- Copy stable/10@285827 to releng/10.2 in preparation for 10.2-RC1

[FreeBSD/releng/10.2.git] / contrib / ntp / sntp / libevent / evdns.c

/* Copyright 2006-2007 Niels Provos
 * Copyright 2007-2012 Nick Mathewson and Niels Provos
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote products
 *    derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/* Based on software by Adam Langly. Adam's original message:
 *
 * 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 "event2/event-config.h"
#include "evconfig-private.h"

#include <sys/types.h>

#ifndef _FORTIFY_SOURCE
#define _FORTIFY_SOURCE 3
#endif

#include <string.h>
#include <fcntl.h>
#ifdef EVENT__HAVE_SYS_TIME_H
#include <sys/time.h>
#endif
#ifdef EVENT__HAVE_STDINT_H
#include <stdint.h>
#endif
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#ifdef EVENT__HAVE_UNISTD_H
#include <unistd.h>
#endif
#include <limits.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdarg.h>
#ifdef _WIN32
#include <winsock2.h>
#include <ws2tcpip.h>
#ifndef _WIN32_IE
#define _WIN32_IE 0x400
#endif
#include <shlobj.h>
#endif

#include "event2/dns.h"
#include "event2/dns_struct.h"
#include "event2/dns_compat.h"
#include "event2/util.h"
#include "event2/event.h"
#include "event2/event_struct.h"
#include "event2/thread.h"

#include "defer-internal.h"
#include "log-internal.h"
#include "mm-internal.h"
#include "strlcpy-internal.h"
#include "ipv6-internal.h"
#include "util-internal.h"
#include "evthread-internal.h"
#ifdef _WIN32
#include <ctype.h>
#include <winsock2.h>
#include <windows.h>
#include <iphlpapi.h>
#include <io.h>
#else
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#endif

#ifdef EVENT__HAVE_NETINET_IN6_H
#include <netinet/in6.h>
#endif

#define EVDNS_LOG_DEBUG EVENT_LOG_DEBUG
#define EVDNS_LOG_WARN EVENT_LOG_WARN
#define EVDNS_LOG_MSG EVENT_LOG_MSG

#ifndef HOST_NAME_MAX
#define HOST_NAME_MAX 255
#endif

#include <stdio.h>

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

#define ASSERT_VALID_REQUEST(req) \
        EVUTIL_ASSERT((req)->handle && (req)->handle->current_req == (req))

#define u64 ev_uint64_t
#define u32 ev_uint32_t
#define u16 ev_uint16_t
#define u8  ev_uint8_t

/* maximum number of addresses from a single packet */
/* that we bother recording */
#define MAX_V4_ADDRS 32
#define MAX_V6_ADDRS 32


#define TYPE_A         EVDNS_TYPE_A
#define TYPE_CNAME     5
#define TYPE_PTR       EVDNS_TYPE_PTR
#define TYPE_SOA       EVDNS_TYPE_SOA
#define TYPE_AAAA      EVDNS_TYPE_AAAA

#define CLASS_INET     EVDNS_CLASS_INET

/* Persistent handle.  We keep this separate from 'struct request' since we
 * need some object to last for as long as an evdns_request is outstanding so
 * that it can be canceled, whereas a search request can lead to multiple
 * 'struct request' instances being created over its lifetime. */
struct evdns_request {
        struct request *current_req;
        struct evdns_base *base;

        int pending_cb; /* Waiting for its callback to be invoked; not
                         * owned by event base any more. */

        /* 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;
};

struct request {
        u8 *request;  /* the dns packet data */
        u8 request_type; /* TYPE_PTR or TYPE_A or TYPE_AAAA */
        unsigned int request_len;
        int reissue_count;
        int tx_count;  /* the number of times that this packet has been sent */
        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 */

        /* these objects are kept in a circular list */
        /* XXX We could turn this into a CIRCLEQ. */
        struct request *next, *prev;

        struct event timeout_event;

        u16 trans_id;  /* the transaction id */
        unsigned request_appended :1;   /* true if the request pointer is data which follows this struct */
        unsigned transmit_me :1;  /* needs to be transmitted */

        /* XXXX This is a horrible hack. */
        char **put_cname_in_ptr; /* store the cname here if we get one. */

        struct evdns_base *base;

        struct evdns_request *handle;
};

struct reply {
        unsigned int type;
        unsigned int have_answer : 1;
        union {
                struct {
                        u32 addrcount;
                        u32 addresses[MAX_V4_ADDRS];
                } a;
                struct {
                        u32 addrcount;
                        struct in6_addr addresses[MAX_V6_ADDRS];
                } aaaa;
                struct {
                        char name[HOST_NAME_MAX];
                } ptr;
        } data;
};

struct nameserver {
        evutil_socket_t socket;  /* a connected UDP socket */
        struct sockaddr_storage address;
        ev_socklen_t addrlen;
        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 */
        /* Outstanding probe request for this nameserver, if any */
        struct evdns_request *probe_request;
        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 */
        struct evdns_base *base;

        /* Number of currently inflight requests: used
         * to track when we should add/del the event. */
        int requests_inflight;
};


/* Represents a local port where we're listening for DNS requests. Right now, */
/* only UDP is supported. */
struct evdns_server_port {
        evutil_socket_t 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;
        struct event_base *event_base;

#ifndef EVENT__DISABLE_THREAD_SUPPORT
        void *lock;
#endif
};

/* 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; /* The RR type */
        u16 class; /* 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 */
        ev_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;
};

struct evdns_base {
        /* An array of n_req_heads circular lists for inflight requests.
         * Each inflight request req is in req_heads[req->trans_id % n_req_heads].
         */
        struct request **req_heads;
        /* A circular list of requests that we're waiting to send, but haven't
         * sent yet because there are too many requests inflight */
        struct request *req_waiting_head;
        /* A circular list of nameservers. */
        struct nameserver *server_head;
        int n_req_heads;

        struct event_base *event_base;

        /* The number of good nameservers that we have */
        int global_good_nameservers;

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

        int global_max_requests_inflight;

        struct timeval global_timeout;  /* 5 seconds by default */
        int global_max_reissues;  /* a reissue occurs when we get some errors from the server */
        int global_max_retransmits;  /* 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 */
        int global_max_nameserver_timeout;
        /* true iff we will use the 0x20 hack to prevent poisoning attacks. */
        int global_randomize_case;

        /* The first time that a nameserver fails, how long do we wait before
         * probing to see if it has returned?  */
        struct timeval global_nameserver_probe_initial_timeout;

        /** Port to bind to for outgoing DNS packets. */
        struct sockaddr_storage global_outgoing_address;
        /** ev_socklen_t for global_outgoing_address. 0 if it isn't set. */
        ev_socklen_t global_outgoing_addrlen;

        struct timeval global_getaddrinfo_allow_skew;

        int getaddrinfo_ipv4_timeouts;
        int getaddrinfo_ipv6_timeouts;
        int getaddrinfo_ipv4_answered;
        int getaddrinfo_ipv6_answered;

        struct search_state *global_search_state;

        TAILQ_HEAD(hosts_list, hosts_entry) hostsdb;

#ifndef EVENT__DISABLE_THREAD_SUPPORT
        void *lock;
#endif

        int disable_when_inactive;
};

struct hosts_entry {
        TAILQ_ENTRY(hosts_entry) next;
        union {
                struct sockaddr sa;
                struct sockaddr_in sin;
                struct sockaddr_in6 sin6;
        } addr;
        int addrlen;
        char hostname[1];
};

static struct evdns_base *current_base = NULL;

struct evdns_base *
evdns_get_global_base(void)
{
        return current_base;
}

/* 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) - evutil_offsetof(struct server_request, base))))

#define REQ_HEAD(base, id) ((base)->req_heads[id % (base)->n_req_heads])

static struct nameserver *nameserver_pick(struct evdns_base *base);
static void evdns_request_insert(struct request *req, struct request **head);
static void evdns_request_remove(struct request *req, struct request **head);
static void nameserver_ready_callback(evutil_socket_t fd, short events, void *arg);
static int evdns_transmit(struct evdns_base *base);
static int evdns_request_transmit(struct request *req);
static void nameserver_send_probe(struct nameserver *const ns);
static void search_request_finished(struct evdns_request *const);
static int search_try_next(struct evdns_request *const req);
static struct request *search_request_new(struct evdns_base *base, struct evdns_request *handle, int type, const char *const name, int flags, evdns_callback_type user_callback, void *user_arg);
static void evdns_requests_pump_waiting_queue(struct evdns_base *base);
static u16 transaction_id_pick(struct evdns_base *base);
static struct request *request_new(struct evdns_base *base, struct evdns_request *handle, int type, const char *name, int flags, evdns_callback_type callback, void *ptr);
static void request_submit(struct request *const 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(evutil_socket_t fd, short events, void *arg);
static int evdns_base_resolv_conf_parse_impl(struct evdns_base *base, int flags, const char *const filename);
static int evdns_base_set_option_impl(struct evdns_base *base,
    const char *option, const char *val, int flags);
static void evdns_base_free_and_unlock(struct evdns_base *base, int fail_requests);
static void evdns_request_timeout_callback(evutil_socket_t fd, short events, void *arg);

static int strtoint(const char *const str);

#ifdef EVENT__DISABLE_THREAD_SUPPORT
#define EVDNS_LOCK(base)  EVUTIL_NIL_STMT_
#define EVDNS_UNLOCK(base) EVUTIL_NIL_STMT_
#define ASSERT_LOCKED(base) EVUTIL_NIL_STMT_
#else
#define EVDNS_LOCK(base)                        \
        EVLOCK_LOCK((base)->lock, 0)
#define EVDNS_UNLOCK(base)                      \
        EVLOCK_UNLOCK((base)->lock, 0)
#define ASSERT_LOCKED(base)                     \
        EVLOCK_ASSERT_LOCKED((base)->lock)
#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 severity, const char *fmt, ...) EVDNS_LOG_CHECK;
static void
evdns_log_(int severity, const char *fmt, ...)
{
        va_list args;
        va_start(args,fmt);
        if (evdns_log_fn) {
                char buf[512];
                int is_warn = (severity == EVDNS_LOG_WARN);
                evutil_vsnprintf(buf, sizeof(buf), fmt, args);
                evdns_log_fn(is_warn, buf);
        } else {
                event_logv_(severity, NULL, fmt, args);
        }
        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(struct evdns_base *base, u16 trans_id) {
        struct request *req = REQ_HEAD(base, trans_id);
        struct request *const started_at = req;

        ASSERT_LOCKED(base);

        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(evutil_socket_t fd, short events, void *arg) {
        struct nameserver *const ns = (struct nameserver *) arg;
        (void)fd;
        (void)events;

        EVDNS_LOCK(ns->base);
        nameserver_send_probe(ns);
        EVDNS_UNLOCK(ns->base);
}

/* 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) {
        struct timeval timeout;
        int i;

        ASSERT_LOCKED(ns->base);
        (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;
        }

#define MAX_PROBE_TIMEOUT 3600
#define TIMEOUT_BACKOFF_FACTOR 3

        memcpy(&timeout, &ns->base->global_nameserver_probe_initial_timeout,
            sizeof(struct timeval));
        for (i=ns->failed_times; i > 0 && timeout.tv_sec < MAX_PROBE_TIMEOUT; --i) {
                timeout.tv_sec *= TIMEOUT_BACKOFF_FACTOR;
                timeout.tv_usec *= TIMEOUT_BACKOFF_FACTOR;
                if (timeout.tv_usec > 1000000) {
                        timeout.tv_sec += timeout.tv_usec / 1000000;
                        timeout.tv_usec %= 1000000;
                }
        }
        if (timeout.tv_sec > MAX_PROBE_TIMEOUT) {
                timeout.tv_sec = MAX_PROBE_TIMEOUT;
                timeout.tv_usec = 0;
        }

        ns->failed_times++;

        if (evtimer_add(&ns->timeout_event, &timeout) < 0) {
                char addrbuf[128];
                log(EVDNS_LOG_WARN,
                    "Error from libevent when adding timer event for %s",
                    evutil_format_sockaddr_port_(
                            (struct sockaddr *)&ns->address,
                            addrbuf, sizeof(addrbuf)));
        }
}

static void
request_swap_ns(struct request *req, struct nameserver *ns) {
        if (ns && req->ns != ns) {
                EVUTIL_ASSERT(req->ns->requests_inflight > 0);
                req->ns->requests_inflight--;
                ns->requests_inflight++;

                req->ns = ns;
        }
}

/* 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;
        struct evdns_base *base = ns->base;
        int i;
        char addrbuf[128];

        ASSERT_LOCKED(base);
        /* if this nameserver has already been marked as failed */
        /* then don't do anything */
        if (!ns->state) return;

        log(EVDNS_LOG_MSG, "Nameserver %s has failed: %s",
            evutil_format_sockaddr_port_(
                    (struct sockaddr *)&ns->address,
                    addrbuf, sizeof(addrbuf)),
            msg);

        base->global_good_nameservers--;
        EVUTIL_ASSERT(base->global_good_nameservers >= 0);
        if (base->global_good_nameservers == 0) {
                log(EVDNS_LOG_MSG, "All nameservers have failed");
        }

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

        if (evtimer_add(&ns->timeout_event,
                &base->global_nameserver_probe_initial_timeout) < 0) {
                log(EVDNS_LOG_WARN,
                    "Error from libevent when adding timer event for %s",
                    evutil_format_sockaddr_port_(
                            (struct sockaddr *)&ns->address,
                            addrbuf, sizeof(addrbuf)));
                /* ???? 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 (!base->global_good_nameservers) return;

        for (i = 0; i < base->n_req_heads; ++i) {
                req = started_at = base->req_heads[i];
                if (req) {
                        do {
                                if (req->tx_count == 0 && req->ns == ns) {
                                        /* still waiting to go out, can be moved */
                                        /* to another server */
                                        request_swap_ns(req, nameserver_pick(base));
                                }
                                req = req->next;
                        } while (req != started_at);
                }
        }
}

static void
nameserver_up(struct nameserver *const ns)
{
        char addrbuf[128];
        ASSERT_LOCKED(ns->base);
        if (ns->state) return;
        log(EVDNS_LOG_MSG, "Nameserver %s is back up",
            evutil_format_sockaddr_port_(
                    (struct sockaddr *)&ns->address,
                    addrbuf, sizeof(addrbuf)));
        evtimer_del(&ns->timeout_event);
        if (ns->probe_request) {
                evdns_cancel_request(ns->base, ns->probe_request);
                ns->probe_request = NULL;
        }
        ns->state = 1;
        ns->failed_times = 0;
        ns->timedout = 0;
        ns->base->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. */
/* when free_handle is one, free the handle as well. */
static void
request_finished(struct request *const req, struct request **head, int free_handle) {
        struct evdns_base *base = req->base;
        int was_inflight = (head != &base->req_waiting_head);
        EVDNS_LOCK(base);
        ASSERT_VALID_REQUEST(req);

        if (head)
                evdns_request_remove(req, head);

        log(EVDNS_LOG_DEBUG, "Removing timeout for request %p", req);
        if (was_inflight) {
                evtimer_del(&req->timeout_event);
                base->global_requests_inflight--;
                req->ns->requests_inflight--;
        } else {
                base->global_requests_waiting--;
        }
        /* it was initialized during request_new / evtimer_assign */
        event_debug_unassign(&req->timeout_event);

        if (req->ns &&
            req->ns->requests_inflight == 0 &&
            req->base->disable_when_inactive) {
                event_del(&req->ns->event);
                evtimer_del(&req->ns->timeout_event);
        }

