# dpkg technical manual

Tom Lees
tom@lpsg.demon.co.uk
Version &apt-product-version;

This document describes the minimum necessary workings for the APT dselect replacement. It gives an overall specification of what its external interface must look like for compatibility, and also gives details of some internal quirks.

1997
Tom Lees

## License Notice

APT and this document are free software; you can redistribute them and/or modify them under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

For more details, on Debian systems, see the file /usr/share/common-licenses/GPL for the full license.

## Quick summary of dpkg's external interface

### Control files

The basic dpkg package control file supports the following major features:-

- 5 types of dependencies:-

  - Pre-Depends, which must be satisfied before a package may be unpacked

  - Depends, which must be satisfied before a package may be configured

  - Recommends, to specify a package which if not installed may severely limit the usefulness of the package

  - Suggests, to specify a package which may increase the productivity of the package

  - Conflicts, to specify a package which must NOT be installed in order for the package to be configured

  - Breaks, to specify a package which is broken by the package and which should therefore not be configured while broken

  Each of these dependencies can specify a version and a dependency on that version, for example "<= 0.5-1", "== 2.7.2-1", etc. The comparators available are:-

  - "<<" - less than

  - "<=" - less than or equal to

  - ">>" - greater than

  - ">=" - greater than or equal to

  - "==" - equal to

- The concept of "virtual packages", which many other packages may provide, using the Provides mechanism. An example of this is the "httpd" virtual package, which all web servers should provide. Virtual package names may be used in dependency headers. However, current policy is that virtual packages do not support version numbers, so dependencies on virtual packages with versions will always fail.

- Several other control fields, such as Package, Version, Description, Section, Priority, etc., which are mainly for classification purposes. The package name must consist entirely of lowercase characters, plus the characters '+', '-', and '.'. Fields can extend across multiple lines - on the second and subsequent lines, there is a space at the beginning instead of a field name and a ':'. Empty lines must consist of the text " .", which will be ignored, as will the initial space for other continuation lines. This feature is usually only used in the Description field.

### The dpkg status area

The "dpkg status area" is the term used to refer to the directory where dpkg keeps its various status files (GNU would have you call it the dpkg shared state directory). This is always, on Debian systems, /var/lib/dpkg. However, the default directory name should not be hard-coded, but #define'd, so that alteration is possible (it is available via configure in dpkg 1.4.0.9 and above). Of course, in a library, code should be allowed to override the default directory, but the default should be part of the library (so that the user may change the dpkg admin dir simply by replacing the library).

Dpkg keeps a variety of files in its status area. These are discussed later on in this document, but a quick summary of the files is here:-

- available - this file contains a concatenation of control information from all the packages which dpkg knows about. This is updated using the dpkg commands "--update-avail <file>", "--merge-avail <file>", and "--clear-avail".

- status - this file contains information on the following things for every package:-

  - Whether it is installed, not installed, unpacked, removed, failed configuration, or half-installed (deconfigured in favour of another package).

  - Whether it is selected as install, hold, remove, or purge.

  - If it is "ok" (no installation problems), or "not-ok".

  - It usually also contains the section and priority (so that dselect may classify packages not in available)

  - For packages which did not initially appear in the "available" file when they were installed, the other control information for them.

  The exact format for the "Status:" field is:

            Status: Want Flag Status

  Where Want may be one of unknown, install, hold, deinstall, purge. Flag may be one of ok, reinstreq. Status may be one of not-installed, config-files, half-installed, unpacked, half-configured and installed. The states are as follows:-

  not-installed

    No files are installed from the package, it has no config files left, it uninstalled cleanly if it ever was installed.

  unpacked

    The basic files have been unpacked (and are listed in /var/lib/dpkg/info/[package].list. There are config files present, but the postinst script has _NOT_ been run.

  half-configured

    The package was installed and unpacked, but the postinst script failed in some way.

  installed

    All files for the package are installed, and the configuration was also successful.

  half-installed

    An attempt was made to remove the package but there was a failure in the prerm script.

  config-files

    The package was "removed", not "purged". The config files are left, but nothing else.

