dns.c 28.2 KB
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/* Copyright 2003-2004 Roger Dingledine.
 * Copyright 2004 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
 * \brief Resolve hostnames in separate processes.
 **/
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/* See http://elvin.dstc.com/ListArchive/elvin-dev/archive/2001/09/msg00027.html
 * 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 "tree.h"
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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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/** 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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/** Linked list of connections waiting for a DNS answer. */
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struct pending_connection_t {
  struct connection_t *conn;
  struct pending_connection_t *next;
};

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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 splay tree, and as a linked
 * list from oldest to newest.
 */
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struct cached_resolve {
  SPLAY_ENTRY(cached_resolve) 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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  struct pending_connection_t *pending_connections;
  struct cached_resolve *next;
};

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static void purge_expired_resolves(uint32_t now);
static int assign_to_dnsworker(connection_t *exitconn);
static void dns_purge_resolve(struct cached_resolve *resolve);
static void dns_found_answer(char *address, uint32_t addr, char outcome);
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static int dnsworker_main(void *data);
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static int spawn_dnsworker(void);
static void spawn_enough_dnsworkers(void);
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static void send_resolved_cell(connection_t *conn, uint8_t answer_type);
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/** Splay tree of cached_resolve objects. */
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static SPLAY_HEAD(cache_tree, cached_resolve) cache_root;
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/** Function to compare hashed resolves on their addresses; used to
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 * implement splay trees. */
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static int compare_cached_resolves(struct cached_resolve *a,
                                   struct cached_resolve *b) {
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  /* make this smarter one day? */
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  return strncasecmp(a->address, b->address, MAX_ADDRESSLEN);
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}

SPLAY_PROTOTYPE(cache_tree, cached_resolve, node, compare_cached_resolves);
SPLAY_GENERATE(cache_tree, cached_resolve, node, compare_cached_resolves);

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/** Initialize the DNS cache. */
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static void init_cache_tree(void) {
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  SPLAY_INIT(&cache_root);
}

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/** Initialize the DNS subsystem; called by the OR process. */
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void dns_init(void) {
  init_cache_tree();
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  last_rotation_time=time(NULL);
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  spawn_enough_dnsworkers();
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}
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/** Linked list of resolved addresses, oldest to newest. */
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static struct cached_resolve *oldest_cached_resolve = NULL;
static struct cached_resolve *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) {
  struct cached_resolve *resolve;
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  struct pending_connection_t *pend;
  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(LOG_DEBUG,"Forgetting old cached resolve (expires %lu)", (unsigned long)resolve->expire);
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    if (resolve->state == CACHE_STATE_PENDING) {
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      log_fn(LOG_WARN,"Bug: Expiring a dns resolve that's still pending. Forgot to cull it?");
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#ifdef TOR_FRAGILE
      tor_assert(0);
#endif
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    }
    if (resolve->pending_connections) {
      log_fn(LOG_WARN, "Closing pending connections on expiring DNS resolve!");
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#ifdef TOR_FRAGILE
      tor_assert(0);
#endif
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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;
        connection_edge_end(pendconn, END_STREAM_REASON_MISC,
                            pendconn->cpath_layer);
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        circuit_detach_stream(circuit_get_by_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 */
    SPLAY_REMOVE(cache_tree, &cache_root, resolve);
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    tor_free(resolve);
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  }
}

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static void send_resolved_cell(connection_t *conn, uint8_t answer_type)
{
  char buf[RELAY_PAYLOAD_SIZE];
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  size_t buflen;
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  buf[0] = answer_type;

  switch (answer_type)
    {
    case RESOLVED_TYPE_IPV4:
      buf[1] = 4;
      set_uint32(buf+2, htonl(conn->addr));
      buflen = 6;
      break;
    case RESOLVED_TYPE_ERROR_TRANSIENT:
    case RESOLVED_TYPE_ERROR:
      buf[1] = 24; /* length of "error resolving hostname" */
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      strlcpy(buf+2, "error resolving hostname", sizeof(buf)-2);
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      buflen = 26;
      break;
    default:
      tor_assert(0);
    }
  connection_edge_send_command(conn, circuit_get_by_conn(conn),
                               RELAY_COMMAND_RESOLVED, buf, buflen,
                               conn->cpath_layer);
}

