RHEL/CentOS 5–6–7–8 Changes
You encounter enormous changes when you change to the next major version of Red Hat's Enterprise Linux product suite. Here are my notes on what changes when you upgrade from one major release of Red Hat Enterprise Linux (or its clone CentOS). In other words, how to upgrade from RHEL (or CentOS) 5 to 6 to 7 to 8. The major releases of RHEL appear further and further apart in time, meaning that the changes are more and more complicated.
Remember that it's not as if Red Hat is changing all of these things. Their installer and their graphical configuration tools, sure, those are Red Hat changes. But much of this is the result of the many underlying projects changing. So this collection of pages also applies to Scientific Linux and Oracle Linux, where the major release numbers 5, 6, 7, 8 align with RHEL and CentOS, and really to any distribution as its components have updated over the past few years.
|RHEL 2.1||26 March 2002|
|RHEL 3||22 October 2003|
|RHEL 4||15 February 2005|
|RHEL 5||14 March 2007|
|RHEL 6||10 November 2010|
|RHEL 7||10 June 2014|
|RHEL 8β||14 Nov 2018|
RHEL 8 beta came out in November 2018, and I have started experimenting with it. For now I will mark my tentative RHEL 8β notes greyed out like this paragraph.
1 — Installation and Exploring the Newly Installed System
The order of events and the logic of the installation completely change with every major release.
It was impossible to do a 5–6 upgrade, but 6–7 should be possible. A 7–8 update might be possible. However, a fresh re-install will have many advantages over upgrading in place.
Linux has always supported much larger systems than I have had to worry about. For example, RHEL 7 had 48/46 bits of virtual/physical memory address, and the kernel did 4 levels of page tables. That limited virtual memory to 256 TiB and physical memory to 64 TiB, although 64 terabytes of RAM was way beyond any hardware I dealt with.
Well, RHEL 8β has moved the goalposts much further out. Its kernel supports 57/52 bits of virtual/physical memory address, and does 5 levels of page tables. So, it supports up to 128 PiB of virtual address space and 4 PiB of physical memory.
RHEL 8β has better support for deployment in containers and in cloud settings. The new Composer lets you created customized images in several formats, including those prepared for deployment onto public cloud providers.
The RHEL 8β web console, Cockpit, lets you create and manage virtual machines. The QEMU full system virtualization control introduces a sandboxing feature with RHEL 8β, limiting the system calls QEMU can make.
The change from RHEL 6 to 7 was the most sweeping.
Not because of what Red Hat did, but because that spans
the change from
and the many
and the Ethernet device name shift that
came with IProute2
The commands and configuration files used to configure RHEL 7 were often completely different from RHEL 6. That didn't happen in going from RHEL 7 to RHEL 8.
RHEL 8 seems (so far, to me)
to be about updating existing components.
Many updates are quite significant:
Kernel 3.10.0 to 4.18.0,
GCC 4.8.5 to 8.2.1,
Python 2.7.5 to 3.6.6 (and 2.7.15),
PHP 5.4.16 to 7.2.11 (and 7.1.20),
Ruby 2.0.0 to 2.5,
OpenSSL 1.0.2k to 1.1.1,
iptables v1.4.21 to v1.8.0 (nf_tables),
qemu-kvm 1.5.3 to 2.12.0,
and others. Others are relatively minor:
OpenSSH 7.4p1 to 7.8p1,
Samba 4.8.3 to 4.9.1,
Bash to 4.2.46 to 4.4.19,
RHEL 8β splits the packaging into two sets: BaseOS (1,688 packages, 930 MB) and AppStream (4,942 packages, 5.1 GB). Versus 9,007 packages and 6.5 GB on the CentOS 7 Everything-1511 ISO.TLS 1.3 with Nginx and OpenSSL
As for web servers, RHEL 7 gave you Apache 2.4.6. RHEL 8 has Apache 2.4.35 and Nginx 1.14.0. Nginx 1.14.0 supports TLS 1.3. Apache 2.4.35 does not. It needs 2.4.37, which became generally available on October 23, just about 3 weeks before RHEL 8β released. I would hope that the final release of RHEL 8 will include Apache 2.4.37 (or better).
X improves from one version to the next. With RHEL 5 you have just whatever was auto-detected during the installation, probably 800×600. RHEL 6 should be close to full screen size, RHEL 7 and later should be full screen size.
RHEL 8β instead uses the
Wayland display server
Package xorg-x11-server-Xwayland provides
while package xorg-x11-server-Xorg provides the classic
Once you get it installed, the configuration of Gnome
within RHEL hides the terminal emulator in ever more
inconvenient corners of the menus.
Gnome 3 supports using the
Windows Start Key*
up a "run this command" text box that allows searching.
It can be confusing, you can bring up a command
by a search for something that isn't really the
This left me confused as to why I could start the
firewall administration tool by typing
at text box, which is really searching on some metadata,
but I couldn't run a command starting
The default user PATH environment variable changes:
sbin components, so remember to always add
--login" to your
su command so your following
system administration commands work!
