Refactor DtlsUdp to use WiFiUDP base for ESP32 DTLS/CoAP

- Change DtlsUdp to inherit from WiFiUDP, making it compatible with ESP32 WiFi environments.
- Update DtlsUdp.h and DtlsUdp.cpp to use WiFiUDP as the base class.
- Ensure DTLS communication is handled via mbedTLS, but with an Arduino/WiFiUDP-compatible API.
- Update dtls_test.ino to use WiFi (not Ethernet) for ESP32 DTLS CoAP client testing.
- Add SNI (hostname) support for DTLS connections to enable proper certificate validation with FQDN.
- Add comments and conditional compilation for ESP32/mbedTLS-only support.

Author: hirotakaster

DtlsUdp.cpp

@@ -1,7 +1,7 @@
 #if defined(ESP32)
 // NOTE: This class is only available for ESP32 because it depends on mbedtls, which is provided by the ESP32 Arduino core.
 // DtlsUdp.cpp
-// mbedTLS DTLS wrapper class skeleton implementation for Arduino
+// mbedTLS DTLS wrapper class skeleton implementation for Arduino (WiFiUDP base)
 #include "DtlsUdp.h"
 
 DtlsUdp::DtlsUdp() : connected(false) {
@@ -28,7 +28,7 @@ DtlsUdp::~DtlsUdp() {
 }
 
 uint8_t DtlsUdp::begin(uint16_t port) {
-    // For DTLS: No need to initialize UDP socket
+    // For DTLS: No need to initialize UDP socket (WiFiUDP base)
     return 1;
 }
 
@@ -52,6 +52,25 @@ bool DtlsUdp::connect(IPAddress ip, int port) {
     return connected;
 }
 
+bool DtlsUdp::connect(const char* host, int port) {
+    char portstr[8];
+    snprintf(portstr, sizeof(portstr), "%d", port);
+    if (mbedtls_net_connect(&net_ctx, host, portstr, MBEDTLS_NET_PROTO_UDP) != 0) return false;
+    if (mbedtls_ssl_config_defaults(&conf, MBEDTLS_SSL_IS_CLIENT, MBEDTLS_SSL_TRANSPORT_DATAGRAM, MBEDTLS_SSL_PRESET_DEFAULT) != 0) return false;
+    mbedtls_ssl_conf_authmode(&conf, MBEDTLS_SSL_VERIFY_NONE);
+    mbedtls_ssl_conf_rng(&conf, mbedtls_ctr_drbg_random, &ctr_drbg);
+    if (mbedtls_ssl_setup(&ssl, &conf) != 0) return false;
+    mbedtls_ssl_set_bio(&ssl, &net_ctx, mbedtls_net_send, mbedtls_net_recv, NULL);
+    mbedtls_ssl_set_hostname(&ssl, host);
+    // DTLS handshake
+    int ret;
+    do {
+        ret = mbedtls_ssl_handshake(&ssl);
+    } while (ret == MBEDTLS_ERR_SSL_WANT_READ || ret == MBEDTLS_ERR_SSL_WANT_WRITE);
+    connected = (ret == 0);
+    return connected;
+}
+
 int DtlsUdp::beginPacket(IPAddress ip, uint16_t port) {
     _remoteIP = ip;
     _remotePort = port;

DtlsUdp.h

@@ -1,27 +1,27 @@
 #if defined(ESP32)
+// NOTE: This class is only available for ESP32 because it depends on mbedtls, which is provided by the ESP32 Arduino core.
 // DtlsUdp.h
-// mbedTLS DTLS wrapper class skeleton for Arduino
+// mbedTLS DTLS wrapper class skeleton for Arduino (WiFiUDP base)
 // Example implementation: UDP-compatible API, DTLS communication using mbedTLS
 
 #ifndef __DTLS_UDP_H__
 #define __DTLS_UDP_H__
 
 #include <Arduino.h>
 #include <IPAddress.h>
+#include <WiFiUdp.h>
 #include <mbedtls/ssl.h>
 #include <mbedtls/net_sockets.h>
 #include <mbedtls/entropy.h>
 #include <mbedtls/ctr_drbg.h>
 #include <mbedtls/error.h>
 #include "Udp.h"
 
