dns.c 36.5 KB
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/* Copyright 2003-2004 Roger Dingledine.
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 * Copyright 2004-2006 Roger Dingledine, Nick Mathewson. */
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/* See LICENSE for licensing information */
/* $Id$ */
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const char dns_c_id[] =
  "$Id$";
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/**
 * \file dns.c
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 * \brief Implements a farm of 'DNS worker' threads or processes to
 * perform DNS lookups for onion routers and cache the results.
 * [This needs to be done in the background because of the lack of a
 * good, ubiquitous asynchronous DNS implementation.]
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 **/
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/* See
 * http://elvin.dstc.com/ListArchive/elvin-dev/archive/2001/09/msg00027.html
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 * for some approaches to asynchronous dns. We will want to switch once one of
 * them becomes more commonly available.
 */

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#include "or.h"
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#include "../common/ht.h"
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#ifdef USE_EVENTDNS
#include "eventdns.h"
#endif
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/** Longest hostname we're willing to resolve. */
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#define MAX_ADDRESSLEN 256

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/** Maximum DNS processes to spawn. */
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#define MAX_DNSWORKERS 100
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/** Minimum DNS processes to spawn. */
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#define MIN_DNSWORKERS 3
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/** If more than this many processes are idle, shut down the extras. */
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#define MAX_IDLE_DNSWORKERS 10
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/** Possible outcomes from hostname lookup: permanent failure,
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 * transient (retryable) failure, and success. */
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#define DNS_RESOLVE_FAILED_TRANSIENT 1
#define DNS_RESOLVE_FAILED_PERMANENT 2
#define DNS_RESOLVE_SUCCEEDED 3

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#ifndef USE_EVENTDNS
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/** How many dnsworkers we have running right now. */
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static int num_dnsworkers=0;
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/** How many of the running dnsworkers have an assigned task right now. */
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static int num_dnsworkers_busy=0;
/** When did we last rotate the dnsworkers? */
static time_t last_rotation_time=0;
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#endif
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/** Linked list of connections waiting for a DNS answer. */
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typedef struct pending_connection_t {
  connection_t *conn;
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  struct pending_connection_t *next;
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} pending_connection_t;
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/** A DNS request: possibly completed, possibly pending; cached_resolve
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 * structs are stored at the OR side in a hash table, and as a linked
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 * list from oldest to newest.
 */
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typedef struct cached_resolve_t {
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  HT_ENTRY(cached_resolve_t) node;
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  char address[MAX_ADDRESSLEN]; /**< The hostname to be resolved. */
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  uint32_t addr; /**< IPv4 addr for <b>address</b>. */
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  char state; /**< 0 is pending; 1 means answer is valid; 2 means resolve
               * failed. */
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#define CACHE_STATE_PENDING 0
#define CACHE_STATE_VALID 1
#define CACHE_STATE_FAILED 2
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  uint32_t expire; /**< Remove items from cache after this time. */
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  uint32_t ttl; /**< What TTL did the nameserver tell us? */
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  pending_connection_t *pending_connections;
  struct cached_resolve_t *next;
} cached_resolve_t;
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static void purge_expired_resolves(uint32_t now);
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static void dns_purge_resolve(cached_resolve_t *resolve);
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static void dns_found_answer(const char *address, uint32_t addr, char outcome,
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                             uint32_t ttl);
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static void send_resolved_cell(connection_t *conn, uint8_t answer_type);
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static int assign_to_dnsworker(connection_t *exitconn);
#ifndef USE_EVENTDNS
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static int dnsworker_main(void *data);
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static int spawn_dnsworker(void);
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static int spawn_enough_dnsworkers(void);
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#endif
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/** Hash table of cached_resolve objects. */
static HT_HEAD(cache_map, cached_resolve_t) cache_root;
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/** Function to compare hashed resolves on their addresses; used to
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 * implement hash tables. */
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static INLINE int
cached_resolves_eq(cached_resolve_t *a, cached_resolve_t *b)
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{
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  /* make this smarter one day? */
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  return !strncmp(a->address, b->address, MAX_ADDRESSLEN);
}

static INLINE unsigned int
cached_resolve_hash(cached_resolve_t *a)
{
  return ht_string_hash(a->address);
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}