  The two last items are only left in dpkg for compatibility - they are understood by it, but never written out in this form.

  Please see the dpkg source code, `lib/parshelp.c`, statusinfos, eflaginfos and wantinfos for more details.

- info - this directory contains files from the control archive of every package currently installed. They are installed with a prefix of "<packagename>.". In addition to this, it also contains a file called <package>.list for every package, which contains a list of files. Note also that the control file is not copied into here; it is instead found as part of status or available.

- methods - this directory is reserved for "method"-specific files - each "method" has a subdirectory underneath this directory (or at least, it can have). In addition, there is another subdirectory "mnt", where misc. filesystems (floppies, CD-ROMs, etc.) are mounted.

- alternatives - directory used by the "update-alternatives" program. It contains one file for each "alternatives" interface, which contains information about all the needed symlinked files for each alternative.

- diversions - file used by the "dpkg-divert" program. Each diversion takes three lines. The first is the package name (or ":" for user diversion), the second the original filename, and the third the diverted filename.

- updates - directory used internally by dpkg. This is discussed later, in the section .

- parts - temporary directory used by dpkg-split

### The dpkg library files

These files are installed under /usr/lib/dpkg (usually), but /usr/local/lib/dpkg is also a possibility (as Debian policy dictates). Under this directory, there is a "methods" subdirectory. The methods subdirectory in turn contains any number of subdirectories for each general method processor (note that one set of method scripts can, and is, used for more than one of the methods listed under dselect).

The following files may be found in each of these subdirectories:-

- names - One line per method, two-digit priority to appear on menu at beginning, followed by a space, the name, and then another space and the short description.

- desc.<name> - Contains the long description displayed by dselect when the cursor is put over the <name> method.

- setup - Script or program which sets up the initial values to be used by this method. Called with first argument as the status area directory (/var/lib/dpkg), second argument as the name of the method (as in the directory name), and the third argument as the option (as in the names file).

- install - Script/program called when the "install" option of dselect is run with this method. Same arguments as for setup.

- update - Script/program called when the "update" option of dselect is run. Same arguments as for setup/install.

### The "dpkg" command-line utility

#### "Documented" command-line interfaces

As yet unwritten. You can refer to the other manuals for now. See dpkg 8.

#### Environment variables which dpkg responds to

- SHELL - used to determine which shell to run.

- CC - used as the C compiler to call to determine the target architecture. The default is "gcc".

- PATH - dpkg checks that it can find at least the following files in the path when it wants to run package installation scripts, and gives an error if it cannot find all of them:-

  - ldconfig

  - start-stop-daemon

  - install-info

  - update-rc.d

#### Assertions

The dpkg utility itself is required for quite a number of packages, even if they have been installed with a tool totally separate from dpkg. The reason for this is that some packages, in their pre-installation scripts, check that your version of dpkg supports certain features. This was broken from the start, and it should have actually been a control file header "Dpkg-requires", or similar. What happens is that the configuration scripts will abort or continue according to the exit code of a call to dpkg, which will stop them from being wrongly configured.

These special command-line options, which simply return as true or false are all prefixed with "--assert-". Here is a list of them (without the prefix):-

- support-predepends - Returns success or failure according to whether a version of dpkg which supports predepends properly (1.1.0 or above) is installed, according to the database.

- working-epoch - Return success or failure according to whether a version of dpkg which supports epochs in version properly (1.4.0.7 or above) is installed, according to the database.

Both these options check the status database to see what version of the "dpkg" package is installed, and check it against a known working version.

#### --predep-package

This strange option is described as follows in the source code:

    /* Print a single package which:
     *  (a) is the target of one or more relevant predependencies.
     *  (b) has itself no unsatisfied pre-dependencies.
     * If such a package is present output is the Packages file entry,
     *  which can be massaged as appropriate.
     * Exit status:
     *  0 = a package printed, OK
     *  1 = no suitable package available
     *  2 = error
     */

On further inspection of the source code, it appears that what is does is this:-

- Looks at the packages in the database which are selected as "install", and are installed.

