Reverse connections in adnl (#545)

Co-authored-by: SpyCheese <mikle98@yandex.ru>

adnl/adnl-address-list.cpp

@@ -185,7 +185,8 @@ td::Ref<AdnlAddressImpl> AdnlAddressImpl::create(const tl_object_ptr<ton_api::ad
       *const_cast<ton_api::adnl_Address *>(addr.get()),
       td::overloaded([&](const ton_api::adnl_address_udp &obj) { res = td::make_ref<AdnlAddressUdp>(obj); },
                      [&](const ton_api::adnl_address_udp6 &obj) { res = td::make_ref<AdnlAddressUdp6>(obj); },
-                     [&](const ton_api::adnl_address_tunnel &obj) { res = td::make_ref<AdnlAddressTunnel>(obj); }));
+                     [&](const ton_api::adnl_address_tunnel &obj) { res = td::make_ref<AdnlAddressTunnel>(obj); },
+                     [&](const ton_api::adnl_address_reverse &obj) { res = td::make_ref<AdnlAddressReverse>(); }));
   return res;
 }
 
@@ -202,7 +203,12 @@ AdnlAddressList::AdnlAddressList(const tl_object_ptr<ton_api::adnl_addressList>
   version_ = static_cast<td::uint32>(addrs->version_);
   std::vector<td::Ref<AdnlAddressImpl>> vec;
   for (auto &addr : addrs->addrs_) {
-    vec.push_back(AdnlAddressImpl::create(addr));
+    auto obj = AdnlAddressImpl::create(addr);
+    if (obj->is_reverse()) {
+      has_reverse_ = true;
+    } else {
+      vec.push_back(std::move(obj));
+    }
   }
   addrs_ = std::move(vec);
   reinit_date_ = addrs->reinit_date_;
@@ -215,6 +221,9 @@ tl_object_ptr<ton_api::adnl_addressList> AdnlAddressList::tl() const {
   for (auto &v : addrs_) {
     addrs.emplace_back(v->tl());
   }
+  if (has_reverse_) {
+    addrs.push_back(create_tl_object<ton_api::adnl_address_reverse>());
+  }
   return create_tl_object<ton_api::adnl_addressList>(std::move(addrs), version_, reinit_date_, priority_, expire_at_);
 }
 

adnl/adnl-address-list.h

@@ -39,6 +39,9 @@ class AdnlAddressImpl : public td::CntObject {
   virtual td::actor::ActorOwn<AdnlNetworkConnection> create_connection(
       td::actor::ActorId<AdnlNetworkManager> network_manager, td::actor::ActorId<Adnl> adnl,
       std::unique_ptr<AdnlNetworkConnection::Callback> callback) const = 0;
+  virtual bool is_reverse() const {
+    return false;
+  }
 
   static td::Ref<AdnlAddressImpl> create(const tl_object_ptr<ton_api::adnl_Address> &addr);
 };
@@ -54,6 +57,7 @@ class AdnlAddressList {
   td::int32 priority_;
   td::int32 expire_at_;
   std::vector<AdnlAddress> addrs_;
+  bool has_reverse_{false};
 
  public:
   static constexpr td::uint32 max_serialized_size() {
@@ -102,6 +106,13 @@ class AdnlAddressList {
 
   static td::Result<AdnlAddressList> create(const tl_object_ptr<ton_api::adnl_addressList> &addr_list);
   td::Status add_udp_address(td::IPAddress addr);
+
+  void set_reverse(bool x = true) {
+    has_reverse_ = x;
+  }
+  bool has_reverse() const {
+    return has_reverse_;
+  }
 };
 
 }  // namespace adnl

adnl/adnl-address-list.hpp

@@ -116,6 +116,31 @@ class AdnlAddressTunnel : public AdnlAddressImpl {
       std::unique_ptr<AdnlNetworkConnection::Callback> callback) const override;
 };
 
