Basic iptables NAT port forwarding

Question

I have three machines: A local PC (public IP 1.2.3.4), an Ubuntu 10 Server box in a datacentre (eth0 on 5.6.7.8 public IP), and a third-party server hosting a website outside of my network (let's say Slashdot on 216.34.181.45).

• Using iptables, how do I access Slashdot from my local machine using 5.6.7.8:8080 ?
• Would this process differ if Slashdot was on the same LAN as my Ubuntu box?
• Can this be done with just NAT PREROUTING/POSTROUTING, or do I need MASQUERADE?

Answer 1

   PC ----- Ubuntu 10 Server ----- Slashdot 
(1.2.3.4)      (5.6.7.8)        (216.34.181.45)

1. Enable the IP forwarding on Ubuntu:

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

   and add the following rules:

   iptables -t nat -A PREROUTING -i eth0 -p tcp -m tcp --dport 8080 -j DNAT \
                                          --to-destination 216.34.181.45:80 
   iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to-source 5.6.7.8
   

2. No.

3. You should use MASQUERADE if the Ubuntu has a dynamic IP:

   iptables -t nat -A POSTROUTING -j MASQUERADE
   

---

You can also use SSH local port forwarding in this case by executing the below command on the Ubuntu:

$ ssh -L 5.6.7.8:8080:216.34.181.45:80 -N user@216.34.181.45

---

There's still another (or more) way to do this. Take a look at the rinetd:

Name       : rinetd
Arch       : i386
Version    : 0.62
Release    : 6.el5.art
Size       : 41 k
Repo       : installed
Summary    : TCP redirection server
URL        : http://www.boutell.com/rinetd
License    : GPL
Description: rinetd is a daemon which redirects TCP connections from one IP address
           : and port to another IP address and port. This daemon is often used to
           : access services behind a firewall.

The configuration is very simple. Add the belows line into /etc/rinetd.conf:

5.6.7.8 8080 216.34.181.45 80

and start:

# /etc/init.d/rinetd start
Starting rinetd:                                           [  OK  ]

It will do everything for you.

Answer 2

Have a read of this as it might have the answer to your question https://help.ubuntu.com/8.04/serverguide/C/firewall.html