Differences between revisions 277 and 278
Revision 277 as of 2009-02-15 11:55:34
Size: 75034
Editor: OsamuAoki
Comment:
Revision 278 as of 2009-02-15 16:29:51
Size: 75035
Editor: OsamuAoki
Comment:
Deletions are marked like this. Additions are marked like this.
Line 543: Line 543:
|| {{{cron}}} || - || - || The {{cron}}}(8) daemon runs processes according to a schedule (in background). || || {{{cron}}} || - || - || The {{{cron}}}(8) daemon runs processes according to a schedule (in background). ||

Do not use Edit(GUI) button.

?TableOfContents(4)

Copyright 2007, 2008 Osamu Aoki GPL, (Please agree to GPL, GPL2, and any version of GPL which is compatible with DSFG if you update any part of wiki page)

Generated HTML is at "[http://people.debian.org/~osamu/pub/getwiki/html/ch10.en.html Debian Reference: Chapter 10. System tips]".

I welcome your contributions to update this wiki page. You must follow these rules:

  • Do not use Edit(GUI) button of MoinMoin.

  • You can update anytime for:
    • grammar errors
    • spelling errors
    • moved URL location
    • package name transition adjustment (emacs23 etc.)
    • clearly broken script.
  • Before updating this wiki content:

System tips

Here, I will 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 support interrupted network connections.

List of programs to support interrupted network connections.

1

2

3

package

popcon

size

description

screen

-

-

terminal multiplexer with VT100/ANSI terminal emulation

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 detach the screen session by any methods such as:

    • brutally unplugging your network connection,
    • typing ^A d ("Control-A" followed by "d") and manually logging out from the remote connection, or

    • typing ^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 will magically reattach all previous screen windows with all actively running programs.

{i} You can save connection fees 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, by default ^A ("Control-A"). All screen commands are entered by typing ^A plus a single key [plus any parameters]. Here are important ones to remember:

List of key bindings for screen.

key binding

meaning

^A ?

show a help screen (display key bindings)

^A c

create a new window and switch to it

^A n

go to next window

^A p

go to previous window

^A 0

go to window number 0

^A 1

go to window number 1

^A w

show a list of windows

^A a

send a Ctrl-A to current window as keyboard input

^A h

write a hardcopy of current window to file

^A H

begin/end logging current window to file

^A ^X

lock the terminal (password protected)

^A d

detach screen session from the terminal

^A DD

detach screen session and log out

See screen(1) for details.

Data recording and presentation

The log daemon

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

  • The kernel log daemon: klogd(8)

  • The system log daemon: syslogd(8)

See @{@thesystemmessage@}@ and @{@thekernelmessage@}@.

Log analyzer

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

List of system log analyzers.

1

2

3

package

popcon

size

description

logwatch

-

-

log analyser with nice output written in Perl

fail2ban

-

-

bans IPs that cause multiple authentication errors

analog

-

-

web server log analyzer

awstats

-

-

powerful and featureful web server log analyzer

sarg

-

-

squid analysis report generator

pflogsumm

-

-

Postfix log entry summarizer

syslog-summary

-

-

summarize the contents of a syslog log file

lire

-

-

full-featured log analyzer and report generator

fwlogwatch

-

-

Firewall log analyzer

squidview

-

-

monitors and analyses squid access.log files

visitors

-

-

fast web server log analyzer

swatch

-

-

Log file viewer with regexp matching, highlighting, & hooks

crm114

-

-

The Controllable Regex Mutilator and Spam Filter (CRM114)

icmpinfo

-

-

Interpret ICMP messages

(!) [http://crm114.sourceforge.net/ CRM114] provides language infrastructure to write fuzzy filters with the [http://www.laurikari.net/tre/ 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 @{@recordingtheshellactivities@}@) to record shell activity produces a file with control characters. This can be avoided by using col(1):

$ script
Script started, file is typescript
  • do whatever ...
  • 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 following instead:

$ sh -i 2>&1 | tee typescript

{i} Some x-terminal-emulator such as gnome-terminal can record. You may wish to extend line buffer for scrollback.

{i} You may use screen(1) with "^A H" (see @{@keybindingsforthescreencommand@}@) to perform recording of console.

{i} 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 @{@thepager@}@) and custom tools for highlighting and formatting @{@highlightingandfingplaintextdata@}@ can display text data nicely, general purpose editors (see @{@thetexteditor@}@) are most versatile and customizable.

{i} For vim(1) and its pager mode alias view(1), ":set hls" will enable 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 value @{@timestamps@}@). 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 lenny.

time style value

locale

display of time and date

iso

any

01-19 00:15

long-iso

any

2009-01-19 00:15

full-iso

any

2009-01-19 00:15:16.000000000 +0900

locale

C

Jan 19 00:15

locale

en_US.UTF-8

2009-01-19 00:15

locale

es_ES.UTF-8

ene 19 00:15

+%d.%m.%y %H:%M

any

19.01.09 00:15

+%d.%b.%y %H:%M

C or en_US.UTF-8

19.Jan.09 00:15

+%d.%b.%y %H:%M

es_ES.UTF-8

19.ene.09 00:15

{i} You can eliminate typing long option on commandline using command alias, e.g. "alias ls='ls --time-style=+%d.%m.%y\ %H:%M'" (see @{@commandalias@}@).

{i} [http://en.wikipedia.org/wiki/ISO_8601 ISO 8601] is followed for these iso-formats.

Colorized shell echo

Shell echo to most modern terminals can be colorized using [http://en.wikipedia.org/wiki/ANSI_escape_code ANSI escape code] (see "/usr/share/doc/xterm/ctlseqs.txt.gz"). E.g.:

$ 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 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).

