tor/src/or/circuitbuild.c

/* Copyright (c) 2001 Matej Pfajfar.
 * Copyright (c) 2001-2004, Roger Dingledine.
 * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
 * Copyright (c) 2007-2013, The Tor Project, Inc. */
/* See LICENSE for licensing information */

/**
 * \file circuitbuild.c
 * \brief The actual details of building circuits.
 **/

#include "or.h"
#include "channel.h"
#include "circpathbias.h"
#include "circuitbuild.h"
#include "circuitlist.h"
#include "circuitstats.h"
#include "circuituse.h"
#include "command.h"
#include "config.h"
#include "confparse.h"
#include "connection.h"
#include "connection_edge.h"
#include "connection_or.h"
#include "control.h"
#include "directory.h"
#include "entrynodes.h"
#include "main.h"
#include "microdesc.h"
#include "networkstatus.h"
#include "nodelist.h"
#include "onion.h"
#include "onion_tap.h"
#include "onion_fast.h"
#include "policies.h"
#include "transports.h"
#include "relay.h"
#include "rephist.h"
#include "router.h"
#include "routerlist.h"
#include "routerparse.h"
#include "routerset.h"
#include "crypto.h"

#ifndef MIN
#define MIN(a,b) ((a)<(b)?(a):(b))
#endif

static channel_t * channel_connect_for_circuit(const tor_addr_t *addr,
                                               uint16_t port,
                                               const char *id_digest);
static int circuit_deliver_create_cell(circuit_t *circ,
                                       const create_cell_t *create_cell,
                                       int relayed);
static int onion_pick_cpath_exit(origin_circuit_t *circ, extend_info_t *exit);
static crypt_path_t *onion_next_hop_in_cpath(crypt_path_t *cpath);
static int onion_extend_cpath(origin_circuit_t *circ);
static int count_acceptable_nodes(smartlist_t *routers);
static int onion_append_hop(crypt_path_t **head_ptr, extend_info_t *choice);
#ifdef CURVE25519_ENABLED
static int circuits_can_use_ntor(void);
#endif

/** This function tries to get a channel to the specified endpoint,
 * and then calls command_setup_channel() to give it the right
 * callbacks.
 */
static channel_t *
channel_connect_for_circuit(const tor_addr_t *addr, uint16_t port,
                            const char *id_digest)
{
  channel_t *chan;

  chan = channel_connect(addr, port, id_digest);
  if (chan) command_setup_channel(chan);

  return chan;
}

/** Iterate over values of circ_id, starting from conn-\>next_circ_id,
 * and with the high bit specified by conn-\>circ_id_type, until we get
 * a circ_id that is not in use by any other circuit on that conn.
 *
 * Return it, or 0 if can't get a unique circ_id.
 */
static circid_t
get_unique_circ_id_by_chan(channel_t *chan)
{
  circid_t test_circ_id;
  circid_t attempts=0;
  circid_t high_bit, max_range;

  tor_assert(chan);

  if (chan->circ_id_type == CIRC_ID_TYPE_NEITHER) {
    log_warn(LD_BUG,
             "Trying to pick a circuit ID for a connection from "
             "a client with no identity.");
    return 0;
  }
  max_range =  (chan->wide_circ_ids) ? (1u<<31) : (1u<<15);
  high_bit = (chan->circ_id_type == CIRC_ID_TYPE_HIGHER) ? max_range : 0;
  do {
    /* Sequentially iterate over test_circ_id=1...max_range until we find a
     * circID such that (high_bit|test_circ_id) is not already used. */
    test_circ_id = chan->next_circ_id++;
    if (test_circ_id == 0 || test_circ_id >= max_range) {
      test_circ_id = 1;
      chan->next_circ_id = 2;
    }
    if (++attempts > max_range) {
      /* Make sure we don't loop forever if all circ_id's are used. This
       * matters because it's an external DoS opportunity.
       */
      log_warn(LD_CIRC,"No unused circ IDs. Failing.");
      return 0;
    }
    test_circ_id |= high_bit;
  } while (circuit_id_in_use_on_channel(test_circ_id, chan));
  return test_circ_id;
}

/** If <b>verbose</b> is false, allocate and return a comma-separated list of
 * the currently built elements of <b>circ</b>. If <b>verbose</b> is true, also
 * list information about link status in a more verbose format using spaces.
 * If <b>verbose_names</b> is false, give nicknames for Named routers and hex
 * digests for others; if <b>verbose_names</b> is true, use $DIGEST=Name style
 * names.
 */
static char *
circuit_list_path_impl(origin_circuit_t *circ, int verbose, int verbose_names)
{
  crypt_path_t *hop;
  smartlist_t *elements;
  const char *states[] = {"closed", "waiting for keys", "open"};
  char *s;

  elements = smartlist_new();

  if (verbose) {
    const char *nickname = build_state_get_exit_nickname(circ->build_state);
    smartlist_add_asprintf(elements, "%s%s circ (length %d%s%s):",
                 circ->build_state->is_internal ? "internal" : "exit",
                 circ->build_state->need_uptime ? " (high-uptime)" : "",
                 circ->build_state->desired_path_len,
                 circ->base_.state == CIRCUIT_STATE_OPEN ? "" : ", last hop ",
                 circ->base_.state == CIRCUIT_STATE_OPEN ? "" :
                 (nickname?nickname:"*unnamed*"));
  }

  hop = circ->cpath;
  do {
    char *elt;
    const char *id;
    const node_t *node;
    if (!hop)
      break;
    if (!verbose && hop->state != CPATH_STATE_OPEN)
      break;
    if (!hop->extend_info)
      break;
    id = hop->extend_info->identity_digest;
    if (verbose_names) {
      elt = tor_malloc(MAX_VERBOSE_NICKNAME_LEN+1);
      if ((node = node_get_by_id(id))) {
        node_get_verbose_nickname(node, elt);
      } else if (is_legal_nickname(hop->extend_info->nickname)) {
        elt[0] = '$';
        base16_encode(elt+1, HEX_DIGEST_LEN+1, id, DIGEST_LEN);
        elt[HEX_DIGEST_LEN+1]= '~';
        strlcpy(elt+HEX_DIGEST_LEN+2,
                hop->extend_info->nickname, MAX_NICKNAME_LEN+1);
      } else {
        elt[0] = '$';
        base16_encode(elt+1, HEX_DIGEST_LEN+1, id, DIGEST_LEN);
      }
    } else { /* ! verbose_names */
      node = node_get_by_id(id);
      if (node && node_is_named(node)) {
        elt = tor_strdup(node_get_nickname(node));
      } else {
        elt = tor_malloc(HEX_DIGEST_LEN+2);
        elt[0] = '$';
        base16_encode(elt+1, HEX_DIGEST_LEN+1, id, DIGEST_LEN);
      }
    }
    tor_assert(elt);
    if (verbose) {
      tor_assert(hop->state <= 2);
      smartlist_add_asprintf(elements,"%s(%s)",elt,states[hop->state]);
      tor_free(elt);
    } else {
      smartlist_add(elements, elt);
    }
    hop = hop->next;
  } while (hop != circ->cpath);

  s = smartlist_join_strings(elements, verbose?" ":",", 0, NULL);
  SMARTLIST_FOREACH(elements, char*, cp, tor_free(cp));
  smartlist_free(elements);
  return s;
}

/** If <b>verbose</b> is false, allocate and return a comma-separated
 * list of the currently built elements of <b>circ</b>.  If
 * <b>verbose</b> is true, also list information about link status in
 * a more verbose format using spaces.
 */
char *
circuit_list_path(origin_circuit_t *circ, int verbose)
{
  return circuit_list_path_impl(circ, verbose, 0);
}

/** Allocate and return a comma-separated list of the currently built elements
 * of <b>circ</b>, giving each as a verbose nickname.
 */
char *
circuit_list_path_for_controller(origin_circuit_t *circ)
{
  return circuit_list_path_impl(circ, 0, 1);
}

/** Log, at severity <b>severity</b>, the nicknames of each router in
 * <b>circ</b>'s cpath. Also log the length of the cpath, and the intended
 * exit point.
 */
void
circuit_log_path(int severity, unsigned int domain, origin_circuit_t *circ)
{
  char *s = circuit_list_path(circ,1);
  tor_log(severity,domain,"%s",s);
  tor_free(s);
}

/** Tell the rep(utation)hist(ory) module about the status of the links
 * in <b>circ</b>.  Hops that have become OPEN are marked as successfully
 * extended; the _first_ hop that isn't open (if any) is marked as
 * unable to extend.
 */
/* XXXX Someday we should learn from OR circuits too. */
void
circuit_rep_hist_note_result(origin_circuit_t *circ)
{
  crypt_path_t *hop;
  const char *prev_digest = NULL;
  hop = circ->cpath;
  if (!hop) /* circuit hasn't started building yet. */
    return;
  if (server_mode(get_options())) {
    const routerinfo_t *me = router_get_my_routerinfo();
    if (!me)
      return;
    prev_digest = me->cache_info.identity_digest;
  }
  do {
    const node_t *node = node_get_by_id(hop->extend_info->identity_digest);
    if (node) { /* Why do we check this?  We know the identity. -NM XXXX */
      if (prev_digest) {
        if (hop->state == CPATH_STATE_OPEN)
          rep_hist_note_extend_succeeded(prev_digest, node->identity);
        else {
          rep_hist_note_extend_failed(prev_digest, node->identity);
          break;
        }
      }
      prev_digest = node->identity;
    } else {
      prev_digest = NULL;
    }
    hop=hop->next;
  } while (hop!=circ->cpath);
}

#ifdef CURVE25519_ENABLED
/** Return 1 iff at least one node in circ's cpath supports ntor. */
static int
circuit_cpath_supports_ntor(const origin_circuit_t *circ)
{
  crypt_path_t *head = circ->cpath, *cpath = circ->cpath;

  cpath = head;
  do {
    if (cpath->extend_info &&
        !tor_mem_is_zero(
            (const char*)cpath->extend_info->curve25519_onion_key.public_key,
            CURVE25519_PUBKEY_LEN))
      return 1;

    cpath = cpath->next;
  } while (cpath != head);

  return 0;
}
#else
#define circuit_cpath_supports_ntor(circ) 0
#endif

/** Pick all the entries in our cpath. Stop and return 0 when we're
 * happy, or return -1 if an error occurs. */
static int
onion_populate_cpath(origin_circuit_t *circ)
{
  int n_tries = 0;
#ifdef CURVE25519_ENABLED
  const int using_ntor = circuits_can_use_ntor();
#else
  const int using_ntor = 0;
#endif

