Sharing Internet on Linux

Hello there, in this post I'm going to show you how to share an Internet connection regardless off you having a public ip or not, you just need to have Internet on the computer that`s going to make the share.

I don't now if this is the most correct way to do it, but I can tell you that it worked on my machine which was running Ubuntu 8.10.

The picture below shows my test scenario, to make it easier for you to map it to your own.

click on the picture for a better viewing!!!

As you can see on the picture above I shared PC1's Internet with PC2, my PC2 was also running Linux but it could also be running Windows, MacOS, etc.

The IP configuration for the link between PC1 and PC2 was done manualy, but you can have an DHCP Server on PC1 dictating the IPs, DNS Servers, Gateway for both PCs.

What we are configuring is an IP Forwanding using the iptables, NAT MASQUERING.

As you can see on the picture you can see the DNS Servers (red an blue) I used on PC2 was the same servers that the wlan0 interface (the one with internet) on PC1, because if PC1 can reach the servers so will PC2.

First configure the interfaces that make the link between PC1 and PC2, you can configure any private network you want, i used the 10.0.0.0/8 network (see picture), gave the IP 10.0.0.1 to PC1, and 10.0.0.2 to PC2 and defined its Gateway as 10.0.0.1 (PC1) in order for it to send it's packets for the internet to PC1. On PC2 don't forget the DNS Servers that are the same that you have on PC1 in the interface with internet (wlan0), in Ubuntu you can find the DNS Servers, by tipping the following on the bash, so type this on PC1:

$ cat /etc/resolv.conf

# Generated by NetworkManager
domain hello.there
search hello.there
nameserver 192.168.1.210
nameserver 192.168.1.216
nameserver 192.168.1.215


Now that you have the link between both PCs configured (2 PCs on the same private network, and DNS Servers on PC2), just type the following on the bash in order to do the share.

$ su

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

# exit

$ sudo iptables -t nat -A POSTROUTING -s 10.0.0.0/8 -o wlan0 -j MASQUERADE

On the last command you if you configured a different private ip range, between the two PCs, you should change the 10.0.0.0/8 onto the one you chose and change the wlan0 to the interface that has the Internet on PC1.

And now you should have Internet both on PC1 and PC2.

Note: If instead of PC2 you have a switch/hub conected to PC1, you can use PC1 to serve multiple PCs (PC2, PC3, PC4, ...), you just need to make the same IP configuration that you did for PC2, only chanaging the the IP (PC2: 10.0.0.2, PC3: 10.0.0.3, PC4: 10.0.0.4) and connect these to the switch.

OR

Another solution to share with more than one PC is PC1 having multiple interfaces onto which PC2, PC3, PC4, ..., connected directly, as above make the same IP configuration that you did for PC2, only changing the the IP (PC2: 10.0.0.2, PC3: 10.0.0.3, PC4: 10.0.0.4) and connect these to PC1.