{i} You can turn off these colorizing aliases in the interactive environment by invoking shell with "TERM=dumb bash".

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.

1

2

3

package

popcon

size

command

xbase-clients

25829

-

xwd(1)

gimp

8489

-

GUI menu

imagemagick

5479

-

import(1)

scrot

134

-

scrot(1)

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.

1

2

3

package

popcon

size

description

etckeeper

-

-

store configuration files and its metadata with [http://en.wikipedia.org/wiki/Git_(software) Git] (default), [http://en.wikipedia.org/wiki/Mercurial_(software) Mercurial], or [http://en.wikipedia.org/wiki/Bazaar_(software) Bazaar]. (new)

changetrack

-

-

store configuration files with [http://en.wikipedia.org/wiki/Revision_Control_System RCS]. (old)

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 @{@gitforrecordingcigurationhistory@}@) but with special hook scripts for more thorough setups.

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

{i} 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.

Data storage tips

Booting your system with Linux [http://en.wikipedia.org/wiki/Live_CD live CDs] or [http://www.debian.org/releases/stable/debian-installer/ debian-installer CDs] in rescue mode make it easy for you to reconfigure data storage on your boot device. See also @{@thebinarydata@}@.

Partition configuration

For 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 [http://en.wikipedia.org/wiki/Master_boot_record Master Boot Record (MBR)] scheme to hold [http://en.wikipedia.org/wiki/Disk_partitioning disk partitioning] data in the first sector, i.e., [http://en.wikipedia.org/wiki/Logical_block_addressing LBA] sector 0 (512 bytes).

(!) Some new PCs with [http://en.wikipedia.org/wiki/Extensible_Firmware_Interface Extensible Firmware Interface (EFI)], including Intel-based Macs, use [http://en.wikipedia.org/wiki/GUID_Partition_Table GUID Partition Table (GPT)] scheme to hold [http://en.wikipedia.org/wiki/Disk_partitioning 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

1

2

3

package

pocon

size

description

[http://en.wikipedia.org/wiki/GUID_Partition_Table GUID Partition Table]

util-linux

-

-

Miscellaneous system utilities including fdisk(8) and cfdisk(8)

Not supported

parted

-

-

The GNU Parted disk partition resizing program

Supported

gparted

-

-

GNOME partition editor based on libparted

Supported

qtparted

-

-

KDE partition editor based on libparted

Supported

gptsync

-

-

Synchronize classic MBR partition table with the GPT one

Supported

<!> 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), ...) and resize2fs(8).

(!) In order to switch between GPT and MBR, you need to erase first few blocks of disk contents directly (see @{@clearfilecontents@}@) and use "parted /dev/sdx mklabel gpt" or "parted /dev/sdx mklabel msdos" to set it. Please note "msdos" is use here for [http://en.wikipedia.org/wiki/Master_boot_record MBR].

Accessing partition using UUID

Although reconfiguration of your partition 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.

{i} You can probe [http://en.wikipedia.org/wiki/Universally_Unique_Identifier UUID] of a block special device with vol_id(8).

Filesystem configuration

For [http://en.wikipedia.org/wiki/Ext3 ext3] filesystem, the e2fsprogs package provides:

The mkfs(8) and fsck(8) commans are provided by the e2fsprogs package as front-ends to various filesystem dependent programs (mkfs.fstype and fsck.fstype). For [http://en.wikipedia.org/wiki/Ext3 ext3] filesystem, they are mkfs.ext3(8) and fsck.ext3(8) (they are hardlinked to mke2fs(8) and e2fsck(8)).

Similar commands are available for each filesystem supported by Linux.

List of filesystem management packages

1

2

3

package

popcon

size

description

e2fsprogs

-

-

Utilities for the [http://en.wikipedia.org/wiki/Ext2 ext2]/[http://en.wikipedia.org/wiki/Ext3 ext3]/[http://en.wikipedia.org/wiki/Ext4 ext4] filesystems.

reiserfsprogs

-

-

Utilities for the [http://en.wikipedia.org/wiki/Reiserfs Reiserfs] filesystem.

dosfstools

-

-

Utilities for the [http://en.wikipedia.org/wiki/File_Allocation_Table FAT] filesystem. (Microsoft: MS-DOS, Windows)

xfsprogs

-

-

Utilities for the [http://en.wikipedia.org/wiki/XFS XFS] filesystem. (SGI: IRIX)

ntfsprogs

-

-

Utilities for the [http://en.wikipedia.org/wiki/NTFS NTFS] filesystem. (Microsoft: Windows NT, ...)

jfsutils

-

-

Utilities for the [http://en.wikipedia.org/wiki/JFS_(file_system) JFS] filesystem. (IBM: AIX, OS/2)

reiser4progs

-

-

Utilities for the [http://en.wikipedia.org/wiki/Reiser4 Reiser4] filesystem.

hfsprogs

-

-

Utilities for [http://en.wikipedia.org/wiki/Hierarchical_File_System HFS] and [http://en.wikipedia.org/wiki/HFS_Plus HFS+] filesystem. (Apple: Mac OS)

btrfs-tools

-

-

Utilities for the [http://en.wikipedia.org/wiki/Btrfs btrfs] filesystem.

{i} [http://en.wikipedia.org/wiki/Ext3 Ext3] filesystem is the default filesystem for the Linux system and strongly recommended to use it unless you have some specific reasons not to. After Linux kernel 2.6.28 (Debian squeeze), [http://en.wikipedia.org/wiki/Ext4 ext4] filesystem will be available and expected to be the default filesystem for the Linux system. [http://en.wikipedia.org/wiki/Btrfs btrfs] filesystem is expected to be the next default filesystem after [http://en.wikipedia.org/wiki/Ext4 ext4] filesystem for the Linux system.

