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Mastering Linux Security and Hardening - Second Edition
Mastering Linux Security and Hardening - Second Edition

Mastering Linux Security and Hardening: Protect your Linux systems from intruders, malware attacks, and other cyber threats, Second Edition

By Donald A. Tevault
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Book Feb 2020 666 pages 2nd Edition
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Product Details

Publication date : Feb 21, 2020
Length 666 pages
Edition : 2nd Edition
Language : English
ISBN-13 : 9781838981778
Vendor :
Linux Foundation
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Mastering Linux Security and Hardening - Second Edition

Running Linux in a Virtual Environment

So, you may be asking yourself: Why do I need to study Linux security? Isn't Linux already secure? After all, it's not Windows. But the fact is, there are many reasons.

It's true that Linux has certain advantages over Windows when it comes to security. These include the following:

  • Unlike Windows, Linux was designed from the ground up as a multiuser operating system. So, user security tends to be a bit better on a Linux system.
  • Linux offers a better separation between administrative users and unprivileged users. This makes it a bit harder for intruders, and it also makes it a bit harder for a user to accidentally infect a Linux machine with something nasty.
  • Linux is much more resistant to viruses and malware infections than Windows is. Certain Linux distributions come with built-in mechanisms, such as SELinux in Red Hat and CentOS, and AppArmor in Ubuntu, that prevent intruders from taking control of a system.
  • Linux is a free and open source software. This allows anyone who has the skill to audit Linux code to hunt for bugs or backdoors.

But even with those advantages, Linux is just like everything else that's been created by mankind. That is, it isn't perfect.

Here are the topics that we'll cover in this chapter:

  • Looking at the threat landscape
  • Why every Linux administrator needs to learn about Linux security
  • A bit about the threat landscape, with some examples of how attackers have, at times, been able to breach Linux systems
  • Resources for keeping up with IT security news
  • Differences between physical, virtual, and cloud setups
  • Setting up Ubuntu Server and CentOS virtual machines with VirtualBox, and installing the Extra Packages for Enterprise Linux (EPEL) repository in the CentOS virtual machine
  • Creating virtual machine snapshots
  • Installing Cygwin on a Windows host so that Windows users can connect to a virtual machine from their Windows hosts
  • Using the Windows 10 Bash shell to access Linux systems
  • How to keep your Linux systems updated

Looking at the threat landscape

If you've kept up with IT technology news over the past few years, you'll likely have seen at least a few articles about how attackers have compromised Linux servers. For example, while it's true that Linux isn't really susceptible to virus infections, there have been several cases where attackers have planted other types of malware on Linux servers. These cases have included the following:

  • Botnet malware: This causes a server to join a botnet that is controlled by a remote attacker. One of the more famous cases involved joining Linux servers to a botnet that launched denial-of-service (DoS) attacks against other networks.
  • Ransomware: This is designed to encrypt user data until the server owner pays a ransom fee. But even after paying the fee, there's no guarantee that the data can be recovered.
  • Cryptocoin mining software: This causes the CPUs of the server on which it's planted to work extra hard and consume more energy. Cryptocoins that get mined go to the accounts of the attackers who planted the software.

And, of course, there have been plenty of breaches that don't involve malware, such as where attackers have found a way to steal user credentials, credit card data, or other sensitive information.

Some security breaches come about because of plain carelessness. Here's an example of where a careless Adobe administrator placed the company's private security key on a public security blog:

Why do security breaches happen?

Regardless of whether you're running Linux, Windows, or whatever else, the reasons for security breaches are usually the same. They could be security bugs in the operating system or security bugs in an application that's running on that operating system. Often, a bug-related security breach could have been prevented had the administrators applied security updates in a timely manner.

Another big issue is poorly configured servers. A standard, out-of-the-box configuration of a Linux server is actually quite insecure and can cause a whole ton of problems. One cause of poorly configured servers is simply the lack of properly trained personnel to securely administer Linux servers. (Of course, that's great news for the readers of this book, because—trust me—there's no lack of well-paying IT security jobs.)

And now, in addition to Linux on servers and desktops, we now have Linux on devices that are part of the Internet of Things (IoT). There have been many security problems with these devices, in large part because people just don't know how to configure them securely.

As we journey through this book, we'll see how to do business the right way, to make our servers as secure as possible.

Keeping up with security news

If you're in the IT business, even if you're not a security administrator, you'll want to keep up with the latest security news. In the age of the internet, that's easy to do.

