📝 Task

In this challenge, you’re still working with UDP, but now there’s a twist:
You’re required to send your message from a specific source port, not just the default random one.

Goal:
From your host at 10.0.0.1, send Hello, World!\n to 10.0.0.2:31337, but your source port must be 31338.

The server only replies if it receives the message from UDP port 31338.

🧠 Background: UDP Source Ports

Normally when you send a UDP message, Linux picks a random source port unless you bind one explicitly.

Key Concept:

s.bind(('', <SOURCE_PORT>))

This tells your OS to use the given port when sending data.

Server Code Analysis (/challenge/run)

server_socket.bind(("0.0.0.0", 31337))  # Server listening on UDP port 31337

client_message, (client_host, client_port) = server_socket.recvfrom(1024)

# Only responds if:
if client_port == 31338 and client_message == b"Hello, World!\n":
    server_socket.sendto(flag.encode(), (client_host, client_port))

So we must:

  • Use destination port 31337
  • Use source port 31338
  • Send exactly Hello, World!\n

Solution (Python)

# udp2.py
import socket

t_ip = '10.0.0.2'      # Target IP (server)
dport = 31337          # Destination port (server)
sport = 31338          # Required source port

msg = b"Hello, World!\n"

with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as s:
    s.bind(('', sport))                      # Set the source port
    s.sendto(msg, (t_ip, dport))             # Send message to server
    data, _ = s.recvfrom(1024)               # Wait for flag
    print("Flag:", data.decode())

Flag

Flag: pwn.college{Itc0GGKtQ7BdQvxVo_g7qra2S89.01M0AjNxwSM0IzMyEzW}