evdns.c

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

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

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

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

#ifdef DNS_USE_FTIME_FOR_ID
#include <sys/timeb.h>
#endif

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

/* #define _POSIX_C_SOURCE 200507 */
#define _GNU_SOURCE

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

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

#ifndef _FORTIFY_SOURCE
#define _FORTIFY_SOURCE 3
#endif

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

#include "evdns.h"
#include "evutil.h"
#include "log.h"
#ifdef WIN32
#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 HAVE_NETINET_IN6_H
#include <netinet/in6.h>
#endif

#define EVDNS_LOG_DEBUG 0
#define EVDNS_LOG_WARN 1

#ifndef HOST_NAME_MAX
#define HOST_NAME_MAX 255
#endif

#include <stdio.h>

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

#ifdef __USE_ISOC99B
/* libevent doesn't work without this */
typedef ev_uint8_t u_char;
typedef unsigned int uint;
#endif
#include <event.h>

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

#ifdef WIN32
#define open _open
#define read _read
#define close _close
#define strdup _strdup
#endif

#define MAX_ADDRS 32  /* maximum number of addresses from a single packet */
/* which we bother recording */

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

#define CLASS_INET     EVDNS_CLASS_INET

#ifdef HAVE_SETFD
#define FD_CLOSEONEXEC(x) do { \
    if (fcntl(x, F_SETFD, 1) == -1) \
        event_warn("fcntl(%d, F_SETFD)", x); \
    } while (0)
#else
#define FD_CLOSEONEXEC(x) (void)0
#endif

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

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

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

    struct event timeout_event;

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

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

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

struct nameserver {
    int socket;  /* a connected UDP socket */
    u32 address;
    u16 port;
    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 */
    // 1服务器活动，0服务器down
    char state;  /* zero if we think that this server is down */
    char choked;  /* true if we have an EAGAIN from this server's socket */
    char write_waiting;  /* true if we are waiting for EV_WRITE events */
};

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

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

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

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

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

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

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

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

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

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

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

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

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

static int global_max_requests_inflight = 64;

static struct timeval global_timeout = {5, 0};  /* 5 seconds */
static int global_max_reissues = 1;  /* a reissue occurs when we get some errors from the server */
static int global_max_retransmits = 3;  /* number of times we'll retransmit a request which timed out */
/* number of timeouts in a row before we consider this server to be down */
static int global_max_nameserver_timeout = 3;

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

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

static int strtoint(const char* const str);

#ifdef WIN32
static int
last_error(int sock)
{
    int optval, optvallen = sizeof(optval);
    int err = WSAGetLastError();
    if (err == WSAEWOULDBLOCK && sock >= 0) {
        if (getsockopt(sock, SOL_SOCKET, SO_ERROR, (void*)&optval,
                       &optvallen))
            return err;
        if (optval)
            return optval;
    }
    return err;

}
static int
error_is_eagain(int err)
{
    return err == EAGAIN || err == WSAEWOULDBLOCK;
}
static int
inet_aton(const char* c, struct in_addr* addr)
{
    ev_uint32_t r;
    if (strcmp(c, "255.255.255.255") == 0) {
        addr->s_addr = 0xffffffffu;
    } else {
        r = inet_addr(c);
        if (r == INADDR_NONE)
            return 0;
        addr->s_addr = r;
    }
    return 1;
}
#else
#define last_error(sock) (errno)
#define error_is_eagain(err) ((err) == EAGAIN)
#endif
#define CLOSE_SOCKET(s) EVUTIL_CLOSESOCKET(s)

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

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

static evdns_debug_log_fn_type evdns_log_fn = NULL;

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

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

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

#define log _evdns_log

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

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

    return NULL;
}

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

    nameserver_send_probe(ns);
}

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    search_request_finished(req);
    global_requests_inflight--;

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

    free(req);

    evdns_requests_pump_waiting_queue();
}

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

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

    return 0;
}

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

        global_requests_waiting--;
        global_requests_inflight++;

