Specialty Linux distributions
Aside from the mainstream Linux distributions, there are several distros that have been purpose-built for a specific set of requirements. They are all built on a more mainstream distro but are tailored to fit a specific set of needs. We'll describe a few here that you are most likely to see or use as a network professional.
Most commercial Network-attached Storage (NAS) and SAN providers are based on Linux or BSD. The front runner on open source NAS/SAN services, at the time of writing, seems to be TrueNAS (formerly FreeNAS) and XigmaNAS (formerly NAS4Free). Both have free and commercial offerings.
Open source firewalls
Networking and security companies offer a wide variety of firewall appliances, most of which are based on Linux or BSD. Many companies do offer free firewalls, some of the more popular being pfSense (free versions and pre-built hardware solutions available), OPNsense (freely available, with donations), and Untangle (which also has a commercial version). Smoothwall is another alternative, with both free and commercial versions available.
In this book, we'll explore using the on-board firewall in Linux to secure individual servers, or to secure a network perimeter.
Kali Linux
Descended from BackTrack, and KNOPPIX before that, Kali Linux is a distribution based on Debian that is focused on information security. The underlying goal of this distribution is to collect as many useful penetration testing and ethical hacking tools as possible on one platform, and then ensure that they all work without interfering with each other. The newer versions of the distribution have focused on maintaining this tool interoperability as the OS and tools get updated (using the apt toolset).
SIFT
SIFT is a distribution authored by the forensics team at the SANS institute, focused on digital forensics and incident response tools and investigations. Similar to Kali, the goal of SIFT is to be a "one-stop shop" for free/open source tools in one field – Digital Forensics and Incident Response (DFIR). Historically, this was a distribution based on Ubuntu, but in recent years, this has changed – SIFT is now also distributed as a script that installs the tools on Ubuntu desktop or Windows Services for Linux (which is Ubuntu-based).
Security Onion
Security Onion is also similar to Kali Linux in that it contains several information security tools, but its focus is more from the defender's point of view. This distribution is centered on threat hunting, network security monitoring, and log management. Some of the tools in this distribution include Suricata, Zeek, and Wazuh, just to name a few.