{i} Some tools allow access to filesystem without Linux kernel support (see @{@manipulatingfilehoutmountingdisk@}@).

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.

<!> It is generally not safe to run fsck on mounted filesystems.

{i} Check files in "/var/log/fsck/" for the result of the fsck(8) command run from the boot script.

{i} Use "shutdown -F -r now" to force to run the fsck(8) command safely on all filesystems including root file system on reboot. See the shutdown(8) manpage for more.

Optimization of filesystem by mount options

Performance and characteristics of a filesystem can be optimized by mount options used on it (see fstab(5) and mount(8)). For example:

  • "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 file system. This option implies "noexec,nosuid,nodev" option combination. (general, used for CD and floppy)

  • "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.

{i} You need to provide kernel boot parameter "rootflags=data=journal" to deploy "data=journal" option for the root file system formatted with ext3fs.

Optimization of filesystem via superblock

Characteristics of a filesystem can be optimized via its superblock using the tune2fs(8) command. For example on "/dev/hda1":

  • Execution of "sudo tune2fs -l /dev/hda1" will display the contents of its filesystem superblock.

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

  • Execution of "sudo tune2fs -j /dev/hda1" will add journaling capability to the filesystem, i.e. filesystem conversion from [http://en.wikipedia.org/wiki/Ext2 ext2] to [http://en.wikipedia.org/wiki/Ext3 ext3]. (Do this on the unmounted filesystem.)

  • Execution of "sudo tune2fs -O extents,uninit_bg,dir_index /dev/hda1 && fsck -pf /dev/hda1" will convert it from [http://en.wikipedia.org/wiki/Ext3 ext3] to [http://en.wikipedia.org/wiki/Ext4 ext4]. (Do this on the unmounted filesystem.)

/!\ Filesystem conversion for the boot device to the [http://en.wikipedia.org/wiki/Ext4 ext4] filesystem should be avoided until [http://bugs.debian.org/511121 GRUB boot loader supports the ext4 filesystem well] and installed Linux Kernel version is newer than 2.6.28.

{i} Despite its name, tune2fs(8) works not only on the [http://en.wikipedia.org/wiki/Ext2 ext2] filesystem but also on the [http://en.wikipedia.org/wiki/Ext3 ext3] and [http://en.wikipedia.org/wiki/Ext4 ext4] filesystems.

Optimization of harddisk

/!\ Please check your hardware and read manpage of hdparam(8) before playing with harddisk configuration because this may be quite dangerous for the data integrity.

You can test disk access speed of a harddisk, e.g. "/dev/hda", by "hdparm -tT /dev/hda". For some harddisk 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 harddisk, 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 head CD-ROM drive by slowing it down with "setcd -x 2.

Using SMART to predict harddisk failure

You can monitor and log your harddisk which is compliant to [http://en.wikipedia.org/wiki/S.M.A.R.T SMART] with the smartd(8) daemon.

  1. Enable [http://en.wikipedia.org/wiki/S.M.A.R.T SMART] feature in [http://en.wikipedia.org/wiki/BIOS BIOS].

  2. Install the smartmontools package.

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

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

  4. Check the output of "smartctl -a /dev/hda" to see if [http://en.wikipedia.org/wiki/S.M.A.R.T SMART] feature is actually enabled.

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

  5. Enable smartd(8) daemon to run by:

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

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

{i} The smartd(8) daemon can be customized with the /etc/smartd.conf file including how to be notified of warnings.

Expand usable storage space via LVM

For partitions created on [http://en.wikipedia.org/wiki/Logical_Volume_Manager_(Linux) Logical Volume Manager (Linux)] 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.

<!> Deployment of the current LVM system may degrade guarantee against filesystem corruption offered by journaled file systems such as ext3fs unless their system performance is sacrificed by disabling write cache of harddisk.

Expand usable storage space by mounting another partition

If you have an empty partition (e.g., "/dev/sdx"), you can format it with mkfs.ext3(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.ext3 /dev/sdx
$ sudo mount -t ext3 /dev/sdx work-dir
$ sudo cp -a old-dir/* work-dir
$ sudo rm -rf old-dir

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

<!> Some software may not function well with "symlink to a directory".

Expand usable storage space using aufs

If you have usable space in another partition (e.g., "/path/to/"), you can create a directory in it and stack that on to a directory where you need space with [http://en.wikipedia.org/wiki/Aufs aufs].

$ sudo mv work-dir old-dir
$ sudo mkdir -p /path/to/emp-dir
$ sudo mount -t aufs -o br:/path/to/emp-dir:old-dir none work-dir

<!> Use of [http://en.wikipedia.org/wiki/Aufs aufs] for long term data storage is not good idea since it is under development and its design change may introduce issues.

{i} In order to use [http://en.wikipedia.org/wiki/Aufs aufs], its utility package aufs-tools and kernel module package for [http://en.wikipedia.org/wiki/Aufs aufs] such as aufs-modules-2.6-amd64 need to be installed.

{i} [http://en.wikipedia.org/wiki/Aufs aufs] is used to provide writable root filesystem by many modern [http://en.wikipedia.org/wiki/Live_CD live CD] projects.

Data encryption tips

Since gaining root privilege is relatively easy with physical access (see @{@securingtherootpassword@}@), it can not secure your private and sensitive data against possible theft of your PC. You must deploy data encryption technology to do it. Although [http://en.wikipedia.org/wiki/GNU_Privacy_Guard GNU privacy guard] (see @{@datasecurityinfrastructure@}@) can encrypt files, it takes some user efforts.

[http://en.wikipedia.org/wiki/Dm-crypt dm-crypt] and [http://ecryptfs.sourceforge.net/ eCryptfs] facilitates automatic data encryption natively via Linux kernel modules with minimal user efforts.

