System tips

System tips#

Here, I describe basic tips to configure and manage systems, mostly from the console.

The screen program#

screen(1) is a very useful tool for people to access remote sites via unreliable or intermittent connections since it supports interrupted network connections.

List of programs to support interrupted network connections

The use scenario for screen(1)#

screen(1) not only allows one terminal window to work with multiple processes, but also allows remote shell process to survive interrupted connections. Here is a typical use scenario of screen(1).

1. You login to a remote machine.

2. You start screen on a single console.

3. You execute multiple programs in screen windows created with ^A c ("Control-A" followed by "c").

4. You switch among the multiple screen windows by ^A n ("Control-A" followed by "n").

5. Suddenly you need to leave your terminal, but you don't want to lose your active work by keeping the connection.

6. You may detach the screen session by any methods.

   • Brutally unplug your network connection

   • Type ^A d ("Control-A" followed by "d") and manually logging out from the remote connection

   • Type ^A DD ("Control-A" followed by "DD") to have screen detach and log you out

7. You log in again to the same remote machine (even from a different terminal).

8. You start screen as "screen -r".

9. screen magically reattaches all previous screen windows with all actively running programs.

[!TIP] You can save connection fees with screen for metered network connections such as dial-up and packet ones, because you can leave a process active while disconnected, and then re-attach it later when you connect again.

Key bindings for the screen command#

In a screen session, all keyboard inputs are sent to your current window except for the command keystroke. All screen command keystrokes are entered by typing ^A ("Control-A") plus a single key [plus any parameters]. Here are important ones to remember.

List of key bindings for screen

See screen(1) for details.

Data recording and presentation#

The log daemon#

Many programs record their activities under the "/var/log/" directory.

• The system log daemon: rsyslogd(8)

See ??? and ???.

Log analyzer#

Here are notable log analyzers ("~Gsecurity::log-analyzer" in aptitude(8)).

List of system log analyzers

[!NOTE] CRM114 provides language infrastructure to write fuzzy filters with the TRE regex library. Its popular use is spam mail filter but it can be used as log analyzer.

Recording the shell activities cleanly#

The simple use of script(1) (see ???) to record shell activity produces a file with control characters. This can be avoided by using col(1) as the following.

$ script
Script started, file is typescript

Do whatever … and press Ctrl-D to exit script.

$ col -bx <typescript >cleanedfile
$ vim cleanedfile

If you don't have script (for example, during the boot process in the initramfs), you can use the following instead.

$ sh -i 2>&1 | tee typescript

[!TIP] Some x-terminal-emulator such as gnome-terminal can record. You may wish to extend line buffer for scrollback.

[!TIP] You may use screen(1) with "^A H" (see Key bindings for the screen command) to perform recording of console.

[!TIP] You may use emacs(1) with "M-x shell", "M-x eshell", or "M-x term" to perform recording of console. You may later use "C-x C-w" to write the buffer to a file.

Customized display of text data#

Although pager tools such as more(1) and less(1) (see ???) and custom tools for highlighting and formatting (see ???) can display text data nicely, general purpose editors (see ???) are most versatile and customizable.

[!TIP] For vim(1) and its pager mode alias view(1), ":set hls" enables highlighted search.

Customized display of time and date#

The default display format of time and date by the "ls -l" command depends on the locale (see ??? for value). The "$LANG" variable is referred first and it can be overridden by the "$LC_TIME" variable.

The actual default display format for each locale depends on the version of the standard C library (the libc6 package) used. I.e., different releases of Debian had different defaults.

If you really wish to customize this display format of time and date beyond the locale, you should set the time style value by the "--time-style" argument or by the "$TIME_STYLE" value (see ls(1), date(1), "info coreutils 'ls invocation'").

Display examples of time and date for the "ls -l" command for wheezy

[!TIP] You can eliminate typing long option on commandline using command alias, e.g. "alias ls='ls --time-style=+%d.%m.%y\ %H:%M'" (see ???).

[!TIP] ISO 8601 is followed for these iso-formats.

Colorized shell echo#

Shell echo to most modern terminals can be colorized using ANSI escape code (see "/usr/share/doc/xterm/ctlseqs.txt.gz").

For example, try the following

$ RED=$(printf "\x1b[31m")
$ NORMAL=$(printf "\x1b[0m")
$ REVERSE=$(printf "\x1b[7m")
$ echo "${RED}RED-TEXT${NORMAL} ${REVERSE}REVERSE-TEXT${NORMAL}"

Colorized commands#

Colorized commands are handy for inspecting their output in the interactive environment. I include the following in my "~/.bashrc".

if [ "$TERM" != "dumb" ]; then
    eval "`dircolors -b`"
    alias ls='ls --color=always'
    alias ll='ls --color=always -l'
    alias la='ls --color=always -A'
    alias less='less -R'
    alias ls='ls --color=always'
    alias grep='grep --color=always'
    alias egrep='egrep --color=always'
    alias fgrep='fgrep --color=always'
    alias zgrep='zgrep --color=always'
else
    alias ll='ls -l'
    alias la='ls -A'
fi

The use of alias limits color effects to the interactive command usage. It has advantage over exporting environment variable "export GREP_OPTIONS='--color=auto'" since color can be seen under pager programs such as less(1). If you wish to suppress color when piping to other programs, use "--color=auto" instead in the above example for "~/.bashrc".

[!TIP] You can turn off these colorizing aliases in the interactive environment by invoking shell with "TERM=dumb bash".

Recording the editor activities for complex repeats#

You can record the editor activities for complex repeats.

For Vim, as follows.

• "qa": start recording typed characters into named register "a".

• … editor activities

• "q": end recording typed characters.

• "@a": execute the contents of register "a".

For Emacs, as follows.

• "C-x (": start defining a keyboard macro.

• … editor activities

• "C-x )": end defining a keyboard macro.

• "C-x e": execute a keyboard macro.

Recording the graphic image of an X application#

There are few ways to record the graphic image of an X application, including an xterm display.

List of graphic image manipulation tools

Recording changes in configuration files#

There are specialized tools to record changes in configuration files with help of DVCS system.

List of packages to record configuration history in VCS

I recommend to use the etckeeper package with git(1) which put entire "/etc" under VCS control. Its installation guide and tutorial are found in "/usr/share/doc/etckeeper/README.gz".

Essentially, running "sudo etckeeper init" initializes the git repository for "/etc" just like the process explained in ??? but with special hook scripts for more thorough setups.

As you change your configuration, you can use git(1) normally to record them. It automatically records changes nicely every time you run package management commands, too.

[!TIP] You can browse the change history of "/etc" by executing "sudo GIT_DIR=/etc/.git gitk" with clear view for new installed packages, removed packages, and version changes of packages.

Monitoring, controlling, and starting program activities#

Program activities can be monitored and controlled using specialized tools.

List of tools for monitoring and controlling program activities

[!TIP] The procps packages provide very basics of monitoring, controlling, and starting program activities. You should learn all of them.

Timing a process#

Display time used by the process invoked by the command.

# time some_command >/dev/null
real    0m0.035s       # time on wall clock (elapsed real time)
user    0m0.000s       # time in user mode
sys     0m0.020s       # time in kernel mode

The scheduling priority#

A nice value is used to control the scheduling priority for the process.

List of nice values for the scheduling priority

# nice  -19 top                                      # very nice
# nice --20 wodim -v -eject speed=2 dev=0,0 disk.img # very fast

Sometimes an extreme nice value does more harm than good to the system. Use this command carefully.

The ps command#

The ps(1) command on a Debian system support both BSD and SystemV features and helps to identify the process activity statically.

List of ps command styles

For the zombie (defunct) children process, you can kill them by the parent process ID identified in the "PPID" field.

The pstree(1) command display a tree of processes.

The top command#

top(1) on the Debian system has rich features and helps to identify what process is acting funny dynamically.

It is an interactive full screen program. You can get its usage help press by pressing the "h"-key and terminate it by pressing the "q"-key.

Listing files opened by a process#

You can list all files opened by a process with a process ID (PID), e.g. 1, by the following.

$ sudo lsof -p 1

PID=1 is usually the init program.

Tracing program activities#

You can trace program activity with strace(1), ltrace(1), or xtrace(1) for system calls and signals, library calls, or communication between X11 client and server.

