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101 lines
3.1 KiB
101 lines
3.1 KiB
/*
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* nodnsd - DNS server, which rejects all requests.
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*
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* The musl library has the misfeature (IMHO), that the resolver
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* does DNS requests to localhost, if no nameservers are defined.
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* As localhost normally doesn't answer DNS requests, the use of
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* unknown hosts leads to a delay until an error is returned.
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*
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* By starting this server all DNS requests are immediately
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* rejected and a delay is avoided.
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*
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* This program is put into the public domain, use it as you like.
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*/
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#include <stdio.h>
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#include <string.h>
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#include <stdlib.h>
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#include <unistd.h>
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#include <arpa/inet.h>
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#include <sys/socket.h>
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#define BUFLEN 1500 /* length of buffer */
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#define PORT 53 /* DNS port */
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#define DNS_FLAG_OFF 2 /* DNS flag offset */
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#define DNS_FLAG_RESP 0x848f /* DNS response field mask */
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#define DNS_FLAG_NXDOM 0x8003 /* DNS response: non-existent domain */
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#define DNS_FLAG_RD 0x0100 /* DNS recursion desired */
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#define DNS_FLAG_RA 0x0080 /* DNS recursion available */
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static void die(char *s) {
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perror(s);
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exit(1);
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}
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int main(int argc, char *argv[]) {
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struct sockaddr_in6 si_me, si_peer;
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socklen_t si_len;
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unsigned char buf[BUFLEN];
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uint16_t dns_flags;
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int ipv6_only;
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int pid;
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int recv_len;
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int sock;
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/* no command line arguments */
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if (argc > 1) {
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fprintf(stderr, "nodnsd - DNS server, which rejects all requests.\n");
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exit(1);
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}
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/* create an UDP socket */
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if ((sock = socket(PF_INET6, SOCK_DGRAM, IPPROTO_UDP)) < 0)
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die("socket");
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/* protocol / address / port for bind() */
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memset(&si_me, 0, sizeof(si_me));
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si_me.sin6_family = AF_INET6;
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si_me.sin6_port = htons(PORT);
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si_me.sin6_addr = in6addr_any;
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/* bind to both IPv4 and IPv6 */
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ipv6_only = 0;
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if (setsockopt(sock, IPPROTO_IPV6, IPV6_V6ONLY,
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&ipv6_only, sizeof(ipv6_only)))
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die("setsockopt");
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/* bind socket to port */
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if (bind(sock , (struct sockaddr*)&si_me, sizeof(si_me) ) < 0)
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die("bind");
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/* background our process */
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pid = fork();
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if (pid < 0) die("fork");
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if (pid > 0) exit(0);
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freopen("/dev/null", "r", stdin);
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if (setsid() < 0) die("setsid");
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/* keep listening for data */
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while (1) {
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/* try to receive some data, this is a blocking call */
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si_len = sizeof(si_peer);
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recv_len = recvfrom(sock, buf, sizeof(buf), 0,
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(struct sockaddr *)&si_peer, &si_len);
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if (recv_len < 0) die("recvfrom");
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/* set DNS response to non-existent domain */
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dns_flags = (buf[DNS_FLAG_OFF] << 8) | buf[DNS_FLAG_OFF+1];
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dns_flags &= ~DNS_FLAG_RESP; /* clear response bits */
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dns_flags |= DNS_FLAG_NXDOM; /* set response */
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if (dns_flags & DNS_FLAG_RD) dns_flags |= DNS_FLAG_RA;
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buf[DNS_FLAG_OFF] = (dns_flags >> 8) & 0xFF;
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buf[DNS_FLAG_OFF+1] = dns_flags & 0xFF;
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/* now send reply to the client */
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if (sendto(sock, buf, recv_len, 0, (struct sockaddr*) &si_peer, si_len) < 0)
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die("sendto");
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}
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close(sock);
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return 0;
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}
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