+class AdnlAddressReverse : public AdnlAddressImpl {
+ public:
+  AdnlAddressReverse *make_copy() const override {
+    return new AdnlAddressReverse();
+  }
+  bool is_public() const override {
+    return true;
+  }
+  td::uint32 serialized_size() const override {
+    return 4;
+  }
+  tl_object_ptr<ton_api::adnl_Address> tl() const override {
+    return create_tl_object<ton_api::adnl_address_reverse>();
+  }
+  td::actor::ActorOwn<AdnlNetworkConnection> create_connection(
+      td::actor::ActorId<AdnlNetworkManager> network_manager, td::actor::ActorId<Adnl> adnl,
+      std::unique_ptr<AdnlNetworkConnection::Callback> callback) const override {
+    LOG(ERROR) << "Cannot create connection for AdnlAddressReverse";
+    return {};
+  }
+  bool is_reverse() const override {
+    return true;
+  }
+};
+
 }  // namespace adnl
 
 }  // namespace ton

adnl/adnl-local-id.cpp

@@ -121,7 +121,7 @@ void AdnlLocalId::update_address_list(AdnlAddressList addr_list) {
 }
 
 void AdnlLocalId::publish_address_list() {
-  if (dht_node_.empty() || addr_list_.empty() || addr_list_.size() == 0) {
+  if (dht_node_.empty() || addr_list_.empty() || (addr_list_.size() == 0 && !addr_list_.has_reverse())) {
     VLOG(ADNL_NOTICE) << this << ": skipping public addr list, because localid (or dht node) not fully initialized";
     return;
   }
@@ -175,6 +175,17 @@ void AdnlLocalId::publish_address_list() {
 
   td::actor::send_closure(keyring_, &keyring::Keyring::sign_message, short_id_.pubkey_hash(), std::move(B),
                           std::move(P));
+
+  if (addr_list_.has_reverse()) {
+    td::actor::send_closure(
+        dht_node_, &dht::Dht::register_reverse_connection, id_, [print_id = print_id()](td::Result<td::Unit> R) {
+          if (R.is_error()) {
+            VLOG(ADNL_NOTICE) << print_id << ": failed to register reverse connection in DHT: " << R.move_as_error();
+          } else {
+            VLOG(ADNL_INFO) << print_id << ": registered reverse connection";
+          }
+        });
+  }
 }
 
 AdnlLocalId::AdnlLocalId(AdnlNodeIdFull id, AdnlAddressList addr_list, td::uint32 mode,

adnl/adnl-peer.cpp

@@ -113,6 +113,7 @@ void AdnlPeerPairImpl::discover() {
 }
 
 void AdnlPeerPairImpl::receive_packet_checked(AdnlPacket packet) {
+  request_reverse_ping_after_ = td::Timestamp::in(15.0);
   auto d = Adnl::adnl_start_time();
   if (packet.dst_reinit_date() > d) {
     VLOG(ADNL_WARNING) << this << ": dropping IN message: too new our reinit date " << packet.dst_reinit_date();
@@ -653,10 +654,15 @@ td::Result<std::pair<td::actor::ActorId<AdnlNetworkConnection>, bool>> AdnlPeerP
   }
 
   if (conns_.size() == 0 && priority_conns_.size() == 0) {
-    return td::Status::Error(ErrorCode::notready, PSTRING()
+    if (has_reverse_addr_) {
-                                                      << "empty network information: version=" << addr_list_.version()
+      request_reverse_ping();
-                                                      << " reinit_date=" << addr_list_.reinit_date()
+      return td::Status::Error(ErrorCode::notready, "waiting for reverse ping");
-                                                      << " real_reinit_date=" << reinit_date_);
+    } else {
+      return td::Status::Error(ErrorCode::notready, PSTRING()
+                                                        << "empty network information: version=" << addr_list_.version()
+                                                        << " reinit_date=" << addr_list_.reinit_date()
+                                                        << " real_reinit_date=" << reinit_date_);
+    }
   }
 