#define MAX_POPULATE_ATTEMPTS 32

  while (1) {
    int r = onion_extend_cpath(circ);
    if (r < 0) {
      log_info(LD_CIRC,"Generating cpath hop failed.");
      return -1;
    }
    if (r == 1) {
      /* This circuit doesn't need/shouldn't be forced to have an ntor hop */
      if (circ->build_state->desired_path_len <= 1 || ! using_ntor)
        return 0;

      /* This circuit has an ntor hop. great! */
      if (circuit_cpath_supports_ntor(circ))
        return 0;

      /* No node in the circuit supports ntor.  Have we already tried too many
       * times? */
      if (++n_tries >= MAX_POPULATE_ATTEMPTS)
        break;

      /* Clear the path and retry */
      circuit_clear_cpath(circ);
    }
  }
  log_warn(LD_CIRC, "I tried for %d times, but I couldn't build a %d-hop "
           "circuit with at least one node that supports ntor.",
           MAX_POPULATE_ATTEMPTS,
           circ->build_state->desired_path_len);

  return -1;
}

/** Create and return a new origin circuit. Initialize its purpose and
 * build-state based on our arguments.  The <b>flags</b> argument is a
 * bitfield of CIRCLAUNCH_* flags. */
origin_circuit_t *
origin_circuit_init(uint8_t purpose, int flags)
{
  /* sets circ->p_circ_id and circ->p_chan */
  origin_circuit_t *circ = origin_circuit_new();
  circuit_set_state(TO_CIRCUIT(circ), CIRCUIT_STATE_CHAN_WAIT);
  circ->build_state = tor_malloc_zero(sizeof(cpath_build_state_t));
  circ->build_state->onehop_tunnel =
    ((flags & CIRCLAUNCH_ONEHOP_TUNNEL) ? 1 : 0);
  circ->build_state->need_uptime =
    ((flags & CIRCLAUNCH_NEED_UPTIME) ? 1 : 0);
  circ->build_state->need_capacity =
    ((flags & CIRCLAUNCH_NEED_CAPACITY) ? 1 : 0);
  circ->build_state->is_internal =
    ((flags & CIRCLAUNCH_IS_INTERNAL) ? 1 : 0);
  circ->base_.purpose = purpose;
  return circ;
}

/** Build a new circuit for <b>purpose</b>. If <b>exit</b>
 * is defined, then use that as your exit router, else choose a suitable
 * exit node.
 *
 * Also launch a connection to the first OR in the chosen path, if
 * it's not open already.
 */
origin_circuit_t *
circuit_establish_circuit(uint8_t purpose, extend_info_t *exit, int flags)
{
  origin_circuit_t *circ;
  int err_reason = 0;

  circ = origin_circuit_init(purpose, flags);

  if (onion_pick_cpath_exit(circ, exit) < 0 ||
      onion_populate_cpath(circ) < 0) {
    circuit_mark_for_close(TO_CIRCUIT(circ), END_CIRC_REASON_NOPATH);
    return NULL;
  }

  control_event_circuit_status(circ, CIRC_EVENT_LAUNCHED, 0);

  if ((err_reason = circuit_handle_first_hop(circ)) < 0) {
    circuit_mark_for_close(TO_CIRCUIT(circ), -err_reason);
    return NULL;
  }
  return circ;
}

/** Start establishing the first hop of our circuit. Figure out what
 * OR we should connect to, and if necessary start the connection to
 * it. If we're already connected, then send the 'create' cell.
 * Return 0 for ok, -reason if circ should be marked-for-close. */
int
circuit_handle_first_hop(origin_circuit_t *circ)
{
  crypt_path_t *firsthop;
  channel_t *n_chan;
  int err_reason = 0;
  const char *msg = NULL;
  int should_launch = 0;

  firsthop = onion_next_hop_in_cpath(circ->cpath);
  tor_assert(firsthop);
  tor_assert(firsthop->extend_info);

  /* now see if we're already connected to the first OR in 'route' */
  log_debug(LD_CIRC,"Looking for firsthop '%s'",
            fmt_addrport(&firsthop->extend_info->addr,
                         firsthop->extend_info->port));

  n_chan = channel_get_for_extend(firsthop->extend_info->identity_digest,
                                  &firsthop->extend_info->addr,
                                  &msg,
                                  &should_launch);

  if (!n_chan) {
    /* not currently connected in a useful way. */
    log_info(LD_CIRC, "Next router is %s: %s",
             safe_str_client(extend_info_describe(firsthop->extend_info)),
             msg?msg:"???");
    circ->base_.n_hop = extend_info_dup(firsthop->extend_info);

    if (should_launch) {
      if (circ->build_state->onehop_tunnel)
        control_event_bootstrap(BOOTSTRAP_STATUS_CONN_DIR, 0);
      n_chan = channel_connect_for_circuit(
          &firsthop->extend_info->addr,
          firsthop->extend_info->port,
          firsthop->extend_info->identity_digest);
      if (!n_chan) { /* connect failed, forget the whole thing */
        log_info(LD_CIRC,"connect to firsthop failed. Closing.");
        return -END_CIRC_REASON_CONNECTFAILED;
      }
    }

    log_debug(LD_CIRC,"connecting in progress (or finished). Good.");
    /* return success. The onion/circuit/etc will be taken care of
     * automatically (may already have been) whenever n_chan reaches
     * OR_CONN_STATE_OPEN.
     */
    return 0;
  } else { /* it's already open. use it. */
    tor_assert(!circ->base_.n_hop);
    circ->base_.n_chan = n_chan;
    log_debug(LD_CIRC,"Conn open. Delivering first onion skin.");
    if ((err_reason = circuit_send_next_onion_skin(circ)) < 0) {
      log_info(LD_CIRC,"circuit_send_next_onion_skin failed.");
      return err_reason;
    }
  }
  return 0;
}

/** Find any circuits that are waiting on <b>or_conn</b> to become
 * open and get them to send their create cells forward.
 *
 * Status is 1 if connect succeeded, or 0 if connect failed.
 */
void
circuit_n_chan_done(channel_t *chan, int status)
{
  smartlist_t *pending_circs;
  int err_reason = 0;

  tor_assert(chan);

  log_debug(LD_CIRC,"chan to %s/%s, status=%d",
            chan->nickname ? chan->nickname : "NULL",
            channel_get_canonical_remote_descr(chan), status);

  pending_circs = smartlist_new();
  circuit_get_all_pending_on_channel(pending_circs, chan);

  SMARTLIST_FOREACH_BEGIN(pending_circs, circuit_t *, circ)
    {
      /* These checks are redundant wrt get_all_pending_on_or_conn, but I'm
       * leaving them in in case it's possible for the status of a circuit to
       * change as we're going down the list. */
      if (circ->marked_for_close || circ->n_chan || !circ->n_hop ||
          circ->state != CIRCUIT_STATE_CHAN_WAIT)
        continue;

      if (tor_digest_is_zero(circ->n_hop->identity_digest)) {
        /* Look at addr/port. This is an unkeyed connection. */
        if (!channel_matches_extend_info(chan, circ->n_hop))
          continue;
      } else {
        /* We expected a key. See if it's the right one. */
        if (tor_memneq(chan->identity_digest,
                   circ->n_hop->identity_digest, DIGEST_LEN))
          continue;
      }
      if (!status) { /* chan failed; close circ */
        log_info(LD_CIRC,"Channel failed; closing circ.");
        circuit_mark_for_close(circ, END_CIRC_REASON_CHANNEL_CLOSED);
        continue;
      }
      log_debug(LD_CIRC, "Found circ, sending create cell.");
      /* circuit_deliver_create_cell will set n_circ_id and add us to
       * chan_circuid_circuit_map, so we don't need to call
       * set_circid_chan here. */
      circ->n_chan = chan;
      extend_info_free(circ->n_hop);
      circ->n_hop = NULL;

      if (CIRCUIT_IS_ORIGIN(circ)) {
        if ((err_reason =
             circuit_send_next_onion_skin(TO_ORIGIN_CIRCUIT(circ))) < 0) {
          log_info(LD_CIRC,
                   "send_next_onion_skin failed; circuit marked for closing.");
          circuit_mark_for_close(circ, -err_reason);
          continue;
          /* XXX could this be bad, eg if next_onion_skin failed because conn
           *     died? */
        }
      } else {
        /* pull the create cell out of circ->n_chan_create_cell, and send it */
        tor_assert(circ->n_chan_create_cell);
        if (circuit_deliver_create_cell(circ, circ->n_chan_create_cell, 1)<0) {
          circuit_mark_for_close(circ, END_CIRC_REASON_RESOURCELIMIT);
          continue;
        }
        tor_free(circ->n_chan_create_cell);
        circuit_set_state(circ, CIRCUIT_STATE_OPEN);
      }
    }
  SMARTLIST_FOREACH_END(circ);

  smartlist_free(pending_circs);
}

/** Find a new circid that isn't currently in use on the circ->n_chan
 * for the outgoing
 * circuit <b>circ</b>, and deliver the cell <b>create_cell</b> to this
 * circuit.  If <b>relayed</b> is true, this is a create cell somebody
 * gave us via an EXTEND cell, so we shouldn't worry if we don't understand
 * it. Return -1 if we failed to find a suitable circid, else return 0.
 */
static int
circuit_deliver_create_cell(circuit_t *circ, const create_cell_t *create_cell,
                            int relayed)
{
  cell_t cell;
  circid_t id;
  int r;

  tor_assert(circ);
  tor_assert(circ->n_chan);
  tor_assert(create_cell);
  tor_assert(create_cell->cell_type == CELL_CREATE ||
             create_cell->cell_type == CELL_CREATE_FAST ||
             create_cell->cell_type == CELL_CREATE2);

