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.

Using UnionfsFuse on Debian/Ubuntu Linux

Unionfs-fuse is a user-space implementation of unionfs that makes it possible to run Linux over a read-only medium, such as CD-ROM. Unionfs-fuse is a convenient alternative to kernel implementations (unionfs, aufs and overlayfs), so users don't have to patch the kernel in order to try unionfs. There are many possible uses of unionfs:

• Create a live CD
  
• Run Linux from a read-only filesystem image, such as a squashfs file
  
• Freeze an existing Linux system and save all changes in the memory
  

Unionfs achieves all this by merging a read-only filesystem and a writeable filesystem into a single virtual filesystem and mount it at a certain mount point. Let's try and apply unionfs-fuse to the third situation above. Basically, we need to create an initramfs file containing unionfs-fuse and a custom init script. The basic procedure is like this:

1. Load kernel drivers necessary to access the underlying storage device
   
2. Mount the read-only filesystem image at /opt
   
3. Mount tmpfs at /opt/tmp and create a directory /opt/tmp/.change
   
4. Use unionfs-fuse to merge the two and mount it at /mnt
   
5. Use chroot and start /sbin/init to boot the new unionfs filesystem
   

To carry about the above steps, an init script might look like this:

#!/bin/dash
# Use the small but functional dash to process this script

# Mount /proc, /sys, /dev and /dev/pts just in case
mount -t proc none /proc
mount -t sysfs none /sys
mount -t devtmpfs none /dev
mkdir -m 755 /dev/pts
mount -t devpts -o gid=5,mode=620 none /dev/pts

# Write commands here to load modules necessary to access a hard drive
modprobe pata_via
modprobe sd_mod

# Mount the Linux filesystem read-only at /opt
mount -r /dev/sda6 /opt

# Mount tmpfs at /opt/tmp
mount -t tmpfs none /opt/tmp
mkdir /opt/tmp/.change

# Create a unionfs mount at /mnt
modprobe fuse
unionfs-fuse -o allow_other,use_ino,suid,dev,nonempty,kernel_cache \
   -o cow,chroot=/opt,max_files=32768 /tmp/.change=RW:/=RO /mnt

# Make sure that init exists and is executable
if [ -x /mnt/sbin/init ]; then
  mount --move /dev /mnt/dev
  mount --move /proc /mnt/proc
  mount --move /sys /mnt/sys

# Start init from the root filesystem with runlevel 5.
  exec chroot /mnt /sbin/init 5
fi

The computer will boot into a virtual unionfs, consisting of writable tmpfs on top of read-only root filesystem. Make sure to modify /etc/rc.local so that it contains the following snippet of code.

for i in `ps ax | grep unionfs | grep -v grep | awk '{print $1}'`; do
  echo $i > /var/run/sendsigs.omit.d/unionfs.$i;
done

This prevents Linux from killing unionfs-fuse during shutdown so the system will properly shut down.

Related Posts

• Manually Creating initrd / initramfs to Boot Linux

Hacker's Guide To Installing Windows 7

Many people want to try the new Windows 7, but they're not too sure if Windows 7 is worth it. After all, it doesn't make sense to buy a new computer to try Windows 7 and then find out it's another lame bloatware. This guide will help an average joe to download, install and activate Windows 7 with freely available tools.

Hardware Requirements of Windows 7

First, you need to decide if your computer is capable of running Windows 7. According to Microsoft, Windows 7 has the following hardware requirements:

• 1 GHz or faster 32-bit or 64-bit processor
  
• 1 GB RAM (32-bit) or 2 GB RAM (64-bit)
  
• 16 GB available hard disk space (32-bit) or 20 GB (64-bit)
  
• DVD-ROM drive or DVD burner
  
• DirectX 9 graphics card with WDDM 1.0 or better driver

That means Windows 7 can run on most computers available. You may have to buy additional memory or a new video card to be able to run Windows 7. See the Windows 7 Compatibility site.

Next, decide which edition of Windows 7 you want to install on your computer. There are many editions of Windows 7, for example, Home Premium, Professional, Enterprise and Ultimate. For most people, Windows 7 Home Premium is good enough.

