dns.c

/* Copyright (c) 2003-2004, Roger Dingledine.
 * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
 * Copyright (c) 2007-2018, The Tor Project, Inc. */
/* See LICENSE for licensing information */

/**
 * \file dns.c
 * \brief Implements a local cache for DNS results for Tor servers.
 * This is implemented as a wrapper around Adam Langley's eventdns.c code.
 * (We can't just use gethostbyname() and friends because we really need to
 * be nonblocking.)
 *
 * There are three main cases when a Tor relay uses dns.c to launch a DNS
 * request:
 *   <ol>
 *    <li>To check whether the DNS server is working more or less correctly.
 *      This happens via dns_launch_correctness_checks().  The answer is
 *      reported in the return value from later calls to
 *      dns_seems_to_be_broken().
 *    <li>When a client has asked the relay, in a RELAY_BEGIN cell, to connect
 *      to a given server by hostname.  This happens via dns_resolve().
 *    <li>When a client has asked the relay, in a RELAY_RESOLVE cell, to look
 *      up a given server's IP address(es) by hostname. This also happens via
 *      dns_resolve().
 *   </ol>
 *
 * Each of these gets handled a little differently.
 *
 * To check for correctness, we look up some hostname we expect to exist and
 * have real entries, some hostnames which we expect to definitely not exist,
 * and some hostnames that we expect to probably not exist.  If too many of
 * the hostnames that shouldn't exist do exist, that's a DNS hijacking
 * attempt.  If too many of the hostnames that should exist have the same
 * addresses as the ones that shouldn't exist, that's a very bad DNS hijacking
 * attempt, or a very naughty captive portal.  And if the hostnames that
 * should exist simply don't exist, we probably have a broken nameserver.
 *
 * To handle client requests, we first check our cache for answers. If there
 * isn't something up-to-date, we've got to launch A or AAAA requests as
 * appropriate.  How we handle responses to those in particular is a bit
 * complex; see dns_lookup() and set_exitconn_info_from_resolve().
 *
 * When a lookup is finally complete, the inform_pending_connections()
 * function will tell all of the streams that have been waiting for the
 * resolve, by calling connection_exit_connect() if the client sent a
 * RELAY_BEGIN cell, and by calling send_resolved_cell() or
 * send_hostname_cell() if the client sent a RELAY_RESOLVE cell.
 **/

#define DNS_PRIVATE

#include "or.h"
#include "circuitlist.h"
#include "circuituse.h"
#include "config.h"
#include "connection.h"
#include "connection_edge.h"
#include "control.h"
#include "crypto_rand.h"
#include "dns.h"
#include "main.h"
#include "policies.h"
#include "relay.h"
#include "router.h"
#include "ht.h"
#include "sandbox.h"

#include "edge_connection_st.h"
#include "or_circuit_st.h"

#include <event2/event.h>
#include <event2/dns.h>

/** How long will we wait for an answer from the resolver before we decide
 * that the resolver is wedged? */
#define RESOLVE_MAX_TIMEOUT 300

/** Our evdns_base; this structure handles all our name lookups. */
static struct evdns_base *the_evdns_base = NULL;

/** Have we currently configured nameservers with eventdns? */
static int nameservers_configured = 0;
/** Did our most recent attempt to configure nameservers with eventdns fail? */
static int nameserver_config_failed = 0;
/** What was the resolv_conf fname we last used when configuring the
 * nameservers? Used to check whether we need to reconfigure. */
static char *resolv_conf_fname = NULL;
/** What was the mtime on the resolv.conf file we last used when configuring
 * the nameservers?  Used to check whether we need to reconfigure. */
static time_t resolv_conf_mtime = 0;

static void purge_expired_resolves(time_t now);
static void dns_found_answer(const char *address, uint8_t query_type,
                             int dns_answer,
                             const tor_addr_t *addr,
                             const char *hostname,
                             uint32_t ttl);
static void add_wildcarded_test_address(const char *address);
static int configure_nameservers(int force);
static int answer_is_wildcarded(const char *ip);
static int evdns_err_is_transient(int err);
static void inform_pending_connections(cached_resolve_t *resolve);
static void make_pending_resolve_cached(cached_resolve_t *cached);

#ifdef DEBUG_DNS_CACHE
static void assert_cache_ok_(void);
#define assert_cache_ok() assert_cache_ok_()
#else
#define assert_cache_ok() STMT_NIL
#endif /* defined(DEBUG_DNS_CACHE) */
static void assert_resolve_ok(cached_resolve_t *resolve);

