setting linux as a gateway

Introduction:

Since my school project need to set up an ethernet environment. I need to create an internal network under virtualbox. The first step is to setup the gateway inside a virtualbox. After google for a while, I finally setting up my linux as a gateway.

Environment :

linux distribution: alpine linux 2.3

kernel version:    3.0

virtualbox network adapter setting:

     1. host-only network=> eth0 for internal network.

     2. bridged network => eth1 for internet.

the connection state is like the following figure:

<figure>

<<internet>>----------<<alpine linux>>------------<<internal>>

            eth1 pppoe                 eth0 NAT

P.S
The reason why I use alpine linux instead of other distribution is that it is tiny but contains the utilities that to set up my environment.
alpine linux download link:
http://alpinelinux.org/
alpine linux installation guide:
http://wiki.alpinelinux.org/wiki/Installation

Content:
I'll write down all the steps that I configure the internet.
include:
1. package requirement
2. configure the network interface
3. setting up the iptables
4. enable packet forwarding
 1.package requirement:

rp-pppoe => pppoe client side program
iptables => firewall
ppp  => ppp deamon
that's all. 

2.configure the network interface
   2.1. the internal network:
      configure the /etc/network/interface
      add the following line to the file.

auto eth0
iface eth0 inet static
address 192.168.56.254
netmask 255.255.255.0
network 192.168.56.0
broadcast 192.168.6.255

      the address, netmask, network and broadcast can be changed according to you network setting. 

   2.2.pppoe configuration:
      type 
      pppoe-setting => to start pppoe configuration. 
      After setting up type 
      pppoe-connect => connect to the internet via pppoe. 
      You can check the result by typing:
      ifconfig | less

3.setting up the iptables:
   type the following command to set the iptables rules:      

   iptables -A FORWARD -o eth1 -i eth0 -s 192.168.56.0/24 -m conntrack --ctstate NEW -j ACCEPT

   iptables -A FORWARD -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT 

   iptables -A POSTROUTING -t nat -j MASQUERADE

4.enable packet forwarding:

    type

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

    type

    cat /proc/sys/net/ipv4/ip_forward => check the result.

reference website:

http://wiki.alpinelinux.org/wiki/Alpine_Linux_package_management#Packages_and_Repositories

http://www.linuxfromscratch.org/blfs/view/6.2.0/connect/other.html

http://tldp.org/HOWTO/DSL-HOWTO/configure.html

http://www.brennan.id.au/05-Broadband_Connectivity.html

https://help.ubuntu.com/community/Internet/ConnectionSharing