Setup a CentOS Linux Router

# enable ipv4 forward 
echo 1 > /proc/sys/net/ipv4/ip_forward

# CentOS 6
iptables -t nat -A POSTROUTING -o XXXX -j MASQUERADE

CentOS/RHEL 7
$ firewall-cmd --add-masquerade --permanent
$ firewall-cmd --reload

Refer to:

• How to Make a CentOS 7 Router 
• Setting up a Linux GW: NATting and forwarding 
• For CentOS 6: Set up CentOS 6 Linux Server as a Router Using Iptables