Add SD query and response to mDNS

tutorials/udp/mdns-sd-client/main.c

@@ -0,0 +1,58 @@
+// Copyright (c) 2026 Cesanta Software Limited
+// All rights reserved
+
+#include "mongoose.h"
+
+static char s_name[] = "Mongoose";
+static char s_service[] = "_http._tcp";
+
+static void mdns_ev_handler(struct mg_connection *c, int ev, void *ev_data) {
+  if (ev == MG_EV_MDNS_RESP) {
+    struct mg_mdns_resp *resp = (struct mg_mdns_resp *) ev_data;
+    if (resp->rr->atype == MG_DNS_RTYPE_PTR &&
+        mg_strcasecmp(mg_str(s_service), resp->sd.srvcproto) == 0) {
+      char addr[40];
+      MG_INFO(("Got a response"));
+      printf("\tSERVICE -> %.*s\n", (int) resp->sd.srvcproto.len,
+             resp->sd.srvcproto.buf);
+      printf("\tNAME -> %.*s\n", (int) resp->name.len, resp->name.buf);
+      if (resp->sd.port != 0)
+        printf("\tPORT -> %u\n", resp->sd.port);
+      if (resp->addr.addr.ip4 != 0) {
+        mg_snprintf(addr, sizeof(addr), "%M", mg_print_ip, &resp->addr);
+        printf("\tADDRESS -> %s\n", addr);
+      }
+      if (resp->sd.txt.buf != NULL)
+        printf("\tTXT -> %.*s\n", (int) resp->sd.txt.len, resp->sd.txt.buf);
+      if (resp->addr.addr.ip4 != 0 && resp->addr.port != 0) {
+        mg_snprintf(addr, sizeof(addr), "http://%M:%u", mg_print_ip, &resp->addr,
+                    mg_ntohs(resp->addr.port));
+        printf("\tURL -> %s\n", addr);
+      } else if (resp->sd.port != 0 && resp->name.buf != NULL) {
+        printf("\tURL -> http://%.*s:%u\n", (int) resp->name.len,
+               resp->name.buf, resp->sd.port);
+      }
+      printf("\n");
+    }
+  }
+  (void) c;
+}
+
+int main(void) {
+  struct mg_mgr mgr;
+  char query[256];
+  static struct mg_connection *c;
+  mg_log_set(MG_LL_DEBUG);  // Set log level
+  mg_mgr_init(&mgr);        // Initialise event manager
+
+  c = mg_mdns_listen(&mgr, mdns_ev_handler, s_name);  // Start mDNS server
+  if (c == NULL) return 1;
+  mg_snprintf(query, sizeof(query), "%s.local", s_service);
+   // Request service providers
+  if (!mg_mdns_query(c, query, MG_DNS_RTYPE_PTR)) return 1;
+
+  for (;;) mg_mgr_poll(&mgr, 1000);  // Event loop
+
+  mg_mgr_free(&mgr);
+  return 0;
+}