You will need to add a rule to the INPUT chain permitting traffic to UDP port 7778, and you must make sure this rule appears before other rules blocking UDP traffic.
The simplest (but not necessarily best) way to accomplish this, is to insert a rule at the top of the chain:
iptables -I INPUT -p udp --dport 7778 -j ACCEPT
The command above must be run as root.
You will also have to save the ruleset to the CentOS configuration files, otherwise the change will be lost next time the system is rebooted. According to the CentOS documentation
, you'll need to run the following command:
/sbin/service iptables save
Again, root privileges are required.