libev/evdns.c

#define DNS_USE_GETTIMEOFDAY_FOR_ID 1
#define HAVE_STRUCT_IN6_ADDR 1

/* 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

#ifdef WIN32
#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>
#ifdef HAVE_STDINT_H
#include <stdint.h>
#endif
#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"
#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

#ifdef HAVE_NETINET_IN6_H
#include <netinet/in6.h>
#endif

#ifdef WIN32
typedef int socklen_t;
#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         EVDNS_TYPE_A
#define TYPE_CNAME     5
#define TYPE_PTR       EVDNS_TYPE_PTR
#define TYPE_AAAA      EVDNS_TYPE_AAAA

#define CLASS_INET     EVDNS_CLASS_INET

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 */
};

#ifndef HAVE_STRUCT_IN6_ADDR
struct in6_addr {
	u8 s6_addr[16];
};
#endif

struct reply {
	unsigned int type;
	unsigned int have_answer;
	union {
		struct {
			u32 addrcount;
			u32 addresses[MAX_ADDRS];
		} a;
		struct {
			u32 addrcount;
			struct in6_addr addresses[MAX_ADDRS];
		} aaaa;
		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 choked;  /* 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;

/* Represents a local port where we're listening for DNS requests. Right now, */
/* only UDP is supported. */
struct evdns_server_port {
	int socket; /* socket we use to read queries and write replies. */
	int refcnt; /* reference count. */
	char choked; /* Are we currently blocked from writing? */
	char closing; /* Are we trying to close this port, pending writes? */
	evdns_request_callback_fn_type user_callback; /* Fn to handle requests */
	void *user_data; /* Opaque pointer passed to user_callback */
	struct event event; /* Read/write event */
	/* circular list of replies that we want to write. */
	struct server_request *pending_replies;
};

/* Represents part of a reply being built.	(That is, a single RR.) */
struct server_reply_item {
	struct server_reply_item *next; /* next item in sequence. */
	char *name; /* name part of the RR */
	u16 type : 16; /* The RR type */
	u16 class : 16; /* The RR class (usually CLASS_INET) */
	u32 ttl; /* The RR TTL */
	char is_name; /* True iff data is a label */
	u16 datalen; /* Length of data; -1 if data is a label */
	void *data; /* The contents of the RR */
};

/* Represents a request that we've received as a DNS server, and holds */
/* the components of the reply as we're constructing it. */
struct server_request {
	/* Pointers to the next and previous entries on the list of replies */
	/* that we're waiting to write.	 Only set if we have tried to respond */
	/* and gotten EAGAIN. */
	struct server_request *next_pending;
	struct server_request *prev_pending;

	u16 trans_id; /* Transaction id. */
	struct evdns_server_port *port; /* Which port received this request on? */
	struct sockaddr_storage addr; /* Where to send the response */
	socklen_t addrlen; /* length of addr */

	int n_answer; /* how many answer RRs have been set? */
	int n_authority; /* how many authority RRs have been set? */
	int n_additional; /* how many additional RRs have been set? */

	struct server_reply_item *answer; /* linked list of answer RRs */
	struct server_reply_item *authority; /* linked list of authority RRs */
	struct server_reply_item *additional; /* linked list of additional RRs */

	/* Constructed response.  Only set once we're ready to send a reply. */
	/* Once this is set, the RR fields are cleared, and no more should be set. */
	char *response;
	size_t response_len;

	/* Caller-visible fields: flags, questions. */
	struct evdns_server_request base;
};

/* helper macro */
#define OFFSET_OF(st, member) ((off_t) (((char*)&((st*)0)->member)-(char*)0))

/* Given a pointer to an evdns_server_request, get the corresponding */
/* server_request. */
#define TO_SERVER_REQUEST(base_ptr)										\
	((struct server_request*)											\
	 (((char*)(base_ptr) - OFFSET_OF(struct server_request, base))))

/* 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 = {5, 0};  /* 5 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);

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);

static int server_request_free(struct server_request *req);
static void server_request_free_answers(struct server_request *req);
static void server_port_free(struct evdns_server_port *port);
static void server_port_ready_callback(int fd, short events, void *arg);

static int strtoint(const char *const str);

