dhcp_server.py

import socket
import json

# DHCP Server - Phase 1
# DHCP (Dynamic Host Configuration Protocol) assigns IP addresses to clients
# when they first join a network. The client sends a DISCOVER broadcast and
# this server responds with an OFFER containing an available IP address.
# UDP is used because the client has no IP yet and cannot perform a TCP handshake.

SERVER_IP   = '127.0.0.1'
SERVER_PORT = 6767       # Standard DHCP uses port 67; we use 6767 to avoid requiring admin rights.
OFFERED_IP  = '127.0.0.2'  # The IP address assigned to all clients in this simulation.


def start_dhcp_server():
    print("starting DHCP server...")

    # SOCK_DGRAM creates a UDP socket.
    server_sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

    # SO_REUSEADDR prevents "Address already in use" errors when restarting during development.
    server_sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

    server_sock.bind((SERVER_IP, SERVER_PORT))
    print(f"DHCP server listening on {SERVER_IP}:{SERVER_PORT}")

    while True:
        print("waiting for DHCP request...")

        raw_data    = None
        client_addr = None
        try:
            raw_data, client_addr = server_sock.recvfrom(1024)
        except Exception as e:
            print(f"error receiving packet: {e}")
            continue

        if raw_data is None:
            continue

        message = raw_data.decode('utf-8')
        print(f"received '{message}' from {client_addr}")

        if message == "DISCOVER":
            print(f"DISCOVER received. sending OFFER (ip={OFFERED_IP})")

            # Build the OFFER response as a JSON object and encode to bytes.
            offer      = {"type": "OFFER", "assigned_ip": OFFERED_IP}
            offer_json = json.dumps(offer)
            offer_bytes = offer_json.encode('utf-8')

            server_sock.sendto(offer_bytes, client_addr)
            print(f"OFFER sent to {client_addr}")

        else:
            print(f"unknown message type: '{message}'. ignoring.")


if __name__ == "__main__":
    start_dhcp_server()