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/** Link <b>r</b> into the tree of address-to-result mappings, and add it to
 * the linked list of resolves-by-age. */
static void
insert_resolve(struct cached_resolve *r)
{
  /* 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;

  SPLAY_INSERT(cache_tree, &cache_root, r);
}

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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.
 */
int dns_resolve(connection_t *exitconn) {
  struct cached_resolve *resolve;
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  struct cached_resolve search;
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  struct 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);
    return 1;
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  }

  /* then take this opportunity to see if there are any expired
   * resolves in the tree. */
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  purge_expired_resolves(now);
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  /* now check the tree to see if 'address' is already there. */
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  strlcpy(search.address, exitconn->address, sizeof(search.address));
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  resolve = SPLAY_FIND(cache_tree, &cache_root, &search);
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  if (resolve) { /* already there */
    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(
                                      sizeof(struct 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_fn(LOG_DEBUG,"Connection (fd %d) waiting for pending DNS resolve of '%s'",
               exitconn->s, 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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        log_fn(LOG_DEBUG,"Connection (fd %d) found cached answer for '%s'",
               exitconn->s, exitconn->address);
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        if (exitconn->purpose == EXIT_PURPOSE_RESOLVE)
          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_fn(LOG_DEBUG,"Connection (fd %d) found cached error for '%s'",
               exitconn->s, exitconn->address);
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        if (exitconn->purpose == EXIT_PURPOSE_RESOLVE)
          send_resolved_cell(exitconn, RESOLVED_TYPE_ERROR);
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        circ = circuit_get_by_conn(exitconn);
        if (circ)
          circuit_detach_stream(circ, exitconn);
        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 */
  resolve = tor_malloc_zero(sizeof(struct cached_resolve));
  resolve->state = CACHE_STATE_PENDING;
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  resolve->expire = now + MAX_DNS_ENTRY_AGE;
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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(struct 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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  return assign_to_dnsworker(exitconn);
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}

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/** Find or spawn a dns worker process to handle resolving
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 * <b>exitconn</b>-\>address; tell that dns worker to begin resolving.
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 */
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static int assign_to_dnsworker(connection_t *exitconn) {
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  connection_t *dnsconn;
  unsigned char len;
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  tor_assert(exitconn->state == EXIT_CONN_STATE_RESOLVING);
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  tor_assert(exitconn->s == -1);
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  spawn_enough_dnsworkers(); /* respawn here, to be sure there are enough */
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  dnsconn = connection_get_by_type_state(CONN_TYPE_DNSWORKER, DNSWORKER_STATE_IDLE);
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  if (!dnsconn) {
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    log_fn(LOG_WARN,"no idle dns workers. Failing.");
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    if (exitconn->purpose == EXIT_PURPOSE_RESOLVE)
      send_resolved_cell(exitconn, RESOLVED_TYPE_ERROR_TRANSIENT);
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    dns_cancel_pending_resolve(exitconn->address); /* also sends end and frees! */
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    return -1;
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  }

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  log_fn(LOG_DEBUG, "Connection (fd %d) needs to resolve '%s'; assigning to DNSWorker (fd %d)",
         exitconn->s, exitconn->address, dnsconn->s);

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  tor_free(dnsconn->address);
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  dnsconn->address = tor_strdup(exitconn->address);
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  dnsconn->state = DNSWORKER_STATE_BUSY;
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  num_dnsworkers_busy++;
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  len = strlen(dnsconn->address);
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  connection_write_to_buf(&len, 1, dnsconn);
  connection_write_to_buf(dnsconn->address, len, dnsconn);
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//  log_fn(LOG_DEBUG,"submitted '%s'", exitconn->address);
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  return 0;
}

