Installing Debian onto USB flash media with everything encrypted.

Posted by DaveV in the Debian section on Sat 9 Jul 2005 at 22:14

This is a simple procedure for installing Debian GNU/Linux onto a USB key flash media. It includes several configuration changes but tries to stay as close to a default debian install as possible.

This is useful for administrators that need to carry sensitive information or people concerned about their privacy.

This was tested on Debian Sid and Knoppix 3.8 with the USB Keys listed below.

Tested Media:

  1. Transcend Jetflash 256MB ( only the base debootstrap install plus a few select packages ).
  2. Apacer Handy Steno HT203 1GB ( very fast, recommended ).

Other reviews of USB Flash media:

  1. ArsTechnica USB 2.0 Hi-Speed Flash Drive Roundup -
  2. ArsTechnica Son of USB 2.0 Hi-Speed Flash Drive Roundup -

Note on Devices: All device names and mappings are as they were detected and I used them on my system. You will need to substitute the correct device as it is detected on your system.

KNOPPIX NOTE: When you see these notes, there are special steps necessary for installing from Knoppix.

Installation Procedure

1. Shred the drive

shred -n 1 -z -v /dev/sdd
(One pass to shred, one pass to zero)

2. Create Partitions

We will create two partitions on the USB key, one for /boot and one for / (root). We do not create a swap partition because that would prematurely age the usb key. You may mount and use swap partitions from the local harddrives ala knoppix but that is up to you.
parted /dev/sdd "mklabel msdos mkpart primary 0 14 mkpart primary 15 -0"

3. Shred rootfs

shred -n 1 -v /dev/sdd2
(zero'd filesystems are bad for encrypted ones.)

4. Load modules if necessary

modprobe dm-crypt
modprobe aes
KNOPPIX NOTE: We need to install a few packages.
apt-get update
apt-get install cryptsetup dmsetup libdevmapper1.01

5. Created mapped crypt device for root

cryptsetup -y create rootfs /dev/sdd2

6. Format filesystems:

Since we can't use journaling filesystems on flash media (premature aging again), we fall back to good old ext2.
mkfs.ext2 /dev/mapper/rootfs
mkfs.ext2 /dev/sdd1
sync ; sync

7. Apply disk labels

We do this so that we can identify our drive when we boot on various systems. Using a strict device mapping often breaks if other usb or flash devices are detected before ours.You may use any label that you like, but you will have to remember to update the initrd (file: /sbin/init)

e2label /dev/sdd1 PRIVDEB_BOOT

8. Make temporary mount points and mount

mkdir /mnt/buildroot/
mount /dev/mapper/rootfs /mnt/buildroot
mkdir /mnt/buildroot/boot
mount /dev/sdd1 /mnt/buildroot/boot

9. Install base files.

debootstrap --arch i386 sid /mnt/buildroot
Note: Installed size is about 160MB at this stage.
KNOPPIX NOTE: We need to copy a few extra devices over.
cp -ap /dev/ub[a-f]* /mnt/buildroot/dev/

10. Enter chroot jail to work on system.

chroot /mnt/buildroot/ /bin/su -

11. Build fstab and mount everything.

Create /etc/fstab file
#/etc/fstab: static file system information.
LABEL=PRIVDEB_BOOT  /boot          ext2    defaults,noatime                   0 2
/dev/mapper/rootfs  /              ext2    defaults,errors=remount-ro,noatime 0 1
proc                /proc          proc    defaults                           0 0
tmpfs               /etc/network/run tmpfs defaults,noatime                   0 0
tmpfs               /tmp           tmpfs   defaults,noatime                   0 0
tmpfs               /var/lock      tmpfs   defaults,noatime                   0 0
tmpfs               /var/log       tmpfs   defaults,noatime                   0 0
tmpfs               /var/run       tmpfs   defaults,noatime                   0 0
tmpfs               /var/tmp       tmpfs   defaults,noatime                   0 0
tmpfs               /home//Scratch tmpfs defaults,noatime               0 0

#Warning:  By mounting /var/log on tmpfs, logs will only be available for the current session.
Mount it all
mount -a

12. Build sources.list

Create /etc/apt/sources.list
deb sid main non-free contrib
deb-src sid main non-free contrib

deb sid non-US/main non-US/non-free non-US/contrib
deb-src sid non-US/main non-US/non-free non-US/contrib

# If you are using debian stable (woody) include the security updates.
# deb sid/updates main non-free contrib

Note: You can install and use apt-spy to test for the fastest downloading mirrors in your area.

