btpd/libevent/evdns.c

/* $Id: evdns.c 6979 2006-08-04 18:31:13Z nickm $ */

/* The original version of this module was written by Adam Langley; for
 * a history of modifications, check out the subversion logs.
 *
 * When editing this module, try to keep it re-mergeable by Adam.  Don't
 * reformat the whitespace, add Tor dependencies, or so on.
 *
 * TODO:
 *   - Support IPv6 and PTR records.
 *   - Replace all externally visible magic numbers with #defined constants.
 *   - Write doccumentation for APIs of all external functions.
 */

/* Async DNS Library
 * Adam Langley <agl@imperialviolet.org>
 * http://www.imperialviolet.org/eventdns.html
 * Public Domain code
 *
 * This software is Public Domain. To view a copy of the public domain dedication,
 * visit http://creativecommons.org/licenses/publicdomain/ or send a letter to
 * Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.
 *
 * I ask and expect, but do not require, that all derivative works contain an
 * attribution similar to:
 * 	Parts developed by Adam Langley <agl@imperialviolet.org>
 *
 * You may wish to replace the word "Parts" with something else depending on
 * the amount of original code.
 *
 * (Derivative works does not include programs which link against, run or include
 * the source verbatim in their source distributions)
 *
 * Version: 0.1b
 */

#include <sys/types.h>
#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

//#define NDEBUG

#ifndef DNS_USE_CPU_CLOCK_FOR_ID
#ifndef DNS_USE_GETTIMEOFDAY_FOR_ID
#ifndef DNS_USE_OPENSSL_FOR_ID
#error Must configure at least one id generation method.
#error Please see the documentation.
#endif
#endif
#endif

// #define _POSIX_C_SOURCE 200507
#define _GNU_SOURCE

#ifdef DNS_USE_CPU_CLOCK_FOR_ID
#ifdef DNS_USE_OPENSSL_FOR_ID
#error Multiple id options selected
#endif
#ifdef DNS_USE_GETTIMEOFDAY_FOR_ID
#error Multiple id options selected
#endif
#include <time.h>
#endif

#ifdef DNS_USE_OPENSSL_FOR_ID
#ifdef DNS_USE_GETTIMEOFDAY_FOR_ID
#error Multiple id options selected
#endif
#include <openssl/rand.h>
#endif

#define _FORTIFY_SOURCE 3

#include <string.h>
#include <fcntl.h>
#include <sys/time.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <assert.h>
#include <unistd.h>
#include <limits.h>
#include <sys/stat.h>
#include <ctype.h>
#include <stdio.h>
#include <stdarg.h>

#include "evdns.h"
#include "log.h"
#ifdef WIN32
#include <windows.h>
#include <winsock2.h>
#include <iphlpapi.h>
#else
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#endif

#define EVDNS_LOG_DEBUG 0
#define EVDNS_LOG_WARN 1

#ifndef HOST_NAME_MAX
#define HOST_NAME_MAX 255
#endif

#ifndef NDEBUG
#include <stdio.h>
#endif

#undef MIN
#define MIN(a,b) ((a)<(b)?(a):(b))

#ifdef __USE_ISOC99B
// libevent doesn't work without this
typedef uint8_t u_char;
typedef unsigned int uint;
#endif
#include <event.h>

#define u64 uint64_t
#define u32 uint32_t
#define u16 uint16_t
#define u8  uint8_t

#define MAX_ADDRS 4  // maximum number of addresses from a single packet
// which we bother recording

#define TYPE_A         1
#define TYPE_CNAME     5
#define TYPE_PTR      12
#define TYPE_AAAA     28

#define CLASS_INET 1

struct request {
	u8 *request;  // the dns packet data
	unsigned int request_len;
	int reissue_count;
	int tx_count;  // the number of times that this packet has been sent
	unsigned int request_type; // TYPE_PTR or TYPE_A
	void *user_pointer;  // the pointer given to us for this request
	evdns_callback_type user_callback;
	struct nameserver *ns;  // the server which we last sent it

	// elements used by the searching code
	int search_index;
	struct search_state *search_state;
	char *search_origname;  // needs to be free()ed
	int search_flags;

	// these objects are kept in a circular list
	struct request *next, *prev;

	struct event timeout_event;

	u16 trans_id;  // the transaction id
	char request_appended;  // true if the request pointer is data which follows this struct
	char transmit_me;  // needs to be transmitted
};

struct reply {
	unsigned int type;
	unsigned int have_answer;
	union {
		struct {
			u32 addrcount;
			u32 addresses[MAX_ADDRS];
		} a;
		struct {
			char name[HOST_NAME_MAX];
		} ptr;
	} data;
};

struct nameserver {
	int socket;  // a connected UDP socket
	u32 address;
	int failed_times;  // number of times which we have given this server a chance
	int timedout;  // number of times in a row a request has timed out
	struct event event;
	// these objects are kept in a circular list
	struct nameserver *next, *prev;
	struct event timeout_event;  // used to keep the timeout for
				     // when we next probe this server.
				     // Valid if state == 0
	char state;  // zero if we think that this server is down
	char choaked;  // true if we have an EAGAIN from this server's socket
	char write_waiting;  // true if we are waiting for EV_WRITE events
};

static struct request *req_head = NULL, *req_waiting_head = NULL;
static struct nameserver *server_head = NULL;

// The number of good nameservers that we have
static int global_good_nameservers = 0;

// inflight requests are contained in the req_head list
// and are actually going out across the network
static int global_requests_inflight = 0;
// requests which aren't inflight are in the waiting list
// and are counted here
static int global_requests_waiting = 0;

static int global_max_requests_inflight = 64;

static struct timeval global_timeout = {3, 0};  // 3 seconds
static int global_max_reissues = 1;  // a reissue occurs when we get some errors from the server
static int global_max_retransmits = 3;  // number of times we'll retransmit a request which timed out
// number of timeouts in a row before we consider this server to be down
static int global_max_nameserver_timeout = 3;

// These are the timeout values for nameservers. If we find a nameserver is down
// we try to probe it at intervals as given below. Values are in seconds.
static const struct timeval global_nameserver_timeouts[] = {{10, 0}, {60, 0}, {300, 0}, {900, 0}, {3600, 0}};
static const int global_nameserver_timeouts_length = sizeof(global_nameserver_timeouts)/sizeof(struct timeval);

const char *const evdns_error_strings[] = {"no error", "The name server was unable to interpret the query", "The name server suffered an internal error", "The requested domain name does not exist", "The name server refused to reply to the request"};

static struct nameserver *nameserver_pick(void);
static void evdns_request_insert(struct request *req, struct request **head);
static void nameserver_ready_callback(int fd, short events, void *arg);
static int evdns_transmit(void);
static int evdns_request_transmit(struct request *req);
static void nameserver_send_probe(struct nameserver *const ns);
static void search_request_finished(struct request *const);
static int search_try_next(struct request *const req);
static int search_request_new(int type, const char *const name, int flags, evdns_callback_type user_callback, void *user_arg);
static void evdns_requests_pump_waiting_queue(void);
static u16 transaction_id_pick(void);
static struct request *request_new(int type, const char *name, int flags, evdns_callback_type callback, void *ptr);
static void request_submit(struct request *req);

#ifdef MS_WINDOWS
static int
last_error(int sock)
{
	int optval, optvallen=sizeof(optval);
	int err = WSAGetLastError();
	if (err == WSAEWOULDBLOCK && sock >= 0) {
		if (getsockopt(sock, SOL_SOCKET, SO_ERROR, (void*)&optval,
			       &optvallen))
			return err;
		if (optval)
			return optval;
	}
	return err;

}
static int
error_is_eagain(int err)
{
	return err == EAGAIN || err == WSAEWOULDBLOCK;
}
static int
inet_aton(const char *c, struct in_addr *addr)
{
	uint32_t r;
	if (strcmp(c, "255.255.255.255") == 0) {
		addr->s_addr = 0xffffffffu;
	} else {
		r = inet_addr(c);
		if (r == INADDR_NONE)
			return 0;
		addr->s_addr = r;
	}
	return 1;
}
#define CLOSE_SOCKET(x) closesocket(x)
#else
#define last_error(sock) (errno)
#define error_is_eagain(err) ((err) == EAGAIN)
#define CLOSE_SOCKET(x) close(x)
#endif

