Sockets UDP & TCP

UDP Sockets with Python – Simple Send & Receive Examples

This guide shows basic UDP client and UDP server implementations in Python, plus how to quickly test both directions using netcat (nc).

Important UDP facts
• Connectionless — no handshake
• No guaranteed delivery
• No guaranteed order
• No flow control
• Best-effort datagram service

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1. UDP Client – Send one message

#!/usr/bin/env python3
import socket

# ────────────────────────────────────────────────
# SETTINGS
# ────────────────────────────────────────────────
TARGET_IP   = "127.0.0.1"
TARGET_PORT = 9191
MESSAGE     = b"Hello from UDP client!\n"

# ────────────────────────────────────────────────
# SOCKET + SEND
# ────────────────────────────────────────────────
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

sock.sendto(MESSAGE, (TARGET_IP, TARGET_PORT))

sock.close()

print(f"Sent → {MESSAGE.decode().rstrip()!r}")

Quick reference – socket flags

• AF_INET → IPv4 addresses
• SOCK_DGRAM → UDP (datagram) mode
• sendto() → requires destination address on every call

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2. Test the client using netcat (as receiver)

In one terminal, start a UDP listener:

nc -luv 9191

Then run the Python client script in another terminal.

→ The message should appear immediately in the nc window.

Flags explained:

• -l listen
• -u UDP mode (very important!)
• -v verbose output

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3. UDP Server – Receive messages in a loop

#!/usr/bin/env python3
import socket

# ────────────────────────────────────────────────
# SETTINGS
# ────────────────────────────────────────────────
BIND_IP   = "127.0.0.1"     # or "0.0.0.0" to accept from anywhere
BIND_PORT = 9191

# ────────────────────────────────────────────────
# SERVER
# ────────────────────────────────────────────────
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

# Reserve the address/port
sock.bind((BIND_IP, BIND_PORT))

print(f"UDP server listening  {BIND_IP}:{BIND_PORT} …")

try:
    while True:
        data, addr = sock.recvfrom(1024)          # buffer size = 1024 bytes
        ip, port = addr

        try:
            text = data.decode(errors="replace").rstrip()
        except:
            text = f"[binary data – {len(data)} bytes]"

        print(f"[{ip}:{port}]  {text!r}")
except KeyboardInterrupt:
    print("\nServer stopped by user.")
finally:
    sock.close()

Core concepts

• bind() is required on servers/receivers
• recvfrom() returns (data, sender_address) tuple
• Without bind() → usually no reliable way to receive on a known port

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4. Test the server using netcat (as sender)

1. Start the Python server first
2. In a second terminal run:

nc -u 127.0.0.1 9191

1. Type messages and press Enter — they should appear in the Python window.

Client-side note
Normal UDP clients almost never call bind()
→ OS automatically assigns a random high-numbered source port

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TCP Sockets with Python

TCP reminder
TCP is connection-oriented: it does a handshake (connect / accept), guarantees delivery, and keeps order.

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TCP Server – Listen, accept one connection, receive once, print

#!/usr/bin/env python3
import socket

ip = "127.0.0.1"
port = 9191

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((ip, port))
s.listen(1)                     # allow 1 waiting connection

conn, addr = s.accept()         # blocks until a client connects
data = conn.recv(1024)          # receive up to 1024 bytes

print("Received:", data.decode().strip())

conn.close()
s.close()

What happens here:
Server binds → listens → waits for exactly one client → receives one message → prints it → closes.

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TCP Client – Connect once, send one message

#!/usr/bin/env python3
import socket

ip = "127.0.0.1"
port = 9191
data = b"Hello, TCP Server!\n"

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((ip, port))           # performs TCP handshake
s.sendall(data)                 # send all bytes (may send in multiple packets)
s.close()

What happens here:
Client creates socket → connects (handshake) → sends message → closes.

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Quick test with netcat

Option 1: Use nc as server, Python as client

First run (in one terminal):

nc -lv 9191

Option 2: Use Python as server, nc as client

First run the Python server script (shown above).

Then in another terminal:

nc 127.0.0.1 9191

• -l = listen (server mode)
• -v = verbose

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