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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)
{
  struct pending_connection_t *pend, *victim;
  struct cached_resolve search;
  struct cached_resolve *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 = SPLAY_FIND(cache_tree, &cache_root, &search);
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  if (!resolve) {
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    log_fn(LOG_NOTICE,"Address '%s' is not pending. Dropping.", 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_fn(LOG_DEBUG, "First connection (fd %d) no longer waiting for resolve of '%s'",
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           conn->s, conn->address);
    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_fn(LOG_DEBUG, "Connection (fd %d) no longer waiting for resolve of '%s'",
               conn->s, conn->address);
        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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/** 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) {
  struct pending_connection_t *pend;
  struct cached_resolve *resolve;

  SPLAY_FOREACH(resolve, cache_tree, &cache_root) {
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    for (pend = resolve->pending_connections;
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         pend;
         pend = pend->next) {
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      tor_assert(pend->conn != conn);
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    }
  }
}

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/** Log an error and abort if any connection waiting for a DNS resolve is
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 * corrupted. */
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void assert_all_pending_dns_resolves_ok(void) {
  struct pending_connection_t *pend;
  struct cached_resolve *resolve;

  SPLAY_FOREACH(resolve, cache_tree, &cache_root) {
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    for (pend = resolve->pending_connections;
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         pend;
         pend = pend->next) {
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      assert_connection_ok(pend->conn, 0);
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      tor_assert(pend->conn->s == -1);
      tor_assert(!connection_in_array(pend->conn));
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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) {
  struct pending_connection_t *pend;
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  struct cached_resolve search;
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  struct cached_resolve *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 = SPLAY_FIND(cache_tree, &cache_root, &search);
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  if (!resolve) {
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    log_fn(LOG_NOTICE,"Address '%s' is not pending. Dropping.", address);
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    return;
  }

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  if (!resolve->pending_connections) {
    /* XXX this should never trigger, but sometimes it does */
    log_fn(LOG_WARN,"Bug: Address '%s' is pending but has no pending connections!", address);
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#ifdef TOR_FRAGILE
    tor_assert(0);
#endif
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    return;
  }
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  tor_assert(resolve->pending_connections);
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  /* mark all pending connections to fail */
  log_fn(LOG_DEBUG, "Failing all connections waiting on DNS resolve of '%s'",
         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_conn(pendconn);
    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 dns_purge_resolve(struct cached_resolve *resolve) {
  struct cached_resolve *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) ;
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    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 tree */
  SPLAY_REMOVE(cache_tree, &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 dns_found_answer(char *address, uint32_t addr, char outcome) {
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  struct pending_connection_t *pend;
  struct cached_resolve search;
  struct cached_resolve *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 = SPLAY_FIND(cache_tree, &cache_root, &search);
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  if (!resolve) {
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    log_fn(LOG_INFO,"Resolved unasked address '%s'; caching anyway.", address);
    resolve = tor_malloc_zero(sizeof(struct cached_resolve));
    resolve->state = (outcome == DNS_RESOLVE_SUCCEEDED) ?
      CACHE_STATE_VALID : CACHE_STATE_FAILED;
    resolve->addr = addr;
    resolve->expire = time(NULL) + MAX_DNS_ENTRY_AGE;
    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_fn(LOG_NOTICE, "Resolved '%s' which was already resolved; ignoring",
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           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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  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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    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);
        /* This detach must happen after we send the end cell. */
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        circuit_detach_stream(circuit_get_by_conn(pendconn), pendconn);
      } 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. */
        circuit_detach_stream(circuit_get_by_conn(pendconn), pendconn);
      }
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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;

        circ = circuit_get_by_conn(pend->conn);
        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;
        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;
        send_resolved_cell(pendconn, RESOLVED_TYPE_IPV4);
        circ = circuit_get_by_conn(pendconn);
        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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/******************************************************************/