First, there are quite a few websites that specialize in network security news. Examples include Packet Storm Security and The Hacker News. Regular tech news sites and Linux news websites, such as Ars Technica, Fudzilla, The Register, ZDNet, and LXer, also carry reports about network security breaches. And, if you'd rather watch videos than read, you'll find plenty of good YouTube channels, such as BeginLinux Guru.

Finally, regardless of which Linux distribution you're using, be sure to keep up with the news and current documentation for your Linux distribution. Distribution maintainers should have a way of letting you know if a security problem crops up in their products.

Links to security news sites are as follows:

Links to general tech news sites are as follows:

You can check out some general Linux learning resources as well as Linux news site:

One thing to always remember as you go through this book is that the only operating system you'll ever see that's totally, 100% secure will be installed on a computer that never gets turned on.

Differences between physical, virtual, and cloud setups

So you can do the hands-on labs, I'll introduce you to the concept of virtual machines. This is just a way of running one operating system within another operating system. So, it doesn't matter whether you're running Windows, macOS, or Linux on your host machine. In any case, you can run a Linux virtual machine that you can use for practice, and that you won't have to worry about if it gets trashed.

Oracle's VirtualBox, which is what we'll be using, is great for what we'll be doing. In an enterprise setting, you'll find other forms of virtualization software that are better suited for use in data centers. In the past, server hardware could only handle doing one thing at a time, which meant that you had to have one server running DNS, another running DHCP, and so on. Nowadays, we have servers with gobs of memory, gobs of drive space, and CPUs with as many as 64 cores each. So, it's now cheaper and more convenient to install multiple virtual machines on each server, with each virtual machine doing its own specific job. This also means that you not only have to worry about security on the physical server that hosts these virtual machines, you also need to worry about the security of each virtual machine. An added problem is that you need to ensure that the virtual machines remain properly isolated from each other, especially ones that contain sensitive data.

And then, there's the cloud. Many different outfits provide cloud services, where a person or a company can spin up an instance of either Windows or their choice of Linux distro. When setting up a Linux distro on a cloud service, there are things that you'll have to do right away to enhance security. (That's something that we'll cover in Chapter 6, SSH Hardening.) And realize that when you set up a server on a cloud service, you'll always have more concerns about proper security, because it will have an interface that connects to the wild and woolly internet. (Your on-premises servers, except for ones that are meant to serve the public, are usually isolated from the internet.)

With our introductory material out of the way, let's get to the real meat of the matter, starting with an introduction to our virtualization software.

Introducing VirtualBox and Cygwin

Whenever I write or teach, I try very hard not to provide students with a cure for insomnia. Throughout this book, you'll see a bit of theory whenever it's necessary, but I mainly like to provide good, practical information. There will also be plenty of step-by-step hands-on labs and an occasional bit of humor.

The best way to do the labs is to use Linux virtual machines. Most of what we'll do can apply to any Linux distribution, but we will also do some things that are specific to either Red Hat Enterprise Linux (RHEL) or Ubuntu Linux. (RHEL is the most popular for enterprise use, while Ubuntu is the most popular for cloud deployments.)

Red Hat is a billion-dollar company, so there's no doubt about where they stand in the Linux market. But since Ubuntu Server is free of charge, we can't judge its popularity strictly on the basis of its parent company's worth. The reality is that Ubuntu Server is the most widely used Linux distribution for deploying cloud-based applications.

See here for details:

Since Red Hat is a fee-based product, we'll substitute CentOS 7 and CentOS 8, which are built from Red Hat source code and are free of charge. (We're using both CentOS 7 and CentOS 8 because there are some differences between them, and both will be supported for quite some time to come.)

For Ubuntu, we'll concentrate on version 18.04, since it's the newest Long Term Support (LTS) version. A new LTS version of Ubuntu comes out in April of every even-numbered year, and non-LTS versions come out in April of every odd-numbered year, and every October. For production use, you'll mainly want to stick with the LTS versions, because the non-LTS versions can sometimes be a bit problematic.

There are several different virtualization platforms that you can use, but my own preferred choice is VirtualBox.

VirtualBox is available for Windows, Linux, and Mac hosts, and is free of charge for all of them. It has features that you have to pay for on other platforms, such as the ability to create snapshots of virtual machines.

