Rebuilding a Debian package starts with getting its source code. The easiest way is to use the apt-get source source-package-name command. This command requires a deb-src line in the /etc/apt/sources.list file, and up-to-date index files (i.e. apt-get update). These conditions should already be met if you followed the instructions from the chapter dealing with APT configuration (see Section 6.1, “Filling in the sources.list File”). Note however, that you'll be downloading the source packages from the Debian version mentioned in the deb-src line. If you need another version, you may need to download it manually from a Debian mirror or from the web site. This involves fetching two or three files (with extensions *.dsc — for Debian Source Control — *.tar.comp, and sometimes *.diff.gz or *.debian.tar.comp — comp taking one value among gz, bz2, lzma or xz depending on the compression tool in use), then run the dpkg-source -x file.dsc command. If the *.dsc file is directly accessible at a given URL, there's an even simpler way to fetch it all, with the dget URL command. This command (which can be found in the devscripts package) fetches the *.dsc file at the given address, then analyzes its contents, and automatically fetches the file or files referenced within. With the -x option, the source package is even unpacked locally after download.

The source of the package is now available in a directory named after the source package and its version (for instance, samba-3.0.24); this is where we'll work on our local changes.

The first thing to do is to change the package version number, so that the rebuilt packages can be distinguished from the original packages provided by Debian. Assuming the current version is 3.0.24-6, we can create version 3.0.24-6.falcot1, which clearly indicates the origin of the package. This makes the package version number higher than the one provided by Debian, so that the package will easily install as an update do the original package. Such a change is best effected with the dch command (Debian CHangelog) from the devscripts package, with an invocation such as dch -v 3.0.24-6.falcot1. This invocation invokes a text editor (sensible-editor — this should be your favorite editor if it's mentioned in the VISUAL or EDITOR environment variables, and default editor otherwise) to allow documenting the differences brought by this rebuild. This editor shows us that dch really did change the debian/changelog file.

When a change in build options is required, changes are made to debian/rules, which drives the steps in the package build process. In the simplest cases, the lines concerning the initial configuration (./configure …) or the actual build ($(MAKE) … or make …) are easy to spot. If these commands are not explicitly called, they are probably a side effect of another explicit command, in which case please refer to their documentation to learn more about how to change the default behavior.

Depending on the local changes to the packages, an update may also be required in the debian/control file, which contains a description of the generated packages. In particular, this file contains Build-Depends lines controlling the list of dependencies that must be fulfilled at package build time. These often refer to versions of packages contained in the distribution the source package comes from, but which may not be available in the distribution used for the rebuild. There is no automated way to determine if a dependency is real or only specified to guarantee that the build should only be attempted with the latest version of a library — this is the only available way to force an autobuilder to use a given package version during build, which is why Debian maintainers frequently use strictly versioned build-dependencies.

If you know for sure that these build-dependencies are too strict, you should feel free to relax them locally. Reading the files which document the standard way of building the software — these files are often called INSTALL — will help you figure out the appropriate dependencies. Ideally, all dependencies should be satisfiable from the distribution used for the rebuild; if they are not, a recursive process starts, whereby the packages mentioned in the Build-Depends field must be backported before the target package can be. Some packages may not need backporting, and can be installed as-is during the build process (a notable example is debhelper). Note that the backporting process can quickly become complex if you are not vigilant. Therefore, backports should be kept to a strict minimum when possible.

When all the needed changes have been applied to the sources, we can start generating the actual binary package (.deb file). The whole process is managed by the dpkg-buildpackage command.

$ dpkg-buildpackage -us -uc
[...]

The previous command can fail if the Build-Depends fields have not been updated, or if the related packages are not installed. In such a case, it is possible to overrule this check by passing the -d option to dpkg-buildpackage. However, explicitly ignoring these dependencies runs the risk of the build process failing at a later stage. Worse, the package may seem to build correctly but fail to run properly: some programs automatically disable some of their features when a required library is not available at build time.

More often than not, Debian developers use a higher-level program such as debuild; this runs dpkg-buildpackage as usual, but it also adds an invocation of a program that runs many checks to validate the generated package against the Debian policy. This script also cleans up the environment so that local environment variables do not “pollute” the package build. The debuild command is one of the tools in the devscripts suite, which share some consistency and configuration to make the maintainers' task easier.