Linux Port Forward

Proxy server: CentOS 6.6 --> IP_a

Backend server: windows server 2003 --> IP_b

Steps:

1. Enable ip_forward

vim /etc/sysctl.conf

net.ipv4.ip_forward=1

sysctl -p

2. Add firewall rules:

iptables -t nat -A PREROUTING -p tcp --dport 21 -j DNAT --to-destination IP_b:21

iptables -t nat -A POSTROUTING -p tcp -d IP_b --dport 21 -j SNAT --to-source IP_a

iptables -t nat -A PREROUTING  -p tcp  --dport 61000:61100 -j DNAT --to IP_b:61000-61100

service iptables save

3. Comment firewall rules:

vim /etc/sysconfig/iptables

#-A FORWARD -j REJECT --reject-with icmp-host-prohibited

4. Restart firewall:

service iptables restart