r14602@catbus: nickm | 2007-08-16 13:33:27 -0400

[tor.git] / src / or / eventdns.c

/* $Id$ */

/* 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:
 *       - Replace all externally visible magic numbers with #defined constants.
 *       - Write documentation for APIs of all external functions.
 */

/* Async DNS Library
 * Adam Langley <agl@imperialviolet.org>
 * 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 "eventdns_tor.h"
#include <sys/types.h>
//#define NDEBUG

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

// #define _POSIX_C_SOURCE 200507
#define _GNU_SOURCE

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

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

#define _FORTIFY_SOURCE 3

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

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

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

#ifdef WIN32
typedef int socklen_t;
#endif

#define EVDNS_LOG_DEBUG 0
#define EVDNS_LOG_WARN 1

#ifndef HOST_NAME_MAX
#define HOST_NAME_MAX 255
#endif

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

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

#if 0
#ifdef __USE_ISOC99B
// libevent doesn't work without this
typedef uint8_t u_char;
typedef unsigned int uint;
#endif
#endif
#include <event.h>

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

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

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

#define CLASS_INET      EVDNS_CLASS_INET

#define CLEAR(x) do { memset((x), 0, sizeof(*(x))); } while(0)

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

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

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

        struct event timeout_event;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

static int global_max_requests_inflight = 64;

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

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

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

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

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

static int strtoint(const char *const str);

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

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

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

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

static evdns_debug_log_fn_type evdns_log_fn = NULL;

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

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

static void _evdns_log(int warn, const char *fmt, ...) EVDNS_LOG_CHECK;
static void
_evdns_log(int warn, const char *fmt, ...)
{
        va_list args;
        static char buf[512];
        if (!evdns_log_fn)
                return;
        va_start(args,fmt);
#ifdef WIN32
        _vsnprintf(buf, sizeof(buf), fmt, args);
#else
        vsnprintf(buf, sizeof(buf), fmt, args);
#endif
        buf[sizeof(buf)-1] = '\0';
        evdns_log_fn(warn, buf);
        va_end(args);
}

#define log _evdns_log

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

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

        return NULL;
}

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

        nameserver_send_probe(ns);
}

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

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

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

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

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

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

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

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

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

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

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

        CLEAR(req);
        free(req);

        evdns_requests_pump_waiting_queue();
}

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

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

        return 0;
}

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

                global_requests_waiting--;
                global_requests_inflight++;

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

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

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

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

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

                switch(error) {
                case DNS_ERR_NOTIMPL:
                case DNS_ERR_REFUSED:
                        // we regard these errors as marking a bad nameserver
                        if (req->reissue_count < global_max_reissues) {
                                char msg[64];
                                snprintf(msg, sizeof(msg), "Bad response %d (%s)",
                                                 error, evdns_err_to_string(error));
                                nameserver_failed(req->ns, msg);
                                if (!request_reissue(req)) return;
                        }
                        break;
                case DNS_ERR_SERVERFAILED:
                        // rcode 2 (servfailed) sometimes means "we are broken" and
                        // sometimes (with some binds) means "that request was very
                        // confusing."  Treat this as a timeout, not a failure.
                        /*XXXX refactor the parts of */
                        log(EVDNS_LOG_DEBUG, "Got a SERVERFAILED from nameserver %s; "
                                "will allow the request to time out.",
                                debug_ntoa(req->ns->address));
                        break;
                default:
                        // we got a good reply from the nameserver
                        nameserver_up(req->ns);
                }

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

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

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

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

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

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

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

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

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

        req = request_find_from_trans_id(trans_id);
        /* if no request, can't do anything. */
        if (!req) return -1;

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

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

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

        reply.type = req->request_type;

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

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

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

                // XXX I'd be more comfortable if we actually checked the name
                // here. -NM
                SKIP_NAME;
                GET16(type);
                GET16(class);
                GET32(ttl);
                GET16(datalength);

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

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

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

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

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

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

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

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

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

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

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

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

        // Ignore answers, authority, and additional.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

        // We have no more pending requests; stop listening for 'writeable' events.
        (void) event_del(&port->event);
        CLEAR(&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);
        CLEAR(&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; int 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, int 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
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));

        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;
        CLEAR(item);
        item->next = NULL;
        if (!(item->name = strdup(name))) {
                CLEAR(item);
                free(item);
                return -1;
        }
        item->type = type;
        item->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);
                                CLEAR(item);
                                free(item);
                                return -1;
                        }
                        item->datalen = -1;
                } else {
                        if (!(item->data = malloc(datalen))) {
                                free(item->name);
                                CLEAR(item);
                                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);
                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_A, 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]->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->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( (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[3] |= 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 err = last_error(port->socket);
                if (! error_is_eagain(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);
                        CLEAR(&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 (req->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);
                        /* XXXX free(victim?) -NM */
                        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]);
        }

