partition table you are using (MBR or GPT), you are likely using the MBR partition table. Most of the time GPT will create a special MBR-style partition (type 0xEE) using the whole disk which will be displayed with the following command:

# fdisk -l /dev/sda

or alternatively:

# sgdisk -p /dev/sda

will show "GPT: not present" if it is not a GPT disk.

Note: If you are trying to rescue an installed system with a live CD, be sure to chroot into it before executing these commands. If you do not chroot first, you must prepend all file paths (not /dev/ paths) with the mount point.

Your boot partition, on which you plan to install Syslinux, must contain a FAT, ext2, ext3, ext4, or Btrfs file system. You should install it on a mounted directory, not a /dev/sdXY device. You do not have to install it on the root directory of a file system, e.g., with device /dev/sda1 mounted on /boot you can install Syslinux in the syslinux directory:

# mkdir /boot/syslinux
# extlinux --install /boot/syslinux 

MBR Partition Table

Next you need to mark your boot partition active in your partition table. Applications capable of doing this include fdisk, cfdisk, sfdisk, parted/gparted. It should look like this:

# fdisk -l /dev/sda

[...]
  Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048      104447       51200   83  Linux
/dev/sda2          104448   625142447   312519000   83  Linux

Install the MBR:

# dd bs=440 count=1 conv=notrunc if=/usr/lib/syslinux/mbr.bin of=/dev/sda

An alternate MBR which Syslinux provides is: altmbr.bin. This MBR does not scan for bootable partitions; instead, the last byte of the MBR is set to a value indicating which partition to boot from. Here is an example of how altmbr.bin can be copied into position:

# printf '\x5' | cat /usr/lib/syslinux/altmbr.bin - | dd bs=440 count=1 iflag=fullblock conv=notrunc of=/dev/sda

In this case, a single byte of value 5 is appended to the contents of altmbr.bin and the resulting 440 bytes are written to the MBR on device sda. Syslinux was installed on the first logical partition (/dev/sda5) of the disk.

GUID Partition Table aka GPT

Bit 2 of the attributes for the /boot partition needs to be set.

# sgdisk /dev/sda --attributes=1:set:2

This would toggle the attribute legacy BIOS bootable on partition 1. To check:

# sgdisk /dev/sda --attributes=1:show
1:2:1 (legacy BIOS bootable)

Install the MBR:

# dd bs=440 conv=notrunc count=1 if=/usr/lib/syslinux/gptmbr.bin of=/dev/sda

Rebooting

When you reboot your system now, you will have a Syslinux prompt. To automatically boot your system or get a boot menu, you still need to create a configuration file.

Configuration

The Syslinux configuration file, syslinux.cfg, should be created in the same directory where you installed Syslinux. In our case, /boot/syslinux/.

The bootloader will look for either syslinux.cfg (preferred) or extlinux.conf

Tip:

• Instead of LINUX, the keyword KERNEL can also be used. KERNEL tries to detect the type of the file, while LINUX always expects a Linux kernel.
• TIMEOUT value is in units of 1/10 of a second.

Examples

Basic Config

This is a simple configuration file that will show a boot: prompt and automatically boot after 5 seconds.

Note: The partition in question needs to be whatever you have as / (root), not /boot.

Configuration:

/boot/syslinux/syslinux.cfg

 PROMPT 1
 TIMEOUT 50
 DEFAULT arch
 
 LABEL arch
         LINUX ../vmlinuz-linux
         APPEND root=/dev/sda2 ro
         INITRD ../initramfs-linux.img
 
 LABEL archfallback
         LINUX ../vmlinuz-linux
         APPEND root=/dev/sda2 ro
         INITRD ../initramfs-linux-fallback.img

If you want to boot directly without seeing a prompt, set PROMPT to 0.

If you want to use UUID for persistent device naming instead of device names, change the APPEND line to your equivalent UUID of the root partition:

APPEND root=UUID=978e3e81-8048-4ae1-8a06-aa727458e8ff ro

If you use encryption LUKS change the APPEND line to use your encrypted volume:

APPEND root=/dev/mapper/<group>-<name> cryptdevice=/dev/sda2:<name> ro

If you are using software RAID using mdadm, change the APPEND line to accommodate your RAID arrays. As an example the following accommodates three RAID 1 array's and sets the appropriate one as root:

APPEND root=/dev/md1 ro md=0,/dev/sda2,/dev/sdb2 md=1,/dev/sda3,/dev/sdb3 md=2,/dev/sda4,/dev/sdb4

