1.2. Relationship with Debian
The Kali Linux distribution is based on Debian Testing. Therefore, most of the packages available in Kali Linux come straight from this Debian repository.
While Kali Linux relies heavily on Debian, it is also entirely independent in the sense that we have our own infrastructure and retain the freedom to make any changes we want.
1.2.1. The Flow of Packages
On the Debian side, the contributors are working every day on updating packages and uploading them to the Debian Unstable distribution. From there, packages migrate to the Debian Testing distribution once the most troublesome bugs have been taken out. The migration process also ensures that no dependencies are broken in Debian Testing. The goal is that Testing is always in a usable (or even releasable!) state.
Debian Testing's goals align quite well with those of Kali Linux so we picked it as the base. To add the Kali-specific packages in the distribution, we follow a two-step process.
First, we take Debian Testing and force-inject our own Kali packages (located in our kali-dev-only repository) to build the kali-dev repository. This repository will break from time to time: for instance, our Kali-specific packages might not be installable until they have been recompiled against newer libraries. In other situations, packages that we have forked might also have to be updated, either to become installable again, or to fix the installability of another package that depends on a newer version of the forked package. In any case, kali-dev is not for end-users.
1.2.2. Managing the Difference with Debian
As a design decision, we try to minimize the number of forked packages as much as possible. However, in order to implement some of Kali's unique features, some changes must be made. To limit the impact of these changes, we strive to send them upstream, either by integrating the feature directly, or by adding the required hooks so that it is straightforward to enable the desired features without further modifying the upstream packages themselves.
The Kali Package Tracker helps us to keep track of our divergence with Debian. At any time, we can look up which package has been forked and whether it is in sync with Debian, or if an update is required. All our packages are maintained in Git repositories hosting a Debian branch and a Kali branch side-by-side. Thanks to this, updating a forked package is a simple two-step process: update the Debian branch and then merge it into the Kali branch.
While the number of forked packages in Kali is relatively low, the number of additional packages is rather high: in April 2017 there were almost 400. Most of these packages are free software complying with the Debian Free Software Guidelines and our ultimate goal would be to maintain those packages within Debian whenever possible. That is why we strive to comply with the Debian Policy and to follow the good packaging practices used in Debian. Unfortunately, there are also quite a few exceptions where proper packaging was nearly impossible to create. As a result of time being scarce, few packages have been pushed to Debian.