List of data encryption utilities.

1

2

3

package

popcon

size

function

cryptsetup

-

-

Utilities for encrypted block device ([http://en.wikipedia.org/wiki/Dm-crypt dm-crypt] / [http://en.wikipedia.org/wiki/Linux_Unified_Key_Setup LUKS])

cryptmount

-

-

Utilities forencrypted block device ([http://en.wikipedia.org/wiki/Dm-crypt dm-crypt] / [http://en.wikipedia.org/wiki/Linux_Unified_Key_Setup LUKS]) with focus on mount/unmount by normal users

ecryptfs-utils

-

-

Utilities for encrypted stacked filesystem ([http://ecryptfs.sourceforge.net/ eCryptfs])

[http://en.wikipedia.org/wiki/Dm-crypt Dm-crypt] is a cryptographic filesystem using [http://en.wikipedia.org/wiki/Device_mapper device-mapper]. [http://en.wikipedia.org/wiki/Device_mapper Device-mapper] maps one block device to another.

[http://ecryptfs.sourceforge.net/ eCryptfs] is another cryptographic filesystem using stacked filesystem. Stacked filesystem stacks itself on top of an existing directory of a mounted filesystem.

<!> Data encryption costs CPU time etc. Please weigh its benefits and costs.

(!) Entire Debian system can be installed on a encrypted disk by the [http://www.debian.org/devel/debian-installer/ debian installer] (lenny or newer) using [http://en.wikipedia.org/wiki/Dm-crypt dm-crypt]/[http://en.wikipedia.org/wiki/Linux_Unified_Key_Setup LUKS] and initramfs.

{i} See @{@datasecurityinfrastructure@}@ for user space encryption utility: [http://en.wikipedia.org/wiki/GNU_Privacy_Guard GNU Privacy Guard].

Removable disk encryption with dm-crypt/LUKS

You can encrypt contents of removable mass storage devices, e.g. USB memory stick on "/dev/sdx", using [http://en.wikipedia.org/wiki/Dm-crypt dm-crypt]/[http://en.wikipedia.org/wiki/Linux_Unified_Key_Setup LUKS]. You simply formatting it as:

# badblocks -c 10240 -s -w -t random -v /dev/sdx
# shred -v -n 1 /dev/sdx
# fdisk /dev/sdx
... "n" "p" "1" "return" "return" "w"
# cryptsetup luksFormat /dev/sdx1
...
# cryptsetup luksOpen /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 @{@removablemassstoragedevice@}@) 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 file format, e.g., ext3 with "mkfs.ext3 /dev/sdx1".

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

Encrypted swap partition with dm-crypt

If your original "/etc/fstab" contains:

/dev/sda7 swap sw 0 0

then you can enable encrypted swap partition using [http://en.wikipedia.org/wiki/Dm-crypt dm-crypt] as

# swapoff -a
# echo "cswap /dev/sda7 /dev/urandom swap" >> /etc/crypttab
# perl -i -p -e "s/\/dev\/sda7/\/dev\/mapper\/cswap/" /etc/fstab
# swapon -a

Automatically encrypting files with eCryptfs

You can encrypt files written under "~/Private/" automatically using [http://ecryptfs.sourceforge.net/ 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.

  • move sensitive data directories to "~/Private/" and make symlinks.

  • do the same for "~/.gnupg" and other directories containing sensitive data.

  • create symlink from "~/.ssh" to "~/Private/.ssh"

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

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

{i} Files and directories with "go-r" permission such as "~/.cvspass", "~/.fetchmailrc", "~/.ssh/identity", "~/.ssh/id_rsa", "~/.ssh/id_dsa", "~/.gnupg/", "~/.gnome2/", ... can be considered sensitive data.

{i} Since [http://ecryptfs.sourceforge.net/ eCryptfs] selectively encrypt only the sensitive files, its system cost is much less than using [http://en.wikipedia.org/wiki/Dm-crypt 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 Pluggable Authentication Module by having active lines in "/etc/pam.d/common-auth" as:

auth required pam_unix.so nullok_secure
auth required pam_ecryptfs.so unwrap

and active lines in "/etc/pam.d/common-session" as:

session required pam_unix.so
session optional pam_ecryptfs.so unwrap

This is quite convienient.

<!> If you use your login password for wrapping encryption keys, your encrypted data are as secure as your user login password (see @{@goodpassword@}@). Unless you are careful to set up a [http://en.wikipedia.org/wiki/Password_strength strong password], your data will be at risk when someone runs [http://en.wikipedia.org/wiki/Password_cracking password cracking] software after stealing your laptop (see @{@securingtherootpassword@}@). The squeeze version of the ecryptfs-utils package comes with option to have independent password for wrapping and to set up user's entire home directory for encryption. This is an actively developed package.

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

1

2

3

package

popcon

size

description

time

-

-

time(1) runs a program to report system resource usages with respect to time.

coreutils

-

-

nice(1) runs a program with modified scheduling priority.

bsdutils

-

-

renice(1) modifies the scheduling priority of a running process.

powertop

-

-

powertop(1) gives information about system power use on Intel-based laptops.

procps

-

-

The "/proc" file system utilities: ps(1), top(1), kill(1), watch(1), ...

psmisc

-

-

The "/proc" file system utilities: killall(1), fuser(1), pstree(1)

cron

-

-

The cron(8) daemon runs processes according to a schedule (in background).

at

-

-

at(1) or batch(1) commands run a job at a specified time or below certain load level.

lsof

-

-

lsof(8) lists open files by a running process using "-p" option.

strace

-

-

strace(1) traces system calls and signals.

ltrace

-

-

ltrace(1) traces library calls.

xtrace

-

-

xtrace(1) traces communication between X11 client and server.

