GitLab

(note, this bug is unverified)

It seems that when a client adds more than one of a bridge's pluggable transport as its bridges, it only ever uses the address and port of the first transport it adds. Therefore, in the current situation, it appears if the first PT is disabled on the server-side (or the connection is blocked on the wire, or connections timeout for whatever reason) and the client attempts to use the second transport, it will try to establish the connection using the ip address and port number of the first transport. It may try connecting using the correct transport, but it'll probably connect to the wrong place.

Specifically, all the transports are added to the bridge list, but we only use the information from the first bridge in the list with a given ID and ignore the rest.

From learned_bridge_descriptor()

    /* Choose the first bridge which either has an ID which matches
     * this routerinfo or the same address and orport if we don't
     * know the ID */
    bridge_info_t *bridge = get_configured_bridge_by_routerinfo(ri);
    time_t now = time(NULL);
    router_set_status(ri->cache_info.identity_digest, 1);

    if (bridge) { /* if we actually want to use this one */
      node_t *node;
      /* it's here; schedule its re-fetch for a long time from now. */
      if (!from_cache)
        download_status_reset(&bridge->fetch_status);                                   

      /* get a *node_t for this bridge, based on its ID */
      node = node_get_mutable_by_id(ri->cache_info.identity_digest);
      tor_assert(node);
      /* Fill in the node_t using the details of the bridge we
       * retrieved above - the first bridge in the list
       */
      rewrite_node_address_for_bridge(bridge, node);

Then, we choose which entry node we want to use, in onion_extend_cpath(), where choose_good_entry_server() returns the *node_t representing the entry guard/bridge we should use, which it gets by ID:

    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);

from populate_live_entry_guards()

  SMARTLIST_FOREACH_BEGIN(all_entry_guards, const entry_guard_t *, entry) {
      const char *msg;
      node = entry_is_live(entry, entry_flags, &msg);

and finally, in extend_info_from_node() where we use the address and port defined in the *node_t:

  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);