Security
Some useful links to explain the concepts of Security in Linux OS
Concepts
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
Rootkits
Fail2Ban
TCP Wrappers
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/
SELinux
Concepts of SELinux
DDoS
Configuration
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/
http://www.ubuntufree.com/how-to-stop-a-ddos-attack-on-ubuntu/
http://ubtutorials.com/tutorial/1139/how-stop-small-ddos-attacks-ubuntu