Time 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 value

scheduling priority

19

lowest priority process (nice)

0

very high priority process for user.

-20

very high priority process for root. (not-nice)

# 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 the Debian support both BSD and SystemV features and helps to identify the process activity statically.

List of ps command styles.

style

typical command

feature

BSD

ps aux

display %CPU %MEM

System V

ps -efH

display PPID

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 has rich features and helps to identify what process is acting funny dynamically.

List of commands for top.

command key

response

h or ?

To show help.

f

To set/reset display field.

o

To reorder display field.

F

To set sort key field.

k

To kill a process.

r

To renice a process.

q

To quit the top command.

List files opened by a process

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

$ sudo lsof -p 1

PID=1 is usually init program.

Trace 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. For example:

$ sudo strace ls
...

Identify processes using files or sockets

You can also identify processes using files or sockets by fuser(1). For example:

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

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

$ 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 [http://en.wikipedia.org/wiki/Transmission_Control_Protocol TCP] connections to [http://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol 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 will display 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 @{@shellloops@}@):
    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 @{@idiomsfortheselectionoffiles@}@ for more advance uses of find(1).

Starting a program from GUI

You can set up to start a process from [http://en.wikipedia.org/wiki/Graphical_user_interface graphical user interface (GUI)].

Under Gnome desktop environment, a program program can be started with proper argument by drag-and-drop of an icon to the launcher icon or by "Open with ..." menu with right clicking. KDE can do the equivalent, too. Here is an example for Gnome to set up mc(1) started in gnome-terminal(1):

  • create an executable program "mc-term" as:

# cat >/usr/local/mc-term <<EOF
#!/bin/sh
gnome-terminal -e "mc $1"
EOF
# chmod 755 /usr/local/mc-term
  • create a desktop launcher
    • right clicking desktop space to select "Create Launcher ..."

      • set "Type" to "Application"

      • set "Name" to "mc"

      • set "Command" to "mc-term %f"

      • click "OK"
  • create an open-with association
    • right click folder to select "Open with Other Application ..."

      • click open "Use a custom command" dialog and enter "mc-term %f"

      • click "Open".

{i} Launcher is a file at "~/Desktop" with ".desktop" as its extension.

Customizing program to be started

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

  • configuration file of the parent program such as "/etc/mc/mc.ext".

  • system configuration menu such as "System" -> "Preferences" -> "Preferred Application" for Gnome.

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

  • the update-alternatives(8) system for programs such as "editor", "view", "x-www-browser", "gnome-www-browser", and "www-browser" (see @{@settingadefaulttexteditor@}@).

  • the "~/.mailcap" and "/etc/mailcap" file contents which associate [http://en.wikipedia.org/wiki/MIME MIME] type with program (see mailcap(5)).

  • the the "~/.mime.types" and "/etc/mime.types" file contents which associate file name extension with [http://en.wikipedia.org/wiki/MIME MIME] type (see run-mailcap(1)).

{i} update-mime(8) updates the "/etc/mailcap" file using "/etc/mailcap.order" file (see mailcap.order(5)).

{i} 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 commands.

Kill 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.

signal value

signal name

function

1

HUP

restart daemon

15

TERM

normal kill

9

KILL

kill hard

Schedule tasks once

Run the at(1) command to schedule a one-time job:

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

Schedule tasks regularly

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

Run the command "crontab -e" to create or edit a crontab file to set up regularly scheduled events.

Example of a crontab 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

{i} For the system not running continuously, install the anacron package to schedule periodic command at the specified intervals as closely as machine-uptime permits.

Alt-SysRq

Insurance against system malfunction is provided by the kernel compile option "Magic ?SysRq key" ([http://en.wikipedia.org/wiki/Secure_attention_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.

key following Alt-?SysRq

function

r

Unraw restores the keyboard after things like X crashes.

0

Changing the console loglevel to 0 reduces error messages.

k

SAK (system attention key) kills all processes on the current virtual console.

e

Send a SIGTERM to all processes, except for init(8).

i

Send a SIGKILL to all processes, except for init(8).

s

Sync all mounted filesystems.

u

Remount all mounted filesystems read-only (umount).

b

Reboot the system without syncing or unmounting.

The combination of "Alt-?SysRq s", "Alt-?SysRq u", and "Alt-?SysRq r" is good for getting out of really bad situations.

See "/usr/share/doc/linux-doc-2.6.*/Documentation/sysrq.txt.gz".

<!> 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" will disable the Alt-?SysRq feature.

{i} 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 will sync and umount all mounted filesystems.

System maintenance tips

Who is logged on

You can check who is logged on to the system with w(1) or who(1).

Warn everyone

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

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

Hardware identification

For the [http://en.wikipedia.org/wiki/Peripheral_Component_Interconnect PCI]-like devices ([http://en.wikipedia.org/wiki/Accelerated_Graphics_Port AGP], [http://en.wikipedia.org/wiki/PCI_Express PCI-Express], [http://en.wikipedia.org/wiki/PC_Card#CardBus CardBus], [http://en.wikipedia.org/wiki/ExpressCard ?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 @{@procfsandsysfs@}@).

List of hardware identification tools.

1

2

3

package

popcon

size

description

pciutils

-

-

Linux PCI Utilities, lspci(8)

usbutils

-

-

Linux USB utilities, lsusb(8)

pcmciautils

-

-

PCMCIA utilities for Linux 2.6, pccardctl(8)

scsitools

-

-

Collection of tools for SCSI hardware management, lsscsi(8)

pnputils

-

-

Plug and Play BIOS utilities, lspnp(8)

procinfo

-

-

Displays system information from "/proc", lsdev(8)

lshw

-

-

Information about hardware configuration, lshw(1)

discover

-

-

Hardware identification system, discover(8)

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.