#define ISSPACE(c) isspace((int)(unsigned char)(c))
#define ISDIGIT(c) isdigit((int)(unsigned char)(c))

#ifndef NDEBUG
static const char *
debug_ntoa(u32 address)
{
	static char buf[32];
	u32 a = ntohl(address);
	sprintf(buf, "%d.%d.%d.%d",
                      (int)(u8)((a>>24)&0xff),
                      (int)(u8)((a>>16)&0xff),
                      (int)(u8)((a>>8 )&0xff),
  		      (int)(u8)((a    )&0xff));
	return buf;
}
#endif

static evdns_debug_log_fn_type evdns_log_fn = NULL;

void
evdns_set_log_fn(evdns_debug_log_fn_type fn)
{
  evdns_log_fn = fn;
}

#ifdef __GNUC__
#define EVDNS_LOG_CHECK  __attribute__ ((format(printf, 2, 3)))
#else
#define EVDNS_LOG_CHECK
#endif

static void _evdns_log(int warn, const char *fmt, ...) EVDNS_LOG_CHECK;
static void
_evdns_log(int warn, const char *fmt, ...)
{
  va_list args;
  static char buf[512];
  if (!evdns_log_fn)
    return;
  va_start(args,fmt);
#ifdef MS_WINDOWS
  _vsnprintf(buf, sizeof(buf), fmt, args);
#else
  vsnprintf(buf, sizeof(buf), fmt, args);
#endif
  buf[sizeof(buf)-1] = '\0';
  evdns_log_fn(warn, buf);
  va_end(args);
}

#define log _evdns_log

// This walks the list of inflight requests to find the
// one with a matching transaction id. Returns NULL on
// failure
static struct request *
request_find_from_trans_id(u16 trans_id) {
	struct request *req = req_head, *const started_at = req_head;

	if (req) {
		do {
			if (req->trans_id == trans_id) return req;
			req = req->next;
		} while (req != started_at);
	}

	return NULL;
}

// a libevent callback function which is called when a nameserver
// has gone down and we want to test if it has came back to life yet
static void
nameserver_prod_callback(int fd, short events, void *arg) {
	struct nameserver *const ns = (struct nameserver *) arg;
        (void)fd;
        (void)events;

	nameserver_send_probe(ns);
}

// a libevent callback which is called when a nameserver probe (to see if
// it has come back to life) times out. We increment the count of failed_times
// and wait longer to send the next probe packet.
static void
nameserver_probe_failed(struct nameserver *const ns) {
	const struct timeval * timeout;
	(void) evtimer_del(&ns->timeout_event);
	if (ns->state == 1) {
		// This can happen if the nameserver acts in a way which makes us mark
		// it as bad and then starts sending good replies.
		return;
	}

	timeout =
	  &global_nameserver_timeouts[MIN(ns->failed_times,
					  global_nameserver_timeouts_length - 1)];
	ns->failed_times++;

	evtimer_set(&ns->timeout_event, nameserver_prod_callback, ns);
	if (evtimer_add(&ns->timeout_event, (struct timeval *) timeout) < 0) {
          log(EVDNS_LOG_WARN,
              "Error from libevent when adding timer event for %s",
              debug_ntoa(ns->address));
          // ???? Do more?
        }
}

// called when a nameserver has been deemed to have failed. For example, too
// many packets have timed out etc
static void
nameserver_failed(struct nameserver *const ns, const char *msg) {
	struct request *req, *started_at;
	// if this nameserver has already been marked as failed
	// then don't do anything
	if (!ns->state) return;

	log(EVDNS_LOG_WARN, "Nameserver %s has failed: %s",
            debug_ntoa(ns->address), msg);
	global_good_nameservers--;
	assert(global_good_nameservers >= 0);
	if (global_good_nameservers == 0) {
		log(EVDNS_LOG_WARN, "All nameservers have failed");
	}

	ns->state = 0;
	ns->failed_times = 1;

	evtimer_set(&ns->timeout_event, nameserver_prod_callback, ns);
	if (evtimer_add(&ns->timeout_event, (struct timeval *) &global_nameserver_timeouts[0]) < 0) {
		log(EVDNS_LOG_WARN,
		    "Error from libevent when adding timer event for %s",
		    debug_ntoa(ns->address));
		// ???? Do more?
        }

	// walk the list of inflight requests to see if any can be reassigned to
	// a different server. Requests in the waiting queue don't have a
	// nameserver assigned yet

	// if we don't have *any* good nameservers then there's no point
	// trying to reassign requests to one
	if (!global_good_nameservers) return;

	req = req_head;
	started_at = req_head;
	if (req) {
		do {
			if (req->tx_count == 0 && req->ns == ns) {
				// still waiting to go out, can be moved
				// to another server
				req->ns = nameserver_pick();
			}
			req = req->next;
		} while (req != started_at);
	}
}

static void
nameserver_up(struct nameserver *const ns) {
	if (ns->state) return;
	log(EVDNS_LOG_WARN, "Nameserver %s is back up",
	    debug_ntoa(ns->address));
	evtimer_del(&ns->timeout_event);
	ns->state = 1;
	ns->failed_times = 0;
	global_good_nameservers++;
}

static void
request_trans_id_set(struct request *const req, const u16 trans_id) {
	req->trans_id = trans_id;
	*((u16 *) req->request) = htons(trans_id);
}

// Called to remove a request from a list and dealloc it.
// head is a pointer to the head of the list it should be
// removed from or NULL if the request isn't in a list.
static void
request_finished(struct request *const req, struct request **head) {
	if (head) {
		if (req->next == req) {
			// only item in the list
			*head = NULL;
		} else {
			req->next->prev = req->prev;
			req->prev->next = req->next;
			if (*head == req) *head = req->next;
		}
	}

	log(EVDNS_LOG_DEBUG, "Removing timeout for request %lx",
	    (unsigned long) req);
	evtimer_del(&req->timeout_event);

	search_request_finished(req);
	global_requests_inflight--;

	if (!req->request_appended) {
		// need to free the request data on it's own
		free(req->request);
	} else {
		// the request data is appended onto the header
		// so everything gets free()ed when we:
	}

	free(req);

	evdns_requests_pump_waiting_queue();
}

// This is called when a server returns a funny error code.
// We try the request again with another server.
//
// return:
//   0 ok
//   1 failed/reissue is pointless
static int
request_reissue(struct request *req) {
	const struct nameserver *const last_ns = req->ns;
	// the last nameserver should have been marked as failing
	// by the caller of this function, therefore pick will try
	// not to return it
	req->ns = nameserver_pick();
	if (req->ns == last_ns) {
		// ... but pick did return it
		// not a lot of point in trying again with the
		// same server
		return 1;
	}

	req->reissue_count++;
	req->tx_count = 0;
	req->transmit_me = 1;

	return 0;
}

// this function looks for space on the inflight queue and promotes
// requests from the waiting queue if it can.
static void
evdns_requests_pump_waiting_queue(void) {
	while (global_requests_inflight < global_max_requests_inflight &&
	    global_requests_waiting) {
		struct request *req;
		// move a request from the waiting queue to the inflight queue
		assert(req_waiting_head);
		if (req_waiting_head->next == req_waiting_head) {
			// only one item in the queue
			req = req_waiting_head;
			req_waiting_head = NULL;
		} else {
			req = req_waiting_head;
			req->next->prev = req->prev;
			req->prev->next = req->next;
			req_waiting_head = req->next;
		}

		global_requests_waiting--;
		global_requests_inflight++;

		req->ns = nameserver_pick();
		request_trans_id_set(req, transaction_id_pick());

		evdns_request_insert(req, &req_head);
		evdns_request_transmit(req);
		evdns_transmit();
	}
}

static void
reply_callback(struct request *const req, u32 ttl, u32 err, struct reply *reply) {
	switch (req->request_type) {
	case TYPE_A:
		if (reply)
			req->user_callback(DNS_ERR_NONE, DNS_IPv4_A,
							   reply->data.a.addrcount, ttl,
						 reply->data.a.addresses,
							   req->user_pointer);
		else
			req->user_callback(err, 0, 0, 0, NULL, req->user_pointer);
		return;
	case TYPE_PTR:
		if (reply) {
			char *name = reply->data.ptr.name;
			req->user_callback(DNS_ERR_NONE, DNS_PTR, 1, ttl,
							   &name, req->user_pointer);
		} else {
			req->user_callback(err, 0, 0, 0, NULL,
							   req->user_pointer);
		}
		return;
	}
	assert(0);
}