   for (auto &conn : priority_conns_) {
@@ -704,11 +710,18 @@ void AdnlPeerPairImpl::update_addr_list(AdnlAddressList addr_list) {
   VLOG(ADNL_INFO) << this << ": updating addr list to version " << addr_list.version() << " size=" << addr_list.size();
 
   const auto addrs = addr_list.addrs();
+  has_reverse_addr_ = addr_list.has_reverse();
+  if (has_reverse_addr_ && addrs.empty()) {
+    return;
+  }
   std::vector<Conn> conns;
   auto &old_conns = priority ? priority_conns_ : conns_;
 
   size_t idx = 0;
   for (const auto &addr : addrs) {
+    if (addr->is_reverse()) {
+      continue;
+    }
     if ((mode_ & static_cast<td::uint32>(AdnlLocalIdMode::direct_only)) && !addr->is_public()) {
       continue;
     }
@@ -944,6 +957,36 @@ void AdnlPeerPairImpl::update_peer_id(AdnlNodeIdFull id) {
   CHECK(!peer_id_.empty());
 }
 
+void AdnlPeerPairImpl::request_reverse_ping() {
+  if (request_reverse_ping_active_ || !request_reverse_ping_after_.is_in_past()) {
+    return;
+  }
+  VLOG(ADNL_INFO) << this << ": requesting reverse ping";
+  request_reverse_ping_after_ = td::Timestamp::in(15.0);
+  request_reverse_ping_active_ = true;
+  td::actor::send_closure(
+      local_actor_, &AdnlLocalId::get_self_node,
+      [SelfId = actor_id(this), peer = peer_id_short_, dht = dht_node_](td::Result<AdnlNode> R) {
+        if (R.is_error()) {
+          td::actor::send_closure(SelfId, &AdnlPeerPairImpl::request_reverse_ping_result, R.move_as_error());
+          return;
+        }
+        td::actor::send_closure(
+            dht, &dht::Dht::request_reverse_ping, R.move_as_ok(), peer, [SelfId](td::Result<td::Unit> R) {
+              td::actor::send_closure(SelfId, &AdnlPeerPairImpl::request_reverse_ping_result, std::move(R));
+            });
+      });
+}
+
+void AdnlPeerPairImpl::request_reverse_ping_result(td::Result<td::Unit> R) {
+  request_reverse_ping_active_ = false;
+  if (R.is_ok()) {
+    VLOG(ADNL_INFO) << this << ": reverse ping requested";
+  } else {
+    VLOG(ADNL_INFO) << this << ": failed to request reverse ping: " << R.move_as_error();
+  }
+}
+
 }  // namespace adnl
 
 }  // namespace ton

adnl/adnl-peer.hpp

@@ -153,6 +153,9 @@ class AdnlPeerPairImpl : public AdnlPeerPair {
     }
   }
 
+  void request_reverse_ping();
+  void request_reverse_ping_result(td::Result<td::Unit> R);
+
   struct Conn {
     class ConnCallback : public AdnlNetworkConnection::Callback {
      public:
@@ -250,6 +253,10 @@ class AdnlPeerPairImpl : public AdnlPeerPair {
   td::Timestamp next_dht_query_at_ = td::Timestamp::never();
   td::Timestamp next_db_update_at_ = td::Timestamp::never();
   td::Timestamp retry_send_at_ = td::Timestamp::never();
+
+  bool has_reverse_addr_ = false;
+  td::Timestamp request_reverse_ping_after_ = td::Timestamp::now();
+  bool request_reverse_ping_active_ = false;
 };
 
 class AdnlPeerImpl : public AdnlPeer {

dht/dht-in.hpp

@@ -66,6 +66,15 @@ class DhtMemberImpl : public DhtMember {
 
   DhtKeyId last_republish_key_ = DhtKeyId::zero();
   DhtKeyId last_check_key_ = DhtKeyId::zero();
+  adnl::AdnlNodeIdShort last_check_reverse_conn_ = adnl::AdnlNodeIdShort::zero();
+
+  struct ReverseConnection {
+    adnl::AdnlNodeIdShort dht_node_;
+    DhtKeyId key_id_;
+    td::Timestamp ttl_;
+  };
+  std::map<adnl::AdnlNodeIdShort, ReverseConnection> reverse_connections_;
+  std::set<adnl::AdnlNodeIdShort> our_reverse_connections_;
 
