mongoose/examples/captive-dns-server/main.c

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2020-12-28 13:25:29 +08:00
// 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;
}