// this processes a parsed reply packet
static void
reply_handle(struct request *const req, u16 flags, u32 ttl, struct reply *reply) {
	int error;
	static const int error_codes[] = {DNS_ERR_FORMAT, DNS_ERR_SERVERFAILED, DNS_ERR_NOTEXIST, DNS_ERR_NOTIMPL, DNS_ERR_REFUSED};

	if (flags & 0x020f || !reply || !reply->have_answer) {
		// there was an error
		if (flags & 0x0200) {
			error = DNS_ERR_TRUNCATED;
		} else {
			u16 error_code = (flags & 0x000f) - 1;
			if (error_code > 4) {
				error = DNS_ERR_UNKNOWN;
			} else {
				error = error_codes[error_code];
			}
		}

		switch(error) {
		case DNS_ERR_SERVERFAILED:
		case DNS_ERR_NOTIMPL:
		case DNS_ERR_REFUSED:
			// we regard these errors as marking a bad nameserver
			if (req->reissue_count < global_max_reissues) {
				char msg[64];
				snprintf(msg, sizeof(msg), "Bad response %d (%s)",
					 error, evdns_err_to_string(error));
				nameserver_failed(req->ns, msg);
				if (!request_reissue(req)) return;
			}
			break;
		default:
			// we got a good reply from the nameserver
			nameserver_up(req->ns);
		}

		if (req->search_state && req->request_type != TYPE_PTR) {
			// if we have a list of domains to search in, try the next one
			if (!search_try_next(req)) {
				// a new request was issued so this request is finished and
				// the user callback will be made when that request (or a
				// child of it) finishes.
				request_finished(req, &req_head);
				return;
			}
		}

		// all else failed. Pass the failure up
		reply_callback(req, 0, error, NULL);
		request_finished(req, &req_head);
	} else {
		// all ok, tell the user
		reply_callback(req, ttl, 0, reply);
		nameserver_up(req->ns);
		request_finished(req, &req_head);
	}
}

static inline int
name_parse(u8 *packet, int length, int *idx, char *name_out, int name_out_len) {
	int name_end = -1;
	int j = *idx;
#define GET32(x) do { if (j + 4 > length) return -1; memcpy(&_t32, packet + j, 4); j += 4; x = ntohl(_t32); } while(0);
#define GET16(x) do { if (j + 2 > length) return -1; memcpy(&_t, packet + j, 2); j += 2; x = ntohs(_t); } while(0);
#define GET8(x) do { if (j >= length) return -1; x = packet[j++]; } while(0);

	char *cp = name_out;
	const char *const end = name_out + name_out_len;

	// Normally, names are a series of length prefixed strings terminated
	// with a length of 0 (the lengths are u8's < 63).
	// However, the length can start with a pair of 1 bits and that
	// means that the next 14 bits are a pointer within the current
	// packet.

	for(;;) {
		u8 label_len;
		if (j >= length) return -1;
		GET8(label_len);
		if (!label_len) break;
		if (label_len & 0xc0) {
			u8 ptr_low;
			GET8(ptr_low);
			if (name_end < 0) name_end = j;
			j = (((int)label_len & 0x3f) << 8) + ptr_low;
			if (j < 0 || j >= length) return -1;
			continue;
		}
		if (label_len > 63) return -1;
		if (cp != name_out) {
			if (cp + 1 >= end) return -1;
			*cp++ = '.';
		}
		if (cp + label_len >= end) return -1;
		memcpy(cp, packet + j, label_len);
		cp += label_len;
		j += label_len;
	}
	if (cp >= end) return -1;
	*cp = '\0';
	if (name_end < 0)
		*idx = j;
	else
		*idx = name_end;
	return 0;
}

// parses a raw packet from the wire
static int
reply_parse(u8 *packet, int length) {
	int j = 0;  // index into packet
	u16 _t;  // used by the macros
	u32 _t32;  // used by the macros
	char tmp_name[256]; // used by the macros

	u16 trans_id, flags, questions, answers, authority, additional, datalength;
	u32 ttl, ttl_r = 0xffffffff;
	struct reply reply;
	struct request *req;
	unsigned int i;

	GET16(trans_id);
	GET16(flags);
	GET16(questions);
	GET16(answers);
	GET16(authority);
	GET16(additional);

	req = request_find_from_trans_id(trans_id);
	if (!req) return -1;
	// XXXX should the other return points also call reply_handle? -NM

	memset(&reply, 0, sizeof(reply));

	if (!(flags & 0x8000)) return -1;  // must be an answer
	if (flags & 0x020f) {
		// there was an error
		reply_handle(req, flags, 0, NULL);
		return -1;
	}
	// if (!answers) return;  // must have an answer of some form

	// This macro skips a name in the DNS reply.
#define SKIP_NAME \
	do { tmp_name[0] = '\0';					\
		if (name_parse(packet, length, &j, tmp_name, sizeof(tmp_name))<0) \
			return -1;					\
	} while(0);

	reply.type = req->request_type;

	// skip over each question in the reply
	for (i = 0; i < questions; ++i) {
		// the question looks like
		//   <label:name><u16:type><u16:class>
		SKIP_NAME;
		j += 4;
		if (j >= length) return -1;
	}

	// now we have the answer section which looks like
	// <label:name><u16:type><u16:class><u32:ttl><u16:len><data...>

	for (i = 0; i < answers; ++i) {
		u16 type, class;

		SKIP_NAME;
		GET16(type);
		GET16(class);
		GET32(ttl);
		GET16(datalength);

		if (type == TYPE_A && class == CLASS_INET) {
			int addrcount, addrtocopy;
			if (req->request_type != TYPE_A) {
				j += datalength; continue;
			}
                        // XXXX do something sane with malformed A answers.
			addrcount = datalength >> 2;
			addrtocopy = MIN(MAX_ADDRS - reply.data.a.addrcount, (unsigned)addrcount);

			ttl_r = MIN(ttl_r, ttl);
			// we only bother with the first four addresses.
			if (j + 4*addrtocopy > length) return -1;
			memcpy(&reply.data.a.addresses[reply.data.a.addrcount],
				   packet + j, 4*addrtocopy);
			j += 4*addrtocopy;
			reply.data.a.addrcount += addrtocopy;
			reply.have_answer = 1;
			if (reply.data.a.addrcount == MAX_ADDRS) break;
		} else if (type == TYPE_PTR && class == CLASS_INET) {
			if (req->request_type != TYPE_PTR) {
				j += datalength; continue;
			}
			if (name_parse(packet, length, &j, reply.data.ptr.name,
						   sizeof(reply.data.ptr.name))<0)
				return -1;
			reply.have_answer = 1;
			break;
		} else {
			// skip over any other type of resource
			j += datalength;
		}
	}

	reply_handle(req, flags, ttl_r, &reply);
	return 0;
#undef SKIP_NAME
#undef GET32
#undef GET16
#undef GET8
}

// Try to choose a strong transaction id which isn't already in flight
static u16
transaction_id_pick(void) {
	for (;;) {
		const struct request *req = req_head, *started_at;
#ifdef DNS_USE_CPU_CLOCK_FOR_ID
		struct timespec ts;
		u16 trans_id;
		if (clock_gettime(CLOCK_MONOTONIC, &ts))
			event_err(1, "clock_gettime");
		trans_id = ts.tv_nsec & 0xffff;
#endif

#ifdef DNS_USE_GETTIMEOFDAY_FOR_ID
		struct timeval tv;
		u16 trans_id;
		gettimeofday(&tv, NULL);
		trans_id = tv.tv_usec & 0xffff;
#endif