  id = get_unique_circ_id_by_chan(circ->n_chan);
  if (!id) {
    log_warn(LD_CIRC,"failed to get unique circID.");
    return -1;
  }
  log_debug(LD_CIRC,"Chosen circID %u.", (unsigned)id);
  circuit_set_n_circid_chan(circ, id, circ->n_chan);

  memset(&cell, 0, sizeof(cell_t));
  r = relayed ? create_cell_format_relayed(&cell, create_cell)
              : create_cell_format(&cell, create_cell);
  if (r < 0) {
    log_warn(LD_CIRC,"Couldn't format create cell");
    return -1;
  }
  cell.circ_id = circ->n_circ_id;

  append_cell_to_circuit_queue(circ, circ->n_chan, &cell,
                               CELL_DIRECTION_OUT, 0);

  if (CIRCUIT_IS_ORIGIN(circ)) {
    /* Update began timestamp for circuits starting their first hop */
    if (TO_ORIGIN_CIRCUIT(circ)->cpath->state == CPATH_STATE_CLOSED) {
      if (circ->n_chan->state != CHANNEL_STATE_OPEN) {
        log_warn(LD_CIRC,
                 "Got first hop for a circuit without an opened channel. "
                 "State: %s.", channel_state_to_string(circ->n_chan->state));
        tor_fragile_assert();
      }

      tor_gettimeofday(&circ->timestamp_began);
    }

    /* mark it so it gets better rate limiting treatment. */
    channel_timestamp_client(circ->n_chan);
  }

  return 0;
}

/** We've decided to start our reachability testing. If all
 * is set, log this to the user. Return 1 if we did, or 0 if
 * we chose not to log anything. */
int
inform_testing_reachability(void)
{
  char dirbuf[128];
  char *address;
  const routerinfo_t *me = router_get_my_routerinfo();
  if (!me)
    return 0;
  address = tor_dup_ip(me->addr);
  control_event_server_status(LOG_NOTICE,
                              "CHECKING_REACHABILITY ORADDRESS=%s:%d",
                              address, me->or_port);
  if (me->dir_port) {
    tor_snprintf(dirbuf, sizeof(dirbuf), " and DirPort %s:%d",
                 address, me->dir_port);
    control_event_server_status(LOG_NOTICE,
                                "CHECKING_REACHABILITY DIRADDRESS=%s:%d",
                                address, me->dir_port);
  }
  log_notice(LD_OR, "Now checking whether ORPort %s:%d%s %s reachable... "
                         "(this may take up to %d minutes -- look for log "
                         "messages indicating success)",
      address, me->or_port,
      me->dir_port ? dirbuf : "",
      me->dir_port ? "are" : "is",
      TIMEOUT_UNTIL_UNREACHABILITY_COMPLAINT/60);

  tor_free(address);
  return 1;
}

/** Return true iff we should send a create_fast cell to start building a given
 * circuit */
static INLINE int
should_use_create_fast_for_circuit(origin_circuit_t *circ)
{
  const or_options_t *options = get_options();
  tor_assert(circ->cpath);
  tor_assert(circ->cpath->extend_info);

  if (!circ->cpath->extend_info->onion_key)
    return 1; /* our hand is forced: only a create_fast will work. */
  if (public_server_mode(options)) {
    /* We're a server, and we know an onion key. We can choose.
     * Prefer to blend our circuit into the other circuits we are
     * creating on behalf of others. */
    return 0;
  }
  if (options->FastFirstHopPK == -1) {
    /* option is "auto", so look at the consensus. */
    return networkstatus_get_param(NULL, "usecreatefast", 1, 0, 1);
  }

  return options->FastFirstHopPK;
}

/** Return true if <b>circ</b> is the type of circuit we want to count
 * timeouts from. In particular, we want it to have not completed yet
 * (already completing indicates we cannibalized it), and we want it to
 * have exactly three hops.
 */
int
circuit_timeout_want_to_count_circ(origin_circuit_t *circ)
{
  return !circ->has_opened
          && circ->build_state->desired_path_len == DEFAULT_ROUTE_LEN;
}

#ifdef CURVE25519_ENABLED
/** Return true if the ntor handshake is enabled in the configuration, or if
 * it's been set to "auto" in the configuration and it's enabled in the
 * consensus. */
static int
circuits_can_use_ntor(void)
{
  const or_options_t *options = get_options();
  if (options->UseNTorHandshake != -1)
    return options->UseNTorHandshake;
  return networkstatus_get_param(NULL, "UseNTorHandshake", 0, 0, 1);
}
#endif

/** Decide whether to use a TAP or ntor handshake for connecting to <b>ei</b>
 * directly, and set *<b>cell_type_out</b> and *<b>handshake_type_out</b>
 * accordingly. */
static void
circuit_pick_create_handshake(uint8_t *cell_type_out,
                              uint16_t *handshake_type_out,
                              const extend_info_t *ei)
{
#ifdef CURVE25519_ENABLED
  if (!tor_mem_is_zero((const char*)ei->curve25519_onion_key.public_key,
                       CURVE25519_PUBKEY_LEN) &&
      circuits_can_use_ntor()) {
    *cell_type_out = CELL_CREATE2;
    *handshake_type_out = ONION_HANDSHAKE_TYPE_NTOR;
    return;
  }
#else
  (void) ei;
#endif

  *cell_type_out = CELL_CREATE;
  *handshake_type_out = ONION_HANDSHAKE_TYPE_TAP;
}

/** Decide whether to use a TAP or ntor handshake for connecting to <b>ei</b>
 * directly, and set *<b>handshake_type_out</b> accordingly. Decide whether,
 * in extending through <b>node</b> to do so, we should use an EXTEND2 or an
 * EXTEND cell to do so, and set *<b>cell_type_out</b> and
 * *<b>create_cell_type_out</b> accordingly. */
static void
circuit_pick_extend_handshake(uint8_t *cell_type_out,
                              uint8_t *create_cell_type_out,
                              uint16_t *handshake_type_out,
                              const node_t *node_prev,
                              const extend_info_t *ei)
{
  uint8_t t;
  circuit_pick_create_handshake(&t, handshake_type_out, ei);
  /* XXXX024 The check for whether the node has a curve25519 key is a bad
   * proxy for whether it can do extend2 cells; once a version that
   * handles extend2 cells is out, remove it. */
  if (node_prev &&
      *handshake_type_out != ONION_HANDSHAKE_TYPE_TAP &&
      (node_has_curve25519_onion_key(node_prev) ||
       (node_prev->rs && node_prev->rs->version_supports_extend2_cells))) {
    *cell_type_out = RELAY_COMMAND_EXTEND2;
    *create_cell_type_out = CELL_CREATE2;
  } else {
    *cell_type_out = RELAY_COMMAND_EXTEND;
    *create_cell_type_out = CELL_CREATE;
  }
}

/** This is the backbone function for building circuits.
 *
 * If circ's first hop is closed, then we need to build a create
 * cell and send it forward.
 *
 * Otherwise, we need to build a relay extend cell and send it
 * forward.
 *
 * Return -reason if we want to tear down circ, else return 0.
 */
int
circuit_send_next_onion_skin(origin_circuit_t *circ)
{
  crypt_path_t *hop;
  const node_t *node;

  tor_assert(circ);

  if (circ->cpath->state == CPATH_STATE_CLOSED) {
    /* This is the first hop. */
    create_cell_t cc;
    int fast;
    int len;
    log_debug(LD_CIRC,"First skin; sending create cell.");
    memset(&cc, 0, sizeof(cc));
    if (circ->build_state->onehop_tunnel)
      control_event_bootstrap(BOOTSTRAP_STATUS_ONEHOP_CREATE, 0);
    else
      control_event_bootstrap(BOOTSTRAP_STATUS_CIRCUIT_CREATE, 0);

    node = node_get_by_id(circ->base_.n_chan->identity_digest);
    fast = should_use_create_fast_for_circuit(circ);
    if (!fast) {
      /* We are an OR and we know the right onion key: we should
       * send a create cell.
       */
      circuit_pick_create_handshake(&cc.cell_type, &cc.handshake_type,
                                    circ->cpath->extend_info);
      note_request("cell: create", 1);
    } else {
      /* We are not an OR, and we're building the first hop of a circuit to a
       * new OR: we can be speedy and use CREATE_FAST to save an RSA operation
       * and a DH operation. */
      cc.cell_type = CELL_CREATE_FAST;
      cc.handshake_type = ONION_HANDSHAKE_TYPE_FAST;
      note_request("cell: create fast", 1);
    }

    len = onion_skin_create(cc.handshake_type,
                            circ->cpath->extend_info,
                            &circ->cpath->handshake_state,
                            cc.onionskin);
    if (len < 0) {
      log_warn(LD_CIRC,"onion_skin_create (first hop) failed.");
      return - END_CIRC_REASON_INTERNAL;
    }
    cc.handshake_len = len;

    if (circuit_deliver_create_cell(TO_CIRCUIT(circ), &cc, 0) < 0)
      return - END_CIRC_REASON_RESOURCELIMIT;

    circ->cpath->state = CPATH_STATE_AWAITING_KEYS;
    circuit_set_state(TO_CIRCUIT(circ), CIRCUIT_STATE_BUILDING);
    log_info(LD_CIRC,"First hop: finished sending %s cell to '%s'",
             fast ? "CREATE_FAST" : "CREATE",
             node ? node_describe(node) : "<unnamed>");
  } else {
    extend_cell_t ec;
    int len;
    tor_assert(circ->cpath->state == CPATH_STATE_OPEN);
    tor_assert(circ->base_.state == CIRCUIT_STATE_BUILDING);
    log_debug(LD_CIRC,"starting to send subsequent skin.");
    hop = onion_next_hop_in_cpath(circ->cpath);
    memset(&ec, 0, sizeof(ec));
    if (!hop) {
      /* done building the circuit. whew. */
      circuit_set_state(TO_CIRCUIT(circ), CIRCUIT_STATE_OPEN);
      if (circuit_timeout_want_to_count_circ(circ)) {
        struct timeval end;
        long timediff;
        tor_gettimeofday(&end);
        timediff = tv_mdiff(&circ->base_.timestamp_began, &end);