You can trace system calls of the ls command as the following.

$ sudo strace ls

Identification of processes using files or sockets#

You can also identify processes using files by fuser(1), e.g. for "/var/log/mail.log" by the following.

$ sudo fuser -v /var/log/mail.log
                     USER        PID ACCESS COMMAND
/var/log/mail.log:   root       2946 F.... rsyslogd

You see that file "/var/log/mail.log" is open for writing by the rsyslogd(8) command.

You can also identify processes using sockets by fuser(1), e.g. for "smtp/tcp" by the following.

$ sudo fuser -v smtp/tcp
                     USER        PID ACCESS COMMAND
smtp/tcp:            Debian-exim   3379 F.... exim4

Now you know your system runs exim4(8) to handle TCP connections to SMTP port (25).

Repeating a command with a constant interval#

watch(1) executes a program repeatedly with a constant interval while showing its output in fullscreen.

$ watch w

This displays who is logged on to the system updated every 2 seconds.

Repeating a command looping over files#

There are several ways to repeat a command looping over files matching some condition, e.g. matching glob pattern "*.ext".

• Shell for-loop method (see ???):

for x in *.ext; do if [ -f "$x"]; then command "$x" ; fi; done

• find(1) and xargs(1) combination:

find . -type f -maxdepth 1 -name '*.ext' -print0 | xargs -0 -n 1 command

• find(1) with "-exec" option with a command:

find . -type f -maxdepth 1 -name '*.ext' -exec command '{}' \;

• find(1) with "-exec" option with a short shell script:

find . -type f -maxdepth 1 -name '*.ext' -exec sh -c "command '{}' && echo 'successful'" \;

The above examples are written to ensure proper handling of funny file names such as ones containing spaces. See ??? for more advance uses of find(1).

Starting a program from GUI#

For the command-line interface (CLI), the first program with the matching name found in the directories specified in the $PATH environment variable is executed. See ???.

For the graphical user interface (GUI) compliant to the freedesktop.org standards, the *.desktop files in the /usr/share/applications/ directory provide necessary attributes for the GUI menu display of each program. See ???.

For example, the chromium.desktop file defines attributes for the "Chromium Web Browser" such as "Name" for the program name, "Exec" for the program execution path and arguments, "Icon" for the icon used, etc. (see the Desktop Entry Specification) as follows:

[Desktop Entry]
Version=1.0
Name=Chromium Web Browser
GenericName=Web Browser
Comment=Access the Internet
Comment[fr]=Explorer le Web
Exec=/usr/bin/chromium %U
Terminal=false
X-MultipleArgs=false
Type=Application
Icon=chromium
Categories=Network;WebBrowser;
MimeType=text/html;text/xml;application/xhtml_xml;x-scheme-handler/http;x-scheme-handler/https;
StartupWMClass=Chromium
StartupNotify=true

This is an oversimplified description. The *.desktop files are scanned as follows.

The desktop environment sets $XDG_DATA_HOME and $XDG_DATA_DIR environment variables. For example, under the GNOME 3:

• $XDG_DATA_HOME is unset. (The default value of $HOME/.local/share is used.)

• $XDG_DATA_DIRS is set to /usr/share/gnome:/usr/local/share/:/usr/share/.

So the base directories (see XDG Base Directory Specification) and the applications directories are as follows.

• $HOME/.local/share/ → $HOME/.local/share/applications/

• /usr/share/gnome/ → /usr/share/gnome/applications/

• /usr/local/share/ → /usr/local/share/applications/

• /usr/share/ → /usr/share/applications/

The *.desktop files are scanned in these applications directories in this order.

[!TIP] A user custom GUI menu entry can be created by adding a *.desktop file in the $HOME/.local/share/applications/ directory.

[!TIP] Similarly, if a *.desktop file is created in the autostart directory under these base directories, the specified program in the *.desktop file is executed automatically when the desktop environment is started. See Desktop Application Autostart Specification.

[!TIP] Similarly, if a *.desktop file is created in the $HOME/Desktop directory and the Desktop environment is configured to support the desktop icon launcher feature, the specified program in it is executed upon clicking the icon. Please note that the actual name of the $HOME/Desktop directory is locale dependent. See xdg-user-dirs-update(1).

Customizing program to be started#

Some programs start another program automatically. Here are check points for customizing this process.

• Application configuration menu:

• GNOME3 desktop: "Settings" → "System" → "Details" → "Default Applications"

• KDE desktop: "K" → "Control Center" → "KDE Components" → "Component Chooser"

• Iceweasel browser: "Edit" → "Preferences" → "Applications"

• mc(1): "/etc/mc/mc.ext"

• Environment variables such as "$BROWSER", "$EDITOR", "$VISUAL", and "$PAGER" (see eviron(7))

• The update-alternatives(1) system for programs such as "editor", "view", "x-www-browser", "gnome-www-browser", and "www-browser" (see ???)

• the "~/.mailcap" and "/etc/mailcap" file contents which associate MIME type with program (see mailcap(5))

• The "~/.mime.types" and "/etc/mime.types" file contents which associate file name extension with MIME type (see run-mailcap(1))

[!TIP] update-mime(8) updates the "/etc/mailcap" file using "/etc/mailcap.order" file (see mailcap.order(5)).

[!TIP] The debianutils package provides sensible-browser(1), sensible-editor(1), and sensible-pager(1) which make sensible decisions on which editor, pager, and web browser to call, respectively. I recommend you to read these shell scripts.

[!TIP] In order to run a console application such as mutt under X as your preferred application, you should create an X application as following and set "/usr/local/bin/mutt-term" as your preferred application to be started as described.

# cat /usr/local/bin/mutt-term <<EOF
#!/bin/sh
gnome-terminal -e "mutt \$@"
EOF
chmod 755 /usr/local/bin/mutt-term

Killing a process#

Use kill(1) to kill (or send a signal to) a process by the process ID.

Use killall(1) or pkill(1) to do the same by the process command name and other attributes.

List of frequently used signals for kill command

Scheduling tasks once#

Run the at(1) command to schedule a one-time job by the following.

$ echo 'command -args'| at 3:40 monday

Scheduling tasks regularly#

Use cron(8) to schedule tasks regularly. See crontab(1) and crontab(5).

You can schedule to run processes as a normal user, e.g. foo by creating a crontab(5) file as "/var/spool/cron/crontabs/foo" with "crontab -e" command.

Here is an example of a crontab(5) file.

# use /bin/sh to run commands, no matter what /etc/passwd says
SHELL=/bin/sh
# mail any output to paul, no matter whose crontab this is
MAILTO=paul
# Min Hour DayOfMonth Month DayOfWeek command (Day... are OR'ed)
# run at 00:05, every day
5  0  *  * *   $HOME/bin/daily.job >> $HOME/tmp/out 2>&1
# run at 14:15 on the first of every month -- output mailed to paul
15 14 1  * *   $HOME/bin/monthly
# run at 22:00 on weekdays(1-5), annoy Joe. % for newline, last % for cc:
0 22 *   * 1-5 mail -s "It's 10pm" joe%Joe,%%Where are your kids?%.%%
23 */2 1 2 *   echo "run 23 minutes after 0am, 2am, 4am ..., on Feb 1"
5  4 *   * sun echo "run at 04:05 every Sunday"
# run at 03:40 on the first Monday of each month
40 3 1-7 * *   [ "$(date +%a)" == "Mon" ] && command -args

[!TIP] For the system not running continuously, install the anacron package to schedule periodic commands at the specified intervals as closely as machine-uptime permits. See anacron(8) and anacrontab(5).

[!TIP] For scheduled system maintenance scripts, you can run them periodically from root account by placing such scripts in "/etc/cron.hourly/", "/etc/cron.daily/", "/etc/cron.weekly/", or "/etc/cron.monthly/". Execution timings of these scripts can be customized by "/etc/crontab" and "/etc/anacrontab".

Alt-SysRq key#

Insurance against system malfunction is provided by the kernel compile option "Magic SysRq key" (SAK key) which is now the default for the Debian kernel. Pressing Alt-SysRq followed by one of the following keys does the magic of rescuing control of the system.