#ifdef WIN32
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);
	snprintf(buf, sizeof(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 WIN32
  _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;
	ns->timedout = 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;
	case TYPE_AAAA:
		if (reply)
			req->user_callback(DNS_ERR_NONE, DNS_IPv6_AAAA,
							   reply->data.aaaa.addrcount, ttl,
							   reply->data.aaaa.addresses,
							   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_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;
		case DNS_ERR_SERVERFAILED:
			/* rcode 2 (servfailed) sometimes means "we are broken" and
			 * sometimes (with some binds) means "that request was very
			 * confusing."  Treat this as a timeout, not a failure. 
			 */
			log(EVDNS_LOG_DEBUG, "Got a SERVERFAILED from nameserver %s; "
				"will allow the request to time out.",
				debug_ntoa(req->ns->address));
			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 int
name_parse(u8 *packet, int length, int *idx, char *name_out, int name_out_len) {
	int name_end = -1;
	int j = *idx;
	int ptr_count = 0;
#define GET32(x) do { if (j + 4 > length) goto err; memcpy(&_t32, packet + j, 4); j += 4; x = ntohl(_t32); } while(0)
#define GET16(x) do { if (j + 2 > length) goto err; memcpy(&_t, packet + j, 2); j += 2; x = ntohs(_t); } while(0)
#define GET8(x) do { if (j >= length) goto err; 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;
			/* Make sure that the target offset is in-bounds. */
			if (j < 0 || j >= length) return -1;
			/* If we've jumped more times than there are characters in the
			 * message, we must have a loop. */
			if (++ptr_count > 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;
 err:
	return -1;
}

/* parses a raw request from a nameserver */
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, questions, answers, authority, additional, datalength;
        u16 flags = 0;
	u32 ttl, ttl_r = 0xffffffff;
	struct reply reply;
	struct request *req = NULL;
	unsigned int i;

	GET16(trans_id);
	GET16(flags);
	GET16(questions);
	GET16(answers);
	GET16(authority);
	GET16(additional);
	(void) authority; /* suppress "unused variable" warnings. */
	(void) additional; /* suppress "unused variable" warnings. */

	req = request_find_from_trans_id(trans_id);
	if (!req) return -1;

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

	/* If it's not an answer, it doesn't correspond to any request. */
	if (!(flags & 0x8000)) return -1;  /* must be an answer */
	if (flags & 0x020f) {
		/* there was an error */
		goto err;
	}
	/* 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) \
			goto err;													\
	} 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) goto err;
	}

	/* 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;
			}
			if ((datalength & 3) != 0) /* not an even number of As. */
			    goto err;
			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) goto err;
			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)
				goto err;
			ttl_r = MIN(ttl_r, ttl);
			reply.have_answer = 1;
			break;
		} else if (type == TYPE_AAAA && class == CLASS_INET) {
			int addrcount, addrtocopy;
			if (req->request_type != TYPE_AAAA) {
				j += datalength; continue;
			}
			if ((datalength & 15) != 0) /* not an even number of AAAAs. */
				goto err;
			addrcount = datalength >> 4;  /* each address is 16 bytes long */
			addrtocopy = MIN(MAX_ADDRS - reply.data.aaaa.addrcount, (unsigned)addrcount);
			ttl_r = MIN(ttl_r, ttl);

			/* we only bother with the first four addresses. */
			if (j + 16*addrtocopy > length) goto err;
			memcpy(&reply.data.aaaa.addresses[reply.data.aaaa.addrcount],
				   packet + j, 16*addrtocopy);
			reply.data.aaaa.addrcount += addrtocopy;
			j += 16*addrtocopy;
			reply.have_answer = 1;
			if (reply.data.aaaa.addrcount == MAX_ADDRS) break;
		} else {
			/* skip over any other type of resource */
			j += datalength;
		}
	}

	reply_handle(req, flags, ttl_r, &reply);
	return 0;
 err:
	if (req)
		reply_handle(req, flags, 0, NULL);
	return -1;
}

/* Parse a raw request (packet,length) sent to a nameserver port (port) from */
/* a DNS client (addr,addrlen), and if it's well-formed, call the corresponding */
/* callback. */
static int
request_parse(u8 *packet, int length, struct evdns_server_port *port, struct sockaddr *addr, socklen_t addrlen)
{
	int j = 0;	/* index into packet */
	u16 _t;	 /* used by the macros */
	char tmp_name[256]; /* used by the macros */

	int i;
	u16 trans_id, flags, questions, answers, authority, additional;
	struct server_request *server_req = NULL;