        /*
         * If the circuit build time is much greater than we would have cut
         * it off at, we probably had a suspend event along this codepath,
         * and we should discard the value.
         */
        if (timediff < 0 ||
            timediff > 2*get_circuit_build_close_time_ms()+1000) {
          log_notice(LD_CIRC, "Strange value for circuit build time: %ldmsec. "
                              "Assuming clock jump. Purpose %d (%s)", timediff,
                     circ->base_.purpose,
                     circuit_purpose_to_string(circ->base_.purpose));
        } else if (!circuit_build_times_disabled()) {
          /* Only count circuit times if the network is live */
          if (circuit_build_times_network_check_live(
              get_circuit_build_times())) {
            circuit_build_times_add_time(get_circuit_build_times_mutable(),
                (build_time_t)timediff);
            circuit_build_times_set_timeout(get_circuit_build_times_mutable());
          }

          if (circ->base_.purpose != CIRCUIT_PURPOSE_C_MEASURE_TIMEOUT) {
            circuit_build_times_network_circ_success(
                get_circuit_build_times_mutable());
          }
        }
      }
      log_info(LD_CIRC,"circuit built!");
      circuit_reset_failure_count(0);

      if (circ->build_state->onehop_tunnel || circ->has_opened) {
        control_event_bootstrap(BOOTSTRAP_STATUS_REQUESTING_STATUS, 0);
      }

      pathbias_count_build_success(circ);
      circuit_rep_hist_note_result(circ);
      circuit_has_opened(circ); /* do other actions as necessary */

      if (!can_complete_circuit && !circ->build_state->onehop_tunnel) {
        const or_options_t *options = get_options();
        can_complete_circuit=1;
        /* FFFF Log a count of known routers here */
        log_notice(LD_GENERAL,
            "Tor has successfully opened a circuit. "
            "Looks like client functionality is working.");
        control_event_bootstrap(BOOTSTRAP_STATUS_DONE, 0);
        control_event_client_status(LOG_NOTICE, "CIRCUIT_ESTABLISHED");
        clear_broken_connection_map(1);
        if (server_mode(options) && !check_whether_orport_reachable()) {
          inform_testing_reachability();
          consider_testing_reachability(1, 1);
        }
      }

      /* We're done with measurement circuits here. Just close them */
      if (circ->base_.purpose == CIRCUIT_PURPOSE_C_MEASURE_TIMEOUT) {
        circuit_mark_for_close(TO_CIRCUIT(circ), END_CIRC_REASON_FINISHED);
      }
      return 0;
    }

    if (tor_addr_family(&hop->extend_info->addr) != AF_INET) {
      log_warn(LD_BUG, "Trying to extend to a non-IPv4 address.");
      return - END_CIRC_REASON_INTERNAL;
    }

    {
      const node_t *prev_node;
      prev_node = node_get_by_id(hop->prev->extend_info->identity_digest);
      circuit_pick_extend_handshake(&ec.cell_type,
                                    &ec.create_cell.cell_type,
                                    &ec.create_cell.handshake_type,
                                    prev_node,
                                    hop->extend_info);
    }

    tor_addr_copy(&ec.orport_ipv4.addr, &hop->extend_info->addr);
    ec.orport_ipv4.port = hop->extend_info->port;
    tor_addr_make_unspec(&ec.orport_ipv6.addr);
    memcpy(ec.node_id, hop->extend_info->identity_digest, DIGEST_LEN);

    len = onion_skin_create(ec.create_cell.handshake_type,
                            hop->extend_info,
                            &hop->handshake_state,
                            ec.create_cell.onionskin);
    if (len < 0) {
      log_warn(LD_CIRC,"onion_skin_create failed.");
      return - END_CIRC_REASON_INTERNAL;
    }
    ec.create_cell.handshake_len = len;

    log_info(LD_CIRC,"Sending extend relay cell.");
    note_request("cell: extend", 1);
    {
      uint8_t command = 0;
      uint16_t payload_len=0;
      uint8_t payload[RELAY_PAYLOAD_SIZE];
      if (extend_cell_format(&command, &payload_len, payload, &ec)<0) {
        log_warn(LD_CIRC,"Couldn't format extend cell");
        return -END_CIRC_REASON_INTERNAL;
      }

      /* send it to hop->prev, because it will transfer
       * it to a create cell and then send to hop */
      if (relay_send_command_from_edge(0, TO_CIRCUIT(circ),
                                       command,
                                       (char*)payload, payload_len,
                                       hop->prev) < 0)
        return 0; /* circuit is closed */
    }
    hop->state = CPATH_STATE_AWAITING_KEYS;
  }
  return 0;
}

/** Our clock just jumped by <b>seconds_elapsed</b>. Assume
 * something has also gone wrong with our network: notify the user,
 * and abandon all not-yet-used circuits. */
void
circuit_note_clock_jumped(int seconds_elapsed)
{
  int severity = server_mode(get_options()) ? LOG_WARN : LOG_NOTICE;
  tor_log(severity, LD_GENERAL, "Your system clock just jumped %d seconds %s; "
      "assuming established circuits no longer work.",
      seconds_elapsed >=0 ? seconds_elapsed : -seconds_elapsed,
      seconds_elapsed >=0 ? "forward" : "backward");
  control_event_general_status(LOG_WARN, "CLOCK_JUMPED TIME=%d",
                               seconds_elapsed);
  can_complete_circuit=0; /* so it'll log when it works again */
  control_event_client_status(severity, "CIRCUIT_NOT_ESTABLISHED REASON=%s",
                              "CLOCK_JUMPED");
  circuit_mark_all_unused_circs();
  circuit_mark_all_dirty_circs_as_unusable();
}

/** Take the 'extend' <b>cell</b>, pull out addr/port plus the onion
 * skin and identity digest for the next hop. If we're already connected,
 * pass the onion skin to the next hop using a create cell; otherwise
 * launch a new OR connection, and <b>circ</b> will notice when the
 * connection succeeds or fails.
 *
 * Return -1 if we want to warn and tear down the circuit, else return 0.
 */
int
circuit_extend(cell_t *cell, circuit_t *circ)
{
  channel_t *n_chan;
  relay_header_t rh;
  extend_cell_t ec;
  const char *msg = NULL;
  int should_launch = 0;

  if (circ->n_chan) {
    log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
           "n_chan already set. Bug/attack. Closing.");
    return -1;
  }
  if (circ->n_hop) {
    log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
           "conn to next hop already launched. Bug/attack. Closing.");
    return -1;
  }

  if (!server_mode(get_options())) {
    log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
           "Got an extend cell, but running as a client. Closing.");
    return -1;
  }

  relay_header_unpack(&rh, cell->payload);

  if (extend_cell_parse(&ec, rh.command,
                        cell->payload+RELAY_HEADER_SIZE,
                        rh.length) < 0) {
    log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
           "Can't parse extend cell. Closing circuit.");
    return -1;
  }

  if (!ec.orport_ipv4.port || tor_addr_is_null(&ec.orport_ipv4.addr)) {
    log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
           "Client asked me to extend to zero destination port or addr.");
    return -1;
  }

  if (tor_addr_is_internal(&ec.orport_ipv4.addr, 0) &&
      !get_options()->ExtendAllowPrivateAddresses) {
    log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
           "Client asked me to extend to a private address");
    return -1;
  }

  /* Check if they asked us for 0000..0000. We support using
   * an empty fingerprint for the first hop (e.g. for a bridge relay),
   * but we don't want to let people send us extend cells for empty
   * fingerprints -- a) because it opens the user up to a mitm attack,
   * and b) because it lets an attacker force the relay to hold open a
   * new TLS connection for each extend request. */
  if (tor_digest_is_zero((const char*)ec.node_id)) {
    log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
           "Client asked me to extend without specifying an id_digest.");
    return -1;
  }

  /* Next, check if we're being asked to connect to the hop that the
   * extend cell came from. There isn't any reason for that, and it can
   * assist circular-path attacks. */
  if (tor_memeq(ec.node_id,
                TO_OR_CIRCUIT(circ)->p_chan->identity_digest,
                DIGEST_LEN)) {
    log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
           "Client asked me to extend back to the previous hop.");
    return -1;
  }

  n_chan = channel_get_for_extend((const char*)ec.node_id,
                                  &ec.orport_ipv4.addr,
                                  &msg,
                                  &should_launch);

  if (!n_chan) {
    log_debug(LD_CIRC|LD_OR,"Next router (%s): %s",
              fmt_addrport(&ec.orport_ipv4.addr,ec.orport_ipv4.port),
              msg?msg:"????");

    circ->n_hop = extend_info_new(NULL /*nickname*/,
                                  (const char*)ec.node_id,
                                  NULL /*onion_key*/,
                                  NULL /*curve25519_key*/,
                                  &ec.orport_ipv4.addr,
                                  ec.orport_ipv4.port);

    circ->n_chan_create_cell = tor_memdup(&ec.create_cell,
                                          sizeof(ec.create_cell));

    circuit_set_state(circ, CIRCUIT_STATE_CHAN_WAIT);

    if (should_launch) {
      /* we should try to open a connection */
      n_chan = channel_connect_for_circuit(&ec.orport_ipv4.addr,
                                           ec.orport_ipv4.port,
                                           (const char*)ec.node_id);
      if (!n_chan) {
        log_info(LD_CIRC,"Launching n_chan failed. Closing circuit.");
        circuit_mark_for_close(circ, END_CIRC_REASON_CONNECTFAILED);
        return 0;
      }
      log_debug(LD_CIRC,"connecting in progress (or finished). Good.");
    }
    /* return success. The onion/circuit/etc will be taken care of
     * automatically (may already have been) whenever n_chan reaches
     * OR_CONN_STATE_OPEN.
     */
    return 0;
  }

  tor_assert(!circ->n_hop); /* Connection is already established. */
  circ->n_chan = n_chan;
  log_debug(LD_CIRC,
            "n_chan is %s",
            channel_get_canonical_remote_descr(n_chan));

  if (circuit_deliver_create_cell(circ, &ec.create_cell, 1) < 0)
    return -1;