   class Callback : public adnl::Adnl::Callback {
    public:
@@ -122,6 +131,10 @@ class DhtMemberImpl : public DhtMember {
   void process_query(adnl::AdnlNodeIdShort src, ton_api::dht_store &query, td::Promise<td::BufferSlice> promise);
   void process_query(adnl::AdnlNodeIdShort src, ton_api::dht_getSignedAddressList &query,
                      td::Promise<td::BufferSlice> promise);
+  void process_query(adnl::AdnlNodeIdShort src, ton_api::dht_registerReverseConnection &query,
+                     td::Promise<td::BufferSlice> promise);
+  void process_query(adnl::AdnlNodeIdShort src, ton_api::dht_requestReversePing &query,
+                     td::Promise<td::BufferSlice> promise);
 
  public:
   DhtMemberImpl(adnl::AdnlNodeIdShort id, std::string db_root, td::actor::ActorId<keyring::Keyring> keyring,
@@ -143,6 +156,12 @@ class DhtMemberImpl : public DhtMember {
   void set_value(DhtValue key_value, td::Promise<td::Unit> result) override;
   td::uint32 distance(DhtKeyId key_id, td::uint32 max_value);
 
+  void register_reverse_connection(adnl::AdnlNodeIdFull client, td::Promise<td::Unit> promise) override;
+  void request_reverse_ping(adnl::AdnlNode target, adnl::AdnlNodeIdShort client,
+                            td::Promise<td::Unit> promise) override;
+  void request_reverse_ping_cont(adnl::AdnlNode target, td::BufferSlice signature, adnl::AdnlNodeIdShort client,
+                                 td::Promise<td::Unit> promise);
+
   td::Status store_in(DhtValue value) override;
   void send_store(DhtValue value, td::Promise<td::Unit> promise);
 

dht/dht-query.cpp

@@ -279,6 +279,131 @@ void DhtQueryStore::store_ready(td::Result<td::BufferSlice> R) {
   }
 }
 