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HT_PROTOTYPE(cache_map, cached_resolve_t, node, cached_resolve_hash,
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             cached_resolves_eq);
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HT_GENERATE(cache_map, cached_resolve_t, node, cached_resolve_hash,
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            cached_resolves_eq, 0.6, malloc, realloc, free);
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/** Initialize the DNS cache. */
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static void
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init_cache_map(void)
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{
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  HT_INIT(&cache_root);
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}

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#ifdef USE_EVENTDNS
static void
eventdns_log_cb(const char *msg)
{
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  if (!strcmpstart(msg, "Resolve requested for") &&
      get_options()->SafeLogging) {
    log(LOG_INFO, LD_EXIT, "eventdns: Resolve requested.");
    return;
  }
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  log(LOG_INFO, LD_EXIT, "eventdns: %s", msg);
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}
#endif

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/** Initialize the DNS subsystem; called by the OR process. */
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void
dns_init(void)
{
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  init_cache_map();
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  dnsworkers_rotate();
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#ifdef USE_EVENTDNS
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  {
    or_options_t *options = get_options();
    eventdns_set_log_fn(eventdns_log_cb);
    if (options->Nameservers && smartlist_len(options->Nameservers)) {
      SMARTLIST_FOREACH(options->Nameservers, const char *, ip,
        {
          struct in_addr in;
          log_info(LD_EXIT, "Parsing /etc/resolv.conf");
          if (tor_inet_aton(ip, &in)) {
            log_info(LD_EXIT, "Adding nameserver '%s'", ip);
            eventdns_nameserver_add(in.s_addr);
          }
        });
    } else {
      log_info(LD_EXIT, "Parsing /etc/resolv.conf");
      eventdns_resolv_conf_parse(DNS_OPTION_NAMESERVERS|DNS_OPTION_MISC,
                                 "/etc/resolv.conf");
    }
  }
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#endif
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}
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uint32_t
dns_clip_ttl(uint32_t ttl)
{
  if (ttl < MIN_DNS_TTL)
    return MIN_DNS_TTL;
  else if (ttl > MAX_DNS_TTL)
    return MAX_DNS_TTL;
  else
    return ttl;
}

static uint32_t
dns_get_expiry_ttl(uint32_t ttl)
{
  if (ttl < MIN_DNS_TTL)
    return MIN_DNS_TTL;
  else if (ttl > MAX_DNS_ENTRY_AGE)
    return MAX_DNS_ENTRY_AGE;
  else
    return ttl;
}

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/** Helper: free storage held by an entry in the DNS cache. */
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static void
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_free_cached_resolve(cached_resolve_t *r)
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{
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  while (r->pending_connections) {
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    pending_connection_t *victim = r->pending_connections;
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    r->pending_connections = victim->next;
    tor_free(victim);
  }
  tor_free(r);
}

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/** Free all storage held in the DNS cache */
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void
dns_free_all(void)
{
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  cached_resolve_t **ptr, **next, *item;
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  for (ptr = HT_START(cache_map, &cache_root); ptr != NULL; ptr = next) {
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    item = *ptr;
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    next = HT_NEXT_RMV(cache_map, &cache_root, ptr);
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    _free_cached_resolve(item);
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  }
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  HT_CLEAR(cache_map, &cache_root);
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}