List of SAK command keys

[!TIP] Read the signal(7), kill(1), and sync(1) manpages to understand the description above.

The combination of "Alt-SysRq s", "Alt-SysRq u", and "Alt-SysRq r" is good for getting out of really bad situations and gaining usable keyboard access without stopping the system.

See "/usr/share/doc/linux-doc-3.*/Documentation/sysrq.txt.gz".

[!CAUTION] The Alt-SysRq feature may be considered a security risk by allowing users access to root-privileged functions. Placing "echo 0 >/proc/sys/kernel/sysrq" in "/etc/rc.local" or "kernel.sysrq = 0" in "/etc/sysctl.conf" disables the Alt-SysRq feature.

[!TIP] From SSH terminal etc., you can use the Alt-SysRq feature by writing to the "/proc/sysrq-trigger". For example, "echo s > /proc/sysrq-trigger; echo u > /proc/sysrq-trigger" from the root shell prompt syncs and umounts all mounted filesystems.

System maintenance tips#

Who is on the system?#

You can check who is on the system by the following.

• who(1) shows who is logged on.

• w(1) shows who is logged on and what they are doing.

• last(1) shows listing of last logged in user.

• lastb(1) shows listing of last bad logged in users.

[!TIP] "/var/run/utmp", and "/var/log/wtmp" hold such user information. See login(1) and utmp(5).

Warning everyone#

You can send message to everyone who is logged on to the system with wall(1) by the following.

$ echo "We are shutting down in 1 hour" | wall

Hardware identification#

For the PCI-like devices (AGP, PCI-Express, CardBus, ExpressCard, etc.), lspci(8) (probably with "-nn" option) is a good start for the hardware identification.

Alternatively, you can identify the hardware by reading contents of "/proc/bus/pci/devices" or browsing directory tree under "/sys/bus/pci" (see ???).

List of hardware identification tools

Hardware configuration#

Although most of the hardware configuration on modern GUI desktop systems such as GNOME and KDE can be managed through accompanying GUI configuration tools, it is a good idea to know some basics methods to configure them.

List of hardware configuration tools

Here, ACPI is a newer framework for the power management system than APM.

[!TIP] CPU frequency scaling on modern system is governed by kernel modules such as acpi_cpufreq.

System and hardware time#

The following sets system and hardware time to MM/DD hh:mm, CCYY.

# date MMDDhhmmCCYY
# hwclock --utc --systohc
# hwclock --show

Times are normally displayed in the local time on the Debian system but the hardware and system time usually use UTC(GMT).

If the hardware (BIOS) time is set to UTC, change the setting to "UTC=yes" in the "/etc/default/rcS".

The following reconfigure the timezone used by the Debian system.

# dpkg-reconfigure tzdata

If you wish to update system time via network, consider to use the NTP service with the packages such as ntp, ntpdate, and chrony.

[!TIP] Under systemd, use systemd-timesyncd for the network time synchronization instead. See systemd-timesyncd(8).

See the following.

• Managing Accurate Date and Time HOWTO

• NTP Public Services Project

• The ntp-doc package

[!TIP] ntptrace(8) in the ntp package can trace a chain of NTP servers back to the primary source.

The terminal configuration#

There are several components to configure character console and ncurses(3) system features.

• The "/etc/terminfo/*/*" file (terminfo(5))

• The "$TERM" environment variable (term(7))

• setterm(1), stty(1), tic(1), and toe(1)

If the terminfo entry for xterm doesn't work with a non-Debian xterm, change your terminal type, "$TERM", from "xterm" to one of the feature-limited versions such as "xterm-r6" when you log in to a Debian system remotely. See "/usr/share/doc/libncurses5/FAQ" for more. "dumb" is the lowest common denominator for "$TERM".

The sound infrastructure#

Device drivers for sound cards for current Linux are provided by Advanced Linux Sound Architecture (ALSA). ALSA provides emulation mode for previous Open Sound System (OSS) for compatibility.

[!TIP] Use "cat /dev/urandom > /dev/audio" or speaker-test(1) to test speaker (^C to stop).

[!TIP] If you can not get sound, your speaker may be connected to a muted output. Modern sound system has many outputs. alsamixer(1) in the alsa-utils package is useful to configure volume and mute settings.

Application softwares may be configured not only to access sound devices directly but also to access them via some standardized sound server system.

List of sound packages

There is usually a common sound engine for each popular desktop environment. Each sound engine used by the application can choose to connect to different sound servers.

Disabling the screen saver#

For disabling the screen saver, use following commands.

List of commands for disabling the screen saver

Disabling beep sounds#

One can always unplug the PC speaker to disable beep sounds. Removing pcspkr kernel module does this for you.

The following prevents the readline(3) program used by bash(1) to beep when encountering an alert character (ASCII=7).

$ echo "set bell-style none">> ~/.inputrc

Memory usage#

There are 2 resources available for you to get the memory usage situation.

• The kernel boot message in the "/var/log/dmesg" contains the total exact size of available memory.

• free(1) and top(1) display information on memory resources on the running system.

Here is an example.

# grep '\] Memory' /var/log/dmesg
[    0.004000] Memory: 990528k/1016784k available (1975k kernel code, 25868k reserved, 931k data, 296k init)
$ free -k
             total       used       free     shared    buffers     cached
Mem:        997184     976928      20256          0     129592     171932
-/+ buffers/cache:     675404     321780
Swap:      4545576          4    4545572

You may be wondering "dmesg tells me a free of 990 MB, and free -k says 320 MB is free. More than 600 MB missing …".

Do not worry about the large size of "used" and the small size of "free" in the "Mem:" line, but read the one under them (675404 and 321780 in the example above) and relax.

For my MacBook with 1GB=1048576k DRAM (video system steals some of this), I see the following.

List of memory sizes reported

System security and integrity check#

Poor system maintenance may expose your system to external exploitation.

For system security and integrity check, you should start with the following.

• The debsums package, see debsums(1) and ???.

• The chkrootkit package, see chkrootkit(1).

• The clamav package family, see clamscan(1) and freshclam(1).

• Debian security FAQ.

• Securing Debian Manual.

List of tools for system security and integrity check

Here is a simple script to check for typical world writable incorrect file permissions.

# find / -perm 777 -a \! -type s -a \! -type l -a \! \( -type d -a -perm 1777 \)

[!CAUTION] Since the debsums package uses MD5 checksums stored locally, it can not be fully trusted as the system security audit tool against malicious attacks.

Data storage tips#

Booting your system with Linux live CDs or debian-installer CDs in rescue mode makes it easy for you to reconfigure data storage on your boot device.

Disk space usage#

The disk space usage can be evaluated by programs provided by the mount, coreutils, and xdu packages:

• mount(8) reports all mounted filesystems (= disks).

• df(1) reports the disk space usage for the file system.

• du(1) reports the disk space usage for the directory tree.

[!TIP] You can feed the output of du(8) to xdu(1x) to produce its graphical and interactive presentation with "du -k . |xdu", "sudo du -k -x / |xdu", etc.

Disk partition configuration#

For disk partition configuration, although fdisk(8) has been considered standard, parted(8) deserves some attention. "Disk partitioning data", "partition table", "partition map", and "disk label" are all synonyms.

Most PCs use the classic Master Boot Record (MBR) scheme to hold disk partitioning data in the first sector, i.e., LBA sector 0 (512 bytes).

[!NOTE] Some new PCs with Extensible Firmware Interface (EFI), including Intel-based Macs, use GUID Partition Table (GPT) scheme to hold disk partitioning data not in the first sector.

Although fdisk(8) has been standard for the disk partitioning tool, parted(8) is replacing it.

List of disk partition management packages

[!CAUTION] Although parted(8) claims to create and to resize filesystem too, it is safer to do such things using best maintained specialized tools such as mkfs(8) (mkfs.msdos(8), mkfs.ext2(8), mkfs.ext3(8), mkfs.ext4(8), …) and resize2fs(8).

[!NOTE] In order to switch between GPT and MBR, you need to erase first few blocks of disk contents directly (see Clearing file contents) and use "parted /dev/sdx mklabel gpt" or "parted /dev/sdx mklabel msdos" to set it. Please note "msdos" is use here for MBR.