+DhtQueryRegisterReverseConnection::DhtQueryRegisterReverseConnection(
+    DhtKeyId key_id, adnl::AdnlNodeIdFull client, td::uint32 ttl, td::BufferSlice signature,
+    DhtMember::PrintId print_id, adnl::AdnlNodeIdShort src, DhtNodesList list, td::uint32 k, td::uint32 a, DhtNode self,
+    bool client_only, td::actor::ActorId<DhtMember> node, td::actor::ActorId<adnl::Adnl> adnl,
+    td::Promise<td::Unit> promise)
+    : print_id_(print_id)
+    , k_(k)
+    , a_(a)
+    , promise_(std::move(promise))
+    , key_id_(key_id)
+    , list_(std::move(list))
+    , self_(std::move(self))
+    , client_only_(client_only) {
+  node_ = node;
+  adnl_ = adnl;
+  src_ = src;
+  query_ = create_serialize_tl_object<ton_api::dht_registerReverseConnection>(client.tl(), ttl, std::move(signature));
+}
+
+void DhtQueryRegisterReverseConnection::start_up() {
+  auto P = td::PromiseCreator::lambda([SelfId = actor_id(this)](td::Result<DhtNodesList> res) {
+    td::actor::send_closure(SelfId, &DhtQueryRegisterReverseConnection::send_queries, std::move(res));
+  });
+
+  auto A = td::actor::create_actor<DhtQueryFindNodes>("FindNodesQuery", key_id_, print_id_, src_, std::move(list_), k_,
+                                                      a_, self_.clone(), client_only_, node_, adnl_, std::move(P));
+  A.release();
+}
+
+void DhtQueryRegisterReverseConnection::send_queries(td::Result<DhtNodesList> R) {
+  if (R.is_error()) {
+    auto S = R.move_as_error();
+    VLOG(DHT_NOTICE) << this << ": failed to get nearest nodes to " << key_id_ << ": " << S;
+    promise_.set_error(std::move(S));
+    stop();
+    return;
+  }
+  auto list = R.move_as_ok();
+
+  remaining_ = static_cast<td::uint32>(list.size());
+  if (remaining_ == 0) {
+    VLOG(DHT_NOTICE) << this << ": failed to get nearest nodes to " << key_id_ << ": no nodes";
+    promise_.set_error(td::Status::Error("no dht nodes"));
+    stop();
+    return;
+  }
+
+  for (auto &node : list.list()) {
+    auto P = td::PromiseCreator::lambda([SelfId = actor_id(this)](td::Result<td::BufferSlice> R) {
+      td::actor::send_closure(SelfId, &DhtQueryRegisterReverseConnection::ready, std::move(R));
+    });
+    td::actor::send_closure(adnl_, &adnl::Adnl::send_query, src_, node.adnl_id().compute_short_id(), "dht regrevcon",
+                            std::move(P), td::Timestamp::in(2.0 + td::Random::fast(0, 20) * 0.1), query_.clone());
+  }
+}
+
+void DhtQueryRegisterReverseConnection::ready(td::Result<td::BufferSlice> R) {
+  if (R.is_error()) {
+    fail_++;
+    VLOG(DHT_INFO) << this << ": failed register reverse connection query: " << R.move_as_error();
+  } else {
+    auto R2 = fetch_tl_object<ton_api::dht_stored>(R.move_as_ok(), true);
+    if (R2.is_error()) {
+      fail_++;
+      VLOG(DHT_WARNING) << this << ": can not parse answer (expected dht.stored): " << R2.move_as_error();
+    } else {
+      success_++;
+    }
+  }
+  CHECK(remaining_ > 0);
+  remaining_--;
+  if (remaining_ == 0) {
+    if (success_ > 0) {
+      promise_.set_value(td::Unit());
+    } else {
+      promise_.set_result(td::Status::Error("failed to make actual query"));
+    }
+    stop();
+  }
+}
+
+void DhtQueryRequestReversePing::send_one_query(adnl::AdnlNodeIdShort id) {
+  td::BufferSlice B;
+  if (client_only_) {
+    B = query_.clone();
+  } else {
+    B = create_serialize_tl_object_suffix<ton_api::dht_query>(query_.as_slice(), self_.tl());
+  }
+  auto P = td::PromiseCreator::lambda([SelfId = actor_id(this), dst = id](td::Result<td::BufferSlice> R) {
+    td::actor::send_closure(SelfId, &DhtQueryRequestReversePing::on_result, std::move(R), dst);
+  });
+  td::actor::send_closure(adnl_, &adnl::Adnl::send_query, get_src(), id, "dht requestReversePing", std::move(P),
+                          td::Timestamp::in(2.0 + td::Random::fast(0, 20) * 0.1), std::move(B));
+}
+
+void DhtQueryRequestReversePing::on_result(td::Result<td::BufferSlice> R, adnl::AdnlNodeIdShort dst) {
+  if (R.is_error()) {
+    VLOG(DHT_INFO) << this << ": failed reverse ping query " << get_src() << "->" << dst << ": " << R.move_as_error();
+    finish_query();
+    return;
+  }
+  auto Res = fetch_tl_object<ton_api::dht_ReversePingResult>(R.move_as_ok(), true);
+  if (Res.is_error()) {
+    VLOG(DHT_WARNING) << this << ": dropping incorrect answer on dht.requestReversePing query from " << dst << ": "
+                      << Res.move_as_error();
+    finish_query();
+    return;
+  }
+
+  auto A = Res.move_as_ok();
+  ton_api::downcast_call(*A, td::overloaded(
+                                 [&](ton_api::dht_reversePingOk &v) {
+                                   promise_.set_value(td::Unit());
+                                   stop();
+                                 },
+                                 [&](ton_api::dht_clientNotFound &v) {
+                                   add_nodes(DhtNodesList{std::move(v.nodes_)});
+                                   finish_query();
+                                 }));
+}
+
+void DhtQueryRequestReversePing::finish(DhtNodesList list) {
+  promise_.set_error(td::Status::Error(ErrorCode::notready, "dht key not found"));
+}
+
 }  // namespace dht
 