/** Hash table of cached_resolve objects. */
static HT_HEAD(cache_map, cached_resolve_t) cache_root;

/** Global: how many IPv6 requests have we made in all? */
static uint64_t n_ipv6_requests_made = 0;
/** Global: how many IPv6 requests have timed out? */
static uint64_t n_ipv6_timeouts = 0;
/** Global: Do we think that IPv6 DNS is broken? */
static int dns_is_broken_for_ipv6 = 0;

/** Function to compare hashed resolves on their addresses; used to
 * implement hash tables. */
static inline int
cached_resolves_eq(cached_resolve_t *a, cached_resolve_t *b)
{
  /* make this smarter one day? */
  assert_resolve_ok(a); // Not b; b may be just a search.
  return !strncmp(a->address, b->address, MAX_ADDRESSLEN);
}

/** Hash function for cached_resolve objects */
static inline unsigned int
cached_resolve_hash(cached_resolve_t *a)
{
  return (unsigned) siphash24g((const uint8_t*)a->address, strlen(a->address));
}

HT_PROTOTYPE(cache_map, cached_resolve_t, node, cached_resolve_hash,
             cached_resolves_eq)
HT_GENERATE2(cache_map, cached_resolve_t, node, cached_resolve_hash,
             cached_resolves_eq, 0.6, tor_reallocarray_, tor_free_)

/** Initialize the DNS cache. */
static void
init_cache_map(void)
{
  HT_INIT(cache_map, &cache_root);
}

/** Helper: called by eventdns when eventdns wants to log something. */
static void
evdns_log_cb(int warn, const char *msg)
{
  const char *cp;
  static int all_down = 0;
  int severity = warn ? LOG_WARN : LOG_INFO;
  if (!strcmpstart(msg, "Resolve requested for") &&
      get_options()->SafeLogging) {
    log_info(LD_EXIT, "eventdns: Resolve requested.");
    return;
  } else if (!strcmpstart(msg, "Search: ")) {
    return;
  }
  if (!strcmpstart(msg, "Nameserver ") && (cp=strstr(msg, " has failed: "))) {
    char *ns = tor_strndup(msg+11, cp-(msg+11));
    const char *colon = strchr(cp, ':');
    tor_assert(colon);
    const char *err = colon+2;
    /* Don't warn about a single failed nameserver; we'll warn with 'all
     * nameservers have failed' if we're completely out of nameservers;
     * otherwise, the situation is tolerable. */
    severity = LOG_INFO;
    control_event_server_status(LOG_NOTICE,
                                "NAMESERVER_STATUS NS=%s STATUS=DOWN ERR=%s",
                                ns, escaped(err));
    tor_free(ns);
  } else if (!strcmpstart(msg, "Nameserver ") &&
             (cp=strstr(msg, " is back up"))) {
    char *ns = tor_strndup(msg+11, cp-(msg+11));
    severity = (all_down && warn) ? LOG_NOTICE : LOG_INFO;
    all_down = 0;
    control_event_server_status(LOG_NOTICE,
                                "NAMESERVER_STATUS NS=%s STATUS=UP", ns);
    tor_free(ns);
  } else if (!strcmp(msg, "All nameservers have failed")) {
    control_event_server_status(LOG_WARN, "NAMESERVER_ALL_DOWN");
    all_down = 1;
  } else if (!strcmpstart(msg, "Address mismatch on received DNS")) {
    static ratelim_t mismatch_limit = RATELIM_INIT(3600);
    const char *src = strstr(msg, " Apparent source");
    if (!src || get_options()->SafeLogging) {
      src = "";
    }
    log_fn_ratelim(&mismatch_limit, severity, LD_EXIT,
                   "eventdns: Received a DNS packet from "
                   "an IP address to which we did not send a request. This "
                   "could be a DNS spoofing attempt, or some kind of "
                   "misconfiguration.%s", src);
    return;
  }
  tor_log(severity, LD_EXIT, "eventdns: %s", msg);
}

/** Helper: passed to eventdns.c as a callback so it can generate random
 * numbers for transaction IDs and 0x20-hack coding. */
static void
dns_randfn_(char *b, size_t n)
{
  crypto_rand(b,n);
}

