The bad guys can masquerade as someone you trust

It is surprisingly easy (you will have the knowledge to do so shortly as you proceed through this text!) to create a packet with an arbitrary source address, packet con- tent, and destination address and then transmit this hand-crafted packet into the Internet, which will dutifully forward the packet to its destination. Imagine the unsuspecting receiver (say an Internet router) who receives such a packet, takes the (false) source address as being truthful, and then performs some command embed- ded in the packet’s contents (say modifies its forwarding table). The ability to inject packets into the Internet with a false source address is known as IP spoofing, and is but one of many ways in which one user can masquerade as another user.

To solve this problem, we will need end-point authentication, that is, a mecha- nism that will allow us to determine with certainty if a message originates from where we think it does. Once again, we encourage you to think about how this can be done for network applications and protocols as you progress through the chapters of this book. We will explore mechanisms for end-point authentication in Chapter 8.

In closing this section, it’s worth considering how the Internet got to be such an insecure place in the first place. The answer, in essence, is that the Internet was orig- inally designed to be that way, based on the model of “a group of mutually trusting users attached to a transparent network” [Blumenthal 2001]—a model in which (by definition) there is no need for security. Many aspects of the original Internet archi- tecture deeply reflect this notion of mutual trust. For example, the ability for one user to send a packet to any other user is the default rather than a requested/granted capability, and user identity is taken at declared face value, rather than being authen- ticated by default.

But today’s Internet certainly does not involve “mutually trusting users.” Nonetheless, today’s users still need to communicate when they don’t necessarily trust each other, may wish to communicate anonymously, may communicate indi- rectly through third parties (e.g., Web caches, which we’ll study in Chapter 2, or mobility-assisting agents, which we’ll study in Chapter 6), and may distrust the hardware, software, and even the air through which they communicate. We now have many security-related challenges before us as we progress through this book: We should seek defenses against sniffing, end-point masquerading, man-in-the- middle attacks, DDoS attacks, malware, and more. We should keep in mind that communication among mutually trusted users is the exception rather than the rule. Welcome to the world of modern computer networking!