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/** Linked list of resolved addresses, oldest to newest. */
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static cached_resolve_t *oldest_cached_resolve = NULL;
static cached_resolve_t *newest_cached_resolve = NULL;
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/** Remove every cached_resolve whose <b>expire</b> time is before <b>now</b>
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 * from the cache. */
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static void
purge_expired_resolves(uint32_t now)
{
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  cached_resolve_t *resolve;
  pending_connection_t *pend;
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  connection_t *pendconn;
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  /* this is fast because the linked list
   * oldest_cached_resolve is ordered by when they came in.
   */
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  while (oldest_cached_resolve && (oldest_cached_resolve->expire < now)) {
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    resolve = oldest_cached_resolve;
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    log_debug(LD_EXIT,
              "Forgetting old cached resolve (address %s, expires %lu)",
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              escaped_safe_str(resolve->address),
              (unsigned long)resolve->expire);
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    if (resolve->state == CACHE_STATE_PENDING) {
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      log_debug(LD_EXIT,
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                "Bug: Expiring a dns resolve %s that's still pending."
                " Forgot to cull it?", escaped_safe_str(resolve->address));
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      tor_fragile_assert();
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    }
    if (resolve->pending_connections) {
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      log_debug(LD_EXIT,
                "Closing pending connections on expiring DNS resolve!");
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      tor_fragile_assert();
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      while (resolve->pending_connections) {
        pend = resolve->pending_connections;
        resolve->pending_connections = pend->next;
        /* Connections should only be pending if they have no socket. */
        tor_assert(pend->conn->s == -1);
        pendconn = pend->conn;
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        connection_edge_end(pendconn, END_STREAM_REASON_TIMEOUT,
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                            pendconn->cpath_layer);
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        circuit_detach_stream(circuit_get_by_edge_conn(pendconn), pendconn);
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        connection_free(pendconn);
        tor_free(pend);
      }
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    }
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    oldest_cached_resolve = resolve->next;
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    if (!oldest_cached_resolve) /* if there are no more, */
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      newest_cached_resolve = NULL; /* then make sure the list's tail knows
                                     * that too */
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    HT_REMOVE(cache_map, &cache_root, resolve);
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    tor_free(resolve);
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  }
}

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/** Send a response to the RESOVLE request of a connection. answer_type must
 *  be one of RESOLVED_TYPE_(IPV4|ERROR|ERROR_TRANSIENT) */
static void
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send_resolved_cell(connection_t *conn, uint8_t answer_type)
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{
  char buf[RELAY_PAYLOAD_SIZE];
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  size_t buflen;
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  uint32_t ttl;
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  buf[0] = answer_type;
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  ttl = dns_clip_ttl(conn->address_ttl);
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  switch (answer_type)
    {
    case RESOLVED_TYPE_IPV4:
      buf[1] = 4;
      set_uint32(buf+2, htonl(conn->addr));
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      set_uint32(buf+6, htonl(ttl));
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      buflen = 10;
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      break;
    case RESOLVED_TYPE_ERROR_TRANSIENT:
    case RESOLVED_TYPE_ERROR:
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      {
        const char *errmsg = "Error resolving hostname";
        int msglen = strlen(errmsg);
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        buf[1] = msglen;
        strlcpy(buf+2, errmsg, sizeof(buf)-2);
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        set_uint32(buf+2+msglen, htonl(ttl));
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        buflen = 6+msglen;
        break;
      }
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    default:
      tor_assert(0);
    }
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  connection_edge_send_command(conn, circuit_get_by_edge_conn(conn),
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                               RELAY_COMMAND_RESOLVED, buf, buflen,
                               conn->cpath_layer);
}

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/** Link <b>r</b> into the hash table of address-to-result mappings, and add it
 * to the linked list of resolves-by-age. */
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static void
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insert_resolve(cached_resolve_t *r)
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{
  /* add us to the linked list of resolves */
  if (!oldest_cached_resolve) {
    oldest_cached_resolve = r;
  } else {
    newest_cached_resolve->next = r;
  }
  newest_cached_resolve = r;

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  HT_INSERT(cache_map, &cache_root, r);
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}

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/** See if we have a cache entry for <b>exitconn</b>-\>address. if so,
 * if resolve valid, put it into <b>exitconn</b>-\>addr and return 1.
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 * If resolve failed, unlink exitconn if needed, free it, and return -1.
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 *
 * Else, if seen before and pending, add conn to the pending list,
 * and return 0.
 *
 * Else, if not seen before, add conn to pending list, hand to
 * dns farm, and return 0.
 */
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int
dns_resolve(connection_t *exitconn)
{
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  cached_resolve_t *resolve;
  cached_resolve_t search;
  pending_connection_t *pending_connection;
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  struct in_addr in;
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  circuit_t *circ;
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  uint32_t now = time(NULL);
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  assert_connection_ok(exitconn, 0);
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  tor_assert(exitconn->s == -1);
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  /* first check if exitconn->address is an IP. If so, we already
   * know the answer. */
  if (tor_inet_aton(exitconn->address, &in) != 0) {
    exitconn->addr = ntohl(in.s_addr);
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    exitconn->address_ttl = DEFAULT_DNS_TTL;
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    if (exitconn->purpose == EXIT_PURPOSE_RESOLVE)
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      send_resolved_cell(exitconn, RESOLVED_TYPE_IPV4);
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    return 1;
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  }