What an odd beginning! There are 12 programs there. Stranger yet,
/usr/lib64/qt4/bin exists with
35 programs, 7 of which have the same names as those under
The newer version is not in the user's path.
I don't know what's going on here...
It's strange that
are included, as they are links to
And I don't know who would use
to house their personal programs.
The same set as RHEL 7, minus the redundant
/sbin, and reordered.
The Bash shell environment on RHEL 8β behaved strangely. It was case insensitive at times, and these two commands worked differently:
$ ls [NO]* ... all files starting with "N" or "O" ... $ ls [N-O]* ... all files starting with "n", "N", "o", or "O" ...
The environment variables
are correctly (for me) set to
on RHEL/CentOS 7.
However, on RHEL 8β only
LANG was set.
Fix: add a line to
$ cat ~/.bash_profile ... lines deleted ... # User specific environment and startup programs export LC_ALL=C
The manual page indices always need to be rebuilt so
man -k will work after the installation.
There will be a cron job to do that, or you can do it
This used the
makewhatis utility for ages,
but with RHEL 7
makes this happen every night.
With RHEL 8β it appears that this is enabled in
but there is no corresponding cron job.
All the same, I got the indices.
I assume they're built at the end of the installation,
and rebuilt when packages are added.
When you upgrade from RHEL 5 to 6, you will notice that the network configuration has become rather complicated. See the IP networking section later for what's behind this. Among other things, be careful to de-select Require addressing for this connection to complete for IPv4 and also IPv6 if you use it. You get more choices about storage starting with RHEL 6, some of it a bit exotic for typical users. If you customize the package selection, watch out! It is very tempting to click the Next button prematurely. That immediately starts an installation process, and the only way to abort it is to reset or power-cycle the machine and start over.
When you upgrade from 6 to 7, it becomes more complicated yet. The graphical configuration tool for networking now includes tabs for 802.1x security and higher performance data center type LAN technology.
The distribution had grown to the point that
RHEL 6 no longer fit onto one single-layer DVD.
DVD1 is the main installation media, it's 4.0 GB for the
RHEL 6 workstation and the single CentOS image.
DVD2 is the "optional" media, it's just 1.2 GB.
DVD2 for the most part contains the
-devel packages, but it also contains some of
the intranet/LAN server packages.
RHEL 5 gave you a choice about using SELinux and the
ip6tables firewall rules.
RHEL 6 and later force these on you,
preconfigured and enabled.
firstboot program that runs on the first
post-installation graphical boot starting with RHEL 6
insists that you create a user.
However, the user creation tool is poorly designed and it
does not let you fully control all the attributes of this user.
Workaround: Create a dummy user to make it
happy, create your real users, and then delete the dummy user.
If you're using VMware, Red Hat has included drivers that give you the benefits of VMware's VMtools, such as the ability to simply move the mouse out of the VM window without first pressing <Ctrl><Alt>. Do not install VMtools, as that can make things worse instead of better. My experience was that VMtools took away my control of the display settings and recreated the mouse and keyboard focus problem. Just install RHEL or CentOS and enjoy.
|RHEL 5||Gnome 2.16||KDE 3.5|
|RHEL 6||Gnome 2.28||KDE 4.3|
|RHEL 7||Gnome 3.8||KDE 4.10|
|RHEL 8β||Gnome 3.28.1||—|
The desktop graphical user interface goes through major changes, especially going from RHEL 6 to 7. Both the default Gnome and the optional KDE graphical interface go through major version changes. RHEL 8β dropped the KDE window manager. Linux Mint also dropped KDE after its version 18.3.
Gnome 3.8 is surprisingly resource-hungry.
You will probably want to put
autospawn = no
file and also in
pulseaudio will always run and
always restart, and sometimes it will impose
a surprising load on the CPU.
By RHEL 7 many more packages had moved to having one main
configuration file plus a collection, like what
xinetd has done for some time.
sudo puts its system-wide
/etc/sudoers and then reads all
the user-specific files
/etc/rsyslog.conf and then
all the files in
This isn't anything specific to Red Hat, it is a Linux-wide
trend that became common between the releases of RHEL 6 and 7.
This is also similar to the shells that for some time
now first use
/etc/profile and then
This is good, take advantage of it.
The intent is that you don't touch the distribution-provided
When the package updates, rpm discovers that the main
configuration file is still in pristine form and you have
and so on to track down and manually merge changes.
Let the distribution's provided configuration file do
whatever they intended, and their your locally created files
can "correct" any system-wide settings you want to change.
With RHEL 7 there is a significant difference between
Many distributions, including earlier RHEL, really
vim, the improved version with more
capabilities, when you typed just
(this was through some trickery with
vim is your friend, it really helps by
coloring the syntax in configuration files, programming
languages, HTML, and much more.
I had the benefit of being in the habit of always typing
vim so I got the better one on non-Linux
OSes like OpenBSD and Solaris.
Adopt my habit!