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

        if (req->handle) {
                EVUTIL_ASSERT(req->handle->current_req == req);

                if (free_handle) {
                        search_request_finished(req->handle);
                        req->handle->current_req = NULL;
                        if (! req->handle->pending_cb) {
                                /* If we're planning to run the callback,
                                 * don't free the handle until later. */
                                mm_free(req->handle);
                        }
                        req->handle = NULL; /* If we have a bug, let's crash
                                             * early */
                } else {
                        req->handle->current_req = NULL;
                }
        }

        mm_free(req);

        evdns_requests_pump_waiting_queue(base);
        EVDNS_UNLOCK(base);
}

/* 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;
        ASSERT_LOCKED(req->base);
        ASSERT_VALID_REQUEST(req);
        /* the last nameserver should have been marked as failing */
        /* by the caller of this function, therefore pick will try */
        /* not to return it */
        request_swap_ns(req, nameserver_pick(req->base));
        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. */
/* */
/* TODO: */
/* add return code, see at nameserver_pick() and other functions. */
static void
evdns_requests_pump_waiting_queue(struct evdns_base *base) {
        ASSERT_LOCKED(base);
        while (base->global_requests_inflight < base->global_max_requests_inflight &&
                   base->global_requests_waiting) {
                struct request *req;

                EVUTIL_ASSERT(base->req_waiting_head);
                req = base->req_waiting_head;

                req->ns = nameserver_pick(base);
                if (!req->ns)
                        return;

                /* move a request from the waiting queue to the inflight queue */
                req->ns->requests_inflight++;

                evdns_request_remove(req, &base->req_waiting_head);

                base->global_requests_waiting--;
                base->global_requests_inflight++;

                request_trans_id_set(req, transaction_id_pick(base));

                evdns_request_insert(req, &REQ_HEAD(base, req->trans_id));
                evdns_request_transmit(req);
                evdns_transmit(base);
        }
}

/* TODO(nickm) document */
struct deferred_reply_callback {
        struct event_callback deferred;
        struct evdns_request *handle;
        u8 request_type;
        u8 have_reply;
        u32 ttl;
        u32 err;
        evdns_callback_type user_callback;
        struct reply reply;
};

static void
reply_run_callback(struct event_callback *d, void *user_pointer)
{
        struct deferred_reply_callback *cb =
            EVUTIL_UPCAST(d, struct deferred_reply_callback, deferred);

        switch (cb->request_type) {
        case TYPE_A:
                if (cb->have_reply)
                        cb->user_callback(DNS_ERR_NONE, DNS_IPv4_A,
                            cb->reply.data.a.addrcount, cb->ttl,
                            cb->reply.data.a.addresses,
                            user_pointer);
                else
                        cb->user_callback(cb->err, 0, 0, cb->ttl, NULL, user_pointer);
                break;
        case TYPE_PTR:
                if (cb->have_reply) {
                        char *name = cb->reply.data.ptr.name;
                        cb->user_callback(DNS_ERR_NONE, DNS_PTR, 1, cb->ttl,
                            &name, user_pointer);
                } else {
                        cb->user_callback(cb->err, 0, 0, cb->ttl, NULL, user_pointer);
                }
                break;
        case TYPE_AAAA:
                if (cb->have_reply)
                        cb->user_callback(DNS_ERR_NONE, DNS_IPv6_AAAA,
                            cb->reply.data.aaaa.addrcount, cb->ttl,
                            cb->reply.data.aaaa.addresses,
                            user_pointer);
                else
                        cb->user_callback(cb->err, 0, 0, cb->ttl, NULL, user_pointer);
                break;
        default:
                EVUTIL_ASSERT(0);
        }

        if (cb->handle && cb->handle->pending_cb) {
                mm_free(cb->handle);
        }

        mm_free(cb);
}

static void
reply_schedule_callback(struct request *const req, u32 ttl, u32 err, struct reply *reply)
{
        struct deferred_reply_callback *d = mm_calloc(1, sizeof(*d));

        if (!d) {
                event_warn("%s: Couldn't allocate space for deferred callback.",
                    __func__);
                return;
        }

        ASSERT_LOCKED(req->base);

        d->request_type = req->request_type;
        d->user_callback = req->user_callback;
        d->ttl = ttl;
        d->err = err;
        if (reply) {
                d->have_reply = 1;
                memcpy(&d->reply, reply, sizeof(struct reply));
        }

        if (req->handle) {
                req->handle->pending_cb = 1;
                d->handle = req->handle;
        }

        event_deferred_cb_init_(
            &d->deferred,
            event_get_priority(&req->timeout_event),
            reply_run_callback,
            req->user_pointer);
        event_deferred_cb_schedule_(
                req->base->event_base,
                &d->deferred);
}

/* this processes a parsed reply packet */
static void
reply_handle(struct request *const req, u16 flags, u32 ttl, struct reply *reply) {
        int error;
        char addrbuf[128];
        static const int error_codes[] = {
                DNS_ERR_FORMAT, DNS_ERR_SERVERFAILED, DNS_ERR_NOTEXIST,
                DNS_ERR_NOTIMPL, DNS_ERR_REFUSED
        };

        ASSERT_LOCKED(req->base);
        ASSERT_VALID_REQUEST(req);

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

                switch (error) {
                case DNS_ERR_NOTIMPL:
                case DNS_ERR_REFUSED:
                        /* we regard these errors as marking a bad nameserver */
                        if (req->reissue_count < req->base->global_max_reissues) {
                                char msg[64];
                                evutil_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"
                                "at %s; will allow the request to time out.",
                            evutil_format_sockaddr_port_(
                                    (struct sockaddr *)&req->ns->address,
                                    addrbuf, sizeof(addrbuf)));
                        /* Call the timeout function */
                        evdns_request_timeout_callback(0, 0, req);
                        return;
                default:
                        /* we got a good reply from the nameserver: it is up. */
                        if (req->handle == req->ns->probe_request) {
                                /* Avoid double-free */
                                req->ns->probe_request = NULL;
                        }

                        nameserver_up(req->ns);
                }

                if (req->handle->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->handle)) {
                                /* 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. */
                                return;
                        }
                }

                /* all else failed. Pass the failure up */
                reply_schedule_callback(req, ttl, error, NULL);
                request_finished(req, &REQ_HEAD(req->base, req->trans_id), 1);
        } else {
                /* all ok, tell the user */
                reply_schedule_callback(req, ttl, 0, reply);
                if (req->handle == req->ns->probe_request)
                        req->ns->probe_request = NULL; /* Avoid double-free */
                nameserver_up(req->ns);
                request_finished(req, &REQ_HEAD(req->base, req->trans_id), 1);
        }
}

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(struct evdns_base *base, u8 *packet, int length) {
        int j = 0, k = 0;  /* index into packet */
        u16 t_;  /* used by the macros */
        u32 t32_;  /* used by the macros */
        char tmp_name[256], cmp_name[256]; /* used by the macros */
        int name_matches = 0;

        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;

        ASSERT_LOCKED(base);

        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(base, trans_id);
        if (!req) return -1;
        EVUTIL_ASSERT(req->base == base);

        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) && (flags & 0x020f) != DNS_ERR_NOTEXIST) {
                /* there was an error and it's not NXDOMAIN */
                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)
#define TEST_NAME                                                       \
        do { tmp_name[0] = '\0';                                        \
                cmp_name[0] = '\0';                                     \
                k = j;                                                  \
                if (name_parse(packet, length, &j, tmp_name,            \
                        sizeof(tmp_name))<0)                            \
                        goto err;                                       \
                if (name_parse(req->request, req->request_len, &k,      \
                        cmp_name, sizeof(cmp_name))<0)                  \
                        goto err;                                       \
                if (base->global_randomize_case) {                      \
                        if (strcmp(tmp_name, cmp_name) == 0)            \
                                name_matches = 1;                       \
                } else {                                                \
                        if (evutil_ascii_strcasecmp(tmp_name, cmp_name) == 0) \
                                name_matches = 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>
                 */
                TEST_NAME;
                j += 4;
                if (j > length) goto err;
        }

        if (!name_matches)
                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_V4_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_V4_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_CNAME) {
                        char cname[HOST_NAME_MAX];
                        if (!req->put_cname_in_ptr || *req->put_cname_in_ptr) {
                                j += datalength; continue;
                        }
                        if (name_parse(packet, length, &j, cname,
                                sizeof(cname))<0)
                                goto err;
                        *req->put_cname_in_ptr = mm_strdup(cname);
                } 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_V6_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_V6_ADDRS) break;
                } else {
                        /* skip over any other type of resource */
                        j += datalength;
                }
        }

        if (!reply.have_answer) {
                for (i = 0; i < authority; ++i) {
                        u16 type, class;
                        SKIP_NAME;
                        GET16(type);
                        GET16(class);
                        GET32(ttl);
                        GET16(datalength);
                        if (type == TYPE_SOA && class == CLASS_INET) {
                                u32 serial, refresh, retry, expire, minimum;
                                SKIP_NAME;
                                SKIP_NAME;
                                GET32(serial);
                                GET32(refresh);
                                GET32(retry);
                                GET32(expire);
                                GET32(minimum);
                                (void)expire;
                                (void)retry;
                                (void)refresh;
                                (void)serial;
                                ttl_r = MIN(ttl_r, ttl);
                                ttl_r = MIN(ttl_r, minimum);
                        } else {
                                /* skip over any other type of resource */
                                j += datalength;
                        }
                }
        }

        if (ttl_r == 0xffffffff)
                ttl_r = 0;

        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, ev_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;

        ASSERT_LOCKED(port);

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

        if (flags & 0x8000) return -1; /* Must not be an answer. */
        flags &= 0x0110; /* Only RD and CD get preserved. */

        server_req = mm_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 = mm_calloc(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 = (int)strlen(tmp_name);
                q = mm_malloc(sizeof(struct evdns_server_question) + namelen);
                if (!q)
                        goto err;
                q->type = type;
                q->dns_question_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++;

        /* Only standard queries are supported. */
        if (flags & 0x7800) {
                evdns_server_request_respond(&(server_req->base), DNS_ERR_NOTIMPL);
                return -1;
        }

        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)
                                mm_free(server_req->base.questions[i]);
                        mm_free(server_req->base.questions);
                }
                mm_free(server_req);
        }
        return -1;

#undef SKIP_NAME
#undef GET32
#undef GET16
#undef GET8
}


void
evdns_set_transaction_id_fn(ev_uint16_t (*fn)(void))
{
}

void
evdns_set_random_bytes_fn(void (*fn)(char *, size_t))
{
}

/* Try to choose a strong transaction id which isn't already in flight */
static u16
transaction_id_pick(struct evdns_base *base) {
        ASSERT_LOCKED(base);
        for (;;) {
                u16 trans_id;
                evutil_secure_rng_get_bytes(&trans_id, sizeof(trans_id));

                if (trans_id == 0xffff) continue;
                /* now check to see if that id is already inflight */
                if (request_find_from_trans_id(base, trans_id) == NULL)
                        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(struct evdns_base *base) {
        struct nameserver *started_at = base->server_head, *picked;
        ASSERT_LOCKED(base);
        if (!base->server_head) return NULL;

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

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

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

/* this is called when a namesever socket is ready for reading */
static void
nameserver_read(struct nameserver *ns) {
        struct sockaddr_storage ss;
        ev_socklen_t addrlen = sizeof(ss);
        u8 packet[1500];
        char addrbuf[128];
        ASSERT_LOCKED(ns->base);

        for (;;) {
                const int r = recvfrom(ns->socket, (void*)packet,
                    sizeof(packet), 0,
                    (struct sockaddr*)&ss, &addrlen);
                if (r < 0) {
                        int err = evutil_socket_geterror(ns->socket);
                        if (EVUTIL_ERR_RW_RETRIABLE(err))
                                return;
                        nameserver_failed(ns,
                            evutil_socket_error_to_string(err));
                        return;
                }
                if (evutil_sockaddr_cmp((struct sockaddr*)&ss,
                        (struct sockaddr*)&ns->address, 0)) {
                        log(EVDNS_LOG_WARN, "Address mismatch on received "
                            "DNS packet.  Apparent source was %s",
                            evutil_format_sockaddr_port_(
                                    (struct sockaddr *)&ss,
                                    addrbuf, sizeof(addrbuf)));
                        return;
                }

                ns->timedout = 0;
                reply_parse(ns->base, 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;
        ev_socklen_t addrlen;
        int r;
        ASSERT_LOCKED(s);

        for (;;) {
                addrlen = sizeof(struct sockaddr_storage);
                r = recvfrom(s->socket, (void*)packet, sizeof(packet), 0,
                                         (struct sockaddr*) &addr, &addrlen);
                if (r < 0) {
                        int err = evutil_socket_geterror(s->socket);
                        if (EVUTIL_ERR_RW_RETRIABLE(err))
                                return;
                        log(EVDNS_LOG_WARN,
                            "Error %s (%d) while reading request.",
                            evutil_socket_error_to_string(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)
{
        struct server_request *req = port->pending_replies;
        ASSERT_LOCKED(port);
        while (req) {
                int r = sendto(port->socket, req->response, (int)req->response_len, 0,
                           (struct sockaddr*) &req->addr, (ev_socklen_t)req->addrlen);
                if (r < 0) {
                        int err = evutil_socket_geterror(port->socket);
                        if (EVUTIL_ERR_RW_RETRIABLE(err))
                                return;
                        log(EVDNS_LOG_WARN, "Error %s (%d) while writing response to port; dropping", evutil_socket_error_to_string(err), err);
                }
                if (server_request_free(req)) {
                        /* we released the last reference to req->port. */
                        return;
                } else {
                        EVUTIL_ASSERT(req != port->pending_replies);
                        req = port->pending_replies;
                }
        }

