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8.1.1. Relationship between APT and dpkg
A Debian package is a compressed archive of a software application. A binary package (a .deb file) contains files that can be directly used (such as programs or documentation), while a source pack- age contains the source code for the software and the instructions required for building a binary package. A Debian package contains the application’s files as well as other metadata including the names of the dependencies the application needs, as well as scripts that enable the execution of commands at different stages in the package’s lifecycle (installation, removal, and upgrades).
The dpkg tool was designed to process and install .deb packages, but if it encountered an unsat- isfied dependency (like a missing library) that would prevent the package from installing, dpkg would simply list the missing dependency, because it had no awareness or built-in logic to find or process the packages that might satisfy those dependencies. The Advanced Package Tool (APT),
including apt and apt-get, were designed to address these shortcomings and could automatically resolve these issues. We will talk about both dpkg and the APT tools in this chapter.
The base command for handling Debian packages on the system is dpkg, which performs installa- tion or analysis of .deb packages and their contents. However, dpkg has only a partial view of the Debian universe: it knows what is installed on the system and whatever you provide on the com- mand line, but knows nothing of the other available packages. As such, it will fail if a dependency is not met. APT addresses the limitations.
APT is a set of tools that help manage Debian packages, or applications on your Debian system. You can use APT to install and remove applications, update packages, and even upgrade your entire system. The magic of APT lies in the fact that it is a complete package management system that will not only install or remove a package, but will consider the requirements and dependencies of the packaged application (and even their requirements and dependencies) and attempt to satisfy them automatically. APT relies on dpkg but APT differs from dpkg, as the former installs the latest package from an online source and works to resolve dependencies while dpkg installs a package located on your local system and does not automatically resolve dependencies.
If you have been around long enough to remember compiling programs with gcc (even with the help of utilities such as make and configure), you likely remember that it was a painful process, especially if the application had several dependencies. By deciphering the various warnings and error messages, you may be able to determine which part of the code was failing and most often that failure was due to a missing library or other dependency. You would then track down that missing library or dependency, correct it, and try again. Then, if you were lucky, the compile would complete, but often the build would fail again, complaining about another broken depen- dency.
APT was designed to help alleviate that problem, collate program requirements and dependencies, and resolve them. This functionality works out-of-the-box on Kali Linux, but it isn’t foolproof. It is important that you understand how Debian and Kali’s packaging system works because you will need to install packages, update software, or troubleshoot problems with packages. You will use APT in your day-to-day work with Kali Linux and in this chapter, we will introduce you to APT and show you how to install, remove, upgrade, and manage packages, and even show you how to move packages between different Linux distributions. We will also talk about graphical tools that leverage APT, show you how to validate the authenticity of packages, and delve into the concept of a rolling distribution, a technique that brings daily updates to your Kali system.
Before we dig in and show you how to use dpkg and APT to install and manage packages, it is important that we delve into some of the inner workings of APT and discuss some terminology surrounding it.
Package Source and The word source can be ambiguous. A source package—a package containing the
Source Package source code of a program—should not be confused with a package source—a reposi- tory (website, FTP server, CD-ROM, local directory, etc.) that contains packages.
Package Source and The word source can be ambiguous. A source package—a package containing the
Source Package source code of a program—should not be confused with a package source—a reposi- tory (website, FTP server, CD-ROM, local directory, etc.) that contains packages.
APT retrieves its packages from a repository, a package storage system or simply, ”package source”. The /etc/apt/sources.list file lists the different repositories (or sources) that pub- lish Debian packages.