#ifdef DNS_USE_OPENSSL_FOR_ID
		u16 trans_id;
		if (RAND_pseudo_bytes((u8 *) &trans_id, 2) == -1) {
			/* // in the case that the RAND call fails we back
			// down to using gettimeofday.
			struct timeval tv;
			gettimeofday(&tv, NULL);
			trans_id = tv.tv_usec & 0xffff; */
			abort();
		}
#endif

		if (trans_id == 0xffff) continue;
		// now check to see if that id is already inflight
		req = started_at = req_head;
		if (req) {
			do {
				if (req->trans_id == trans_id) break;
				req = req->next;
			} while (req != started_at);
		}
		// we didn't find it, so this is a good id
		if (req == started_at) return trans_id;
	}
}

// choose a namesever to use. This function will try to ignore
// nameservers which we think are down and load balance across the rest
// by updating the server_head global each time.
static struct nameserver *
nameserver_pick(void) {
	struct nameserver *started_at = server_head, *picked;
	if (!server_head) return NULL;

	// if we don't have any good nameservers then there's no
	// point in trying to find one.
	if (!global_good_nameservers) {
		server_head = server_head->next;
		return server_head;
	}

	// remember that nameservers are in a circular list
	for (;;) {
		if (server_head->state) {
			// we think this server is currently good
			picked = server_head;
			server_head = server_head->next;
			return picked;
		}

		server_head = server_head->next;
		if (server_head == started_at) {
			// all the nameservers seem to be down
			// so we just return this one and hope for the
			// best
			assert(global_good_nameservers == 0);
			picked = server_head;
			server_head = server_head->next;
			return picked;
		}
	}
}

// this is called when a namesever socket is ready for reading
static void
nameserver_read(struct nameserver *ns) {
	u8 packet[1500];

	for (;;) {
          	const int r = recv(ns->socket, packet, sizeof(packet), 0);
		if (r < 0) {
			int err = last_error(ns->socket);
			if (error_is_eagain(err)) return;
			nameserver_failed(ns, strerror(err));
			return;
		}
		reply_parse(packet, r);
	}
}

// set if we are waiting for the ability to write to this server.
// if waiting is true then we ask libevent for EV_WRITE events, otherwise
// we stop these events.
static void
nameserver_write_waiting(struct nameserver *ns, char waiting) {
	if (ns->write_waiting == waiting) return;

	ns->write_waiting = waiting;
	(void) event_del(&ns->event);
	event_set(&ns->event, ns->socket, EV_READ | (waiting ? EV_WRITE : 0) | EV_PERSIST,
			nameserver_ready_callback, ns);
	if (event_add(&ns->event, NULL) < 0) {
          log(EVDNS_LOG_WARN, "Error from libevent when adding event for %s",
              debug_ntoa(ns->address));
          // ???? Do more?
        }
}

// a callback function. Called by libevent when the kernel says that
// a nameserver socket is ready for writing or reading
static void
nameserver_ready_callback(int fd, short events, void *arg) {
	struct nameserver *ns = (struct nameserver *) arg;
        (void)fd;

	if (events & EV_WRITE) {
		ns->choaked = 0;
		if (!evdns_transmit()) {
			nameserver_write_waiting(ns, 0);
		}
	}
	if (events & EV_READ) {
		nameserver_read(ns);
	}
}

// Converts a string to a length-prefixed set of DNS labels.
// @buf must be strlen(name)+2 or longer. name and buf must
// not overlap. name_len should be the length of name
//
// Input: abc.def
// Output: <3>abc<3>def<0>
//
// Returns the length of the data. negative on error
//   -1  label was > 63 bytes
//   -2  name was > 255 bytes
static int
dnsname_to_labels(u8 *const buf, const char *name, const int name_len) {
	const char *end = name + name_len;
	int j = 0;  // current offset into buf

	if (name_len > 255) return -2;

	for (;;) {
		const char *const start = name;
		name = strchr(name, '.');
		if (!name) {
			const unsigned int label_len = end - start;
			if (label_len > 63) return -1;
			buf[j++] = label_len;

			memcpy(buf + j, start, end - start);
			j += end - start;
			break;
		} else {
			// append length of the label.
			const unsigned int label_len = name - start;
			if (label_len > 63) return -1;
			buf[j++] = label_len;

			memcpy(buf + j, start, name - start);
			j += name - start;
			// hop over the '.'
			name++;
		}
	}

	// the labels must be terminated by a 0.
	// It's possible that the name ended in a .
	// in which case the zero is already there
	if (!j || buf[j-1]) buf[j++] = 0;
	return j;
}

// Finds the length of a dns request for a DNS name of the given
// length. The actual request may be smaller than the value returned
// here
static int
evdns_request_len(const int name_len) {
	return 96 + // length of the DNS standard header
		name_len + 2 +
		4;  // space for the resource type
}

// build a dns request packet into buf. buf should be at least as long
// as evdns_request_len told you it should be.
//
// Returns the amount of space used. Negative on error.
static int
evdns_request_data_build(const char *const name, const int name_len,
    const u16 trans_id, const u16 type, const u16 class,
    u8 *const buf, size_t buf_len) {
	int j = 0;  // current offset into buf
	u16 _t;  // used by the macros
	u8 *labels;
	int labels_len;

#define APPEND16(x) do { \
  if (j + 2 > buf_len) \
    return (-1); \
  _t = htons(x); \
  memcpy(buf + j, &_t, 2); j += 2; \
} while(0)
	
	APPEND16(trans_id);
	APPEND16(0x0100);  // standard query, recusion needed
	APPEND16(1);  // one question
	APPEND16(0);  // no answers
	APPEND16(0);  // no authority
	APPEND16(0);  // no additional

	labels = (u8 *) malloc(name_len + 2);
        if (labels == NULL)
		return (-1);
	labels_len = dnsname_to_labels(labels, name, name_len);
	if (labels_len < 0) {
		free(labels);
		return (labels_len);
	}
	if ((size_t)(j + labels_len) > buf_len) {
		free(labels);
		return (-1);
	}
	memcpy(buf + j, labels, labels_len);
	j += labels_len;
	free(labels);
	
	APPEND16(type);
	APPEND16(class);
#undef APPEND16

	return (j);
}

// this is a libevent callback function which is called when a request
// has timed out.
static void
evdns_request_timeout_callback(int fd, short events, void *arg) {
	struct request *const req = (struct request *) arg;
        (void) fd;
        (void) events;

	log(EVDNS_LOG_DEBUG, "Request %lx timed out", (unsigned long) arg);

	req->ns->timedout++;
	if (req->ns->timedout > global_max_nameserver_timeout) {
		nameserver_failed(req->ns, "request timed out.");
	}

	(void) evtimer_del(&req->timeout_event);
	if (req->tx_count >= global_max_retransmits) {
		// this request has failed
		reply_callback(req, 0, DNS_ERR_TIMEOUT, NULL);
		request_finished(req, &req_head);
	} else {
		// retransmit it
		evdns_request_transmit(req);
	}
}

// try to send a request to a given server.
//
// return:
//   0 ok
//   1 temporary failure
//   2 other failure
static int
evdns_request_transmit_to(struct request *req, struct nameserver *server) {
	const int r = send(server->socket, req->request, req->request_len, 0);
	if (r < 0) {
		int err = last_error(server->socket);
		if (error_is_eagain(err)) return 1;
		nameserver_failed(req->ns, strerror(err));
		return 2;
	} else if (r != (int)req->request_len) {
		return 1;  // short write
	} else {
		return 0;
	}
}

// try to send a request, updating the fields of the request
// as needed
//
// return:
//   0 ok
//   1 failed
static int
evdns_request_transmit(struct request *req) {
	int retcode = 0, r;

