To Install the Same Set of Packages as Another Debian Machine

I know this is a well-known trick with the Debian package system, but I'm going to sum it up here. Suppose that you want to install the same set of packages on box A as box B. At box B, type the following command to get the list of installed packages.

dpkg --get-selections > select.txt

Then, take select.txt to box A and run on box A:

apt-get update
dpkg --set-selections < select.txt
apt-get -u dselect-upgrade

Share your Linux desktop with x11vnc

x11vnc is a pretty useful program that you can use to share your Linux desktop with someone far away. To start x11vnc while using X11 windows, issue the following command:

x11vnc -display :0

This command will work in most cases, assuming there is only one instance of X11 Windows server running. However, the display number could be different. To be sure, type the following command:

xdpyinfo | head

x11vnc will output a lot of information, including the port number it runs on:

The VNC desktop is:      bootcd:2
PORT=5902

Subtracting 5900 from the port number (5902), you'll get the display number (2). Use this number to connect to your desktop remotely. I use gvncviewer.

gvncviewer 192.168.200.83:2

Administrator To Monitor Someone's Desktop with X11VNC

Let's assume that you are a system administrator with root privilege and want to monitor a user's X11 Windows desktop. You can't just run “x11vnc -display :0” because you are not the user currently using display :0. In that case, you need access to the X11 Windows' xauth file which is located at one of the following locations:

• /var/lib/gdm
  
• /var/lib/kdm
  
• /var/lib/xdm/authdir/authfiles
  
• /home/USER/.Xauthority
  

Once you locate the xauth file associated with the user's X11 desktop, you can use x11vnc to monitor his desktop activity.

x11vnc -display :0 -auth /var/lib/gdm/\:0.Xauth

Then, on your desktop as administrator, connect to the user's desktop with a vnc client.

gvncviewer 192.168.200.83:2

x11vnc with SSL

x11vnc supports various connection schemes, including SSL. On Debian and Ubuntu, type the following command to create a SSL certificate for use with x11vnc.

cat /etc/ssl/private/ssl-cert-snakeoil.key /etc/ssl/certs/ssl-cert-snakeoil.pem > /tmp/x11vnc.pem

Then, start x11vnc with -ssl option to run x11vnc in secure mode.

x11vnc -display :0 -auth /var/lib/gdm/\:0.Xauth -ssl /tmp/x11vnc.pem

ssvnc and gvncviewer can be used to connect to x11vnc via SSL connection. The following example command starts x11vnc in anonymous SSL mode that doesn't require X509 exchange.

x11vnc -display :0 -xauth /var/lib/xdm/authdir/authfiles/A\:0-R2x5Db -xkb -nopw -nc_cr -ssl /tmp/x11vnc.pem -vencrypt newdh:nox509:support

Setting Up XDM on Debian/Ubuntu Linux

XDM, short for X Display Manager, is my favorite login manager for its simplicity. Sure, gdm and kdm are visually pleasant, but why do I have to install all the dependencies if xdm provides the same functionality as them? After all, xdm can launch GNOME and KDE for you — you just need to fix your ~/.xsession file. This post will serve as my notes on installation, configuration and customization of xdm.

All configuration of xdm, except for distribution-specific settings, is done by modifying the files in the /etc/X11/xdm directory. For now, I'll just go over what I have in my xdm files.

/etc/X11/xdm/Xaccess

This file is used to tell xdm who can use xdm. Therefore, if you are happy to let anyone access the X-Windows desktop locally or remotely, then you would just put an asterisk (*) in Xaccess as I did.

*

If you are very keen on security, you would put only the following in Xaccess. This only allows local users to have access to xdm.

localhost
127.0.0.1

However, even if Xaccess is set up as above, remote xdm login may still be possible with SSH X11 forwarding.