- It then looks at the Pre-Depends information for each of these packages from the available file. When it find a package for which any of the pre-dependencies are not satisfied, it breaks from the loop through the packages.

- It then looks through the unsatisfied pre-dependencies, and looks for packages which would satisfy this pre-dependency, stopping on the first it finds. If it finds none, it bombs out with an error.

- It then continues this for every dependency of the initial package.

Eventually, it writes out the record of all the packages to satisfy the pre-dependencies. This is used by the disk method to make sure that its dependency ordering is correct. What happens is that all pre-depending packages are first installed, then it runs dpkg -iGROEB on the directory, which installs in the order package files are found. Since pre-dependencies mean that a package may not even be unpacked unless they are satisfied, it is necessary to do this (usually, since all the package files are unpacked in one phase, the configured in another, this is not needed).

## dpkg-deb and .deb file internals

This chapter describes the internals to the "dpkg-deb" tool, which is used by "dpkg" as a back-end. dpkg-deb has its own tar extraction functions, which is the source of many problems, as it does not support long filenames, using extension blocks.

### The .deb archive format

The main principal of the new-format Debian archive (I won't describe the old format - for that have a look at deb-old.5), is that the archive really is an archive - as used by "ar" and friends. However, dpkg-deb uses this format internally, rather than calling "ar". Inside this archive, there are usually the following members:-

- debian-binary

- control.tar.gz

- data.tar.gz

The debian-binary member consists simply of the string "2.0", indicating the format version. control.tar.gz contains the control files (and scripts), and the data.tar.gz contains the actual files to populate the filesystem with. Both tarfiles extract straight into the current directory. Information on the tar formats can be found in the GNU tar info page. Since dpkg-deb calls "tar -cf" to build packages, the Debian packages use the GNU extensions.

### The dpkg-deb command-line

dpkg-deb documents itself thoroughly with its '--help' command-line option. However, I am including a reference to these for completeness. dpkg-deb supports the following options:-

- --build (-b) <dir> - builds a .deb archive, takes a directory which contains all the files as an argument. Note that the directory <dir>/DEBIAN will be packed separately into the control archive.

- --contents (-c) <debfile> - Lists the contents of the "data.tar.gz" member.

- --control (-e) <debfile> - Extracts the control archive into a directory called DEBIAN. Alternatively, with another argument, it will extract it into a different directory.

- --info (-I) <debfile> - Prints the contents of the "control" file in the control archive to stdout. Alternatively, giving it other arguments will cause it to print the contents of those files instead.

- --field (-f) <debfile> <field> ... - Prints any number of fields from the "control" file. Giving it extra arguments limits the fields it prints to only those specified. With no command-line arguments other than a filename, it is equivalent to -I and just the .deb filename.

- --extract (-x) <debfile> <dir> - Extracts the data archive of a debian package under the directory <dir>.

- --vextract (-X) <debfile> <dir> - Same as --extract, except it is equivalent of giving tar the '-v' option - it prints the filenames as it extracts them.

- --fsys-tarfile <debfile> - This option outputs a gunzip'd version of data.tar.gz to stdout.

- --new - sets the archive format to be used to the new Debian format

- --old - sets the archive format to be used to the old Debian format

- --debug - Tells dpkg-deb to produce debugging output

- --nocheck - Tells dpkg-deb not to check the sanity of the control file

- --help (-h) - Gives a help message

- --version - Shows the version number

- --licence/--license (UK/US spellings) - Shows a brief outline of the GPL

#### Internal checks used by dpkg-deb when building packages

Here is a list of the internal checks used by dpkg-deb when building packages. It is in the order they are done.

