Understanding Debian Packages (2014)

Miriam Ruiz <miriam@debian.org>
Understanding Debian Packages
A very brief introduction to what's inside Debian binary packages

2 / 36
Contents
What is a Package?
Questions and Answers
What's inside a package
Relationship between packages
The scripts inside the packages
Debian Configuration: Debconf

4 / 36
What is a Package?
● Minimal Unit of Installation and Removal
● A Debian package can include:
● The files needed to provide some functionality to the system
(software, artwork, configuration files, program data, etc)
● Some administrative information regarding that package:
Description, Dependencies, Maintainer, etc.
● The copyright/licensing information about the contents
included in that package, though this can't be read until the
package is open.
● Configuration scripts to install/deinstall/upgrade/purge the
software
● Checksums to check if the files contained in the file have
not been modified. Keep in mind that these are not provided
for security against attacks.
● A mechanism (debconf) to configure some parameters
about the package.

5 / 36
What can you do with a Package?
● Install it: dpkg ––install <file.deb>
● Show information about it: dpkg ––info <file.deb>
● List its contents: dpkg ––contents <file.deb>
● Extract its files: dpkg ––extract <file.deb> <directory>
● Extract and show files: dpkg ––vextract <file.deb> <directory>
● Lists the installed packages: dpkg ––list <pattern>
● List the files installed from a package: dpkg ––listfiles <package>
● Shows information about a package: dpkg ––print-avail <package>
● Packages where a given filename is found: dpkg ––search <filename>
● Shows the status of a fiven package: dpkg ––status <package>
● Remove package: dpkg ––remove <package>
● Remove it and purge its configuration: sudo dpkg ––purge <package>
See: https://www.debian.org/doc/manuals/debian-reference/ch02.en.html

6 / 36
What's inside a Package?

7 / 36
How is a package inside?
Files Metadata (aka. DEBIAN)
Package

8 / 36
How are these things stored?
Package
data.tar + compression control.tar + compr.debian-binary
2.0
BSD ar file (uncompressed)
See: https://www.debian.org/doc/manuals/debian-faq/ch-pkg_basics

9 / 36
How can I extract the contents?
● Extract the archived contents: ar x <file.deb>
● Result: control.tar.gz data.tar.gz debian-binary
● Standards version of the binary package : cat debian-binary
● Which files are distributed: tvfJ data.tar.xz
● Which metadata files are included: tvfz control.tar.gz
● Some files that we can find: control, prerm. postrm, preinst,
postinst, shlibs, conffiles, config, templates,
md5sums
● It is easier to work with the dpkg-deb command:
● Show information about a package: dpkg ––info <file.deb>
● Show a control field: dpkg ––field <file.deb> <field>
● Show a control file: dpkg ––info <file.deb> <control_file>
● Show the data contents: dpkg ––contents <file.deb>
● Extract all: dpkg ––raw-extract <file.deb> <directory>
● Build a package: dpkg ––build <directory> <directory>

10 / 36
The control data of a package

11 / 36
Control Fields: DEBIAN/control
Package: [package name]
Version: [version-deb]
Architecture: [all or architecture_id. Try 'dpkg-architecture -L']
Maintainer: [Name <email@debian.org>]
Installed-Size: [est. inst. size in bytes, div. By 1024, rounded up]
Pre-Depends: [packages needed before even starting the installation]
Depends: [absolute dependency]
Recommends: [strong, but not absolute, dependency]
Suggests: [one package may be more useful with one or more others]
Enhances: [can enhance the functionality of another package]
Breaks: [will not be unpacked unless other package is deconfigured]
Conflicts: [will not be unpacked in the system at the same time]
Provides: [virtual packages]
Replaces: [overwrites files in other packages or replaces them]
Section: [application area]
Priority: [extra, optional, standard, important, required, essential]
Homepage: [web page]
Description: short description
This is the long description of the package
See: https://www.debian.org/doc/debian-policy/ch-controlfields.html