        /* We have no more pending requests; stop listening for 'writeable' events. */
        (void) event_del(&port->event);
        event_assign(&port->event, port->event_base,
                                 port->socket, EV_READ | EV_PERSIST,
                                 server_port_ready_callback, port);

        if (event_add(&port->event, NULL) < 0) {
                log(EVDNS_LOG_WARN, "Error from libevent when adding event for DNS server.");
                /* ???? Do more? */
        }
}

/* 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) {
        ASSERT_LOCKED(ns->base);
        if (ns->write_waiting == waiting) return;

        ns->write_waiting = waiting;
        (void) event_del(&ns->event);
        event_assign(&ns->event, ns->base->event_base,
            ns->socket, EV_READ | (waiting ? EV_WRITE : 0) | EV_PERSIST,
            nameserver_ready_callback, ns);
        if (event_add(&ns->event, NULL) < 0) {
                char addrbuf[128];
                log(EVDNS_LOG_WARN, "Error from libevent when adding event for %s",
                    evutil_format_sockaddr_port_(
                            (struct sockaddr *)&ns->address,
                            addrbuf, sizeof(addrbuf)));
                /* ???? 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(evutil_socket_t fd, short events, void *arg) {
        struct nameserver *ns = (struct nameserver *) arg;
        (void)fd;

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

/* a callback function. Called by libevent when the kernel says that */
/* a server socket is ready for writing or reading. */
static void
server_port_ready_callback(evutil_socket_t fd, short events, void *arg) {
        struct evdns_server_port *port = (struct evdns_server_port *) arg;
        (void) fd;

        EVDNS_LOCK(port);
        if (events & EV_WRITE) {
                port->choked = 0;
                server_port_flush(port);
        }
        if (events & EV_READ) {
                server_port_read(port);
        }
        EVDNS_UNLOCK(port);
}

/* This is an inefficient representation; only use it via the dnslabel_table_*
 * functions, so that is can be safely replaced with something smarter later. */
#define MAX_LABELS 128
/* Structures used to implement name compression */
struct dnslabel_entry { char *v; off_t pos; };
struct dnslabel_table {
        int n_labels; /* number of current entries */
        /* map from name to position in message */
        struct dnslabel_entry labels[MAX_LABELS];
};

/* Initialize dnslabel_table. */
static void
dnslabel_table_init(struct dnslabel_table *table)
{
        table->n_labels = 0;
}

/* Free all storage held by table, but not the table itself. */
static void
dnslabel_clear(struct dnslabel_table *table)
{
        int i;
        for (i = 0; i < table->n_labels; ++i)
                mm_free(table->labels[i].v);
        table->n_labels = 0;
}

/* return the position of the label in the current message, or -1 if the label */
/* hasn't been used yet. */
static int
dnslabel_table_get_pos(const struct dnslabel_table *table, const char *label)
{
        int i;
        for (i = 0; i < table->n_labels; ++i) {
                if (!strcmp(label, table->labels[i].v))
                        return table->labels[i].pos;
        }
        return -1;
}

/* remember that we've used the label at position pos */
static int
dnslabel_table_add(struct dnslabel_table *table, const char *label, off_t pos)
{
        char *v;
        int p;
        if (table->n_labels == MAX_LABELS)
                return (-1);
        v = mm_strdup(label);
        if (v == NULL)
                return (-1);
        p = table->n_labels++;
        table->labels[p].v = v;
        table->labels[p].pos = pos;

        return (0);
}

/* Converts a string to a length-prefixed set of DNS labels, starting */
/* at buf[j]. name and buf must not overlap. name_len should be the length */
/* of name.      table is optional, and is used for compression. */
/* */
/* Input: abc.def */
/* Output: <3>abc<3>def<0> */
/* */
/* Returns the first index after the encoded name, or negative on error. */
/*       -1      label was > 63 bytes */
/*       -2      name too long to fit in buffer. */
/* */
static off_t
dnsname_to_labels(u8 *const buf, size_t buf_len, off_t j,
                                  const char *name, const size_t name_len,
                                  struct dnslabel_table *table) {
        const char *end = name + name_len;
        int ref = 0;
        u16 t_;

#define APPEND16(x) do {                                                \
                if (j + 2 > (off_t)buf_len)                             \
                        goto overflow;                                  \
                t_ = htons(x);                                          \
                memcpy(buf + j, &t_, 2);                                \
                j += 2;                                                 \
        } while (0)
#define APPEND32(x) do {                                                \
                if (j + 4 > (off_t)buf_len)                             \
                        goto overflow;                                  \
                t32_ = htonl(x);                                        \
                memcpy(buf + j, &t32_, 4);                              \
                j += 4;                                                 \
        } while (0)

        if (name_len > 255) return -2;

        for (;;) {
                const char *const start = name;
                if (table && (ref = dnslabel_table_get_pos(table, name)) >= 0) {
                        APPEND16(ref | 0xc000);
                        return j;
                }
                name = strchr(name, '.');
                if (!name) {
                        const size_t label_len = end - start;
                        if (label_len > 63) return -1;
                        if ((size_t)(j+label_len+1) > buf_len) return -2;
                        if (table) dnslabel_table_add(table, start, j);
                        buf[j++] = (ev_uint8_t)label_len;

                        memcpy(buf + j, start, label_len);
                        j += (int) label_len;
                        break;
                } else {
                        /* append length of the label. */
                        const size_t label_len = name - start;
                        if (label_len > 63) return -1;
                        if ((size_t)(j+label_len+1) > buf_len) return -2;
                        if (table) dnslabel_table_add(table, start, j);
                        buf[j++] = (ev_uint8_t)label_len;

                        memcpy(buf + j, start, label_len);
                        j += (int) label_len;
                        /* 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;
 overflow:
        return (-2);
}

/* 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 size_t
evdns_request_len(const size_t 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 size_t name_len,
    const u16 trans_id, const u16 type, const u16 class,
    u8 *const buf, size_t buf_len) {
        off_t j = 0;  /* current offset into buf */
        u16 t_;  /* used by the macros */

        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 */

        j = dnsname_to_labels(buf, buf_len, j, name, name_len, NULL);
        if (j < 0) {
                return (int)j;
        }

        APPEND16(type);
        APPEND16(class);

        return (int)j;
 overflow:
        return (-1);
}

/* exported function */
struct evdns_server_port *
evdns_add_server_port_with_base(struct event_base *base, evutil_socket_t socket, int flags, evdns_request_callback_fn_type cb, void *user_data)
{
        struct evdns_server_port *port;
        if (flags)
                return NULL; /* flags not yet implemented */
        if (!(port = mm_malloc(sizeof(struct evdns_server_port))))
                return NULL;
        memset(port, 0, sizeof(struct evdns_server_port));


        port->socket = socket;
        port->refcnt = 1;
        port->choked = 0;
        port->closing = 0;
        port->user_callback = cb;
        port->user_data = user_data;
        port->pending_replies = NULL;
        port->event_base = base;

        event_assign(&port->event, port->event_base,
                                 port->socket, EV_READ | EV_PERSIST,
                                 server_port_ready_callback, port);
        if (event_add(&port->event, NULL) < 0) {
                mm_free(port);
                return NULL;
        }
        EVTHREAD_ALLOC_LOCK(port->lock, EVTHREAD_LOCKTYPE_RECURSIVE);
        return port;
}

struct evdns_server_port *
evdns_add_server_port(evutil_socket_t socket, int flags, evdns_request_callback_fn_type cb, void *user_data)
{
        return evdns_add_server_port_with_base(NULL, socket, flags, cb, user_data);
}

/* exported function */
void
evdns_close_server_port(struct evdns_server_port *port)
{
        EVDNS_LOCK(port);
        if (--port->refcnt == 0) {
                EVDNS_UNLOCK(port);
                server_port_free(port);
        } else {
                port->closing = 1;
        }
}

/* exported function */
int
evdns_server_request_add_reply(struct evdns_server_request *req_, int section, const char *name, int type, int class, int ttl, int datalen, int is_name, const char *data)
{
        struct server_request *req = TO_SERVER_REQUEST(req_);
        struct server_reply_item **itemp, *item;
        int *countp;
        int result = -1;

        EVDNS_LOCK(req->port);
        if (req->response) /* have we already answered? */
                goto done;

        switch (section) {
        case EVDNS_ANSWER_SECTION:
                itemp = &req->answer;
                countp = &req->n_answer;
                break;
        case EVDNS_AUTHORITY_SECTION:
                itemp = &req->authority;
                countp = &req->n_authority;
                break;
        case EVDNS_ADDITIONAL_SECTION:
                itemp = &req->additional;
                countp = &req->n_additional;
                break;
        default:
                goto done;
        }
        while (*itemp) {
                itemp = &((*itemp)->next);
        }
        item = mm_malloc(sizeof(struct server_reply_item));
        if (!item)
                goto done;
        item->next = NULL;
        if (!(item->name = mm_strdup(name))) {
                mm_free(item);
                goto done;
        }
        item->type = type;
        item->dns_question_class = class;
        item->ttl = ttl;
        item->is_name = is_name != 0;
        item->datalen = 0;
        item->data = NULL;
        if (data) {
                if (item->is_name) {
                        if (!(item->data = mm_strdup(data))) {
                                mm_free(item->name);
                                mm_free(item);
                                goto done;
                        }
                        item->datalen = (u16)-1;
                } else {
                        if (!(item->data = mm_malloc(datalen))) {
                                mm_free(item->name);
                                mm_free(item);
                                goto done;
                        }
                        item->datalen = datalen;
                        memcpy(item->data, data, datalen);
                }
        }

        *itemp = item;
        ++(*countp);
        result = 0;
done:
        EVDNS_UNLOCK(req->port);
        return result;
}

/* exported function */
int
evdns_server_request_add_a_reply(struct evdns_server_request *req, const char *name, int n, const void *addrs, int ttl)
{
        return evdns_server_request_add_reply(
                  req, EVDNS_ANSWER_SECTION, name, TYPE_A, CLASS_INET,
                  ttl, n*4, 0, addrs);
}

/* exported function */
int
evdns_server_request_add_aaaa_reply(struct evdns_server_request *req, const char *name, int n, const void *addrs, int ttl)
{
        return evdns_server_request_add_reply(
                  req, EVDNS_ANSWER_SECTION, name, TYPE_AAAA, CLASS_INET,
                  ttl, n*16, 0, addrs);
}

/* exported function */
int
evdns_server_request_add_ptr_reply(struct evdns_server_request *req, struct in_addr *in, const char *inaddr_name, const char *hostname, int ttl)
{
        u32 a;
        char buf[32];
        if (in && inaddr_name)
                return -1;
        else if (!in && !inaddr_name)
                return -1;
        if (in) {
                a = ntohl(in->s_addr);
                evutil_snprintf(buf, sizeof(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));
                inaddr_name = buf;
        }
        return evdns_server_request_add_reply(
                  req, EVDNS_ANSWER_SECTION, inaddr_name, TYPE_PTR, CLASS_INET,
                  ttl, -1, 1, hostname);
}

/* exported function */
int
evdns_server_request_add_cname_reply(struct evdns_server_request *req, const char *name, const char *cname, int ttl)
{
        return evdns_server_request_add_reply(
                  req, EVDNS_ANSWER_SECTION, name, TYPE_CNAME, CLASS_INET,
                  ttl, -1, 1, cname);
}

/* exported function */
void
evdns_server_request_set_flags(struct evdns_server_request *exreq, int flags)
{
        struct server_request *req = TO_SERVER_REQUEST(exreq);
        req->base.flags &= ~(EVDNS_FLAGS_AA|EVDNS_FLAGS_RD);
        req->base.flags |= flags;
}

static int
evdns_server_request_format_response(struct server_request *req, int err)
{
        unsigned char buf[1500];
        size_t buf_len = sizeof(buf);
        off_t j = 0, r;
        u16 t_;
        u32 t32_;
        int i;
        u16 flags;
        struct dnslabel_table table;

        if (err < 0 || err > 15) return -1;

        /* Set response bit and error code; copy OPCODE and RD fields from
         * question; copy RA and AA if set by caller. */
        flags = req->base.flags;
        flags |= (0x8000 | err);

        dnslabel_table_init(&table);
        APPEND16(req->trans_id);
        APPEND16(flags);
        APPEND16(req->base.nquestions);
        APPEND16(req->n_answer);
        APPEND16(req->n_authority);
        APPEND16(req->n_additional);

        /* Add questions. */
        for (i=0; i < req->base.nquestions; ++i) {
                const char *s = req->base.questions[i]->name;
                j = dnsname_to_labels(buf, buf_len, j, s, strlen(s), &table);
                if (j < 0) {
                        dnslabel_clear(&table);
                        return (int) j;
                }
                APPEND16(req->base.questions[i]->type);
                APPEND16(req->base.questions[i]->dns_question_class);
        }

        /* Add answer, authority, and additional sections. */
        for (i=0; i<3; ++i) {
                struct server_reply_item *item;
                if (i==0)
                        item = req->answer;
                else if (i==1)
                        item = req->authority;
                else
                        item = req->additional;
                while (item) {
                        r = dnsname_to_labels(buf, buf_len, j, item->name, strlen(item->name), &table);
                        if (r < 0)
                                goto overflow;
                        j = r;

                        APPEND16(item->type);
                        APPEND16(item->dns_question_class);
                        APPEND32(item->ttl);
                        if (item->is_name) {
                                off_t len_idx = j, name_start;
                                j += 2;
                                name_start = j;
                                r = dnsname_to_labels(buf, buf_len, j, item->data, strlen(item->data), &table);
                                if (r < 0)
                                        goto overflow;
                                j = r;
                                t_ = htons( (short) (j-name_start) );
                                memcpy(buf+len_idx, &t_, 2);
                        } else {
                                APPEND16(item->datalen);
                                if (j+item->datalen > (off_t)buf_len)
                                        goto overflow;
                                memcpy(buf+j, item->data, item->datalen);
                                j += item->datalen;
                        }
                        item = item->next;
                }
        }

        if (j > 512) {
overflow:
                j = 512;
                buf[2] |= 0x02; /* set the truncated bit. */
        }

        req->response_len = j;

        if (!(req->response = mm_malloc(req->response_len))) {
                server_request_free_answers(req);
                dnslabel_clear(&table);
                return (-1);
        }
        memcpy(req->response, buf, req->response_len);
        server_request_free_answers(req);
        dnslabel_clear(&table);
        return (0);
}

/* exported function */
int
evdns_server_request_respond(struct evdns_server_request *req_, int err)
{
        struct server_request *req = TO_SERVER_REQUEST(req_);
        struct evdns_server_port *port = req->port;
        int r = -1;

