recvdns.c by vincesafe based on Paul Krzyzanowski's work

/* recvdns.c version alpha 1
This code is under CC BY-NC-SA 3.0 license.
For more information, please visit the link below:
https://creativecommons.org/licenses/by-nc-sa/3.0/legalcode

Ce code est sous licence CC BY-NC-SA 3.0.
Plus d'informations à l'adresse suivante :
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By vincesafe,
based on "demo-udp-03: udp-recv: a simple udp server" by Paul Krzyzanowski
http://vincesafe.fr/
https://twitter.com/vincesafe

*/

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <netdb.h>
#include <sys/socket.h>
#include <arpa/inet.h>

#define SERVICE_PORT 53
#define BUFSIZE 2048

int
main(int argc, char **argv)
{
    struct sockaddr_in myaddr;  /* our address */
    struct sockaddr_in remaddr; /* remote address */
    socklen_t addrlen = sizeof(remaddr);        /* length of addresses */
    int recvlen;            /* # bytes received */
    int fd;             /* our socket */
    unsigned char buf[BUFSIZE]; /* receive buffer */
    int i;

    /* create a UDP socket */

    if ((fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
        perror("cannot create socket\n");
        return 0;
    }

    /* bind the socket to any valid IP address and a specific port (set by the client) */

    memset((char *)&myaddr, 0, sizeof(myaddr));
    myaddr.sin_family = AF_INET;
    myaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    myaddr.sin_port = htons(SERVICE_PORT);

    if (bind(fd, (struct sockaddr *)&myaddr, sizeof(myaddr)) < 0) {
        perror("bind failed");
        return 0;
    }

    /* file to store domain */
    FILE *f = NULL;
    f = fopen("req", "wb");
    if(f == NULL) return -1;

    /* now loop, receiving data and printing what we received */
    for (;;) {
        printf("waiting on port %d\n", SERVICE_PORT);
        recvlen = recvfrom(fd, buf, BUFSIZE, 0, (struct sockaddr *)&remaddr, &addrlen);
        printf("received %d bytes\n", recvlen);
        if (recvlen > 0) {
            buf[recvlen] = 0;
            printf("received message: \"%s\"\n", buf);
            for(i = 0; i < recvlen; i++)
                fputc(buf[i], f);
            break;
        }
    }
    /* never exits */
}