  return 0;
}

/** Initialize cpath-\>{f|b}_{crypto|digest} from the key material in
 * key_data.  key_data must contain CPATH_KEY_MATERIAL bytes, which are
 * used as follows:
 *   - 20 to initialize f_digest
 *   - 20 to initialize b_digest
 *   - 16 to key f_crypto
 *   - 16 to key b_crypto
 *
 * (If 'reverse' is true, then f_XX and b_XX are swapped.)
 */
int
circuit_init_cpath_crypto(crypt_path_t *cpath, const char *key_data,
                          int reverse)
{
  crypto_digest_t *tmp_digest;
  crypto_cipher_t *tmp_crypto;

  tor_assert(cpath);
  tor_assert(key_data);
  tor_assert(!(cpath->f_crypto || cpath->b_crypto ||
             cpath->f_digest || cpath->b_digest));

  cpath->f_digest = crypto_digest_new();
  crypto_digest_add_bytes(cpath->f_digest, key_data, DIGEST_LEN);
  cpath->b_digest = crypto_digest_new();
  crypto_digest_add_bytes(cpath->b_digest, key_data+DIGEST_LEN, DIGEST_LEN);

  if (!(cpath->f_crypto =
        crypto_cipher_new(key_data+(2*DIGEST_LEN)))) {
    log_warn(LD_BUG,"Forward cipher initialization failed.");
    return -1;
  }
  if (!(cpath->b_crypto =
        crypto_cipher_new(key_data+(2*DIGEST_LEN)+CIPHER_KEY_LEN))) {
    log_warn(LD_BUG,"Backward cipher initialization failed.");
    return -1;
  }

  if (reverse) {
    tmp_digest = cpath->f_digest;
    cpath->f_digest = cpath->b_digest;
    cpath->b_digest = tmp_digest;
    tmp_crypto = cpath->f_crypto;
    cpath->f_crypto = cpath->b_crypto;
    cpath->b_crypto = tmp_crypto;
  }

  return 0;
}

/** A "created" cell <b>reply</b> came back to us on circuit <b>circ</b>.
 * (The body of <b>reply</b> varies depending on what sort of handshake
 * this is.)
 *
 * Calculate the appropriate keys and digests, make sure KH is
 * correct, and initialize this hop of the cpath.
 *
 * Return - reason if we want to mark circ for close, else return 0.
 */
int
circuit_finish_handshake(origin_circuit_t *circ,
                         const created_cell_t *reply)
{
  char keys[CPATH_KEY_MATERIAL_LEN];
  crypt_path_t *hop;
  int rv;

  if ((rv = pathbias_count_build_attempt(circ)) < 0)
    return rv;

  if (circ->cpath->state == CPATH_STATE_AWAITING_KEYS) {
    hop = circ->cpath;
  } else {
    hop = onion_next_hop_in_cpath(circ->cpath);
    if (!hop) { /* got an extended when we're all done? */
      log_warn(LD_PROTOCOL,"got extended when circ already built? Closing.");
      return - END_CIRC_REASON_TORPROTOCOL;
    }
  }
  tor_assert(hop->state == CPATH_STATE_AWAITING_KEYS);

  {
    if (onion_skin_client_handshake(hop->handshake_state.tag,
                                    &hop->handshake_state,
                                    reply->reply, reply->handshake_len,
                                    (uint8_t*)keys, sizeof(keys),
                                    (uint8_t*)hop->rend_circ_nonce) < 0) {
      log_warn(LD_CIRC,"onion_skin_client_handshake failed.");
      return -END_CIRC_REASON_TORPROTOCOL;
    }
  }

  onion_handshake_state_release(&hop->handshake_state);

  if (circuit_init_cpath_crypto(hop, keys, 0)<0) {
    return -END_CIRC_REASON_TORPROTOCOL;
  }

  hop->state = CPATH_STATE_OPEN;
  log_info(LD_CIRC,"Finished building circuit hop:");
  circuit_log_path(LOG_INFO,LD_CIRC,circ);
  control_event_circuit_status(circ, CIRC_EVENT_EXTENDED, 0);

  return 0;
}

/** We received a relay truncated cell on circ.
 *
 * Since we don't send truncates currently, getting a truncated
 * means that a connection broke or an extend failed. For now,
 * just give up: force circ to close, and return 0.
 */
int
circuit_truncated(origin_circuit_t *circ, crypt_path_t *layer, int reason)
{
//  crypt_path_t *victim;
//  connection_t *stream;

  tor_assert(circ);
  tor_assert(layer);

  /* XXX Since we don't send truncates currently, getting a truncated
   *     means that a connection broke or an extend failed. For now,
   *     just give up.
   */
  circuit_mark_for_close(TO_CIRCUIT(circ),
          END_CIRC_REASON_FLAG_REMOTE|reason);
  return 0;

#if 0
  while (layer->next != circ->cpath) {
    /* we need to clear out layer->next */
    victim = layer->next;
    log_debug(LD_CIRC, "Killing a layer of the cpath.");

    for (stream = circ->p_streams; stream; stream=stream->next_stream) {
      if (stream->cpath_layer == victim) {
        log_info(LD_APP, "Marking stream %d for close because of truncate.",
                 stream->stream_id);
        /* no need to send 'end' relay cells,
         * because the other side's already dead
         */
        connection_mark_unattached_ap(stream, END_STREAM_REASON_DESTROY);
      }
    }

    layer->next = victim->next;
    circuit_free_cpath_node(victim);
  }

  log_info(LD_CIRC, "finished");
  return 0;
#endif
}

/** Given a response payload and keys, initialize, then send a created
 * cell back.
 */
int
onionskin_answer(or_circuit_t *circ,
                 const created_cell_t *created_cell,
                 const char *keys,
                 const uint8_t *rend_circ_nonce)
{
  cell_t cell;
  crypt_path_t *tmp_cpath;

  if (created_cell_format(&cell, created_cell) < 0) {
    log_warn(LD_BUG,"couldn't format created cell (type=%d, len=%d)",
             (int)created_cell->cell_type, (int)created_cell->handshake_len);
    return -1;
  }
  cell.circ_id = circ->p_circ_id;

  tmp_cpath = tor_malloc_zero(sizeof(crypt_path_t));
  tmp_cpath->magic = CRYPT_PATH_MAGIC;

  circuit_set_state(TO_CIRCUIT(circ), CIRCUIT_STATE_OPEN);

  log_debug(LD_CIRC,"init digest forward 0x%.8x, backward 0x%.8x.",
            (unsigned int)get_uint32(keys),
            (unsigned int)get_uint32(keys+20));
  if (circuit_init_cpath_crypto(tmp_cpath, keys, 0)<0) {
    log_warn(LD_BUG,"Circuit initialization failed");
    tor_free(tmp_cpath);
    return -1;
  }
  circ->n_digest = tmp_cpath->f_digest;
  circ->n_crypto = tmp_cpath->f_crypto;
  circ->p_digest = tmp_cpath->b_digest;
  circ->p_crypto = tmp_cpath->b_crypto;
  tmp_cpath->magic = 0;
  tor_free(tmp_cpath);

  memcpy(circ->rend_circ_nonce, rend_circ_nonce, DIGEST_LEN);

  circ->is_first_hop = (created_cell->cell_type == CELL_CREATED_FAST);

  append_cell_to_circuit_queue(TO_CIRCUIT(circ),
                               circ->p_chan, &cell, CELL_DIRECTION_IN, 0);
  log_debug(LD_CIRC,"Finished sending '%s' cell.",
            circ->is_first_hop ? "created_fast" : "created");

  if (!channel_is_local(circ->p_chan) &&
      !channel_is_outgoing(circ->p_chan)) {
    /* record that we could process create cells from a non-local conn
     * that we didn't initiate; presumably this means that create cells
     * can reach us too. */
    router_orport_found_reachable();
  }

  return 0;
}

/** Choose a length for a circuit of purpose <b>purpose</b>: three + the
 * number of endpoints that would give something away about our destination.
 *
 * If the routerlist <b>nodes</b> doesn't have enough routers
 * to handle the desired path length, return -1.
 */
static int
new_route_len(uint8_t purpose, extend_info_t *exit, smartlist_t *nodes)
{
  int num_acceptable_routers;
  int routelen;

  tor_assert(nodes);

  routelen = DEFAULT_ROUTE_LEN;
  if (exit &&
      purpose != CIRCUIT_PURPOSE_TESTING &&
      purpose != CIRCUIT_PURPOSE_S_ESTABLISH_INTRO)
    routelen++;

  num_acceptable_routers = count_acceptable_nodes(nodes);

  log_debug(LD_CIRC,"Chosen route length %d (%d/%d routers suitable).",
            routelen, num_acceptable_routers, smartlist_len(nodes));

  if (num_acceptable_routers < routelen) {
    log_info(LD_CIRC,
             "Not enough acceptable routers (%d/%d). Discarding this circuit.",
             num_acceptable_routers, routelen);
    return -1;
  }

  return routelen;
}

/** Return a newly allocated list of uint16_t * for each predicted port not
 * handled by a current circuit. */
static smartlist_t *
circuit_get_unhandled_ports(time_t now)
{
  smartlist_t *dest = rep_hist_get_predicted_ports(now);
  circuit_remove_handled_ports(dest);
  return dest;
}

/** Return 1 if we already have circuits present or on the way for
 * all anticipated ports. Return 0 if we should make more.
 *
 * If we're returning 0, set need_uptime and need_capacity to
 * indicate any requirements that the unhandled ports have.
 */
int
circuit_all_predicted_ports_handled(time_t now, int *need_uptime,
                                    int *need_capacity)
{
  int i, enough;
  uint16_t *port;
  smartlist_t *sl = circuit_get_unhandled_ports(now);
  smartlist_t *LongLivedServices = get_options()->LongLivedPorts;
  tor_assert(need_uptime);
  tor_assert(need_capacity);
  // Always predict need_capacity
  *need_capacity = 1;
  enough = (smartlist_len(sl) == 0);
  for (i = 0; i < smartlist_len(sl); ++i) {
    port = smartlist_get(sl, i);
    if (smartlist_contains_int_as_string(LongLivedServices, *port))
      *need_uptime = 1;
    tor_free(port);
  }
  smartlist_free(sl);
  return enough;
}