	// if we fail to send this packet then this flag marks it
	// for evdns_transmit
	req->transmit_me = 1;
	if (req->trans_id == 0xffff) abort();

	if (req->ns->choaked) {
		// don't bother trying to write to a socket
		// which we have had EAGAIN from
		return 1;
	}

	r = evdns_request_transmit_to(req, req->ns);
	switch (r) {
	case 1:
		// temp failure
		req->ns->choaked = 1;
		nameserver_write_waiting(req->ns, 1);
		return 1;
	case 2:
		// failed in some other way
		retcode = 1;
		// fall through
	default:
		// all ok
		log(EVDNS_LOG_DEBUG,
		    "Setting timeout for request %lx", (unsigned long) req);
		evtimer_set(&req->timeout_event, evdns_request_timeout_callback, req);
		if (evtimer_add(&req->timeout_event, &global_timeout) < 0) {
                  log(EVDNS_LOG_WARN,
		      "Error from libevent when adding timer for request %lx",
                      (unsigned long) req);
                  // ???? Do more?
                }
		req->tx_count++;
		req->transmit_me = 0;
		return retcode;
	}
}

static void
nameserver_probe_callback(int result, char type, int count, int ttl, void *addresses, void *arg) {
	struct nameserver *const ns = (struct nameserver *) arg;
        (void) type;
        (void) count;
        (void) ttl;
        (void) addresses;

	if (result == DNS_ERR_NONE || result == DNS_ERR_NOTEXIST) {
		// this is a good reply
		nameserver_up(ns);
	} else nameserver_probe_failed(ns);
}

static void
nameserver_send_probe(struct nameserver *const ns) {
	struct request *req;
	// here we need to send a probe to a given nameserver
	// in the hope that it is up now.

  	log(EVDNS_LOG_DEBUG, "Sending probe to %s", debug_ntoa(ns->address));

	req = request_new(TYPE_A, "www.google.com", DNS_QUERY_NO_SEARCH, nameserver_probe_callback, ns);
        if (!req) return;
	// we force this into the inflight queue no matter what
	request_trans_id_set(req, transaction_id_pick());
	req->ns = ns;
	request_submit(req);
}

// returns:
//   0 didn't try to transmit anything
//   1 tried to transmit something
static int
evdns_transmit(void) {
	char did_try_to_transmit = 0;

	if (req_head) {
		struct request *const started_at = req_head, *req = req_head;
		// first transmit all the requests which are currently waiting
		do {
			if (req->transmit_me) {
				did_try_to_transmit = 1;
				evdns_request_transmit(req);
			}

			req = req->next;
		} while (req != started_at);
	}

	return did_try_to_transmit;
}

// exported function
int
evdns_count_nameservers(void)
{
	const struct nameserver *server = server_head;
	int n = 0;
	if (!server)
		return 0;
	do {
		++n;
		server = server->next;
	} while (server != server_head);
	return n;
}

// exported function
int
evdns_clear_nameservers_and_suspend(void)
{
	struct nameserver *server = server_head, *started_at = server_head;
	struct request *req = req_head, *req_started_at = req_head;

	if (!server)
		return 0;
	while (1) {
		struct nameserver *next = server->next;
		(void) event_del(&server->event);
		(void) evtimer_del(&server->timeout_event);
		if (server->socket >= 0)
			CLOSE_SOCKET(server->socket);
		free(server);
		if (next == started_at)
			break;
		server = next;
	}
	server_head = NULL;
	global_good_nameservers = 0;

	while (req) {
		struct request *next = req->next;
		req->tx_count = req->reissue_count = 0;
		req->ns = NULL;
		// ???? What to do about searches?
		(void) evtimer_del(&req->timeout_event);
		req->trans_id = 0;
		req->transmit_me = 0;

		global_requests_waiting++;
		evdns_request_insert(req, &req_waiting_head);
		/* We want to insert these suspended elements at the front of
		 * the waiting queue, since they were pending before any of
		 * the waiting entries were added.  This is a circular list,
		 * so we can just shift the start back by one.*/
		req_waiting_head = req_waiting_head->prev;

		if (next == req_started_at)
			break;
		req = next;
	}
	req_head = NULL;
	global_requests_inflight = 0;

	return 0;
}


// exported function
int
evdns_resume(void)
{
	evdns_requests_pump_waiting_queue();
	return 0;
}

// exported function
int
evdns_nameserver_add(unsigned long int address) {
	// first check to see if we already have this nameserver

	const struct nameserver *server = server_head, *const started_at = server_head;
	struct nameserver *ns;
	struct sockaddr_in sin;
	int err = 0;
	if (server) {
		do {
			if (server->address == address) return 3;
			server = server->next;
		} while (server != started_at);
	}

	ns = (struct nameserver *) malloc(sizeof(struct nameserver));
        if (!ns) return -1;

	memset(ns, 0, sizeof(struct nameserver));

	ns->socket = socket(PF_INET, SOCK_DGRAM, 0);
	if (ns->socket < 0) { err = 1; goto out1; }
#ifdef MS_WINDOWS
        {
		u_long nonblocking = 1;
		ioctlsocket(ns->socket, FIONBIO, &nonblocking);
	}
#else
        fcntl(ns->socket, F_SETFL, O_NONBLOCK);
#endif
	sin.sin_addr.s_addr = address;
	sin.sin_port = htons(53);
	sin.sin_family = AF_INET;
	if (connect(ns->socket, (struct sockaddr *) &sin, sizeof(sin)) != 0) {
		err = 2;
		goto out2;
	}

	ns->address = address;
	ns->state = 1;
	event_set(&ns->event, ns->socket, EV_READ | EV_PERSIST, nameserver_ready_callback, ns);
	if (event_add(&ns->event, NULL) < 0) {
          err = 2;
          goto out2;
        }

	log(EVDNS_LOG_DEBUG, "Added nameserver %s", debug_ntoa(address));

	// insert this nameserver into the list of them
	if (!server_head) {
		ns->next = ns->prev = ns;
		server_head = ns;
	} else {
		ns->next = server_head->next;
		ns->prev = server_head;
		server_head->next = ns;
		if (server_head->prev == server_head) {
			server_head->prev = ns;
		}
	}

	global_good_nameservers++;

	return 0;

out2:
	CLOSE_SOCKET(ns->socket);
out1:
	free(ns);
	log(EVDNS_LOG_WARN, "Unable to add nameserver %s: error %d", debug_ntoa(address), err);
	return err;
}

// exported function
int
evdns_nameserver_ip_add(const char *ip_as_string) {
	struct in_addr ina;
	if (!inet_aton(ip_as_string, &ina)) return 4;
	return evdns_nameserver_add(ina.s_addr);
}

// insert into the tail of the queue
static void
evdns_request_insert(struct request *req, struct request **head) {
	if (!*head) {
		*head = req;
		req->next = req->prev = req;
		return;
	}

	req->prev = (*head)->prev;
	req->prev->next = req;
	req->next = *head;
	(*head)->prev = req;
}

static int
string_num_dots(const char *s) {
	int count = 0;
	while ((s = strchr(s, '.'))) {
		s++;
		count++;
	}
	return count;
}

static struct request *
request_new(int type, const char *name, int flags,
    evdns_callback_type callback, void *user_ptr) {
	const char issuing_now =
	    (global_requests_inflight < global_max_requests_inflight) ? 1 : 0;

	const int name_len = strlen(name);
	const int request_max_len = evdns_request_len(name_len);
	const u16 trans_id = issuing_now ? transaction_id_pick() : 0xffff;
	// the request data is alloced in a single block with the header
	struct request *const req =
	    (struct request *) malloc(sizeof(struct request) + request_max_len);
	int rlen;
        (void) flags;

        if (!req) return NULL;
	memset(req, 0, sizeof(struct request));

	// request data lives just after the header
	req->request = ((u8 *) req) + sizeof(struct request);
	// denotes that the request data shouldn't be free()ed
	req->request_appended = 1;
	rlen = evdns_request_data_build(name, name_len, trans_id,
	    type, CLASS_INET, req->request, request_max_len);
	if (rlen < 0)
		goto err1;
	req->request_len = rlen;
	req->trans_id = trans_id;
	req->tx_count = 0;
	req->request_type = type;
	req->user_pointer = user_ptr;
	req->user_callback = callback;
	req->ns = issuing_now ? nameserver_pick() : NULL;
	req->next = req->prev = NULL;

	return req;
err1:
	free(req);
	return NULL;
}

static void
request_submit(struct request *const req) {
	if (req->ns) {
		// if it has a nameserver assigned then this is going
		// straight into the inflight queue
		evdns_request_insert(req, &req_head);
		global_requests_inflight++;
		evdns_request_transmit(req);
	} else {
		evdns_request_insert(req, &req_waiting_head);
		global_requests_waiting++;
	}
}

