1 files changed, 244 insertions, 0 deletions

diff --git a/dns.c b/dns.c
new file mode 100644
index 0000000..97ea778
--- /dev/null
+++ b/dns.c
@@ -0,0 +1,244 @@
+#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));
+	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->fd != -1)
+		close (dns->fd);
+	free(dns->domain);
+	free(dns);
+}
+#if __INCLUDE_LEVEL__ == 0
+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