/** Initialize the DNS subsystem; called by the OR process. */
int
dns_init(void)
{
  init_cache_map();
  evdns_set_random_bytes_fn(dns_randfn_);
  if (server_mode(get_options())) {
    int r = configure_nameservers(1);
    return r;
  }
  return 0;
}

/** Called when DNS-related options change (or may have changed).  Returns -1
 * on failure, 0 on success. */
int
dns_reset(void)
{
  const or_options_t *options = get_options();
  if (! server_mode(options)) {

    if (!the_evdns_base) {
      if (!(the_evdns_base = evdns_base_new(tor_libevent_get_base(), 0))) {
        log_err(LD_BUG, "Couldn't create an evdns_base");
        return -1;
      }
    }

    evdns_base_clear_nameservers_and_suspend(the_evdns_base);
    evdns_base_search_clear(the_evdns_base);
    nameservers_configured = 0;
    tor_free(resolv_conf_fname);
    resolv_conf_mtime = 0;
  } else {
    if (configure_nameservers(0) < 0) {
      return -1;
    }
  }
  return 0;
}

/** Return true iff the most recent attempt to initialize the DNS subsystem
 * failed. */
int
has_dns_init_failed(void)
{
  return nameserver_config_failed;
}

/** Helper: Given a TTL from a DNS response, determine what TTL to give the
 * OP that asked us to resolve it, and how long to cache that record
 * ourselves. */
uint32_t
dns_clip_ttl(uint32_t ttl)
{
  /* This logic is a defense against "DefectTor" DNS-based traffic
   * confirmation attacks, as in https://nymity.ch/tor-dns/tor-dns.pdf .
   * We only give two values: a "low" value and a "high" value.
   */
  if (ttl < MIN_DNS_TTL_AT_EXIT)
    return MIN_DNS_TTL_AT_EXIT;
  else
    return MAX_DNS_TTL_AT_EXIT;
}

/** Helper: free storage held by an entry in the DNS cache. */
static void
free_cached_resolve_(cached_resolve_t *r)
{
  if (!r)
    return;
  while (r->pending_connections) {
    pending_connection_t *victim = r->pending_connections;
    r->pending_connections = victim->next;
    tor_free(victim);
  }
  if (r->res_status_hostname == RES_STATUS_DONE_OK)
    tor_free(r->result_ptr.hostname);
  r->magic = 0xFF00FF00;
  tor_free(r);
}

/** Compare two cached_resolve_t pointers by expiry time, and return
 * less-than-zero, zero, or greater-than-zero as appropriate. Used for
 * the priority queue implementation. */
static int
compare_cached_resolves_by_expiry_(const void *_a, const void *_b)
{
  const cached_resolve_t *a = _a, *b = _b;
  if (a->expire < b->expire)
    return -1;
  else if (a->expire == b->expire)
    return 0;
  else
    return 1;
}

/** Priority queue of cached_resolve_t objects to let us know when they
 * will expire. */
static smartlist_t *cached_resolve_pqueue = NULL;

static void
cached_resolve_add_answer(cached_resolve_t *resolve,
                          int query_type,
                          int dns_result,
                          const tor_addr_t *answer_addr,
                          const char *answer_hostname,
                          uint32_t ttl)
{
  if (query_type == DNS_PTR) {
    if (resolve->res_status_hostname != RES_STATUS_INFLIGHT)
      return;

    if (dns_result == DNS_ERR_NONE && answer_hostname) {
      resolve->result_ptr.hostname = tor_strdup(answer_hostname);
      resolve->res_status_hostname = RES_STATUS_DONE_OK;
    } else {
      resolve->result_ptr.err_hostname = dns_result;
      resolve->res_status_hostname = RES_STATUS_DONE_ERR;
    }
    resolve->ttl_hostname = ttl;
  } else if (query_type == DNS_IPv4_A) {
    if (resolve->res_status_ipv4 != RES_STATUS_INFLIGHT)
      return;

    if (dns_result == DNS_ERR_NONE && answer_addr &&
        tor_addr_family(answer_addr) == AF_INET) {
      resolve->result_ipv4.addr_ipv4 = tor_addr_to_ipv4h(answer_addr);
      resolve->res_status_ipv4 = RES_STATUS_DONE_OK;
    } else {
      resolve->result_ipv4.err_ipv4 = dns_result;
      resolve->res_status_ipv4 = RES_STATUS_DONE_ERR;
    }
    resolve->ttl_ipv4 = ttl;
  } else if (query_type == DNS_IPv6_AAAA) {
    if (resolve->res_status_ipv6 != RES_STATUS_INFLIGHT)
      return;