Accessing partition using UUID#

Although reconfiguration of your partition or activation order of removable storage media may yield different names for partitions, you can access them consistently. This is also helpful if you have multiple disks and your BIOS doesn't give them consistent device names.

• mount(8) with "-U" option can mount a block device using UUID, instead of using its file name such as "/dev/sda3".

• "/etc/fstab" (see fstab(5)) can use UUID.

• Boot loaders (???) may use UUID too.

[!TIP] You can probe UUID of a block special device with blkid(8).

[!TIP] Device nodes of devices such as removable storage media can be made static by using udev rules, if needed. See ???.

LVM2#

LVM2 is a logical volume manager for the Linux kernel. With LVM2, disk partitions can be created on logical volumes instead of the physical harddisks.

LVM requires the following.

• device-mapper support in the Linux kernel (default for Debian kernels)

• the userspace device-mapper support library (libdevmapper* package)

• the userspace LVM2 tools (lvm2 package)

Please start learning LVM2 from the following manpages.

• lvm(8): Basics of LVM2 mechanism (list of all LVM2 commands)

• lvm.conf(5): Configuration file for LVM2

• lvs(8): Report information about logical volumes

• vgs(8): Report information about volume groups

• pvs(8): Report information about physical volumes

Filesystem configuration#

For ext4 filesystem, the e2fsprogs package provides the following.

• mkfs.ext4(8) to create new ext4 filesystem

• fsck.ext4(8) to check and to repair existing ext4 filesystem

• tune2fs(8) to configure superblock of ext4 filesystem

• debugfs(8) to debug ext4 filesystem interactively. (It has undel command to recover deleted files.)

The mkfs(8) and fsck(8) commands are provided by the e2fsprogs package as front-ends to various filesystem dependent programs (mkfs.fstype and fsck.fstype). For ext4 filesystem, they are mkfs.ext4(8) and fsck.ext4(8) (they are symlinked to mke2fs(8) and e2fsck(8)).

Similar commands are available for each filesystem supported by Linux.

List of filesystem management packages

[!TIP] Ext4 filesystem is the default filesystem for the Linux system and strongly recommended to use it unless you have some specific reasons not to.

[!TIP] Btrfs filesystem is available in Linux kernel 3.2 (Debian wheezy). It is expected to be the next default filesystem after the ext4 filesystem.

[!WARNING] You should not use the Btrfs filesystem for your critical data yet before it acquires the live kernel space fsck(8) feature and the boot loader support.

[!TIP] Some tools allow access to filesystem without Linux kernel support (see Manipulating files without mounting disk).

Filesystem creation and integrity check#

The mkfs(8) command creates the filesystem on a Linux system. The fsck(8) command provides the filesystem integrity check and repair on a Linux system.

Debian now defaults to no periodic fsck after filesystem creation.

[!CAUTION] It is generally not safe to run fsck on mounted filesystems.

[!TIP] You can run the fsck(8) command safely on all filesystems including root filesystem on reboot by setting "enable_periodic_fsck" in "/etc/mke2fs.conf" and the max mount count to 0 using "tune2fs -c0 /dev/<partition_name>". See mke2fs.conf(5) and tune2fs(8).

[!TIP] Check files in "/var/log/fsck/" for the result of the fsck(8) command run from the boot script.

Optimization of filesystem by mount options#

The basic static filesystem configuration is given by "/etc/fstab". For example,

# <file system> <mount point>   <type>  <options>       <dump>  <pass>
proc            /proc           proc    defaults        0       0
UUID=709cbe4c-80c1-56db-8ab1-dbce3146d2f7 / ext4 noatime,errors=remount-ro 0 1
UUID=817bae6b-45d2-5aca-4d2a-1267ab46ac23 none swap sw  0       0
/dev/scd0       /media/cdrom0   udf,iso9660 user,noauto 0       0

[!TIP] UUID (see Accessing partition using UUID) may be used to identify a block device instead of normal block device names such as "/dev/sda1", "/dev/sda2", …

Performance and characteristics of a filesystem can be optimized by mount options used on it (see fstab(5) and mount(8)). Notable ones are the following.

• "defaults" option implies default options: "rw,suid,dev,exec,auto,nouser,async". (general)

• "noatime" or "relatime" option is very effective for speeding up the read access. (general)

• "user" option allows an ordinary user to mount the filesystem. This option implies "noexec,nosuid,nodev" option combination. (general, used for CDs or usb storage devices)

• "noexec,nodev,nosuid" option combination is used to enhance security. (general)

• "noauto" option limits mounting by explicit operation only. (general)

• "data=journal" option for ext3fs can enhance data integrity against power failure with some loss of write speed.

[!TIP] You need to provide kernel boot parameter (see ???), e.g. "rootflags=data=journal" to deploy a non-default journaling mode for the root filesystem. For lenny, the default jounaling mode is "rootflags=data=ordered". For squeeze, it is "rootflags=data=writeback".

Optimization of filesystem via superblock#

Characteristics of a filesystem can be optimized via its superblock using the tune2fs(8) command.

• Execution of "sudo tune2fs -l /dev/hda1" displays the contents of the filesystem superblock on "/dev/hda1".

• Execution of "sudo tune2fs -c 50 /dev/hda1" changes frequency of filesystem checks (fsck execution during boot-up) to every 50 boots on "/dev/hda1".

• Execution of "sudo tune2fs -j /dev/hda1" adds journaling capability to the filesystem, i.e. filesystem conversion from ext2 to ext3 on "/dev/hda1". (Do this on the unmounted filesystem.)

• Execution of "sudo tune2fs -O extents,uninit_bg,dir_index /dev/hda1 && fsck -pf /dev/hda1" converts it from ext3 to ext4 on "/dev/hda1". (Do this on the unmounted filesystem.)

[!TIP] Despite its name, tune2fs(8) works not only on the ext2 filesystem but also on the ext3 and ext4 filesystems.

Optimization of hard disk#

[!WARNING] Please check your hardware and read manpage of hdparam(8) before playing with hard disk configuration because this may be quite dangerous for the data integrity.

You can test disk access speed of a hard disk, e.g. "/dev/hda", by "hdparm -tT /dev/hda". For some hard disk connected with (E)IDE, you can speed it up with "hdparm -q -c3 -d1 -u1 -m16 /dev/hda" by enabling the "(E)IDE 32-bit I/O support", enabling the "using_dma flag", setting "interrupt-unmask flag", and setting the "multiple 16 sector I/O" (dangerous!).

You can test write cache feature of a hard disk, e.g. "/dev/sda", by "hdparm -W /dev/sda". You can disable its write cache feature with "hdparm -W 0 /dev/sda".

You may be able to read badly pressed CDROMs on modern high speed CD-ROM drive by slowing it down with "setcd -x 2".

Optimization of solid state drive#

Performance and disk wear of the solid state drive (SSD) can be optimized as follows.

• Use the latest Linux kernel. (>= 3.2)

• Reduce disk writes for read disk accesses.

• Set "noatime" or "relatime" mount option in /etc/fstab.

• Enable the TRIM command.

• Set "discard" mount option in /etc/fstab for the ext4 filesystem, swap partition, Btrfs, etc. See fstab(5).

• Set "discard" option in /etc/lvm/lvm.conf for LVM. See lvm.conf(5).

• Set "discard" option in /etc/crypttab for dm-crypt. See crypttab(5).

• Enable the SSD optimized disk space allocation scheme.

• Set "ssd" mount option in /etc/fstab for the Btrfs.

• Make system flush data to the disk every 10 minutes for laptop PCs.

• Set "commit=600" mount option in /etc/fstab. See fstab(5).

• Set pm-utils to use laptop-mode even under AC operation. See Debian BTS #659260.

[!WARNING] Changing flushing interval from normal 5 seconds to 10 minutes makes your data vulnerable to the power failure.

Using SMART to predict hard disk failure#

You can monitor and log your hard disk which is compliant to SMART with the smartd(8) daemon.

1. Enable SMART feature in BIOS.

2. Install the smartmontools package.

3. Identify your hard disk drives by listing them with df(1).

   • Let's assume a hard disk drive to be monitored as "/dev/hda".