12 / 36
Relationship with other packages
● Pre-Depends: Forces dpkg to complete installation of the packages named before
even starting the installation of the package which declares the pre-dependency
● Depends: A package will not be configured unless all of the packages listed in its
Depends field have been correctly configured (unless there is a circular
dependency).
●
Recommends: A strong, but not absolute, dependency. Packages that would be
found together with this one in all but unusual installations.
● Suggests: Packages that are related to this one and can perhaps enhance its
usefulness, but that installing this one without them is perfectly reasonable.
● Enhances: Similar to Suggests but works in the opposite direction. A package can
enhance the functionality of another package.
● Breaks: The package won't be unpacked unless the broken package is deconfigured
first, and it will refuse to allow the broken package to be reconfigured.
● Conflicts: The packages won't be unpacked on the system at the same time.
● Replaces: If the overwriting package declares that it Replaces the one containing
the file being overwritten, then dpkg will replace the file from the old package with
that from the new. Normally, Breaks should be used in conjunction with Replaces.
● Provides: A virtual package is one which appears in the Provides control field of
another package. The effect is as if the package(s) which provide a particular virtual
package name had been listed by name everywhere the virtual package name
appears.
See: https://www.debian.org/doc/manuals/maint-guide/dother.en.html#conffiles

13 / 36
What are conffiles (DEBIAN/conffiles)?
● List of configuration packages thet dpkg will not overwrite when the package
is upgraded
● Those files are usually placed in /etc/
● To determine exactly which files are preserved during an upgrade, for a
package you have already installed, you can run:
● dpkg ––status <package>
● To see the conffiles in a debian package file, you can do:
● dpkg ––info <file.deb> conffiles
See: https://www.debian.org/doc/manuals/maint-guide/dother.en.html#conffiles

14 / 36
Checksums and signatures
● The file DEBIAN/md5sums holded a relationship of all the files included in
data.tar.xz file and their corresponding MD5 checksum once extracted. This
data is not meant for security reasons, because they would be easily
modifiable, but as a way to check if the files have been modified.
● You can use, for example debsums -a <package>, to check if you have
modified a file in your system, that is supposed to be managed by dpkg.
● The security is managed via cryptographic signatures at an APT level, and
not at an individual package level.
● You can sign and verify individual packages via debsigs and debsig-verify, or
dpkg-sig, although I don't think it is too widely spread. I have never tried it.
See: https://www.debian.org/doc/debian-policy/ap-pkg-controlfields.html
See: https://wiki.debian.org/SecureApt
See: http://purplefloyd.wordpress.com/2009/02/05/signing-deb-packages/

15 / 36
Scripts that handle package changes

16 / 36
preinst, postinst, prerm, and postrm scripts
● Preinst: This script executes before that package will be unpacked from its
Debian archive (".deb") file. Many 'preinst' scripts stop services for packages
which are being upgraded until their installation or upgrade is completed.
● Postinst: This script typically completes any required configuration of the
package foo once foo has been unpacked from its Debian archive (".deb")
file. Often, 'postinst' scripts ask the user for input, and/or warn the user that if
they accept default values, they should remember to go back and
reconfigure that package afterwards. Many 'postinst' scripts execute any
commands necessary to start or restart a service once a new package has
been installed or upgraded.
● Prerm: This script typically stops any daemons which are associated with a
package. It is executed before the removal of files associated with the
package.
● Postrm: This script typically modifies links or other files associated with foo,
and/or removes files created by the package.
See: http://www.marga.com.ar/~marga/debian/diagrams/
See: http://people.debian.org/~srivasta/MaintainerScripts.html

20 / 36
Removing and purging a package

21 / 36
Installing an already configured package

23 / 36
DPKG Triggers
● Triggers are used to ensure that during the standard package-management
process certain operations always run, but not more than necessary.
● Trigger-using packages can be classified in two behavioral categories:
● Consumers: packages which declare triggers and thus can be
"triggered"
● Producers: packages which activate triggers (explicitly or implicitly)
● When a consumer is triggered, its postinst script is run with the arguments:
● postinst triggered "<trigger1> ... <triggerN>"
See: http://www.seanius.net/blog/2009/09/dpkg-triggers-howto/
See: http://stackoverflow.com/questions/15276535/dpkg-how-to-use-trigger

24 / 36
States in which Debian Packages can be
● Not-installed: The package is not installed on your system.
● Config-files: Only the configuration files of the package exist on the system.
● Half-installed: The installation of the package has been started, but not
completed for some reason.
● Unpacked: The package is unpacked, but not configured.
● Half-configured: The package is unpacked and configuration has been
started, but not yet completed for some reason.
● Triggers-awaited: The package awaits trigger processing by another
package.
● Triggers-pending: The package has been triggered.
● Installed: The package is unpacked and configured OK.