  /* then take this opportunity to see if there are any expired
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   * resolves in the hash table. */
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  purge_expired_resolves(now);
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  /* lower-case exitconn->address, so it's in canonical form */
  tor_strlower(exitconn->address);

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  /* now check the hash table to see if 'address' is already there. */
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  strlcpy(search.address, exitconn->address, sizeof(search.address));
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  resolve = HT_FIND(cache_map, &cache_root, &search);
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  if (resolve && resolve->expire > now) { /* already there */
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    switch (resolve->state) {
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      case CACHE_STATE_PENDING:
        /* add us to the pending list */
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        pending_connection = tor_malloc_zero(
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                                      sizeof(pending_connection_t));
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        pending_connection->conn = exitconn;
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        pending_connection->next = resolve->pending_connections;
        resolve->pending_connections = pending_connection;
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        log_debug(LD_EXIT,"Connection (fd %d) waiting for pending DNS "
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                  "resolve of %s",
                  exitconn->s, escaped_safe_str(exitconn->address));
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        exitconn->state = EXIT_CONN_STATE_RESOLVING;
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        return 0;
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      case CACHE_STATE_VALID:
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        exitconn->addr = resolve->addr;
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        exitconn->address_ttl = resolve->ttl;
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        log_debug(LD_EXIT,"Connection (fd %d) found cached answer for %s",
                  exitconn->s, escaped_safe_str(exitconn->address));
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        if (exitconn->purpose == EXIT_PURPOSE_RESOLVE)
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          send_resolved_cell(exitconn, RESOLVED_TYPE_IPV4);
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        return 1;
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      case CACHE_STATE_FAILED:
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        log_debug(LD_EXIT,"Connection (fd %d) found cached error for %s",
                  exitconn->s, escaped_safe_str(exitconn->address));
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        if (exitconn->purpose == EXIT_PURPOSE_RESOLVE)
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          send_resolved_cell(exitconn, RESOLVED_TYPE_ERROR);
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        circ = circuit_get_by_edge_conn(exitconn);
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        if (circ)
          circuit_detach_stream(circ, exitconn);
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        if (!exitconn->marked_for_close)
          connection_free(exitconn);
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        return -1;
    }
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    tor_assert(0);
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  }
  /* not there, need to add it */
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  resolve = tor_malloc_zero(sizeof(cached_resolve_t));
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  resolve->state = CACHE_STATE_PENDING;
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  resolve->expire = now + DEFAULT_DNS_TTL; /* this will get replaced. */
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  strlcpy(resolve->address, exitconn->address, sizeof(resolve->address));
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  /* add us to the pending list */
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  pending_connection = tor_malloc_zero(sizeof(pending_connection_t));
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  pending_connection->conn = exitconn;
  resolve->pending_connections = pending_connection;
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  exitconn->state = EXIT_CONN_STATE_RESOLVING;
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  insert_resolve(resolve);
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  log_debug(LD_EXIT,"Assigning question %s to dnsworker.",
            escaped_safe_str(exitconn->address));
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  return assign_to_dnsworker(exitconn);
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}

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/** Log an error and abort if conn is waiting for a DNS resolve.
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 */
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void
assert_connection_edge_not_dns_pending(connection_t *conn)
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{
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  pending_connection_t *pend;
  cached_resolve_t **resolve;
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  HT_FOREACH(resolve, cache_map, &cache_root) {
    for (pend = (*resolve)->pending_connections;
         pend;
         pend = pend->next) {
      tor_assert(pend->conn != conn);
    }
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  }
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}
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/** Log an error and abort if any connection waiting for a DNS resolve is
 * corrupted. */
void
assert_all_pending_dns_resolves_ok(void)
{
  pending_connection_t *pend;
  cached_resolve_t **resolve;
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  HT_FOREACH(resolve, cache_map, &cache_root) {
    for (pend = (*resolve)->pending_connections;
         pend;
         pend = pend->next) {
      assert_connection_ok(pend->conn, 0);
      tor_assert(pend->conn->s == -1);
      tor_assert(!connection_in_array(pend->conn));
    }
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  }
}

