Network bridging in VirtualBox

For those of you working on the ARP project you need to send and receive packets form your Ubuntu VM to the actual physical network. In order to do this you would need to setup network adapter to Bridged networking. In this mode you can sniff traffic on the actual physical network from within your VM and also inject packets into it.

Note: You should turn of Vbox DHCP server before you proceed. In order to do so follow my older post http://zaidmunir.blogspot.com/2010/07/how-to-enabledisable-virtualboxs-dhcp.html

Follow these steps for a hassle free setup.
1. Shut down VM.
2. Open its settings in VBox and goto settings->network
3. Over here click on all the adapter tabs and make sure that enable adapter is unchecked. You should only check this for the adapter you wish to use. In our case adapter 1.
4. In the 'Attached to' field select 'Bridged adapter' and in the 'name' field select the name of the adapter you want to make the bridge with.
5. Now run your VM and find out the name for your interface using the command ifconfig. In this example it is assumed to be eth1.
6. In case the DHCP server on your local LAN physical LAN is working follow step 7 or jump to step 8.
7. Now open and edit the /etc/network/interfaces file. It should look something like this

Preview:

auto lo
iface lo inet loopback

auto eth1
iface eth1 inet dynamic

And now you can try pinging the host machine and machines on the physical network and all should be well :)

8. See the network settings for your host machine. Now we wish to assign the guest machine an IP which is on the same network as that of the host machine. In my case the host machine has the following configuration

Preview:

IP 172.16.1.10
subnet 255.255.0.0
default gateway 172.16.1.1
DNS 172.16.1.1

So my /etc/network/interfaces file looks something like this

Preview:

auto lo
iface lo inet loopback

auto eth1
iface eth1 inet static
address 172.16.1.103
netmask    255.255.0.0
gateway 172.16.1.1

9. You will now be able to ping the host machine and others machines on the physical network. But you will still not be able to browse through the web. For this you need to make an entry for the DNS server in /etc/resolv.conf

Make the following entries in the file

Preview:

search com
nameserver 172.16.1.1

Note: the entries in your case should correspond to your particular network settings.

Making ARP packets

In order to successfully make your ARP request packet, you need to understand the ARP header. You need to declare an appropriate structure for it in your code. Be advised, the ARP header follows immediately after the Ethernet header.

There had been some confusion regarding the values of different fields, please refer to the following screen shot if in confusion. You should be very clear that in order to broadcast the packet on layer 2, you need to adjust the destination MAC in the Ethernet header only, whereas for the value regarding destination hardware address in ARP header, pay special attention to the field destination MAC address in the ARP header in the image attached.



Also observe the total number of bytes received/captured. You will also need to adjust this value accordingly in your code when you send down the packet, you would need to adjust the parameters of pcap_sendpacket (...) accordingly. Refer to my older post for the code related part.

Libpcap

I have written a small sample code to get you started. You can download it here. to compile the code: gcc code.c -lpcap
and then execute the output file as: sudo ./a.out

Here is a code which generates an ICMP packet, you should follow the structure given in this code. Compile and run the code as mentioned above.