1

2

3

package

popcon

size

description

hal

-

-

Hardware Abstraction Layer, lshal(1)

console-tools

-

Linux console font and keytable utilities.

x11-xserver-utils

-

X server utilities. xset(1) and xmodmap(1).

acpid

24513

-

Daemon to manage events delivered by the Advanced Configuration and Power Interface (ACPI)

acpi

2563

-

Utilities for ACPI devices

apmd

1222

-

Daemon to manage events delivered by the Advanced Power Management (APM)

powersaved

1038

-

Daemon to manage battery, temperature, ac, cpufreq (?SpeedStep, Powernow!) control and monitor with ACPI and APM supports.

noflushd

95

-

Allow idle hard disks to spin down

sleepd

75

-

Puts a laptop to sleep during inactivity

hdparm

5192

-

Hard disk access optimization. Very effective but dangerous. You must read hdparm(8) first.

smartmontools

3526

-

Control and monitor storage systems using S.M.A.R.T.

setserial

2619

-

Collection of tools for serial port management.

memtest86+

406

-

Collection of tools for memory hardware management.

scsitools

185

-

Collection of tools for SCSI hardware management.

tpconfig

276

-

A program to configure touchpad devices

setcd

82

-

Compact disc drive access optimization.

big-cursor

*121

-

Larger mouse cursors for X

lspowertweak

-

-

Simple front end to powertweak, lspowertweak(8)

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

System and hardware time

The following will set 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.

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

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

{i} 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 2.6 are provided by [http://en.wikipedia.org/wiki/Advanced_Linux_Sound_Architecture Advanced Linux Sound Architecture (ALSA)]. ALSA provides emulation mode for previous [http://en.wikipedia.org/wiki/Open_Sound_System Open Sound System (OSS)] for compatibility.

Run "dpkg-reconfigure linux-sound-base" to select the sound system to use ALSA via blacklisting of kernel modules. Unless you have very new sound hardware, udev infrastructure should configure your sound system.

{i} Use "cat /dev/urandom > /dev/audio" or speaker-test(1) to test speaker. (^C to stop)

{i} 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

1

2

3

package

pocon

size

description

linux-sound-base

-

-

Base package for ALSA and OSS sound systems

alsa-base

-

-

ALSA driver configuration files

alsa-utils

-

-

Utilities for configuring and using ALSA

oss-compat

-

-

OSS compatibility under ALSA preventing "/dev/dsp not found" errors

esound-common

-

-

[http://en.wikipedia.org/wiki/Enlightened_Sound_Daemon Enlightened Sound Daemon (ESD)] common (Enlightenment and GNOME)

esound

-

-

[http://en.wikipedia.org/wiki/Enlightened_Sound_Daemon Enlightened Sound Daemon (ESD)] server (Enlightenment and GNOME)

esound-clients

-

-

[http://en.wikipedia.org/wiki/Enlightened_Sound_Daemon Enlightened Sound Daemon (ESD)] client (Enlightenment and GNOME)

libesd-alsa0

-

-

[http://en.wikipedia.org/wiki/Enlightened_Sound_Daemon Enlightened Sound Daemon (ESD)] library Enlightenment and GNOME)

libesd0

-

-

[http://en.wikipedia.org/wiki/Enlightened_Sound_Daemon Enlightened Sound Daemon (ESD)] library (Enlightenment and GNOME) - OSS

arts

-

-

[http://en.wikipedia.org/wiki/ARts aRts] server (KDE)

libarts1c2a

-

-

[http://en.wikipedia.org/wiki/ARts aRts] library (KDE)

libartsc0

-

-

[http://en.wikipedia.org/wiki/ARts aRts] library (KDE)

jackd

-

-

[http://en.wikipedia.org/wiki/JACK_Audio_Connection_Kit JACK Audio Connection Kit. (JACK)] server (low latency)

libjack0

-

-

[http://en.wikipedia.org/wiki/JACK_Audio_Connection_Kit JACK Audio Connection Kit. (JACK)] library (low latency)

libjack0.100.0-0

-

-

[http://en.wikipedia.org/wiki/JACK_Audio_Connection_Kit JACK Audio Connection Kit. (JACK)] library (low latency)

nas

-

-

[http://en.wikipedia.org/wiki/Network_Audio_System Network Audio System (NAS)] server

libaudio2

-

-

[http://en.wikipedia.org/wiki/Network_Audio_System Network Audio System (NAS)] library

pulseaudio

-

-

[http://en.wikipedia.org/wiki/PulseAudio PulseAudio] server, replacement for ESD

libpulse0

-

-

[http://en.wikipedia.org/wiki/PulseAudio PulseAudio] client library, replacement for ESD

libpulsecore5

-

-

[http://en.wikipedia.org/wiki/PulseAudio PulseAudio] server library, replacement for ESD

libgstreamer0.10-0

-

-

[http://en.wikipedia.org/wiki/GStreamer GStreamer]: Gnome sound engine

libxine1

-

-

[http://en.wikipedia.org/wiki/Xine xine]: KDE older sound engine

libphonon4

-

-

[http://en.wikipedia.org/wiki/Phonon_(KDE) Phonon]: KDE new sound engine

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.

Disable the screen saver

For disabling the screen saver, use following commands.

List of commands for disabling the screen saver.

environment

command

The Linux console

setterm -powersave off

The X Window by turning off screensaver

xset s off

The X Window by disabling dpms

xset -dpms

The X Window by GUI configuration of screen saver

xscreensaver-command -prefs

Disable the sound (beep)

One can always unplug the PC speaker. ;-) Removing pcspkr kernel module does this for you.