        EVDNS_LOCK(port);
        if (!req->response) {
                if ((r = evdns_server_request_format_response(req, err))<0)
                        goto done;
        }

        r = sendto(port->socket, req->response, (int)req->response_len, 0,
                           (struct sockaddr*) &req->addr, (ev_socklen_t)req->addrlen);
        if (r<0) {
                int sock_err = evutil_socket_geterror(port->socket);
                if (EVUTIL_ERR_RW_RETRIABLE(sock_err))
                        goto done;

                if (port->pending_replies) {
                        req->prev_pending = port->pending_replies->prev_pending;
                        req->next_pending = port->pending_replies;
                        req->prev_pending->next_pending =
                                req->next_pending->prev_pending = req;
                } else {
                        req->prev_pending = req->next_pending = req;
                        port->pending_replies = req;
                        port->choked = 1;

                        (void) event_del(&port->event);
                        event_assign(&port->event, port->event_base, port->socket, (port->closing?0:EV_READ) | EV_WRITE | EV_PERSIST, server_port_ready_callback, port);

                        if (event_add(&port->event, NULL) < 0) {
                                log(EVDNS_LOG_WARN, "Error from libevent when adding event for DNS server");
                        }

                }

                r = 1;
                goto done;
        }
        if (server_request_free(req)) {
                r = 0;
                goto done;
        }

        if (port->pending_replies)
                server_port_flush(port);

        r = 0;
done:
        EVDNS_UNLOCK(port);
        return r;
}

/* Free all storage held by RRs in req. */
static void
server_request_free_answers(struct server_request *req)
{
        struct server_reply_item *victim, *next, **list;
        int i;
        for (i = 0; i < 3; ++i) {
                if (i==0)
                        list = &req->answer;
                else if (i==1)
                        list = &req->authority;
                else
                        list = &req->additional;

                victim = *list;
                while (victim) {
                        next = victim->next;
                        mm_free(victim->name);
                        if (victim->data)
                                mm_free(victim->data);
                        mm_free(victim);
                        victim = next;
                }
                *list = NULL;
        }
}

/* Free all storage held by req, and remove links to it. */
/* return true iff we just wound up freeing the server_port. */
static int
server_request_free(struct server_request *req)
{
        int i, rc=1, lock=0;
        if (req->base.questions) {
                for (i = 0; i < req->base.nquestions; ++i)
                        mm_free(req->base.questions[i]);
                mm_free(req->base.questions);
        }

        if (req->port) {
                EVDNS_LOCK(req->port);
                lock=1;
                if (req->port->pending_replies == req) {
                        if (req->next_pending && req->next_pending != req)
                                req->port->pending_replies = req->next_pending;
                        else
                                req->port->pending_replies = NULL;
                }
                rc = --req->port->refcnt;
        }

        if (req->response) {
                mm_free(req->response);
        }

        server_request_free_answers(req);

        if (req->next_pending && req->next_pending != req) {
                req->next_pending->prev_pending = req->prev_pending;
                req->prev_pending->next_pending = req->next_pending;
        }

        if (rc == 0) {
                EVDNS_UNLOCK(req->port); /* ????? nickm */
                server_port_free(req->port);
                mm_free(req);
                return (1);
        }
        if (lock)
                EVDNS_UNLOCK(req->port);
        mm_free(req);
        return (0);
}

/* Free all storage held by an evdns_server_port.  Only called when  */
static void
server_port_free(struct evdns_server_port *port)
{
        EVUTIL_ASSERT(port);
        EVUTIL_ASSERT(!port->refcnt);
        EVUTIL_ASSERT(!port->pending_replies);
        if (port->socket > 0) {
                evutil_closesocket(port->socket);
                port->socket = -1;
        }
        (void) event_del(&port->event);
        event_debug_unassign(&port->event);
        EVTHREAD_FREE_LOCK(port->lock, EVTHREAD_LOCKTYPE_RECURSIVE);
        mm_free(port);
}

/* exported function */
int
evdns_server_request_drop(struct evdns_server_request *req_)
{
        struct server_request *req = TO_SERVER_REQUEST(req_);
        server_request_free(req);
        return 0;
}

/* exported function */
int
evdns_server_request_get_requesting_addr(struct evdns_server_request *req_, struct sockaddr *sa, int addr_len)
{
        struct server_request *req = TO_SERVER_REQUEST(req_);
        if (addr_len < (int)req->addrlen)
                return -1;
        memcpy(sa, &(req->addr), req->addrlen);
        return req->addrlen;
}

#undef APPEND16
#undef APPEND32

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

        (void) fd;
        (void) events;

        log(EVDNS_LOG_DEBUG, "Request %p timed out", arg);
        EVDNS_LOCK(base);

        if (req->tx_count >= req->base->global_max_retransmits) {
                struct nameserver *ns = req->ns;
                /* this request has failed */
                log(EVDNS_LOG_DEBUG, "Giving up on request %p; tx_count==%d",
                    arg, req->tx_count);
                reply_schedule_callback(req, 0, DNS_ERR_TIMEOUT, NULL);

                request_finished(req, &REQ_HEAD(req->base, req->trans_id), 1);
                nameserver_failed(ns, "request timed out.");
        } else {
                /* retransmit it */
                log(EVDNS_LOG_DEBUG, "Retransmitting request %p; tx_count==%d",
                    arg, req->tx_count);
                (void) evtimer_del(&req->timeout_event);
                request_swap_ns(req, nameserver_pick(base));
                evdns_request_transmit(req);

                req->ns->timedout++;
                if (req->ns->timedout > req->base->global_max_nameserver_timeout) {
                        req->ns->timedout = 0;
                        nameserver_failed(req->ns, "request timed out.");
                }
        }

        EVDNS_UNLOCK(base);
}

/* 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) {
        int r;
        ASSERT_LOCKED(req->base);
        ASSERT_VALID_REQUEST(req);

        if (server->requests_inflight == 1 &&
                req->base->disable_when_inactive &&
                event_add(&server->event, NULL) < 0) {
                return 1;
        }

        r = sendto(server->socket, (void*)req->request, req->request_len, 0,
            (struct sockaddr *)&server->address, server->addrlen);
        if (r < 0) {
                int err = evutil_socket_geterror(server->socket);
                if (EVUTIL_ERR_RW_RETRIABLE(err))
                        return 1;
                nameserver_failed(req->ns, evutil_socket_error_to_string(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;

        ASSERT_LOCKED(req->base);
        ASSERT_VALID_REQUEST(req);
        /* if we fail to send this packet then this flag marks it */
        /* for evdns_transmit */
        req->transmit_me = 1;
        EVUTIL_ASSERT(req->trans_id != 0xffff);

        if (!req->ns)
        {
                /* unable to transmit request if no nameservers */
                return 1;
        }

        if (req->ns->choked) {
                /* 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->choked = 1;
                nameserver_write_waiting(req->ns, 1);
                return 1;
        case 2:
                /* failed to transmit the request entirely. */
                retcode = 1;
                /* fall through: we'll set a timeout, which will time out,
                 * and make us retransmit the request anyway. */
        default:
                /* all ok */
                log(EVDNS_LOG_DEBUG,
                    "Setting timeout for request %p, sent to nameserver %p", req, req->ns);
                if (evtimer_add(&req->timeout_event, &req->base->global_timeout) < 0) {
                        log(EVDNS_LOG_WARN,
                      "Error from libevent when adding timer for request %p",
                            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_CANCEL) {
                /* We canceled this request because the nameserver came up
                 * for some other reason.  Do not change our opinion about
                 * the nameserver. */
                return;
        }

        EVDNS_LOCK(ns->base);
        ns->probe_request = NULL;
        if (result == DNS_ERR_NONE || result == DNS_ERR_NOTEXIST) {
                /* this is a good reply */
                nameserver_up(ns);
        } else {
                nameserver_probe_failed(ns);
        }
        EVDNS_UNLOCK(ns->base);
}

static void
nameserver_send_probe(struct nameserver *const ns) {
        struct evdns_request *handle;
        struct request *req;
        char addrbuf[128];
        /* here we need to send a probe to a given nameserver */
        /* in the hope that it is up now. */

        ASSERT_LOCKED(ns->base);
        log(EVDNS_LOG_DEBUG, "Sending probe to %s",
            evutil_format_sockaddr_port_(
                    (struct sockaddr *)&ns->address,
                    addrbuf, sizeof(addrbuf)));
        handle = mm_calloc(1, sizeof(*handle));
        if (!handle) return;
        req = request_new(ns->base, handle, TYPE_A, "google.com", DNS_QUERY_NO_SEARCH, nameserver_probe_callback, ns);
        if (!req) {
                mm_free(handle);
                return;
        }
        ns->probe_request = handle;
        /* we force this into the inflight queue no matter what */
        request_trans_id_set(req, transaction_id_pick(ns->base));
        req->ns = ns;
        request_submit(req);
}

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

        ASSERT_LOCKED(base);
        for (i = 0; i < base->n_req_heads; ++i) {
                if (base->req_heads[i]) {
                        struct request *const started_at = base->req_heads[i], *req = started_at;
                        /* 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_base_count_nameservers(struct evdns_base *base)
{
        const struct nameserver *server;
        int n = 0;

        EVDNS_LOCK(base);
        server = base->server_head;
        if (!server)
                goto done;
        do {
                ++n;
                server = server->next;
        } while (server != base->server_head);
done:
        EVDNS_UNLOCK(base);
        return n;
}

int
evdns_count_nameservers(void)
{
        return evdns_base_count_nameservers(current_base);
}

/* exported function */
int
evdns_base_clear_nameservers_and_suspend(struct evdns_base *base)
{
        struct nameserver *server, *started_at;
        int i;

        EVDNS_LOCK(base);
        server = base->server_head;
        started_at = base->server_head;
        if (!server) {
                EVDNS_UNLOCK(base);
                return 0;
        }
        while (1) {
                struct nameserver *next = server->next;
                (void) event_del(&server->event);
                if (evtimer_initialized(&server->timeout_event))
                        (void) evtimer_del(&server->timeout_event);
                if (server->probe_request) {
                        evdns_cancel_request(server->base, server->probe_request);
                        server->probe_request = NULL;
                }
                if (server->socket >= 0)
                        evutil_closesocket(server->socket);
                mm_free(server);
                if (next == started_at)
                        break;
                server = next;
        }
        base->server_head = NULL;
        base->global_good_nameservers = 0;

        for (i = 0; i < base->n_req_heads; ++i) {
                struct request *req, *req_started_at;
                req = req_started_at = base->req_heads[i];
                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;

                        base->global_requests_waiting++;
                        evdns_request_insert(req, &base->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.*/
                        base->req_waiting_head = base->req_waiting_head->prev;

                        if (next == req_started_at)
                                break;
                        req = next;
                }
                base->req_heads[i] = NULL;
        }

        base->global_requests_inflight = 0;

        EVDNS_UNLOCK(base);
        return 0;
}

int
evdns_clear_nameservers_and_suspend(void)
{
        return evdns_base_clear_nameservers_and_suspend(current_base);
}


/* exported function */
int
evdns_base_resume(struct evdns_base *base)
{
        EVDNS_LOCK(base);
        evdns_requests_pump_waiting_queue(base);
        EVDNS_UNLOCK(base);

        return 0;
}

int
evdns_resume(void)
{
        return evdns_base_resume(current_base);
}

static int
evdns_nameserver_add_impl_(struct evdns_base *base, const struct sockaddr *address, int addrlen) {
        /* first check to see if we already have this nameserver */

        const struct nameserver *server = base->server_head, *const started_at = base->server_head;
        struct nameserver *ns;
        int err = 0;
        char addrbuf[128];

        ASSERT_LOCKED(base);
        if (server) {
                do {
                        if (!evutil_sockaddr_cmp((struct sockaddr*)&server->address, address, 1)) return 3;
                        server = server->next;
                } while (server != started_at);
        }
        if (addrlen > (int)sizeof(ns->address)) {
                log(EVDNS_LOG_DEBUG, "Addrlen %d too long.", (int)addrlen);
                return 2;
        }

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

        memset(ns, 0, sizeof(struct nameserver));
        ns->base = base;

        evtimer_assign(&ns->timeout_event, ns->base->event_base, nameserver_prod_callback, ns);

        ns->socket = evutil_socket_(address->sa_family,
            SOCK_DGRAM|EVUTIL_SOCK_NONBLOCK|EVUTIL_SOCK_CLOEXEC, 0);
        if (ns->socket < 0) { err = 1; goto out1; }

        if (base->global_outgoing_addrlen &&
            !evutil_sockaddr_is_loopback_(address)) {
                if (bind(ns->socket,
                        (struct sockaddr*)&base->global_outgoing_address,
                        base->global_outgoing_addrlen) < 0) {
                        log(EVDNS_LOG_WARN,"Couldn't bind to outgoing address");
                        err = 2;
                        goto out2;
                }
        }

        memcpy(&ns->address, address, addrlen);
        ns->addrlen = addrlen;
        ns->state = 1;
        event_assign(&ns->event, ns->base->event_base, ns->socket,
                                 EV_READ | EV_PERSIST, nameserver_ready_callback, ns);
        if (!base->disable_when_inactive && event_add(&ns->event, NULL) < 0) {
                err = 2;
                goto out2;
        }

        log(EVDNS_LOG_DEBUG, "Added nameserver %s as %p",
            evutil_format_sockaddr_port_(address, addrbuf, sizeof(addrbuf)), ns);

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

        base->global_good_nameservers++;

        return 0;

out2:
        evutil_closesocket(ns->socket);
out1:
        event_debug_unassign(&ns->event);
        mm_free(ns);
        log(EVDNS_LOG_WARN, "Unable to add nameserver %s: error %d",
            evutil_format_sockaddr_port_(address, addrbuf, sizeof(addrbuf)), err);
        return err;
}

/* exported function */
int
evdns_base_nameserver_add(struct evdns_base *base, unsigned long int address)
{
        struct sockaddr_in sin;
        int res;
        memset(&sin, 0, sizeof(sin));
        sin.sin_addr.s_addr = address;
        sin.sin_port = htons(53);
        sin.sin_family = AF_INET;
        EVDNS_LOCK(base);
        res = evdns_nameserver_add_impl_(base, (struct sockaddr*)&sin, sizeof(sin));
        EVDNS_UNLOCK(base);
        return res;
}

int
evdns_nameserver_add(unsigned long int address) {
        if (!current_base)
                current_base = evdns_base_new(NULL, 0);
        return evdns_base_nameserver_add(current_base, address);
}

static void
sockaddr_setport(struct sockaddr *sa, ev_uint16_t port)
{
        if (sa->sa_family == AF_INET) {
                ((struct sockaddr_in *)sa)->sin_port = htons(port);
        } else if (sa->sa_family == AF_INET6) {
                ((struct sockaddr_in6 *)sa)->sin6_port = htons(port);
        }
}

static ev_uint16_t
sockaddr_getport(struct sockaddr *sa)
{
        if (sa->sa_family == AF_INET) {
                return ntohs(((struct sockaddr_in *)sa)->sin_port);
        } else if (sa->sa_family == AF_INET6) {
                return ntohs(((struct sockaddr_in6 *)sa)->sin6_port);
        } else {
                return 0;
        }
}

