RHEL6 - IP Forwarding

sysctl -w net.ipv4.ip_forward=1

echo "net.ipv4.ip_forward = 1" >> /etc/sysctl.conf

reference
http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/6/html/Security_Guide/sect-Security_Guide-Firewalls-FORWARD_and_NAT_Rules.html