4. Check the output of "smartctl -a /dev/hda" to see if SMART feature is actually enabled.

   • If not, enable it by "smartctl -s on -a /dev/hda".

5. Enable smartd(8) daemon to run by the following.

   • uncomment "start_smartd=yes" in the "/etc/default/smartmontools" file.

   • restart the smartd(8) daemon by "sudo /etc/init.d/smartmontools restart".

[!TIP] The smartd(8) daemon can be customized with the /etc/smartd.conf file including how to be notified of warnings.

Specify temporary storage directory via \$TMPDIR#

Applications create temporary files normally under the temporary storage directory "/tmp". If "/tmp" does not provide enough space, you can specify such temporary storage directory via the $TMPDIR variable for well-behaving programs.

Expansion of usable storage space via LVM#

For partitions created on Logical Volume Manager (LVM) (Linux feature) at install time, they can be resized easily by concatenating extents onto them or truncating extents from them over multiple storage devices without major system reconfiguration.

Expansion of usable storage space by mounting another partition#

If you have an empty partition (e.g., "/dev/sdx"), you can format it with mkfs.ext4(1) and mount(8) it to a directory where you need more space. (You need to copy original data contents.)

$ sudo mv work-dir old-dir
$ sudo mkfs.ext4 /dev/sdx
$ sudo mount -t ext4 /dev/sdx work-dir
$ sudo cp -a old-dir/* work-dir
$ sudo rm -rf old-dir

[!TIP] You may alternatively mount an empty disk image file (see Making the empty disk image file) as a loop device (see Mounting the disk image file). The actual disk usage grows with the actual data stored.

Expansion of usable storage space by bind-mounting another directory#

If you have an empty directory (e.g., "/path/to/emp-dir") on another partition with usable space, you can mount(8) it with "--bind" option to a directory (e.g., "work-dir") where you need more space.

$ sudo mount --bind /path/to/emp-dir work-dir

Expansion of usable storage space by overlay-mounting another directory#

If you have usable space in another partition (e.g., "/path/to/empty" and "/path/to/work"), you can create a directory in it and stack that on to an old directory (e.g., "/path/to/old") where you need space using the OverlayFS for Linux kernel 3.18 or newer (Debian Stretch 9.0 or newer).

$ sudo mount -t overlay overlay \
  -olowerdir=/path/to/old-dir,upperdir=/path/to/empty,workdir=/path/to/work

Here, "/path/to/empty" and "/path/to/work" should be on the RW-enabled partition to write on "/path/to/old".

Expansion of usable storage space using symlink#

[!CAUTION] This is a deprecated method. Some software may not function well with "symlink to a directory". Instead, use the "mounting" approaches described in the above.

If you have an empty directory (e.g., "/path/to/emp-dir") in another partition with usable space, you can create a symlink to the directory with ln(8).

$ sudo mv work-dir old-dir
$ sudo mkdir -p /path/to/emp-dir
$ sudo ln -sf /path/to/emp-dir work-dir
$ sudo cp -a old-dir/* work-dir
$ sudo rm -rf old-dir

[!WARNING] Do not use "symlink to a directory" for directories managed by the system such as "/opt". Such a symlink may be overwritten when the system is upgraded.

The disk image#

Here, we discuss manipulations of the disk image.

Making the disk image file#

The disk image file, "disk.img", of an unmounted device, e.g., the second SCSI or serial ATA drive "/dev/sdb", can be made using cp(1) or dd(1) by the following.

# cp /dev/sdb disk.img
# dd if=/dev/sdb of=disk.img

The disk image of the traditional PC's master boot record (MBR) (see Disk partition configuration) which reside on the first sector on the primary IDE disk can be made by using dd(1) by the following.

# dd if=/dev/hda of=mbr.img bs=512 count=1
# dd if=/dev/hda of=mbr-nopart.img bs=446 count=1
# dd if=/dev/hda of=mbr-part.img skip=446 bs=1 count=66

• "mbr.img": The MBR with the partition table

• "mbr-nopart.img": The MBR without the partition table

• "mbr-part.img": The partition table of the MBR only

If you have an SCSI or serial ATA device as the boot disk, substitute "/dev/hda" with "/dev/sda".

If you are making an image of a disk partition of the original disk, substitute "/dev/hda" with "/dev/hda1" etc.

Writing directly to the disk#

The disk image file, "disk.img" can be written to an unmounted device, e.g., the second SCSI drive "/dev/sdb" with matching size, by the following.

# dd if=disk.img of=/dev/sdb

Similarly, the disk partition image file, "partition.img" can be written to an unmounted partition, e.g., the first partition of the second SCSI drive "/dev/sdb1" with matching size, by the following.

# dd if=partition.img of=/dev/sdb1

Mounting the disk image file#

The disk image "partition.img" containing a single partition image can be mounted and unmounted by using the loop device as follows.

# losetup -v -f partition.img
Loop device is /dev/loop0
# mkdir -p /mnt/loop0
# mount -t auto /dev/loop0 /mnt/loop0
...hack...hack...hack
# umount /dev/loop0
# losetup -d /dev/loop0

This can be simplified as follows.

# mkdir -p /mnt/loop0
# mount -t auto -o loop partition.img /mnt/loop0
...hack...hack...hack
# umount partition.img

Each partition of the disk image "disk.img" containing multiple partitions can be mounted by using the loop device. Since the loop device does not manage partitions by default, we need to reset it as follows.

# modinfo -p loop # verify kernel capability
max_part:Maximum number of partitions per loop device
max_loop:Maximum number of loop devices
# losetup -a # verify nothing using the loop device
# rmmod loop
# modprobe loop max_part=16

Now, the loop device can manage up to 16 partitions.

# losetup -v -f disk.img
Loop device is /dev/loop0
# fdisk -l /dev/loop0

Disk /dev/loop0: 5368 MB, 5368709120 bytes
255 heads, 63 sectors/track, 652 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x452b6464

      Device Boot      Start         End      Blocks   Id  System
/dev/loop0p1               1         600     4819468+  83  Linux
/dev/loop0p2             601         652      417690   83  Linux
# mkdir -p /mnt/loop0p1
# mount -t ext4 /dev/loop0p1 /mnt/loop0p1
# mkdir -p /mnt/loop0p2
# mount -t ext4 /dev/loop0p2 /mnt/loop0p2
...hack...hack...hack
# umount /dev/loop0p1
# umount /dev/loop0p2
# losetup -d /dev/loop0

Alternatively, similar effects can be done by using the device mapper devices created by kpartx(8) from the kpartx package as follows.

# kpartx -a -v disk.img
...
# mkdir -p /mnt/loop0p2
# mount -t ext4 /dev/mapper/loop0p2 /mnt/loop0p2
...
...hack...hack...hack
# umount /dev/mapper/loop0p2
...
# kpartx -d /mnt/loop0

[!NOTE] You can mount a single partition of such disk image with loop device using offset to skip MBR etc., too. But this is more error prone.

Cleaning a disk image file#

A disk image file, "disk.img" can be cleaned of all removed files into clean sparse image "new.img" by the following.

# mkdir old; mkdir new
# mount -t auto -o loop disk.img old
# dd bs=1 count=0 if=/dev/zero of=new.img seek=5G
# mount -t auto -o loop new.img new
# cd old
# cp -a --sparse=always ./ ../new/
# cd ..
# umount new.img
# umount disk.img

If "disk.img" is in ext2, ext3 or ext4, you can also use zerofree(8) from the zerofree package as follows.

# losetup -f -v disk.img
Loop device is /dev/loop3
# zerofree /dev/loop3
# cp --sparse=always disk.img new.img

Making the empty disk image file#

The empty disk image "disk.img" which can grow up to 5GiB can be made using dd(1) as follows.

$ dd bs=1 count=0 if=/dev/zero of=disk.img seek=5G

You can create an ext4 filesystem on this disk image "disk.img" using the loop device as follows.

# losetup -f -v disk.img
Loop device is /dev/loop1
# mkfs.ext4 /dev/loop1
...hack...hack...hack
# losetup -d /dev/loop1
$ du  --apparent-size -h disk.img
5.0G  disk.img
$ du -h disk.img
83M disk.img

For "disk.img", its file size is 5.0 GiB and its actual disk usage is mere 83MiB. This discrepancy is possible since ext4 can hold sparse file.