    if (dns_result == DNS_ERR_NONE && answer_addr &&
        tor_addr_family(answer_addr) == AF_INET6) {
      memcpy(&resolve->result_ipv6.addr_ipv6,
             tor_addr_to_in6(answer_addr),
             sizeof(struct in6_addr));
      resolve->res_status_ipv6 = RES_STATUS_DONE_OK;
    } else {
      resolve->result_ipv6.err_ipv6 = dns_result;
      resolve->res_status_ipv6 = RES_STATUS_DONE_ERR;
    }
    resolve->ttl_ipv6 = ttl;
  }
}

/** Return true iff there are no in-flight requests for <b>resolve</b>. */
static int
cached_resolve_have_all_answers(const cached_resolve_t *resolve)
{
  return (resolve->res_status_ipv4 != RES_STATUS_INFLIGHT &&
          resolve->res_status_ipv6 != RES_STATUS_INFLIGHT &&
          resolve->res_status_hostname != RES_STATUS_INFLIGHT);
}

/** Set an expiry time for a cached_resolve_t, and add it to the expiry
 * priority queue */
static void
set_expiry(cached_resolve_t *resolve, time_t expires)
{
  tor_assert(resolve && resolve->expire == 0);
  if (!cached_resolve_pqueue)
    cached_resolve_pqueue = smartlist_new();
  resolve->expire = expires;
  smartlist_pqueue_add(cached_resolve_pqueue,
                       compare_cached_resolves_by_expiry_,
                       offsetof(cached_resolve_t, minheap_idx),
                       resolve);
}

/** Free all storage held in the DNS cache and related structures. */
void
dns_free_all(void)
{
  cached_resolve_t **ptr, **next, *item;
  assert_cache_ok();
  if (cached_resolve_pqueue) {
    SMARTLIST_FOREACH(cached_resolve_pqueue, cached_resolve_t *, res,
      {
        if (res->state == CACHE_STATE_DONE)
          free_cached_resolve_(res);
      });
  }
  for (ptr = HT_START(cache_map, &cache_root); ptr != NULL; ptr = next) {
    item = *ptr;
    next = HT_NEXT_RMV(cache_map, &cache_root, ptr);
    free_cached_resolve_(item);
  }
  HT_CLEAR(cache_map, &cache_root);
  smartlist_free(cached_resolve_pqueue);
  cached_resolve_pqueue = NULL;
  tor_free(resolv_conf_fname);
}

/** Remove every cached_resolve whose <b>expire</b> time is before or
 * equal to <b>now</b> from the cache. */
static void
purge_expired_resolves(time_t now)
{
  cached_resolve_t *resolve, *removed;
  pending_connection_t *pend;
  edge_connection_t *pendconn;

  assert_cache_ok();
  if (!cached_resolve_pqueue)
    return;

  while (smartlist_len(cached_resolve_pqueue)) {
    resolve = smartlist_get(cached_resolve_pqueue, 0);
    if (resolve->expire > now)
      break;
    smartlist_pqueue_pop(cached_resolve_pqueue,
                         compare_cached_resolves_by_expiry_,
                         offsetof(cached_resolve_t, minheap_idx));

    if (resolve->state == CACHE_STATE_PENDING) {
      log_debug(LD_EXIT,
                "Expiring a dns resolve %s that's still pending. Forgot to "
                "cull it? DNS resolve didn't tell us about the timeout?",
                escaped_safe_str(resolve->address));
    } else if (resolve->state == CACHE_STATE_CACHED) {
      log_debug(LD_EXIT,
                "Forgetting old cached resolve (address %s, expires %lu)",
                escaped_safe_str(resolve->address),
                (unsigned long)resolve->expire);
      tor_assert(!resolve->pending_connections);
    } else {
      tor_assert(resolve->state == CACHE_STATE_DONE);
      tor_assert(!resolve->pending_connections);
    }

    if (resolve->pending_connections) {
      log_debug(LD_EXIT,
                "Closing pending connections on timed-out DNS resolve!");
      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(!SOCKET_OK(pend->conn->base_.s));
        pendconn = pend->conn;
        /* Prevent double-remove */
        pendconn->base_.state = EXIT_CONN_STATE_RESOLVEFAILED;
        if (!pendconn->base_.marked_for_close) {
          connection_edge_end(pendconn, END_STREAM_REASON_TIMEOUT);
          circuit_detach_stream(circuit_get_by_edge_conn(pendconn), pendconn);
          connection_free_(TO_CONN(pendconn));
        }
        tor_free(pend);
      }
    }