/* exported function */
int
evdns_base_nameserver_ip_add(struct evdns_base *base, const char *ip_as_string) {
        struct sockaddr_storage ss;
        struct sockaddr *sa;
        int len = sizeof(ss);
        int res;
        if (evutil_parse_sockaddr_port(ip_as_string, (struct sockaddr *)&ss,
                &len)) {
                log(EVDNS_LOG_WARN, "Unable to parse nameserver address %s",
                        ip_as_string);
                return 4;
        }
        sa = (struct sockaddr *) &ss;
        if (sockaddr_getport(sa) == 0)
                sockaddr_setport(sa, 53);

        EVDNS_LOCK(base);
        res = evdns_nameserver_add_impl_(base, sa, len);
        EVDNS_UNLOCK(base);
        return res;
}

int
evdns_nameserver_ip_add(const char *ip_as_string) {
        if (!current_base)
                current_base = evdns_base_new(NULL, 0);
        return evdns_base_nameserver_ip_add(current_base, ip_as_string);
}

int
evdns_base_nameserver_sockaddr_add(struct evdns_base *base,
    const struct sockaddr *sa, ev_socklen_t len, unsigned flags)
{
        int res;
        EVUTIL_ASSERT(base);
        EVDNS_LOCK(base);
        res = evdns_nameserver_add_impl_(base, sa, len);
        EVDNS_UNLOCK(base);
        return res;
}

int
evdns_base_get_nameserver_addr(struct evdns_base *base, int idx,
    struct sockaddr *sa, ev_socklen_t len)
{
        int result = -1;
        int i;
        struct nameserver *server;
        EVDNS_LOCK(base);
        server = base->server_head;
        for (i = 0; i < idx && server; ++i, server = server->next) {
                if (server->next == base->server_head)
                        goto done;
        }
        if (! server)
                goto done;

        if (server->addrlen > len) {
                result = (int) server->addrlen;
                goto done;
        }

        memcpy(sa, &server->address, server->addrlen);
        result = (int) server->addrlen;
done:
        EVDNS_UNLOCK(base);
        return result;
}

/* remove from the queue */
static void
evdns_request_remove(struct request *req, struct request **head)
{
        ASSERT_LOCKED(req->base);
        ASSERT_VALID_REQUEST(req);

#if 0
        {
                struct request *ptr;
                int found = 0;
                EVUTIL_ASSERT(*head != NULL);

                ptr = *head;
                do {
                        if (ptr == req) {
                                found = 1;
                                break;
                        }
                        ptr = ptr->next;
                } while (ptr != *head);
                EVUTIL_ASSERT(found);

                EVUTIL_ASSERT(req->next);
        }
#endif

        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;
        }
        req->next = req->prev = NULL;
}

/* insert into the tail of the queue */
static void
evdns_request_insert(struct request *req, struct request **head) {
        ASSERT_LOCKED(req->base);
        ASSERT_VALID_REQUEST(req);
        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(struct evdns_base *base, struct evdns_request *handle, int type,
            const char *name, int flags, evdns_callback_type callback,
            void *user_ptr) {

        const char issuing_now =
            (base->global_requests_inflight < base->global_max_requests_inflight) ? 1 : 0;

        const size_t name_len = strlen(name);
        const size_t request_max_len = evdns_request_len(name_len);
        const u16 trans_id = issuing_now ? transaction_id_pick(base) : 0xffff;
        /* the request data is alloced in a single block with the header */
        struct request *const req =
            mm_malloc(sizeof(struct request) + request_max_len);
        int rlen;
        char namebuf[256];
        (void) flags;

        ASSERT_LOCKED(base);

        if (!req) return NULL;

        if (name_len >= sizeof(namebuf)) {
                mm_free(req);
                return NULL;
        }

        memset(req, 0, sizeof(struct request));
        req->base = base;

        evtimer_assign(&req->timeout_event, req->base->event_base, evdns_request_timeout_callback, req);

        if (base->global_randomize_case) {
                unsigned i;
                char randbits[(sizeof(namebuf)+7)/8];
                strlcpy(namebuf, name, sizeof(namebuf));
                evutil_secure_rng_get_bytes(randbits, (name_len+7)/8);
                for (i = 0; i < name_len; ++i) {
                        if (EVUTIL_ISALPHA_(namebuf[i])) {
                                if ((randbits[i >> 3] & (1<<(i & 7))))
                                        namebuf[i] |= 0x20;
                                else
                                        namebuf[i] &= ~0x20;
                        }
                }
                name = namebuf;
        }

        /* 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(base) : NULL;
        req->next = req->prev = NULL;
        req->handle = handle;
        if (handle) {
                handle->current_req = req;
                handle->base = base;
        }

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

static void
request_submit(struct request *const req) {
        struct evdns_base *base = req->base;
        ASSERT_LOCKED(base);
        ASSERT_VALID_REQUEST(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(base, req->trans_id));

                base->global_requests_inflight++;
                req->ns->requests_inflight++;

                evdns_request_transmit(req);
        } else {
                evdns_request_insert(req, &base->req_waiting_head);
                base->global_requests_waiting++;
        }
}

/* exported function */
void
evdns_cancel_request(struct evdns_base *base, struct evdns_request *handle)
{
        struct request *req;

        if (!handle->current_req)
                return;

        if (!base) {
                /* This redundancy is silly; can we fix it? (Not for 2.0) XXXX */
                base = handle->base;
                if (!base)
                        base = handle->current_req->base;
        }

        EVDNS_LOCK(base);
        if (handle->pending_cb) {
                EVDNS_UNLOCK(base);
                return;
        }

        req = handle->current_req;
        ASSERT_VALID_REQUEST(req);

        reply_schedule_callback(req, 0, DNS_ERR_CANCEL, NULL);
        if (req->ns) {
                /* remove from inflight queue */
                request_finished(req, &REQ_HEAD(base, req->trans_id), 1);
        } else {
                /* remove from global_waiting head */
                request_finished(req, &base->req_waiting_head, 1);
        }
        EVDNS_UNLOCK(base);
}

/* exported function */
struct evdns_request *
evdns_base_resolve_ipv4(struct evdns_base *base, const char *name, int flags,
    evdns_callback_type callback, void *ptr) {
        struct evdns_request *handle;
        struct request *req;
        log(EVDNS_LOG_DEBUG, "Resolve requested for %s", name);
        handle = mm_calloc(1, sizeof(*handle));
        if (handle == NULL)
                return NULL;
        EVDNS_LOCK(base);
        if (flags & DNS_QUERY_NO_SEARCH) {
                req =
                        request_new(base, handle, TYPE_A, name, flags,
                                    callback, ptr);
                if (req)
                        request_submit(req);
        } else {
                search_request_new(base, handle, TYPE_A, name, flags,
                    callback, ptr);
        }
        if (handle->current_req == NULL) {
                mm_free(handle);
                handle = NULL;
        }
        EVDNS_UNLOCK(base);
        return handle;
}

int evdns_resolve_ipv4(const char *name, int flags,
                                           evdns_callback_type callback, void *ptr)
{
        return evdns_base_resolve_ipv4(current_base, name, flags, callback, ptr)
                ? 0 : -1;
}


/* exported function */
struct evdns_request *
evdns_base_resolve_ipv6(struct evdns_base *base,
    const char *name, int flags,
    evdns_callback_type callback, void *ptr)
{
        struct evdns_request *handle;
        struct request *req;
        log(EVDNS_LOG_DEBUG, "Resolve requested for %s", name);
        handle = mm_calloc(1, sizeof(*handle));
        if (handle == NULL)
                return NULL;
        EVDNS_LOCK(base);
        if (flags & DNS_QUERY_NO_SEARCH) {
                req = request_new(base, handle, TYPE_AAAA, name, flags,
                                  callback, ptr);
                if (req)
                        request_submit(req);
        } else {
                search_request_new(base, handle, TYPE_AAAA, name, flags,
                    callback, ptr);
        }
        if (handle->current_req == NULL) {
                mm_free(handle);
                handle = NULL;
        }
        EVDNS_UNLOCK(base);
        return handle;
}

int evdns_resolve_ipv6(const char *name, int flags,
    evdns_callback_type callback, void *ptr) {
        return evdns_base_resolve_ipv6(current_base, name, flags, callback, ptr)
                ? 0 : -1;
}

struct evdns_request *
evdns_base_resolve_reverse(struct evdns_base *base, const struct in_addr *in, int flags, evdns_callback_type callback, void *ptr) {
        char buf[32];
        struct evdns_request *handle;
        struct request *req;
        u32 a;
        EVUTIL_ASSERT(in);
        a = ntohl(in->s_addr);
        evutil_snprintf(buf, sizeof(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));
        handle = mm_calloc(1, sizeof(*handle));
        if (handle == NULL)
                return NULL;
        log(EVDNS_LOG_DEBUG, "Resolve requested for %s (reverse)", buf);
        EVDNS_LOCK(base);
        req = request_new(base, handle, TYPE_PTR, buf, flags, callback, ptr);
        if (req)
                request_submit(req);
        if (handle->current_req == NULL) {
                mm_free(handle);
                handle = NULL;
        }
        EVDNS_UNLOCK(base);
        return (handle);
}

int evdns_resolve_reverse(const struct in_addr *in, int flags, evdns_callback_type callback, void *ptr) {
        return evdns_base_resolve_reverse(current_base, in, flags, callback, ptr)
                ? 0 : -1;
}

struct evdns_request *
evdns_base_resolve_reverse_ipv6(struct evdns_base *base, const struct in6_addr *in, int flags, evdns_callback_type callback, void *ptr) {
        /* 32 nybbles, 32 periods, "ip6.arpa", NUL. */
        char buf[73];
        char *cp;
        struct evdns_request *handle;
        struct request *req;
        int i;
        EVUTIL_ASSERT(in);
        cp = buf;
        for (i=15; i >= 0; --i) {
                u8 byte = in->s6_addr[i];
                *cp++ = "0123456789abcdef"[byte & 0x0f];
                *cp++ = '.';
                *cp++ = "0123456789abcdef"[byte >> 4];
                *cp++ = '.';
        }
        EVUTIL_ASSERT(cp + strlen("ip6.arpa") < buf+sizeof(buf));
        memcpy(cp, "ip6.arpa", strlen("ip6.arpa")+1);
        handle = mm_calloc(1, sizeof(*handle));
        if (handle == NULL)
                return NULL;
        log(EVDNS_LOG_DEBUG, "Resolve requested for %s (reverse)", buf);
        EVDNS_LOCK(base);
        req = request_new(base, handle, TYPE_PTR, buf, flags, callback, ptr);
        if (req)
                request_submit(req);
        if (handle->current_req == NULL) {
                mm_free(handle);
                handle = NULL;
        }
        EVDNS_UNLOCK(base);
        return (handle);
}

int evdns_resolve_reverse_ipv6(const struct in6_addr *in, int flags, evdns_callback_type callback, void *ptr) {
        return evdns_base_resolve_reverse_ipv6(current_base, in, flags, callback, ptr)
                ? 0 : -1;
}

/* ================================================================= */
/* 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 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;
                        mm_free(dom);
                }
                mm_free(state);
        }
}

static struct search_state *
search_state_new(void) {
        struct search_state *state = (struct search_state *) mm_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(struct evdns_base *base) {
        search_state_decref(base->global_search_state);

        base->global_search_state = search_state_new();
}

/* exported function */
void
evdns_base_search_clear(struct evdns_base *base)
{
        EVDNS_LOCK(base);
        search_postfix_clear(base);
        EVDNS_UNLOCK(base);
}

void
evdns_search_clear(void) {
        evdns_base_search_clear(current_base);
}

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

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

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

        base->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(struct evdns_base *base) {
        struct search_domain *cur, *prev = NULL, *next;
        ASSERT_LOCKED(base);
        cur = base->global_search_state->head;
        while (cur) {
                next = cur->next;
                cur->next = prev;
                prev = cur;
                cur = next;
        }

        base->global_search_state->head = prev;
}

/* exported function */
void
evdns_base_search_add(struct evdns_base *base, const char *domain) {
        EVDNS_LOCK(base);
        search_postfix_add(base, domain);
        EVDNS_UNLOCK(base);
}
void
evdns_search_add(const char *domain) {
        evdns_base_search_add(current_base, domain);
}

/* exported function */
void
evdns_base_search_ndots_set(struct evdns_base *base, const int ndots) {
        EVDNS_LOCK(base);
        if (!base->global_search_state) base->global_search_state = search_state_new();
        if (base->global_search_state)
                base->global_search_state->ndots = ndots;
        EVDNS_UNLOCK(base);
}
void
evdns_search_ndots_set(const int ndots) {
        evdns_base_search_ndots_set(current_base, ndots);
}

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

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

/* warning: returns malloced string */
static char *
search_make_new(const struct search_state *const state, int n, const char *const base_name) {
        const size_t 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 *) mm_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 */
        EVUTIL_ASSERT(0);
        return NULL; /* unreachable; stops warnings in some compilers. */
}

static struct request *
search_request_new(struct evdns_base *base, struct evdns_request *handle,
                   int type, const char *const name, int flags,
                   evdns_callback_type user_callback, void *user_arg) {
        ASSERT_LOCKED(base);
        EVUTIL_ASSERT(type == TYPE_A || type == TYPE_AAAA);
        EVUTIL_ASSERT(handle->current_req == NULL);
        if ( ((flags & DNS_QUERY_NO_SEARCH) == 0) &&
             base->global_search_state &&
                 base->global_search_state->num_domains) {
                /* we have some domains to search */
                struct request *req;
                if (string_num_dots(name) >= base->global_search_state->ndots) {
                        req = request_new(base, handle, type, name, flags, user_callback, user_arg);
                        if (!req) return NULL;
                        handle->search_index = -1;
                } else {
                        char *const new_name = search_make_new(base->global_search_state, 0, name);
                        if (!new_name) return NULL;
                        req = request_new(base, handle, type, new_name, flags, user_callback, user_arg);
                        mm_free(new_name);
                        if (!req) return NULL;
                        handle->search_index = 0;
                }
                EVUTIL_ASSERT(handle->search_origname == NULL);
                handle->search_origname = mm_strdup(name);
                if (handle->search_origname == NULL) {
                        /* XXX Should we dealloc req? If yes, how? */
                        if (req)
                                mm_free(req);
                        return NULL;
                }
                handle->search_state = base->global_search_state;
                handle->search_flags = flags;
                base->global_search_state->refcount++;
                request_submit(req);
                return req;
        } else {
                struct request *const req = request_new(base, handle, type, name, flags, user_callback, user_arg);
                if (!req) return NULL;
                request_submit(req);
                return req;
        }
}