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

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

static void
reply_callback(struct request* const req, u32 ttl, u32 err, struct reply* reply)
{
    switch (req->request_type) {
    case TYPE_A:
        if (reply)
            req->user_callback(DNS_ERR_NONE, DNS_IPv4_A,
                               reply->data.a.addrcount, ttl,
                               reply->data.a.addresses,
                               req->user_pointer);
        else
            req->user_callback(err, 0, 0, 0, NULL, req->user_pointer);
        return;
    case TYPE_PTR:
        if (reply) {
            char* name = reply->data.ptr.name;
            req->user_callback(DNS_ERR_NONE, DNS_PTR, 1, ttl,
                               &name, req->user_pointer);
        } else {
            req->user_callback(err, 0, 0, 0, NULL,
                               req->user_pointer);
        }
        return;
    case TYPE_AAAA:
        if (reply)
            req->user_callback(DNS_ERR_NONE, DNS_IPv6_AAAA,
                               reply->data.aaaa.addrcount, ttl,
                               reply->data.aaaa.addresses,
                               req->user_pointer);
        else
            req->user_callback(err, 0, 0, 0, NULL, req->user_pointer);
        return;
    }
    assert(0);
}

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

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

        switch (error) {
        case DNS_ERR_NOTIMPL:
        case DNS_ERR_REFUSED:
            /* we regard these errors as marking a bad nameserver */
            if (req->reissue_count < global_max_reissues) {
                char msg[64];
                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 %s; "
                "will allow the request to time out.",
                debug_ntoa(req->ns->address));
            break;
        default:
            /* we got a good reply from the nameserver */
            nameserver_up(req->ns);
        }

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

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

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;

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

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

    for (;;) {
        u8 label_len;
        if (j >= length) return -1;
        GET8(label_len);
        if (!label_len) break;
        if (label_len & 0xc0) {
            u8 ptr_low;
            GET8(ptr_low);
            if (name_end < 0) name_end = j;
            j = (((int)label_len & 0x3f) << 8) + ptr_low;
            /* Make sure that the target offset is in-bounds. */
            if (j < 0 || j >= length) return -1;
            /* If we've jumped more times than there are characters in the
             * message, we must have a loop. */
            if (++ptr_count > length) return -1;
            continue;
        }
        if (label_len > 63) return -1;
        if (cp != name_out) {
            if (cp + 1 >= end) return -1;
            *cp++ = '.';
        }
        if (cp + label_len >= end) return -1;
        memcpy(cp, packet + j, label_len);
        cp += label_len;
        j += label_len;
    }
    if (cp >= end) return -1;
    *cp = '\0';
    if (name_end < 0)
        *idx = j;
    else
        *idx = name_end;
    return 0;
err:
    return -1;
}

/* parses a raw request from a nameserver */
static int
reply_parse(u8* packet, int length)
{
    int j = 0, 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 */

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

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

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

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

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

    /* This macro skips a name in the DNS reply. */
#define SKIP_NAME \
    do { tmp_name[0] = '\0';                \
        if (name_parse(packet, length, &j, tmp_name, sizeof(tmp_name))<0)\
            goto err;               \
    } while(0)
#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 (memcmp(tmp_name, cmp_name, strlen (tmp_name)) != 0) \
            return (-1); /* we ignore mismatching names */  \
    } 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;
    }

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

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

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

        if (type == TYPE_A && class == CLASS_INET) {
            int addrcount, addrtocopy;
            if (req->request_type != TYPE_A) {
                j += datalength;
                continue;
            }
            if ((datalength & 3) != 0) /* not an even number of As. */
                goto err;
            addrcount = datalength >> 2;
            addrtocopy = MIN(MAX_ADDRS - reply.data.a.addrcount, (unsigned)addrcount);