13. System adjustments this file is cached as drives are scanned. Since the scan only takes a few seconds, you dont lose much by not caching or setting the default cache to /dev/null. This file generates an error on boot if the cache file has different device mappings than are currently detected. By removing the cache and forcing a fresh scan every time, the error is eliminated.
rm -f /etc/*
ln -s /dev/null /etc/
mtab: This file is written a lot and may prematurely age parts of the flash media and the information can simply be accessed from /proc directly.
rm -f /etc/mtab
ln -s /proc/mounts /etc/mtab

Set Hostname
vi /etc/hostname
Set /etc/hosts with localhost + hostname
vim /etc/hosts localhost.localdoman localhost 

14. Install additional required packages

apt-get update
apt-get install cryptsetup dmsetup libdevmapper1.01
apt-get install discover1 libdiscover1
apt-get install module-init-tools equivs cramfsprogs
apt-get clean

15. Install custom mkinitrd script and equiv package

Create mkinitrd.dmcrypt-usb file in /usr/local/sbin

# Filename:      mkinitrd.dmcrypt-usb
# Maintainer:    Dave Vehrs 

# Help
    -c           Temporary directory to build image in.
    -k           Keep temporary directory used to build image.
    -l    Use  to indenify boot partition.
    -o  Write to outfie
    -d,-m,-r     Included for fake support of default mkinitrd script
                 (anything passed to them is discarded).
See for more info.

function display_shelp {  
	echo; echo "Usage $0 [OPTION]...<-o outfile> [version]"
	sed --silent -e '/HELP_STEXT$/,/^HELP_STEXT/p' "$0" | sed -e '/HELP_STEXT/d'

# Set defaults

# Parse command line.
# if version + other options not specified, exit.
if [ $# -eq 0 ] ; then
    exit 1

while [ $# -ge 1 ] ; do
	case $1 in 
        -c  ) CRAMFSDIR=$2          ; shift ; shift ;;
        -d  ) dir_conf=$2           ; shift ; shift ;;
        -k  ) keep_temp=1           ; shift ;;
        -l  ) BOOT_LABEL=$2         ; shift ; shift ;;
        -m  ) cmd_mkinitrd=$2       ; shift ; shift ;;
        -o  ) outfile_name=$2       ; shift ; shift ;;
        -r  ) initrd_root=$2        ; shift ; shift ;;
		*   ) VERSION=$1            ; shift ;; 

# Exit if version not specified
if [ -z "$VERSION" ] ; then
    echo "Error: You need to specify a kernel version to build for."
    exit 1

# Start build...
echo "Build directory tree."
install -d $CRAMFSDIR/{bin,dev/mapper,etc,proc,mnt,sbin}

echo "Copy binaries from /bin."
# Copy /bin binaries over and any require libraries.
files_bin="bash grep mount umount mkdir mknod sed sleep uname"
for file in $files_bin ; do 
    install /bin/$file $CRAMFSDIR/bin/$file
    for lib in $( ldd /bin/$file | awk '{print $3}' | grep -v fffe000 ) ; do
        install -d $CRAMFSDIR/${lib%/*}
        install $lib $CRAMFSDIR/$lib

echo "Copy binaries from /usr/bin."
# Copy /usr/bin binaries over and any require libraries.
files_usrbin="find mawk"
for file in $files_usrbin ; do 
    install /usr/bin/$file $CRAMFSDIR/bin/$file
    for lib in $( ldd /usr/bin/$file | awk '{print $3}' | grep -v fffe000 ); do
        install -d $CRAMFSDIR/${lib%/*}
        install $lib $CRAMFSDIR/$lib

echo "Copy binaries from /sbin."
# Copy /sbin binaries over and any require libraries.
files_sbin="cryptsetup e2label modprobe pivot_root"
for file in $files_sbin ; do 
    install /sbin/$file $CRAMFSDIR/sbin/$file
    for lib in $( ldd /sbin/$file | awk '{print $3}' | grep -v fffe000 ) ; do
        install -d $CRAMFSDIR/${lib%/*}
        install $lib $CRAMFSDIR/$lib

# Add common links
ln -s bash /tmp/cramfs/bin/sh
ln -s mawk /tmp/cramfs/bin/awk

echo "Copy devices over."
# Copy devices over
cp -apL /dev/{console,hd,initrd,null,ram,scd,sd}* $CRAMFSDIR/dev/

echo "Copy modules over."
# Copy modules over
modules="aes-i586 dm-crypt sd_mod sr_mod ehci-hcd uhci-hcd ohci-hcd sl811-hcd usbhid usbkbd usb-storage vesafb fbcon ext2 unix"
for mod in $modules; do
  for ko in $( modprobe --set-version $VERSION --show-depends $mod | cut -b8- ) ; do
    install -d $CRAMFSDIR/${ko%/*}
    install $ko $CRAMFSDIR/$ko

cp -apL /lib/modules/$VERSION/modules.* $CRAMFSDIR/lib/modules/$VERSION/

echo "Copy /etc files over."
# Copy required config files over
cp -apr /etc/modprobe.d $CRAMFSDIR/etc/
echo "Copy custom init over."
# Copy custom init file.  (see below)
cat <$CRAMFSDIR/sbin/init