The following will prevent the readline(3) program used by bash(1) to beep when encountering "\a" (ASCII=7):

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

Memory usage

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.

$ grep '^Memory' /var/log/dmesg
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

For my MacBook with 1GB=1048576k DRAM (video system steals some of this):

List of memory sizes reported.

report

size

Total size in dmesg

1016784k = 1GB - 31792k

Free in dmesg

990528k

Total under shell

997184k

Free under shell

20256k

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 below) and relax.

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:

List of tools for system security and integrity check

1

2

3

package

popcon

size

description

logcheck

-

-

This mails anomalies in the system logfiles to the administrator

debsums

-

-

This verifies installed package files against MD5 checksums.

chkrootkit

-

-

[http://en.wikipedia.org/wiki/Rootkit Rootkit] detector.

clamav

-

-

Anti-virus utility for Unix - command-line interface.

tiger

-

-

Report system security vulnerabilities

tripwire

-

-

File and directory integrity checker

john

-

-

Active password cracking tool

aide

-

-

Advanced Intrusion Detection Environment - static binary

bastille

-

-

Security hardening tool

integrit

-

-

A file integrity verification program

crack

-

-

Password guessing program

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

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

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

The kernel

Debian distributes modularized Linux kernel as packages for supported architectures.

Linux kernel 2.6

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

  • Devices are created by the udev system (see @{@theudevsystem@}@).
  • Read/write accesses to IDE CD/DVD devices do not use the ide-scsi module.

  • Network packet filtering functions use iptable kernel modules.

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.

(!) 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. The module-assistant package helps users to build and install module package(s) easily for one or more custom kernels with m-a(8).

Kernel and module compile

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

1

2

3

package

popcon

size

description

build-essential

-

-

essential packages for building Debian packages: make, gcc, ...

bzip2

-

-

compress and decompress utilities for bz2 files

libncurses5-dev

-

-

developer's libraries and docs for ncurses

git-core

-

-

git: distributed revision control system used by the Linux kernel

fakeroot

-

-

provide fakeroot environment for building package as non-root

initramfs-tools

-

-

tool to build an initramfs (Debian specific)

kernel-package

-

-

tool to build Linux kernel packages (Debian specific)

module-assistant

-

-

tool to help build module packages (Debian specific)

devscripts

-

-

helper scripts for a Debian Package maintainer (Debian specific)

linux-tree-2.6.*

-

-

Linux kernel source tree for building Debian kernel images (Debian specific)

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

/!\ 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.)

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

Kernel source compile: Debian standard method

The Debian standard method for compiling kernel source to create a custom kernel package uses make-kpkg(1). The official documentation is in (the bottom of) "/usr/share/doc/kernel-package/README.gz". See kernel-pkg.conf(5) and kernel-img.conf(5) for customization.

Here is an example for amd64 system:

# aptitude install linux-tree-<version>
$ cd /usr/src
$ tar -xjvf linux-source-<version>.tar.bz2
$ cd linux-source-<version>
$ cp /boot/config-<oldversion> .config
$ make menuconfig
 ...
$ make-kpkg clean
$ fakeroot make-kpkg --append_to_version -amd64 --initrd --revision=rev.01 kernel_image modules_image
$ cd ..
# dpkg -i linux-image*.deb
  • reboot to new kernel with "shutdown -r now" .

<!> When you intend to create a non-modularized kernel compiled only for one machine, invoke make-kpkg without "--initrd" option since initrd is not used. Invocation of "make oldconfig" and "make dep" are not required since "make-kpkg kernel_image" invokes them.

Module source compile: Debian standard method

The Debian standard method for creating and installing a custom module package for a custom kernel package uses module-assistant(8) and module-source packages. For example, following will build the unionfs kernel module package and installs it.

$ sudo aptitude install module-assistant
...
$ sudo aptitude install unionfs-source unionfs-tools unionfs-utils
$ sudo m-a update
$ sudo m-a prepare
$ sudo m-a auto-install unionfs
...
$ sudo apt-get autoremove

Kernel source compile: classic method

You can still build [http://www.kernel.org/ Linux kernel from the pristine sources] with the classic method. You must take care the details of the system configuration manually.

$ cd /usr/src
$ wget http://www.kernel.org/pub/linux/kernel/v2.6/linux-<version>.tar.bz2
$ tar -xjvf linux-<version>.tar.bz2
$ cd linux-<version>
$ cp /boot/config-<version> .config
$ make menuconfig
 ...
$ make dep; make bzImage
$ make modules
# cp ./arch/x86_64/boot/bzImage /boot/vmlinuz-<version>
# make modules_install
# depmod -a
# update-initramfs -c -k <version>
  • set up bootloader
    • edit "/etc/lilo.conf" and run "/sbin/lilo", if you use lilo.

    • edit "/boot/grub/menu.lst", if you use grub.

  • reboot to new kernel with "shutdown -r now".

Non-free hardware drivers

Although most of hardware drivers are available as free software and as a part of the Debian system, you may need to load some non-free external drivers to support some hardwares, such as Winmodem, on your system.

Check pertinent resources:

Chroot system

chroot(8) is most basic way to run different instances of the GNU/Linux environment on a single system simultaneously without rebooting. I will explain simple [http://en.wikipedia.org/wiki/Chroot chroot] systems in the following as examples.

<!> Examples below assumes both parent system and chroot system share the same CPU architecture.

For serious chroot setup with the detail configuration, such as i386 chroot system under amd64 parent system, please consider to use the specialized schroot package.

Run a different Debian distribution with chroot

A chroot Debian environment can easily be created by debootstrap(8) or cdebootstrap(1).