Text Boot menu

Syslinux also allows you to use a boot menu. To use it, copy the menu COM32 module to your Syslinux directory:

# cp /usr/lib/syslinux/menu.c32 /boot/syslinux/

If /boot is in the same partition as /usr, a symlink will also work:

# ln -s /usr/lib/syslinux/menu.c32 /boot/syslinux/

Configuration:

/boot/syslinux/syslinux.cfg

 UI menu.c32
 PROMPT 0
 
 MENU TITLE Boot Menu
 TIMEOUT 50
 DEFAULT arch
 
 LABEL arch
         MENU LABEL Arch Linux
         LINUX ../vmlinuz-linux
         APPEND root=/dev/sda2 ro
         INITRD ../initramfs-linux.img
 
 LABEL archfallback
         MENU LABEL Arch Linux Fallback
         LINUX ../vmlinuz-linux
         APPEND root=/dev/sda2 ro
         INITRD ../initramfs-linux-fallback.img

For more details about the menu system, see the Syslinux documentation.

Graphical Boot menu

Syslinux also allows you to use a graphical boot menu. To use it, copy the vesamenu COM32 module to your Syslinux folder:

# cp /usr/lib/syslinux/vesamenu.c32 /boot/syslinux/

If /boot is the same partition as /, a symlink will also work:

# ln -s /usr/lib/syslinux/vesamenu.c32 /boot/syslinux/

This config uses the same menu design as the Arch Install CD. The background file can be found there too.

Configuration:

/boot/syslinux/syslinux.cfg

 UI vesamenu.c32
 DEFAULT arch
 PROMPT 0
 MENU TITLE Boot Menu
 MENU BACKGROUND splash.png
 TIMEOUT 50
 
 MENU WIDTH 78
 MENU MARGIN 4
 MENU ROWS 5
 MENU VSHIFT 10
 MENU TIMEOUTROW 13
 MENU TABMSGROW 11
 MENU CMDLINEROW 11
 MENU HELPMSGROW 16
 MENU HELPMSGENDROW 29
 
 # Refer to http://www.syslinux.org/wiki/index.php/Comboot/menu.c32
 
 MENU COLOR border       30;44   #40ffffff #a0000000 std
 MENU COLOR title        1;36;44 #9033ccff #a0000000 std
 MENU COLOR sel          7;37;40 #e0ffffff #20ffffff all
 MENU COLOR unsel        37;44   #50ffffff #a0000000 std
 MENU COLOR help         37;40   #c0ffffff #a0000000 std
 MENU COLOR timeout_msg  37;40   #80ffffff #00000000 std
 MENU COLOR timeout      1;37;40 #c0ffffff #00000000 std
 MENU COLOR msg07        37;40   #90ffffff #a0000000 std
 MENU COLOR tabmsg       31;40   #30ffffff #00000000 std
 
 
 LABEL arch
         MENU LABEL Arch Linux
         LINUX ../vmlinuz-linux
         APPEND root=/dev/sda2 ro
         INITRD ../initramfs-linux.img
 
 
 LABEL archfallback
         MENU LABEL Arch Linux Fallback
         LINUX ../vmlinuz-linux
         APPEND root=/dev/sda2 ro
         INITRD ../initramfs-linux-fallback.img

Since Syslinux 3.84, vesamenu.c32 supports the MENU RESOLUTION $WIDTH $HEIGHT directive. To use it, insert MENU RESOLUTION 1440 900 into your config for a 1440x900 resolution. The background picture has to have exactly the right resolution, however, as Syslinux will otherwise refuse to load the menu.

Auto Boot

If you don't want to see the Syslinux menu at all, comment out all UI commands and make sure there is a DEFAULT set in your syslinux.cfg.

Chainloading

If you want to chainload other operating systems (such as Windows) or boot loaders, copy (or symlink) the chain.c32 module to the Syslinux directory (for details, see the instructions in the previous section). Then create a section in the configuration file:

/boot/syslinux/syslinux.cfg

...
 LABEL windows
         MENU LABEL Windows
         COM32 chain.c32
         APPEND hd0 3
...

hd0 3 is the third partition on the first BIOS drive - drives are counted from zero, but partitions are counted from one.

