How do I assign the firewall mark 21 to FTP traffic on a Linux Virtual Server (LVS) cluster?

Before assigning any iptables rules for File Transfer Protocol (FTP) service, check the existing network packet filtering rules.

Below are rules which assign the same firewall mark 21 to FTP traffic. For these rules to work properly, you must also use the VIRTUAL SERVER subsection of Piranha Configuration Tool to configure a virtual server for port 21 w...