administration Archives

How to list installed packages on RHEL 8 / CentOS 8 Linux

20 September 2019 by Răzvan T. Coloja

There might come a time when you want to know if you have already installed a certain package on your RHEL 8 / CentOS 8. An application to be manually installed might require certain dependencies to work so you’ll have to check beforehand if these are satisfied. Or you might want to compile something and want to make sure you have all the right header libraries on your system.

In this tutorial you will learn:

How to use yum and dnf in RHEL 8 / CentOS 8 to list all packages
How to use the repoquery command to accomplish the same task
How to use the rpm command to list all packages installed on RHEL 8 / CentOS 8
How to filter the output of each command to seek certain packages

Use the repoquery command to list all available packages in RHEL 8 / CentOS 8

How to Install Java on RHEL 8 / CentOS 8 Linux

13 September 2019 by Nick Congleton

Java is incredibly popular on servers, and if you plan on using RHEL 8 / CentOS 8, you’ll need to install it. There are a couple of ways to install Java on RHEL, both from the open source OpenJDK packages and directly from Oracle.

In this tutorial you will learn:

How to Install OpenJDK 8
How to Install OpenJDK 11
How to Install Oracle Java 8 JRE
How to Install Oracle Java 8 JDK
How to Switch Java Versions

How to Install an RPM package on RHEL 8 / CentOS 8 Linux

13 September 2019 by Nick Congleton

There are a few different ways that you can install an RPM package on RHEL 8 / CentOS 8 as oppose to package installation from a systems repository. They each have their own merits, but DNF should probably be your first choice in most situations. It’s also good to remember that, for stability’s sake, it’s a good idea to limit your external RPM installs as much as possible.

In this tutorial you will learn:

How to Install an RPM with DNF
How to Install an RPM with Yum
How to Install an RPM with RPM

How to create a simple encrypted Two-Way SSH tunnel

13 September 2019 by Lubos Rendek

Why would you need to create a simple Two-Way SSH tunnel? In your Linux system administration job have you ever found your self in a situation in which you cannot SSH to a any of your servers/hosts that may be behind a firewall, NAT or otherwise obstructed from an easy access.

In order to gain the access, you would need to reconfigure the firewall or create VPN which could be an enormous overhead just because you need to execute few commands from now and then. With Two-Way SSH tunnel you can connect to any destination under a single condition, which is, the ability to ssh login from the destination to the source.

If you can do that, you can as well reverse login from source to destination even if it is behind firewall or NAT.

In this tutorial you will learn:

How to create Two-Way SSH tunnel

How to setup NRPE for client side monitoring

12 September 2019 by Thomas Sandmann

Nrpe, or Nagios Remote Plugin Executor, is the client side service of a monitoring setup. The monitoring server will send commands to the client, which listens passively when got no work to do. Upon incoming command, the nrpe checks it’s local configuration, and executes the plugin configured with the command, then sends back the results to the server for processing. You can read more about the server side installation in the Nagios installation guide, while this guide will focus on the client side.

In this tutorial you will learn:

How to install NRPE on Debian/Red Hat based distributions
How to configure NRPE to accept commands from the server
How to configure a custom check on the server and client side

How to Debug Bash Scripts

6 September 2019 by Mike Ward

There are techniques from traditional programming environments that can help.
Some basic tools like using an editor with syntax highlighting will help as well.
There are builtin options that Bash provides to make debugging and your everyday Linux System Administration job easier.

In this article you will learn some useful methods of debugging Bash scripts:

How to use traditonal techniques
How to use the xtrace option
How to use other Bash options
How to use trap

How To enable the EPEL Repository on RHEL 8 / CentOS 8 Linux

21 August 2019 by Egidio Docile

Although it’s been a while since the release of Red Hat Enterprise Linux 8, the corresponding version of the EPEL repository (Extra Packages for Enterprise Linux) was only released few days ago. The repository contains packages that are not provided by the official software sources, as for example extundelete, an utility to recover deleted files from ext3/4 filesystems. Until now the solution to install those software was to build it from source or to use the previous version of EPEL (less than ideal). In this tutorial we will see how to add EPEL8 to RHEL 8 / CentOS 8.

In this tutorial you will learn:

How to add the EPEL8 repository to RHEL 8 / CentOS 8
How to check all the packages contained in the EPEL8 repository

How to Install Puppet on RHEL 8 / CentOS 8

17 August 2019 by Nick Congleton

IT admins rely on Puppet to manage complex deployments every day. If your network is built on Red Hat systems, you’re going to need to install Puppet on RHEL 8 / CentOS 8. Puppet Labs provides a repository and packages, so the whole thing should be relatively hassle free.

In this tutorial you will learn:

How to Enable the Puppet Repo
How to Install Puppet for Servers/Puppet Masters
How to Update Servers/Puppet Masters
How to Install Puppet for Clients
How to Update Puppet Clients

Changing the IP address in RHEL 8 from within GNOME is a matter of a few clicks

How to change IP address on RHEL 8 / CentOS 8 Linux

10 August 2019 by Răzvan T. Coloja

Most IPs these days are automatically assigned through DHCP by either your ISP or your home router. But there may come a time when you wish to opt for a static IP address and you will want to change it. It may be that you are configuring a home LAN and you don’t want to use DHCP or simply want a static IP address that you can access from outside your home. In this tutorial we will learn how we can change the IP address in RHEL 8 / CentOS 8 Linus system.

In this tutorial you will learn:

How to change the IP address in RHEL 8 / CentOS 8 using GNOME
How to change the IP address by editing configuration files
How to manually change your DNS server
How to restart networking in RHEL 8 / CentOS 8

User is not in the sudoers file error message on RHEL 8 / CentOS 8 Linux

RHEL 8 / CentOS 8 add user to sudoers

10 August 2019 by Lubos Rendek

The sudo command allows regular users to execute commands with administrative/root privileges. By adding any user to predefined sudo group wheel will grant root privileges to execute any command as root user. Any attempt to use the sudo command for the non-sudo user will result in:

user is not in the sudoers file.  This incident will be reported.

In this tutorial you will learn:

How to create sudo user on RHEL 8 / CentOS 8 system.
How to add existing user to sudoers.

Opening and Closing firewall ports on RHEL 8 / CentOS 8 using firewall-cmd command.

How to open and close ports on RHEL 8 / CentOS 8 Linux

10 August 2019 by Lubos Rendek

Firewalld is a powerful and yet simple to use tool to manage a firewall on RHEL 8 / CentOS 8 Server or GNOME workstation. Firewalld allows to manage open or close ports using predefined services as well as open and close user custom ports.

In this tutorial you will learn:

How to open and close ports on RHEL 8 / CentOS 8 with Firewalld.
How to open and close services on RHEL 8 / CentOS 8.
How to reload firewall configuration.
How to list open ports or services.

RHEL 8 - HTTP & HTTPS protocol services. Open ports 80 & 443

RHEL 8 / CentOS 8 open HTTP port 80 and HTTPS port 443 with firewalld

10 August 2019 by Lubos Rendek

This article explains how to open HTTP port 80 and HTTPS port 443 on RHEL 8 / CentOS 8 system with the firewalld firewall. HTTP and HTTPS protocols are primarily used by web services such as, but not limited to, Apache or Nginx web serves.

For more information about the firewalld firewall visit our introduction guide to firewalld syntax and usage guide.

In this tutorial you will learn:

How to open HTTP port 80 and HTTPS port 443.
How to open HTTP port 80 and HTTPS port 443 permanently.
How to list currently open ports/services.
How to close/remove HTTP port 80 and HTTPS port 443.