https://satishdotpatel.github.io/ha-with-keepalived-and-conntrackd/
https://backreference.org/2013/04/03/firewall-ha-with-conntrackd-and-keepalived/index.html
https://serverfault.com/questions/1030236/when-does-iptables-conntrack-module-track-states-of-packets