 }  // namespace ton

dht/dht-query.hpp

@@ -165,6 +165,62 @@ class DhtQueryStore : public td::actor::Actor {
   }
 };
 
+class DhtQueryRegisterReverseConnection : public td::actor::Actor {
+ private:
+  DhtMember::PrintId print_id_;
+  td::uint32 k_;
+  td::uint32 a_;
+  td::Promise<td::Unit> promise_;
+  td::actor::ActorId<DhtMember> node_;
+  td::actor::ActorId<adnl::Adnl> adnl_;
+  adnl::AdnlNodeIdShort src_;
+  DhtKeyId key_id_;
+  td::BufferSlice query_;
+  td::uint32 success_ = 0;
+  td::uint32 fail_ = 0;
+  td::uint32 remaining_;
+  DhtNodesList list_;
+  DhtNode self_;
+  bool client_only_;
+
+ public:
+  DhtQueryRegisterReverseConnection(DhtKeyId key_id, adnl::AdnlNodeIdFull client, td::uint32 ttl,
+                                    td::BufferSlice signature, DhtMember::PrintId print_id, adnl::AdnlNodeIdShort src,
+                                    DhtNodesList list, td::uint32 k, td::uint32 a, DhtNode self, bool client_only,
+                                    td::actor::ActorId<DhtMember> node, td::actor::ActorId<adnl::Adnl> adnl,
+                                    td::Promise<td::Unit> promise);
+  void send_queries(td::Result<DhtNodesList> R);
+  void ready(td::Result<td::BufferSlice> R);
+  void start_up() override;
+  DhtMember::PrintId print_id() const {
+    return print_id_;
+  }
+};
+
+class DhtQueryRequestReversePing : public DhtQuery {
+ private:
+  td::Promise<td::Unit> promise_;
+  td::BufferSlice query_;
+
+ public:
+  DhtQueryRequestReversePing(adnl::AdnlNodeIdShort client, adnl::AdnlNode target, td::BufferSlice signature,
+                             DhtMember::PrintId print_id, adnl::AdnlNodeIdShort src, DhtNodesList list, td::uint32 k,
+                             td::uint32 a, DhtNode self, bool client_only, td::actor::ActorId<DhtMember> node,
+                             td::actor::ActorId<adnl::Adnl> adnl, td::Promise<td::Unit> promise)
+      : DhtQuery(DhtMember::get_reverse_connection_key(client).compute_key_id(), print_id, src, std::move(list), k, a,
+                 std::move(self), client_only, node, adnl)
+      , promise_(std::move(promise))
+      , query_(create_serialize_tl_object<ton_api::dht_requestReversePing>(target.tl(), std::move(signature),
+                                                                           client.bits256_value(), k)) {
+  }
+  void send_one_query(adnl::AdnlNodeIdShort id) override;
+  void on_result(td::Result<td::BufferSlice> R, adnl::AdnlNodeIdShort dst);
+  void finish(DhtNodesList list) override;
+  std::string get_name() const override {
+    return "request remote ping";
+  }
+};
+
 inline td::StringBuilder &operator<<(td::StringBuilder &sb, const DhtQuery &dht) {
   sb << dht.print_id();
   return sb;

dht/dht.cpp

@@ -90,8 +90,11 @@ void DhtMemberImpl::start_up() {
                                     ton_api::dht_findValue::ID,
                                     ton_api::dht_store::ID,
                                     ton_api::dht_ping::ID,
+                                    ton_api::dht_registerReverseConnection::ID,
+                                    ton_api::dht_requestReversePing::ID,
                                     ton_api::dht_query::ID,
-                                    ton_api::dht_message::ID};
+                                    ton_api::dht_message::ID,
+                                    ton_api::dht_requestReversePingCont::ID};
 
