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<div>
<h1 class="title"><a id="ch2"/>Chapter 2. dpkg-deb and .deb file internals</h1>
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<div class="toc">
<p>
<strong>Table of Contents</strong>
</p>
<dl class="toc">
<dt>
<span class="section">
<a href="ch2.html#s2.1">2.1. The .deb archive format</a>
</span>
</dt>
<dt>
<span class="section">
<a href="ch2.html#s2.2">2.2. The dpkg-deb command-line</a>
</span>
</dt>
</dl>
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<p>
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.
</p>
<div class="section">
<div class="titlepage">
<div>
<div>
<h2 class="title"><a id="s2.1"/>2.1. The .deb archive format</h2>
</div>
</div>
</div>
<p>
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:-
</p>
<div class="itemizedlist">
<ul class="itemizedlist">
<li class="listitem">
<p>
debian-binary
</p>
</li>
<li class="listitem">
<p>
control.tar.gz
</p>
</li>
<li class="listitem">
<p>
data.tar.gz
</p>
</li>
</ul>
</div>
<p>
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.
</p>
</div>
<div class="section">
<div class="titlepage">
<div>
<div>
<h2 class="title"><a id="s2.2"/>2.2. The dpkg-deb command-line</h2>
</div>
</div>
</div>
<p>
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:-
</p>
<div class="itemizedlist">
<ul class="itemizedlist">
<li class="listitem">
<p>
--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.
</p>
</li>
<li class="listitem">
<p>
--contents (-c) <debfile> - Lists the contents of the "data.tar.gz"
member.
</p>
</li>
<li class="listitem">
<p>
--control (-e) <debfile> - Extracts the control archive into a directory
called DEBIAN. Alternatively, with another argument, it will extract it into a
different directory.
</p>
</li>
<li class="listitem">
<p>
--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.
</p>
</li>
<li class="listitem">
<p>
--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.
</p>
</li>
<li class="listitem">
<p>
--extract (-x) <debfile> <dir> - Extracts the data archive of a
debian package under the directory <dir>.
</p>
</li>
<li class="listitem">
<p>
--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.
</p>
</li>
<li class="listitem">
<p>
--fsys-tarfile <debfile> - This option outputs a gunzip'd version of
data.tar.gz to stdout.
</p>
</li>
<li class="listitem">
<p>
--new - sets the archive format to be used to the new Debian format
</p>
</li>
<li class="listitem">
<p>
--old - sets the archive format to be used to the old Debian format
</p>
</li>
<li class="listitem">
<p>
--debug - Tells dpkg-deb to produce debugging output
</p>
</li>
<li class="listitem">
<p>
--nocheck - Tells dpkg-deb not to check the sanity of the control file
</p>
</li>
<li class="listitem">
<p>
--help (-h) - Gives a help message
</p>
</li>
<li class="listitem">
<p>
--version - Shows the version number
</p>
</li>
<li class="listitem">
<p>
--licence/--license (UK/US spellings) - Shows a brief outline of the GPL
</p>
</li>
</ul>
</div>
<div class="section">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="s2.2.1"/>2.2.1. Internal checks used by dpkg-deb when building packages</h3>
</div>
</div>
</div>
<p>
Here is a list of the internal checks used by dpkg-deb when building
packages. It is in the order they are done.
</p>
<div class="itemizedlist">
<ul class="itemizedlist">
<li class="listitem">
<p>
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.
</p>
</li>
<li class="listitem">
<p>
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:-
</p>
<div class="itemizedlist">
<ul class="itemizedlist">
<li class="listitem">
<p>
The package name is checked to see if it contains any invalid characters (see
<a class="xref" href="ch1.html#control" title="1.1. Control files">Section 1.1, “Control files”</a> for this).
</p>
</li>
<li class="listitem">
<p>
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.
</p>
</li>
<li class="listitem">
<p>
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.
</p>
</li>
<li class="listitem">
<p>
dpkg-deb then checks that the control file contains a valid version number.
</p>
</li>
</ul>
</div>
</li>
<li class="listitem">
<p>
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.
</p>
</li>
<li class="listitem">
<p>
Next, dpkg-deb checks for the <dir>/DEBIAN directory. It complains if it
doesn't exist, or if it has permissions < 0755, or > 0775.
</p>
</li>
<li class="listitem">
<p>
It then checks that all the files in this subdir are either symlinks or plain
files, and have permissions between 0555 and 0775.
</p>
</li>
<li class="listitem">
<p>
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.
</p>
</li>
</ul>
</div>
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