HOWTO: Share internet connection over wireless network with DHCP

[sciyoshi]

This HOWTO is based on this forum post, but also explains how to set up internet connection sharing with wireless and DHCP.

Say you want to use your desktop as a router, so that the topology looks something like this:

internet -> [eth0 - desktop - wlan0] -> laptop

with the laptop being assigned a dynamic IP address. To do this (replace eth0 and wlan0 appropriately for your setup - eth0 is the connection to the internet, and wlan0 is the wireless for your LAN):

1. First, install the packages dnsmasq, ipmasq, and dhcp3-server, either with Synaptic or with the following command line:
   

   sudo apt-get install dnsmasq ipmasq dhcp3-server

   This may warn you that dhcp3-server could not be started - as we haven't edited the configuration yet, this is normal .
   
2. Assign a static IP address for the wireless card on the desktop machine by editing the file /etc/network/interfaces. Add these lines to the end of the file (if you already see wlan0 somewhere else, delete that first):
   

   auto wlan0
   iface wlan0 inet static
        address 192.168.0.1
        netmask 255.255.255.0
        broadcast 192.168.0.255
        wireless-mode ad-hoc
        wireless-essid YOUR-NETWORK-SSID-HERE

   You can also add encryption to this - see "man interfaces" for more details.
   
3. Edit the file /etc/default/dhcp3-server by finding the line with INTERFACES="" and replacing it with
   

   INTERFACES="wlan0"

   This tells the DHCP server to listen on the local network for connections.
   
4. Open the file /etc/dhcp3/dhcpd.conf. Find the following lines:
   

   option domain-name "example.org";
   option domain-name-servers ns1.example.org, ns2.example.org;
   
   default-lease-time 600;
   max-lease-time 7200;

   and replace them with:
   

   #option domain-name "example.org";
   #option domain-name-servers ns1.example.org, ns2.example.org;
   
   #default-lease-time 600;
   #max-lease-time 7200;

   by adding a pound sign to the beginning of the lines. Then paste this at the end of the file:
   

   subnet 192.168.0.0 netmask 255.255.255.0 {
     range 192.168.0.100 192.168.0.200;
     option domain-name-servers 192.168.0.1;
   #  option domain-name "internal.example.org";
     option routers 192.168.0.1;
     option broadcast-address 192.168.0.255;
     default-lease-time 600;
     max-lease-time 7200;
   }

5. Set up IP masquerading and forwarding:
   

   sudo iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
   sudo sh -c "echo 1 > /proc/sys/net/ipv4/ip_forward"
   sudo dpkg-reconfigure ipmasq

   The default answers should be fine.
   
6. Set up DNS masquerading:
   

   sudo dpkg-reconfigure dnsmasq

7. Start everything up!
   

   sudo ifdown wlan0
   sudo ifup wlan0
   sudo /etc/init.d/dnsmasq restart
   sudo /etc/init.d/dhcp3-server restart

If everything went well, you should now be able to get internet from a laptop just by connecting to this network!

Note: If the settings aren't saved after a reboot, try this:

sudo sh -c "echo \"net.ipv4.ip_forward = 1\" >> /etc/sysctl.conf"

Hope you enjoyed, post any problems and questions here