   for (auto it : methods) {
     td::actor::send_closure(adnl_, &adnl::Adnl::subscribe, id_, adnl::Adnl::int_to_bytestring(it),
@@ -131,8 +134,11 @@ void DhtMemberImpl::tear_down() {
                                     ton_api::dht_findValue::ID,
                                     ton_api::dht_store::ID,
                                     ton_api::dht_ping::ID,
+                                    ton_api::dht_registerReverseConnection::ID,
+                                    ton_api::dht_requestReversePing::ID,
                                     ton_api::dht_query::ID,
-                                    ton_api::dht_message::ID};
+                                    ton_api::dht_message::ID,
+                                    ton_api::dht_requestReversePingCont::ID};
 
   for (auto it : methods) {
     td::actor::send_closure(adnl_, &adnl::Adnl::unsubscribe, id_, adnl::Adnl::int_to_bytestring(it));
@@ -299,6 +305,61 @@ void DhtMemberImpl::process_query(adnl::AdnlNodeIdShort src, ton_api::dht_getSig
   get_self_node(std::move(P));
 }
 
+static td::BufferSlice register_reverse_connection_to_sign(adnl::AdnlNodeIdShort client, adnl::AdnlNodeIdShort dht_id,
+                                                           td::uint32 ttl) {
+  td::BufferSlice result(32 + 32 + 4);
+  td::MutableSlice s = result.as_slice();
+  s.copy_from(client.as_slice());
+  s.remove_prefix(32);
+  s.copy_from(dht_id.as_slice());
+  s.remove_prefix(32);
+  s.copy_from(std::string(reinterpret_cast<char *>(&ttl), 4));
+  return result;
+}
+
+void DhtMemberImpl::process_query(adnl::AdnlNodeIdShort src, ton_api::dht_registerReverseConnection &query,
+                                  td::Promise<td::BufferSlice> promise) {
+  td::uint32 ttl = query.ttl_, now = (td::uint32)td::Clocks::system();
+  if (ttl <= now) {
+    return;
+  }
+  PublicKey pub{query.node_};
+  adnl::AdnlNodeIdShort client_id{pub.compute_short_id()};
+  td::BufferSlice to_sign = register_reverse_connection_to_sign(client_id, src, ttl);
+  TRY_RESULT_PROMISE(promise, encryptor, pub.create_encryptor());
+  TRY_STATUS_PROMISE(promise, encryptor->check_signature(to_sign, query.signature_));
+  DhtKeyId key_id = get_reverse_connection_key(client_id).compute_key_id();
+  reverse_connections_[client_id] = ReverseConnection{src, key_id, td::Timestamp::at_unix(std::min(ttl, now + 300))};
+  promise.set_value(create_serialize_tl_object<ton_api::dht_stored>());
+}
+
+void DhtMemberImpl::process_query(adnl::AdnlNodeIdShort src, ton_api::dht_requestReversePing &query,
+                                  td::Promise<td::BufferSlice> promise) {
+  adnl::AdnlNodeIdShort client{query.client_};
+  auto it = reverse_connections_.find(client);
+  if (it != reverse_connections_.end()) {
+    if (it->second.ttl_.is_in_past()) {
+      reverse_connections_.erase(it);
+    } else {
+      PublicKey pub{query.target_->id_};
+      TRY_RESULT_PROMISE(promise, encryptor, pub.create_encryptor());
+      TRY_STATUS_PROMISE(promise,
+                         encryptor->check_signature(serialize_tl_object(query.target_, true), query.signature_));
+      td::actor::send_closure(adnl_, &adnl::Adnl::send_message, id_, it->second.dht_node_,
+                              create_serialize_tl_object<ton_api::dht_requestReversePingCont>(
+                                  std::move(query.target_), std::move(query.signature_), query.client_));
+      promise.set_result(create_serialize_tl_object<ton_api::dht_reversePingOk>());
+      return;
+    }
+  }
+  auto k = static_cast<td::uint32>(query.k_);
+  if (k > max_k()) {
+    k = max_k();
+  }
+  auto R = get_nearest_nodes(get_reverse_connection_key(client).compute_key_id(), k);
+  promise.set_value(create_serialize_tl_object<ton_api::dht_clientNotFound>(R.tl()));
+}
+
 void DhtMemberImpl::receive_query(adnl::AdnlNodeIdShort src, td::BufferSlice data,
                                   td::Promise<td::BufferSlice> promise) {
   if (client_only_) {
@@ -369,6 +430,27 @@ void DhtMemberImpl::receive_ping(DhtKeyId key, DhtNode result) {
 }
 
