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 its free-of-charge clones, and AppArmor in Ubuntu and SUSE, that help prevent intruders from taking control of a system.
• Linux is 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 Red Hat-type virtual machines with VirtualBox, and installing the Extra Packages for Enterprise Linux (EPEL) repository in the Red Hat-type virtual machines
• 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 10/11 Bash shell to access Linux systems
• How to keep your Linux systems updated

Let’s begin by talking about threats.

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. Here are some examples:

• 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.

Now, let’s talk a bit more about security breaches.

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 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. One thing we can do is to keep up with security-related 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 distro you’re using, be sure to keep up with the news and current documentation for your Linux distro. Distro maintainers should have a way of letting you know if a security problem crops up in their products.

Here are some links to some good security-related websites:

• Packet Storm Security: https://packetstormsecurity.com/
• The Hacker News: https://thehackernews.com/

Here are some links to more generalized tech websites:

• Ars Technica: https://arstechnica.com/
• Fudzilla: https://www.fudzilla.com/
• The Register: https://www.theregister.co.uk/
• ZDNet: https://www.zdnet.com/

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

• LXer: http://lxer.com/
• BeginLinux Guru on YouTube: https://www.youtube.com/channel/UC88eard_2sz89an6unmlbeA

(Full disclosure: I am the world-famous BeginLinux Guru.)

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.

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 96 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 but 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 a 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 7, 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.

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 distro, 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.) SUSE is the third big enterprise Linux distro. We won’t be doing too much with SUSE, but on occasion, I’ll point out some of its little quirks.

Since Red Hat is a fee-based product, we’ll substitute CentOS 7, AlmaLinux8, and AlmaLinux9, which are built from Red Hat source code and are free of charge. (We’re using all three of these distros because there are some differences between them, and all of them will be supported for quite some time to come.) CentOS and AlmaLinux offer various download images. You’ll want to download the DVD images, because they contain necessary things that are missing from the minimal images. Specifically, download these image files:

• CentOS 7: CentOS-7-x86_64-DVD-2009.iso
• AlmaLinux 8: AlmaLinux-8-latest-x86_64-dvd.iso
• AlmaLinux 9: AlmaLinux-9-latest-x86_64-dvd.iso

For Ubuntu, we’ll concentrate on version 22.04, since it’s the newest Long Term Support (LTS) version. (We’ll also take an occasional look at Ubuntu 20.04, since it’s still supported and there are a few differences between it and 22.04.) 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’s also available for Solaris hosts, but I doubt that many of you will be running that.) 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, such as Cygwin, or just use the Bash shell that’s built into Windows 10/11 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 https://www.virtualbox.org/.
2. Download the installation .iso files for Ubuntu Server 22.04, CentOS 7, AlmaLinux8, and AlmaLinux9. You can get them from https://ubuntu.com/, https://almalinux.org/, and https://www.centos.org/.
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 here:

Figure 1.1:Create the virtual drive

4. Start the new virtual machine. Click on the folder icon that’s beside the Location dialog box and navigate to the directory where you stored the .iso files that you downloaded. Choose either the Ubuntu ISO file, the CentOS ISO file, or one of the AlmaLinux ISO files, as shown in the following screenshot. (If the ISO file doesn’t show up in the list, click the Add button in the top-left corner to add it.)

Figure 1.2: Choose the .iso file

5. 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 and AlmaLinux virtual machines, choose to install a Server without a graphical interface. (Later on, we’ll go through at least one exercise that will require a desktop interface for an AlmaLinux machine. You can create a virtual machine with a graphical interface at that time.)
6. When installing Ubuntu, choose Try or Install Ubuntu Server when you get to this screen:

Figure 1.3: Installing Ubuntu

7. Repeat the procedure for the other Linux distros.
8. Update the Ubuntu virtual machine with these two commands:

   sudo apt update
   sudo apt dist-upgrade
   

9. Hold off on updating the CentOS and AlmaLinux virtual machines because we’ll do that in the next exercise.
10. For Ubuntu, choose to install the OpenSSH Server on the SSH setup screen.

