Friday, 5 February 2010

Syslinux from Linux!

This post tells you how to launch syslinux from a Master Boot Record (MBR).

Recently I was locked out of a customer-provided laptop with their development environment, and access to their source code repository via vpn.  I suspect their domain controller propagated an update last time I was on the vpn which has locked me out.

They're overseas and about 12 hours flight time away, so with their permission I used ntpasswd to reset the Administrator password.  The boot CD (downloadable as an iso) uses syslinux, which is fine, except that instead of wasting CD-Rs I like to use USB keys.

I copied the contents to a blank FAT32-formatted usb key, but it has no boot sector yet.  I installed grub and tried to make a grub menu file from the syslinux.cfg with these tips for converting a syslinux .cfg file to a grub .conf file.  I failed because the syslinux.cfg has the line:
 append rw vga=1 initrd=initrd.cgz,scsi.cgz

And I don't know how to append the two cgz's into one grub initrd line.  Normally grub uses an initrd like this:
 initrd /initrd.cgz

So I decided to install syslinux from linux.  The man page makes it look easy:
 syslinux [-sfr] [-d directory] [-o offset] device

When I ran "syslinux /dev/sdd1" and booted the laptop with this usb key, it just gave me a blank cursor blink.  This is because the laptop is looking in the MBR of the usb key and finding nothing.  The syslinux man page shows some hints:
Booting from a FAT 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  pro-
cedure  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. A sample master boot sector (mbr.bin) is included with SYSLINUX.
Well, that's nice to know, but how do I put that on my usb key?  Follow these steps:

1. Start with a FAT32 formatted usb key (it can have other data on it) and some syslinux-based boot image.  I'm using the latest ntpasswd iso cd080802.  Unpack the contents to the root of the usb key.

2. Copy a boot sector to the code image of the MBR of your to-be-booted usb key:
sudo dd if=/usr/share/syslinux/mbr.bin of=/dev/sdd bs=440 count=1
  • /dev/sdd is my usb key
  • Look at Wikipedia for an explanation of the MBR layout
  • your mbr.bin might be in a different location.  It should come installed with syslinux

3. Run syslinux to make the partition bootable:
 sudo syslinux /dev/sdd1

4. Mark the partition as bootable (may not be necessary)
 sudo fdisk /dev/sdd

Select a, 1, w to make the first partition bootable.  CHECK THESE OPTIONS FIRST!

5. Insert USB key to your PC / laptop and boot (so long as your BIOS is setup and capable!)
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