            ttl_r = MIN(ttl_r, ttl);
            /* we only bother with the first four addresses. */
            if (j + 4 * addrtocopy > length) goto err;
            memcpy(&reply.data.a.addresses[reply.data.a.addrcount],
                   packet + j, 4 * addrtocopy);
            j += 4 * addrtocopy;
            reply.data.a.addrcount += addrtocopy;
            reply.have_answer = 1;
            if (reply.data.a.addrcount == MAX_ADDRS) break;
        } else if (type == TYPE_PTR && class == CLASS_INET) {
            if (req->request_type != TYPE_PTR) {
                j += datalength;
                continue;
            }
            if (name_parse(packet, length, &j, reply.data.ptr.name,
                           sizeof(reply.data.ptr.name)) < 0)
                goto err;
            ttl_r = MIN(ttl_r, ttl);
            reply.have_answer = 1;
            break;
        } else if (type == TYPE_AAAA && class == CLASS_INET) {
            int addrcount, addrtocopy;
            if (req->request_type != TYPE_AAAA) {
                j += datalength;
                continue;
            }
            if ((datalength & 15) != 0) /* not an even number of AAAAs. */
                goto err;
            addrcount = datalength >> 4;  /* each address is 16 bytes long */
            addrtocopy = MIN(MAX_ADDRS - reply.data.aaaa.addrcount, (unsigned)addrcount);
            ttl_r = MIN(ttl_r, ttl);

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

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

/* Parse a raw request (packet,length) sent to a nameserver port (port) from */
/* a DNS client (addr,addrlen), and if it's well-formed, call the corresponding */
/* callback. */
// 解析成功数据包之后回调port->user_callback进行通知
static int
request_parse(u8* packet, int length, struct evdns_server_port* port, struct sockaddr* addr, socklen_t addrlen)
{
    int j = 0;  /* index into packet */
    u16 _t;  /* used by the macros */
    char tmp_name[256]; /* used by the macros */

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

    /* Get the header fields */
    // 解析数据包，确认packet是否足够长
    GET16(trans_id);
    GET16(flags);
    GET16(questions);
    GET16(answers);
    GET16(authority);
    GET16(additional);

    if (flags & 0x8000) return -1; /* Must not be an answer. */
    flags &= 0x0110; /* Only RD and CD get preserved. */
    // NOTE:怎么没看到server_req的释放?会调用TO_SERVER_REQUEST将server_req->base偏移到头指针获得
    server_req = malloc(sizeof(struct server_request));
    if (server_req == NULL) return -1;
    memset(server_req, 0, sizeof(struct server_request));

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

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

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

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

static u16
default_transaction_id_fn(void)
{
    u16 trans_id;
#ifdef DNS_USE_CPU_CLOCK_FOR_ID
    struct timespec ts;
    static int clkid = -1;
    if (clkid == -1) {
        clkid = CLOCK_REALTIME;
#ifdef CLOCK_MONOTONIC
        if (clock_gettime(CLOCK_MONOTONIC, &ts) != -1)
            clkid = CLOCK_MONOTONIC;
#endif
    }
    if (clock_gettime(clkid, &ts) == -1)
        event_err(1, "clock_gettime");
    trans_id = ts.tv_nsec & 0xffff;
#endif

#ifdef DNS_USE_FTIME_FOR_ID
    struct _timeb tb;
    _ftime(&tb);
    trans_id = tb.millitm & 0xffff;
#endif

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

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

static ev_uint16_t (*trans_id_function)(void) = default_transaction_id_fn;

void
evdns_set_transaction_id_fn(ev_uint16_t (*fn)(void))
{
    if (fn)
        trans_id_function = fn;
    else
        trans_id_function = default_transaction_id_fn;
}

/* Try to choose a strong transaction id which isn't already in flight */
static u16
transaction_id_pick(void)
{
    for (;;) {
        u16 trans_id = trans_id_function();

        if (trans_id == 0xffff) continue;

        if (request_find_from_trans_id(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(void)
{
    struct nameserver* started_at = server_head, *picked;
    if (!server_head) return NULL;

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

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

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

static int
address_is_correct(struct nameserver* ns, struct sockaddr* sa, socklen_t slen)
{
    struct sockaddr_in* sin = (struct sockaddr_in*) sa;
    if (sa->sa_family != AF_INET || slen != sizeof(struct sockaddr_in))
        return 0;
    if (sin->sin_addr.s_addr != ns->address)
        return 0;
    return 1;
}