/etc/X11/xdm/Xresources

Xresources file is used to fine-tune the behavior and appearance of XDM. Here is what I put in my Xresources file:

Xcursor.theme: redglass

xlogin*login.translations: #override \
 Ctrl<Key>R: abort-display()\n\
 <Key>F1: set-session-argument(failsafe) finish-field()\n\
 <Key>Delete: delete-character()\n\
 <Key>Left: move-backward-character()\n\
 <Key>Right: move-forward-character()\n\
 <Key>Home: move-to-begining()\n\
 <Key>End: move-to-end()\n\
 Ctrl<Key>KP_Enter: set-session-argument(failsafe) finish-field()\n\
 <Key>KP_Enter: set-session-argument() finish-field()\n\
 Ctrl<Key>Return: set-session-argument(failsafe) finish-field()\n\
 <Key>Return: set-session-argument() finish-field()

xlogin*greeting: Welcome to CLIENTHOST
xlogin*namePrompt: Login:\040
xlogin*passwdPrompt: Password:\040
xlogin*fail: Login incorrect

xlogin*greetFace: Serif-21:bold:italic
xlogin*face: Sans-15
xlogin*promptFace: Sans-15:bold
xlogin*failFace: Sans-16:bold

xlogin*geometry: 600x400
xlogin*borderWidth: 1
xlogin*frameWidth: 5
xlogin*innerFramesWidth: 2
xlogin*shdColor: grey30
xlogin*hiColor: grey90
xlogin*background: grey
xlogin*greetColor: Blue3
xlogin*failColor: red
*Foreground: black
*Background: #fffff0

xlogin*logoFileName: /usr/share/X11/xdm/pixmaps/penguin_doll_200x300.xpm
xlogin*useShape: true
xlogin*logoPadding: 10

I think the settings are self-explanatory. This is how xdm would look with the above settings. The penguin image was taken from here.

/etc/X11/xdm/Xresources

The Xresources file allows you to specify how many X servers to run and their command arguments. In my case, I run Xorg and Xvfb. Xorg is the standard X server, and Xvfb is a server that runs totally in memory without depending on the video hardware. To see what's going on with Xvfb, I use x11vnc and gvncviewer.

# Default X.org server
:0 local /usr/bin/Xorg :0 vt7 -br -dpi 96 -nolisten TCP

:1 local /usr/local/bin/Xvfb :1 -dpi 96 -screen 0 1024x768x16

/etc/X11/xdm/Xsetup

Xsetup allows you to write any command to run when xdm displays the login screen. In my case, I run gm to set up the background image and run x11vnc to remotely access Xvfb via VNC.

#!/bin/sh
#
# This script is run as root before showing login widget.

gm display -window root /usr/share/wallpapers/Board_Tile_Gray.jpg
xkbcomp -dflts -I/usr/share/X11/xkb /etc/X11/xkb/server-0.xkm $DISPLAY

PID_XVFB=$(pidof Xvfb)
PID_X11VNC=$(pidof x11vnc)
if [ -n "$PID_XVFB" -a -z "$PID_X11VNC" ]; then
  x11vnc -display :1 -bg -xkb -nopw -nc_cr -forever -listen localhost -auth $(find /var/lib/xdm/authdir/authfiles -name \*:1\*) > /var/log/x11vnc.log 2>&1
fi

/etc/X11/xdm/xdm-config

In xdm-config, there is only one line that I bother to change. This line enables remote xdm logins.

! SECURITY: do not listen for XDMCP or Chooser requests
! Comment out this line if you want to manage X terminals with xdm
DisplayManager.requestPort:     177

/etc/X11/xdm/xdm-options

I think xdm-options is Debian specific. I don't think I made any change in this file.

# configuration options for xdm
# See xdm.options(5) for an explanation of the available options.

no-ignore-nologin
no-restart-on-upgrade
no-start-on-install
use-sessreg

Resetting XDM

There are other files in /etc/X11/xdm directory that you can safely leave as is. After you make any change with the files, have xdm reread the config files.

kill -1 $(pidof xdm)

If you want to force xdm to kill all running sessions and restart, enter the following command.