	/* Get the header fields */
	GET16(trans_id);
	GET16(flags);
	GET16(questions);
	GET16(answers);
	GET16(authority);
	GET16(additional);

	if (flags & 0x8000) return -1; /* Must not be an answer. */
	if (flags & 0x7800) return -1; /* only standard queries are supported */
	flags &= 0x0300; /* Only TC and RD get preserved. */

	server_req = malloc(sizeof(struct server_request));
	if (server_req == NULL) return -1;
	memset(server_req, 0, sizeof(struct server_request));

	server_req->trans_id = trans_id;
	memcpy(&server_req->addr, addr, addrlen);
	server_req->addrlen = addrlen;

	server_req->base.flags = flags;
	server_req->base.nquestions = 0;
	server_req->base.questions = malloc(sizeof(struct evdns_server_question *) * questions);
	if (server_req->base.questions == NULL)
		goto err;

	for (i = 0; i < questions; ++i) {
		u16 type, class;
		struct evdns_server_question *q;
		int namelen;
		if (name_parse(packet, length, &j, tmp_name, sizeof(tmp_name))<0)
			goto err;
		GET16(type);
		GET16(class);
		namelen = strlen(tmp_name);
		q = malloc(sizeof(struct evdns_server_question) + namelen);
		if (!q)
			goto err;
		q->type = type;
		q->class = class;
		memcpy(q->name, tmp_name, namelen+1);
		server_req->base.questions[server_req->base.nquestions++] = q;
	}

	/* Ignore answers, authority, and additional. */

	server_req->port = port;
	port->refcnt++;
	port->user_callback(&(server_req->base), port->user_data);

	return 0;
err:
	if (server_req) {
		if (server_req->base.questions) {
			for (i = 0; i < server_req->base.nquestions; ++i)
				free(server_req->base.questions[i]);
			free(server_req->base.questions);
		}
		free(server_req);
	}
	return -1;

#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;
#ifdef CLOCK_MONOTONIC
		if (clock_gettime(CLOCK_MONOTONIC, &ts) == -1)
#else
		if (clock_gettime(CLOCK_REALTIME, &ts) == -1)
#endif
			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;
		}
		ns->timedout = 0;
		reply_parse(packet, r);
	}
}

/* Read a packet from a DNS client on a server port s, parse it, and */
/* act accordingly. */
static void
server_port_read(struct evdns_server_port *s) {
	u8 packet[1500];
	struct sockaddr_storage addr;
	socklen_t addrlen;
	int r;

	for (;;) {
		addrlen = sizeof(struct sockaddr_storage);
		r = recvfrom(s->socket, packet, sizeof(packet), 0,
					 (struct sockaddr*) &addr, &addrlen);
		if (r < 0) {
			int err = last_error(s->socket);
			if (error_is_eagain(err)) return;
			log(EVDNS_LOG_WARN, "Error %s (%d) while reading request.",
				strerror(err), err);
			return;
		}
		request_parse(packet, r, s, (struct sockaddr*) &addr, addrlen);
	}
}

/* Try to write all pending replies on a given DNS server port. */
static void
server_port_flush(struct evdns_server_port *port)
{
	while (port->pending_replies) {
		struct server_request *req = port->pending_replies;
		int r = sendto(port->socket, req->response, req->response_len, 0,
			   (struct sockaddr*) &req->addr, req->addrlen);
		if (r < 0) {
			int err = last_error(port->socket);
			if (error_is_eagain(err))
				return;
			log(EVDNS_LOG_WARN, "Error %s (%d) while writing response to port; dropping", strerror(err), err);
		}
		if (server_request_free(req)) {
			/* we released the last reference to req->port. */
			return;
		}
	}

	/* We have no more pending requests; stop listening for 'writeable' events. */
	(void) event_del(&port->event);
	event_set(&port->event, port->socket, EV_READ | EV_PERSIST,
			  server_port_ready_callback, port);
	if (event_add(&port->event, NULL) < 0) {