Book Image

Linux Shell Scripting Cookbook, Second Edition - Second Edition

Book Image

Linux Shell Scripting Cookbook, Second Edition - Second Edition

Overview of this book

The shell remains one of the most powerful tools on a computer system — yet a large number of users are unaware of how much one can accomplish with it. Using a combination of simple commands, we will see how to solve complex problems in day to day computer usage.Linux Shell Scripting Cookbook, Second Edition will take you through useful real-world recipes designed to make your daily life easy when working with the shell. The book shows the reader how to effectively use the shell to accomplish complex tasks with ease.The book discusses basics of using the shell, general commands and proceeds to show the reader how to use them to perform complex tasks with ease.Starting with the basics of the shell, we will learn simple commands with their usages allowing us to perform operations on files of different kind. The book then proceeds to explain text processing, web interaction and concludes with backups, monitoring and other sysadmin tasks.Linux Shell Scripting Cookbook, Second Edition serves as an excellent guide to solving day to day problems using the shell and few powerful commands together to create solutions.
Table of Contents (16 chapters)
Linux Shell Scripting Cookbook
Credits
About the Authors
About the Reviewers
www.PacktPub.com
Preface
Index

Basic firewall using iptables


A firewall is a network service which is used to filter network traffic for unwanted traffic, block it, and allow the desired traffic to pass. The most powerful tool on Linux is iptables, which has kernel integration in recent versions of the kernels.

How to do it...

iptables is present, by default, on all modern Linux distributions. We will see how to configure iptables for common scenarios.

  1. Block traffic to a specific IP address:

    #iptables -A OUTPUT -d 8.8.8.8 -j DROP
    

    If you run PING 8.8.8.8 in another terminal before running the iptables command, you will see this:

    PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
    64 bytes from 8.8.8.8: icmp_req=1 ttl=56 time=221 ms
    64 bytes from 8.8.8.8: icmp_req=2 ttl=56 time=221 ms
    ping: sendmsg: Operation not permitted
    ping: sendmsg: Operation not permitted
    

    Here, the ping fails the third time because we used the iptables command to drop all traffic to 8.8.8.8.

  2. Block traffic to a specific port:

    #iptables -A OUTPUT -p tcp -dport...