Some of the labs that we'll be doing will require you to simulate creating a connection from your host machine to a remote Linux server. If your host machine is either a Linux or a Mac machine, you'll just be able to open the Terminal and use the built-in Secure Shell (SSH) tools. If your host machine is running Windows, you'll need to install some sort of Bash shell, which you can do by either installing Cygwin or by using the Bash shell that's built into Windows 10 Pro.

Installing a virtual machine in VirtualBox

For those of you who've never used VirtualBox, here's a quick guide to get you going:

  1. Download and install VirtualBox and the VirtualBox Extension Pack. You can get them from
  2. Download the installation .iso files for Ubuntu Server 18.04, CentOS 7, and CentOS 8. You can get them from and (Note that for Ubuntu 18.04, you'll need to use this alternate installer. The default installer that you get from the main Download page lacks some of the features that you'll need to complete the exercises.)
  3. Start VirtualBox and click the New icon at the top of the screen. Fill out the information where requested. Increase the virtual drive size to 20 GB, but leave everything else as the default settings, as shown in the following screenshot:
  1. Start the new virtual machine. Click on the folder icon at the bottom-left corner of the dialog box and navigate to the directory where you stored the .iso files that you downloaded. Choose either the Ubuntu ISO file or the CentOS ISO file, as shown in the following screenshot:
  1. Click the Start button on the dialog box to start installing the operating system. Note that, for Ubuntu Server, you won't be installing a desktop interface. For the CentOS 7 virtual machine, choose either the KDE desktop or the GNOME desktop, as you desire. For CentOS 8, your only desktop choice is GNOME. (We'll go through at least one exercise that will require a desktop interface for the CentOS machine.)
  2. When installing Ubuntu, choose Install Ubuntu Server when you get to the following screen:
  1. Repeat the procedure for the other Linux distributions.
  2. Update the Ubuntu virtual machine by entering the following commands:
sudo apt update
sudo apt dist-upgrade
  1. Hold off on updating the CentOS virtual machine because we'll do that in the next exercise.
  2. For Ubuntu, choose No automatic updates on the Configuring tasks screen, and choose to install the OpenSSH Server on the Software selection screen.
When installing Ubuntu, you'll be asked to create a normal user account and password for yourself. It won't ask you to create a root user password, but will instead automatically add you to the sudo group so that you'll have admin privileges.

When you get to the user account creation screen of the CentOS installer, be sure to check the Make this user administrator box for your own user account, since it isn't checked by default. It will offer you the chance to create a password for the root user, but that's entirely optional—in fact, I never do.

The user account creation screen of the RHEL 8 installer—which looks the same as the one on CentOS 7 and CentOS 8—is shown here:

For Ubuntu 18.04, you'll go through several self-explanatory screens to set up your real name, a username, and a password. The Ubuntu installer will automatically add your user account to the sudo group, which will give you full administrator privileges.

Here's the user account creation screen for Ubuntu 18.04:

So, now, let's change gears and move on to CentOS 7.

Installing the EPEL repository on the CentOS 7 virtual machine

While the Ubuntu package repositories have pretty much everything that you need for this course, the CentOS package repositories are—shall we say—lacking. To have the packages that you'll need for the CentOS hands-on labs, you'll need to install the EPEL repository. (The EPEL project is run by the Fedora team.) When you install third-party repositories on Red Hat and CentOS systems, you'll also need to install a priorities package and edit the .repo files to set the proper priorities for each repository. This will prevent packages from the third-party repository from overwriting official Red Hat and CentOS packages if they just happen to have the same name. The following steps will help you install the required packages and edit the .repo files:

  1. The two packages that you'll need to install EPEL are in the normal CentOS 7 repositories. Run the following command:
sudo yum install yum-plugin-priorities epel-release
  1. When the installation completes, navigate to the /etc/yum.repos.d directory, and open the CentOS-Base.repo file in your favorite text editor. After the last line of the base, updates, and extras sections, add the line priority=1. After the last line of the centosplus section, add the line priority=2. Save the file and close the editor. Each of the sections that you've edited should look something like this (except with the appropriate name and priority number):
name=CentOS-$releasever - Base
  1. Open the epel.repo file for editing. After the last line of the epel section, add the line priority=10. After the last line of each remaining section, add the line priority=11.
  2. Update the system and then create a list of the installed and available packages by running the following commands:
sudo yum upgrade
sudo yum list > yum_list.txt

Now, let's move on to CentOS 8.

