mongoose/docs/dns/server_example.md

title
DNS server example

To create a DNS server, follow this pattern:

1. Create a listening UDP connection by calling mg_bind() or mg_bind_opt()
2. Call mg_set_protocol_dns() for that listening connection. That attaches a built-in DNS event handler which parses incoming data and triggers DNS-specific events.
3. Create an event handler function.

Here is an example of a simpe DNS server. It is a captive DNS server, meaning that it replies with the same IP address on all queries, regardless of what exactly host name is being resolved. Error checking is omitted for the sake of clarity:

#include "mongoose.h"
#include <stdio.h>

static const char *s_listening_addr = "udp://:5353";

static void ev_handler(struct mg_connection *nc, int ev, void *ev_data) {
  struct mg_dns_message *msg;
  struct mg_dns_resource_record *rr;
  struct mg_dns_reply reply;
  int i;

  switch (ev) {
    case MG_DNS_MESSAGE: {
      struct mbuf reply_buf;
      mbuf_init(&reply_buf, 512);
      msg = (struct mg_dns_message *) ev_data;
      reply = mg_dns_create_reply(&reply_buf, msg);

      for (i = 0; i < msg->num_questions; i++) {
        char rname[256];
        rr = &msg->questions[i];
        mg_dns_uncompress_name(msg, &rr->name, rname, sizeof(rname) - 1);
        fprintf(stdout, "Q type %d name %s\n", rr->rtype, rname);
        if (rr->rtype == MG_DNS_A_RECORD) {
          in_addr_t addr = inet_addr("127.0.0.1");
          mg_dns_reply_record(&reply, rr, NULL, rr->rtype, 10, &addr, 4);
        }
      }

      mg_dns_send_reply(nc, &reply);
      nc->flags |= MG_F_SEND_AND_CLOSE;
      mbuf_free(&reply_buf);
      break;
    }
  }
}

int main(int argc, char *argv[]) {
  struct mg_mgr mgr;
  struct mg_connection *c;

  mg_mgr_init(&mgr, NULL);
  c = mg_bind(&mgr, s_listening_addr, ev_handler);
  mg_set_protocol_dns(c);

  for (;;) {
    mg_mgr_poll(&mgr, 1000);
  }
  mg_mgr_free(&mgr);

  return 0;
}

See full Captive DNS server example.