- First, the output Debian archive argument, if it is given, is checked using stat. If it is a directory, an internal flag is set. This check is only made if the archive name is specified explicitly on the command-line. If the argument was not given, the default is the directory name, with ".deb" appended.

- Next, the control file is checked, unless the --nocheck flag was specified on the command-line. dpkg-deb will bomb out if the second argument to --build was a directory, and --nocheck was specified. Note that dpkg-deb will not be able to determine the name of the package in this case. In the control file, the following things are checked:-

  - The package name is checked to see if it contains any invalid characters (see for this).

  - The priority field is checked to see if it uses standard values, and user-defined values are warned against. However, note that this check is now redundant, since the control file no longer contains the priority - the changes file now does this.

  - The control file fields are then checked against the standard list of fields which appear in control files, and any "user-defined" fields are reported as warnings.

  - dpkg-deb then checks that the control file contains a valid version number.

- After this, in the case where a directory was specified to build the .deb file in, the filename is created as "directory/pkg_ver.deb" or "directory/pkg_ver_arch.deb", depending on whether the control file contains an architecture field.

- Next, dpkg-deb checks for the <dir>/DEBIAN directory. It complains if it doesn't exist, or if it has permissions < 0755, or > 0775.

- It then checks that all the files in this subdir are either symlinks or plain files, and have permissions between 0555 and 0775.

- The conffiles file is then checked to see if the filenames are too long. Warnings are produced for each that is. After this, it checks that the package provides initial copies of each of these conffiles, and that they are all plain files.

## dpkg internals

This chapter describes the internals of dpkg itself. Although the low-level formats are quite simple, what dpkg does in certain cases often does not make sense.

### Updates

This describes the /var/lib/dpkg/updates directory. The function of this directory is somewhat strange, and seems only to be used internally. A function called cleanupdates is called whenever the database is scanned. This function in turn uses scandir 3, to sort the files in this directory. Files who names do not consist entirely of digits are discarded. dpkg also causes a fatal error if any of the filenames are different lengths.

After having scanned the directory, dpkg in turn parses each file the same way it parses the status file (they are sorted by the scandir to be in numerical order). After having done this, it then writes the status information back to the "status" file, and removes all the "updates" files.

These files are created internally by dpkg's "checkpoint" function, and are cleaned up when dpkg exits cleanly.

Judging by the use of the updates directory I would call it a Journal. Inorder to efficiently ensure the complete integrity of the status file dpkg will "checkpoint" or journal all of it's activities in the updates directory. By merging the contents of the updates directory (in order!!) against the original status file it can get the precise current state of the system, even in the event of a system failure while dpkg is running.

The other option would be to sync-rewrite the status file after each operation, which would kill performance.

It is very important that any program that uses the status file abort if the updates directory is not empty! The user should be informed to run dpkg manually (what options though??) to correct the situation.

### What happens when dpkg reads the database

First, the status file is read. This gives dpkg an initial idea of the packages that are there. Next, the updates files are read in, overriding the status file, and if necessary, the status file is re-written, and updates files are removed. Finally, the available file is read. The available file is read with flags which preclude dpkg from updating any status information from it, though - installed version, etc., and is also told to record that the packages it reads this time are available, not installed.

More information on updates is given above.

### How dpkg compares version numbers

Version numbers consist of three parts: the epoch, the upstream version, and the Debian revision. Dpkg compares these parts in that order. If the epochs are different, it returns immediately, and so on.

However, the important part is how it compares the versions which are essentially stored as just strings. These are compared in two distinct parts: those consisting of numerical characters (which are evaluated, and then compared), and those consisting of other characters. When comparing non-numerical parts, they are compared as the character values (ASCII), but non-alphabetical characters are considered "greater than" alphabetical ones. Also note that longer strings (after excluding differences where numerical values are equal) are considered "greater than" shorter ones.

Here are a few examples of how these rules apply:-

    15 > 10
    0010 == 10

    d.r > dsr
    32.d.r == 0032.d.r
    d.rnr < d.rnrn