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

    for (;;) {
        const int r = recvfrom(ns->socket, packet, sizeof(packet), 0,
                               (struct sockaddr*)&ss, &addrlen);
        if (r < 0) {
            int err = last_error(ns->socket);
            if (error_is_eagain(err)) return;
            nameserver_failed(ns, strerror(err));
            return;
        }
        if (!address_is_correct(ns, (struct sockaddr*)&ss, addrlen)) {
            log(EVDNS_LOG_WARN, "Address mismatch on received "
                "DNS packet.");
            return;
        }
        ns->timedout = 0;
        reply_parse(packet, r);
    }
}

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

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

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

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

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

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

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

/* 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(int fd, short events, void* arg)
{
    struct evdns_server_port* port = (struct evdns_server_port*) arg;
    (void) fd;

    if (events & EV_WRITE) {
        port->choked = 0;
        server_port_flush(port);
    }
    if (events & EV_READ) {
        server_port_read(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)
        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 = 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 int 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 unsigned int 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++] = label_len;

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

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

    /* the labels must be terminated by a 0. */
    /* It's possible that the name ended in a . */
    /* in which case the zero is already there */
    if (!j || buf[j - 1]) buf[j++] = 0;
    return j;
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 int
evdns_request_len(const int name_len)
{
    return 96 + /* length of the DNS standard header */
           name_len + 2 +
           4;  /* space for the resource type */
}

/* build a dns request packet into buf. buf should be at least as long */
/* as evdns_request_len told you it should be. */
/* */
/* Returns the amount of space used. Negative on error. */
static int
evdns_request_data_build(const char* const name, const int name_len,
                         const u16 trans_id, const u16 type, const u16 class,
                         u8* const buf, size_t buf_len)
{
    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 */
// 添加一个响应dns请求服务器
struct evdns_server_port*
evdns_add_server_port(int socket, int is_tcp, evdns_request_callback_fn_type cb, void* user_data)
{
    struct evdns_server_port* port;
    if (!(port = malloc(sizeof(struct evdns_server_port))))
        return NULL;
    memset(port, 0, sizeof(struct evdns_server_port));

    // 不支持tcp
    assert(!is_tcp); /* TCP sockets not yet implemented */
    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;

    event_set(&port->event, port->socket, EV_READ | EV_PERSIST,
              server_port_ready_callback, port);
    event_add(&port->event, NULL); /* check return. */
    return port;
}

/* exported function */
void
evdns_close_server_port(struct evdns_server_port* port)
{
    if (--port->refcnt == 0)
        server_port_free(port);
    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;

    if (req->response) /* have we already answered? */
        return (-1);

    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:
        return (-1);
    }
    while (*itemp) {
        itemp = &((*itemp)->next);
    }
    item = malloc(sizeof(struct server_reply_item));
    if (!item)
        return -1;
    item->next = NULL;
    if (!(item->name = strdup(name))) {
        free(item);
        return -1;
    }
    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 = strdup(data))) {
                free(item->name);
                free(item);
                return -1;
            }
            item->datalen = (u16) - 1;
        } else {
            if (!(item->data = malloc(datalen))) {
                free(item->name);
                free(item);
                return -1;
            }
            item->datalen = datalen;
            memcpy(item->data, data, datalen);
        }
    }

    *itemp = item;
    ++(*countp);
    return 0;
}

/* exported function */
int
evdns_server_request_add_a_reply(struct evdns_server_request* req, const char* name, int n, 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, 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];
    assert(in || inaddr_name);
    assert(!(in && inaddr_name));
    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);
}


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 = 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;
    if (!req->response) {
        if ((r = evdns_server_request_format_response(req, err)) < 0)
            return r;
    }

    r = sendto(port->socket, req->response, req->response_len, 0,
               (struct sockaddr*) &req->addr, req->addrlen);
    if (r < 0) {
        int sock_err = last_error(port->socket);
        if (! error_is_eagain(sock_err))
            return -1;

        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_set(&port->event, 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");
            }