[!TIP] The actual disk usage of sparse file grows with data which are written to it.

Using similar operation on devices created by the loop device or the device mapper devices as Mounting the disk image file, you can partition this disk image "disk.img" using parted(8) or fdisk(8), and can create filesystem on it using mkfs.ext4(8), mkswap(8), etc.

Making the ISO9660 image file#

The ISO9660 image file, "cd.iso", from the source directory tree at "source_directory" can be made using genisoimage(1) provided by cdrkit by the following.

#  genisoimage -r -J -T -V volume_id -o cd.iso source_directory

Similarly, the bootable ISO9660 image file, "cdboot.iso", can be made from debian-installer like directory tree at "source_directory" by the following.

#  genisoimage -r -o cdboot.iso -V volume_id \
   -b isolinux/isolinux.bin -c isolinux/boot.cat \
   -no-emul-boot -boot-load-size 4 -boot-info-table source_directory

Here Isolinux boot loader (see ???) is used for booting.

You can calculate the md5sum value and make the ISO9660 image directly from the CD-ROM device as follows.

$ isoinfo -d -i /dev/cdrom
CD-ROM is in ISO 9660 format
...
Logical block size is: 2048
Volume size is: 23150592
...
# dd if=/dev/cdrom bs=2048 count=23150592 conv=notrunc,noerror | md5sum
# dd if=/dev/cdrom bs=2048 count=23150592 conv=notrunc,noerror > cd.iso

[!WARNING] You must carefully avoid ISO9660 filesystem read ahead bug of Linux as above to get the right result.

Writing directly to the CD/DVD-R/RW#

[!TIP] DVD is only a large CD to wodim(1) provided by cdrkit.

You can find a usable device by the following.

# wodim --devices

Then the blank CD-R is inserted to the CD drive, and the ISO9660 image file, "cd.iso" is written to this device, e.g., "/dev/hda", using wodim(1) by the following.

# wodim -v -eject dev=/dev/hda cd.iso

If CD-RW is used instead of CD-R, do this instead by the following.

# wodim -v -eject blank=fast dev=/dev/hda cd.iso

[!TIP] If your desktop system mounts CDs automatically, unmount it by "sudo umount /dev/hda" from console before using wodim(1).

Mounting the ISO9660 image file#

If "cd.iso" contains an ISO9660 image, then the following manually mounts it to "/cdrom".

# mount -t iso9660 -o ro,loop cd.iso /cdrom

[!TIP] Modern desktop system may mount removable media such as ISO9660 formatted CD automatically (see ???).

The binary data#

Here, we discuss direct manipulations of the binary data on storage media.

Viewing and editing binary data#

The most basic viewing method of binary data is to use "od -t x1" command.

List of packages which view and edit binary data

[!TIP] HEX is used as an acronym for hexadecimal format with radix 16. OCTAL is for octal format with radix 8. ASCII is for American Standard Code for Information Interchange, i.e., normal English text code. EBCDIC is for Extended Binary Coded Decimal Interchange Code used on IBM mainframe operating systems.

Manipulating files without mounting disk#

There are tools to read and write files without mounting disk.

List of packages to manipulate files without mounting disk

Data redundancy#

Software RAID systems offered by the Linux kernel provide data redundancy in the kernel filesystem level to achieve high levels of storage reliability.

There are tools to add data redundancy to files in application program level to achieve high levels of storage reliability, too.

List of tools to add data redundancy to files

Data file recovery and forensic analysis#

There are tools for data file recovery and forensic analysis.

List of packages for data file recovery and forensic analysis

[!TIP] You can undelete files on the ext2 filesystem using list_deleted_inodes and undel commands of debugfs(8) in the e2fsprogs package.

Splitting a large file into small files#

When a data is too big to backup as a single file, you can backup its content after splitting it into, e.g. 2000MiB chunks and merge those chunks back into the original file later.

$ split -b 2000m large_file
$ cat x* >large_file

[!CAUTION] Please make sure you do not have any files starting with "x" to avoid name crashes.

Clearing file contents#

In order to clear the contents of a file such as a log file, do not use rm(1) to delete the file and then create a new empty file, because the file may still be accessed in the interval between commands. The following is the safe way to clear the contents of the file.

$ :>file_to_be_cleared

Dummy files#

The following commands create dummy or empty files.

$ dd if=/dev/zero    of=5kb.file bs=1k count=5
$ dd if=/dev/urandom of=7mb.file bs=1M count=7
$ touch zero.file
$ : > alwayszero.file

You should find following files.

• "5kb.file" is 5KB of zeros.

• "7mb.file" is 7MB of random data.

• "zero.file" may be a 0 byte file. If it existed, its mtime is updated while its content and its length are kept.

• "alwayszero.file" is always a 0 byte file. If it existed, its mtime is updated and its content is reset.

Erasing an entire hard disk#

There are several ways to completely erase data from an entire hard disk like device, e.g., USB memory stick at "/dev/sda".

[!CAUTION] Check your USB memory stick location with mount(8) first before executing commands here. The device pointed by "/dev/sda" may be SCSI hard disk or serial-ATA hard disk where your entire system resides.

Erase all the disk content by resetting data to 0 with the following.

# dd if=/dev/zero of=/dev/sda

Erase everything by overwriting with random data as follows.

# dd if=/dev/urandom of=/dev/sda

Erase everything by overwriting with random data very efficiently as follows.

# shred -v -n 1 /dev/sda

Since dd(1) is available from the shell of many bootable Linux CDs such as Debian installer CD, you can erase your installed system completely by running an erase command from such media on the system hard disk, e.g., "/dev/hda", "/dev/sda", etc.

Erasing unused area of an hard disk#

Unused area on an hard disk (or USB memory stick), e.g. "/dev/sdb1" may still contain erased data themselves since they are only unlinked from the filesystem. These can be cleaned by overwriting them.

# mount -t auto /dev/sdb1 /mnt/foo
# cd /mnt/foo
# dd if=/dev/zero of=junk
dd: writing to `junk': No space left on device
...
# sync
# umount /dev/sdb1

[!WARNING] This is usually good enough for your USB memory stick. But this is not perfect. Most parts of erased filenames and their attributes may be hidden and remain in the filesystem.

Undeleting deleted but still open files#

Even if you have accidentally deleted a file, as long as that file is still being used by some application (read or write mode), it is possible to recover such a file.

For example, try the following

$ echo foo > bar
$ less bar
$ ps aux | grep ' less[ ]'
bozo    4775  0.0  0.0  92200   884 pts/8    S+   00:18   0:00 less bar
$ rm bar
$ ls -l /proc/4775/fd | grep bar
lr-x------ 1 bozo bozo 64 2008-05-09 00:19 4 -> /home/bozo/bar (deleted)
$ cat /proc/4775/fd/4 >bar
$ ls -l
-rw-r--r-- 1 bozo bozo 4 2008-05-09 00:25 bar
$ cat bar
foo

Execute on another terminal (when you have the lsof package installed) as follows.

$ ls -li bar
2228329 -rw-r--r-- 1 bozo bozo 4 2008-05-11 11:02 bar
$ lsof |grep bar|grep less
less 4775 bozo 4r REG 8,3 4 2228329 /home/bozo/bar
$ rm bar
$ lsof |grep bar|grep less
less 4775 bozo 4r REG 8,3 4 2228329 /home/bozo/bar (deleted)
$ cat /proc/4775/fd/4 >bar
$ ls -li bar
2228302 -rw-r--r-- 1 bozo bozo 4 2008-05-11 11:05 bar
$ cat bar
foo

Searching all hardlinks#

Files with hardlinks can be identified by "ls -li".

$ ls -li
total 0
2738405 -rw-r--r-- 1 root root 0 2008-09-15 20:21 bar
2738404 -rw-r--r-- 2 root root 0 2008-09-15 20:21 baz
2738404 -rw-r--r-- 2 root root 0 2008-09-15 20:21 foo

Both "baz" and "foo" have link counts of "2" (>1) showing them to have hardlinks. Their inode numbers are common "2738404". This means they are the same hardlinked file. If you do not happen to find all hardlinked files by chance, you can search it by the inode, e.g., "2738404" as the following.