Installing the EPEL repository on the CentOS 8 virtual machine

To install the EPEL repository on CentOS 8, all you have to do is run the following command:

sudo dnf install epel-release

There's no priorities package as there is on CentOS 7 and earlier, so we won't have to worry about configuring the repository priorities.

When the package installation is complete, create a list of available software packages with the following commands:

sudo dnf upgrade
sudo dnf list > dnf_list.txt

Next, let's configure our network.

Configuring a network for VirtualBox virtual machines

Some of our training scenarios will require you to simulate creating a connection to a remote server. You would do this by using your host machine to connect to a virtual machine. When you first create a virtual machine on VirtualBox, the networking is set to NAT mode. In order to connect to the virtual machine from the host, you'll need to set the virtual machine's network adapter to Bridged Adapter mode. Here's how you can do this:

  1. Shut down any virtual machines that you've already created.
  2. On the VirtualBox Manager screen, open the Settings dialog for a virtual machine.
  3. Click the Network menu item, and change the Attached to setting from NAT to Bridged Adapter, as shown in the following screenshot:
  1. Expand the Advanced item, and change the Promiscuous Mode setting to Allow All, as shown in the following screenshot:
  1. Restart the virtual machine and set it to use a static IP address.
If you assign static IP addresses from the high end of your subnet range, it will be easier to prevent conflicts with low-number IP addresses that get handed out from your internet gateway.

Creating a virtual machine snapshot with VirtualBox

One of the beautiful things about working with virtual machines is that you can create a snapshot and roll back to it if you mess something up. With VirtualBox, that's easy to do, by following these steps:

  1. At the top right-hand corner of the VirtualBox Manager screen, click the Snapshots button.
  1. Further left on the screen, click on the Take icon to bring up the snapshot dialog box. Either fill in the desired Snapshot Name or accept the default name. Optionally, you can create a description, as shown in the following screenshot:
  1. After you've made changes to the virtual machine, you can roll back to the snapshot by shutting down the virtual machine, then highlighting the Snapshot Name, and clicking on the Restore button.

Using Cygwin to connect to your virtual machines

If your host machine is either a Linux or Mac machine, you'll simply open the host's Terminal and use the tools that are already there to connect to the virtual machine. But if you're running a Windows machine, you'll need some sort of Bash shell and its networking tools. Windows 10 Pro now comes with a Bash shell that's been provided by the Ubuntu folk, and you can use that if you desire. But if you don't have Windows 10 Pro, or if you prefer to use something else, you might consider Cygwin.

Cygwin, a project of the Red Hat company, is a free open source Bash shell that's built for Windows. It's free of charge and easy to install.

Installing Cygwin on your Windows host

Here's a quick how-to to get you going with Cygwin:

  1. In your host machine's browser, download the appropriate setup*.exe file for your version of Windows from
  2. Double-click on the setup icon to begin the installation. For the most part, just accept the defaults until you get to the Package selection screen. (The one exception will be the screen where you select a download mirror.)
  3. At the top of the Package selection screen, select Category from the View menu.
  4. Expand the Net category, as shown in the following screenshot:
  1. Scroll down until you see the openssh package. Under the New column, click on Skip (this causes a version number to appear in place of the Skip), as shown in the following screenshot:
  1. After you have selected the proper package, your screen should look like this:
  1. In the bottom right-hand corner, click Next. If a Resolving Dependencies screen pops up, click Next on it as well.
  1. Keep the setup file that you downloaded, because you'll use it later to either install more software packages or to update Cygwin. (When you open Cygwin, any updated packages will show up on the Pending view on the View menu.)
  2. Once you open Cygwin from the Windows Start menu, you can resize it as you desire, and use either the Ctrl + + or Ctrl + - key combinations to resize the font.

Next, we'll look at the Windows 10 Bash shell.

Using Windows 10 Pro Bash shell to interface with Linux virtual machines

If you're using either Windows 10 Pro or Windows 10 Enterprise, you already have an SSH client built into your operating system.

So, let's see how to do this:

  1. To get to it, you can open the traditional Command Prompt from the Windows System menu, like so:
  1. Then, just type in the SSH commands the same as you would from a Mac or Linux machine, like this:
  1. A better option is to use Windows PowerShell instead of the normal Command Prompt. Get to it as you see here:
  1. As before, let's use it to log in to my Orange Pi device, as shown here:

If you have the choice, go with PowerShell instead of Command Prompt. PowerShell is a bit closer to the Linux Bash shell experience, and you'll be much happier with it.

