Tech Juice

Introduction

Scapy is a powerful interactive packet manipulation tool, packet generator, network scanner, network discovery tool, and packet sniffer. It is written in the Python, and is installed by default on Backtrack 4+. On Ubuntu it can be installed using this command:

sudo apt-get install scapy

The official scapy documentation is located here, and you may also need a Python Cheat Sheet.

Scapy Basics

Execute scapy at the command-line to run the Python interpreter with the scapy libraries loaded.

Start up scapy and run the ls() command. This will list all supported packet types.

$ scapy
Welcome to Scapy (2.0.0.5 beta)
>>> ls()
ARP        : ARP
ASN1_Packet : None
BOOTP      : BOOTP
CookedLinux : cooked linux
DHCP       : DHCP options
...more

List all available functions using lsc():

>>> lsc()
 sr               : Send and receive packets at layer 3
 sr1              : Send packets at layer 3 and return only the first answer
 srp              : Send and receive packets at layer 2
 srp1             : Send and receive packets at layer 2 and return only the first answer
 srloop           : Send a packet at layer 3 in loop and print the answer each time

The ls() command can do much more. Show the contents of the IP structure with ls(IP)…

Introduction

The Address Resolution Protocol, or ARP, is used for resolution of network-layer addresses (IP) to link-layer addresses (MAC). This post describes the basics of the ARP protocol, viewing and manipulating your arp cache is discussed in the next post.

Other posts on the ARP protocol are available here:

• Viewing and manipulating the ARP cache on Windows and Linux.
• An introduction to ARP cache poisoning.

ARP Basics

When a system wants to send data to another computer, it prepares an IP packet with the appropriate destination IP. This packet is passed down to the link-layer (usually Ethernet). This layer needs to encapsulate the IP packet in an Ethernet frame before it can be sent.

An Ethernet frame must be addressed to a destination hardware address (MAC address). But which MAC address corresponds with the destination IP address? This is looked up in the ARP cache. The ARP cache contains mappings of the form (IP - MAC).

Source and Destination Hosts on the Same Local Network

The sending host will first look up the destination IP address in its routing table. If the destination IP is on the same physical network (subnet) then there are no routers between the host…