Add captive dns server example

examples/captive-dns-server/Makefile

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+PROG ?= example
+CFLAGS ?= -DMG_IO_SIZE=8192 -DMG_ENABLE_LINES
+
+all: $(PROG)
+	$(DEBUGGER) ./$(PROG) $(ARGS)
+
+$(PROG): main.c
+	$(CC) ../../mongoose.c -I../.. -W -Wall $(CFLAGS) -o $(PROG) main.c
+
+clean:
+	rm -rf $(PROG) *.o *.dSYM *.gcov *.gcno *.gcda *.obj *.exe *.ilk *.pdb

examples/captive-dns-server/main.c

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+// Copyright (c) 2020 Cesanta Software Limited
+// All rights reserved
+//
+// Example captive DNS server (captive portal).
+//    1. Run `make`. This builds and starts a server on port 5533
+//    2. Run `dig -t A www.google.com -4 @localhost -p 5533`
+
+#include "mongoose.h"
+
+static const char *s_listen_url = "udp://0.0.0.0:5533";
+
+// DNS answer section. We response with IP 1.2.3.4 - you can change it
+//  in the last 4 bytes of this array
+uint8_t answer[] = {
+    0xc0, 0x0c,          // Point to the name in the DNS question
+    0,    1,             // 2 bytes - record type, A
+    0,    1,             // 2 bytes - address class, INET
+    0,    0,    0, 120,  // 4 bytes - TTL
+    0,    4,             // 2 bytes - address length
+    1,    2,    3, 4     // 4 bytes - IP address
+};
+
+static void fn(struct mg_connection *c, int ev, void *ev_data, void *fn_data) {
+  if (ev == MG_EV_READ) {
+    struct mg_dns_rr rr;  // Parse first question, offset 12 is header size
+    size_t n = mg_dns_parse_rr(c->recv.buf, c->recv.len, 12, true, &rr);
+    if (n > 0) {
+      char buf[12 + n + sizeof(answer)];
+      struct mg_dns_header *h = (struct mg_dns_header *) buf;
+      memset(buf, 0, sizeof(buf));  // Clear the whole datagram
+      h->txnid = ((struct mg_dns_header *) c->recv.buf)->txnid;  // Copy tnxid
+      h->num_questions = mg_htons(1);  // We use only the 1st question
+      h->num_answers = mg_htons(1);    // And only one answer
+      h->flags = mg_htons(0x8400);     // Authoritative response
+      memcpy(buf + sizeof(*h), c->recv.buf + sizeof(*h), n);  // Copy question
+      memcpy(buf + sizeof(*h) + n, answer, sizeof(answer));   // And answer
+      mg_send(c, buf, sizeof(buf));                           // And send it!
+    }
+    mg_iobuf_delete(&c->recv, c->recv.len);
+  }
+  (void) fn_data;
+  (void) ev_data;
+}
+
+int main(void) {
+  struct mg_mgr mgr;
+
+  mg_log_set("3");                          // Set log level
+  mg_mgr_init(&mgr);                        // Initialise event manager
+  mg_listen(&mgr, s_listen_url, fn, NULL);  // Start DNS server
+  for (;;) mg_mgr_poll(&mgr, 1000);         // Event loop
+  mg_mgr_free(&mgr);
+
+  return 0;
+}