Windows 7 Direct Downloads and Bittorrents

It is recommended that you use aria2, Halogenware Retriever or Free Download Manager to download Windows 7 so you can resume a download in the event of connection problems. On the Web, you can download DVD images of Windows 7 from the following locations:

• Windows 7 Home Premium SP1 (x86 English) X17-24208.iso
  
  MD5: c5bb99b2f1a9e7a5b4fbc6e3eff70882
  
  SHA1: 22df3e5a80f8dbf014c2776a01b1cd9f24d83233
  
  CRC32: 9c5dc931
  
• Windows 7 Home Premium SP1 (x64 English) X17-24209.iso
  
  MD5: 971843a457b6e0db0af61258cbe7256a
  
  SHA1: b4821f46a171708f5f8f8a0ef48fc16529437961
  
  CRC32: d236fd70
  
• Windows 7 Professional SP1 (x86 English) X17-24280.iso
  
  MD5: befd4f1e00ec8da2effbd789f4e66fbf
  
  SHA1: 917a542b0541054bb9c2a06a11a46aed6943856b
  
  CRC32: 20078d8d
  
• Windows 7 Professional SP1 (x64 English) X17-24281.iso
  
  MD5: 338f3245d68527db47b8a44e55317d0a
  
  SHA1: 5ed2584110e03f498db4458ba9fafd5a7ef602ed
  
  CRC32: 74f3cb73
  
• Windows 7 Ultimate SP1 (x86 English) X17-24394.iso
  
  CRC32: C2966895
  
  MD5: 24F3A45D43D7C532AA3126CC094C61BD
  
  SHA1 : 92c1ada4ff09c76ec2f1974940624cab7f822f62
  
  SHA2: BBB5E0BAF3659F6DEC96B50381CBB05C09F066675EE639324B728215D5D47C43
  
• Windows 7 Ultimate SP1 (x64 English) X17-24395.iso
  
  CRC32: 8589EE18
  
  MD5: 56A26636EC667799F5A7F42F142C772D
  
  SHA1: 1693b6cb50b90d96fc3c04e4329604feba88cd51
  
  SHA2: 9A2F3972FBBBB4974255CCCC26CDA9B1A75D9D0179867D6111162AC33F0CB693
  
• Windows 7 Home Premium (x86 English) X15-65732.iso
  
  SHA1: CC9D8220B2179E784D85BF1EA98D2EE2190D534F
  
  ISO/CRC: 5DF6DBA0
  
• Windows 7 Home Premium (x64 English) X15-65733.iso
  
  SHA1: 336779EA6B65F63E11A609B4D021439C47AB315B
  
  ISO/CRC: 56D954E4
  
• Windows 7 Professional (x86 English) X15-65804.iso
  
  SHA1: 697FA06554502FB21D30275273B25747299C020D
  
  ISO/CRC: 578725D1
  
• Windows 7 Professional (x64 English) X15-65805.iso
  
  SHA1: 50127304441A793EE51B3F501289F6599A559E9F
  
  ISO/CRC: 502C42C1
  
• Windows-7-SP1-EN-MSDN-TechNet.torrent

I like using aria2 for download. Get my aria2 win32 build from here. To use aria2c to download Windows 7 Home Premium, open the Command Prompt and type the following command.

aria2c --file-allocation=prealloc http://msft-dnl.digitalrivercontent.net/msvista/pub/X15-65732/X15-65732.iso

To stop the download, press Ctrl+C twice. Later, you can resume download with the command:

aria2c -c http://msft-dnl.digitalrivercontent.net/msvista/pub/X15-65732/X15-65732.iso

After the download has finished, verify the checksums of the download with md5deep or HashCheck shell extension.

This is optional. If you want to create a single All-in-One DVD that can be used to install Home Basic, Home Premium, Professional and Ultimate edition, download and run eicfg_remover. This tool removes ei.cfg from the ISO file.

Then, burn the image to a blank DVD. You need to have a DVD burner and a blank DVD, of course. For free DVD-burning software, you can get ImgBurn, InfraRecorder or CDBurnerXP.