    if (resolve->state == CACHE_STATE_CACHED ||
        resolve->state == CACHE_STATE_PENDING) {
      removed = HT_REMOVE(cache_map, &cache_root, resolve);
      if (removed != resolve) {
        log_err(LD_BUG, "The expired resolve we purged didn't match any in"
                " the cache. Tried to purge %s (%p); instead got %s (%p).",
                resolve->address, (void*)resolve,
                removed ? removed->address : "NULL", (void*)removed);
      }
      tor_assert(removed == resolve);
    } else {
      /* This should be in state DONE. Make sure it's not in the cache. */
      cached_resolve_t *tmp = HT_FIND(cache_map, &cache_root, resolve);
      tor_assert(tmp != resolve);
    }
    if (resolve->res_status_hostname == RES_STATUS_DONE_OK)
      tor_free(resolve->result_ptr.hostname);
    resolve->magic = 0xF0BBF0BB;
    tor_free(resolve);
  }

  assert_cache_ok();
}

/* argument for send_resolved_cell only, meaning "let the answer type be ipv4
 * or ipv6 depending on the connection's address". */
#define RESOLVED_TYPE_AUTO 0xff

/** Send a response to the RESOLVE request of a connection.
 * <b>answer_type</b> must be one of
 * RESOLVED_TYPE_(AUTO|ERROR|ERROR_TRANSIENT|).
 *
 * If <b>circ</b> is provided, and we have a cached answer, send the
 * answer back along circ; otherwise, send the answer back along
 * <b>conn</b>'s attached circuit.
 */
MOCK_IMPL(STATIC void,
send_resolved_cell,(edge_connection_t *conn, uint8_t answer_type,
                    const cached_resolve_t *resolved))
{
  char buf[RELAY_PAYLOAD_SIZE], *cp = buf;
  size_t buflen = 0;
  uint32_t ttl;

  buf[0] = answer_type;
  ttl = dns_clip_ttl(conn->address_ttl);

  switch (answer_type)
    {
    case RESOLVED_TYPE_AUTO:
      if (resolved && resolved->res_status_ipv4 == RES_STATUS_DONE_OK) {
        cp[0] = RESOLVED_TYPE_IPV4;
        cp[1] = 4;
        set_uint32(cp+2, htonl(resolved->result_ipv4.addr_ipv4));
        set_uint32(cp+6, htonl(ttl));
        cp += 10;
      }
      if (resolved && resolved->res_status_ipv6 == RES_STATUS_DONE_OK) {
        const uint8_t *bytes = resolved->result_ipv6.addr_ipv6.s6_addr;
        cp[0] = RESOLVED_TYPE_IPV6;
        cp[1] = 16;
        memcpy(cp+2, bytes, 16);
        set_uint32(cp+18, htonl(ttl));
        cp += 22;
      }
      if (cp != buf) {
        buflen = cp - buf;
        break;
      } else {
        answer_type = RESOLVED_TYPE_ERROR;
        /* fall through. */
      }
      /* Falls through. */
    case RESOLVED_TYPE_ERROR_TRANSIENT:
    case RESOLVED_TYPE_ERROR:
      {
        const char *errmsg = "Error resolving hostname";
        size_t msglen = strlen(errmsg);

        buf[0] = answer_type;
        buf[1] = msglen;
        strlcpy(buf+2, errmsg, sizeof(buf)-2);
        set_uint32(buf+2+msglen, htonl(ttl));
        buflen = 6+msglen;
        break;
      }
    default:
      tor_assert(0);
      return;
    }
  // log_notice(LD_EXIT, "Sending a regular RESOLVED reply: ");

  connection_edge_send_command(conn, RELAY_COMMAND_RESOLVED, buf, buflen);
}

/** Send a response to the RESOLVE request of a connection for an in-addr.arpa
 * address on connection <b>conn</b> which yielded the result <b>hostname</b>.
 * The answer type will be RESOLVED_HOSTNAME.
 *
 * If <b>circ</b> is provided, and we have a cached answer, send the
 * answer back along circ; otherwise, send the answer back along
 * <b>conn</b>'s attached circuit.
 */
MOCK_IMPL(STATIC void,
send_resolved_hostname_cell,(edge_connection_t *conn,
                             const char *hostname))
{
  char buf[RELAY_PAYLOAD_SIZE];
  size_t buflen;
  uint32_t ttl;
  size_t namelen = strlen(hostname);
  tor_assert(hostname);

