nodnsd/nodnsd.c

/*
 * nodnsd - DNS server, which rejects all requests.
 *
 * The musl library has the misfeature (IMHO), that the resolver
 * does DNS requests to localhost, if no nameservers are defined.
 * As localhost normally doesn't answer DNS requests, the use of
 * unknown hosts leads to a delay until an error is returned.
 *
 * By starting this server all DNS requests are immediately
 * rejected and a delay is avoided.
 *
 * This program is put into the public domain, use it as you like.
 */
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>

#define BUFLEN		1500	/* length of buffer */
#define PORT		53	/* DNS port */

#define DNS_FLAG_OFF	2	/* DNS flag offset */
#define DNS_FLAG_RESP	0x848f	/* DNS response field mask */
#define DNS_FLAG_NXDOM	0x8003	/* DNS response: non-existent domain */
#define DNS_FLAG_RD	0x0100	/* DNS recursion desired */
#define DNS_FLAG_RA	0x0080	/* DNS recursion available */

static void die(char *s) {
    perror(s);
    exit(1);
}

int main(int argc, char *argv[]) {
    struct sockaddr_in6 si_me, si_peer;
    socklen_t si_len;
    unsigned char buf[BUFLEN];
    uint16_t dns_flags;
    int ipv6_only;
    int pid;
    int recv_len;
    int sock;

    /* no command line arguments */
    if (argc > 1) {
        fprintf(stderr, "nodnsd - DNS server, which rejects all requests.\n");
        exit(1);
    }

    /* create an UDP socket */
    if ((sock = socket(PF_INET6, SOCK_DGRAM, IPPROTO_UDP)) < 0)
        die("socket");

    /* protocol / address / port for bind() */
    memset(&si_me, 0, sizeof(si_me));
    si_me.sin6_family = AF_INET6;
    si_me.sin6_port = htons(PORT);
    si_me.sin6_addr = in6addr_any;

    /* bind to both IPv4 and IPv6 */
    ipv6_only = 0;
    if (setsockopt(sock, IPPROTO_IPV6, IPV6_V6ONLY,
                   &ipv6_only, sizeof(ipv6_only)))
        die("setsockopt");

    /* bind socket to port */
    if  (bind(sock , (struct sockaddr*)&si_me, sizeof(si_me) ) < 0)
        die("bind");

    /* background our process */
    pid = fork();
    if (pid < 0) die("fork");
    if (pid > 0) exit(0);
    freopen("/dev/null", "r", stdin);
    if (setsid() < 0) die("setsid");

    /* keep listening for data */
    while (1) {
        /* try to receive some data, this is a blocking call */
        si_len = sizeof(si_peer);
        recv_len = recvfrom(sock, buf, sizeof(buf), 0,
                            (struct sockaddr *)&si_peer, &si_len);
        if (recv_len < 0) die("recvfrom");

        /* set DNS response to non-existent domain */
        dns_flags = (buf[DNS_FLAG_OFF] << 8) | buf[DNS_FLAG_OFF+1];
        dns_flags &= ~DNS_FLAG_RESP;		/* clear response bits */
        dns_flags |= DNS_FLAG_NXDOM;		/* set response */
        if (dns_flags & DNS_FLAG_RD) dns_flags |= DNS_FLAG_RA;
        buf[DNS_FLAG_OFF]   = (dns_flags >> 8) & 0xFF;
        buf[DNS_FLAG_OFF+1] = dns_flags & 0xFF;

        /* now send reply to the client */
        if (sendto(sock, buf, recv_len, 0, (struct sockaddr*) &si_peer, si_len) < 0)
            die("sendto");
    }

    close(sock);
    return 0;
}