HTTP

Overview

Terms defined: client, Domain Name System (DNS), HTTP request, HTTP response, HyperText Transfer Protocol (HTTP), Internet Protocol, IP address, port, server, socket, Transmission Control Protocol

Outline

Typical web application has clients and servers
- Client initiates communication by sending a message and waiting for a response
- Server waits for requests and then replies to them
Communicate using Internet Protocol (IP), and specifically Transmission Control Protocol (TCP/IP)
- Makes communication between computers look like reading and writing files
A socket is one end of a point-to-point channel
- Consists of an IP address (identifies machine) and a port on that machine
- IP address is four 8-bit numbers(e.g., 93.184.216.34)
- Domain Name System (DNS) matches these to symbolic names like example.com
- Port is a number in the range 0-65535
- Only one process can use a port at a time

Using Sockets

socketserver module provides:
- TCPServer class to manage incoming connections
- BaseRequestHandler class that does everything except process the incoming data
TCPServer creates a new handler each time it gets a connection and calls that object's handle method
simple_server.py reads up to 1024 bytes of data and returns a message

"""Basic socket server."""

import socketserver


CHUNK_SIZE = 1024
SERVER_ADDRESS = ("localhost", 5000)


class MyHandler(socketserver.BaseRequestHandler):
    def handle(self):
        data = self.request.recv(CHUNK_SIZE)
        cli = self.client_address[0]
        msg = f"server received {len(data)} bytes from {cli}"
        print(msg)
        print(data)
        self.request.sendall(bytes(msg, "utf-8"))


if __name__ == "__main__":
    server = socketserver.TCPServer(SERVER_ADDRESS, MyHandler)
    server.serve_forever()

Use netcat to open a connection and send some text with send_with_nc.sh

echo "testing" | nc 127.0.0.1 5000

HTTP Request and Response

HyperText Transfer Protocol (HTTP) is deliberately simple
- Send request with path, headers, and body
- Get response with status, headers, and content
Minimal request is just GET /index.html HTTP/1.1
- Method
- Path
- Protocol version
http_request_headers.txt shows more realistic request

GET /index.html HTTP/1.1
Accept: text/html
Accept-Language: en, fr
If-Modified-Since: 16-May-2024

http_response.txt shows possible response

HTTP/1.1 200 OK
Date: Thu, 16 June 2023 12:28:53 GMT
Content-Type: text/html
Content-Length: 53

<html>
<body>
<h1>Hello, World!</h1>
</body>
</html>

http_server.py responds to GET with same page every time.

from http.server import BaseHTTPRequestHandler, HTTPServer

PAGE = """<html><body><p>path: {path}</p></body></html>"""

class RequestHandler(BaseHTTPRequestHandler):
    def do_GET(self):
        page = PAGE.format(path=self.path)
        content = bytes(page, "utf-8")
        self.send_response(200)
        self.send_header("Content-Type", "text/html; charset=utf-8")
        self.send_header("Content-Length", str(len(content)))
        self.end_headers()
        self.wfile.write(content)

if __name__ == "__main__":
    server_address = ("localhost", 5000)
    server = HTTPServer(server_address, RequestHandler)
    server.serve_forever()