Sharing Internet Modem via Lan on Backtrack (Linux)

To get internet form other computer, first we set our ip, gateway and DNS in our computer.

The text which blue color is our computer (computer A). And the red color is the other computer which connect to the internet (computer I).

1. First, in computer I we look for network information about ppp0.

# ifconfig ppp0
ppp0 Link encap:Point-to-Point Protocol
inet addr:10.77.221.111 P-t-P:192.168.11.10 Mask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1
RX packets:21275 errors:4 dropped:0 overruns:0 frame:0
TX packets:21823 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:3
RX bytes:24204058 (24.2 MB) TX bytes:2716284 (2.7 MB)

The conclusion, ip ppp0 computer I is 10.77.221.111.

2. We set ip for our computer. In my computer:

Ip : 192.168.100.2
Gateway : 192.168.100.1
Prefered DNS server : 10.4.22.1
Alternate DNS server : 10.4.21.3
(we can see for DNS from Computer I with "cat /etc/resolv.conf")

And than, on linux we can set like this:

# ifconfig eth0 down
# ifconfig eth0 192.168.100.2 netmask 255.255.255.0
# route add default gw 192.168.100.1 eth0
# nano /etc/resolv.conf

nameserver 10.4.22.1
nameserver 10.4.21.3

# ifconfig eth0 up

3. In computer I, we must set ip interface for eth0 (lan network)

# ifconfig eth0 192.168.100.1

4. For shared internet, we set the rule and ipforward via eth0,

# iptables -A FORWARD -i eth0 -p ALL -s 192.168.100.2/24 -d 10.77.221.111 -j ACCEPT

then set NAT (Network Addres Translation):

# iptables -t nat -A POSTROUTING -s 192.168.100.1/24 -j MASQUERADE

so we can give the last command for ip forward

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

Finally we can check the network from our computer (computer A) to access the internet.