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/** Remove <b>conn</b> from the list of connections waiting for conn-\>address.
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 */
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void
connection_dns_remove(connection_t *conn)
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{
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  pending_connection_t *pend, *victim;
  cached_resolve_t search;
  cached_resolve_t *resolve;
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  tor_assert(conn->type == CONN_TYPE_EXIT);
  tor_assert(conn->state == EXIT_CONN_STATE_RESOLVING);

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  strlcpy(search.address, conn->address, sizeof(search.address));
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  resolve = HT_FIND(cache_map, &cache_root, &search);
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  if (!resolve) {
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    log_notice(LD_BUG, "Address %s is not pending. Dropping.",
               escaped_safe_str(conn->address));
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    return;
  }

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  tor_assert(resolve->pending_connections);
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  assert_connection_ok(conn,0);

  pend = resolve->pending_connections;

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  if (pend->conn == conn) {
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    resolve->pending_connections = pend->next;
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    tor_free(pend);
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    log_debug(LD_EXIT, "First connection (fd %d) no longer waiting "
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              "for resolve of %s",
              conn->s, escaped_safe_str(conn->address));
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    return;
  } else {
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    for ( ; pend->next; pend = pend->next) {
      if (pend->next->conn == conn) {
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        victim = pend->next;
        pend->next = victim->next;
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        tor_free(victim);
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        log_debug(LD_EXIT,
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                  "Connection (fd %d) no longer waiting for resolve of %s",
                  conn->s, escaped_safe_str(conn->address));
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        return; /* more are pending */
      }
    }
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    tor_assert(0); /* not reachable unless onlyconn not in pending list */
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  }
}

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/** Mark all connections waiting for <b>address</b> for close.  Then cancel
 * the resolve for <b>address</b> itself, and remove any cached results for
 * <b>address</b> from the cache.
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 */
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void
dns_cancel_pending_resolve(char *address)
{
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  pending_connection_t *pend;
  cached_resolve_t search;
  cached_resolve_t *resolve;
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  connection_t *pendconn;
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  circuit_t *circ;
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  strlcpy(search.address, address, sizeof(search.address));
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  resolve = HT_FIND(cache_map, &cache_root, &search);
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  if (!resolve) {
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    log_notice(LD_BUG,"Address %s is not pending. Dropping.",
               escaped_safe_str(address));
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    return;
  }

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  if (!resolve->pending_connections) {
    /* XXX this should never trigger, but sometimes it does */
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    log_warn(LD_BUG,
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             "Bug: Address %s is pending but has no pending connections!",
             escaped_safe_str(address));
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    tor_fragile_assert();
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    return;
  }
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  tor_assert(resolve->pending_connections);
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  /* mark all pending connections to fail */
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  log_debug(LD_EXIT,
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             "Failing all connections waiting on DNS resolve of %s",
             escaped_safe_str(address));
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  while (resolve->pending_connections) {
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    pend = resolve->pending_connections;
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    pend->conn->state = EXIT_CONN_STATE_RESOLVEFAILED;
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    pendconn = pend->conn;
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    tor_assert(pendconn->s == -1);
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    if (!pendconn->marked_for_close) {
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      connection_edge_end(pendconn, END_STREAM_REASON_RESOURCELIMIT,
                          pendconn->cpath_layer);
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    }
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    circ = circuit_get_by_edge_conn(pendconn);
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    if (circ)
      circuit_detach_stream(circ, pendconn);
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    connection_free(pendconn);
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    resolve->pending_connections = pend->next;
    tor_free(pend);
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  }

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  dns_purge_resolve(resolve);
}

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/** Remove <b>resolve</b> from the cache.
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 */
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static void
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dns_purge_resolve(cached_resolve_t *resolve)
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{
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  cached_resolve_t *tmp;
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  /* remove resolve from the linked list */
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  if (resolve == oldest_cached_resolve) {
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    oldest_cached_resolve = resolve->next;
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    if (oldest_cached_resolve == NULL)
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      newest_cached_resolve = NULL;
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  } else {
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    /* FFFF make it a doubly linked list if this becomes too slow */
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    for (tmp=oldest_cached_resolve; tmp && tmp->next != resolve; tmp=tmp->next)
      ;
    tor_assert(tmp); /* it's got to be in the list, or we screwed up somewhere
                      * else */
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    tmp->next = resolve->next; /* unlink it */

