The Red Hat saga — RHEL, Fedora, CentOS, Rocky and AlmaLinux

Objective

Understand the Red Hat family, the most widespread in large companies: who does what between RHEL, Fedora, CentOS, Rocky and AlmaLinux, why CentOS disappeared, and what changes from one version of RHEL to the next. This is the common thread of the Red Hat track of this course.

Key points

• RHEL (Red Hat Enterprise Linux) is the enterprise version, sold by subscription (support + updates), with 10 years of support per version.
• Fedora (2003) is the community distribution where new features are prepared; CentOS Stream is the preview of the next RHEL update.
• CentOS (2004) was a free rebuild of RHEL. At the end of 2020, Red Hat ended CentOS Linux 8 early (end of 2021); CentOS 7 ended on 30 June 2024.
• Rocky Linux and AlmaLinux (2021) took over: free of charge and compatible with RHEL.
• Each version of RHEL brings a major change: systemd in 7, dnf in 8, OpenSSL 3 in 9, image mode in 10…

1. From Red Hat Linux to RHEL (1994-2003)

In lesson 0.5 you saw the birth of Red Hat Linux in 1994. For nearly ten years, it was a single distribution, both free of charge (downloadable) and sold in a box with a manual and support. It went up to Red Hat Linux 9 (2003).

The problem: a new version came out about every six months. A bank cannot reinstall its servers twice a year. So in 2002 Red Hat launched Red Hat Linux Advanced Server 2.1, soon renamed Red Hat Enterprise Linux: a stable version, certified with major software (Oracle, SAP…) and with manufacturers' hardware (Dell, HP, IBM), and maintained for years.

In 2003, Red Hat permanently separated its two audiences:

Beware of a common misconception: the RHEL subscription does not pay for a "licence" in the Windows sense. The code is free software (GPL and others). The customer pays for support, updates, certifications and access to Red Hat's tools.

2. CentOS: RHEL for free (2004-2014)

Because RHEL is made of free software, Red Hat publishes its source code. Volunteers then had a simple idea: rebuild this code, remove Red Hat's logos and trademarks, and distribute the result free of charge.

This was CentOS (Community ENTerprise Operating System), whose first versions came out in 2004. The result was identical to RHEL "bug for bug", but without official support. Other projects did the same thing, such as Scientific Linux (Fermilab and CERN laboratories, discontinued with version 7).

CentOS quickly became the favourite free server:

• of web hosting companies (the very widespread cPanel hosting panel long required CentOS or RHEL);
• of startups, which wanted RHEL without paying;
• of administrators, who trained on CentOS to pass the RHCSA exam;
• of companies, for their test servers (with RHEL in production).

In January 2014, Red Hat hired the CentOS team and took the project under its wing. To many, CentOS then seemed secure for a long time.

3. CentOS Stream and the shock of December 2020

In September 2019, with CentOS 8, Red Hat also launched CentOS Stream: a version that moves forward continuously, just ahead of the next RHEL update.

BEFORE (2004-2020): CentOS follows RHEL

  Fedora ──► RHEL 8.3 ──► (a few weeks) ──► CentOS 8.3
                     source code published, then rebuilt

AFTER (2021 →): CentOS Stream comes before RHEL

  Fedora ──► CentOS Stream 9 ──► RHEL 9.x
              (what will go into the next minor RHEL update)

On 8 December 2020, Red Hat announced that CentOS Linux 8, promised until 2029, would end on 31 December 2021. From then on, the CentOS project would focus on CentOS Stream.

For users, it was a shock:

• thousands of companies had installed CentOS 8 believing they had eight years ahead of them;
• CentOS Stream does not suit everyone: it receives changes before RHEL, so it is no longer an exact clone of an already validated RHEL version.

CentOS Linux 7, for its part, kept its original end date: 30 June 2024. Many CentOS 7 servers were still running on that date, and some still are: you may come across them.

4. Rocky Linux and AlmaLinux (2021)

The community reacted within days. Two projects were born to take over the role of CentOS: a free, compatible rebuild of RHEL.

To move from CentOS 8 to one of them, each project provides an in-place migration script (migrate2rocky, almalinux-deploy), with no reinstallation. Other alternatives exist: Oracle Linux (a RHEL rebuild since 2006), CentOS Stream itself, or RHEL directly.

Red Hat also relaxed its offer: since 2021, the free Red Hat Developer Subscription for Individuals allows an individual to use RHEL on a small number of machines (up to 16).

June 2023: a new twist

In June 2023, Red Hat announced that CentOS Stream would become the only public source of RHEL-related code: the exact source code of each RHEL version would no longer be freely published, and would be reserved for customers.