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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) {
  log_fn(LOG_WARN,"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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  tor_assert(conn);
  tor_assert(conn->type == CONN_TYPE_DNSWORKER);
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  if (conn->state != DNSWORKER_STATE_BUSY && buf_datalen(conn->inbuf)) {
    log_fn(LOG_WARN,"Bug: read data from an idle dns worker.  Please report.");
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#ifdef TOR_FRAGILE
    tor_assert(0);
#endif
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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_fn(LOG_DEBUG, "DNSWorker (fd %d) returned answer for '%s'",
         conn->s, conn->address);

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  tor_assert(success >= DNS_RESOLVE_FAILED_TRANSIENT);
  tor_assert(success <= DNS_RESOLVE_SUCCEEDED);
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  dns_found_answer(conn->address, ntohl(addr), success);
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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.
 **/
void dnsworkers_rotate(void)
{
  connection_t *dnsconn;
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  log_fn(LOG_INFO, "Rotating DNS workers.");
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  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);
  spawn_enough_dnsworkers();
}

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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) {
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  char address[MAX_ADDRESSLEN];
  unsigned char address_len;
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  char answer[5];
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  uint32_t ip;
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  int *fdarray = data;
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  int fd;
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  int result;
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  /* log_fn(LOG_NOTICE,"After spawn: fdarray @%d has %d:%d", (int)fdarray, fdarray[0],fdarray[1]); */

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  fd = fdarray[1]; /* this side is ours */
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#ifndef TOR_IS_MULTITHREADED
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  tor_close_socket(fdarray[0]); /* this is the side of the socketpair the parent uses */
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  connection_free_all(); /* so the child doesn't hold the parent's fd's open */
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  handle_signals(0); /* ignore interrupts from the keyboard, etc */
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#endif
  tor_free(data);
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  for (;;) {
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    int r;
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    if ((r = recv(fd, &address_len, 1, 0)) != 1) {
      if (r == 0) {
        log_fn(LOG_INFO,"DNS worker exiting because Tor process closed connection (either pruned idle dnsworker or died).");
      } else {
        log_fn(LOG_INFO,"DNS worker exiting because of error on connection to Tor process.");
        log_fn(LOG_INFO,"(Error on %d was %s)", fd, tor_socket_strerror(tor_socket_errno(fd)));
      }
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      spawn_exit();
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    }

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    if (address_len && read_all(fd, address, address_len, 1) != address_len) {
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      log_fn(LOG_ERR,"read hostname failed. Child exiting.");
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      spawn_exit();
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    }
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    address[address_len] = 0; /* null terminate it */
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    result = tor_lookup_hostname(address, &ip);
    /* Make 0.0.0.0 an error, so that we can use "0" to mean "no addr") */
    if (!ip)
      result = -1;
    switch (result) {
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      case 1:
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        /* XXX result can never be 1, because we set it to -1 above on error */
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        log_fn(LOG_INFO,"Could not resolve dest addr %s (transient).",address);
        answer[0] = DNS_RESOLVE_FAILED_TRANSIENT;
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        break;
      case -1:
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        log_fn(LOG_INFO,"Could not resolve dest addr %s (permanent).",address);
        answer[0] = DNS_RESOLVE_FAILED_PERMANENT;
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        break;
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      case 0:
        log_fn(LOG_INFO,"Resolved address '%s'.",address);
        answer[0] = DNS_RESOLVE_SUCCEEDED;
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        break;
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    }
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    set_uint32(answer+1, ip);
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    if (write_all(fd, answer, 5, 1) != 5) {
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      log_fn(LOG_ERR,"writing answer failed. Child exiting.");
      spawn_exit();
    }
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  }
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  return 0; /* windows wants this function to return an int */
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}

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/** Launch a new DNS worker; return 0 on success, -1 on failure.
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 */
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static int spawn_dnsworker(void) {
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  int *fdarray;
  int fd;
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  connection_t *conn;