If you are unsure about which drive your BIOS thinks is "first", you can instead use the MBR identifier, or if you are using GPT, the filesystem labels. To use the MBR identifier, run the command

# fdisk -l /dev/sdb

 Disk /dev/sdb: 128.0 GB, 128035676160 bytes 
 255 heads, 63 sectors/track, 15566 cylinders, total 250069680 sectors
 Units = sectors of 1 * 512 = 512 bytes
 Sector size (logical/physical): 512 bytes / 512 bytes
 I/O size (minimum/optimal): 512 bytes / 512 bytes
 Disk identifier: 0xf00f1fd3
  
 Device Boot      Start         End      Blocks   Id  System
 /dev/sdb1            2048     4196351     2097152    7  HPFS/NTFS/exFAT
 /dev/sdb2         4196352   250066943   122935296    7  HPFS/NTFS/exFAT

replacing /dev/sdb with the drive you wish to chainload. Using the hexadecimal number under Disk identifier: 0xf00f1fd3 in this case, the syntax in syslinux.cfg is

/boot/syslinux/syslinux.cfg

...
 LABEL windows
         MENU LABEL Windows
         COM32 chain.c32
         APPEND mbr:0xf00f1fd3
...

For more details about chainloading, see the Syslinux wiki.

If you have GRUB installed on the same partition, you can chainload it by using:

/boot/syslinux/syslinux.cfg

...
 LABEL grub2
        MENU LABEL Grub2
        COM32 chain.c32
        append file=../grub/boot.img
...

This may be required for booting from ISO images.

Chainloading other Linux systems

Chainloading another bootloader such as Windows' is pretty obvious, as there is a definite bootloader to chain to. But with Syslinux, it is only able to load files residing on the same partition as the configuration file. Thus, if you have another version of Linux on a separate partition, without a shared /boot, it becomes necessary to employ Extlinux. Essentially, Extlinux can be installed on the partition superblock and be called as a separate bootloader from the MBR installed by Syslinux. Extlinux is part of the Syslinux project and is included with the syslinux package.

The following instructions assume you have Syslinux installed already. These instructions will also assume that the typical Arch Linux configuration path of /boot/syslinux is being used and the chainloaded / is on /dev/sda3.

From a booted Linux (likely the partition that Syslinux is set up to boot), mount the other root partition to your desired mount point. In this example this will be /mnt. Also, if a separate /boot partition is used on the second operating system, that will also need to be mounted. The example assumes this is /dev/sda2.

# mount /dev/sda3 /mnt
# mount /dev/sda2 /mnt/boot (only necessary for separate /boot)

Install Extlinux and copy necessary *.c32 files

# extlinux -i /mnt/boot/syslinux
# cp /usr/lib/syslinux/{chain,menu}.c32 /mnt/boot/syslinux

Create /mnt/boot/syslinux/syslinux.cfg. Below is an example:

/boot/syslinux/syslinux.cfg on /dev/sda3

timeout 10

ui menu.c32


label Other Linux
    linux /boot/vmlinuz-linux
    initrd /boot/initramfs-linux.img
    append root=/dev/sda3 ro quiet


label MAIN
    com32 chain.c32
    append hd0 0

taken from Djgera's user wiki page.

Using memtest

Install memtest+ from the official repositories.

Use this LABEL section to launch memtest:

/boot/syslinux/syslinux.cfg

...
 LABEL memtest
         MENU LABEL Memtest86+
         LINUX ../memtest86+/memtest.bin
...

HDT

HDT (Hardware Detection Tool) displays hardware information. Like before, the .c32 file has to be copied or symlinked from /boot/syslinux/. For PCI info, either copy or symlink /usr/share/hwdata/pci.ids to /boot/syslinux/pci.ids and add the following to your configuration file:

/boot/syslinux/syslinux.cfg

 LABEL hdt
         MENU LABEL Hardware Info
         COM32 hdt.c32

Reboot and power off

Use the following sections to reboot or power off your machine:

/boot/syslinux/syslinux.cfg

 LABEL reboot
         MENU LABEL Reboot
         COM32 reboot.c32
 
 LABEL poweroff
         MENU LABEL Power Off
         COMBOOT poweroff.com

Clear Menu

To clear the screen when exiting the menu, add the following line:

/boot/syslinux/syslinux.cfg

 MENU CLEAR

Keyboard remapping

If you often have to edit your boot parameters, you might want to remap your keyboard layout. This allows you to enter "=", "/" and other characters easily on a non-US keyboard.