Cygwin versus Windows Bash shell

Both Cygwin and the SSH client that's built into Windows 10 have their pros and cons. In favor of Cygwin, you can install a variety of packages to customize it pretty much any way you want. Also, if you're stuck using Windows 10 Home Edition or—heaven forbid—Windows 7, you can use Cygwin.

In favor of the Windows 10 built-in SSH client, there's the fact that on the Pro and Enterprise editions of Windows 10, it's already there. Also, it's much easier to use if you need to access your normal Windows folders because Cygwin traps you in its own sandboxed directory structure.

Keeping the Linux systems updated

Spend some time perusing the Common Vulnerabilities and Exposures database, and you'll soon see why it's so important to keep your systems updated. Yes, indeed, you'll even find that there have been security flaws with our beloved Linux, as shown in the following screenshot:

Updating a Linux system only requires one or two simple commands, and it's generally faster and less painful than updating a Windows system.

You can find the Common Vulnerabilities and Exposures database here:

All of you conscientious, dedicated Linux administrators will definitely want to become familiar with this site.

Updating Debian-based systems

Let's take a look at how to update Debian-based systems:

  1. On Debian and its many children, including Ubuntu, run two commands, as shown here:
sudo apt update
sudo apt dist-upgrade
  1. Occasionally, you'll also need to remove some old packages that are no longer needed. How will you know? Easy. When you log in to the system, a message will appear on the command line. To remove these old packages, just run the following command:
sudo apt auto-remove

Next, we will configure auto updates for Ubuntu.

Configuring auto updates for Ubuntu

A new feature of Ubuntu 18.04 LTS that wasn't in Ubuntu 16.04 LTS is that you can configure it to automatically install security updates. You can see that here on the installer screen:

I must confess, though, that I have mixed feelings about this. I mean, it's nice that the security updates get installed without me having to do anything, but a lot of those updates require that the system be rebooted before they can take effect. By default, Ubuntu systems don't automatically reboot after an update is installed. If you keep it that way, you'll see a message about it when you log in to the system. But if you prefer, you can set Ubuntu to automatically reboot after it automatically updates itself. Here's how to do this:

  1. Go into the /etc/apt/apt.conf.d directory and open the 50unattended-upgrades file in your favorite text editor. In the vicinity of line 68, you'll see a line that says the following:
//Unattended-Upgrade::Automatic-Reboot "false";
  1. Uncomment the line by removing the leading slashes, and change false to true, like so:
Unattended-Upgrade::Automatic-Reboot "true";
  1. With this new configuration, Ubuntu will now reboot itself immediately after the automatic update process has completed. If you'd rather have the machine reboot at a specific time, scroll down to about line 73, where you'll see this line of code:
//Unattended-Upgrade::Automatic-Reboot-Time "02:00";
  1. Since this line is commented out with its pair of leading slashes, it currently has no effect. To have the machine reboot at 2:00 A.M., just uncomment this line. To have it reboot at, say, 10:00 P.M., uncomment the line and change the time to 22:00, like so:
Unattended-Upgrade::Automatic-Reboot-Time "22:00";

Of course, there's that old, basic precept that thou shalt not install system updates on a production system without first testing them on a test system. Any operating system vendor can occasionally supply you with problematic updates, and that has included Ubuntu. (I know what you're saying: Preach it, Donnie.) Ubuntu's automatic update feature is in direct opposition to that basic precept. If automatic updates have been enabled, disabling them is quite easy, if you choose to do so:

  1. To disable automatic updates, just go into the /etc/apt/apt.conf.d directory and open the 20auto-upgrades file in your favorite text editor. What you'll see is this:
APT::Periodic::Update-Package-Lists "1";
APT::Periodic::Unattended-Upgrade "1";
  1. Change the parameter for that second line to 0, so that the file will now look like this:
APT::Periodic::Update-Package-Lists "1";
APT::Periodic::Unattended-Upgrade "0";

Now, the system will still check for updates and show a message at the login screen when any are available, but it won't automatically install them. And of course, it should go without saying that you need to check your systems on a regular basis to see if updates are available. If you do prefer to leave automatic updates enabled, be sure to either enable automatic rebooting or to log in to the system at least a couple of times a week to see if it needs to be rebooted.