// exported function
int evdns_resolve_ipv4(const char *name, int flags,
    evdns_callback_type callback, void *ptr) {
	log(EVDNS_LOG_DEBUG, "Resolve requested for %s", name);
	if (flags & DNS_QUERY_NO_SEARCH) {
		struct request *const req =
			request_new(TYPE_A, name, flags, callback, ptr);
		if (req == NULL)
			return (1);
		request_submit(req);
		return (0);
	} else {
		return (search_request_new(TYPE_A, name, flags, callback, ptr));
	}
}

int evdns_resolve_reverse(struct in_addr *in, int flags, evdns_callback_type callback, void *ptr) {
	char buf[32];
	struct request *req;
	u32 a;
	assert(in);
	a = ntohl(in->s_addr);
	sprintf(buf, "%d.%d.%d.%d.in-addr.arpa",
			(int)(u8)((a	)&0xff),
			(int)(u8)((a>>8 )&0xff),
			(int)(u8)((a>>16)&0xff),
			(int)(u8)((a>>24)&0xff));
	log(EVDNS_LOG_DEBUG, "Resolve requested for %s (reverse)", buf);
	req = request_new(TYPE_PTR, buf, flags, callback, ptr);
	if (!req) return 1;
	request_submit(req);
	return 0;
}

/////////////////////////////////////////////////////////////////////
// Search support
//
// the libc resolver has support for searching a number of domains
// to find a name. If nothing else then it takes the single domain
// from the gethostname() call.
//
// It can also be configured via the domain and search options in a
// resolv.conf.
//
// The ndots option controls how many dots it takes for the resolver
// to decide that a name is non-local and so try a raw lookup first.

struct search_domain {
	int len;
	struct search_domain *next;
	// the text string is appended to this structure
};

struct search_state {
	int refcount;
	int ndots;
	int num_domains;
	struct search_domain *head;
};

static struct search_state *global_search_state = NULL;

static void
search_state_decref(struct search_state *const state) {
	if (!state) return;
	state->refcount--;
	if (!state->refcount) {
		struct search_domain *next, *dom;
		for (dom = state->head; dom; dom = next) {
			next = dom->next;
			free(dom);
		}
		free(state);
	}
}

static struct search_state *
search_state_new(void) {
	struct search_state *state = (struct search_state *) malloc(sizeof(struct search_state));
        if (!state) return NULL;
	memset(state, 0, sizeof(struct search_state));
	state->refcount = 1;
	state->ndots = 1;

	return state;
}

static void
search_postfix_clear(void) {
	search_state_decref(global_search_state);

	global_search_state = search_state_new();
}

// exported function
void
evdns_search_clear(void) {
	search_postfix_clear();
}

static void
search_postfix_add(const char *domain) {
	int domain_len;
	struct search_domain *sdomain;
	while (domain[0] == '.') domain++;
	domain_len = strlen(domain);

	if (!global_search_state) global_search_state = search_state_new();
        if (!global_search_state) return;
	global_search_state->num_domains++;

	sdomain = (struct search_domain *) malloc(sizeof(struct search_domain) + domain_len);
        if (!sdomain) return;
	memcpy( ((u8 *) sdomain) + sizeof(struct search_domain), domain, domain_len);
	sdomain->next = global_search_state->head;
	sdomain->len = domain_len;

	global_search_state->head = sdomain;
}

// reverse the order of members in the postfix list. This is needed because,
// when parsing resolv.conf we push elements in the wrong order
static void
search_reverse(void) {
	struct search_domain *cur, *prev = NULL, *next;
	cur = global_search_state->head;
	while (cur) {
		next = cur->next;
		cur->next = prev;
		prev = cur;
		cur = next;
	}

	global_search_state->head = prev;
}

// exported function
void
evdns_search_add(const char *domain) {
	search_postfix_add(domain);
}

// exported function
void
evdns_search_ndots_set(const int ndots) {
	if (!global_search_state) global_search_state = search_state_new();
        if (!global_search_state) return;
	global_search_state->ndots = ndots;
}

static void
search_set_from_hostname(void) {
	char hostname[HOST_NAME_MAX + 1], *domainname;

	search_postfix_clear();
	if (gethostname(hostname, sizeof(hostname))) return;
	domainname = strchr(hostname, '.');
	if (!domainname) return;
	search_postfix_add(domainname);
}

// warning: returns malloced string
static char *
search_make_new(const struct search_state *const state, int n, const char *const base_name) {
	const int base_len = strlen(base_name);
	const char need_to_append_dot = base_name[base_len - 1] == '.' ? 0 : 1;
	struct search_domain *dom;

	for (dom = state->head; dom; dom = dom->next) {
		if (!n--) {
			// this is the postfix we want
			// the actual postfix string is kept at the end of the structure
			const u8 *const postfix = ((u8 *) dom) + sizeof(struct search_domain);
			const int postfix_len = dom->len;
			char *const newname = (char *) malloc(base_len + need_to_append_dot + postfix_len + 1);
                        if (!newname) return NULL;
			memcpy(newname, base_name, base_len);
			if (need_to_append_dot) newname[base_len] = '.';
			memcpy(newname + base_len + need_to_append_dot, postfix, postfix_len);
			newname[base_len + need_to_append_dot + postfix_len] = 0;
			return newname;
		}
	}

	// we ran off the end of the list and still didn't find the requested string
	abort();
}

static int
search_request_new(int type, const char *const name, int flags, evdns_callback_type user_callback, void *user_arg) {
	assert(type == TYPE_A);
	if ( ((flags & DNS_QUERY_NO_SEARCH) == 0) &&
	     global_search_state &&
		 global_search_state->num_domains) {
		// we have some domains to search
		struct request *req;
		if (string_num_dots(name) >= global_search_state->ndots) {
			req = request_new(type, name, flags, user_callback, user_arg);
			if (!req) return 1;
			req->search_index = -1;
		} else {
			char *const new_name = search_make_new(global_search_state, 0, name);
                        if (!new_name) return 1;
			req = request_new(type, new_name, flags, user_callback, user_arg);
			free(new_name);
			if (!req) return 1;
			req->search_index = 0;
		}
		req->search_origname = strdup(name);
		req->search_state = global_search_state;
		req->search_flags = flags;
		global_search_state->refcount++;
		request_submit(req);
		return 0;
	} else {
		struct request *const req = request_new(type, name, flags, user_callback, user_arg);
		if (!req) return 1;
		request_submit(req);
		return 0;
	}
}

// this is called when a request has failed to find a name. We need to check
// if it is part of a search and, if so, try the next name in the list
// returns:
//   0 another request has been submitted
//   1 no more requests needed
static int
search_try_next(struct request *const req) {
	if (req->search_state) {
		// it is part of a search
		char *new_name;
		struct request *newreq;
		req->search_index++;
		if (req->search_index >= req->search_state->num_domains) {
			// no more postfixes to try, however we may need to try
			// this name without a postfix
			if (string_num_dots(req->search_origname) < req->search_state->ndots) {
				// yep, we need to try it raw
				struct request *const newreq = request_new(req->request_type, req->search_origname, req->search_flags, req->user_callback, req->user_pointer);
				log(EVDNS_LOG_DEBUG, "Search: trying raw query %s", req->search_origname);
				if (newreq) {
					request_submit(newreq);
					return 0;
				}
			}
			return 1;
		}

		new_name = search_make_new(req->search_state, req->search_index, req->search_origname);
                if (!new_name) return 1;
		log(EVDNS_LOG_DEBUG, "Search: now trying %s (%d)", new_name, req->search_index);
		newreq = request_new(req->request_type, new_name, req->search_flags, req->user_callback, req->user_pointer);
		free(new_name);
		if (!newreq) return 1;
		newreq->search_origname = req->search_origname;
		req->search_origname = NULL;
		newreq->search_state = req->search_state;
		newreq->search_flags = req->search_flags;
		newreq->search_index = req->search_index;
		newreq->search_state->refcount++;
		request_submit(newreq);
		return 0;
	}
	return 1;
}

static void
search_request_finished(struct request *const req) {
	if (req->search_state) {
		search_state_decref(req->search_state);
		req->search_state = NULL;
	}
	if (req->search_origname) {
		free(req->search_origname);
		req->search_origname = NULL;
	}
}