  tor_assert(namelen < 256);
  ttl = dns_clip_ttl(conn->address_ttl);

  buf[0] = RESOLVED_TYPE_HOSTNAME;
  buf[1] = (uint8_t)namelen;
  memcpy(buf+2, hostname, namelen);
  set_uint32(buf+2+namelen, htonl(ttl));
  buflen = 2+namelen+4;

  // log_notice(LD_EXIT, "Sending a reply RESOLVED reply: %s", hostname);
  connection_edge_send_command(conn, RELAY_COMMAND_RESOLVED, buf, buflen);
  // log_notice(LD_EXIT, "Sent");
}

/** 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.
 * If resolve failed, free exitconn and return -1.
 *
 * (For EXIT_PURPOSE_RESOLVE connections, send back a RESOLVED error cell
 * on returning -1.  For EXIT_PURPOSE_CONNECT connections, there's no
 * need to send back an END cell, since connection_exit_begin_conn will
 * do that for us.)
 *
 * If we have a cached answer, send the answer back along <b>exitconn</b>'s
 * circuit.
 *
 * 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.
 *
 * Exitconn's on_circuit field must be set, but exitconn should not
 * yet be linked onto the n_streams/resolving_streams list of that circuit.
 * On success, link the connection to n_streams if it's an exit connection.
 * On "pending", link the connection to resolving streams.  Otherwise,
 * clear its on_circuit field.
 */
int
dns_resolve(edge_connection_t *exitconn)
{
  or_circuit_t *oncirc = TO_OR_CIRCUIT(exitconn->on_circuit);
  int is_resolve, r;
  int made_connection_pending = 0;
  char *hostname = NULL;
  cached_resolve_t *resolve = NULL;
  is_resolve = exitconn->base_.purpose == EXIT_PURPOSE_RESOLVE;

  r = dns_resolve_impl(exitconn, is_resolve, oncirc, &hostname,
                       &made_connection_pending, &resolve);

  switch (r) {
    case 1:
      /* We got an answer without a lookup -- either the answer was
       * cached, or it was obvious (like an IP address). */
      if (is_resolve) {
        /* Send the answer back right now, and detach. */
        if (hostname)
          send_resolved_hostname_cell(exitconn, hostname);
        else
          send_resolved_cell(exitconn, RESOLVED_TYPE_AUTO, resolve);
        exitconn->on_circuit = NULL;
      } else {
        /* Add to the n_streams list; the calling function will send back a
         * connected cell. */
        exitconn->next_stream = oncirc->n_streams;
        oncirc->n_streams = exitconn;
      }
      break;
    case 0:
      /* The request is pending: add the connection into the linked list of
       * resolving_streams on this circuit. */
      exitconn->base_.state = EXIT_CONN_STATE_RESOLVING;
      exitconn->next_stream = oncirc->resolving_streams;
      oncirc->resolving_streams = exitconn;
      break;
    case -2:
    case -1:
      /* The request failed before it could start: cancel this connection,
       * and stop everybody waiting for the same connection. */
      if (is_resolve) {
        send_resolved_cell(exitconn,
             (r == -1) ? RESOLVED_TYPE_ERROR : RESOLVED_TYPE_ERROR_TRANSIENT,
             NULL);
      }

      exitconn->on_circuit = NULL;

      dns_cancel_pending_resolve(exitconn->base_.address);

      if (!made_connection_pending && !exitconn->base_.marked_for_close) {
        /* If we made the connection pending, then we freed it already in
         * dns_cancel_pending_resolve().  If we marked it for close, it'll
         * get freed from the main loop.  Otherwise, can free it now. */
        connection_free_(TO_CONN(exitconn));
      }
      break;
    default:
      tor_assert(0);
  }