/* 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 evdns_request *const handle) {
        struct request *req = handle->current_req;
        struct evdns_base *base = req->base;
        struct request *newreq;
        ASSERT_LOCKED(base);
        if (handle->search_state) {
                /* it is part of a search */
                char *new_name;
                handle->search_index++;
                if (handle->search_index >= handle->search_state->num_domains) {
                        /* no more postfixes to try, however we may need to try */
                        /* this name without a postfix */
                        if (string_num_dots(handle->search_origname) < handle->search_state->ndots) {
                                /* yep, we need to try it raw */
                                newreq = request_new(base, NULL, req->request_type, handle->search_origname, handle->search_flags, req->user_callback, req->user_pointer);
                                log(EVDNS_LOG_DEBUG, "Search: trying raw query %s", handle->search_origname);
                                if (newreq) {
                                        search_request_finished(handle);
                                        goto submit_next;
                                }
                        }
                        return 1;
                }

                new_name = search_make_new(handle->search_state, handle->search_index, handle->search_origname);
                if (!new_name) return 1;
                log(EVDNS_LOG_DEBUG, "Search: now trying %s (%d)", new_name, handle->search_index);
                newreq = request_new(base, NULL, req->request_type, new_name, handle->search_flags, req->user_callback, req->user_pointer);
                mm_free(new_name);
                if (!newreq) return 1;
                goto submit_next;
        }
        return 1;

submit_next:
        request_finished(req, &REQ_HEAD(req->base, req->trans_id), 0);
        handle->current_req = newreq;
        newreq->handle = handle;
        request_submit(newreq);
        return 0;
}

static void
search_request_finished(struct evdns_request *const handle) {
        ASSERT_LOCKED(handle->current_req->base);
        if (handle->search_state) {
                search_state_decref(handle->search_state);
                handle->search_state = NULL;
        }
        if (handle->search_origname) {
                mm_free(handle->search_origname);
                handle->search_origname = NULL;
        }
}

/* ================================================================= */
/* Parsing resolv.conf files */

static void
evdns_resolv_set_defaults(struct evdns_base *base, int flags) {
        /* if the file isn't found then we assume a local resolver */
        ASSERT_LOCKED(base);
        if (flags & DNS_OPTION_SEARCH) search_set_from_hostname(base);
        if (flags & DNS_OPTION_NAMESERVERS) evdns_base_nameserver_ip_add(base,"127.0.0.1");
}

#ifndef EVENT__HAVE_STRTOK_R
static char *
strtok_r(char *s, const char *delim, char **state) {
        char *cp, *start;
        start = cp = s ? s : *state;
        if (!cp)
                return NULL;
        while (*cp && !strchr(delim, *cp))
                ++cp;
        if (!*cp) {
                if (cp == start)
                        return NULL;
                *state = NULL;
                return start;
        } else {
                *cp++ = '\0';
                *state = cp;
                return start;
        }
}
#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;
}

/* Parse a number of seconds into a timeval; return -1 on error. */
static int
evdns_strtotimeval(const char *const str, struct timeval *out)
{
        double d;
        char *endptr;
        d = strtod(str, &endptr);
        if (*endptr) return -1;
        if (d < 0) return -1;
        out->tv_sec = (int) d;
        out->tv_usec = (int) ((d - (int) d)*1000000);
        if (out->tv_sec == 0 && out->tv_usec < 1000) /* less than 1 msec */
                return -1;
        return 0;
}

/* helper version of atoi that returns -1 on error and clips to bounds. */
static int
strtoint_clipped(const char *const str, int min, int max)
{
        int r = strtoint(str);
        if (r == -1)
                return r;
        else if (r<min)
                return min;
        else if (r>max)
                return max;
        else
                return r;
}

static int
evdns_base_set_max_requests_inflight(struct evdns_base *base, int maxinflight)
{
        int old_n_heads = base->n_req_heads, n_heads;
        struct request **old_heads = base->req_heads, **new_heads, *req;
        int i;

        ASSERT_LOCKED(base);
        if (maxinflight < 1)
                maxinflight = 1;
        n_heads = (maxinflight+4) / 5;
        EVUTIL_ASSERT(n_heads > 0);
        new_heads = mm_calloc(n_heads, sizeof(struct request*));
        if (!new_heads)
                return (-1);
        if (old_heads) {
                for (i = 0; i < old_n_heads; ++i) {
                        while (old_heads[i]) {
                                req = old_heads[i];
                                evdns_request_remove(req, &old_heads[i]);
                                evdns_request_insert(req, &new_heads[req->trans_id % n_heads]);
                        }
                }
                mm_free(old_heads);
        }
        base->req_heads = new_heads;
        base->n_req_heads = n_heads;
        base->global_max_requests_inflight = maxinflight;
        return (0);
}

/* exported function */
int
evdns_base_set_option(struct evdns_base *base,
    const char *option, const char *val)
{
        int res;
        EVDNS_LOCK(base);
        res = evdns_base_set_option_impl(base, option, val, DNS_OPTIONS_ALL);
        EVDNS_UNLOCK(base);
        return res;
}

static inline int
str_matches_option(const char *s1, const char *optionname)
{
        /* Option names are given as "option:" We accept either 'option' in
         * s1, or 'option:randomjunk'.  The latter form is to implement the
         * resolv.conf parser. */
        size_t optlen = strlen(optionname);
        size_t slen = strlen(s1);
        if (slen == optlen || slen == optlen - 1)
                return !strncmp(s1, optionname, slen);
        else if (slen > optlen)
                return !strncmp(s1, optionname, optlen);
        else
                return 0;
}

static int
evdns_base_set_option_impl(struct evdns_base *base,
    const char *option, const char *val, int flags)
{
        ASSERT_LOCKED(base);
        if (str_matches_option(option, "ndots:")) {
                const int ndots = strtoint(val);
                if (ndots == -1) return -1;
                if (!(flags & DNS_OPTION_SEARCH)) return 0;
                log(EVDNS_LOG_DEBUG, "Setting ndots to %d", ndots);
                if (!base->global_search_state) base->global_search_state = search_state_new();
                if (!base->global_search_state) return -1;
                base->global_search_state->ndots = ndots;
        } else if (str_matches_option(option, "timeout:")) {
                struct timeval tv;
                if (evdns_strtotimeval(val, &tv) == -1) return -1;
                if (!(flags & DNS_OPTION_MISC)) return 0;
                log(EVDNS_LOG_DEBUG, "Setting timeout to %s", val);
                memcpy(&base->global_timeout, &tv, sizeof(struct timeval));
        } else if (str_matches_option(option, "getaddrinfo-allow-skew:")) {
                struct timeval tv;
                if (evdns_strtotimeval(val, &tv) == -1) return -1;
                if (!(flags & DNS_OPTION_MISC)) return 0;
                log(EVDNS_LOG_DEBUG, "Setting getaddrinfo-allow-skew to %s",
                    val);
                memcpy(&base->global_getaddrinfo_allow_skew, &tv,
                    sizeof(struct timeval));
        } else if (str_matches_option(option, "max-timeouts:")) {
                const int maxtimeout = strtoint_clipped(val, 1, 255);
                if (maxtimeout == -1) return -1;
                if (!(flags & DNS_OPTION_MISC)) return 0;
                log(EVDNS_LOG_DEBUG, "Setting maximum allowed timeouts to %d",
                        maxtimeout);
                base->global_max_nameserver_timeout = maxtimeout;
        } else if (str_matches_option(option, "max-inflight:")) {
                const int maxinflight = strtoint_clipped(val, 1, 65000);
                if (maxinflight == -1) return -1;
                if (!(flags & DNS_OPTION_MISC)) return 0;
                log(EVDNS_LOG_DEBUG, "Setting maximum inflight requests to %d",
                        maxinflight);
                evdns_base_set_max_requests_inflight(base, maxinflight);
        } else if (str_matches_option(option, "attempts:")) {
                int retries = strtoint(val);
                if (retries == -1) return -1;
                if (retries > 255) retries = 255;
                if (!(flags & DNS_OPTION_MISC)) return 0;
                log(EVDNS_LOG_DEBUG, "Setting retries to %d", retries);
                base->global_max_retransmits = retries;
        } else if (str_matches_option(option, "randomize-case:")) {
                int randcase = strtoint(val);
                if (!(flags & DNS_OPTION_MISC)) return 0;
                base->global_randomize_case = randcase;
        } else if (str_matches_option(option, "bind-to:")) {
                /* XXX This only applies to successive nameservers, not
                 * to already-configured ones.  We might want to fix that. */
                int len = sizeof(base->global_outgoing_address);
                if (!(flags & DNS_OPTION_NAMESERVERS)) return 0;
                if (evutil_parse_sockaddr_port(val,
                        (struct sockaddr*)&base->global_outgoing_address, &len))
                        return -1;
                base->global_outgoing_addrlen = len;
        } else if (str_matches_option(option, "initial-probe-timeout:")) {
                struct timeval tv;
                if (evdns_strtotimeval(val, &tv) == -1) return -1;
                if (tv.tv_sec > 3600)
                        tv.tv_sec = 3600;
                if (!(flags & DNS_OPTION_MISC)) return 0;
                log(EVDNS_LOG_DEBUG, "Setting initial probe timeout to %s",
                    val);
                memcpy(&base->global_nameserver_probe_initial_timeout, &tv,
                    sizeof(tv));
        }
        return 0;
}

int
evdns_set_option(const char *option, const char *val, int flags)
{
        if (!current_base)
                current_base = evdns_base_new(NULL, 0);
        return evdns_base_set_option(current_base, option, val);
}

static void
resolv_conf_parse_line(struct evdns_base *base, 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);
        ASSERT_LOCKED(base);
        if (!first_token) return;

        if (!strcmp(first_token, "nameserver") && (flags & DNS_OPTION_NAMESERVERS)) {
                const char *const nameserver = NEXT_TOKEN;

                if (nameserver)
                        evdns_base_nameserver_ip_add(base, nameserver);
        } else if (!strcmp(first_token, "domain") && (flags & DNS_OPTION_SEARCH)) {
                const char *const domain = NEXT_TOKEN;
                if (domain) {
                        search_postfix_clear(base);
                        search_postfix_add(base, domain);
                }
        } else if (!strcmp(first_token, "search") && (flags & DNS_OPTION_SEARCH)) {
                const char *domain;
                search_postfix_clear(base);

                while ((domain = NEXT_TOKEN)) {
                        search_postfix_add(base, domain);
                }
                search_reverse(base);
        } else if (!strcmp(first_token, "options")) {
                const char *option;
                while ((option = NEXT_TOKEN)) {
                        const char *val = strchr(option, ':');
                        evdns_base_set_option_impl(base, option, val ? val+1 : "", flags);
                }
        }
#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_base_resolv_conf_parse(struct evdns_base *base, int flags, const char *const filename) {
        int res;
        EVDNS_LOCK(base);
        res = evdns_base_resolv_conf_parse_impl(base, flags, filename);
        EVDNS_UNLOCK(base);
        return res;
}

static char *
evdns_get_default_hosts_filename(void)
{
#ifdef _WIN32
        /* Windows is a little coy about where it puts its configuration
         * files.  Sure, they're _usually_ in C:\windows\system32, but
         * there's no reason in principle they couldn't be in
         * W:\hoboken chicken emergency\
         */
        char path[MAX_PATH+1];
        static const char hostfile[] = "\\drivers\\etc\\hosts";
        char *path_out;
        size_t len_out;

        if (! SHGetSpecialFolderPathA(NULL, path, CSIDL_SYSTEM, 0))
                return NULL;
        len_out = strlen(path)+strlen(hostfile)+1;
        path_out = mm_malloc(len_out);
        evutil_snprintf(path_out, len_out, "%s%s", path, hostfile);
        return path_out;
#else
        return mm_strdup("/etc/hosts");
#endif
}

static int
evdns_base_resolv_conf_parse_impl(struct evdns_base *base, int flags, const char *const filename) {
        size_t n;
        char *resolv;
        char *start;
        int err = 0;

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

        if (flags & DNS_OPTION_HOSTSFILE) {
                char *fname = evdns_get_default_hosts_filename();
                evdns_base_load_hosts(base, fname);
                if (fname)
                        mm_free(fname);
        }

        if ((err = evutil_read_file_(filename, &resolv, &n, 0)) < 0) {
                if (err == -1) {
                        /* No file. */
                        evdns_resolv_set_defaults(base, flags);
                        return 1;
                } else {
                        return 2;
                }
        }

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

        if (!base->server_head && (flags & DNS_OPTION_NAMESERVERS)) {
                /* no nameservers were configured. */
                evdns_base_nameserver_ip_add(base, "127.0.0.1");
                err = 6;
        }
        if (flags & DNS_OPTION_SEARCH && (!base->global_search_state || base->global_search_state->num_domains == 0)) {
                search_set_from_hostname(base);
        }

        mm_free(resolv);
        return err;
}

int
evdns_resolv_conf_parse(int flags, const char *const filename) {
        if (!current_base)
                current_base = evdns_base_new(NULL, 0);
        return evdns_base_resolv_conf_parse(current_base, flags, filename);
}


#ifdef _WIN32
/* Add multiple nameservers from a space-or-comma-separated list. */
static int
evdns_nameserver_ip_add_line(struct evdns_base *base, const char *ips) {
        const char *addr;
        char *buf;
        int r;
        ASSERT_LOCKED(base);
        while (*ips) {
                while (isspace(*ips) || *ips == ',' || *ips == '\t')
                        ++ips;
                addr = ips;
                while (isdigit(*ips) || *ips == '.' || *ips == ':' ||
                    *ips=='[' || *ips==']')
                        ++ips;
                buf = mm_malloc(ips-addr+1);
                if (!buf) return 4;
                memcpy(buf, addr, ips-addr);
                buf[ips-addr] = '\0';
                r = evdns_base_nameserver_ip_add(base, buf);
                mm_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(struct evdns_base *base)
{
        /* 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;

        ASSERT_LOCKED(base);
        if (!(handle = evutil_load_windows_system_library_(
                        TEXT("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 = mm_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) {
                mm_free(buf);
                buf = mm_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;
                }
        }