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  fdarray = tor_malloc(sizeof(int)*2);
  if (tor_socketpair(AF_UNIX, SOCK_STREAM, 0, fdarray) < 0) {
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    log(LOG_ERR, "Couldn't construct socketpair: %s",
        tor_socket_strerror(tor_socket_errno(-1)));
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    tor_cleanup();
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    tor_free(fdarray);
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    exit(1);
  }

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  /* log_fn(LOG_NOTICE,"Before spawn: fdarray @%d has %d:%d", (int)fdarray, fdarray[0],fdarray[1]); */

  fd = fdarray[0]; /* We copy this out here, since dnsworker_main may free fdarray */
  spawn_func(dnsworker_main, (void*)fdarray);
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  log_fn(LOG_DEBUG,"just spawned a worker.");
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#ifndef TOR_IS_MULTITHREADED
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  tor_close_socket(fdarray[1]); /* we don't need the worker's side of the pipe */
  tor_free(fdarray);
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#endif
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  conn = connection_new(CONN_TYPE_DNSWORKER);

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  set_socket_nonblocking(fd);
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  /* set up conn so it's got all the data we need to remember */
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  conn->s = fd;
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  conn->address = tor_strdup("<unused>");
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  if (connection_add(conn) < 0) { /* no space, forget it */
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    log_fn(LOG_WARN,"connection_add failed. Giving up.");
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    connection_free(conn); /* this closes fd */
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    return -1;
  }

  conn->state = DNSWORKER_STATE_IDLE;
  connection_start_reading(conn);

  return 0; /* success */
}

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/** If we have too many or too few DNS workers, spawn or kill some.
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 */
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static void spawn_enough_dnsworkers(void) {
  int num_dnsworkers_needed; /* aim to have 1 more than needed,
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                           * but no less than min and no more than max */
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  connection_t *dnsconn;

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  /* XXX This may not be the best strategy. Maybe we should queue pending
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   *     requests until the old ones finish or time out: otherwise, if
   *     the connection requests come fast enough, we never get any DNS done. -NM
   * XXX But if we queue them, then the adversary can pile even more
   *     queries onto us, blocking legitimate requests for even longer.
   *     Maybe we should compromise and only kill if it's been at it for
   *     more than, e.g., 2 seconds. -RD
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   */
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  if (num_dnsworkers_busy == MAX_DNSWORKERS) {
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    /* We always want at least one worker idle.
     * So find the oldest busy worker and kill it.
     */
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    dnsconn = connection_get_by_type_state_lastwritten(CONN_TYPE_DNSWORKER,
                                                       DNSWORKER_STATE_BUSY);
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    tor_assert(dnsconn);
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    log_fn(LOG_WARN, "%d DNS workers are spawned; all are busy. Killing one.",
           MAX_DNSWORKERS);
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    connection_mark_for_close(dnsconn);
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    num_dnsworkers_busy--;
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    num_dnsworkers--;
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  }
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  if (num_dnsworkers_busy >= MIN_DNSWORKERS)
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    num_dnsworkers_needed = num_dnsworkers_busy+1;
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  else
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    num_dnsworkers_needed = MIN_DNSWORKERS;
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  while (num_dnsworkers < num_dnsworkers_needed) {
    if (spawn_dnsworker() < 0) {
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      log(LOG_WARN,"spawn_enough_dnsworkers(): spawn failed!");
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      return;
    }
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    num_dnsworkers++;
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  }

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  while (num_dnsworkers > num_dnsworkers_busy+MAX_IDLE_DNSWORKERS) { /* too many idle? */
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    /* cull excess workers */
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    log_fn(LOG_NOTICE,"%d of %d dnsworkers are idle. Killing one.",
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           num_dnsworkers-num_dnsworkers_needed, num_dnsworkers);
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    dnsconn = connection_get_by_type_state(CONN_TYPE_DNSWORKER, DNSWORKER_STATE_IDLE);
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    tor_assert(dnsconn);
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    connection_mark_for_close(dnsconn);
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    num_dnsworkers--;
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  }
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}