#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <signal.h>
#include <unistd.h>
#include "domain2name.c" // this is from http://git.sijanec.eu/sijanec/dnsfind
enum type {
A = 1,
Ns,
Md,
Mf,
Cname,
Soa,
Mb,
M,
Mr,
Null,
Wks,
Ptr,
Hinfo,
Minfo,
Mx,
Txt
};
enum class {
In = 1,
Cs,
Ch,
He,
Any = 255
};
#define RESPONSE (1 << 15)
#define QUESTION (0 << 15)
#define QUERY (0 << 11)
#define IQUERY (1 << 11)
#define STATUS (1 << 12)
#define AA (1 << 10)
#define TC (1 << 9)
#define RD (1 << 8)
#define RA (1 << 7)
#define SUCCESS (0 << 0)
#define FORMAT_ERROR (1 << 0)
#define SERVFAIL (1 << 1)
#define NXDOMAIN (FORMAT_ERROR | SERVFAIL)
#define NI (1 << 2)
#define FORBIDDEN (NXDOMAIN | FORMAT_ERROR)
struct header {
uint16_t xid __attribute__((packed));
uint16_t flags __attribute__((packed));
uint16_t qdcount __attribute__((packed));
uint16_t ancount __attribute__((packed));
uint16_t nscount __attribute__((packed));
uint16_t arcount __attribute__((packed));
char data[]; /* ignored for char[] */
} __attribute__((packed));
struct rr { /* name is omitted, first byte of struct is first byte of type */
uint16_t type __attribute__((packed));
uint16_t class __attribute__((packed));
uint32_t ttl __attribute__((packed));
uint16_t len __attribute__((packed));
char data[]; /* ignored for char[] */
} __attribute__((packed));
struct question {
uint16_t type __attribute__((packed));
uint16_t class __attribute__((packed));
} __attribute__((packed));
enum dns_loglevel {
DNS_DEBUG,
DNS_INFO,
DNS_WARN,
DNS_ERROR
};
typedef void (* dns_log_handler) (void * const, const enum dns_loglevel, const char * const, const char * const);
struct dns {
int fd;
char * domain; // this is as it appears in the packet
dns_log_handler log_handler;
void * log_userdata;
struct sockaddr_in sockaddr;
};
typedef struct dns dns;
static void dns_default_log_handler (void * const u __attribute__((unused)),
const enum dns_loglevel l, const char * a, const char * m) {
char * n = "unspec";
switch (l) {
case DNS_DEBUG:
n = "DEBUG";
break;
case DNS_INFO:
n = "INFO";
break;
case DNS_WARN:
n = "WARN";
break;
case DNS_ERROR:
n = "ERROR";
break;
}
fprintf(stderr, "%s %s %s\n", n, a, m);
}
struct dns * dns_init (void) {
struct dns * dns = calloc(1, sizeof(struct dns));
if (!dns)
return NULL;
dns->fd = -1;
dns->domain = strdup(" call dns_set_domain to set the domain");
dns->domain[0] = strlen(dns->domain)-1;
dns->log_handler = dns_default_log_handler;
dns->sockaddr.sin_family = AF_INET;
dns->sockaddr.sin_port = htons(53);
dns->sockaddr.sin_addr.s_addr = INADDR_ANY;
return dns;
}
static void dns_set_domain (struct dns * dns, const char * domain) { // static functions are
int required = domain2name_len(domain, strlen(domain)); // visible inside the same
if (required <= 0) // translation unit - they
return; // are visible across
free(dns->domain); // included files
domain2name((dns->domain = malloc(required)), domain, strlen(domain));
}
static void dns_set_log_handler (struct dns * dns, dns_log_handler log_handler) {
if (!log_handler)
log_handler = dns_default_log_handler;
dns->log_handler = log_handler;
}
static void dns_set_log_userdata (struct dns * dns, void * log_userdata) {
dns->log_userdata = log_userdata;
}
static void dns_set_port (struct dns * dns, int port) {
dns->sockaddr.sin_port = htons(port);
}
static void dns_set_ip (struct dns * dns, const char * ip) {
