Fix DTLS server example consuming the ClientHello before handshake (F-3481)

The DTLS branch called bind_socket.recvfrom(1) before creating the
context. On UDP that removes the entire first datagram (the client's
ClientHello) from the queue and discards everything past the first
byte, so wolfSSL_accept() had nothing to consume and the handshake
only recovered after the client's retransmit timer. The captured
from_addr was also reused for every iteration of the -i loop.

Replace it with a peek_peer_address() helper that uses MSG_PEEK to read
the source address without consuming the datagram, and move the peek
into the accept loop so the address is refreshed per connection.

examples/server.py

@@ -115,6 +115,21 @@ def get_DTLSmethod(index):
     )[index]
 
 
+# Large enough to peek a DTLS ClientHello source address.
+PEEK_BUFSIZE = 1500
+
+
+def peek_peer_address(sock):
+    """
+    Return the source address of the next pending datagram without removing
+    it from the socket queue. MSG_PEEK leaves the datagram (the DTLS
+    ClientHello) intact so wolfSSL_accept() can consume it during the
+    handshake.
+    """
+    _, from_addr = sock.recvfrom(PEEK_BUFSIZE, socket.MSG_PEEK)
+    return from_addr
+
+
 def main():
     args = build_arg_parser().parse_args()
     # DTLS connection over UDP
@@ -124,7 +139,6 @@ def main():
             args.v = 1
         bind_socket = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, 0)
         bind_socket.bind(("" if args.b else "localhost", args.p))
-        data, from_addr = bind_socket.recvfrom(1)
         context = wolfssl.SSLContext(get_DTLSmethod(args.v), server_side=True)
     # SSL/TLS connection over TCP
     else:
@@ -156,6 +170,9 @@ def main():
         try:
             secure_socket = None
             if args.u:
+                # Peek the client's address for this connection without
+                # consuming the ClientHello datagram needed by the handshake.
+                from_addr = peek_peer_address(bind_socket)
                 secure_socket = context.wrap_socket(bind_socket)
             else:
                 new_socket, from_addr = bind_socket.accept()

tests/test_dtls_server_example.py

@@ -0,0 +1,73 @@
+# -*- coding: utf-8 -*-
+#
+# test_dtls_server_example.py
+#
+# Copyright (C) 2006-2020 wolfSSL Inc.
+#
+# This file is part of wolfSSL. (formerly known as CyaSSL)
+#
+# wolfSSL is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# wolfSSL is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA
+
+# pylint: disable=missing-docstring, invalid-name, import-error
+
+import os
+import sys
+import socket
+
+import pytest
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "examples"))
+import server as server_example  # noqa: E402
+
+
+@pytest.fixture
+def udp_pair():
+    srv = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+    srv.bind(("localhost", 0))
+    cli = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+    try:
+        yield srv, cli, srv.getsockname()
+    finally:
+        srv.close()
+        cli.close()
+
+
+def test_peek_peer_address_returns_source(udp_pair):
+    srv, cli, srv_addr = udp_pair
+    cli.bind(("localhost", 0))
+    cli.sendto(b"clienthello-payload", srv_addr)
+
+    addr = server_example.peek_peer_address(srv)
+
+    assert addr == cli.getsockname()
+
+
+def test_peek_peer_address_does_not_consume_datagram(udp_pair):
+    """
+    Regression test for F-3481: peeking the client's address before the
+    DTLS handshake must leave the ClientHello datagram intact. The previous
+    example used recvfrom(1), which consumed the datagram and discarded
+    everything past the first byte, breaking the handshake.
+    """
+    srv, cli, srv_addr = udp_pair
+    payload = b"X" * 256  # stand-in for a DTLS ClientHello record
+    cli.sendto(payload, srv_addr)
+
+    server_example.peek_peer_address(srv)
+
+    # The datagram must still be fully available for wolfSSL_accept().
+    srv.settimeout(2)
+    data, _ = srv.recvfrom(4096)
+    assert data == payload