First you have to create a compatible keymap (for example a German one):

# cp /usr/share/kbd/keymaps/i386/qwerty/us.map.gz ./
# cp /usr/share/kbd/keymaps/i386/qwertz/de.map.gz ./
# gunzip {de,us}.map.gz
# mv de.{,k}map
# mv us.{,k}map
# keytab-lilo de > de.ktl

Copy de.ktl as root to /boot/syslinux/ and set ownership to root:

# chown root:root /boot/syslinux/de.ktl

Now edit syslinux.conf and add:

/boot/syslinux/syslinux.cfg

 KBDMAP de.ktl

Hiding the menu

Use the option:

/boot/syslinux/syslinux.cfg

 MENU HIDDEN

to hide the menu while displaying only the timeout. Press any key to bring up the menu.

Troubleshooting

Using the Syslinux prompt

You can type in the LABEL name of the entry that you want to boot (as per your syslinux.cfg). If you used the example configurations, just type:

boot: arch

If you get an error that the configuration file could not be loaded, you can pass your needed boot parameters, e.g.:

boot: ../vmlinuz-linux root=/dev/sda2 ro initrd=../initramfs-linux.img

If you do not have access to boot: in ramfs, and therefore temporarily unable to boot kernel again,

1. Create a temporary directory, in order to mount your root partition (if it does not exist already):

 # mkdir -p /new_root

2. Mount / under /new_root (in case /boot/ is on the same partition, otherwise you will need to mount them both):

Note: If /boot is on its own ext2 partition then busybox cannot mount it.

 # mount /dev/sd[a-z][1-9] /new_root

3. Use vim and edit syslinux.cfg again to suit your needs and save file.

4. Reboot.

No Default or UI found on some computers

Certain motherboard manufacturers have less compatibility for booting from USB devices than others. While an ext4 formatted USB drive may boot on a more recent computer, some computers may hang if the boot partition containing the kernel and initrd are not on a FAT16 partition. To prevent an older machine from loading ldlinux and failing to read syslinux.cfg, use cfdisk to create a FAT16 partition (<=2GB) and format using dosfstools:

# mkfs.msdos -F 16 /dev/sda1

then install and configure Syslinux.

Missing Operating System

If you get this message, check if the partition that contains /boot has the boot flag enabled. If the flag is enabled, then perhaps this partition starts at sector 1 rather than sector 63 or 2048. Check this with fdisk -l. If it starts at sector 1, you can move the partition(s) with gparted from a rescue disk. Or, if you have a separate boot partition, you can back up /boot with

# cp -a /boot /boot.bak

and then boot up with the Arch install disk. Next, use cfdisk to delete the /boot partition, and recreate it. This time it should begin at the proper sector, 63. Now mount your partitions and chroot into your mounted system, as described in the beginners guide. Restore /boot with the command

# cp -a /boot.bak/* /boot

Check if /etc/fstab is correct, run:

# syslinux-install_update -iam

and reboot.

Windows boots up, ignoring Syslinux

Solution: Make sure the partition that contains /boot has the boot flag enabled. Also, make sure the boot flag is not enabled on the Windows partition. See the installation section above.

The MBR that comes with Syslinux looks for the first active partition that has the boot flag set. The Windows partition was likely found first and had the boot flag set. If you wanted, you could use the MBR that Windows or MS-DOS fdisk provides.

Menu entries do nothing

You select a menu entry and it does nothing, it just "refreshes" the menu. This usually means that you have an error in your syslinux.cfg file. Hit Template:Keypress to edit your boot parameters. Alternatively, press Template:Keypress and type in the LABEL of your boot entry (e.g. arch).

Cannot remove ldlinux.sys

The ldlinux.sys file has the immutable attribute set, which prevents it from being deleted or overwritten. This is because the sector location of the file must not change or else Syslinux has to be reinstalled. To remove it, run:

# chattr -i /boot/syslinux/ldlinux.sys
# rm /boot/syslinux/ldlinux.sys

White block in upper left corner when using vesamenu

Problem: As of linux-3.0, the modesetting driver tries to keep the current contents of the screen after changing the resolution (at least it does so with my Intel, when having Syslinux in text mode). It seems that this goes wrong when combined with the vesamenu module in Syslinux (the white block is actually an attempt to keep the Syslinux menu, but the driver fails to capture the picture from vesa graphics mode).

If you have a custom resolution and a vesamenu with early modesetting, try to append the following in syslinux.cfg to remove the white block and continue in graphics mode:

APPEND root=/dev/sda6 ro 5 vga=current quiet splash

See also

• Official Website
• PXELinux configuration