/etc/init.d/xdm restart

That's all I have to write about xdm for now. In summary, xdm is a simple, yet functional login manager that is easy to set up.

My Live CD based on Debian Linux Sid

I created a Live CD from my Debian Linux installation. It is based on the unstable version (Sid). However, the live CD still lacks the capability to automatically configure X-Windows and the network, and uses IceWM for minimalist desktop. It can be downloaded from Google Drive. This Live CD has many uses, such as:

• To troubleshoot and fix computer problems
  
• To create backups and restore them
  
• To partition a hard drive before installing Windows or Linux
  
• To set up GRUB bootloader or enter boot commands
  
• To install Debian, Ubuntu or Fedora
  
• To access a Windows partition when it is unbootable
  
• To survey and diagnose wireless and wired networks
  
• To enjoy the Internet with the Web, IM, IRC and VoIP
  
• To run a simple FTP or HTTP server
  
• To participate in peer-to-peer networks
  
• To do basic word processing and spreadsheet
  
• To listen to music or watch movies
  
• To rip CD's and DVD's

The snapshot above shows idesk icons on the desktop. Idesk is a neat application that displays icons on the desktop for you to quickly launch. Currently, I use IceWM instead of GNOME or KDE desktop to save space on the CD. So idesk is used to provide desktop icons for IceWM.

The snapshot above shows mlterm running. mlterm is my favorite terminal application. It allows me to use many languages.

Gparted is a cool application to have on a live CD. It lets you divide the hard drive into many partitions so you can run multiple operating systems.

IceWeasel is rebranded Firefox from Debian. Sorry, Chrome is not included.

LibreOffice is a fork of OpenOffice.org suite. I use it to create documents or read Microsoft Office documents.

Specifying the locale at boot time

To see your own language with the live CD, select an entry from the menu, click Tab and append LANG= option to the command line. For example, if you speak French, append LANG=fr_FR.UTF-8.

Debian Linux: Simple steps to recover files from /lost+found

The following steps illustrate how one can use simple shell commands to recover files from the /lost+found directory in Linux. This assumes that you actually have some files in /lost+found.

1. First, concatenate all the *.md5sums files in /var/lib/dpkg/info into a single file:

   
   

   cd /var/lib/dpkg/info
   cat *.md5sums | sort -k 2 > /tmp/all.md5

   
   
   

2. Go to the /lost+found directory. fsck may have placed some files here after fixing the Linux partition. If you don't find any file here, you can relax and skip the following steps. Run md5sum on files in /lost+found.

   
   

   cd /lost+found
   md5sum * | sort > /tmp/lost.md5

   
   
   

3. Put only the md5sum values into a temporary file, called 0.txt.

   
   

   awk '{print $1}' /tmp/lost.md5 > /tmp/0.txt

   
   
   

4. Search all.md5 for the md5sum values in 0.txt and save the results in 1.txt.

   
   

   for f in $(cat /tmp/0.txt); do grep $f all.md5 >> /tmp/1.txt; done

   
   
   

5. Put the names of files in /lost+found into a temporary fie 2.txt.

   
   

   awk '{print $2}' /tmp/lost.md5 > /tmp/2.txt

   
   
   

6. Move the lost+found files to their original locations.

   
   

   cd /
   for $f in $(cat /tmp/2.txt); do MD5=$(grep $f /dev/shm/lost.md5 | awk '{print $1}'); ORIGIN=$(grep $MD5 /tmp/1.txt | awk '{print $2}'); mv /lost+found/$f /$ORIGIN; done

   
   

Checking Integrity of A Debian/Ubuntu System

Sometimes, a Linux filesystem becomes corrupted, system files are damaged, or some crucial files get lost. This often happens, regardless of which filesystem (ext2, ext3, ext4, jfs, reiserfs, reiser4, or xfs) is used. There are many possible reasons, such as:

• Unstable hardware, for example, memory or hard drive problem
  
• Overheat, power surge, quake or another environmental disaster
  
• Buggy software, such as a bug in the kernel or the filesystem driver
  
• Compromised security, for example, network intrusion or attack
  
• Worm or virus infection

Files in Linux systems can be categorized into the following three:

1. Verifiable System Files
   
   In Linux systems that are managed by packages (such as Debian and Ubuntu), these files are installed by packages and make up the bulk of the filesystem. These files reside in such directories as /bin, /lib, /sbin and /usr. They are usually static, which means they don't normally change except when the system is updated, or locally compiled binaries are installed.
   
