From 2bbac982e1ed128fac80c42a783f28cb7dbde643 Mon Sep 17 00:00:00 2001
From: Fletcher Dunn <fletcherd@valvesoftware.com>
Date: Wed, 13 May 2026 14:05:35 -0700
Subject: [PATCH] Add a STUN server

(cherry picked from commit af470f47f68b6000cfe2e31390acc797e3fb8383)
---
 tests/stun_server.py | 63 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)
 create mode 100644 tests/stun_server.py

diff --git a/tests/stun_server.py b/tests/stun_server.py
new file mode 100644
index 0000000..043bace
--- /dev/null
+++ b/tests/stun_server.py
@@ -0,0 +1,63 @@
+#!/usr/bin/env python3
+"""
+Minimal STUN server implementing RFC 5389 Binding Request/Response.
+
+Other STUN message types (TURN Allocate, Send/Data/Channel, etc.) are
+silently ignored.  Message integrity (HMAC-SHA1) is not implemented.
+"""
+
+import argparse
+import socket
+import struct
+import sys
+
+STUN_MAGIC_COOKIE       = 0x2112A442
+MSG_BINDING_REQUEST     = 0x0001
+MSG_BINDING_SUCCESS     = 0x0101
+ATTR_XOR_MAPPED_ADDRESS = 0x0020
+
+def run(host, port):
+    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+    sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+    sock.bind((host, port))
+    print("STUN server listening on %s:%d" % (host, port), flush=True)
+
+    while True:
+        try:
+            data, addr = sock.recvfrom(2048)
+            if len(data) < 20:
+                print("Dropped runt packet (%d bytes) from %s:%d" % (len(data), addr[0], addr[1]), flush=True)
+                continue
+            msg_type, _msg_len, magic = struct.unpack_from('!HHI', data)
+            if magic != STUN_MAGIC_COOKIE:
+                print("Dropped packet from %s:%d: bad magic 0x%08x" % (addr[0], addr[1], magic), flush=True)
+                continue
+            if msg_type != MSG_BINDING_REQUEST:
+                print("Dropped packet from %s:%d: unexpected message type 0x%04x" % (addr[0], addr[1], msg_type), flush=True)
+                continue
+            print("Binding request from %s:%d" % (addr[0], addr[1]), flush=True)
+            transaction_id = data[8:20]
+
+            # XOR-MAPPED-ADDRESS (IPv4)
+            xport = addr[1] ^ (STUN_MAGIC_COOKIE >> 16)
+            xip   = struct.unpack('!I', socket.inet_aton(addr[0]))[0] ^ STUN_MAGIC_COOKIE
+            attr_body = struct.pack('!BBH I', 0x00, 0x01, xport, xip)
+            attr = struct.pack('!HH', ATTR_XOR_MAPPED_ADDRESS, len(attr_body)) + attr_body
+
+            response = struct.pack('!HHI', MSG_BINDING_SUCCESS, len(attr), STUN_MAGIC_COOKIE) \
+                       + transaction_id + attr
+            sock.sendto(response, addr)
+        except KeyboardInterrupt:
+            break
+        except Exception as e:
+            print("Error: %s" % e, file=sys.stderr, flush=True)
+
+if __name__ == '__main__':
+    parser = argparse.ArgumentParser(
+        description='Minimal STUN server (RFC 5389 Binding Request only; no message integrity)')
+    parser.add_argument('--host', default='0.0.0.0',
+                        help='IP address to bind (default: 0.0.0.0)')
+    parser.add_argument('--port', type=int, default=3478,
+                        help='UDP port to listen on (default: 3478)')
+    args = parser.parse_args()
+    run(args.host, args.port)