        }

        return 1;
    }
    if (server_request_free(req))
        return 0;

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

    return 0;
}

/* 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;
            free(victim->name);
            if (victim->data)
                free(victim->data);
            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;
    if (req->base.questions) {
        for (i = 0; i < req->base.nquestions; ++i)
            free(req->base.questions[i]);
        free(req->base.questions);
    }

    if (req->port) {
        if (req->port->pending_replies == req) {
            if (req->next_pending)
                req->port->pending_replies = req->next_pending;
            else
                req->port->pending_replies = NULL;
        }
        rc = --req->port->refcnt;
    }

    if (req->response) {
        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) {
        server_port_free(req->port);
        free(req);
        return (1);
    }
    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)
{
    assert(port);
    assert(!port->refcnt);
    assert(!port->pending_replies);
    if (port->socket > 0) {
        CLOSE_SOCKET(port->socket);
        port->socket = -1;
    }
    (void) event_del(&port->event);
    /* XXXX actually free the port? -NM */
}

/* 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(int fd, short events, void* arg)
{
    struct request* const req = (struct request*) arg;
    (void) fd;
    (void) events;

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

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

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

/* try to send a request to a given server. */
/* */
/* return: */
/*   0 ok */
/*   1 temporary failure */
/*   2 other failure */
static int
evdns_request_transmit_to(struct request* req, struct nameserver* server)
{
    struct sockaddr_in sin;
    int r;
    memset(&sin, 0, sizeof(sin));
    sin.sin_addr.s_addr = req->ns->address;
    sin.sin_port = req->ns->port;
    sin.sin_family = AF_INET;

    r = sendto(server->socket, req->request, req->request_len, 0,
               (struct sockaddr*)&sin, sizeof(sin));
    if (r < 0) {
        int err = last_error(server->socket);
        if (error_is_eagain(err)) return 1;
        nameserver_failed(req->ns, strerror(err));
        return 2;
    } else if (r != (int)req->request_len) {
        return 1;  /* short write */
    } else {
        return 0;
    }
}

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

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

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

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

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

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

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

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

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

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

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

    return did_try_to_transmit;
}

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

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

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

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

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

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

    return 0;
}


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

static int
_evdns_nameserver_add_impl(unsigned long int address, int port)
{
    /* first check to see if we already have this nameserver */

    const struct nameserver* server = server_head, *const started_at = server_head;
    struct nameserver* ns;
    int err = 0;
    // 确认未添加到服务器列表
    if (server) {
        do {
            if (server->address == address) return 3;
            server = server->next;
        } while (server != started_at);
    }

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

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

    evtimer_set(&ns->timeout_event, nameserver_prod_callback, ns);
    // UDP端口
    ns->socket = socket(PF_INET, SOCK_DGRAM, 0);
    if (ns->socket < 0) {
        err = 1;
        goto out1;
    }
    FD_CLOSEONEXEC(ns->socket);
    evutil_make_socket_nonblocking(ns->socket);
    // nameserver 特有信息，地址端口
    ns->address = address;
    ns->port = htons(port);
    ns->state = 1;
    event_set(&ns->event, ns->socket, EV_READ | EV_PERSIST, nameserver_ready_callback, ns);
    if (event_add(&ns->event, NULL) < 0) {
        err = 2;
        goto out2;
    }

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

    /* insert this nameserver into the list of them */
    if (!server_head) {
        // 开始链表为空
        ns->next = ns->prev = ns;
        server_head = ns;
    } else {
        // 插在第二个节点?
        ns->next = server_head->next;
        ns->prev = server_head;
        // 是不是少了 server->next->prev = ns还是故意让其他链表的所有
        server_head->next = ns;

        if (server_head->prev == server_head) {
            server_head->prev = ns;// 指向最后一个
        }
    }

    global_good_nameservers++;

    return 0;

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

/* exported function */
// 默认端口为53
int
evdns_nameserver_add(unsigned long int address)
{
    return _evdns_nameserver_add_impl(address, 53);
}