        EVUTIL_ASSERT(fixed);
        added_any = 0;
        ns = &(fixed->DnsServerList);
        while (ns) {
                r = evdns_nameserver_ip_add_line(base, 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;
                } else {
                        ++added_any;
                        log(EVDNS_LOG_DEBUG,"Successfully added %s as nameserver",ns->IpAddress.String);
                }

                ns = ns->Next;
        }

        if (!added_any) {
                log(EVDNS_LOG_DEBUG, "No nameservers added.");
                if (status == 0)
                        status = -1;
        } else {
                status = 0;
        }

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

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

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

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

        mm_free(buf);
        return status;
}

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

static int
load_nameservers_from_registry(struct evdns_base *base)
{
        int found = 0;
        int r;
#define TRY(k, name) \
        if (!found && config_nameserver_from_reg_key(base,k,TEXT(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);                                          \
        }

        ASSERT_LOCKED(base);

        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, TEXT("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_base_config_windows_nameservers(struct evdns_base *base)
{
        int r;
        char *fname;
        if (base == NULL)
                base = current_base;
        if (base == NULL)
                return -1;
        EVDNS_LOCK(base);
        fname = evdns_get_default_hosts_filename();
        log(EVDNS_LOG_DEBUG, "Loading hosts entries from %s", fname);
        evdns_base_load_hosts(base, fname);
        if (fname)
                mm_free(fname);

        if (load_nameservers_with_getnetworkparams(base) == 0) {
                EVDNS_UNLOCK(base);
                return 0;
        }
        r = load_nameservers_from_registry(base);

        EVDNS_UNLOCK(base);
        return r;
}

int
evdns_config_windows_nameservers(void)
{
        if (!current_base) {
                current_base = evdns_base_new(NULL, 1);
                return current_base == NULL ? -1 : 0;
        } else {
                return evdns_base_config_windows_nameservers(current_base);
        }
}
#endif

struct evdns_base *
evdns_base_new(struct event_base *event_base, int flags)
{
        struct evdns_base *base;

        if (evutil_secure_rng_init() < 0) {
                log(EVDNS_LOG_WARN, "Unable to seed random number generator; "
                    "DNS can't run.");
                return NULL;
        }

        /* Give the evutil library a hook into its evdns-enabled
         * functionality.  We can't just call evdns_getaddrinfo directly or
         * else libevent-core will depend on libevent-extras. */
        evutil_set_evdns_getaddrinfo_fn_(evdns_getaddrinfo);

        base = mm_malloc(sizeof(struct evdns_base));
        if (base == NULL)
                return (NULL);
        memset(base, 0, sizeof(struct evdns_base));
        base->req_waiting_head = NULL;

        EVTHREAD_ALLOC_LOCK(base->lock, EVTHREAD_LOCKTYPE_RECURSIVE);
        EVDNS_LOCK(base);

        /* Set max requests inflight and allocate req_heads. */
        base->req_heads = NULL;

        evdns_base_set_max_requests_inflight(base, 64);

        base->server_head = NULL;
        base->event_base = event_base;
        base->global_good_nameservers = base->global_requests_inflight =
                base->global_requests_waiting = 0;

        base->global_timeout.tv_sec = 5;
        base->global_timeout.tv_usec = 0;
        base->global_max_reissues = 1;
        base->global_max_retransmits = 3;
        base->global_max_nameserver_timeout = 3;
        base->global_search_state = NULL;
        base->global_randomize_case = 1;
        base->global_getaddrinfo_allow_skew.tv_sec = 3;
        base->global_getaddrinfo_allow_skew.tv_usec = 0;
        base->global_nameserver_probe_initial_timeout.tv_sec = 10;
        base->global_nameserver_probe_initial_timeout.tv_usec = 0;

        TAILQ_INIT(&base->hostsdb);

#define EVDNS_BASE_ALL_FLAGS (0x8001)
        if (flags & ~EVDNS_BASE_ALL_FLAGS) {
                flags = EVDNS_BASE_INITIALIZE_NAMESERVERS;
                log(EVDNS_LOG_WARN,
                    "Unrecognized flag passed to evdns_base_new(). Assuming "
                    "you meant EVDNS_BASE_INITIALIZE_NAMESERVERS.");
        }
#undef EVDNS_BASE_ALL_FLAGS

        if (flags & EVDNS_BASE_INITIALIZE_NAMESERVERS) {
                int r;
#ifdef _WIN32
                r = evdns_base_config_windows_nameservers(base);
#else
                r = evdns_base_resolv_conf_parse(base, DNS_OPTIONS_ALL, "/etc/resolv.conf");
#endif
                if (r == -1) {
                        evdns_base_free_and_unlock(base, 0);
                        return NULL;
                }
        }
        if (flags & EVDNS_BASE_DISABLE_WHEN_INACTIVE) {
                base->disable_when_inactive = 1;
        }

        EVDNS_UNLOCK(base);
        return base;
}

int
evdns_init(void)
{
        struct evdns_base *base = evdns_base_new(NULL, 1);
        if (base) {
                current_base = base;
                return 0;
        } else {
                return -1;
        }
}

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";
        case DNS_ERR_CANCEL: return "dns request canceled";
        case DNS_ERR_NODATA: return "no records in the reply";
        default: return "[Unknown error code]";
    }
}

static void
evdns_nameserver_free(struct nameserver *server)
{
        if (server->socket >= 0)
        evutil_closesocket(server->socket);
        (void) event_del(&server->event);
        event_debug_unassign(&server->event);
        if (server->state == 0)
                (void) event_del(&server->timeout_event);
        if (server->probe_request) {
                evdns_cancel_request(server->base, server->probe_request);
                server->probe_request = NULL;
        }
        event_debug_unassign(&server->timeout_event);
        mm_free(server);
}

static void
evdns_base_free_and_unlock(struct evdns_base *base, int fail_requests)
{
        struct nameserver *server, *server_next;
        struct search_domain *dom, *dom_next;
        int i;

        /* Requires that we hold the lock. */

        /* TODO(nickm) we might need to refcount here. */

        for (server = base->server_head; server; server = server_next) {
                server_next = server->next;
                evdns_nameserver_free(server);
                if (server_next == base->server_head)
                        break;
        }
        base->server_head = NULL;
        base->global_good_nameservers = 0;

        for (i = 0; i < base->n_req_heads; ++i) {
                while (base->req_heads[i]) {
                        if (fail_requests)
                                reply_schedule_callback(base->req_heads[i], 0, DNS_ERR_SHUTDOWN, NULL);
                        request_finished(base->req_heads[i], &REQ_HEAD(base, base->req_heads[i]->trans_id), 1);
                }
        }
        while (base->req_waiting_head) {
                if (fail_requests)
                        reply_schedule_callback(base->req_waiting_head, 0, DNS_ERR_SHUTDOWN, NULL);
                request_finished(base->req_waiting_head, &base->req_waiting_head, 1);
        }
        base->global_requests_inflight = base->global_requests_waiting = 0;


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

        {
                struct hosts_entry *victim;
                while ((victim = TAILQ_FIRST(&base->hostsdb))) {
                        TAILQ_REMOVE(&base->hostsdb, victim, next);
                        mm_free(victim);
                }
        }

        mm_free(base->req_heads);

        EVDNS_UNLOCK(base);
        EVTHREAD_FREE_LOCK(base->lock, EVTHREAD_LOCKTYPE_RECURSIVE);

        mm_free(base);
}

void
evdns_base_free(struct evdns_base *base, int fail_requests)
{
        EVDNS_LOCK(base);
        evdns_base_free_and_unlock(base, fail_requests);
}

void
evdns_base_clear_host_addresses(struct evdns_base *base)
{
        struct hosts_entry *victim;
        EVDNS_LOCK(base);
        while ((victim = TAILQ_FIRST(&base->hostsdb))) {
                TAILQ_REMOVE(&base->hostsdb, victim, next);
                mm_free(victim);
        }
        EVDNS_UNLOCK(base);
}

void
evdns_shutdown(int fail_requests)
{
        if (current_base) {
                struct evdns_base *b = current_base;
                current_base = NULL;
                evdns_base_free(b, fail_requests);
        }
        evdns_log_fn = NULL;
}

static int
evdns_base_parse_hosts_line(struct evdns_base *base, char *line)
{
        char *strtok_state;
        static const char *const delims = " \t";
        char *const addr = strtok_r(line, delims, &strtok_state);
        char *hostname, *hash;
        struct sockaddr_storage ss;
        int socklen = sizeof(ss);
        ASSERT_LOCKED(base);

#define NEXT_TOKEN strtok_r(NULL, delims, &strtok_state)

        if (!addr || *addr == '#')
                return 0;

        memset(&ss, 0, sizeof(ss));
        if (evutil_parse_sockaddr_port(addr, (struct sockaddr*)&ss, &socklen)<0)
                return -1;
        if (socklen > (int)sizeof(struct sockaddr_in6))
                return -1;

        if (sockaddr_getport((struct sockaddr*)&ss))
                return -1;

        while ((hostname = NEXT_TOKEN)) {
                struct hosts_entry *he;
                size_t namelen;
                if ((hash = strchr(hostname, '#'))) {
                        if (hash == hostname)
                                return 0;
                        *hash = '\0';
                }

                namelen = strlen(hostname);

                he = mm_calloc(1, sizeof(struct hosts_entry)+namelen);
                if (!he)
                        return -1;
                EVUTIL_ASSERT(socklen <= (int)sizeof(he->addr));
                memcpy(&he->addr, &ss, socklen);
                memcpy(he->hostname, hostname, namelen+1);
                he->addrlen = socklen;

                TAILQ_INSERT_TAIL(&base->hostsdb, he, next);

                if (hash)
                        return 0;
        }

        return 0;
#undef NEXT_TOKEN
}

static int
evdns_base_load_hosts_impl(struct evdns_base *base, const char *hosts_fname)
{
        char *str=NULL, *cp, *eol;
        size_t len;
        int err=0;

        ASSERT_LOCKED(base);

        if (hosts_fname == NULL ||
            (err = evutil_read_file_(hosts_fname, &str, &len, 0)) < 0) {
                char tmp[64];
                strlcpy(tmp, "127.0.0.1   localhost", sizeof(tmp));
                evdns_base_parse_hosts_line(base, tmp);
                strlcpy(tmp, "::1   localhost", sizeof(tmp));
                evdns_base_parse_hosts_line(base, tmp);
                return err ? -1 : 0;
        }

        /* This will break early if there is a NUL in the hosts file.
         * Probably not a problem.*/
        cp = str;
        for (;;) {
                eol = strchr(cp, '\n');

                if (eol) {
                        *eol = '\0';
                        evdns_base_parse_hosts_line(base, cp);
                        cp = eol+1;
                } else {
                        evdns_base_parse_hosts_line(base, cp);
                        break;
                }
        }

        mm_free(str);
        return 0;
}

int
evdns_base_load_hosts(struct evdns_base *base, const char *hosts_fname)
{
        int res;
        if (!base)
                base = current_base;
        EVDNS_LOCK(base);
        res = evdns_base_load_hosts_impl(base, hosts_fname);
        EVDNS_UNLOCK(base);
        return res;
}

/* A single request for a getaddrinfo, either v4 or v6. */
struct getaddrinfo_subrequest {
        struct evdns_request *r;
        ev_uint32_t type;
};

/* State data used to implement an in-progress getaddrinfo. */
struct evdns_getaddrinfo_request {
        struct evdns_base *evdns_base;
        /* Copy of the modified 'hints' data that we'll use to build
         * answers. */
        struct evutil_addrinfo hints;
        /* The callback to invoke when we're done */
        evdns_getaddrinfo_cb user_cb;
        /* User-supplied data to give to the callback. */
        void *user_data;
        /* The port to use when building sockaddrs. */
        ev_uint16_t port;
        /* The sub_request for an A record (if any) */
        struct getaddrinfo_subrequest ipv4_request;
        /* The sub_request for an AAAA record (if any) */
        struct getaddrinfo_subrequest ipv6_request;

        /* The cname result that we were told (if any) */
        char *cname_result;

        /* If we have one request answered and one request still inflight,
         * then this field holds the answer from the first request... */
        struct evutil_addrinfo *pending_result;
        /* And this event is a timeout that will tell us to cancel the second
         * request if it's taking a long time. */
        struct event timeout;

        /* And this field holds the error code from the first request... */
        int pending_error;
        /* If this is set, the user canceled this request. */
        unsigned user_canceled : 1;
        /* If this is set, the user can no longer cancel this request; we're
         * just waiting for the free. */
        unsigned request_done : 1;
};

/* Convert an evdns errors to the equivalent getaddrinfo error. */
static int
evdns_err_to_getaddrinfo_err(int e1)
{
        /* XXX Do this better! */
        if (e1 == DNS_ERR_NONE)
                return 0;
        else if (e1 == DNS_ERR_NOTEXIST)
                return EVUTIL_EAI_NONAME;
        else
                return EVUTIL_EAI_FAIL;
}

/* Return the more informative of two getaddrinfo errors. */
static int
getaddrinfo_merge_err(int e1, int e2)
{
        /* XXXX be cleverer here. */
        if (e1 == 0)
                return e2;
        else
                return e1;
}

static void
free_getaddrinfo_request(struct evdns_getaddrinfo_request *data)
{
        /* DO NOT CALL this if either of the requests is pending.  Only once
         * both callbacks have been invoked is it safe to free the request */
        if (data->pending_result)
                evutil_freeaddrinfo(data->pending_result);
        if (data->cname_result)
                mm_free(data->cname_result);
        event_del(&data->timeout);
        mm_free(data);
        return;
}

static void
add_cname_to_reply(struct evdns_getaddrinfo_request *data,
    struct evutil_addrinfo *ai)
{
        if (data->cname_result && ai) {
                ai->ai_canonname = data->cname_result;
                data->cname_result = NULL;
        }
}

/* Callback: invoked when one request in a mixed-format A/AAAA getaddrinfo
 * request has finished, but the other one took too long to answer. Pass
 * along the answer we got, and cancel the other request.
 */
static void
evdns_getaddrinfo_timeout_cb(evutil_socket_t fd, short what, void *ptr)
{
        int v4_timedout = 0, v6_timedout = 0;
        struct evdns_getaddrinfo_request *data = ptr;