2. Changeable System Files
   
   These files are auxiliary system files for system configuration, initialization or customization, and system data (such as logs and cache). They reside in /boot, /etc, /opt, /srv and /var.
   
3. User Data
   
   These files are created and used by superuser (a.k.a root) and normal users, or software-generated during casual user activities. Typically, they are in /home, /media, /mnt and /root.
   

This post focuses on verifiable system files (installed by packages). When the filesystem becomes corrupted (but not completely unreadable), it is possible to verify and restore the system integrity by using package checksums. Before you continue, make sure to fsck the filesystem.

e2fsck -r -v /dev/sda7

In this example, /dev/sda7 points to an ext2 partition we're going to check. Be aware that you cannot fsck a mounted filesystem. Therefore, boot with a Debian Live CD (or a Ubuntu CD) and run fsck. After you've performed fsck, there may be some files created in the /lost+found directory. We'll deal with them later. First, mount the filesystem.

mount -t ext2 /dev/sda7 /mnt

Go to /var/lib/dpkg/info. Then, concatenate all the md5sums files. Most, if not all, Debian and Ubuntu packages come with a md5sum file that we can use to check the integrity of the package and the files installed by the package.

cd /var/lib/dpkg/info
cat *.md5sums | sort > /dev/shm/all.md5

all.md5 has md5 checksums of all the files installed on the system. Now, check the files on the Debian/Ubuntu system against the concatenated md5sums file.

cd /
md5sum -c /dev/shm/all.md5 > /dev/shm/check.txt 2>&1

/dev/shm/check.txt now contains the results of the integrity check. It looks like this:

bin/bash: OK
bin/bunzip2: OK
bin/bzcat: FAILED

In this example, /bin/bzcat is damaged. To find all the missing or damaged files, use a command like this one:

grep -v ': OK$' /dev/shm/check.txt

Let's reinstall this file. First, find out which package this file belongs to.

dpkg -S /bin/bzcat

We'll see the following result.

bzip2: /bin/bzcat

Now we know that we need to reinstall bzip2. Let's download the package.

dpkg -p bzip2 | grep 'Filename: '

This command will let us know the name of the package to download. Use wget to download it.

wget ftp://ftp.us.debian.org/debian/pool/main/b/bzip2/bzip2_1.0.5-4_i386.deb

You can just reinstall the package.

dpkg -i bzip2_1.0.5-4_i386.deb

Or, you can just extract one file:

dpkg --fsys-tarfile bzip2_1.0.5-4_i386.deb | tar xf - ./bin/bzcat

Alternatively,

dpkg --fsys-tarfile bzip2_1.0.5-4_i386.deb | tar xOf - ./bin/bzcat > /mnt/bin/bzcat

To restore a file from the /lost+found directory, you can also use the MD5SUMS file. First, run md5sum on files in /lost+found.

cd /lost+found
md5sum *

You may get an output like this.

9aaa2176d20c1b1203e3abbac55a2513  #124531

To find out what #124531 file is originally, find its md5 checksum from the all.md5 file above.

grep 9aaa /dev/shm/all.md5

You'll get a result like this.