# find /path/to/mount/point -xdev -inum 2738404

Invisible disk space consumption#

All deleted but open files consume disk space although they are not visible from normal du(1). They can be listed with their size by the following.

# lsof -s -X / |grep deleted

Data encryption tips#

With physical access to your PC, anyone can easily gain root privilege and access all the files on your PC (see ???). This means that login password system can not secure your private and sensitive data against possible theft of your PC. You must deploy data encryption technology to do it. Although GNU privacy guard (see ???) can encrypt files, it takes some user efforts.

dm-crypt and eCryptfs facilitates automatic data encryption natively via Linux kernel modules with minimal user efforts.

List of data encryption utilities

Dm-crypt is a cryptographic filesystem using device-mapper. Device-mapper maps one block device to another.

eCryptfs is another cryptographic filesystem using stacked filesystem. Stacked filesystem stacks itself on top of an existing directory of a mounted filesystem.

[!CAUTION] Data encryption costs CPU time etc. Please weigh its benefits and costs.

[!NOTE] Entire Debian system can be installed on a encrypted disk by the debian-installer (lenny or newer) using dm-crypt/LUKS and initramfs.

[!TIP] See ??? for user space encryption utility: GNU Privacy Guard.

Removable disk encryption with dm-crypt/LUKS#

You can encrypt contents of removable mass devices, e.g. USB memory stick on "/dev/sdx", using dm-crypt/LUKS. You simply format it as the following.

# badblocks -c 1024 -s -w -t random -v /dev/sdx
# fdisk /dev/sdx
... "n" "p" "1" "return" "return" "w"
# cryptsetup luksFormat /dev/sdx1
...
# cryptsetup open --type luks /dev/sdx1 sdx1
...
# ls -l /dev/mapper/
total 0
crw-rw---- 1 root root  10, 60 2008-10-04 18:44 control
brw-rw---- 1 root disk 254,  0 2008-10-04 23:55 sdx1
# mkfs.vfat /dev/mapper/sdx1
...
# cryptsetup luksClose sdx1

Then, it can be mounted just like normal one on to "/media/<disk_label>", except for asking password (see ???) under modern desktop environment, such as GNOME using gnome-mount(1). The difference is that every data written to it is encrypted. You may alternatively format media in different filesystem, e.g., ext4 with "mkfs.ext4 /dev/mapper/sdx1".

[!NOTE] If you are really paranoid for the security of data, you may need to overwrite multiple times (the "badblocks" command in the above example). This operation is very time consuming though.

Encrypted swap partition with dm-crypt#

Let's assume that your original "/etc/fstab" contains the following.

/dev/sda7 swap sw 0 0

You can enable encrypted swap partition using dm-crypt by as the following.

# aptitude install cryptsetup
# swapoff -a
# echo "cswap /dev/sda7 /dev/urandom swap" >> /etc/crypttab
# perl -i -p -e "s/\/dev\/sda7/\/dev\/mapper\/cswap/" /etc/fstab
# /etc/init.d/cryptdisks restart
 ...
# swapon -a

Mounting encrypted disk with dm-crypt/LUKS#

An encrypted disk partition created with dm-crypt/LUKS on "/dev/sdc5" can be mounted onto "/mnt" as follows:

$ sudo cryptsetup open /dev/sdc5 ninja --type luks
Enter passphrase for /dev/sdc5: ****
$ sudo lvm
lvm> lvscan
  inactive          '/dev/ninja-vg/root' [13.52 GiB] inherit
  inactive          '/dev/ninja-vg/swap_1' [640.00 MiB] inherit
  ACTIVE            '/dev/goofy/root' [180.00 GiB] inherit
  ACTIVE            '/dev/goofy/swap' [9.70 GiB] inherit
lvm> lvchange -a y /dev/ninja-vg/root
lvm> exit
  Exiting.
$ sudo mount /dev/ninja-vg/root /mnt

Automatically encrypting files with eCryptfs#

You can encrypt files written under "~/Private/" automatically using eCryptfs and the ecryptfs-utils package.

• Run ecryptfs-setup-private(1) and set up "~/Private/" by following prompts.

• Activate "~/Private/" by running ecryptfs-mount-private(1).

• Move sensitive data files to "~/Private/" and make symlinks as needed.

• Candidates: "~/.fetchmailrc", "~/.ssh/identity", "~/.ssh/id_rsa", "~/.ssh/id_dsa" and other files with "go-rwx"

• Move sensitive data directories to a subdirectory in "~/Private/" and make symlinks as needed.

• Candidates: "~/.gnupg" and other directories with "go-rwx"

• Create symlink from "~/Desktop/Private/" to "~/Private/" for easier desktop operations.

• Deactivate "~/Private/" by running ecryptfs-umount-private(1).

• Activate "~/Private/" by issuing "ecryptfs-mount-private" as you need encrypted data.

[!TIP] Since eCryptfs selectively encrypt only the sensitive files, its system cost is much less than using dm-crypt on the entire root or "/home" device. It does not require any special on-disk storage allocation effort but cannot keep all filesystem metadata confidential.

Automatically mounting eCryptfs#

If you use your login password for wrapping encryption keys, you can automate mounting eCryptfs via PAM (Pluggable Authentication Modules).

Insert the following line just before "pam_permit.so" in "/etc/pam.d/common-auth".

auth required pam_ecryptfs.so unwrap

Insert the following line just at the last line in "/etc/pam.d/common-session".

session optional pam_ecryptfs.so unwrap

Insert the following line at first active line in "/etc/pam.d/common-password".

password required pam_ecryptfs.so

This is quite convenient.

[!WARNING] Configuration errors of PAM may lock you out of your own system. See ???.

[!CAUTION] If you use your login password for wrapping encryption keys, your encrypted data are as secure as your user login password (see ???). Unless you are careful to set up a strong password, your data is at risk when someone runs password cracking software after stealing your laptop (see ???).

The kernel#

Debian distributes modularized Linux kernel as packages for supported architectures.

Linux kernel 2.6/3.x#

There are few notable features on Linux kernel 2.6/3.x compared to 2.4.

• Devices are created by the udev system (see ???).

• Read/write accesses to IDE CD/DVD devices do not use the ide-scsi module.

• Network packet filtering functions use iptables kernel modules.

The version bump from Linux 2.6.39 to Linux 3.0 is not about major technological changes but about the 20th anniversary.

Kernel parameters#

Many Linux features are configurable via kernel parameters as follows.

• Kernel parameters initialized by the bootloader (see ???)

• Kernel parameters changed by sysctl(8) at runtime for ones accessible via sysfs (see ???)

• Module parameters set by arguments of modprobe(8) when a module is activated (see Mounting the disk image file)

See "kernel-parameters.txt(.gz)" and other related documents in the Linux kernel documentation ("/usr/share/doc/linux-doc-3.*/Documentation/filesystems/*") provided by the linux-doc-3.* package.

Kernel headers#

Most normal programs don't need kernel headers and in fact may break if you use them directly for compiling. They should be compiled against the headers in "/usr/include/linux" and "/usr/include/asm" provided by the libc6-dev package (created from the glibc source package) on the Debian system.

[!NOTE] For compiling some kernel-specific programs such as the kernel modules from the external source and the automounter daemon (amd), you must include path to the corresponding kernel headers, e.g. "-I/usr/src/linux-particular-version/include/", to your command line. module-assistant(8) (or its short form m-a) helps users to build and install module package(s) easily for one or more custom kernels.

Compiling the kernel and related modules#

Debian has its own method of compiling the kernel and related modules.

List of key packages to be installed for the kernel recompilation on the Debian system

If you use initrd in ???, make sure to read the related information in initramfs-tools(8), update-initramfs(8), mkinitramfs(8) and initramfs.conf(5).

[!WARNING] Do not put symlinks to the directories in the source tree (e.g. "/usr/src/linux*") from "/usr/include/linux" and "/usr/include/asm" when compiling the Linux kernel source. (Some outdated documents suggest this.)

[!NOTE] When compiling the latest Linux kernel on the Debian stable system, the use of backported latest tools from the Debian unstable may be needed.

