Computer Networking Basics

The presenter aims to explain computer networking basics, including protocol layers like Ethernet and TCP/IP, which can initially be confusing.
The OSI layer model is introduced as an abstract concept, and the explanation will bridge the gap to practical aspects like sockets, bits, and bytes.

Computer A and Computer B want to exchange a text message.
The simplest way for humans is to hand over a physical piece of paper, which corresponds to the physical layer in computer networking.
Computers use electricity to send 0s and 1s over a wire or radio waves, following a specific protocol for the sender and receiver to understand the message.

The internet involves many computers, requiring a way to address and find them.
The IP layer uses IP addresses as identifiers for sender and receiver.
A switch (akin to a postal service) is used to help find the correct computer using the IP address.
The sender (e.g., 192.168.178.2) needs to know the recipient's IP address (e.g., 192.168.178.3).
An IP header containing source and destination IPs is attached to the data before sending.

The packet (IP header + data) is transmitted as 0s and 1s to a switch.
The switch's physical layer receives the packet and passes it to the IP layer.
The switch checks the destination IP address and forwards the packet to the appropriate port.
Note: In reality, Ethernet frames and MAC addresses are used at this stage, but this detail is simplified for the explanation.
The target computer's physical layer reassembles the packet and hands it to the IP layer.
The IP layer verifies that the destination IP matches its own and extracts the data.

Since multiple programs run on each computer, a mechanism is needed to direct the data to the correct program.
The TCP layer includes port numbers to specify which program should receive the data.
The TCP protocol also manages error correction and ensures data delivery, but the focus is on the port number for this explanation.
A port number references a specific program (e.g., a web server running on port 80).

A TCP header is created, containing the source port (sender's program number) and the destination port (e.g., 80 for the web server).
The text data is attached to this TCP header.
The TCP packet is then handed to the IP layer, which adds an IP header with source and destination IPs.
The complete packet is sent through the physical layer to the switch and then to the target computer.
The target computer's IP layer identifies that the packet is for itself and passes the TCP packet to the TCP layer.
The TCP layer identifies the destination port (80) and forwards the data to the web server process.

The layered approach is beneficial as it compartmentalizes different functions, allowing for isolated development and thinking.
Each layer solves a specific problem: physical transmission, addressing, program identification.
Stacking protocols allows developers to focus on specific layers without needing to understand the entire stack.
The complexity is abstracted away, allowing developers to treat lower layers as "black boxes".

Developers can focus on sending data to a specific IP address and port, treating the underlying layers as a black box that magically delivers the data.
This black box thinking applies to every layer, where each layer only focuses on its specific task without needing to understand the layers above or below.
The OSI layer model depicts this, with each layer performing a well-defined task and communicating directly with its peer on the other side.

Operating systems and software libraries implement the black box abstraction.
Creating a TCP socket (e.g., in Python or C on Linux) allows developers to send data without handling lower-level details.
System calls (kernel functions) are used to instruct the operating system to manage the network stack.
Within the Linux kernel, functions handle building TCP headers and passing packets to the IP layer and network device.
These layers are essentially functions implemented by the kernel or software libraries.

Netcat is a utility that simplifies sending data over a network by calling the necessary kernel functions.
Wireshark provides insight into the actual network packets, showing TCP headers, IP headers, and the raw bytes being transmitted.
Wireshark helps visualize the layers and headers in a practical context.

The concept of layering is present in everyday life, such as making a phone call.
Voice is converted to electrical signals, transmitted over wires, and converted back to voice at the recipient's end.
The complex chain is abstracted away, allowing users to think of it as a direct connection.

Accept black boxes to improve efficiency and focus on specific problems.
However, as a hacker, it's important to understand what's inside the black boxes and how the layers interact.
Protocol layers are not truly isolated; they are interconnected functions passing data back and forth.

HTTP request smuggling vulnerabilities arise from the interplay between HTTP and TCP.
Understanding the relationship between TCP and HTTP is crucial to understand this vulnerability.
Request smuggling involves how servers use TCP for multiple HTTP requests and responses.