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    if (newest_cached_resolve == resolve)
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      newest_cached_resolve = tmp;
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  }
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  /* remove resolve from the map */
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  HT_REMOVE(cache_map, &cache_root, resolve);
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  tor_free(resolve);
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}

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/** Called on the OR side when a DNS worker tells us the outcome of a DNS
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 * resolve: tell all pending connections about the result of the lookup, and
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 * cache the value.  (<b>address</b> is a NUL-terminated string containing the
 * address to look up; <b>addr</b> is an IPv4 address in host order;
 * <b>outcome</b> is one of
 * DNS_RESOLVE_{FAILED_TRANSIENT|FAILED_PERMANENT|SUCCEEDED}.
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 */
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static void
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dns_found_answer(const char *address, uint32_t addr, char outcome, uint32_t ttl)
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{
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  pending_connection_t *pend;
  cached_resolve_t search;
  cached_resolve_t *resolve;
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  connection_t *pendconn;
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  circuit_t *circ;
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  strlcpy(search.address, address, sizeof(search.address));
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  resolve = HT_FIND(cache_map, &cache_root, &search);
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  if (!resolve) {
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    log_info(LD_EXIT,"Resolved unasked address %s; caching anyway.",
             escaped_safe_str(address));
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    resolve = tor_malloc_zero(sizeof(cached_resolve_t));
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    resolve->state = (outcome == DNS_RESOLVE_SUCCEEDED) ?
      CACHE_STATE_VALID : CACHE_STATE_FAILED;
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    strlcpy(resolve->address, address, sizeof(resolve->address));
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    resolve->addr = addr;
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    resolve->expire = time(NULL) + dns_get_expiry_ttl(ttl);
    resolve->ttl = ttl;
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    insert_resolve(resolve);
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    return;
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  }

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  if (resolve->state != CACHE_STATE_PENDING) {
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    /* XXXX Maybe update addr? or check addr for consistency? Or let
     * VALID replace FAILED? */
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    log_notice(LD_EXIT, "Resolved %s which was already resolved; ignoring",
               escaped_safe_str(address));
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    tor_assert(resolve->pending_connections == NULL);
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    return;
  }
  /* Removed this assertion: in fact, we'll sometimes get a double answer
   * to the same question.  This can happen when we ask one worker to resolve
   * X.Y.Z., then we cancel the request, and then we ask another worker to
   * resolve X.Y.Z. */
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  /* tor_assert(resolve->state == CACHE_STATE_PENDING); */
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  resolve->addr = addr;
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  resolve->expire = time(NULL) + dns_get_expiry_ttl(ttl);
  resolve->ttl = ttl;
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  if (outcome == DNS_RESOLVE_SUCCEEDED)
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    resolve->state = CACHE_STATE_VALID;
  else
    resolve->state = CACHE_STATE_FAILED;

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  while (resolve->pending_connections) {
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    pend = resolve->pending_connections;
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    assert_connection_ok(pend->conn,time(NULL));
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    pend->conn->addr = resolve->addr;
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    pend->conn->address_ttl = resolve->ttl;
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    pendconn = pend->conn; /* don't pass complex things to the
                              connection_mark_for_close macro */
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    if (resolve->state == CACHE_STATE_FAILED) {
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      /* prevent double-remove. */
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      pendconn->state = EXIT_CONN_STATE_RESOLVEFAILED;
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      if (pendconn->purpose == EXIT_PURPOSE_CONNECT) {
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        connection_edge_end(pendconn, END_STREAM_REASON_RESOLVEFAILED,
                            pendconn->cpath_layer);
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        /* This detach must happen after we send the end cell. */
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        circuit_detach_stream(circuit_get_by_edge_conn(pendconn), pendconn);
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      } else {
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        send_resolved_cell(pendconn, RESOLVED_TYPE_ERROR);
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        /* This detach must happen after we send the resolved cell. */
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        circuit_detach_stream(circuit_get_by_edge_conn(pendconn), pendconn);
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      }
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      connection_free(pendconn);
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    } else {
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      if (pendconn->purpose == EXIT_PURPOSE_CONNECT) {
        /* prevent double-remove. */
        pend->conn->state = EXIT_CONN_STATE_CONNECTING;