9aaa2176d20c1b1203e3abbac55a2513  bin/bzip2

Now you can just move it to its place.

mv \#124531 /mnt/bin/bzip2

After you restore all damaged files and restore files from /lost+found, you can find missing files in the system. Go to /var/lib/dpkg/info again and concatenate all the list files.

cd /var/lib/dpkg/info
cat *.list | sort | uniq > /dev/shm/all.txt

The .list files in the /var/lib/dpkg/info directore show the list of files installed by packages. Let's find what's missing from the system.

cd /
for f in $(cat /dev/shm/all.txt ); do test -e "$f" || echo "$f" >> /dev/shm/nonexist.txt ; done

The file /dev/shm/nonexist.txt will show which files are missing from the system. You can then replace the missing files as done previously.

Use cdebootstrap to install Debian/Ubuntu Linux

Using the official CD from Debian or Ubuntu to install a fresh new system can be tedious if you just want a minimal or customized system. An alternative way to install Debian or Ubuntu is to use cdebootstrap. cdebootstrap is a simple command-line program that downloads packages from a Debian/Ubuntu archive, unpacks them into a mounted filesystem, and set 'em up. But it's not a full-fledged installer, so you need to take care of partitioning and setting up bootloader in order to complete the installation.

In order to use cdebootstrap, you need to have an already-running Linux system. However, any Linux live CD will suffice. Fortunately, I have created my own Debian Live CD with included cdebootstrap. I booted my Debian Live and opened mlterm.

If you don't have cdebootstrap, there are many ways to install it. Download a cdebootstrap-static package from http://packages.debian.org/sid/cdebootstrap-static that matches your architecture. If you are running an rpm-based system, such as Fedora, use ar, tar and gzip to unpack the package. For example:

wget http://ftp.us.debian.org/debian/pool/main/c/cdebootstrap/cdebootstrap-static_0.5.5_i386.deb

ar xv cdebootstrap-static_0.5.5_i386.deb

gzip -dc data.tar.gz > /tmp/data.tar

cd /; tar xvf /tmp/data.tar

Before running cdebootstrap, make sure that you have an empty partition and have mounted it. GParted can be used to create an empty partition. In the following example, I used cdebootstrap-static to install Ubuntu 6.06 Dapper into a filesystem mounted on /mnt.

cdebootstrap-static --allow-unauthenticated --flavour=minimal dapper /mnt http://archive.ubuntu.com/ubuntu

--allow-unauthenticated option is used in case ubuntu-archive-keyring or debian-archive-keyring is not installed. There are three flavours to choose from: minimal, standard and build. For suites other than dapper, you can choose one among etch, lenny, squeeze, sid, breezy, dapper, edgy, feisty, gutsy, hardy, intrepid, oldstable, stable, testing, unstable.. The URL at the end of command line is optional. For Debian, you can type http://archive.debian.org/debian.

cdebootstrap downloaded and installed the following set of packages into the new filesystem mounted at /mnt. This list is for a minimal flavour.

apt
base-files
base-passwd
bash
belocs-locales-bin
bsdutils
coreutils
debconf
debconf-i18n
debianutils
diff
dpkg
e2fslibs
e2fsprogs
findutils
gcc-4.0-base
gnupg
grep
gzip
hostname
initscripts
libacl1
libattr1
libblkid1
libbz2-1.0
libc6
libcap1
libcomerr2
libdb4.3
libgcc1
libgcrypt11
libgnutls12
libgpg-error0
libldap2
liblocale-gettext-perl
liblzo1
libncurses5
libopencdk8
libpam-foreground
libpam-modules
libpam-runtime
libpam0g
libreadline5
libsasl2
libsasl2-modules
libselinux1
libsepol1
libslang2
libss2
libssl0.9.8
libstdc++6
libtasn1-2
libtext-charwidth-perl
libtext-iconv-perl
libtext-wrapi18n-perl
libusb-0.1-4
libuuid1
locales
login
lsb-base
makedev
mawk
mount
ncurses-base
ncurses-bin
perl-base
python-minimal
python2.4-minimal
readline-common
sed
sysv-rc
sysvinit
tar
ubuntu-keyring
util-linux
zlib1g

In order to fine-tune the newly installed system, chroot into it and install additional packages.

chroot /mnt /bin/bash