/** Return 1 if <b>node</b> can handle one or more of the ports in
 * <b>needed_ports</b>, else return 0.
 */
static int
node_handles_some_port(const node_t *node, smartlist_t *needed_ports)
{ /* XXXX MOVE */
  int i;
  uint16_t port;

  for (i = 0; i < smartlist_len(needed_ports); ++i) {
    addr_policy_result_t r;
    /* alignment issues aren't a worry for this dereference, since
       needed_ports is explicitly a smartlist of uint16_t's */
    port = *(uint16_t *)smartlist_get(needed_ports, i);
    tor_assert(port);
    if (node)
      r = compare_tor_addr_to_node_policy(NULL, port, node);
    else
      continue;
    if (r != ADDR_POLICY_REJECTED && r != ADDR_POLICY_PROBABLY_REJECTED)
      return 1;
  }
  return 0;
}

/** Return true iff <b>conn</b> needs another general circuit to be
 * built. */
static int
ap_stream_wants_exit_attention(connection_t *conn)
{
  entry_connection_t *entry;
  if (conn->type != CONN_TYPE_AP)
    return 0;
  entry = TO_ENTRY_CONN(conn);

  if (conn->state == AP_CONN_STATE_CIRCUIT_WAIT &&
      !conn->marked_for_close &&
      !(entry->want_onehop) && /* ignore one-hop streams */
      !(entry->use_begindir) && /* ignore targeted dir fetches */
      !(entry->chosen_exit_name) && /* ignore defined streams */
      !connection_edge_is_rendezvous_stream(TO_EDGE_CONN(conn)) &&
      !circuit_stream_is_being_handled(TO_ENTRY_CONN(conn), 0,
                                       MIN_CIRCUITS_HANDLING_STREAM))
    return 1;
  return 0;
}

/** Return a pointer to a suitable router to be the exit node for the
 * general-purpose circuit we're about to build.
 *
 * Look through the connection array, and choose a router that maximizes
 * the number of pending streams that can exit from this router.
 *
 * Return NULL if we can't find any suitable routers.
 */
static const node_t *
choose_good_exit_server_general(int need_uptime, int need_capacity)
{
  int *n_supported;
  int n_pending_connections = 0;
  smartlist_t *connections;
  int best_support = -1;
  int n_best_support=0;
  const or_options_t *options = get_options();
  const smartlist_t *the_nodes;
  const node_t *node=NULL;

  connections = get_connection_array();

  /* Count how many connections are waiting for a circuit to be built.
   * We use this for log messages now, but in the future we may depend on it.
   */
  SMARTLIST_FOREACH(connections, connection_t *, conn,
  {
    if (ap_stream_wants_exit_attention(conn))
      ++n_pending_connections;
  });
//  log_fn(LOG_DEBUG, "Choosing exit node; %d connections are pending",
//         n_pending_connections);
  /* Now we count, for each of the routers in the directory, how many
   * of the pending connections could possibly exit from that
   * router (n_supported[i]). (We can't be sure about cases where we
   * don't know the IP address of the pending connection.)
   *
   * -1 means "Don't use this router at all."
   */
  the_nodes = nodelist_get_list();
  n_supported = tor_malloc(sizeof(int)*smartlist_len(the_nodes));
  SMARTLIST_FOREACH_BEGIN(the_nodes, const node_t *, node) {
    const int i = node_sl_idx;
    if (router_digest_is_me(node->identity)) {
      n_supported[i] = -1;
//      log_fn(LOG_DEBUG,"Skipping node %s -- it's me.", router->nickname);
      /* XXX there's probably a reverse predecessor attack here, but
       * it's slow. should we take this out? -RD
       */
      continue;
    }
    if (!node_has_descriptor(node)) {
      n_supported[i] = -1;
      continue;
    }
    if (!node->is_running || node->is_bad_exit) {
      n_supported[i] = -1;
      continue; /* skip routers that are known to be down or bad exits */
    }
    if (node_get_purpose(node) != ROUTER_PURPOSE_GENERAL) {
      /* never pick a non-general node as a random exit. */
      n_supported[i] = -1;
      continue;
    }
    if (routerset_contains_node(options->ExcludeExitNodesUnion_, node)) {
      n_supported[i] = -1;
      continue; /* user asked us not to use it, no matter what */
    }
    if (options->ExitNodes &&
        !routerset_contains_node(options->ExitNodes, node)) {
      n_supported[i] = -1;
      continue; /* not one of our chosen exit nodes */
    }

    if (node_is_unreliable(node, need_uptime, need_capacity, 0)) {
      n_supported[i] = -1;
      continue; /* skip routers that are not suitable.  Don't worry if
                 * this makes us reject all the possible routers: if so,
                 * we'll retry later in this function with need_update and
                 * need_capacity set to 0. */
    }
    if (!(node->is_valid || options->AllowInvalid_ & ALLOW_INVALID_EXIT)) {
      /* if it's invalid and we don't want it */
      n_supported[i] = -1;
//      log_fn(LOG_DEBUG,"Skipping node %s (index %d) -- invalid router.",
//             router->nickname, i);
      continue; /* skip invalid routers */
    }
    if (options->ExcludeSingleHopRelays &&
        node_allows_single_hop_exits(node)) {
      n_supported[i] = -1;
      continue;
    }
    if (node_exit_policy_rejects_all(node)) {
      n_supported[i] = -1;
//      log_fn(LOG_DEBUG,"Skipping node %s (index %d) -- it rejects all.",
//             router->nickname, i);
      continue; /* skip routers that reject all */
    }
    n_supported[i] = 0;
    /* iterate over connections */
    SMARTLIST_FOREACH_BEGIN(connections, connection_t *, conn) {
      if (!ap_stream_wants_exit_attention(conn))
        continue; /* Skip everything but APs in CIRCUIT_WAIT */
      if (connection_ap_can_use_exit(TO_ENTRY_CONN(conn), node)) {
        ++n_supported[i];
//        log_fn(LOG_DEBUG,"%s is supported. n_supported[%d] now %d.",
//               router->nickname, i, n_supported[i]);
      } else {
//        log_fn(LOG_DEBUG,"%s (index %d) would reject this stream.",
//               router->nickname, i);
      }
    } SMARTLIST_FOREACH_END(conn);
    if (n_pending_connections > 0 && n_supported[i] == 0) {
      /* Leave best_support at -1 if that's where it is, so we can
       * distinguish it later. */
      continue;
    }
    if (n_supported[i] > best_support) {
      /* If this router is better than previous ones, remember its index
       * and goodness, and start counting how many routers are this good. */
      best_support = n_supported[i]; n_best_support=1;
//      log_fn(LOG_DEBUG,"%s is new best supported option so far.",
//             router->nickname);
    } else if (n_supported[i] == best_support) {
      /* If this router is _as good_ as the best one, just increment the
       * count of equally good routers.*/
      ++n_best_support;
    }
  } SMARTLIST_FOREACH_END(node);
  log_info(LD_CIRC,
           "Found %d servers that might support %d/%d pending connections.",
           n_best_support, best_support >= 0 ? best_support : 0,
           n_pending_connections);

  /* If any routers definitely support any pending connections, choose one
   * at random. */
  if (best_support > 0) {
    smartlist_t *supporting = smartlist_new();

    SMARTLIST_FOREACH(the_nodes, const node_t *, node, {
      if (n_supported[node_sl_idx] == best_support)
        smartlist_add(supporting, (void*)node);
    });

    node = node_sl_choose_by_bandwidth(supporting, WEIGHT_FOR_EXIT);
    smartlist_free(supporting);
  } else {
    /* Either there are no pending connections, or no routers even seem to
     * possibly support any of them.  Choose a router at random that satisfies
     * at least one predicted exit port. */

    int attempt;
    smartlist_t *needed_ports, *supporting;

    if (best_support == -1) {
      if (need_uptime || need_capacity) {
        log_info(LD_CIRC,
                 "We couldn't find any live%s%s routers; falling back "
                 "to list of all routers.",
                 need_capacity?", fast":"",
                 need_uptime?", stable":"");
        tor_free(n_supported);
        return choose_good_exit_server_general(0, 0);
      }
      log_notice(LD_CIRC, "All routers are down or won't exit%s -- "
                 "choosing a doomed exit at random.",
                 options->ExcludeExitNodesUnion_ ? " or are Excluded" : "");
    }
    supporting = smartlist_new();
    needed_ports = circuit_get_unhandled_ports(time(NULL));
    for (attempt = 0; attempt < 2; attempt++) {
      /* try once to pick only from routers that satisfy a needed port,
       * then if there are none, pick from any that support exiting. */
      SMARTLIST_FOREACH_BEGIN(the_nodes, const node_t *, node) {
        if (n_supported[node_sl_idx] != -1 &&
            (attempt || node_handles_some_port(node, needed_ports))) {
//          log_fn(LOG_DEBUG,"Try %d: '%s' is a possibility.",
//                 try, router->nickname);
          smartlist_add(supporting, (void*)node);
        }
      } SMARTLIST_FOREACH_END(node);

      node = node_sl_choose_by_bandwidth(supporting, WEIGHT_FOR_EXIT);
      if (node)
        break;
      smartlist_clear(supporting);
      /* If we reach this point, we can't actually support any unhandled
       * predicted ports, so clear all the remaining ones. */
      if (smartlist_len(needed_ports))
        rep_hist_remove_predicted_ports(needed_ports);
    }
    SMARTLIST_FOREACH(needed_ports, uint16_t *, cp, tor_free(cp));
    smartlist_free(needed_ports);
    smartlist_free(supporting);
  }

  tor_free(n_supported);
  if (node) {
    log_info(LD_CIRC, "Chose exit server '%s'", node_describe(node));
    return node;
  }
  if (options->ExitNodes) {
    log_warn(LD_CIRC,
             "No specified %sexit routers seem to be running: "
             "can't choose an exit.",
             options->ExcludeExitNodesUnion_ ? "non-excluded " : "");
  }
  return NULL;
}