# Filename:      /sbin/init
# Dependencies:  awk, bash, cryptsetup, e2label, find, grep, modprobe 
#                mount, pivot_root, sed, sleep and uname.
# This file generated by mkinitrd.dmcrypt-usb by Dave Vehrs.
set -e

# Set vars
unset pass part_boot part_rootfs major minor label

# Mount /proc
/bin/mount -n -t proc none /proc

# Mount /dev/mapper on tmpfs
/bin/mount -o rw -n -t tmpfs none /dev/mapper

# Modules to load
CORE_MODULES="unix ide-core scsi_mod sd_mod sr_mod mbcache ext2"
DISPLAY_MODULES="vesafb fbcon"
CRYPT_MODULES="aes-i586 dm-mod dm-crypt"
USB_MODULES="ehci-hcd ohci-hcd uhci-hcd sl811-hcd usbcore usbhid usbkbd usb-storage"

# Load Modules
if [ -e /lib/modules/\$(/bin/uname -r) ] ; then 
    echo "initrd: loading modules."
        /bin/find /lib/modules/\$(/bin/uname -r) -name \$module.ko -exec /sbin/modprobe \$module \;

# Test to be sure the procfs is mounted, if not exit. 
if [ ! -e /proc/devices ] ; then
    echo "initrd: procfs not found: please create \$control manually."
    exit 1

major=\$(/bin/sed -n 's/^ *\\([0-9]\+\\) \+misc$/\1/p' /proc/devices)
minor=\$(/bin/sed -n "s/^ *\\([0-9]\+\\) \+\$dm_name\\\$/\1/p" /proc/misc)

# Test to be sure dm_mod loaded
if [ -z "\$major" -o -z "\$minor" ] ; then
    echo "initrd: \$dm_name kernel module not loaded: can't create \$control."
    exit 1
# Create new control device.
echo "initrd: creating \$control character device with major:\$major minor:\$minor."
/bin/mknod --mode=600 \$control c \$major \$minor

# Sleep to let kernel finish loading.  15 seconds is enough on most systems.  
echo "initrd: sleeping for 15 seconds so kernel can finish detecting devices."
/bin/sleep 5
echo "initrd: sleeping for 10 more seconds..."
/bin/sleep 5
echo "initrd: sleeping for 5 more seconds..."
/bin/sleep 5
echo "initrd: awake...."

# Search for boot partition label.  When usb media is detected by the operating
# system seems to migrate a little depending on what port you connect to on the
# mainboard and what if any other devices are connected and where.  To
# compensate for that, we search for the label on our boot partition.
echo "initrd: searching for boot partition label."
for device in \$( /bin/grep sd[a-h]1 /proc/partitions | /bin/awk '{print \$4}' ) ; do
    label=\$( /sbin/e2label /dev/\$device 2>/dev/null )
    if [ ! -z "\$label" ] ; then
        if [ "\$label" == "$BOOT_LABEL" ] ; then

# Exit if boot partition not found.
if [ -z "\$part_boot" ] ; then
    echo "initrd: error -- boot partition label not found (\$part_boot)."
    exit 1

# Assign rootfs variable from boot (i.e. if boot is on /dev/sda1, this will
# set part_rootfs to /dev/sda2).
part_rootfs=\$( echo \$part_boot | /bin/sed -e 's/1/2/' )

# Unmount /proc
/bin/umount /proc

# Prompt for password
echo -en "\\nplease enter password for rootfs filesystem: "
read -s pass
echo -e

# Attempt mounting
echo "initrd: attempting to mount rootfs."
echo \$pass | /sbin/cryptsetup create rootfs \$part_rootfs
/bin/mount -r -n -t ext2 /dev/mapper/rootfs /mnt

# Loop for bad password attempts
while [ \$? -ne 0  ] ; do
    # Remove old crypt mount.
    /sbin/cryptsetup remove rootfs
    # Test for max tries.
    if [ \$count -ge 5 ] ; then
        echo -e "\\ninitrd: too many bad guesses.  aborting."
        exit 1
        count=\$(( \$count + 1 ))
    # Reprompt for password
    echo -e "\\ninitrd: error -- rootfs mount failed." 
    echo -n "please re-enter password: "
    read -s pass
    # Reattempt mounting
    echo \$pass | /sbin/cryptsetup create rootfs \$part_rootfs
    /bin/mount -r -n -t ext2 /dev/mapper/rootfs /mnt

unset pass

echo "initrd: rootfs successfully mounted."