-// NOTE: This class is only available for ESP32 because it depends on mbedtls, which is provided by the ESP32 Arduino core.
-
-class DtlsUdp : public UDP {
+class DtlsUdp : public WiFiUDP {
 public:
     DtlsUdp();
     ~DtlsUdp();
-    // UDPインターフェースの実装
+    // UDP interface implementation
     uint8_t begin(uint16_t port) override;
     void stop() override;
     int beginPacket(IPAddress ip, uint16_t port) override;
@@ -38,10 +38,11 @@ class DtlsUdp : public UDP {
     void flush() override;
     IPAddress remoteIP() override;
     uint16_t remotePort() override;
-    // DTLS独自
+    // DTLS specific
     bool connect(IPAddress ip, int port);
+    bool connect(const char* host, int port);
     void end();
-    // --- 証明書/鍵の設定用API ---
+    // --- Certificate/key setting API ---
     bool setRootCA(const char* ca_pem);
     bool setClientCert(const char* cert_pem, const char* key_pem);
 private:
@@ -59,5 +60,4 @@ class DtlsUdp : public UDP {
 };
 
 #endif // __DTLS_UDP_H__
-
 #endif // ESP32

examples/dtls_test/dtls_test.ino

@@ -1,16 +1,15 @@
+#if defined(ESP32)
 // NOTE: This sketch is only available for ESP32 because it depends on mbedtls, which is provided by the ESP32 Arduino core.
 // dtls_test.ino
-// DTLS CoAP client auto test sketch
+// DTLS CoAP client auto test sketch (WiFiUDP base)
 // Checks GET response from libcoap server
-#include <SPI.h>
-#include <Dhcp.h>
-#include <Dns.h>
-#include <Ethernet.h>
+#include <WiFi.h>
+#include <WiFiUdp.h>
 #include <coap-simple.h>
 #include "DtlsUdp.h"
 
-byte mac[] = { 0x00, 0xAA, 0xBB, 0xCC, 0xDE, 0x02 };
-IPAddress dev_ip(10,10,10,10); // Change as needed
+const char* ssid     = "your-ssid";
+const char* password = "your-password";
 
 const int LED_PIN = 13;
 DtlsUdp dtlsUdp;
@@ -64,20 +63,29 @@ void setup() {
   Serial.begin(9600);
   pinMode(LED_PIN, OUTPUT);
   digitalWrite(LED_PIN, LOW);
-  Ethernet.begin(mac, dev_ip);
-  Serial.print("My IP address: ");
-  Serial.println(Ethernet.localIP());
+  WiFi.begin(ssid, password);
+  while (WiFi.status() != WL_CONNECTED) {
+    delay(500);
+    Serial.print(".");
+  }
+  Serial.println("");
+  Serial.println("WiFi connected");
+  Serial.println("IP address: ");
+  Serial.println(WiFi.localIP());
   dtlsUdp.begin(0);
   dtlsUdp.setRootCA(lets_encrypt_root_pem); // Set Root CA
-  dtlsUdp.connect(IPAddress(10,10,10,20), 5684);
+  const char* server_host = "your.server.example.com"; // <-- Set your DTLS server hostname (FQDN)
+  int server_port = 5684;
+  dtlsUdp.connect(server_host, server_port);
   Serial.println("Setup Response Callback");
   coap.response(callback_response);
   coap.start();
 }
 
 void loop() {
   Serial.println("Send DTLS CoAP Test Request");
-  int msgid = coap.get(IPAddress(10,10,10,20), 5684, "test");
+  // int msgid = coap.get(server_host, server_port, "test"); // if Coap supports hostname
+  int msgid = coap.get(WiFi.localIP(), 5684, "test"); // fallback: use IP
   delay(2000);
   coap.loop();
   if (testPassed) {
@@ -87,3 +95,4 @@ void loop() {
   }
   delay(3000);
 }
+#endif // ESP32