/////////////////////////////////////////////////////////////////////
// Parsing resolv.conf files

static void
evdns_resolv_set_defaults(int flags) {
	// if the file isn't found then we assume a local resolver
	if (flags & DNS_OPTION_SEARCH) search_set_from_hostname();
	if (flags & DNS_OPTION_NAMESERVERS) evdns_nameserver_ip_add("127.0.0.1");
}

#ifndef HAVE_STRTOK_R
static char *
strtok_r(char *s, const char *delim, char **state) {
	return strtok(s, delim);
}
#endif

// helper version of atoi which returns -1 on error
static int
strtoint(const char *const str) {
	char *endptr;
	const int r = strtol(str, &endptr, 10);
	if (*endptr) return -1;
	return r;
}

static void
resolv_conf_parse_line(char *const start, int flags) {
	char *strtok_state;
	static const char *const delims = " \t";
#define NEXT_TOKEN strtok_r(NULL, delims, &strtok_state)

	char *const first_token = strtok_r(start, delims, &strtok_state);
	if (!first_token) return;

	if (!strcmp(first_token, "nameserver")) {
		const char *const nameserver = NEXT_TOKEN;
		struct in_addr ina;

		if (inet_aton(nameserver, &ina)) {
			// address is valid
			evdns_nameserver_add(ina.s_addr);
		}
	} else if (!strcmp(first_token, "domain") && (flags & DNS_OPTION_SEARCH)) {
		const char *const domain = NEXT_TOKEN;
		if (domain) {
			search_postfix_clear();
			search_postfix_add(domain);
		}
	} else if (!strcmp(first_token, "search") && (flags & DNS_OPTION_SEARCH)) {
		const char *domain;
		search_postfix_clear();

		while ((domain = NEXT_TOKEN)) {
			search_postfix_add(domain);
		}
		search_reverse();
	} else if (!strcmp(first_token, "options")) {
		const char *option;

		while ((option = NEXT_TOKEN)) {
			if (!strncmp(option, "ndots:", 6)) {
				const int ndots = strtoint(&option[6]);
				if (ndots == -1) continue;
				if (!(flags & DNS_OPTION_SEARCH)) continue;
				log(EVDNS_LOG_DEBUG, "Setting ndots to %d", ndots);
				if (!global_search_state) global_search_state = search_state_new();
                                if (!global_search_state) return;
				global_search_state->ndots = ndots;
			} else if (!strncmp(option, "timeout:", 8)) {
				const int timeout = strtoint(&option[8]);
				if (timeout == -1) continue;
				if (!(flags & DNS_OPTION_MISC)) continue;
				log(EVDNS_LOG_DEBUG, "Setting timeout to %d", timeout);
				global_timeout.tv_sec = timeout;
			} else if (!strncmp(option, "attempts:", 9)) {
				int retries = strtoint(&option[9]);
				if (retries == -1) continue;
				if (retries > 255) retries = 255;
				if (!(flags & DNS_OPTION_MISC)) continue;
				log(EVDNS_LOG_DEBUG, "Setting retries to %d", retries);
				global_max_retransmits = retries;
			}
		}
	}
#undef NEXT_TOKEN
}

// exported function
// returns:
//   0 no errors
//   1 failed to open file
//   2 failed to stat file
//   3 file too large
//   4 out of memory
//   5 short read from file
int
evdns_resolv_conf_parse(int flags, const char *const filename) {
	struct stat st;
	int fd;
	u8 *resolv;
	char *start;
	int err = 0;

	log(EVDNS_LOG_DEBUG, "Parsing resolv.conf file %s", filename);

	fd = open(filename, O_RDONLY);
	if (fd < 0) {
		evdns_resolv_set_defaults(flags);
		return 1;
	}

	if (fstat(fd, &st)) { err = 2; goto out1; }
	if (!st.st_size) {
		evdns_resolv_set_defaults(flags);
		err = 0;
		goto out1;
	}
	if (st.st_size > 65535) { err = 3; goto out1; }  // no resolv.conf should be any bigger

	resolv = (u8 *) malloc(st.st_size + 1);
	if (!resolv) { err = 4; goto out1; }

	if (read(fd, resolv, st.st_size) != st.st_size) { err = 5; goto out2; }
	resolv[st.st_size] = 0;  // we malloced an extra byte

	start = (char *) resolv;
	for (;;) {
		char *const newline = strchr(start, '\n');
		if (!newline) {
			resolv_conf_parse_line(start, flags);
			break;
		} else {
			*newline = 0;
			resolv_conf_parse_line(start, flags);
			start = newline + 1;
		}
	}

	if (!server_head && (flags & DNS_OPTION_NAMESERVERS)) {
		// no nameservers were configured.
		evdns_nameserver_ip_add("127.0.0.1");
	}
	if (flags & DNS_OPTION_SEARCH && (!global_search_state || global_search_state->num_domains == 0)) {
		search_set_from_hostname();
	}

out2:
	free(resolv);
out1:
	close(fd);
	return err;
}

#ifdef MS_WINDOWS
// Add multiple nameservers from a space-or-comma-separated list.
static int
evdns_nameserver_ip_add_line(const char *ips) {
	const char *addr;
	char *buf;
	int r;
	while (*ips) {
		while (ISSPACE(*ips) || *ips == ',' || *ips == '\t')
			++ips;
		addr = ips;
		while (ISDIGIT(*ips) || *ips == '.')
			++ips;
		buf = malloc(ips-addr+1);
		if (!buf) return 4;
		memcpy(buf, addr, ips-addr);
		buf[ips-addr] = '\0';
		r = evdns_nameserver_ip_add(buf);
		free(buf);
		if (r) return r;
	}
	return 0;
}

typedef DWORD(WINAPI *GetNetworkParams_fn_t)(FIXED_INFO *, DWORD*);

// Use the windows GetNetworkParams interface in iphlpapi.dll to
// figure out what our nameservers are.
static int
load_nameservers_with_getnetworkparams(void)
{
	// Based on MSDN examples and inspection of  c-ares code.
	FIXED_INFO *fixed;
	HMODULE handle = 0;
	ULONG size = sizeof(FIXED_INFO);
	void *buf = NULL;
	int status = 0, r, added_any;
	IP_ADDR_STRING *ns;
	GetNetworkParams_fn_t fn;

	if (!(handle = LoadLibrary("iphlpapi.dll"))) {
		log(EVDNS_LOG_WARN, "Could not open iphlpapi.dll");
		status = -1;
		goto done;
	}
	if (!(fn = (GetNetworkParams_fn_t) GetProcAddress(handle, "GetNetworkParams"))) {
		log(EVDNS_LOG_WARN, "Could not get address of function.");
		status = -1;
		goto done;
	}

	buf = malloc(size);
	if (!buf) { status = 4; goto done; }
	fixed = buf;
	r = fn(fixed, &size);
	if (r != ERROR_SUCCESS && r != ERROR_BUFFER_OVERFLOW) {
		status = -1;
		goto done;
	}
	if (r != ERROR_SUCCESS) {
		free(buf);
		buf = malloc(size);
		if (!buf) { status = 4; goto done; }
		fixed = buf;
		r = fn(fixed, &size);
		if (r != ERROR_SUCCESS) {
			log(EVDNS_LOG_DEBUG, "fn() failed.");
			status = -1;
			goto done;
		}
	}

	assert(fixed);
	added_any = 0;
	ns = &(fixed->DnsServerList);
	while (ns) {
		r = evdns_nameserver_ip_add_line(ns->IpAddress.String);
		if (r) {
			log(EVDNS_LOG_DEBUG,"Could not add nameserver %s to list,error: %d",
				(ns->IpAddress.String),(int)GetLastError());
			status = r;
			goto done;
		} else {
			log(EVDNS_LOG_DEBUG,"Succesfully added %s as nameserver",ns->IpAddress.String);
		}