25 / 36

26 / 36
● Debconf is a backend database, with a frontend that talks to it and presents
an interface to the user. There can be many different types of frontends, from
plain text to a web frontend.
● The frontend also talks to a special config script in the control section of a
debian package, and it can talk to postinst scripts and other scripts as
well, all using a special protocol. These scripts tell the frontend what values
they need from the database, and the frontend asks the user questions to
get those values if they aren't set.
● All the configuration information is stored in a special database that defines
a hierarchy of information. Each package receives its own space in the
hierarchy and is free to use a flat space, or divide its space further into sub-
hierarchies.
● If multiple packages share a common purpose they may use a shared
toplevel hierarchy, preferably with the same name as a shared (virtual)
package name.
● Each variable in the configuration space has some information associated
with it, and it has a value.
See: http://www.fifi.org/doc/debconf-doc/tutorial.html
See: https://www.debian.org/doc/packaging-manuals/debconf_specification.html

27 / 36

28 / 36

29 / 36
Debconf script: DEBIAN/config
#!/bin/sh
set -e
. /usr/share/debconf/confmodule
if [ -f /usr/share/dbconfig-common/dpkg/config.mysql ]; then
. /usr/share/dbconfig-common/dpkg/config.mysql
if ! dbc_go phpmyadmin $@ ; then
echo 'Automatic configuration using dbconfig-common
failed!'
fi
fi
db_version 2.0
db_input high phpmyadmin/reconfigure-webserver || true
if [ ! -f /etc/phpmyadmin/htpasswd.setup ]; then
db_input low phpmyadmin/setup-username || true
db_input low phpmyadmin/setup-password || true
fi
db_go || true

30 / 36
Debconf templates: DEBIAN/templates
Template: hostname
Type: string
Default: debian
Description: unqualified hostname for this computer
This is the name by which this computer will be known on the network.
It
has to be a unique name in your domain.
Template: domain
Type: string
Description: domain for this computer
This is the domain your computer is a member of. Typically it is
something like "mycompany.com" or "myuniversity.edu".

31 / 36
Configuration templates: DEBIAN/templates
debian package, and it can talk to postinst scripts and other scripts as
well, all using a special protocol. These scripts tell the frontend what values
they need from the database, and the frontend asks the user questions to
get those values if they aren't set.
● All the configuration information is stored in a special database that defines
a hierarchy of information. Each package receives its own space in the
hierarchy and is free to use a flat space, or divide its space further into sub-
hierarchies.
● If multiple packages share a common purpose they may use a shared
toplevel hierarchy, preferably with the same name as a shared (virtual)
package name.
● Each variable in the configuration space has some information associated
with it, and it has a value.
● You can see its contents using the command: debconf-show

32 / 36
debian package, and it can talk to postinst scripts and other scripts as well,
all using a special protocol. These scripts tell the frontend what values they
need from the database, and the frontend asks the user questions to get
those values if they aren't set.

34 / 36
Some Links
● Debian Policy Manual:
● https://www.debian.org/doc/debian-policy/
● Basics of the Debian Package Management System:
● https://www.debian.org/doc/manuals/debian-faq/ch-pkg_basics
● Debian Administrator's Handbook:
● http://debian-handbook.info/browse/stable/sect.manipulating-packages-with-dpkg.html
● Getting Information About Packages with APT:
● http://newbiedoc.sourceforge.net/tutorials/apt-get-intro/info.html
● Maintainer Scripts:
● http://people.debian.org/~srivasta/MaintainerScripts.html
● Debconf Specification:
● https://www.debian.org/doc/packaging-manuals/debconf_specification.html
● The Debconf Programmer's Tutorial
● http://www.fifi.org/doc/debconf-doc/tutorial.html
●

36 / 36
Copyright © 2014, Miriam Ruiz
This work is licensed under the Creative
Commons Attribution-Share Alike 3.0
(CC-by-sa 3.0) license. You can use, copy,
modify, merge, remix, distribute, display,
perform, sublicense and/or sale it freely under
the conditions defined in that license.
See http://creativecommons.org/licenses/by-sa/3.0/

Understanding Debian Packages (2014)

More Related Content

Viewers also liked

Similar to Understanding Debian Packages (2014)

More from Miriam Ruiz

Recently uploaded

Understanding Debian Packages (2014)