CentOS 6 - Port forwarding

I have the following scenario:

1. My computer
2. CentOS 6.5 intermediate server IP_CENTOS
3. Third party service: https://IP_SERVICE/domain/service-path 

From my computer I can reach the CentOS server, but not the third party service.

From CentOS server you can reach the third party service.

I want to use CentOS server as a bridge in order to forward traffic from my computer to the third party service and vice versa.

Set configuration

Step 1

Enable ip forward in iptables:

 
    echo 1 >/proc/sys/net/ipv4/ip_forward

Step 2

Configure forwarding:

    iptables -t nat -A PREROUTING -p tcp --dport 9443 -j DNAT --to-destination [IP_SERVICE]:443
    iptables -t nat -A POSTROUTING -p tcp -d [IP_SERVICE] --dport 443 -j SNAT --to-source [IP_CENTOS]

Here you have:

• 9443 is the fake port used in CentOS server.
• IP_SERVICE is the IP address of the third party service
• 443 is the default HTTPS port, as the third party service is in https://xxxxxx
• IP_CENTOS is the IP address of the CentOS server

Step 3

Save the rules:

    service iptables save
    service iptables reload

You can check the configuration as follow:

    iptables -t nat --line-numbers -L

    more /etc/sysconfig/iptables

Test

From My Computer you can check to access to https://IP_CENTOS:9443/domain/service-path 

Rollback configuration

In order to restore configuration and delete all previous work, do as follows:

    echo 0 >/proc/sys/net/ipv4/ip_forward

Check the rules and delete them one by one:

    iptables -t nat --line-numbers -L

    iptables -t nat -D PREROUTING|POSTROUTING [num]