Introduce DNS-SD using mDNS

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

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+// Copyright (c) 2022 Cesanta Software Limited
+// All rights reserved
+
+#include "mongoose.h"
+
+// A TXT record data containing JSON, if actually needed.
+// The whole record must not exceed 256 bytes, so keep this small
+char txt[] = "\x15{\"property\": \"value\"}";
+// string length followed by string data, data length = 21 + 1 = 22 bytes
+
+// DNS-SD service records
+static const struct mg_dnssd_record srvcs[] = {
+    {{"_myservice._tcp", 15}, {"", 0}, 9876},
+    {{"_myservice._udp", 15}, {txt, 22}, 9876}};
+
+// DNS-SD service records database
+struct dnssd_db {
+  struct mg_dnssd_record *srvcs;  // service record data
+  uint8_t num;                    // number of records in db
+};
+
+static const struct dnssd_db db_ = {
+    (struct mg_dnssd_record *) srvcs,
+    sizeof(srvcs) / sizeof(struct mg_dnssd_record)};
+
+static struct mg_dnssd_record *dnssd_lookup(const struct dnssd_db *db,
+                                            struct mg_str *name) {
+  unsigned int i;
+  for (i = 0; i < db->num; i++) {
+    if (mg_strcasecmp(db->srvcs[i].srvcproto, *name) == 0) break;
+  }
+  if (i == db->num) return NULL;
+  return &db->srvcs[i];
+}
+
+// we can do more complex stuff, this is a simple implementation
+static void fn(struct mg_connection *c, int ev, void *ev_data) {
+  struct mg_mdns_req *req = (struct mg_mdns_req *) ev_data;
+  if (ev == MG_EV_MDNS_PTR) {  // do we serve this service ?
+    struct mg_dnssd_record *r;
+    if (req->is_listing) return;  // TODO(): handle services listing
+    r = dnssd_lookup(&db_, &req->reqname);
+    if (r == NULL) return;
+    req->r = r;
+    req->is_resp = true;
+  } else if (ev == MG_EV_MDNS_SRV || ev == MG_EV_MDNS_TXT) {
+    // Mongoose already checked the host name for us, otherwise we need to do
+    // that ourselves first, then check for service name in db
+    struct mg_dnssd_record *r = dnssd_lookup(&db_, &req->reqname);
+    if (r == NULL) return;
+    req->r = r;
+    req->is_resp = true;
+  } else if (ev == MG_EV_MDNS_A) {  // is this us ?
+    // Mongoose already checked the host name for us, otherwise we need to do
+    // that ourselves
+  }
+  (void) c;
+}
+
+int main(void) {
+  uint32_t response_ip = inet_addr("192.168.69.11");
+  struct mg_mgr mgr;
+  static struct mg_connection *c;
+  mg_log_set(MG_LL_DEBUG);  // Set log level
+  mg_mgr_init(&mgr);        // Initialise event manager
+
+  // Desired name must NOT have any dots in it, nor a domain
+  c = mg_mdns_listen(&mgr, fn, "Mongoose");  // Start mDNS server
+  if (c == NULL) return 1;
+  // if not using our built-in TCP/IP stack, pass the IP address you want to
+  // use as a response, this depends on your underlying TCP/IP stack and number
+  // of interfaces available
+  memcpy(c->data, &response_ip, sizeof(response_ip));
+
+  for (;;) mg_mgr_poll(&mgr, 1000);  // Event loop
+
+  mg_mgr_free(&mgr);
+  return 0;
+}