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Mastering CentOS 7 Linux Server
Mastering CentOS 7 Linux Server

Mastering CentOS 7 Linux Server:

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Mastering CentOS 7 Linux Server

Chapter 2. Security

In this chapter, we will find different tools and utilities that we can use to secure the CentOS system we are using. Security is the most important part for a system or server and thus, a System Administrator's job is always to keep the system up to date and secure from different kind of attacks that happen on a regular basis on servers.

We will discuss several tools here starting from SELinux to other security tools and measures that we can take on a CentOS 7 system. Let's dive into them one by one.

In this chapter, we will look into:

  • SELinux and its tools
  • Installing SELinux
  • Domain transition
  • SELinux users
  • SELinux audit logs and troubleshooting

Introducing SELinux

Security Enhanced Linux (SELinux) is a set of kernel modifications and user-space tools that have been present in CentOS for quite a long time. It is a kind of mechanism that supports Mandatory Access Control security policies, which were initially developed by the US National Security Agency and later released in the public domain to protect computer systems from malicious intrusion and tampering.

Not many System Administrators use SELinux. Commonly, people are reluctant to learn about SELinux and just disable it directly. However, a properly configured SELinux system can reduce the security risks to a great extent.

SELinux implements Mandatory Access Control (MAC), which works on top of already available Discretionary Access Control (DAC) on CentOS 7. DAC is the traditional security model that we have on Linux systems where we have three entities: User, Group, and Others who can have a combination of read, write, and execute permission for files and directories. By...

Domain transition

Now, let's find out how a process accesses other processes.

Let's consider that the vsftpd process is running; if it's not started, we can start it using the following command:

systemctl start vsftpd

The vsftpd process is started by the systemd process; this is a replacement of the Sys V init process and runs within a context of init_t:

ps -eZ | grep init
Domain transition

The systemd process running under the init_t domain is very short lived; it invokes /usr/sbin/vsftpd, which has a type context ftpd_exec_t, and when this binary executable starts, it becomes the vsftpd service itself and runs in the ftpd_t domain.

Domain transition

So, here's the systemd process running under the init_t domain executing a binary file with the ftpd_exec_t type. The binary file then starts a service within the ftpd_t domain.

Domain transition is followed by three strict rules:

  • The parent process of the source domain must have the permission to execute the application between both the domains
  • The file context...

SELinux users

As explained before, SELinux users are different from normal Linux users. SELinux users are defined in the policy that's loaded into the memory at boot time, and there are only a few of these users.

After SELinux is enforced, each regular user account is mapped to a SELinux user account. There can be multiple user accounts mapped to the same SELinux user. This enables the normal user account to inherit the permission of its SELinux counterpart.

To view the mapping, we will run the following command:

semanage login -l
SELinux users

Here, we will find that there are only three Login Names as shown in the preceding screenshot representing the Linux user accounts. Any Linux user is mapped to the entry shown as __default__ here. The user root is not mapped to default, instead it has its own entry and there is system_u for the running processes or services. The second column indicates the SELinux user they are mapped to. Normal user accounts and roots are mapped to unconfined_u, whereas the...

Restricting access to su or sudo

We can restrict a user from running the su or sudo commands by changing the user's SELinux user mapping like this:

semanage login -a -s user_u test

The preceding command will change the Linux test user's mapping to user_u and will not allow the su or sudo commands access.

Note

This will only take effect when the user is not logged in.

Restricting permissions to run scripts

To restrict the Linux test user's ability to run scripts we have to do two things. First, we change the user's mapping to guest_u, the same way as we did previously:

semanage login -a -s guest_u test

By default, SELinux allows users mapped to guest_t to execute scripts from their home directories. We can confirm the same using the following command:

getsebool allow_guest_exec_content

It will show that guest_exec_content is on. So, the second step is that we disable the guest_exec_content using this:

setsebool allow_guest_exec_content off

Now, the test user for whom we changed...

SELinux audit logs

In CentOS 7, we should look into two files for SELinux-related errors and alerts; they are as follows:

  • /var/log/audit/audit.log
  • /var/log/messages

Introducing SELinux


Security Enhanced Linux (SELinux) is a set of kernel modifications and user-space tools that have been present in CentOS for quite a long time. It is a kind of mechanism that supports Mandatory Access Control security policies, which were initially developed by the US National Security Agency and later released in the public domain to protect computer systems from malicious intrusion and tampering.

Not many System Administrators use SELinux. Commonly, people are reluctant to learn about SELinux and just disable it directly. However, a properly configured SELinux system can reduce the security risks to a great extent.

SELinux implements Mandatory Access Control (MAC), which works on top of already available Discretionary Access Control (DAC) on CentOS 7. DAC is the traditional security model that we have on Linux systems where we have three entities: User, Group, and Others who can have a combination of read, write, and execute permission for files and directories. By default...

Domain transition


Now, let's find out how a process accesses other processes.

Let's consider that the vsftpd process is running; if it's not started, we can start it using the following command:

systemctl start vsftpd

The vsftpd process is started by the systemd process; this is a replacement of the Sys V init process and runs within a context of init_t:

ps -eZ | grep init

The systemd process running under the init_t domain is very short lived; it invokes /usr/sbin/vsftpd, which has a type context ftpd_exec_t, and when this binary executable starts, it becomes the vsftpd service itself and runs in the ftpd_t domain.