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        circ = circuit_get_by_edge_conn(pend->conn);
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        tor_assert(circ);
        /* unlink pend->conn from resolving_streams, */
        circuit_detach_stream(circ, pend->conn);
        /* and link it to n_streams */
        pend->conn->next_stream = circ->n_streams;
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        pend->conn->on_circuit = circ;
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        circ->n_streams = pend->conn;

        connection_exit_connect(pend->conn);
      } else {
        /* prevent double-remove.  This isn't really an accurate state,
         * but it does the right thing. */
        pendconn->state = EXIT_CONN_STATE_RESOLVEFAILED;
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        send_resolved_cell(pendconn, RESOLVED_TYPE_IPV4);
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        circ = circuit_get_by_edge_conn(pendconn);
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        tor_assert(circ);
        circuit_detach_stream(circ, pendconn);
        connection_free(pendconn);
      }
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    }
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    resolve->pending_connections = pend->next;
    tor_free(pend);
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  }
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  if (outcome == DNS_RESOLVE_FAILED_TRANSIENT) { /* remove from cache */
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    dns_purge_resolve(resolve);
  }
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}

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#ifndef USE_EVENTDNS
/** Find or spawn a dns worker process to handle resolving
 * <b>exitconn</b>-\>address; tell that dns worker to begin resolving.
 */
static int
assign_to_dnsworker(connection_t *exitconn)
{
  connection_t *dnsconn;
  unsigned char len;

  tor_assert(exitconn->state == EXIT_CONN_STATE_RESOLVING);
  tor_assert(exitconn->s == -1);

  /* respawn here, to be sure there are enough */
  if (spawn_enough_dnsworkers() < 0) {
    goto err;
  }

  dnsconn = connection_get_by_type_state(CONN_TYPE_DNSWORKER,
                                         DNSWORKER_STATE_IDLE);

  if (!dnsconn) {
    log_warn(LD_EXIT,"no idle dns workers. Failing.");
    if (exitconn->purpose == EXIT_PURPOSE_RESOLVE)
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      send_resolved_cell(exitconn, RESOLVED_TYPE_ERROR_TRANSIENT);
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    goto err;
  }

  log_debug(LD_EXIT,
            "Connection (fd %d) needs to resolve %s; assigning "
            "to DNSWorker (fd %d)", exitconn->s,
            escaped_safe_str(exitconn->address), dnsconn->s);

  tor_free(dnsconn->address);
  dnsconn->address = tor_strdup(exitconn->address);
  dnsconn->state = DNSWORKER_STATE_BUSY;
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  /* touch the lastwritten timestamp, since that's how we check to
   * see how long it's been since we asked the question, and sometimes
   * we check before the first call to connection_handle_write(). */
  dnsconn->timestamp_lastwritten = time(NULL);
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  num_dnsworkers_busy++;

  len = strlen(dnsconn->address);
  connection_write_to_buf((char*)&len, 1, dnsconn);
  connection_write_to_buf(dnsconn->address, len, dnsconn);

  return 0;
err:
  dns_cancel_pending_resolve(exitconn->address); /* also sends end and frees */
  return -1;
}

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/******************************************************************/

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/*
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 * Connection between OR and dnsworker
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 */
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/** Write handler: called when we've pushed a request to a dnsworker. */
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int
connection_dns_finished_flushing(connection_t *conn)
{
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  tor_assert(conn);
  tor_assert(conn->type == CONN_TYPE_DNSWORKER);
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  connection_stop_writing(conn);
  return 0;
}

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int
connection_dns_reached_eof(connection_t *conn)
{
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  log_warn(LD_EXIT,"Read eof. Worker died unexpectedly.");
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  if (conn->state == DNSWORKER_STATE_BUSY) {
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    /* don't cancel the resolve here -- it would be cancelled in
     * connection_about_to_close_connection(), since conn is still
     * in state BUSY
     */
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    num_dnsworkers_busy--;
  }
  num_dnsworkers--;
  connection_mark_for_close(conn);
  return 0;
}