The user account creation screen of the AlmaLinux 9 installer—which looks the same as the one on CentOS 7 and AlmaLinux 8—is shown here:

Figure 1.4: User creation for AlmaLinux

For Ubuntu 22.04, you’ll see just one self-explanatory screen 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 22.04:

Figure 1.5: Ubuntu user creation

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 and AlmaLinux package repositories are—shall we say—lacking. To have the packages that you’ll need for the CentOS and AlmaLinux 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 7 and CentOS 7 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. To install them, just run this command:

   sudo yum install yum-plugin-priorities epel-release
   

2. 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:

    [base]
     name=CentOS-$releasever – Base
     mirrorlist=http://mirrorlist.centos.org/?
     release=$releasever&arch=$basearch&repo=os&infra=$infra
     #baseurl=http://mirror.centos.org/centos/
     $releasever/os/$basearch/
     gpgcheck=1
     gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
     priority=1
   

3. 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.
4. 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 AlmaLinux.

Installing the EPEL repository on the AlmaLinux 8/9 virtual machines

To install the EPEL repository on AlmaLinux, all you have to do is run this 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, update the system and create a list of available software packages with these two 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. Change the Attached to setting from NAT to Bridged Adapter, and change the Promiscuous Mode setting to Allow All, as shown in this screenshot:

Figure 1.6: Configuring the network

4. Restart the virtual machine and set it to use a static IP address.

   Tip

   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. From the Machine menu of the VirtualBox Manager screen, select Tools/Snapshots.
2. Further right 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 you see in this screenshot:

Figure 1.7: Taking a snapshot

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. Windows 10 and Windows 11, even in the base Home Edition, now come with a Secure Shell client that’s built into both the normal Command Prompt and PowerShell, and you can use that if you desire. But if you’d prefer to use something that comes closer to the actual Linux experience, 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 http://www.cygwin.com/.
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:

Figure 1.8: Installing Cygwin packages

5. 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 Skip), as you see in this screenshot:

Figure 1.9: Select the OpenSSH package

6. After you have selected the proper package, your screen should look like this:

Figure 1.10: After selecting the OpenSSH package

7. In the bottom right-hand corner, click Next. If a Resolving Dependencies screen pops up, click Next on it as well.
8. 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.)
9. 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 10/11 Bash shell.

Using the Windows 10 SSH client to interface with Linux virtual machines

If you’re using Windows 10, 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:

Figure 1.11: Windows 10 Command Prompt

2. Then, just type in the SSH commands the same as you would from a Mac or Linux machine, like this:

Figure 1.12: SSH remote from Windows Command Prompt

3. A better option is to use Windows PowerShell instead of the normal Command Prompt. Get to it as you see here:

Figure 1.13: PowerShell command prompt

4. As before, let’s use it to log in to my Orange Pi device, as you see here:

Figure 1.14: Remote login from PowerShell

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.

Using the Windows 11 SSH client to interface with Linux virtual machines

You’ll work with Windows 11 the same way, except that the menu entries for the Command Prompt and PowerShell are in different places. The Command Prompt now has its own Terminal item on the main menu, and PowerShell is now under the Windows Tools submenu. Windows 11 also has a third option, which is a built-in Ubuntu virtual machine. You’ll see an icon for that in the bottom taskbar.

Cygwin versus the Windows shell

Both Cygwin and the SSH client that’s built into Windows 10/11 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, Cygwin stores the SSH known_hosts and keys files in the .ssh directory of the user’s home directory, which is where you’d expect to find them if you’re used to working with Linux. If you use the SSH client that’s built into Windows, you’ll have to search for these files in other locations.

In favor of the Windows 10/11 built-in SSH client, there’s the fact that 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.

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 you can see here:

Figure 1.15: Common Vulnerabilities and Exposures

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

Next, let’s look at updating the Debian-based systems, which include Ubuntu.

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
   

2. 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 this command:

   sudo apt auto-remove
   

Next, we will configure auto updates for Ubuntu.

