IPTables
Some useful links to explain the concepts of IPTables
Concepts
Configuration
https://linuxconfig.org/collection-of-basic-linux-firewall-iptables-rules
http://blog.ls20.com/securing-your-server-using-ipset-and-dynamic-blocklists/
http://www.cyberciti.biz/faq/how-do-i-block-an-ip-on-my-linux-server/
http://www.tecmint.com/basic-guide-on-iptables-linux-firewall-tips-commands/
http://www.cyberciti.biz/faq/rhel-fedorta-linux-iptables-firewall-configuration-tutorial/
http://www.howtogeek.com/177621/the-beginners-guide-to-iptables-the-linux-firewall/
https://www.digitalocean.com/community/tutorials/how-to-list-and-delete-iptables-firewall-rules
https://www.digitalocean.com/community/tutorials/7-security-measures-to-protect-your-servers
https://www.frozentux.net/iptables-tutorial/chunkyhtml/x1309.html
https://jadendreamer.wordpress.com/2013/04/18/linux-tutorial-blocking-ip-addresses-from-your-server/
http://www.linuxjournal.com/content/advanced-firewall-configurations-ipset
Tuning and Hardening
IP Conntrack
http://www.cyberciti.biz/faq/ip_conntrack-table-ful-dropping-packet-error/
http://blackbird.si/ip_conntrack-table-full-dropping-packet-conclusions-about-connection-tracking/
https://www.scalescale.com/tips/nginx/ip_conntrack-table-full-dropping-packet/
http://www.rigacci.org/wiki/lib/exe/fetch.php/doc/appunti/linux/sa/iptables/conntrack.html
https://bobcares.com/blog/2011/04/21/ip-connection-tracking-ip_conntrack/
https://timanovsky.wordpress.com/2009/04/10/tuning-linux-firewall-connection-tracker-ip_conntrack/