/* exported function */
int
evdns_nameserver_ip_add(const char* ip_as_string)
{
    struct in_addr ina;
    int port;
    char buf[20];
    const char* cp;
    // IP:端口
    cp = strchr(ip_as_string, ':');
    if (!cp) {
        cp = ip_as_string;
        port = 53;
    } else {
        port = strtoint(cp + 1);
        if (port < 0 || port > 65535) {
            return 4;
        }
        if ((cp - ip_as_string) >= (int)sizeof(buf)) {
            return 4;
        }
        memcpy(buf, ip_as_string, cp - ip_as_string);
        buf[cp - ip_as_string] = '\0';
        cp = buf;
    }
    // cp 指向ip
    if (!inet_aton(cp, &ina)) {
        return 4;
    }
    return _evdns_nameserver_add_impl(ina.s_addr, port);
}

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

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

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

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

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

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

    evtimer_set(&req->timeout_event, evdns_request_timeout_callback, req);

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

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

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

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

/* exported function */
int evdns_resolve_ipv6(const char* name, int flags,
                       evdns_callback_type callback, void* ptr)
{
    log(EVDNS_LOG_DEBUG, "Resolve requested for %s", name);
    if (flags & DNS_QUERY_NO_SEARCH) {
        struct request* const req =
            request_new(TYPE_AAAA, name, flags, callback, ptr);
        if (req == NULL)
            return (1);
        request_submit(req);
        return (0);
    } else {
        return (search_request_new(TYPE_AAAA, name, flags, callback, ptr));
    }
}

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

int evdns_resolve_reverse_ipv6(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 request* req;
    int i;
    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++ = '.';
    }
    assert(cp + strlen("ip6.arpa") < buf + sizeof(buf));
    memcpy(cp, "ip6.arpa", strlen("ip6.arpa") + 1);
    log(EVDNS_LOG_DEBUG, "Resolve requested for %s (reverse)", buf);
    req = request_new(TYPE_PTR, buf, flags, callback, ptr);
    if (!req) return 1;
    request_submit(req);
    return 0;
}

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

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

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

static struct search_state* global_search_state = NULL;

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

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

    return state;
}

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

    global_search_state = search_state_new();
}

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

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

    // 第一次初始化
    if (!global_search_state) global_search_state = search_state_new();
    if (!global_search_state) return;
    global_search_state->num_domains++;

    sdomain = (struct search_domain*) malloc(sizeof(struct search_domain) + domain_len/* 尾巴存储domail */);
    if (!sdomain) return;
    // 拷贝domain到尾巴
    memcpy( ((u8*) sdomain) + sizeof(struct search_domain), domain, domain_len);
    // 头插法
    sdomain->next = global_search_state->head;
    sdomain->len = domain_len;

    global_search_state->head = sdomain;
}

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

    global_search_state->head = prev;
}

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

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

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

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

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

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

    /* we ran off the end of the list and still didn't find the requested string */
    abort();
    return NULL; /* unreachable; stops warnings in some compilers. */
}

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

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

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

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

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

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

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

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

/* 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;
}

/* exported function */
int
evdns_set_option(const char* option, const char* val, int flags)
{
    if (!strncmp(option, "ndots:", 6)) {
        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 (!global_search_state) global_search_state = search_state_new();
        if (!global_search_state) return -1;
        global_search_state->ndots = ndots;
    } else if (!strncmp(option, "timeout:", 8)) {
        const int timeout = strtoint(val);
        if (timeout == -1) return -1;
        if (!(flags & DNS_OPTION_MISC)) return 0;
        log(EVDNS_LOG_DEBUG, "Setting timeout to %d", timeout);
        global_timeout.tv_sec = timeout;
    } else if (!strncmp(option, "max-timeouts:", 12)) {
        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);
        global_max_nameserver_timeout = maxtimeout;
    } else if (!strncmp(option, "max-inflight:", 13)) {
        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);
        global_max_requests_inflight = maxinflight;
    } else if (!strncmp(option, "attempts:", 9)) {
        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);
        global_max_retransmits = retries;
    }
    return 0;
}