  tor_free(hostname);
  return r;
}

/** Helper function for dns_resolve: same functionality, but does not handle:
 *     - marking connections on error and clearing their on_circuit
 *     - linking connections to n_streams/resolving_streams,
 *     - sending resolved cells if we have an answer/error right away,
 *
 * Return -2 on a transient error. If it's a reverse resolve and it's
 * successful, sets *<b>hostname_out</b> to a newly allocated string
 * holding the cached reverse DNS value.
 *
 * Set *<b>made_connection_pending_out</b> to true if we have placed
 * <b>exitconn</b> on the list of pending connections for some resolve; set it
 * to false otherwise.
 *
 * Set *<b>resolve_out</b> to a cached resolve, if we found one.
 */
MOCK_IMPL(STATIC int,
dns_resolve_impl,(edge_connection_t *exitconn, int is_resolve,
                 or_circuit_t *oncirc, char **hostname_out,
                 int *made_connection_pending_out,
                 cached_resolve_t **resolve_out))
{
  cached_resolve_t *resolve;
  cached_resolve_t search;
  pending_connection_t *pending_connection;
  int is_reverse = 0;
  tor_addr_t addr;
  time_t now = time(NULL);
  int r;
  assert_connection_ok(TO_CONN(exitconn), 0);
  tor_assert(!SOCKET_OK(exitconn->base_.s));
  assert_cache_ok();
  tor_assert(oncirc);
  *made_connection_pending_out = 0;

  /* first check if exitconn->base_.address is an IP. If so, we already
   * know the answer. */
  if (tor_addr_parse(&addr, exitconn->base_.address) >= 0) {
    if (tor_addr_family(&addr) == AF_INET ||
        tor_addr_family(&addr) == AF_INET6) {
      tor_addr_copy(&exitconn->base_.addr, &addr);
      exitconn->address_ttl = DEFAULT_DNS_TTL;
      return 1;
    } else {
      /* XXXX unspec? Bogus? */
      return -1;
    }
  }

  /* If we're a non-exit, don't even do DNS lookups. */
  if (router_my_exit_policy_is_reject_star())
    return -1;

  if (address_is_invalid_destination(exitconn->base_.address, 0)) {
    tor_log(LOG_PROTOCOL_WARN, LD_EXIT,
        "Rejecting invalid destination address %s",
        escaped_safe_str(exitconn->base_.address));
    return -1;
  }

  /* then take this opportunity to see if there are any expired
   * resolves in the hash table. */
  purge_expired_resolves(now);

  /* lower-case exitconn->base_.address, so it's in canonical form */
  tor_strlower(exitconn->base_.address);

  /* Check whether this is a reverse lookup.  If it's malformed, or it's a
   * .in-addr.arpa address but this isn't a resolve request, kill the
   * connection.
   */
  if ((r = tor_addr_parse_PTR_name(&addr, exitconn->base_.address,
                                              AF_UNSPEC, 0)) != 0) {
    if (r == 1) {
      is_reverse = 1;
      if (tor_addr_is_internal(&addr, 0)) /* internal address? */
        return -1;
    }

    if (!is_reverse || !is_resolve) {
      if (!is_reverse)
        log_info(LD_EXIT, "Bad .in-addr.arpa address \"%s\"; sending error.",
                 escaped_safe_str(exitconn->base_.address));
      else if (!is_resolve)
        log_info(LD_EXIT,
                 "Attempt to connect to a .in-addr.arpa address \"%s\"; "
                 "sending error.",
                 escaped_safe_str(exitconn->base_.address));

      return -1;
    }
    //log_notice(LD_EXIT, "Looks like an address %s",
    //exitconn->base_.address);
  }
  exitconn->is_reverse_dns_lookup = is_reverse;

  /* now check the hash table to see if 'address' is already there. */
  strlcpy(search.address, exitconn->base_.address, sizeof(search.address));
  resolve = HT_FIND(cache_map, &cache_root, &search);
  if (resolve && resolve->expire > now) { /* already there */
    switch (resolve->state) {
      case CACHE_STATE_PENDING:
        /* add us to the pending list */
        pending_connection = tor_malloc_zero(
                                      sizeof(pending_connection_t));
        pending_connection->conn = exitconn;
        pending_connection->next = resolve->pending_connections;
        resolve->pending_connections = pending_connection;
        *made_connection_pending_out = 1;
        log_debug(LD_EXIT,"Connection (fd "TOR_SOCKET_T_FORMAT") waiting "
                  "for pending DNS resolve of %s", exitconn->base_.s,
                  escaped_safe_str(exitconn->base_.address));
        return 0;
      case CACHE_STATE_CACHED:
        log_debug(LD_EXIT,"Connection (fd "TOR_SOCKET_T_FORMAT") found "
                  "cached answer for %s",
                  exitconn->base_.s,
                  escaped_safe_str(resolve->address));