/** Read handler: called when we get data from a dnsworker. See
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 * if we have a complete answer.  If so, call dns_found_answer on the
 * result.  If not, wait.  Returns 0. */
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int
connection_dns_process_inbuf(connection_t *conn)
{
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  char success;
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  uint32_t addr;
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  int ttl;
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  tor_assert(conn);
  tor_assert(conn->type == CONN_TYPE_DNSWORKER);
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  if (conn->state != DNSWORKER_STATE_BUSY && buf_datalen(conn->inbuf)) {
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    log_warn(LD_BUG,
             "Bug: read data (%d bytes) from an idle dns worker (fd %d, "
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             "address %s). Please report.", (int)buf_datalen(conn->inbuf),
             conn->s, escaped_safe_str(conn->address));
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    tor_fragile_assert();
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    /* Pull it off the buffer anyway, or it will just stay there.
     * Keep pulling things off because sometimes we get several
     * answers at once (!). */
    while (buf_datalen(conn->inbuf)) {
      connection_fetch_from_buf(&success,1,conn);
      connection_fetch_from_buf((char *)&addr,sizeof(uint32_t),conn);
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      log_warn(LD_EXIT,"Discarding idle dns answer (success %d, addr %d.)",
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               success, addr);
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    }
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    return 0;
  }
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  if (buf_datalen(conn->inbuf) < 5) /* entire answer available? */
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    return 0; /* not yet */
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  tor_assert(conn->state == DNSWORKER_STATE_BUSY);
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  tor_assert(buf_datalen(conn->inbuf) == 5);
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  connection_fetch_from_buf(&success,1,conn);
  connection_fetch_from_buf((char *)&addr,sizeof(uint32_t),conn);
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  log_debug(LD_EXIT, "DNSWorker (fd %d) returned answer for %s",
            conn->s, escaped_safe_str(conn->address));
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  tor_assert(success >= DNS_RESOLVE_FAILED_TRANSIENT);
  tor_assert(success <= DNS_RESOLVE_SUCCEEDED);
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  ttl = (success == DNS_RESOLVE_FAILED_TRANSIENT) ? 0 : MAX_DNS_ENTRY_AGE;
  dns_found_answer(conn->address, ntohl(addr), success, ttl);
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  tor_free(conn->address);
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  conn->address = tor_strdup("<idle>");
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  conn->state = DNSWORKER_STATE_IDLE;
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  num_dnsworkers_busy--;
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  if (conn->timestamp_created < last_rotation_time) {
    connection_mark_for_close(conn);
    num_dnsworkers--;
    spawn_enough_dnsworkers();
  }
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  return 0;
}

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/** Close and re-open all idle dnsworkers; schedule busy ones to be closed
 * and re-opened once they're no longer busy.
 **/
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void
dnsworkers_rotate(void)
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{
  connection_t *dnsconn;
  while ((dnsconn = connection_get_by_type_state(CONN_TYPE_DNSWORKER,
                                                 DNSWORKER_STATE_IDLE))) {
    connection_mark_for_close(dnsconn);
    num_dnsworkers--;
  }
  last_rotation_time = time(NULL);
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  if (server_mode(get_options()))
    spawn_enough_dnsworkers();
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}

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/** Implementation for DNS workers; this code runs in a separate
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 * execution context.  It takes as its argument an fdarray as returned
 * by socketpair(), and communicates via fdarray[1].  The protocol is
 * as follows:
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 *    - The OR says:
 *         - ADDRESSLEN [1 byte]
 *         - ADDRESS    [ADDRESSLEN bytes]
 *    - The DNS worker does the lookup, and replies:
 *         - OUTCOME    [1 byte]
 *         - IP         [4 bytes]
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 *
 * OUTCOME is one of DNS_RESOLVE_{FAILED_TRANSIENT|FAILED_PERMANENT|SUCCEEDED}.
 * IP is in host order.
 *
 * The dnsworker runs indefinitely, until its connection is closed or an error
 * occurs.
 */
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static int
dnsworker_main(void *data)
{