/** Return a pointer to a suitable router to be the exit node for the
 * circuit of purpose <b>purpose</b> that we're about to build (or NULL
 * if no router is suitable).
 *
 * For general-purpose circuits, pass it off to
 * choose_good_exit_server_general()
 *
 * For client-side rendezvous circuits, choose a random node, weighted
 * toward the preferences in 'options'.
 */
static const node_t *
choose_good_exit_server(uint8_t purpose,
                        int need_uptime, int need_capacity, int is_internal)
{
  const or_options_t *options = get_options();
  router_crn_flags_t flags = CRN_NEED_DESC;
  if (need_uptime)
    flags |= CRN_NEED_UPTIME;
  if (need_capacity)
    flags |= CRN_NEED_CAPACITY;

  switch (purpose) {
    case CIRCUIT_PURPOSE_C_GENERAL:
      if (options->AllowInvalid_ & ALLOW_INVALID_MIDDLE)
        flags |= CRN_ALLOW_INVALID;
      if (is_internal) /* pick it like a middle hop */
        return router_choose_random_node(NULL, options->ExcludeNodes, flags);
      else
        return choose_good_exit_server_general(need_uptime,need_capacity);
    case CIRCUIT_PURPOSE_C_ESTABLISH_REND:
      if (options->AllowInvalid_ & ALLOW_INVALID_RENDEZVOUS)
        flags |= CRN_ALLOW_INVALID;
      return router_choose_random_node(NULL, options->ExcludeNodes, flags);
  }
  log_warn(LD_BUG,"Unhandled purpose %d", purpose);
  tor_fragile_assert();
  return NULL;
}

/** Log a warning if the user specified an exit for the circuit that
 * has been excluded from use by ExcludeNodes or ExcludeExitNodes. */
static void
warn_if_last_router_excluded(origin_circuit_t *circ, const extend_info_t *exit)
{
  const or_options_t *options = get_options();
  routerset_t *rs = options->ExcludeNodes;
  const char *description;
  uint8_t purpose = circ->base_.purpose;

  if (circ->build_state->onehop_tunnel)
    return;

  switch (purpose)
    {
    default:
    case CIRCUIT_PURPOSE_OR:
    case CIRCUIT_PURPOSE_INTRO_POINT:
    case CIRCUIT_PURPOSE_REND_POINT_WAITING:
    case CIRCUIT_PURPOSE_REND_ESTABLISHED:
      log_warn(LD_BUG, "Called on non-origin circuit (purpose %d, %s)",
               (int)purpose,
               circuit_purpose_to_string(purpose));
      return;
    case CIRCUIT_PURPOSE_C_GENERAL:
      if (circ->build_state->is_internal)
        return;
      description = "requested exit node";
      rs = options->ExcludeExitNodesUnion_;
      break;
    case CIRCUIT_PURPOSE_C_INTRODUCING:
    case CIRCUIT_PURPOSE_C_INTRODUCE_ACK_WAIT:
    case CIRCUIT_PURPOSE_C_INTRODUCE_ACKED:
    case CIRCUIT_PURPOSE_S_ESTABLISH_INTRO:
    case CIRCUIT_PURPOSE_S_CONNECT_REND:
    case CIRCUIT_PURPOSE_S_REND_JOINED:
    case CIRCUIT_PURPOSE_TESTING:
      return;
    case CIRCUIT_PURPOSE_C_ESTABLISH_REND:
    case CIRCUIT_PURPOSE_C_REND_READY:
    case CIRCUIT_PURPOSE_C_REND_READY_INTRO_ACKED:
    case CIRCUIT_PURPOSE_C_REND_JOINED:
      description = "chosen rendezvous point";
      break;
    case CIRCUIT_PURPOSE_CONTROLLER:
      rs = options->ExcludeExitNodesUnion_;
      description = "controller-selected circuit target";
      break;
    }

  if (routerset_contains_extendinfo(rs, exit)) {
    /* We should never get here if StrictNodes is set to 1. */
    if (options->StrictNodes) {
      log_warn(LD_BUG, "Using %s '%s' which is listed in ExcludeNodes%s, "
               "even though StrictNodes is set. Please report. "
               "(Circuit purpose: %s)",
               description, extend_info_describe(exit),
               rs==options->ExcludeNodes?"":" or ExcludeExitNodes",
               circuit_purpose_to_string(purpose));
    } else {
      log_warn(LD_CIRC, "Using %s '%s' which is listed in "
               "ExcludeNodes%s, because no better options were available. To "
               "prevent this (and possibly break your Tor functionality), "
               "set the StrictNodes configuration option. "
               "(Circuit purpose: %s)",
               description, extend_info_describe(exit),
               rs==options->ExcludeNodes?"":" or ExcludeExitNodes",
               circuit_purpose_to_string(purpose));
    }
    circuit_log_path(LOG_WARN, LD_CIRC, circ);
  }

  return;
}

/** Decide a suitable length for circ's cpath, and pick an exit
 * router (or use <b>exit</b> if provided). Store these in the
 * cpath. Return 0 if ok, -1 if circuit should be closed. */
static int
onion_pick_cpath_exit(origin_circuit_t *circ, extend_info_t *exit)
{
  cpath_build_state_t *state = circ->build_state;

  if (state->onehop_tunnel) {
    log_debug(LD_CIRC, "Launching a one-hop circuit for dir tunnel.");
    state->desired_path_len = 1;
  } else {
    int r = new_route_len(circ->base_.purpose, exit, nodelist_get_list());
    if (r < 1) /* must be at least 1 */
      return -1;
    state->desired_path_len = r;
  }

  if (exit) { /* the circuit-builder pre-requested one */
    warn_if_last_router_excluded(circ, exit);
    log_info(LD_CIRC,"Using requested exit node '%s'",
             extend_info_describe(exit));
    exit = extend_info_dup(exit);
  } else { /* we have to decide one */
    const node_t *node =
      choose_good_exit_server(circ->base_.purpose, state->need_uptime,
                              state->need_capacity, state->is_internal);
    if (!node) {
      log_warn(LD_CIRC,"failed to choose an exit server");
      return -1;
    }
    exit = extend_info_from_node(node, 0);
    tor_assert(exit);
  }
  state->chosen_exit = exit;
  return 0;
}

/** Give <b>circ</b> a new exit destination to <b>exit</b>, and add a
 * hop to the cpath reflecting this. Don't send the next extend cell --
 * the caller will do this if it wants to.
 */
int
circuit_append_new_exit(origin_circuit_t *circ, extend_info_t *exit)
{
  cpath_build_state_t *state;
  tor_assert(exit);
  tor_assert(circ);

  state = circ->build_state;
  tor_assert(state);
  extend_info_free(state->chosen_exit);
  state->chosen_exit = extend_info_dup(exit);

  ++circ->build_state->desired_path_len;
  onion_append_hop(&circ->cpath, exit);
  return 0;
}

/** Take an open <b>circ</b>, and add a new hop at the end, based on
 * <b>info</b>. Set its state back to CIRCUIT_STATE_BUILDING, and then
 * send the next extend cell to begin connecting to that hop.
 */
int
circuit_extend_to_new_exit(origin_circuit_t *circ, extend_info_t *exit)
{
  int err_reason = 0;
  warn_if_last_router_excluded(circ, exit);

  tor_gettimeofday(&circ->base_.timestamp_began);

  circuit_append_new_exit(circ, exit);
  circuit_set_state(TO_CIRCUIT(circ), CIRCUIT_STATE_BUILDING);
  if ((err_reason = circuit_send_next_onion_skin(circ))<0) {
    log_warn(LD_CIRC, "Couldn't extend circuit to new point %s.",
             extend_info_describe(exit));
    circuit_mark_for_close(TO_CIRCUIT(circ), -err_reason);
    return -1;
  }

  // XXX: Should cannibalized circuits be dirty or not? Not easy to say..

  return 0;
}

/** Return the number of routers in <b>routers</b> that are currently up
 * and available for building circuits through.
 */
static int
count_acceptable_nodes(smartlist_t *nodes)
{
  int num=0;

  SMARTLIST_FOREACH_BEGIN(nodes, const node_t *, node) {
    //    log_debug(LD_CIRC,
//              "Contemplating whether router %d (%s) is a new option.",
//              i, r->nickname);
    if (! node->is_running)
//      log_debug(LD_CIRC,"Nope, the directory says %d is not running.",i);
      continue;
    if (! node->is_valid)
//      log_debug(LD_CIRC,"Nope, the directory says %d is not valid.",i);
      continue;
    if (! node_has_descriptor(node))
      continue;
      /* XXX This clause makes us count incorrectly: if AllowInvalidRouters
       * allows this node in some places, then we're getting an inaccurate
       * count. For now, be conservative and don't count it. But later we
       * should try to be smarter. */
    ++num;
  } SMARTLIST_FOREACH_END(node);

//    log_debug(LD_CIRC,"I like %d. num_acceptable_routers now %d.",i, num);

  return num;
}

/** Add <b>new_hop</b> to the end of the doubly-linked-list <b>head_ptr</b>.
 * This function is used to extend cpath by another hop.
 */
void
onion_append_to_cpath(crypt_path_t **head_ptr, crypt_path_t *new_hop)
{
  if (*head_ptr) {
    new_hop->next = (*head_ptr);
    new_hop->prev = (*head_ptr)->prev;
    (*head_ptr)->prev->next = new_hop;
    (*head_ptr)->prev = new_hop;
  } else {
    *head_ptr = new_hop;
    new_hop->prev = new_hop->next = new_hop;
  }
}

/** A helper function used by onion_extend_cpath(). Use <b>purpose</b>
 * and <b>state</b> and the cpath <b>head</b> (currently populated only
 * to length <b>cur_len</b> to decide a suitable middle hop for a
 * circuit. In particular, make sure we don't pick the exit node or its
 * family, and make sure we don't duplicate any previous nodes or their
 * families. */
static const node_t *
choose_good_middle_server(uint8_t purpose,
                          cpath_build_state_t *state,
                          crypt_path_t *head,
                          int cur_len)
{
  int i;
  const node_t *r, *choice;
  crypt_path_t *cpath;
  smartlist_t *excluded;
  const or_options_t *options = get_options();
  router_crn_flags_t flags = CRN_NEED_DESC;
  tor_assert(CIRCUIT_PURPOSE_MIN_ <= purpose &&
             purpose <= CIRCUIT_PURPOSE_MAX_);

  log_debug(LD_CIRC, "Contemplating intermediate hop: random choice.");
  excluded = smartlist_new();
  if ((r = build_state_get_exit_node(state))) {
    nodelist_add_node_and_family(excluded, r);
  }
  for (i = 0, cpath = head; i < cur_len; ++i, cpath=cpath->next) {
    if ((r = node_get_by_id(cpath->extend_info->identity_digest))) {
      nodelist_add_node_and_family(excluded, r);
    }
  }

  if (state->need_uptime)
    flags |= CRN_NEED_UPTIME;
  if (state->need_capacity)
    flags |= CRN_NEED_CAPACITY;
  if (options->AllowInvalid_ & ALLOW_INVALID_MIDDLE)
    flags |= CRN_ALLOW_INVALID;
  choice = router_choose_random_node(excluded, options->ExcludeNodes, flags);
  smartlist_free(excluded);
  return choice;
}