        *resolve_out = resolve;

        return set_exitconn_info_from_resolve(exitconn, resolve, hostname_out);
      case CACHE_STATE_DONE:
        log_err(LD_BUG, "Found a 'DONE' dns resolve still in the cache.");
        tor_fragile_assert();
    }
    tor_assert(0);
  }
  tor_assert(!resolve);
  /* not there, need to add it */
  resolve = tor_malloc_zero(sizeof(cached_resolve_t));
  resolve->magic = CACHED_RESOLVE_MAGIC;
  resolve->state = CACHE_STATE_PENDING;
  resolve->minheap_idx = -1;
  strlcpy(resolve->address, exitconn->base_.address, sizeof(resolve->address));

  /* add this connection to the pending list */
  pending_connection = tor_malloc_zero(sizeof(pending_connection_t));
  pending_connection->conn = exitconn;
  resolve->pending_connections = pending_connection;
  *made_connection_pending_out = 1;

  /* Add this resolve to the cache and priority queue. */
  HT_INSERT(cache_map, &cache_root, resolve);
  set_expiry(resolve, now + RESOLVE_MAX_TIMEOUT);

  log_debug(LD_EXIT,"Launching %s.",
            escaped_safe_str(exitconn->base_.address));
  assert_cache_ok();

  return launch_resolve(resolve);
}

/** Given an exit connection <b>exitconn</b>, and a cached_resolve_t
 * <b>resolve</b> whose DNS lookups have all either succeeded or failed,
 * update the appropriate fields (address_ttl and addr) of <b>exitconn</b>.
 *
 * The logic can be complicated here, since we might have launched both
 * an A lookup and an AAAA lookup, and since either of those might have
 * succeeded or failed, and since we want to answer a RESOLVE cell with
 * a full answer but answer a BEGIN cell with whatever answer the client
 * would accept <i>and</i> we could still connect to.
 *
 * If this is a reverse lookup, set *<b>hostname_out</b> to a newly allocated
 * copy of the name resulting hostname.
 *
 * Return -2 on a transient error, -1 on a permenent error, and 1 on
 * a successful lookup.
 */
MOCK_IMPL(STATIC int,
set_exitconn_info_from_resolve,(edge_connection_t *exitconn,
                                const cached_resolve_t *resolve,
                                char **hostname_out))
{
  int ipv4_ok, ipv6_ok, answer_with_ipv4, r;
  uint32_t begincell_flags;
  const int is_resolve = exitconn->base_.purpose == EXIT_PURPOSE_RESOLVE;
  tor_assert(exitconn);
  tor_assert(resolve);

  if (exitconn->is_reverse_dns_lookup) {
    exitconn->address_ttl = resolve->ttl_hostname;
    if (resolve->res_status_hostname == RES_STATUS_DONE_OK) {
      *hostname_out = tor_strdup(resolve->result_ptr.hostname);
      return 1;
    } else {
      return -1;
    }
  }

  /* If we're here then the connection wants one or either of ipv4, ipv6, and
   * we can give it one or both. */
  if (is_resolve) {
    begincell_flags = BEGIN_FLAG_IPV6_OK;
  } else {
    begincell_flags = exitconn->begincell_flags;
  }

  ipv4_ok = (resolve->res_status_ipv4 == RES_STATUS_DONE_OK) &&
    ! (begincell_flags & BEGIN_FLAG_IPV4_NOT_OK);
  ipv6_ok = (resolve->res_status_ipv6 == RES_STATUS_DONE_OK) &&
    (begincell_flags & BEGIN_FLAG_IPV6_OK) &&
    get_options()->IPv6Exit;

  /* Now decide which one to actually give. */
  if (ipv4_ok && ipv6_ok && is_resolve) {
    answer_with_ipv4 = 1;
  } else if (ipv4_ok && ipv6_ok) {
    /* If we have both, see if our exit policy has an opinion. */
    const uint16_t port = exitconn->base_.port;
    int ipv4_allowed, ipv6_allowed;
    tor_addr_t a4, a6;
    tor_addr_from_ipv4h(&a4, resolve->result_ipv4.addr_ipv4);
    tor_addr_from_in6(&a6, &resolve->result_ipv6.addr_ipv6);
    ipv4_allowed = !router_compare_to_my_exit_policy(&a4, port);
    ipv6_allowed =</