The rebuilds had to adapt:

• AlmaLinux chose to be binary-compatible (ABI) with RHEL, rather than a "bug for bug" copy: software designed for RHEL works on it, but there may be small differences;
• Rocky Linux continues to aim for identity with RHEL, obtaining the code through other legal means;
• CIQ (the company behind Rocky), Oracle and SUSE created OpenELA in August 2023, an association that publishes RHEL-compatible source code.

For you, as an administrator, the consequence is simple: what you learn on Rocky or Alma applies to RHEL, and vice versa. The commands, the configuration files and the RHCSA exam are the same.

5. The family today

             new features, every 6 months
  FEDORA  ─────────────────────────────────►   (the laboratory)
     │  about every 3 years, Fedora serves as the base for
     ▼
  CENTOS STREAM N  ───────────────────────►   (the next RHEL update, ahead of time)
     │
     ▼
  RHEL N.0, N.1, N.2…                          (the product, 10 years of support)
     │  compatible code
     ▼
  ROCKY LINUX N.x   ALMALINUX N.x   ORACLE LINUX N.x   (free of charge, compatible)

In 2018, IBM announced the acquisition of Red Hat for about 34 billion dollars, completed in July 2019. Red Hat remains an autonomous division, which continues to develop RHEL and to contribute massively to Linux, systemd, GNOME, Podman…

6. The life cycle of a RHEL version

Each major version of RHEL has a lifetime of about ten years, in two main phases, plus a paid option:

Minor versions come out about every six months: 9.0, 9.1, 9.2… Rocky and Alma follow the same schedule, with a slight delay.

By version

This table is the reason for the Red Hat track of this course: every important lesson there has its "By version" section, because in companies you will still come across RHEL or CentOS 6 and 7 servers alongside RHEL 9 and 10.

# On RHEL 6 / CentOS 6                        # On RHEL 7 and later
service sshd restart                          systemctl restart sshd
chkconfig sshd on                             systemctl enable sshd
iptables -L                                   firewall-cmd --list-all
yum install httpd                             dnf install httpd   (RHEL 8+)

7. The lesson timeline

1994  Red Hat Linux
1999  Red Hat goes public
2002  Red Hat Linux Advanced Server 2.1 → soon "RHEL"
2003  Red Hat Linux 9, the last one; RHEL 3; Fedora Core 1
2004  First CentOS versions
2007  RHEL 5
2010  RHEL 6
2014  Red Hat hires the CentOS team (January); RHEL 7 (June)
2019  RHEL 8 (May); IBM completes the acquisition (July); CentOS Stream (September)
2020  8 December: early end of CentOS Linux 8 announced
2021  AlmaLinux 8.3 (March), Rocky Linux 8.4 (June); end of CentOS Linux 8 (31 December)
2022  RHEL 9 (May), followed by Rocky 9 and AlmaLinux 9
2023  RHEL source code reserved for customers (June); OpenELA (August)
2024  End of CentOS Linux 7 (30 June)
2025  RHEL 10 (May), followed by AlmaLinux 10 and Rocky 10

Common pitfalls

• Saying "CentOS" when talking about a recent Red Hat server. CentOS Linux no longer exists: a CentOS 7 still in service has received no free fixes since July 2024. It is a security risk that should be reported.
• Confusing CentOS Stream with the old CentOS. Stream is ahead of RHEL, not a copy of a validated RHEL version.
• Believing that Rocky and Alma are "less professional". They are used in production by large organisations. The difference with RHEL lies in support and vendor certifications, not in the commands.
• Applying a RHEL 6 procedure on RHEL 9 (or the reverse). Always check the version (cat /etc/redhat-release) before following a tutorial.

Lab

You are on Rocky Linux 9. Check its kinship with RHEL:

grep -E '^(ID|ID_LIKE|VERSION_ID)=' /etc/os-release
cat /etc/redhat-release       # present on the whole Red Hat family
rpm -qa | wc -l               # number of installed RPM packages
rpm -q rocky-release          # the package that "signs" the distribution

The tasks: note the family declared by ID_LIKE, the line of /etc/redhat-release, the number of packages and the exact version of rocky-release. Then click Check.

Quiz

1. What is the difference between Fedora and RHEL?
2. What was CentOS, and why was it so popular?
3. What did Red Hat announce on 8 December 2020?
4. Where does CentOS Stream stand in relation to RHEL?
5. In which version of RHEL did systemctl and firewalld appear? And dnf?

Reference sources: Red Hat Enterprise Linux life cycle and release notes (Red Hat documentation, CC BY-SA 3.0), official announcements from CentOS, Rocky Linux and AlmaLinux. Original MasterLinux text.