Chapter 4 : Obtaining System Installation Media | 3. Preparing Files for USB Memory Stick Booting - 7 February 2012 - Blog - | LINUX - SECURITY |
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Main » 2012 » February » 7 » Chapter 4 : Obtaining System Installation Media | 3. Preparing Files for USB Memory Stick Booting
11:51 PM
Chapter 4 : Obtaining System Installation Media | 3. Preparing Files for USB Memory Stick Booting

Preparing Files for USB Memory Stick Booting

There are two installation methods possible when booting from USB stick. The first is to install completely from the network. The second is to also copy a CD image onto the USB stick and use that as a source for packages, possibly in combination with a mirror. This second method is the more common.

For the first installation method you'll need to download an installer image from the netboot directory (at the location mentioned in the section called "Where to Find Installation Images”) and use the "flexible way explained below to copy the files to the USB stick.

Installation images for the second installation method can be found in the hd-media directory and either the "easy way or the "flexible way can be used to copy the image to the USB stick. For this installation method you will also need to download a CD image. The installation image and the CD image must be based on the same release of debian-installer. If they do not match you are likely to get errors[6] during the installation.

To prepare the USB stick, you will need a system where GNU/Linux is already running and where USB is supported. With current GNU/Linux systems the USB stick should be automatically recognized when you insert it. If it is not you should check that the usb-storage kernel module is loaded. When the USB stick is inserted, it will be mapped to a device named /dev/sdX, where the "X is a letter in the range a-z. You should be able to see to which device the USB stick was mapped by running the command dmesg after inserting it. To write to your stick, you may have to turn off its write protection switch.


[Warning]

The procedures described in this section will destroy anything already on the device! Make very sure that you use the correct device name for your USB stick. If you use the wrong device the result could be that all information on for example a hard disk could be lost.

Note that the USB stick should be at least 8 MB in size. If you follow the section called "Copying the files — the flexible way” and want to include an Ubuntu ISO image on the stick, you will need enough space for the ISO as well.

Copying the files — the easy way

There is an all-in-one file netboot/boot.img.gz which contains all the installer files (including the kernel) as well as syslinux and its configuration file.

To use this image simply extract it directly to a partition on your USB stick:

# zcat boot.img.gz > /dev/sdX1

Copying the files — the flexible way

If you like more flexibility or just want to know what's going on, you should use the following method to put the files on your stick. One advantage of using this method is that — if the capacity of your USB stick is large enough — you have the option of copying a full CD ISO image to it.

Partitioning the USB stick

We will show how to set up the memory stick to use the first partition, instead of the entire device.

[Note]

Since most USB sticks come pre-configured with a single FAT16 partition, you probably won't have to repartition or reformat the stick. If you have to do that anyway, use cfdisk or any other partitioning tool to create a FAT16 partition[7], and then create the filesystem using:

# mkdosfs /dev/sdX1

Take care that you use the correct device name for your USB stick. The mkdosfs command is contained in the dosfstools Ubuntu package.

In order to start the kernel after booting from the USB stick, we will put a boot loader on the stick. Although any boot loader (e.g. lilo) should work, it's convenient to use syslinux, since it uses a FAT16 partition and can be reconfigured by just editing a text file. Any operating system which supports the FAT file system can be used to make changes to the configuration of the boot loader.

To put syslinux on the FAT16 partition on your USB stick, install the syslinux and mtools packages on your system, and do:

# syslinux /dev/sdX1

Again, take care that you use the correct device name. The partition must not be mounted when starting syslinux. This procedure writes a boot sector to the partition and creates the file ldlinux.sys which contains the boot loader code.

Adding the installer image

Mount the partition (mount /dev/sdX1 /mnt) and copy the following installer image files to the stick:

  • vmlinuz or linux (kernel binary)

  • initrd.gz (initial ramdisk image)

You can choose between either the regular version or the graphical version of the installer. The latter can be found in the gtk subdirectory. If you want to rename the files, please note that syslinux can only process DOS (8.3) file names.

Next you should create a syslinux.cfg configuration file, which at a bare minimum should contain the following two lines (change the name of the kernel binary to "linux if you used a netboot image):

default vmlinuz
append initrd=initrd.gz

For the graphical installer you should add video=vesa:ywrap,mtrr vga=788 to the second line.

If you used an hd-media image, you should now copy an Ubuntu ISO image[8] onto the stick. When you are done, unmount the USB memory stick (umount /mnt).

Booting the USB stick

[Warning]

If your system refuses to boot from the memory stick, the stick may contain an invalid master boot record (MBR). To fix this, use the install-mbr command from the package mbr:

# install-mbr /dev/sdX
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