Some useful links to explain the concepts of Security in Linux OS
- https://www.thefanclub.co.za/how-to/how-secure-ubuntu-1604-lts-server-part-1-basics
- https://github.com/imthenachoman/How-To-Secure-A-Linux-Server
- https://dev.to/danlebrero/kerberos-explained-in-pictures
- https://www.digitalocean.com/community/tutorials/how-to-use-rkhunter-to-guard-against-rootkits-on-an-ubuntu-vps
- http://www.woktron.com/secure/knowledgebase/79/Installation-Rootkit-Hunter-rkhunter-on-CentOS-5-and-6.html
- http://www.tecmint.com/install-rootkit-hunter-scan-for-rootkits-backdoors-in-linux/
- https://www.digitalocean.com/community/tutorials/how-to-protect-ssh-with-fail2ban-on-centos-6
- https://www.digitalocean.com/community/tutorials/how-fail2ban-works-to-protect-services-on-a-linux-server
- http://www.tecmint.com/install-fail2ban-on-rhel-centos-fedora/
- http://www.the-art-of-web.com/system/fail2ban-log/
- http://www.dghost.com/techno/internet/the-power-of-fail2ban
- http://www.fail2ban.org/wiki/index.php/MANUAL_0_8
- http://aarvik.dk/prevent-intrusion-with-fail2ban/
- https://jamalahmed.wordpress.com/2010/03/19/using-etchosts-allow-and-etchosts-deny-to-secure-unix/
- http://static.closedsrc.org/articles/dn-articles/hosts_allow.html
- http://www.tecmint.com/secure-linux-tcp-wrappers-hosts-allow-deny-restrict-access/
- http://www.aboutlinux.info/2005/10/using-tcp-wrappers-to-secure-linux.html
- http://www.cyberciti.biz/faq/tcp-wrappers-hosts-allow-deny-tutorial/
- http://bastille-linux.sourceforge.net/
- https://grsecurity.net/index.php
- https://www.digitalocean.com/community/tutorial_series/an-introduction-to-selinux-on-centos-7
- http://www.linuxpathfinder.com/how-to-disable-selinux-temporarily-or-permanently
- http://www.revsys.com/writings/quicktips/turn-off-selinux.html
- http://www.digitalattackmap.com/understanding-ddos/
- https://blog.cloudflare.com/the-ddos-that-almost-broke-the-internet/
- https://www.incapsula.com/ddos/ddos-attacks/
- http://bencane.com/2013/01/14/mitigating-dos-attacks-with-a-null-or-blackhole-route-on-linux/
- http://www.mkyong.com/linux/how-to-block-attackers-ip-with-null-route-command/
- http://www.opensourceforu.com/2011/04/securing-apache-part-8-dos-ddos-attacks/
- https://developers.google.com/speed/public-dns/docs/security#introduction-dns-security-threats-and-mitigations
- http://www.tldp.org/HOWTO/Firewall-Piercing/index.html
- http://www.ubuntufree.com/how-to-stop-a-ddos-attack-on-ubuntu/
- http://ubtutorials.com/tutorial/1139/how-stop-small-ddos-attacks-ubuntu