ok if the iptables command works you can try this command
put a ping on a customer machine
$ for ((;
) ; do sleep 2; ping -c 3 -w 2 220.127.116.11;done
this will keep it running
check with 'tcpdump' on the server
# tcpdump -i eth0 icmp
# iptables -t nat -A POSTROUTING -s 10.10.10.0/24 --d ! 10.10.10.0/24 o eth0 -j MASQUERADE
then try again the tcpdump command from above
do you see any difference, if the iptables command shows the stuff from before then it is fine. That service is probably just configuring iptables. I don't remember but i think there as some wrapper scripts on redhat that will also exist on centos.
but first try to get it working before trying to make it persistent
does the ping work after the iptables line above (the part after the # symbol that I put there to show it needs to be executed as root.