        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);
                CLEAR(req);
                free(req);
                return (1);
        }
        CLEAR(req);
        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);
        CLEAR(&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;
}

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

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

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

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

        if (req->ns->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;
                break;
        default:
                // transmitted; we need to check for timeout.
                log(EVDNS_LOG_DEBUG,
                        "Setting timeout for request %lx", (unsigned long) req);
                evtimer_set(&req->timeout_event, evdns_request_timeout_callback, req);
                if (evtimer_add(&req->timeout_event, &global_timeout) < 0) {
                        log(EVDNS_LOG_WARN,
                                "Error from libevent when adding timer for request %lx",
                                (unsigned long) req);
                        // ???? Do more?
                }
        }

        req->tx_count++;
        req->transmit_me = 0;
        return retcode;
}

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

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

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

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

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

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

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

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

        return did_try_to_transmit;
}

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

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

        if (!server)
                return 0;
        while (1) {
                struct nameserver *next = server->next;
                (void) event_del(&server->event);
                CLEAR(&server->event);
                (void) evtimer_del(&server->timeout_event);
                CLEAR(&server->timeout_event);
                if (server->socket >= 0)
                        CLOSE_SOCKET(server->socket);
                CLEAR(server);
                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);
                CLEAR(&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;
        struct sockaddr_in sin;
        int err = 0;
        if (server) {
                do {
                        if (server->address == address) return 3;
                        server = server->next;
                } while (server != started_at);
        }

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

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

        ns->socket = socket(PF_INET, SOCK_DGRAM, 0);
        if (ns->socket < 0) { err = 1; goto out1; }
#ifdef WIN32
        {
                u_long nonblocking = 1;
                ioctlsocket(ns->socket, FIONBIO, &nonblocking);
        }
#else
        fcntl(ns->socket, F_SETFL, O_NONBLOCK);
#endif
        sin.sin_addr.s_addr = address;
        sin.sin_port = htons(port);
        sin.sin_family = AF_INET;
        if (connect(ns->socket, (struct sockaddr *) &sin, sizeof(sin)) != 0) {
                err = 2;
                goto out2;
        }

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

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

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

        global_good_nameservers++;

        return 0;

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

// exported function
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;
        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;
        }
        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));

        // 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:
        CLEAR(req);
        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(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);
        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(struct in6_addr *in, int flags, evdns_callback_type callback, void *ptr) {
        char buf[64];
        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;
                        CLEAR(dom);
                        free(dom);
                }
                CLEAR(state);
                free(state);
        }
}

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

        return state;
}

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

        global_search_state = search_state_new();
}

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

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

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

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

        global_search_state->head = sdomain;
}

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

        global_search_state->head = prev;
}

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

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

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

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

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

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

        // we ran off the end of the list and still didn't find the requested string
        abort();
        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
                                struct request *const newreq = request_new(req->request_type, req->search_origname, req->search_flags, req->user_callback, req->user_pointer);
                                log(EVDNS_LOG_DEBUG, "Search: trying raw query %s", req->search_origname);
                                if (newreq) {
                                        request_submit(newreq);
                                        return 0;
                                }
                        }
                        return 1;
                }

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

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

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

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

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

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

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

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

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

                while ((domain = NEXT_TOKEN)) {
                        search_postfix_add(domain);
                }
                search_reverse();
        } else if (!strcmp(first_token, "options")) {
                const char *option;
                while ((option = NEXT_TOKEN)) {
                        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; goto out1; }  // no resolv.conf should be any bigger

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

    n = 0;
        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.

        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.
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;

        /* XXXX Possibly, we should hardcode the location of this DLL. */
        if (!(handle = LoadLibrary("iphlpapi.dll"))) {
                log(EVDNS_LOG_WARN, "Could not open iphlpapi.dll");
                //right now status = 0, doesn't that mean "good" - mikec
                status = -1;
                goto done;
        }
        if (!(fn = (GetNetworkParams_fn_t) GetProcAddress(handle, "GetNetworkParams"))) {
                log(EVDNS_LOG_WARN, "Could not get address of function.");
                //same as above
                status = -1;
                goto done;
        }

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

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

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

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

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

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

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

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

        free(buf);
        return status;
}

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

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

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

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

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

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

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

int
evdns_init(void)
{
                int res = 0;
#ifdef WIN32
                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);
                CLEAR(server);
                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;
                        CLEAR(dom);
                        free(dom);
                }
                CLEAR(global_search_state);
                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]->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]->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]->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);
                fcntl(sock, F_SETFL, O_NONBLOCK);
                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

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