  1. If you want to see if there are any security-related updates available, but don't want to see any non-security updates, use the unattended-upgrade command, like so:
sudo unattended-upgrade --dry-run -d
  1. To manually install the security-related updates without installing non-security updates, just run the following line of code:
sudo unattended-upgrade -d
If you're running some form of desktop Ubuntu on a workstation that gets shut down after every use, you can enable the automatic updates if you like, but there's no need to enable automatic reboots.

Also, if you're running a non-Ubuntu flavor of Debian, which would include Raspbian for the Raspberry Pi, you can give it the same functionality as Ubuntu by installing the unattended-upgrades package. Just run the following line of code:

sudo apt install unattended-upgrades

You can also use the apt command to install only the security updates, but it would require piping the apt output into a convoluted set of text filters in order to mask the non-security updates. Using the unattended-upgrade command is much easier.

I said before that we should always test updates on a test system before we install them on a production system, and that certainly does hold true for corporate servers. But what do we do when we have a whole bunch of IoT devices that we need to keep updated, especially if these devices are all over the place out in the field and in consumer devices?

In the wonderful world of IoT, the ARM CPU versions of Ubuntu, Raspbian, and Debian are the most popular Linux distros for use on the various Pi devices, including the ubiquitous Raspberry Pi. If you have lots of IoT devices in the field and in consumer devices, you might not have direct control over them once they've been deployed or sold. They still need to be kept updated, so setting up unattended updates with automatic rebooting would certainly be advantageous. But keep in mind that in the world of IoT, we have to be concerned about safety as well as security. So, for example, if you have devices that are set up as some sort of critical, safety-related industrial controller, then you most likely don't want the device to automatically reboot after doing automatic updates. But if you're a television vendor who installs Linux on smart televisions, then definitely set them up to automatically update and to automatically reboot themselves after an update.

Updating Red Hat 7-based systems

With Red Hat-based systems, which include CentOS and Oracle Linux, there's no automatic update mechanism that you can set up during installation. So, with the default configuration, you'll need to perform updates yourself:

  1. To update a Red Hat 7-based system, just run this one command:
sudo yum upgrade
  1. Sometimes, you might just want to see if there are any security-related updates that are ready to be installed. Do that by running the following command:
sudo yum updateinfo list updates security
  1. If any security updates are available, you'll see them at the end of the command output. On the system that I just tested, there was only one security update available, which looks like this:
FEDORA-EPEL-2019-d661b588d2 Low/Sec. nagios-common-4.4.3-1.el7.x86_64

updateinfo list done
  1. If the only thing you want to install is just the security updates, run the following command:
sudo yum upgrade --security
  1. Now, let's say that you need a CentOS system to automatically update itself. You're in luck because there's a package for that. Install and enable it, and start it by running the following commands:
sudo yum install yum-cron

sudo systemctl enable --now yum-cron
  1. To configure it, go into the /etc/yum directory, and edit the yum-cron.conf file. At the top of the file, you'll see this:
# What kind of update to use:
# default = yum upgrade
# security = yum --security upgrade
# security-severity:Critical = yum --sec-severity=Critical upgrade
# minimal = yum --bugfix update-minimal
# minimal-security = yum --security update-minimal
# minimal-security-severity:Critical = --sec-severity=Critical update-minimal
update_cmd = default

This lists the various types of upgrades we can do. The last line shows that we're set to update everything.

  1. Let's say that you only want security updates to get applied automatically. Just change the last line to the following:
update_cmd = security
  1. On lines 15 and 20, you'll see this line:
download_updates = yes
apply_updates = no

This indicates that by default, yum-cron is only set to automatically download updates, but not to install them.

  1. If you want the updates to get automatically installed, change the apply_updates parameter to yes.
Note that unlike Ubuntu, there's no setting to make the system automatically reboot itself after an update.
  1. Finally, let's look at the mail settings for yum-cron, which you'll find on lines 48 through 57 of the yum-cron.conf file, as shown here:
# The address to send email messages from.
# NOTE: 'localhost' will be replaced with the value of system_name.
email_from = root@localhost

# List of addresses to send messages to.
email_to = root

# Name of the host to connect to to send email messages.
email_host = localhost

As you can see, the email_to = line is set to send messages to the root user account. If you want to receive messages at your own account, just change it here.