// linux /etc/resolv.conf 文件单行解析
static void
resolv_conf_parse_line(char* const start, int flags)
{
    char* strtok_state;
    static const char* const delims = " \t";
#define NEXT_TOKEN strtok_r(NULL, delims, &strtok_state)

    char* const first_token = strtok_r(start, delims, &strtok_state);
    if (!first_token) return;
    // search 和 domain不能共存，使用最后一个
    if (!strcmp(first_token, "nameserver") && (flags & DNS_OPTION_NAMESERVERS)) {
        // nameserver 114.114.114.114
        const char* const nameserver = NEXT_TOKEN;
        struct in_addr ina;

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

        while ((domain = NEXT_TOKEN)) {
            search_postfix_add(domain);
        }
        search_reverse();
    } else if (!strcmp(first_token, "options")) {
        const char* option;
        while ((option = NEXT_TOKEN)) {
            const char* val = strchr(option, ':');
            evdns_set_option(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_resolv_conf_parse(int flags, const char* const filename)
{
    struct stat st;
    int fd, n, r;
    u8* resolv;
    char* start;
    int err = 0;

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

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

    if (fstat(fd, &st)) {
        err = 2;
        goto out1;
    }
    if (!st.st_size) {
        evdns_resolv_set_defaults(flags);
        err = (flags & DNS_OPTION_NAMESERVERS) ? 6 : 0;
        goto out1;
    }
    if (st.st_size > 65535) {
        err = 3;    /* no resolv.conf should be any bigger */
        goto out1;
    }

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

    n = 0;
    // 读取整个文件
    while ((r = read(fd, resolv + n, (size_t)st.st_size - n)) > 0) {
        n += r;
        if (n == st.st_size)
            break;
        assert(n < st.st_size);
    }
    if (r < 0) {
        err = 5;
        goto out2;
    }
    resolv[n] = 0;   /* we malloced an extra byte; this should be fine. */
    // 分行
    start = (char*) resolv;
    for (;;) {
        char* const newline = strchr(start, '\n');
        if (!newline) {
            resolv_conf_parse_line(start, flags);
            break;
        } else {
            *newline = 0;
            resolv_conf_parse_line(start, flags);
            start = newline + 1;
        }
    }

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

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

#ifdef WIN32
/* Add multiple nameservers from a space-or-comma-separated list. */
static int
evdns_nameserver_ip_add_line(const char* ips)
{
    const char* addr;
    char* buf;
    int r;
    while (*ips) {
        while (ISSPACE(*ips) || *ips == ',' || *ips == '\t')
            ++ips;
        addr = ips;
        while (ISDIGIT(*ips) || *ips == '.' || *ips == ':')
            ++ips;
        buf = malloc(ips - addr + 1);
        if (!buf) return 4;
        memcpy(buf, addr, ips - addr);
        buf[ips - addr] = '\0';
        r = evdns_nameserver_ip_add(buf);
        free(buf);
        if (r) return r;
    }
    return 0;
}

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

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

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

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

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

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

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

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

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

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

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

    free(buf);
    return status;
}

// windows下从注册表获取丹尼斯服务器
#define SERVICES_KEY "System\\CurrentControlSet\\Services\\"
#define WIN_NS_9X_KEY  SERVICES_KEY "VxD\\MSTCP"
#define WIN_NS_NT_KEY  SERVICES_KEY "Tcpip\\Parameters"

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

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

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

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

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

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

int
evdns_init(void)
{
    int res = 0;
#ifdef WIN32
    res = evdns_config_windows_nameservers();
#else
    res = evdns_resolv_conf_parse(DNS_OPTIONS_ALL, "/etc/resolv.conf");
#endif