So, here's the systemd process running under the init_t domain executing a binary file with the ftpd_exec_t type. The binary file then starts a service within the ftpd_t domain.

Domain transition is followed by three strict rules:

  • The parent process of the source domain must have the permission to execute the application between both the domains

  • The file context for that application...

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Key benefits

  • Learn how to efficiently set up and manage a Linux server using one of the best suited technologies for this purpose, CentOS 7
  • Personalize your Linux server and familiarize yourself with the latest tools and utilities setup provided by the new CentOS distribution
  • Follow a step-by-step tutorial through the configuration of the requested services with the capacity to personalize them as per your needs

Description

Most server infrastructures are equipped with at least one Linux server that provides many essential services, both for a user's demands and for the infrastructure itself. Setting up a sustainable Linux server is one of the most demanding tasks for a system administrator to perform. However, learning multiple, new technologies to meet all of their needs is time-consuming. CentOS 7 is the brand new version of the CentOS Linux system under the RPM (Red Hat) family. It is one of the most widely-used operating systems, being the choice of many organizations across the world. With the help of this book, you will explore the best practices and administration tools of CentOS 7 Linux server along with implementing some of the most common Linux services. We start by explaining the initial steps you need to carry out after installing CentOS 7 by briefly explaining the concepts related to users, groups, and right management, along with some basic system security measures. Next, you will be introduced to the most commonly used services and shown in detail how to implement and deploy them so they can be used by internal or external users. Soon enough, you will be shown how to monitor the server. We will then move on to master the virtualization and cloud computing techniques. Finally, the book wraps up by explaining configuration management and some security tweaks. All these topics and more are covered in this comprehensive guide, which briefly demonstrates the latest changes to all of the services and tools with the recent shift from CentOS 6 to CentOS 7.

Who is this book for?

If you are a Linux system administrator with an intermediate administration level, this is your opportunity to master the brand new distribution of CentOS. If you wish to possess a fully sustainable Linux server, with all its new tools and tweaks, that serves a variety of services to your users and customers, this book is ideal for you. It is your ticket to easily adapt to all the changes made in the latest shift.

What you will learn

  • Manage CentOS 7 users, groups, and root access privileges
  • Enhance the server's security through its firewall and prevent the most common attacks from penetrating or disabling the server
  • Explore and implement the common, useful services that a CentOS 7 server can provide
  • Monitor your server infrastructure for system or hardware issues
  • Create and configure a virtual machine using virtualization technologies
  • Implement a cloud computing solution on a single node system
  • Get an introduction to the configuration management tools and their usage
  • Discover the importance of the tools that provide remote connection, server service security, and system and process monitoring tools
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Publication date : Jan 29, 2016
Length: 298 pages
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Language : English
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Table of Contents

10 Chapters
1. Advanced User Management Chevron down icon Chevron up icon
2. Security Chevron down icon Chevron up icon
3. Linux for Different Purposes Chevron down icon Chevron up icon
4. Mail Server with Postfix Chevron down icon Chevron up icon
5. Monitoring and Logging Chevron down icon Chevron up icon
6. Virtualization Chevron down icon Chevron up icon
7. Cloud Computing Chevron down icon Chevron up icon
8. Configuration Management Chevron down icon Chevron up icon
9. Some Additional Tricks and Tools Chevron down icon Chevron up icon
Index Chevron down icon Chevron up icon

Customer reviews

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Rating distribution
Full star icon Full star icon Full star icon Half star icon Empty star icon 3.7
(16 Ratings)
5 star 43.8%
4 star 18.8%
3 star 12.5%
2 star 12.5%
1 star 12.5%
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RAJIV LODHA Nov 19, 2017
Full star icon Full star icon Full star icon Full star icon Full star icon 5
excellent
Amazon Verified review Amazon
Get_UmJ13 Feb 03, 2017
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Great Reference
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Bill Jones Apr 08, 2016
Full star icon Full star icon Full star icon Full star icon Full star icon 5
I would agree with another reviewer first that it does say "Mastering" yet does fall short on the title. However the book covers enough of the core OS to claim some competency with CentOS 7 so I didn't deduct stars for it, note that if you've been working in the field for a long period of time a lot of this book will be just skipped over, so keep that in mind. I wanted to use it as a refresher into the CentOS 7 HPC and HAC deployment book I found by Packt as well, perfect combination. Certainly recommend this book to anyone wanting to enter the realm of CentOS 7 and wanting to administer it effectively, as well as know nearly all of the ins and outs one would expect you to know if you're looking for a job in Systems Engineering as a Linux Administrator.
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Giuseppe Dec 01, 2017
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Manuale scritto chiaramente da professionisti del settore, usando termini tecnici e spiegazioni semplici ed ordinate, che lo rendono accessibile a chiunque, perfino ai meno ferrati su Linux. Il costo è giustificato dalla qualità del libro.
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ruben Apr 06, 2016
Full star icon Full star icon Full star icon Full star icon Full star icon 5
All these topics and more are covered in this comprehensive guide, which briefly demonstrates the latest changes to all of the services and tools with the recent shift from CentOS 6 to CentOS 7.I agree with this book, it covers all the topics with this important guide.
Amazon Verified review Amazon
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