  1. To see the messages, you'll need to install a mail reader program, if one isn't already installed. (It hasn't been installed if you chose Minimal installation when you installed the operating system.) Your best bet is to install mutt, like so:
sudo yum install mutt
  1. When you open mutt and look at a message, you'll see something like this:
  1. As with all operating systems, certain updates will require that the system be restarted. And how do you know when the system needs to be restarted? With the needs-restarting command, of course. First, though, you need to make sure that needs-restarting is installed on your system. Do that with the following line of code:
sudo yum install yum-utils

Once the package is installed, there are three ways to use needs-restarting. If you just run the command without any option switches, you'll see the services that need to be restarted and the packages that require you to reboot the machine. You can also use the -s or -r options, as shown here:



sudo needs-restarting

This shows the services that need to be restarted, and the reasons why the system might need to be rebooted.

sudo needs-restarting -s

This only shows the services that need to be restarted.

sudo needs-restarting -r

This only shows the reasons why the system needs to be rebooted.

Next, we will be updating Red Hat 8-based systems.

Updating Red Hat 8-based systems

The old yum utility has been around for practically forever, and it's been a good, hard-working utility. But it does have its occasional quirks, and at times it can be excruciatingly slow. But not to worry. Our heroes at Red Hat have finally done something about that, by replacing yum with dnf. Now, dnf has been tested on the Fedora distro for the past few years, and it's now a part of the RHEL 8 family. So, when you work with your CentOS 8 virtual machines, you'll use dnf instead of yum. Let's see how to do this:

  1. For the most part, you use dnf the same way that you'd use yum, with the same arguments and options. For example, to do a system upgrade, just run the following command:
sudo dnf upgrade
  1. The main functional difference between yum and dnf is that dnf has a different automatic update mechanism. Instead of installing the yum-cron package, you'll now install the dnf-automatic package, like so:
sudo dnf install dnf-automatic
  1. In the /etc/dnf directory, you'll see the automatic.conf file, which you'll configure the same way as you did the yum-cron.conf file for CentOS 7. Instead of working as a cron job, as the old yum-cron did, dnf-automatic works with a systemd timer. When you first install dnf-automatic, the timer is disabled. Enable it and start it by running the following line of code:
sudo systemctl enable --now dnf-automatic.timer
  1. Verify that it's running by typing the following line of code:
sudo systemctl status dnf-automatic.timer
  1. If it started successfully, you should see something like this:
[donnie@redhat-8 ~]$ sudo systemctl status dnf-automatic.timer
dnf-automatic.timer - dnf-automatic timer
Loaded: loaded (/usr/lib/systemd/system/dnf-automatic.timer; enabled; vendor preset: disabled)
Active: active (waiting) since Sun 2019-07-07 19:17:14 EDT; 13s ago
Trigger: Sun 2019-07-07 19:54:49 EDT; 37min left

Jul 07 19:17:14 redhat-8 systemd[1]: Started dnf-automatic timer.
[donnie@redhat-8 ~]$
For more details about dnf-automatic, type the following command:

man dnf-automatic

And that's all there is to it.

Automatic updating sounds like a good thing, right? Well, it is in some circumstances. On my own personal Linux workstations, I always like to turn it off. That's because it drives me crazy whenever I want to install a package, and the machine tells me that I have to wait until the update process finishes. In an enterprise, it might also be desirable to disable automatic updates, so that administrators can have more control over the update process.

There are special considerations about doing updates in an enterprise environment. Let's look at them next.

Managing updates in an enterprise

When you first install any Linux distro, it will be configured to access its own package repositories. This allows the user to install any software package or install updates directly from these normal distro repositories. This is great for home or small business use, but not so great for the enterprise.

In an enterprise setting, there are two additional considerations:

  • You want to restrict what packages the end users are allowed to install.
  • You always want to test updates on a separate test network before allowing them to be installed on a production network.

For these reasons, enterprises will often set up their own repository servers that only have approved packages and approved updates. All other machines on the network will be configured to pull their packages and updates from them, rather than from the normal distro repository. (We won't go into how to set up on-premises repository servers here, because that's a topic better suited for a Linux administration book.)

Ubuntu has always been one of the more innovative Linux distros, but it's also had more than its fair share of quality-control problems. In its early days, there was at least one Ubuntu update that completely broke the operating system, requiring the user to re-install the operating system. So, yeah, in any mission-critical environment, test those updates before putting them into production.

I think that that's about it for our introductory chapter. Let's wrap things up in a summary, shall we?