# Now that the encrypted media is readable, shift the root to it and continue
# the boot cycle by running its init.
cd /mnt
/sbin/pivot_root . initrd
exec /usr/sbin/chroot . /sbin/init
chown root:root $CRAMFSDIR/sbin/init
chmod 755 $CRAMFSDIR/sbin/init

# make cramfs file
if [ -z "$outfile_name" ] ; then
    mkcramfs $CRAMFSDIR ./initrd-$VERSION.img
    mkcramfs $CRAMFSDIR $outfile_name

# Cleanup
if [ $keep_temp -eq 0 ] ; then
    rm -rf $CRAMFSDIR
Set permissions, and links.
chown root.root /usr/local/sbin/mkinitrd.dmcrypt-usb
chmod 750 /usr/local/sbin/mkinitrd.dmcrypt-usb
ln -s /usr/local/sbin/mkinitrd.dmcrypt-usb /usr/sbin/mkinitrd
Next we need to install an equivs package to let the package system know that we installed this ourselves and not to install initrd-tools
cd /tmp
equivs-control initrd-tools
Edit the generated template so that it looks like:
Section: misc
Priority: optional
Package: initrd-tools
Build equivs package
equivs-build initrd-tools
Install the package
dpkg -i initrd-tools_1.0_all.deb
For more information about equivs, see the APT howto at: APT-Howto: Equivs

16. Remove unwanted locales

Be very careful configuring and running localepurge. It is very easy to delete too many locales.
apt-get install localepurge
apt-get clean
For more information about localepurge, see the APT howto at: APT-Howto: localepurge

17. Install kernel

WARNING: Kernels prior to 2.6.10 had a bug in the dm_crypt modules that potentially could reveal data. Only use 2.6.10 or better.
apt-get install kernel-image-2.6.11-1-686
apt-get clean
Note: Install size is approximately 184MB now. If you want to install a kernel built from source you can. After you install it, run /sbin/mkinitrd to build the /boot/initrd file. When you run /sbin/mkinitrd, it may print several FATAL errors regarding modules that it cannot find. If you built these modules into the kernel then you can ignore the error messages. If you omitted the modules, this is your warning to go build them as modules or into the kernel. Required modules: dm_crypt, aes, ide_core, scsi_mod, sd_mod, ehci-hcd, ohci-hcd, uhci-hcd, sl811-hcd, usb-storage, usb-hid, dm_mod, cramfs

18. Install optional packages

apt-get install vim irsii-text mutt fetchmail antiword screen
apt-get install exuberant-ctags less procmail
apt-get install python2.3 python2.3-pexpect python2.3-fuse
apt-get install xserver-common xserver-xfree86 xbase-clients xfree86-common
apt-get install ion3 -or- blackbox -or- fluxbox -or- icewm
apt-get install xterm
apt-get install memtest86+
Note: All this is approximately 300mb installed (with dependencies).

19. Install grub

apt-get install grub
grub-install /dev/sdd
mkdir /boot/grub
 root (hd1,0)
 setup (hd1)
Create /boot/grub/menu.lst file
# default num
default         0

# timeout sec
timeout         5

# pretty colours
color green/black black/green

title   Debian GNU/Linux-2.6.11-1-686
root    (hd0,0)
kernel  /vmlinuz-2.6.11-1-686 root=/dev/ram0 init=/sbin/init vga=794
initrd  /initrd.img-2.6.11-1-686

title   Debian GNU/Linux-2.6.11-1-686 (Rescue/Single)
root    (hd0,0)
kernel  /vmlinuz-2.6.11-1-686 root=/dev/ram0 init=/sbin/init single
initrd  /initrd.img-2.6.11-1-686
title   Memtest86+
root    (hd0,0)
kernel  /memtest86+.bin

20. Add User accounts


Copy an existing /etc/group, /etc/passwd, and /etc/shadow file over from another system (this has to be done from outside the chroot directory).


Add users locally.

  1. set root password
    passwd root
  2. add local user
  3. repeat step 2 as necessary

21. Exit Jail

umount -a
umount /proc

22. Unmount and remove crypt mapping

umount /mnt/buildroot/
cryptsetup remove rootfs

23. Reboot to test media

shutdown -r now


1. Make sure required modules are loaded.

modprobe dm_crypt
modprobe aes-i586  (or aes)
modprobe usb-storage

2. Insert USB key into port

3. Create device mapping and mount

cryptsetup create rootfs /dev/sdd2
mount /dev/mapper/rootfs /mnt/buildroot

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