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59 lines
2.2 KiB
C
59 lines
2.2 KiB
C
// Copyright (c) 2020 Cesanta Software Limited
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// All rights reserved
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//
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// Example captive DNS server (captive portal).
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// 1. Run `make`. This builds and starts a server on port 5533
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// 2. Run `dig -t A www.google.com -4 @localhost -p 5533`
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#include "mongoose.h"
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static const char *s_listen_url = "udp://0.0.0.0:5533";
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// DNS answer section. We response with IP 1.2.3.4 - you can change it
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// in the last 4 bytes of this array
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uint8_t answer[] = {
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0xc0, 0x0c, // Point to the name in the DNS question
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0, 1, // 2 bytes - record type, A
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0, 1, // 2 bytes - address class, INET
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0, 0, 0, 120, // 4 bytes - TTL
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0, 4, // 2 bytes - address length
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1, 2, 3, 4 // 4 bytes - IP address
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};
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static void fn(struct mg_connection *c, int ev, void *ev_data, void *fn_data) {
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if (ev == MG_EV_OPEN) {
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c->is_hexdumping = 1;
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} else if (ev == MG_EV_READ) {
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struct mg_dns_rr rr; // Parse first question, offset 12 is header size
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size_t n = mg_dns_parse_rr(c->recv.buf, c->recv.len, 12, true, &rr);
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MG_INFO(("DNS request parsed, result=%d", (int) n));
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if (n > 0) {
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char buf[512];
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struct mg_dns_header *h = (struct mg_dns_header *) buf;
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memset(buf, 0, sizeof(buf)); // Clear the whole datagram
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h->txnid = ((struct mg_dns_header *) c->recv.buf)->txnid; // Copy tnxid
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h->num_questions = mg_htons(1); // We use only the 1st question
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h->num_answers = mg_htons(1); // And only one answer
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h->flags = mg_htons(0x8400); // Authoritative response
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memcpy(buf + sizeof(*h), c->recv.buf + sizeof(*h), n); // Copy question
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memcpy(buf + sizeof(*h) + n, answer, sizeof(answer)); // And answer
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mg_send(c, buf, 12 + n + sizeof(answer)); // And send it!
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}
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mg_iobuf_del(&c->recv, 0, c->recv.len);
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}
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(void) fn_data;
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(void) ev_data;
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}
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int main(void) {
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struct mg_mgr mgr;
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mg_log_set(MG_LL_DEBUG); // Set log level
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mg_mgr_init(&mgr); // Initialise event manager
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mg_listen(&mgr, s_listen_url, fn, NULL); // Start DNS server
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for (;;) mg_mgr_poll(&mgr, 1000); // Event loop
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mg_mgr_free(&mgr);
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return 0;
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}
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