So, we've made a good start to our journey into Linux security and hardening. In this chapter, we looked at why it's just as important to know about securing and hardening Linux systems as it is to know how to secure and harden Windows systems. We provided a few examples of how a poorly configured Linux system can be compromised, and we mentioned that learning about Linux security could be good for your career. We then looked at some special considerations for setting up Linux servers either as virtual machines, on a cloud service. After that, we looked at how to set up a virtualized lab environment using VirtualBox, Cygwin, and the Windows 10 Bash shell. We wrapped things up with a quick look at how to keep your Linux systems updated.

In the next chapter, we'll look at locking down user accounts, and ensuring that the wrong people never get administrative privileges. I'll see you there.


  1. Because Linux is more securely designed than Windows, we never have to worry about Linux security.

A. True
B. False

  1. Which of the following is true about Linux on IoT devices?

A. There are too many of them.
B. They're taking over the world.
C. Too many of them are configured insecurely.
D. They're so securely configured, they'll put security practitioners out of their jobs.

  1. Which of the following is true about automatic operating system updates in an enterprise?

A. You should always leave them enabled.
B. They violate the basic precept of testing updates on a test network before
installing them on a production network.
C. Unlike with manual updates, you never have to reboot a system after automatic
D. For IoT devices, it isn't useful to enable automatic updates.

Further reading

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Key benefits

  • Deliver a system that reduces the risk of being hacked
  • Explore a variety of advanced Linux security techniques with the help of hands-on labs
  • Master the art of securing a Linux environment with this end-to-end practical guide


From creating networks and servers to automating the entire working environment, Linux has been extremely popular with system administrators for the last couple of decades. However, security has always been a major concern. With limited resources available in the Linux security domain, this book will be an invaluable guide in helping you get your Linux systems properly secured. Complete with in-depth explanations of essential concepts, practical examples, and self-assessment questions, this book begins by helping you set up a practice lab environment and takes you through the core functionalities of securing Linux. You'll practice various Linux hardening techniques and advance to setting up a locked-down Linux server. As you progress, you will also learn how to create user accounts with appropriate privilege levels, protect sensitive data by setting permissions and encryption, and configure a firewall. The book will help you set up mandatory access control, system auditing, security profiles, and kernel hardening, and finally cover best practices and troubleshooting techniques to secure your Linux environment efficiently. By the end of this Linux security book, you will be able to confidently set up a Linux server that will be much harder for malicious actors to compromise.

What you will learn

Create locked-down user accounts with strong passwords Configure firewalls with iptables, UFW, nftables, and firewalld Protect your data with different encryption technologies Harden the secure shell service to prevent security break-ins Use mandatory access control to protect against system exploits Harden kernel parameters and set up a kernel-level auditing system Apply OpenSCAP security profiles and set up intrusion detection Configure securely the GRUB 2 bootloader and BIOS/UEFI

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Product Details

Publication date : Feb 21, 2020
Length 666 pages
Edition : 2nd Edition
Language : English
ISBN-13 : 9781838981778
Vendor :
Linux Foundation

Table of Contents

20 Chapters
Preface Chevron down icon Chevron up icon
1. Section 1: Setting up a Secure Linux System Chevron down icon Chevron up icon
2. Running Linux in a Virtual Environment Chevron down icon Chevron up icon
3. Securing User Accounts Chevron down icon Chevron up icon
4. Securing Your Server with a Firewall - Part 1 Chevron down icon Chevron up icon
5. Securing Your Server with a Firewall - Part 2 Chevron down icon Chevron up icon
6. Encryption Technologies Chevron down icon Chevron up icon
7. SSH Hardening Chevron down icon Chevron up icon
8. Section 2: Mastering File and Directory Access Control (DAC) Chevron down icon Chevron up icon
9. Mastering Discretionary Access Control Chevron down icon Chevron up icon
10. Access Control Lists and Shared Directory Management Chevron down icon Chevron up icon
11. Section 3: Advanced System Hardening Techniques Chevron down icon Chevron up icon
12. Implementing Mandatory Access Control with SELinux and AppArmor Chevron down icon Chevron up icon
13. Kernel Hardening and Process Isolation Chevron down icon Chevron up icon
14. Scanning, Auditing, and Hardening Chevron down icon Chevron up icon
15. Logging and Log Security Chevron down icon Chevron up icon
16. Vulnerability Scanning and Intrusion Detection Chevron down icon Chevron up icon
17. Security Tips and Tricks for the Busy Bee Chevron down icon Chevron up icon
18. Assessments Chevron down icon Chevron up icon
19. Other Books You May Enjoy Chevron down icon Chevron up icon

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