        /* Cancel any pending requests, and note which one */
        if (data->ipv4_request.r) {
                /* XXXX This does nothing if the request's callback is already
                 * running (pending_cb is set). */
                evdns_cancel_request(NULL, data->ipv4_request.r);
                v4_timedout = 1;
                EVDNS_LOCK(data->evdns_base);
                ++data->evdns_base->getaddrinfo_ipv4_timeouts;
                EVDNS_UNLOCK(data->evdns_base);
        }
        if (data->ipv6_request.r) {
                /* XXXX This does nothing if the request's callback is already
                 * running (pending_cb is set). */
                evdns_cancel_request(NULL, data->ipv6_request.r);
                v6_timedout = 1;
                EVDNS_LOCK(data->evdns_base);
                ++data->evdns_base->getaddrinfo_ipv6_timeouts;
                EVDNS_UNLOCK(data->evdns_base);
        }

        /* We only use this timeout callback when we have an answer for
         * one address. */
        EVUTIL_ASSERT(!v4_timedout || !v6_timedout);

        /* Report the outcome of the other request that didn't time out. */
        if (data->pending_result) {
                add_cname_to_reply(data, data->pending_result);
                data->user_cb(0, data->pending_result, data->user_data);
                data->pending_result = NULL;
        } else {
                int e = data->pending_error;
                if (!e)
                        e = EVUTIL_EAI_AGAIN;
                data->user_cb(e, NULL, data->user_data);
        }

        data->user_cb = NULL; /* prevent double-call if evdns callbacks are
                               * in-progress. XXXX It would be better if this
                               * weren't necessary. */

        if (!v4_timedout && !v6_timedout) {
                /* should be impossible? XXXX */
                free_getaddrinfo_request(data);
        }
}

static int
evdns_getaddrinfo_set_timeout(struct evdns_base *evdns_base,
    struct evdns_getaddrinfo_request *data)
{
        return event_add(&data->timeout, &evdns_base->global_getaddrinfo_allow_skew);
}

static inline int
evdns_result_is_answer(int result)
{
        return (result != DNS_ERR_NOTIMPL && result != DNS_ERR_REFUSED &&
            result != DNS_ERR_SERVERFAILED && result != DNS_ERR_CANCEL);
}

static void
evdns_getaddrinfo_gotresolve(int result, char type, int count,
    int ttl, void *addresses, void *arg)
{
        int i;
        struct getaddrinfo_subrequest *req = arg;
        struct getaddrinfo_subrequest *other_req;
        struct evdns_getaddrinfo_request *data;

        struct evutil_addrinfo *res;

        struct sockaddr_in sin;
        struct sockaddr_in6 sin6;
        struct sockaddr *sa;
        int socklen, addrlen;
        void *addrp;
        int err;
        int user_canceled;

        EVUTIL_ASSERT(req->type == DNS_IPv4_A || req->type == DNS_IPv6_AAAA);
        if (req->type == DNS_IPv4_A) {
                data = EVUTIL_UPCAST(req, struct evdns_getaddrinfo_request, ipv4_request);
                other_req = &data->ipv6_request;
        } else {
                data = EVUTIL_UPCAST(req, struct evdns_getaddrinfo_request, ipv6_request);
                other_req = &data->ipv4_request;
        }

        EVDNS_LOCK(data->evdns_base);
        if (evdns_result_is_answer(result)) {
                if (req->type == DNS_IPv4_A)
                        ++data->evdns_base->getaddrinfo_ipv4_answered;
                else
                        ++data->evdns_base->getaddrinfo_ipv6_answered;
        }
        user_canceled = data->user_canceled;
        if (other_req->r == NULL)
                data->request_done = 1;
        EVDNS_UNLOCK(data->evdns_base);

        req->r = NULL;

        if (result == DNS_ERR_CANCEL && ! user_canceled) {
                /* Internal cancel request from timeout or internal error.
                 * we already answered the user. */
                if (other_req->r == NULL)
                        free_getaddrinfo_request(data);
                return;
        }

        if (data->user_cb == NULL) {
                /* We already answered.  XXXX This shouldn't be needed; see
                 * comments in evdns_getaddrinfo_timeout_cb */
                free_getaddrinfo_request(data);
                return;
        }

        if (result == DNS_ERR_NONE) {
                if (count == 0)
                        err = EVUTIL_EAI_NODATA;
                else
                        err = 0;
        } else {
                err = evdns_err_to_getaddrinfo_err(result);
        }

        if (err) {
                /* Looks like we got an error. */
                if (other_req->r) {
                        /* The other request is still working; maybe it will
                         * succeed. */
                        /* XXXX handle failure from set_timeout */
                        evdns_getaddrinfo_set_timeout(data->evdns_base, data);
                        data->pending_error = err;
                        return;
                }

                if (user_canceled) {
                        data->user_cb(EVUTIL_EAI_CANCEL, NULL, data->user_data);
                } else if (data->pending_result) {
                        /* If we have an answer waiting, and we weren't
                         * canceled, ignore this error. */
                        add_cname_to_reply(data, data->pending_result);
                        data->user_cb(0, data->pending_result, data->user_data);
                        data->pending_result = NULL;
                } else {
                        if (data->pending_error)
                                err = getaddrinfo_merge_err(err,
                                    data->pending_error);
                        data->user_cb(err, NULL, data->user_data);
                }
                free_getaddrinfo_request(data);
                return;
        } else if (user_canceled) {
                if (other_req->r) {
                        /* The other request is still working; let it hit this
                         * callback with EVUTIL_EAI_CANCEL callback and report
                         * the failure. */
                        return;
                }
                data->user_cb(EVUTIL_EAI_CANCEL, NULL, data->user_data);
                free_getaddrinfo_request(data);
                return;
        }

        /* Looks like we got some answers. We should turn them into addrinfos
         * and then either queue those or return them all. */
        EVUTIL_ASSERT(type == DNS_IPv4_A || type == DNS_IPv6_AAAA);

        if (type == DNS_IPv4_A) {
                memset(&sin, 0, sizeof(sin));
                sin.sin_family = AF_INET;
                sin.sin_port = htons(data->port);

                sa = (struct sockaddr *)&sin;
                socklen = sizeof(sin);
                addrlen = 4;
                addrp = &sin.sin_addr.s_addr;
        } else {
                memset(&sin6, 0, sizeof(sin6));
                sin6.sin6_family = AF_INET6;
                sin6.sin6_port = htons(data->port);

                sa = (struct sockaddr *)&sin6;
                socklen = sizeof(sin6);
                addrlen = 16;
                addrp = &sin6.sin6_addr.s6_addr;
        }

        res = NULL;
        for (i=0; i < count; ++i) {
                struct evutil_addrinfo *ai;
                memcpy(addrp, ((char*)addresses)+i*addrlen, addrlen);
                ai = evutil_new_addrinfo_(sa, socklen, &data->hints);
                if (!ai) {
                        if (other_req->r) {
                                evdns_cancel_request(NULL, other_req->r);
                        }
                        data->user_cb(EVUTIL_EAI_MEMORY, NULL, data->user_data);
                        if (res)
                                evutil_freeaddrinfo(res);

                        if (other_req->r == NULL)
                                free_getaddrinfo_request(data);
                        return;
                }
                res = evutil_addrinfo_append_(res, ai);
        }

        if (other_req->r) {
                /* The other request is still in progress; wait for it */
                /* XXXX handle failure from set_timeout */
                evdns_getaddrinfo_set_timeout(data->evdns_base, data);
                data->pending_result = res;
                return;
        } else {
                /* The other request is done or never started; append its
                 * results (if any) and return them. */
                if (data->pending_result) {
                        if (req->type == DNS_IPv4_A)
                                res = evutil_addrinfo_append_(res,
                                    data->pending_result);
                        else
                                res = evutil_addrinfo_append_(
                                    data->pending_result, res);
                        data->pending_result = NULL;
                }

                /* Call the user callback. */
                add_cname_to_reply(data, res);
                data->user_cb(0, res, data->user_data);

                /* Free data. */
                free_getaddrinfo_request(data);
        }
}

static struct hosts_entry *
find_hosts_entry(struct evdns_base *base, const char *hostname,
    struct hosts_entry *find_after)
{
        struct hosts_entry *e;

        if (find_after)
                e = TAILQ_NEXT(find_after, next);
        else
                e = TAILQ_FIRST(&base->hostsdb);

        for (; e; e = TAILQ_NEXT(e, next)) {
                if (!evutil_ascii_strcasecmp(e->hostname, hostname))
                        return e;
        }
        return NULL;
}

static int
evdns_getaddrinfo_fromhosts(struct evdns_base *base,
    const char *nodename, struct evutil_addrinfo *hints, ev_uint16_t port,
    struct evutil_addrinfo **res)
{
        int n_found = 0;
        struct hosts_entry *e;
        struct evutil_addrinfo *ai=NULL;
        int f = hints->ai_family;

        EVDNS_LOCK(base);
        for (e = find_hosts_entry(base, nodename, NULL); e;
            e = find_hosts_entry(base, nodename, e)) {
                struct evutil_addrinfo *ai_new;
                ++n_found;
                if ((e->addr.sa.sa_family == AF_INET && f == PF_INET6) ||
                    (e->addr.sa.sa_family == AF_INET6 && f == PF_INET))
                        continue;
                ai_new = evutil_new_addrinfo_(&e->addr.sa, e->addrlen, hints);
                if (!ai_new) {
                        n_found = 0;
                        goto out;
                }
                sockaddr_setport(ai_new->ai_addr, port);
                ai = evutil_addrinfo_append_(ai, ai_new);
        }
        EVDNS_UNLOCK(base);
out:
        if (n_found) {
                /* Note that we return an empty answer if we found entries for
                 * this hostname but none were of the right address type. */
                *res = ai;
                return 0;
        } else {
                if (ai)
                        evutil_freeaddrinfo(ai);
                return -1;
        }
}

struct evdns_getaddrinfo_request *
evdns_getaddrinfo(struct evdns_base *dns_base,
    const char *nodename, const char *servname,
    const struct evutil_addrinfo *hints_in,
    evdns_getaddrinfo_cb cb, void *arg)
{
        struct evdns_getaddrinfo_request *data;
        struct evutil_addrinfo hints;
        struct evutil_addrinfo *res = NULL;
        int err;
        int port = 0;
        int want_cname = 0;

        if (!dns_base) {
                dns_base = current_base;
                if (!dns_base) {
                        log(EVDNS_LOG_WARN,
                            "Call to getaddrinfo_async with no "
                            "evdns_base configured.");
                        cb(EVUTIL_EAI_FAIL, NULL, arg); /* ??? better error? */
                        return NULL;
                }
        }

        /* If we _must_ answer this immediately, do so. */
        if ((hints_in && (hints_in->ai_flags & EVUTIL_AI_NUMERICHOST))) {
                res = NULL;
                err = evutil_getaddrinfo(nodename, servname, hints_in, &res);
                cb(err, res, arg);
                return NULL;
        }

        if (hints_in) {
                memcpy(&hints, hints_in, sizeof(hints));
        } else {
                memset(&hints, 0, sizeof(hints));
                hints.ai_family = PF_UNSPEC;
        }

        evutil_adjust_hints_for_addrconfig_(&hints);

        /* Now try to see if we _can_ answer immediately. */
        /* (It would be nice to do this by calling getaddrinfo directly, with
         * AI_NUMERICHOST, on plaforms that have it, but we can't: there isn't
         * a reliable way to distinguish the "that wasn't a numeric host!" case
         * from any other EAI_NONAME cases.) */
        err = evutil_getaddrinfo_common_(nodename, servname, &hints, &res, &port);
        if (err != EVUTIL_EAI_NEED_RESOLVE) {
                cb(err, res, arg);
                return NULL;
        }

        /* If there is an entry in the hosts file, we should give it now. */
        if (!evdns_getaddrinfo_fromhosts(dns_base, nodename, &hints, port, &res)) {
                cb(0, res, arg);
                return NULL;
        }

        /* Okay, things are serious now. We're going to need to actually
         * launch a request.
         */
        data = mm_calloc(1,sizeof(struct evdns_getaddrinfo_request));
        if (!data) {
                cb(EVUTIL_EAI_MEMORY, NULL, arg);
                return NULL;
        }

        memcpy(&data->hints, &hints, sizeof(data->hints));
        data->port = (ev_uint16_t)port;
        data->ipv4_request.type = DNS_IPv4_A;
        data->ipv6_request.type = DNS_IPv6_AAAA;
        data->user_cb = cb;
        data->user_data = arg;
        data->evdns_base = dns_base;

        want_cname = (hints.ai_flags & EVUTIL_AI_CANONNAME);

        /* If we are asked for a PF_UNSPEC address, we launch two requests in
         * parallel: one for an A address and one for an AAAA address.  We
         * can't send just one request, since many servers only answer one
         * question per DNS request.
         *
         * Once we have the answer to one request, we allow for a short
         * timeout before we report it, to see if the other one arrives.  If
         * they both show up in time, then we report both the answers.
         *
         * If too many addresses of one type time out or fail, we should stop
         * launching those requests. (XXX we don't do that yet.)
         */

        if (hints.ai_family != PF_INET6) {
                log(EVDNS_LOG_DEBUG, "Sending request for %s on ipv4 as %p",
                    nodename, &data->ipv4_request);

                data->ipv4_request.r = evdns_base_resolve_ipv4(dns_base,
                    nodename, 0, evdns_getaddrinfo_gotresolve,
                    &data->ipv4_request);
                if (want_cname && data->ipv4_request.r)
                        data->ipv4_request.r->current_req->put_cname_in_ptr =
                            &data->cname_result;
        }
        if (hints.ai_family != PF_INET) {
                log(EVDNS_LOG_DEBUG, "Sending request for %s on ipv6 as %p",
                    nodename, &data->ipv6_request);

                data->ipv6_request.r = evdns_base_resolve_ipv6(dns_base,
                    nodename, 0, evdns_getaddrinfo_gotresolve,
                    &data->ipv6_request);
                if (want_cname && data->ipv6_request.r)
                        data->ipv6_request.r->current_req->put_cname_in_ptr =
                            &data->cname_result;
        }

        evtimer_assign(&data->timeout, dns_base->event_base,
            evdns_getaddrinfo_timeout_cb, data);

        if (data->ipv4_request.r || data->ipv6_request.r) {
                return data;
        } else {
                mm_free(data);
                cb(EVUTIL_EAI_FAIL, NULL, arg);
                return NULL;
        }
}

void
evdns_getaddrinfo_cancel(struct evdns_getaddrinfo_request *data)
{
        EVDNS_LOCK(data->evdns_base);
        if (data->request_done) {
                EVDNS_UNLOCK(data->evdns_base);
                return;
        }
        event_del(&data->timeout);
        data->user_canceled = 1;
        if (data->ipv4_request.r)
                evdns_cancel_request(data->evdns_base, data->ipv4_request.r);
        if (data->ipv6_request.r)
                evdns_cancel_request(data->evdns_base, data->ipv6_request.r);
        EVDNS_UNLOCK(data->evdns_base);
}