Configuring auto updates for Ubuntu

When you first install Ubuntu 22.04, automatic updates are turned on by default. To verify that, you’ll first check the status of the unattended-upgrades service, like so:

donnie@ubuntu2204-packt:~$ systemctl status unattended-upgrades
● unattended-upgrades.service - Unattended Upgrades Shutdown
     Loaded: loaded (/lib/systemd/system/unattended-upgrades.service; enabled; vendor preset: enabled)
     Active: active (running) since Sat 2022-10-08 19:25:54 UTC; 52min ago
. . .
. . .
donnie@ubuntu2204-packt:~$

Then, look in the /etc/apt/apt.conf.d/20auto-upgrades file. If auto-updating is enabled, you’ll see this:

APT::Periodic::Update-Package-Lists "1";
APT::Periodic::Unattended-Upgrade "1";

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 into the system. But if you prefer, you can set Ubuntu to automatically reboot after it automatically updates itself. Here’s how to do it:

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 67, you’ll see a line that says:

   //Unattended-Upgrade::Automatic-Reboot "false";
   

2. Uncomment the line by removing the leading slashes, and change false to true, like so:

   Unattended-Upgrade::Automatic-Reboot "true";
   

3. 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 103, where you’ll see this:

   //Unattended-Upgrade::Automatic-Reboot-Time "02:00";
   

4. 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";
   

5. 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. Here’s what you’ll see:

   APT::Periodic::Update-Package-Lists "1";
   APT::Periodic::Unattended-Upgrade "1";
   

6. 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";
   

7. 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
   

8. To manually install the security-related updates without installing non-security updates, just run:

   sudo unattended-upgrade -d
   

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.

Next, let’s look at updating the RHEL 7 systems.

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
   

2. Sometimes, you might just want to see if there are any security-related updates that are ready to be installed. Do that by running this command:

   sudo yum updateinfo list updates security
   

3. 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
   

4. If the only thing you want to install is the security updates, run this command:

   sudo yum upgrade --security
   

5. 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 these two commands:

   sudo yum install yum-cron
   sudo systemctl enable --now yum-cron
   

6. 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:

   [commands]
   # 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.

7. Let’s say that you only want security updates to get applied automatically. Just change the last line to the following:

   update_cmd = security
   

8. On lines 15 and 20, you’ll see this:

   download_updates = yes
   apply_updates = no
   

9. This indicates that by default, yum-cron is only set to automatically download updates, but not to install them.
10. 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.

11. 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:

     [email]
    # 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 on your own account, just change it here.

12. 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
    

13. When you open mutt and look at a message, you’ll see something like this:

Figure 1.16: Mutt mail client

14. 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 this command:

    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:

Next, we will be updating Red Hat 8/9-based systems.

Updating Red Hat 8/9-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. So, when you work with your AlmaLinux 8/9 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 do:

   sudo dnf upgrade
   

2. 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
   

3. 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 this command:

   sudo systemctl enable --now dnf-automatic.timer
   

4. Verify that it’s running with this command:

   sudo systemctl status dnf-automatic.timer
   

5. If it started successfully, you should see something like this when you check the status:

   [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 ~]$
   

man dnf-automatic

And that’s all there is to it.

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 software packages and 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 topic is better suited for a Linux administration book.)

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

We’ve made a good start with 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 or on a cloud service.

After that, we looked at how to set up a virtualized lab environment using VirtualBox, Cygwin, and the Windows 10/11 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.
   1. True
   2. False
2. Which of the following is true about Linux on IoT devices?
   1. There are too many of them.
   2. They’re taking over the world.
   3. Too many of them are configured insecurely.
   4. They’re so securely configured that they’ll put security practitioners out of their jobs.
3. Which of the following is true about automatic operating system updates in an enterprise?
   1. You should always leave them enabled.
   2. They violate the basic precept of testing updates on a test network before installing them on a production network.
   3. Unlike with manual updates, you never have to reboot a system after automatic updates.
   4. For IoT devices, it isn’t useful to enable automatic updates.

Here are some handy resources for your viewing pleasure:

• Linux Security: https://linuxsecurity.com/
• The official VirtualBox website: https://www.virtualbox.org/
• The official CentOS page: https://www.centos.org/
• RHEL documentation (this also works for CentOS and AlmaLinux): https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/9
• Enabling automatic updates in RHEL 7 and CentOS 7: https://linuxaria.com/howto/enabling-automatic-updates-in-centos-7-and-rhel-7
• Managing and Monitoring Security Updates for RHEL 8: https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/managing_and_monitoring_security_updates/index

1. b
2. c
3. b

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