if (!ip) {
dns->sockaddr.sin_addr.s_addr = INADDR_ANY;
return;
}
inet_aton(ip, &dns->sockaddr.sin_addr);
}
static void dns_run_once (struct dns * dns) {
#define BUFLEN 4096
char buf[BUFLEN];
if (dns->fd == -1) {
if ((dns->fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1) {
sprintf(buf, "socket failed with %s", strerror(errno));
dns->log_handler(dns->log_userdata, DNS_ERROR, "dns", buf);
return;
}
int ž = 1;
if (setsockopt(dns->fd, SOL_SOCKET, SO_BROADCAST, &ž, sizeof(ž)) == -1) {
sprintf(buf, "setsockopt failed with %s", strerror(errno));
dns->log_handler(dns->log_userdata, DNS_ERROR, "dns", buf);
close(dns->fd);
dns->fd = -1;
return;
}
if (bind(dns->fd, (struct sockaddr *) &dns->sockaddr, sizeof(struct sockaddr))) {
sprintf(buf, "bind failed with %s", strerror(errno));
dns->log_handler(dns->log_userdata, DNS_ERROR, "dns", buf);
close(dns->fd);
dns->fd = -1;
return;
}
}
struct sockaddr_in sender;
socklen_t sendl = sizeof sender;
int size = recvfrom(dns->fd, buf, 65535, MSG_DONTWAIT, (struct sockaddr *) &sender, &sendl);
if (size == -1) {
if (errno != EWOULDBLOCK) {
sprintf(buf, "recvfrom failed with %s", strerror(errno));
dns->log_handler(dns->log_userdata, DNS_ERROR, "dns", buf);
close(dns->fd);
dns->fd = -1;
return;
}
return;
}
struct header * header = (struct header *) buf; // time for some overwriting
header->flags = htons(RESPONSE | QUERY | AA | SUCCESS);
header->ancount = htons(1); // we keep question number intact, as we send all qs back
header->nscount = 0;
header->arcount = 0;
buf[BUFLEN-1] = '\0'; // strstr and strlen on untrusted data!
char * ouranswerisat = memmem(buf, size, "in-addr", strlen("in-addr"));
if (!ouranswerisat) {
dns->log_handler(dns->log_userdata, DNS_INFO, "dns",
"received request without 'in-addr' string ... weird.");
return;
}
ouranswerisat += 17;
struct rr * rr = (struct rr *) (ouranswerisat + strlen(header->data) + 1);
if (ouranswerisat + strlen(header->data) + 1 + sizeof(struct rr)
+ strlen(dns->domain) + 1 >= buf + BUFLEN) {
dns->log_handler(dns->log_userdata, DNS_WARN, "dns", "sent packet would be to big");
return;
}
strcpy(ouranswerisat, header->data);
rr->type = htons(Ptr);
rr->class = htons(In);
rr->ttl = 0;
rr->len = htons(strlen(dns->domain)+1);
strcpy(rr->data, dns->domain);
int len = rr->data - buf + strlen(dns->domain) + 1;
if (sendto(dns->fd, buf, len, 0, (struct sockaddr *) &sender, sizeof(sender)) == -1) {
sprintf(buf, "sendto failed with %s", strerror(errno));
dns->log_handler(dns->log_userdata, DNS_ERROR, "dns", buf);
close(dns->fd);
dns->fd = -1;
return;
}
char dst[INET_ADDRSTRLEN];
const char * resp = inet_ntop(AF_INET, &sender.sin_addr, dst, INET_ADDRSTRLEN);
sprintf(buf, "successfully sent DNS reply to %s", resp ? resp : "[inet_ntop failed]");
dns->log_handler(dns->log_userdata, DNS_ERROR, "dns", buf);
}
static void dns_free (struct dns * dns) {
if (!dns)
return;
if (dns->fd != -1)
close (dns->fd);
free(dns->domain);
free(dns);
}
#if IX_DNS_MAIN
int shouldexit = 0;
void handler (int signal __attribute__((unused))) {
shouldexit++;
}
int main (int argc, char ** argv) {
if (argc != 1+1) {
fprintf(stderr, "usage: %s respond.to.ptr.with.this.domain.\n", argv[0]);
return 1;
}
signal(SIGINT, handler);
signal(SIGTERM, handler);
dns * dns = dns_init();
dns_set_domain(dns, argv[1]);
while (!shouldexit) {
dns_run_once(dns);
}
dns_free(dns);
}
#endif
