Tcpdump command examples.

Introduction This post is about tcpdump – packet analyzer that comes preinstalled with any modern Linux distribution. tcpdump allows us to capture packets and either display them in real time or save them to a file for later analysis. We will look at several examples of using tcpdump to learn which options it has and

Introduction to iptables

Introduction In this tutorial we will learn how to configure iptables which is a built-in Linux firewall. I will use Ubuntu 18.04 for demo, but you can use any other Linux distro. Before we get into actual configuration we will learn some theory behind iptables. iptables allows us to filter packets based on IP addresses,

Custom background image on Cisco IP Phone

Introduction This is a short article on how to create a custom background image on Cisco IP phone managed by Cisco Unified Communications Manager(CUCM). We will consider IP Phone Cisco 8851 and CUCM 11.5, but the provided instructions are applicable to any IP Phone managed by CUCM. Background Image Format The first thing we need

Shell Scripting Guide

Introduction In this guide we are going to learn how to use BASH shell scripting. Shell scripting is usually used for tasks such as file manipulation, program execution and task automation. Shell scripts contain the same commands you normally run in a terminal, it just allows you to group them together and run them in

How to create a custom recording for IVR greeting in FreePBX?

Introduction In this short tutorial we are going to create a custom recording for IVR greeting. These recordings are called Announcements/System Recordings (these terms are used interchangeably) in FreePBX. Whenever you create an IVR application you select which audio file should be played back to the callers in Announcement field. How to create a custom

How to create a systemd service?

Introduction In this guide we will create a systemd service and make it automatically start on a boot. We have already seen how we can make commands execute on a boot using cron scheduler in this post. Creating systemd service file is an alternative way of doing this, and since systemd is becoming more and

Regular Expressions

In this guide we are going to learn how to use Regular Expressions (regex) to match any pattern or text. This is often used in text editors, CLI commands(like grep or awk) and programming languages. What is regex? Regex are patterns used to match/find strings in a text. You might think that you can use

Using grep command in Linux

What is grep ? Grep stands for Global Regular Expression Print. Grep utility is usually used to find lines in a file/files containing particular strings and substrings. Grep comes preinstalled on any modern Linux distribution, so we can start using it without having to install anything. Before we start looking at usage examples of grep

Using awk command in Linux

What is awk? Awk is a programming language which has loops, arrays, functions etc., but nowadays mostly used only for “one-liners”. Awk is not used for creating complex programs, but it  is great when we need to add some additional functionality to our standard Linux commands. We will have a look at some of its