/** Pick a good entry server for the circuit to be built according to
 * <b>state</b>.  Don't reuse a chosen exit (if any), don't use this
 * router (if we're an OR), and respect firewall settings; if we're
 * configured to use entry guards, return one.
 *
 * If <b>state</b> is NULL, we're choosing a router to serve as an entry
 * guard, not for any particular circuit.
 */
/* XXXX024 I'd like to have this be static again, but entrynodes.c needs it. */
const node_t *
choose_good_entry_server(uint8_t purpose, cpath_build_state_t *state)
{
  const node_t *choice;
  smartlist_t *excluded;
  const or_options_t *options = get_options();
  router_crn_flags_t flags = CRN_NEED_GUARD|CRN_NEED_DESC;
  const node_t *node;

  if (state && options->UseEntryGuards &&
      (purpose != CIRCUIT_PURPOSE_TESTING || options->BridgeRelay)) {
    /* This request is for an entry server to use for a regular circuit,
     * and we use entry guard nodes.  Just return one of the guard nodes.  */
    return choose_random_entry(state);
  }

  excluded = smartlist_new();

  if (state && (node = build_state_get_exit_node(state))) {
    /* Exclude the exit node from the state, if we have one.  Also exclude its
     * family. */
    nodelist_add_node_and_family(excluded, node);
  }
  if (firewall_is_fascist_or()) {
    /* Exclude all ORs that we can't reach through our firewall */
    smartlist_t *nodes = nodelist_get_list();
    SMARTLIST_FOREACH(nodes, const node_t *, node, {
      if (!fascist_firewall_allows_node(node))
        smartlist_add(excluded, (void*)node);
    });
  }
  /* and exclude current entry guards and their families, if applicable */
  /*XXXX025 use the using_as_guard flag to accomplish this.*/
  if (options->UseEntryGuards) {
    SMARTLIST_FOREACH(get_entry_guards(), const entry_guard_t *, entry,
      {
        if ((node = node_get_by_id(entry->identity))) {
          nodelist_add_node_and_family(excluded, node);
        }
      });
  }

  if (state) {
    if (state->need_uptime)
      flags |= CRN_NEED_UPTIME;
    if (state->need_capacity)
      flags |= CRN_NEED_CAPACITY;
  }
  if (options->AllowInvalid_ & ALLOW_INVALID_ENTRY)
    flags |= CRN_ALLOW_INVALID;

  choice = router_choose_random_node(excluded, options->ExcludeNodes, flags);
  smartlist_free(excluded);
  return choice;
}

/** Return the first non-open hop in cpath, or return NULL if all
 * hops are open. */
static crypt_path_t *
onion_next_hop_in_cpath(crypt_path_t *cpath)
{
  crypt_path_t *hop = cpath;
  do {
    if (hop->state != CPATH_STATE_OPEN)
      return hop;
    hop = hop->next;
  } while (hop != cpath);
  return NULL;
}

/** Choose a suitable next hop in the cpath <b>head_ptr</b>,
 * based on <b>state</b>. Append the hop info to head_ptr.
 *
 * Return 1 if the path is complete, 0 if we successfully added a hop,
 * and -1 on error.
 */
static int
onion_extend_cpath(origin_circuit_t *circ)
{
  uint8_t purpose = circ->base_.purpose;
  cpath_build_state_t *state = circ->build_state;
  int cur_len = circuit_get_cpath_len(circ);
  extend_info_t *info = NULL;

  if (cur_len >= state->desired_path_len) {
    log_debug(LD_CIRC, "Path is complete: %d steps long",
              state->desired_path_len);
    return 1;
  }

  log_debug(LD_CIRC, "Path is %d long; we want %d", cur_len,
            state->desired_path_len);

  if (cur_len == state->desired_path_len - 1) { /* Picking last node */
    info = extend_info_dup(state->chosen_exit);
  } else if (cur_len == 0) { /* picking first node */
    const node_t *r = choose_good_entry_server(purpose, state);
    if (r) {
      /* If we're a client, use the preferred address rather than the
         primary address, for potentially connecting to an IPv6 OR
         port. */
      info = extend_info_from_node(r, server_mode(get_options()) == 0);
      tor_assert(info);
    }
  } else {
    const node_t *r =
      choose_good_middle_server(purpose, state, circ->cpath, cur_len);
    if (r) {
      info = extend_info_from_node(r, 0);
      tor_assert(info);
    }
  }

  if (!info) {
    log_warn(LD_CIRC,"Failed to find node for hop %d of our path. Discarding "
             "this circuit.", cur_len);
    return -1;
  }

  log_debug(LD_CIRC,"Chose router %s for hop %d (exit is %s)",
            extend_info_describe(info),
            cur_len+1, build_state_get_exit_nickname(state));

  onion_append_hop(&circ->cpath, info);
  extend_info_free(info);
  return 0;
}

/** Create a new hop, annotate it with information about its
 * corresponding router <b>choice</b>, and append it to the
 * end of the cpath <b>head_ptr</b>. */
static int
onion_append_hop(crypt_path_t **head_ptr, extend_info_t *choice)
{
  crypt_path_t *hop = tor_malloc_zero(sizeof(crypt_path_t));

  /* link hop into the cpath, at the end. */
  onion_append_to_cpath(head_ptr, hop);

  hop->magic = CRYPT_PATH_MAGIC;
  hop->state = CPATH_STATE_CLOSED;

  hop->extend_info = extend_info_dup(choice);

  hop->package_window = circuit_initial_package_window();
  hop->deliver_window = CIRCWINDOW_START;

  return 0;
}

/** Allocate a new extend_info object based on the various arguments. */
extend_info_t *
extend_info_new(const char *nickname, const char *digest,
                crypto_pk_t *onion_key,
                const curve25519_public_key_t *curve25519_key,
                const tor_addr_t *addr, uint16_t port)
{
  extend_info_t *info = tor_malloc_zero(sizeof(extend_info_t));
  memcpy(info->identity_digest, digest, DIGEST_LEN);
  if (nickname)
    strlcpy(info->nickname, nickname, sizeof(info->nickname));
  if (onion_key)
    info->onion_key = crypto_pk_dup_key(onion_key);
#ifdef CURVE25519_ENABLED
  if (curve25519_key)
    memcpy(&info->curve25519_onion_key, curve25519_key,
           sizeof(curve25519_public_key_t));
#else
  (void)curve25519_key;
#endif
  tor_addr_copy(&info->addr, addr);
  info->port = port;
  return info;
}

/** Allocate and return a new extend_info that can be used to build a
 * circuit to or through the node <b>node</b>. Use the primary address
 * of the node (i.e. its IPv4 address) unless
 * <b>for_direct_connect</b> is true, in which case the preferred
 * address is used instead. May return NULL if there is not enough
 * info about <b>node</b> to extend to it--for example, if there is no
 * routerinfo_t or microdesc_t.
 **/
extend_info_t *
extend_info_from_node(const node_t *node, int for_direct_connect)
{
  tor_addr_port_t ap;

  if (node->ri == NULL && (node->rs == NULL || node->md == NULL))
    return NULL;

  if (for_direct_connect)
    node_get_pref_orport(node, &ap);
  else
    node_get_prim_orport(node, &ap);

  log_debug(LD_CIRC, "using %s for %s",
            fmt_addrport(&ap.addr, ap.port),
            node->ri ? node->ri->nickname : node->rs->nickname);

  if (node->ri)
    return extend_info_new(node->ri->nickname,
                             node->identity,
                             node->ri->onion_pkey,
                             node->ri->onion_curve25519_pkey,
                             &ap.addr,
                             ap.port);
  else if (node->rs && node->md)
    return extend_info_new(node->rs->nickname,
                             node->identity,
                             node->md->onion_pkey,
                             node->md->onion_curve25519_pkey,
                             &ap.addr,
                             ap.port);
  else
    return NULL;
}

/** Release storage held by an extend_info_t struct. */
void
extend_info_free(extend_info_t *info)
{
  if (!info)
    return;
  crypto_pk_free(info->onion_key);
  tor_free(info);
}

/** Allocate and return a new extend_info_t with the same contents as
 * <b>info</b>. */
extend_info_t *
extend_info_dup(extend_info_t *info)
{
  extend_info_t *newinfo;
  tor_assert(info);
  newinfo = tor_malloc(sizeof(extend_info_t));
  memcpy(newinfo, info, sizeof(extend_info_t));
  if (info->onion_key)
    newinfo->onion_key = crypto_pk_dup_key(info->onion_key);
  else
    newinfo->onion_key = NULL;
  return newinfo;
}

/** Return the routerinfo_t for the chosen exit router in <b>state</b>.
 * If there is no chosen exit, or if we don't know the routerinfo_t for
 * the chosen exit, return NULL.
 */
const node_t *
build_state_get_exit_node(cpath_build_state_t *state)
{
  if (!state || !state->chosen_exit)
    return NULL;
  return node_get_by_id(state->chosen_exit->identity_digest);
}

/** Return the nickname for the chosen exit router in <b>state</b>. If
 * there is no chosen exit, or if we don't know the routerinfo_t for the
 * chosen exit, return NULL.
 */
const char *
build_state_get_exit_nickname(cpath_build_state_t *state)
{
  if (!state || !state->chosen_exit)
    return NULL;
  return state->chosen_exit->nickname;
}