[!NOTE] The dynamic kernel module support (DKMS) is a new distribution independent framework designed to allow individual kernel modules to be upgraded without changing the whole kernel. This is used for the maintenance of out-of-tree modules. This also makes it very easy to rebuild modules as you upgrade kernels.

Compiling the kernel source: Debian Kernel Team recommendation#

For building custom kernel binary packages from the upstream kernel source, you should use the "deb-pkg" target provided by it.

$ sudo apt-get build-dep linux
$ cd /usr/src
$ wget http://www.kernel.org/pub/linux/kernel/v3.11/linux-<version>.tar.bz2
$ tar -xjvf linux-<version>.tar.bz2
$ cd linux-<version>
$ cp /boot/config-<version> .config
$ make menuconfig
 ...
$ make deb-pkg

[!TIP] The linux-source-\<version> package provides the Linux kernel source with Debian patches as "/usr/src/linux-<version>.tar.bz2".

For building specific binary packages from the Debian kernel source package, you should use the "binary-arch_<architecture>_<featureset>_<flavour>" targets in "debian/rules.gen".

$ sudo apt-get build-dep linux
$ apt-get source linux
$ cd linux-3.*
$ fakeroot make -f debian/rules.gen binary-arch_i386_none_686

See further information:

• Debian Wiki: KernelFAQ

• Debian Wiki: DebianKernel

• Debian Linux Kernel Handbook: https://kernel-handbook.debian.net

Hardware drivers and firmware#

The hardware driver is the code running on the target system. Most hardware drivers are available as free software now and are included in the normal Debian kernel packages in the main area.

• GPU driver

• Intel GPU driver (main)

• AMD/ATI GPU driver (main)

• NVIDIA GPU driver (main for nouveau driver, and non-free for binary-only drivers supported by the vendor.)

• Softmodem driver

• martian-modem and sl-modem-dkms packages (non-free)

The firmware is the code or data loaded on the device (e.g., CPU microcode, rendering code running on GPU, or FPGA / CPLD data, …). Some firmware packages are available as free software but many firmware packages are not available as free software since they contain sourceless binary data.

• firmware-linux-free (main)

• firmware-linux-nonfree (non-free)

• firmware-linux-* (non-free)

• *-firmware (non-free)

• intel-microcode (non-free)

• amd64-microcode (non-free)

Please note that non-free and contrib packages are not part of the Debian system. The access configuration to enable and to disable the non-free and contrib areas is described in ???. You should be aware of negatives associated with the use of the non-free and contrib packages as described in ???.

Virtualized system#

Use of virtualized system enables us to run multiple instances of system simultaneously on a single hardware.

[!TIP] See http://wiki.debian.org/SystemVirtualization.

Virtualization tools#

There are several system virtualization and emulation related packages in Debian beyond simple chroot. Some packages also help you to setup such system.

List of virtualization tools

See Wikipedia article Comparison of platform virtual machines for detail comparison of different platform virtualization solutions.

Virtualization work flow#

[!NOTE] Some functionalities described here are only available in squeeze or later.

[!NOTE] Default Debian kernels support KVM since lenny.

Typical work flow for virtualization involves several steps.

• Create an empty filesystem (a file tree or a disk image).

• The file tree can be created by "mkdir -p /path/to/chroot".

• The raw disk image file can be created with dd(1) (see Making the disk image file and Making the empty disk image file).

• qemu-img(1) can be used to create and convert disk image files supported by QEMU.

• The raw and VMDK file format can be used as common format among virtualization tools.

• Mount the disk image with mount(8) to the filesystem (optional).

• For the raw disk image file, mount it as loop device or device mapper devices (see Mounting the disk image file).

• For disk images supported by QEMU, mount them as network block device (see Mounting the virtual disk image file).

• Populate the target filesystem with required system data.

• The use of programs such as debootstrap and cdebootstrap helps with this process (see Chroot system).

• Use installers of OSs under the full system emulation.

• Run a program under a virtualized environment.

• chroot provides basic virtualized environment enough to compile programs, run console applications, and run daemons in it.

• QEMU provides cross-platform CPU emulation.

• QEMU with KVM provides full system emulation by the hardware-assisted virtualization.

• VirtualBox provides full system emulation on i386 and amd64 with or without the hardware-assisted virtualization.

Mounting the virtual disk image file#

For the raw disk image file, see The disk image.

For other virtual disk image files, you can use qemu-nbd(8) to export them using network block device protocol and mount them using the nbd kernel module.

qemu-nbd(8) supports disk formats supported by QEMU: QEMU supports following disk formats: raw, qcow2, qcow, vmdk, vdi, bochs, cow (user-mode Linux copy-on-write), parallels, dmg, cloop, vpc, vvfat (virtual VFAT), and host_device.

The network block device can support partitions in the same way as the loop device (see Mounting the disk image file). You can mount the first partition of "disk.img" as follows.

# modprobe nbd max_part=16
# qemu-nbd -v -c /dev/nbd0 disk.img
...
# mkdir /mnt/part1
# mount /dev/nbd0p1 /mnt/part1

[!TIP] You may export only the first partition of "disk.img" using "-P 1" option to qemu-nbd(8).

Chroot system#

chroot(8) offers most basic way to run different instances of the GNU/Linux environment on a single system simultaneously without rebooting.

[!CAUTION] Examples below assumes both parent system and chroot system share the same CPU architecture.

You can learn how to setup and use chroot(8) by running pbuilder(8) program under script(1) as follows.

$ sudo mkdir /sid-root
$ sudo pbuilder --create --no-targz --debug --buildplace /sid-root

You see how debootstrap(8) or cdebootstrap(1) populate system data for sid environment under "/sid-root".

[!TIP] These debootstrap(8) or cdebootstrap(1) are used to install Debian by the Debian Installer. These can also be used to install Debian to a system without using a Debian install disk, but instead from another GNU/Linux distribution.

$ sudo pbuilder --login --no-targz  --debug --buildplace /sid-root

You see how a system shell running under sid environment is created as the following.

1. Copy local configuration ("/etc/hosts", "/etc/hostname", "/etc/resolv.conf")

2. Mount "/proc" filesystem

3. Mount "/dev/pts" filesystem

4. Create "/usr/sbin/policy-rc.d" which always exits with 101

5. Run "chroot /sid-root bin/bash -c 'exec -a -bash bin/bash'"

[!NOTE] Some programs under chroot may require access to more files from the parent system to function than pbuilder provides. For example, "/sys", "/etc/passwd", "/etc/group", "/var/run/utmp", "/var/log/wtmp", etc. may need to be bind-mounted or copied.

[!NOTE] The "/usr/sbin/policy-rc.d" file prevents daemon programs to be started automatically on the Debian system. See "/usr/share/doc/sysv-rc/README.policy-rc.d.gz".

[!TIP] The original purpose of the specialized chroot package, pbuilder is to construct a chroot system and builds a package inside the chroot. It is an ideal system to use to check that a package's build-dependencies are correct, and to be sure that unnecessary and wrong build dependencies do not exist in the resulting package.

[!TIP] Similar schroot package may give you an idea to run i386 chroot system under amd64 parent system.

Multiple desktop systems#

I recommend you to use QEMU or VirtualBox on a Debian stable system to run multiple desktop systems safely using virtualization. These enable you to run desktop applications of Debian unstable and testing without usual risks associated with them.

Since pure QEMU is very slow, it is recommended to accelerate it with KVM when the host system support it.

The virtual disk image "virtdisk.qcow2" containing a Debian system for QEMU can be created using debian-installer: Small CDs as follows.

$ wget http://cdimage.debian.org/debian-cd/5.0.3/amd64/iso-cd/debian-503-amd64-netinst.iso
$ qemu-img create -f qcow2 virtdisk.qcow2 5G
$ qemu -hda virtdisk.qcow2 -cdrom debian-503-amd64-netinst.iso -boot d -m 256
...

See more tips at Debian wiki: QEMU.

VirtualBox comes with Qt GUI tools and quite intuitive. Its GUI and command line tools are explained in VirtualBox User Manual and VirtualBox User Manual (PDF).

[!TIP] Running other GNU/Linux distributions such as Ubuntu and Fedora under virtualization is a great way to learn configuration tips. Other proprietary OSs may be run nicely under this GNU/Linux virtualization, too.