Sample dns server written in C with libuv

dns-server.c

/*
 * Licensed under the MIT License <https://opensource.org/licenses/MIT>.
 * Copyright (c) 2017 Matsuz <xiangsong.zeng@gmail.com>
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>

#include "uv.h"

static uint8_t DNSRsp[] = {
    0x00, 0x00, // Transaction ID
    0x80, 0x00, // flags |QR:1|Opcode:0|AA:0|TC:0|RD:0|RA:0|Z:0|Rcode:0|
    0, 1,       // Number of Questions
    0, 1,       // Number of Answer RRs
    0, 0,       // Number of Authority RRs
    0, 0,       // Number of Additional RRs
};
static uint8_t qrecord[] = {
    1, 'g', 2, 'c', 'n', 0, // query name ("g.cn")
    0x00, 0x01,             // query type (A)
    0x00, 0x01,             // query class (IN)
};
static uint8_t arecord[] = {
    0xc0, 0x0c,       // query name
    0x00, 0x01,       // query type (A)
    0x00, 0x01,       // query class (IN)
    0, 0, 0, 0,       // ttl
    0, 4,             // data length
    203, 208, 51, 81, // data (ipv4 address)
};

static uv_loop_t *loop;
static uv_udp_t server;
static struct sockaddr_in addr;

static void alloc_buffer(uv_handle_t *handle, size_t suggested_size, uv_buf_t *buf);
static void on_recv(uv_udp_t *handle, ssize_t nread, const uv_buf_t *buf, const struct sockaddr *addr, unsigned flags);
static void on_send(uv_udp_send_t *req, int status);

int main()
{
    loop = uv_default_loop();

    uv_udp_init(loop, &server);

    /* Bind the UDP handle to an IP address and port. */
    uv_ip4_addr("127.0.0.1", 53, &addr);
    uv_udp_bind(&server, (const struct sockaddr *)&addr, UV_UDP_REUSEADDR);

    /* Prepare for receiving data. */
    uv_udp_recv_start(&server, alloc_buffer, on_recv);

    uv_run(loop, UV_RUN_DEFAULT);

    return 0;
}

static void
alloc_buffer(uv_handle_t *handle,
             size_t suggested_size,
             uv_buf_t *buf)
{
    buf->base = malloc(suggested_size);
    buf->len = suggested_size;
}

static void
on_recv(uv_udp_t *handle,
        ssize_t nread,
        const uv_buf_t *rcvbuf,
        const struct sockaddr *addr,
        unsigned flags)
{
    if (nread <= 0)
        return;

    // fprintf(stderr, "Size: %lu, Addr: %p\n", nread, rcvbuf->base);

    uv_udp_send_t *req = malloc(sizeof(*req));
    uv_buf_t sndbuf; /* Buffer for send data. */

    /* #### construct response data start #### */
    sndbuf.len = sizeof(DNSRsp) + sizeof(qrecord) + sizeof(arecord);
    sndbuf.base = malloc(sndbuf.len);
    memset(sndbuf.base, 0, sndbuf.len); // fill 0 to buf

    memcpy(sndbuf.base, DNSRsp, sizeof(DNSRsp));                                      // DNS Header
    memcpy(sndbuf.base + sizeof(DNSRsp), qrecord, sizeof(qrecord));                   // Query Record
    memcpy(sndbuf.base + sizeof(DNSRsp) + sizeof(qrecord), arecord, sizeof(arecord)); // Answer Record

    memcpy(sndbuf.base, rcvbuf->base, 2); // Transaction ID
    /* #### construct response data end #### */

    fprintf(stderr, "UV_UDP_SEND_STATUS: %d\n", uv_udp_send(req, handle, &sndbuf, 1, addr, on_send));

    // printf("free  :%lu %p\n", rcvbuf->len, rcvbuf->base);
    free(rcvbuf->base);
    free(sndbuf.base);
}

static void
on_send(uv_udp_send_t *req, int status)
{
    // assert(status == 0);
    fprintf(stderr, "Status: %d\n", status);
    free(req);
}