		added_any++;
		ns = ns->Next;
	}

	if (!added_any) {
		log(EVDNS_LOG_DEBUG, "No nameservers added.");
		status = -1;
	}

 done:
	if (buf)
		free(buf);
	if (handle)
		FreeLibrary(handle);
	return status;
}

static int
config_nameserver_from_reg_key(HKEY key, const char *subkey)
{
	char *buf;
	DWORD bufsz = 0, type = 0;
	int status = 0;

	if (RegQueryValueEx(key, subkey, 0, &type, NULL, &bufsz)
	    != ERROR_MORE_DATA)
		return -1;
	if (!(buf = malloc(bufsz)))
		return -1;

	if (RegQueryValueEx(key, subkey, 0, &type, (LPBYTE)buf, &bufsz)
	    == ERROR_SUCCESS && bufsz > 1) {
		status = evdns_nameserver_ip_add_line(buf);
	}

	free(buf);
	return status;
}

#define SERVICES_KEY "System\\CurrentControlSet\\Services\\"
#define WIN_NS_9X_KEY  SERVICES_KEY "VxD\\MSTCP"
#define WIN_NS_NT_KEY  SERVICES_KEY "Tcpip\\Parameters"

static int
load_nameservers_from_registry(void)
{
	int found = 0;
	int r;
#define TRY(k, name) \
	if (!found && config_nameserver_from_reg_key(k,name) == 0) {	\
		log(EVDNS_LOG_DEBUG,"Found nameservers in %s/%s",#k,name); \
		found = 1;						\
	} else if (!found) {						\
		log(EVDNS_LOG_DEBUG,"Didn't find nameservers in %s/%s", \
		    #k,#name);						\
	}

	if (((int)GetVersion()) > 0) { /* NT */
		HKEY nt_key = 0, interfaces_key = 0;

		if (RegOpenKeyEx(HKEY_LOCAL_MACHINE, WIN_NS_NT_KEY, 0,
				 KEY_READ, &nt_key) != ERROR_SUCCESS) {
			log(EVDNS_LOG_DEBUG,"Couldn't open nt key, %d",(int)GetLastError());
			return -1;
		}
		r = RegOpenKeyEx(nt_key, "Interfaces", 0,
			     KEY_QUERY_VALUE|KEY_ENUMERATE_SUB_KEYS,
			     &interfaces_key);
		if (r != ERROR_SUCCESS) {
			log(EVDNS_LOG_DEBUG,"Couldn't open interfaces key, %d",(int)GetLastError());
			return -1;
		}
		TRY(nt_key, "NameServer");
		TRY(nt_key, "DhcpNameServer");
		TRY(interfaces_key, "NameServer");
		TRY(interfaces_key, "DhcpNameServer");
		RegCloseKey(interfaces_key);
		RegCloseKey(nt_key);
	} else {
		HKEY win_key = 0;
		if (RegOpenKeyEx(HKEY_LOCAL_MACHINE, WIN_NS_9X_KEY, 0,
				 KEY_READ, &win_key) != ERROR_SUCCESS) {
			log(EVDNS_LOG_DEBUG, "Couldn't open registry key, %d", (int)GetLastError());
			return -1;
		}
		TRY(win_key, "NameServer");
		RegCloseKey(win_key);
	}

	if (found == 0) {
		log(EVDNS_LOG_WARN,"Didn't find any nameservers.");
	}

	return found ? 0 : -1;
#undef TRY
}

int
evdns_config_windows_nameservers(void)
{
	if (load_nameservers_with_getnetworkparams() == 0)
		return 0;
	return load_nameservers_from_registry();
}
#endif

int
evdns_init()
{
	int res = 0;
#ifdef MS_WINDOWS
	evdns_config_windows_nameservers(void);
#else
	res = evdns_resolv_conf_parse(DNS_OPTIONS_ALL, "/etc/resolv.conf");
#endif

	return (res);
}

const char *
evdns_err_to_string(int err)
{
    switch (err) {
	case DNS_ERR_NONE: return "no error";
	case DNS_ERR_FORMAT: return "misformatted query";
	case DNS_ERR_SERVERFAILED: return "server failed";
	case DNS_ERR_NOTEXIST: return "name does not exist";
	case DNS_ERR_NOTIMPL: return "query not implemented";
	case DNS_ERR_REFUSED: return "refused";

	case DNS_ERR_TRUNCATED: return "reply truncated or ill-formed";
	case DNS_ERR_UNKNOWN: return "unknown";
	case DNS_ERR_TIMEOUT: return "request timed out";
	case DNS_ERR_SHUTDOWN: return "dns subsystem shut down";
	default: return "[Unknown error code]";
    }
}

void
evdns_shutdown(int fail_requests)
{
	struct nameserver *server, *server_next;
	struct search_domain *dom, *dom_next;

	while (req_head) {
		if (fail_requests)
			reply_callback(req_head, 0, DNS_ERR_SHUTDOWN, NULL);
		request_finished(req_head, &req_head);
	}
	while (req_waiting_head) {
		if (fail_requests)
			reply_callback(req_waiting_head, 0, DNS_ERR_SHUTDOWN, NULL);
		request_finished(req_waiting_head, &req_waiting_head);
	}
	global_requests_inflight = global_requests_waiting = 0;

	for (server = server_head; server; server = server_next) {
		server_next = server->next;
		if (server->socket >= 0)
			CLOSE_SOCKET(server->socket);
		(void) event_del(&server->event);
		free(server);
		if (server_next == server_head)
			break;
	}
	server_head = NULL;
	global_good_nameservers = 0;

	if (global_search_state) {
		for (dom = global_search_state->head; dom; dom = dom_next) {
			dom_next = dom->next;
			free(dom);
		}
		free(global_search_state);
		global_search_state = NULL;
	}
	evdns_log_fn = NULL;
}

#ifdef EVDNS_MAIN
void
main_callback(int result, char type, int count, int ttl,
			  void *addrs, void *orig) {
	char *n = (char*)orig;
	int i;
	for (i = 0; i < count; ++i) {
		if (type == DNS_IPv4_A) {
			printf("%s: %s\n", n, debug_ntoa(((u32*)addrs)[i]));
		} else if (type == DNS_PTR) {
			printf("%s: %s\n", n, ((char**)addrs)[i]);
		}
	}
	if (!count) {
		printf("%s: No answer (%d)\n", n, result);
	}
	fflush(stdout);
}

void
logfn(const char *msg) {
  fprintf(stderr, "%s\n", msg);
}
int
main(int c, char **v) {
	int idx;
	int reverse = 0, verbose = 1;
	if (c<2) {
		fprintf(stderr, "syntax: %s [-x] [-v] hostname\n", v[0]);
		return 1;
	}
	idx = 1;
	while (idx < c && v[idx][0] == '-') {
		if (!strcmp(v[idx], "-x"))
			reverse = 1;
		else if (!strcmp(v[idx], "-v"))
			verbose = 1;
		else
			fprintf(stderr, "Unknown option %s\n", v[idx]);
		++idx;
	}
	event_init();
	if (verbose)
		evdns_set_log_fn(logfn);
	evdns_resolv_conf_parse(DNS_OPTION_NAMESERVERS, "/etc/resolv.conf");
	for (; idx < c; ++idx) {
		if (reverse) {
			struct in_addr addr;
			if (!inet_aton(v[idx], &addr)) {
				fprintf(stderr, "Skipping non-IP %s\n", v[idx]);
				continue;
			}
			fprintf(stderr, "resolving %s...\n",v[idx]);
			evdns_resolve_reverse(&addr, 0, main_callback, v[idx]);
		} else {
			fprintf(stderr, "resolving (fwd) %s...\n",v[idx]);
			evdns_resolve_ipv4(v[idx], 0, main_callback, v[idx]);
		}
	}
	fflush(stdout);
	event_dispatch();
	return 0;
}
#endif