For example, the following will create a sid chroot on "/sid-root" while having fast Internet access:

main # debootstrap sid /sid-root http://ftp.debian.org/debian/
  • watch it download the whole system

main # echo "proc-sid    /sid-root/proc     proc   none 0 0" >> /etc/fstab
main # echo "devpts-sid  /sid-root/dev/pts  devpts defaults 0 0" >> /etc/fstab
main # mount -a
main # cp -f /etc/passwd /sid-root/etc/passwd
main # cp -f /etc/shadow /sid-root/etc/shadow
main # cp -f /etc/group  /sid-root/etc/group
main # cp -f /etc/hosts  /sid-root/etc/hosts
main # chroot /sid-root /bin/bash
chroot # cd /dev; /sbin/MAKEDEV generic ; cd -
chroot # vi /etc/apt/sources.list
  • point the source to unstable

chroot # aptitude update
...
chroot # aptitude install locales
...
  • add "en_US.UTF-8" as locale and make it default

Do you want to continue? [Y/n/?] y
chroot # aptitude install mc vim
...
Do you want to continue? [Y/n/?] y
...
chroot # exit
main #

At this point you should have a fully working Debian sid system, where you can play around without fear of affecting your main Debian installation.

<!> If you use bind mount for directories such as home directory in the chroot, you must be careful for its side effects. I heard people lost their home directory after executing "rm -rf /sid-root" without unbinding their home directory in the chroot. A bind mount is not normally visible with df(1), you need to execute "df -a" to see it.

This debootstrap trick can also be used to [http://www.debian.org/releases/stable/installmanual install Debian] to a system without using a Debian install disk, but instead from another GNU/Linux distribution.

Setting up login for chroot

Typing "chroot /sid-root /bin/bash" is easy, but it retains all sorts of environment variables that you may not want, and has other issues. A much better approach is to run another login process on a separate virtual terminal where you can log in to the chroot directly.

Since on default Debian systems tty1 to tty6 run Linux consoles and tty7 runs the X Window System, let's set up tty8 for a chrooted console as an example. After creating a chroot system, type from the root shell of the main system:

main # echo "8:23:respawn:/usr/sbin/chroot /sid-root /sbin/getty 38400 tty8"  >> /etc/inittab
main # init q
  • reload init

Setting up X for chroot

You want to run the latest X and GNOME safely in your chroot? That's entirely possible! The following example will make GDM run on virtual terminal vt9.

First install a chroot system. From the root of the main system, copy key configuration files to the chroot system.

main # cp /etc/X11/xorg.conf /sid-root/etc/X11/xorg.conf
main # chroot /sid-root
chroot # cd /dev; /sbin/MAKEDEV generic ; cd -
chroot # aptitude install gdm gnome x-window-system
chroot # vim /etc/gdm/gdm.conf
  • change "[servers]" section with "s/vt7/vt9/" to make the first virtual console in the chroot from vt7 to vt9.

chroot # /etc/init.d/gdm start

Now you can easily switch back and forth between full X environments in your chroot and your main system just by switching between Linux virtual terminals; e.g. by using Ctrl-Alt-F7 and Ctrl-Alt-F9. Have fun!

Run other distributions with chroot

A chroot environment for another Linux distribution can easily be created. You install a system into separate partitions using the installer of the other distribution. If its root partition is in "/dev/hda9":

main # cd / ; mkdir /other-dist
main # mount -t ext3 /dev/hda9 /other-dist
main # chroot /other-dist /bin/bash

Build packages under chroot

There is a more specialized chroot package, pbuilder, which constructs 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 will not exist in the resulting package.

Other virtualization tools

There are several system [http://en.wikipedia.org/wiki/Virtualization virtualization] and [http://en.wikipedia.org/wiki/Emulator emulation] related packages in Debian beyond simple [http://en.wikipedia.org/wiki/Chroot chroot].

List of virtualization tools

1

2

3

package

pocon

size

description

schroot

-

-

Specialized tool for executing Debian binary packages in chroot

sbuild

-

-

Tool for building Debian binary packages from Debian sources

pbuilder

-

-

Personal package builder for Debian packages

debootstrap

-

-

Bootstrap a basic Debian system (written in sh)

cdebootstrap

-

-

Bootstrap a Debian system (written in C)

rootstrap

-

-

A tool for building complete Linux filesystem images

user-mode-linux

-

-

[http://en.wikipedia.org/wiki/User-mode_Linux User-mode Linux] (kernel)

xen-tools

-

-

Tools to manage debian [http://en.wikipedia.org/wiki/Xen XEN] virtual server

bochs

-

-

[http://en.wikipedia.org/wiki/Bochs Bochs]: IA-32 PC emulator

qemu

-

-

[http://en.wikipedia.org/wiki/Qemu Qemu]: fast generic processor emulator

virtualbox-ose

-

-

[http://en.wikipedia.org/wiki/VirtualBox VirtualBox]: x86 virtualization solution on i386 and amd64

wine

-

-

[http://en.wikipedia.org/wiki/Wine_(software) Wine]: Windows API Implementation (standard suite)

dosbox

-

-

[http://en.wikipedia.org/wiki/DOSBox DOSBox]: x86 emulator with Tandy/Herc/CGA/EGA/VGA/SVGA graphics, sound and DOS

util-vserver

-

-

[http://en.wikipedia.org/wiki/Linux-VServer Linux-VServer] virtual private servers - user-space tools

vzctl

-

-

[http://en.wikipedia.org/wiki/OpenVZ OpenVZ] server virtualization solution - control tools

vzquota

-

-

[http://en.wikipedia.org/wiki/OpenVZ OpenVZ] server virtualization solution - quota tools

See Wikipedia article [http://en.wikipedia.org/wiki/Comparison_of_virtual_machines] for detail comparison of different virtualization solutions.