    return (res);
}

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

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

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

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

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

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

#ifdef EVDNS_MAIN
void
main_callback(int result, char type, int count, int ttl,
              void* addrs, void* orig)
{
    char* n = (char*)orig;
    int i;
    for (i = 0; i < count; ++i) {
        if (type == DNS_IPv4_A) {
            printf("%s: %s\n", n, debug_ntoa(((u32*)addrs)[i]));
        } else if (type == DNS_PTR) {
            printf("%s: %s\n", n, ((char**)addrs)[i]);
        }
    }
    if (!count) {
        printf("%s: No answer (%d)\n", n, result);
    }
    fflush(stdout);
}
void
evdns_server_callback(struct evdns_server_request* req, void* data)
{
    int i, r;
    (void)data;
    /* dummy; give 192.168.11.11 as an answer for all A questions,
     *  give foo.bar.example.com as an answer for all PTR questions. */
    for (i = 0; i < req->nquestions; ++i) {
        u32 ans = htonl(0xc0a80b0bUL);
        if (req->questions[i]->type == EVDNS_TYPE_A &&
            req->questions[i]->dns_question_class == EVDNS_CLASS_INET) {
            printf(" -- replying for %s (A)\n", req->questions[i]->name);
            r = evdns_server_request_add_a_reply(req, req->questions[i]->name,
                                                 1, &ans, 10);
            if (r < 0)
                printf("eeep, didn't work.\n");
        } else if (req->questions[i]->type == EVDNS_TYPE_PTR &&
                   req->questions[i]->dns_question_class == EVDNS_CLASS_INET) {
            printf(" -- replying for %s (PTR)\n", req->questions[i]->name);
            r = evdns_server_request_add_ptr_reply(req, NULL, req->questions[i]->name,
                                                   "foo.bar.example.com", 10);
        } else {
            printf(" -- skipping %s [%d %d]\n", req->questions[i]->name,
                   req->questions[i]->type, req->questions[i]->dns_question_class);
        }
    }

    r = evdns_request_respond(req, 0);
    if (r < 0)
        printf("eeek, couldn't send reply.\n");
}

void
logfn(int is_warn, const char* msg)
{
    (void) is_warn;
    fprintf(stderr, "%s\n", msg);
}
int
main(int c, char** v)
{
    int idx;
    int reverse = 0, verbose = 1, servertest = 0;
    if (c < 2) {
        fprintf(stderr, "syntax: %s [-x] [-v] hostname\n", v[0]);
        fprintf(stderr, "syntax: %s [-servertest]\n", v[0]);
        return 1;
    }
    idx = 1;
    while (idx < c && v[idx][0] == '-') {
        if (!strcmp(v[idx], "-x"))
            reverse = 1;
        else if (!strcmp(v[idx], "-v"))
            verbose = 1;
        else if (!strcmp(v[idx], "-servertest"))
            servertest = 1;
        else
            fprintf(stderr, "Unknown option %s\n", v[idx]);
        ++idx;
    }
    event_init();
    if (verbose)
        evdns_set_log_fn(logfn);
    evdns_resolv_conf_parse(DNS_OPTION_NAMESERVERS, "/etc/resolv.conf");
    if (servertest) {
        int sock;
        struct sockaddr_in my_addr;
        sock = socket(PF_INET, SOCK_DGRAM, 0);
        evutil_make_socket_nonblocking(sock);
        my_addr.sin_family = AF_INET;
        my_addr.sin_port = htons(10053);
        my_addr.sin_addr.s_addr = INADDR_ANY;
        if (bind(sock, (struct sockaddr*)&my_addr, sizeof(my_addr)) < 0) {
            perror("bind");
            exit(1);
        }
        evdns_add_server_port(sock, 0, evdns_server_callback, NULL);
    }
    for (; idx < c; ++idx) {
        if (reverse) {
            struct in_addr addr;
            if (!inet_aton(v[idx], &addr)) {
                fprintf(stderr, "Skipping non-IP %s\n", v[idx]);
                continue;
            }
            fprintf(stderr, "resolving %s...\n", v[idx]);
            evdns_resolve_reverse(&addr, 0, main_callback, v[idx]);
        } else {
            fprintf(stderr, "resolving (fwd) %s...\n", v[idx]);
            evdns_resolve_ipv4(v[idx], 0, main_callback, v[idx]);
        }
    }
    fflush(stdout);
    event_dispatch();
    return 0;
}
#endif