 void DhtMemberImpl::receive_message(adnl::AdnlNodeIdShort src, td::BufferSlice data) {
+  auto F = fetch_tl_object<ton_api::dht_requestReversePingCont>(data, true);
+  if (F.is_ok()) {
+    auto S = [&]() -> td::Status {
+      auto f = F.move_as_ok();
+      adnl::AdnlNodeIdShort client{f->client_};
+      if (!our_reverse_connections_.count(client)) {
+        return td::Status::Error(PSTRING() << ": unknown id for reverse ping: " << client);
+      }
+      TRY_RESULT_PREFIX(node, adnl::AdnlNode::create(f->target_), "failed to parse node: ");
+      TRY_RESULT_PREFIX(encryptor, node.pub_id().pubkey().create_encryptor(), "failed to create encryptor: ");
+      TRY_STATUS_PREFIX(encryptor->check_signature(serialize_tl_object(f->target_, true), f->signature_),
+                        "invalid signature: ");
+      VLOG(DHT_INFO) << this << ": sending reverse ping to " << node.compute_short_id();
+      td::actor::send_closure(adnl_, &adnl::Adnl::add_peer, client, node.pub_id(), node.addr_list());
+      td::actor::send_closure(adnl_, &adnl::Adnl::send_message, client, node.compute_short_id(), td::BufferSlice());
+      return td::Status::OK();
+    }();
+    if (S.is_error()) {
+      VLOG(DHT_INFO) << this << ": " << S;
+    }
+  }
 }
 
 void DhtMemberImpl::set_value(DhtValue value, td::Promise<td::Unit> promise) {
@@ -396,6 +478,70 @@ void DhtMemberImpl::get_value_in(DhtKeyId key, td::Promise<DhtValue> result) {
   get_self_node(std::move(P));
 }
 
+void DhtMemberImpl::register_reverse_connection(adnl::AdnlNodeIdFull client, td::Promise<td::Unit> promise) {
+  auto client_short = client.compute_short_id();
+  td::uint32 ttl = (td::uint32)td::Clocks::system() + 300;
+  our_reverse_connections_.insert(client_short);
+  auto key_id = get_reverse_connection_key(client_short).compute_key_id();
+  td::actor::send_closure(keyring_, &keyring::Keyring::sign_message, client_short.pubkey_hash(),
+                          register_reverse_connection_to_sign(client_short, id_, ttl),
+                          [=, print_id = print_id(), list = get_nearest_nodes(key_id, k_), SelfId = actor_id(this),
+                           promise = std::move(promise)](td::Result<td::BufferSlice> R) mutable {
+                            TRY_RESULT_PROMISE_PREFIX(promise, signature, std::move(R), "Failed to sign: ");
+                            td::actor::send_closure(SelfId, &DhtMemberImpl::get_self_node,
+                                                    [=, list = std::move(list), signature = std::move(signature),
+                                                     promise = std::move(promise)](td::Result<DhtNode> R) mutable {
+                                                      R.ensure();
+                                                      td::actor::create_actor<DhtQueryRegisterReverseConnection>(
+                                                          "RegisterReverseQuery", key_id, std::move(client), ttl,
+                                                          std::move(signature), print_id, id_, std::move(list), k_, a_,
+                                                          R.move_as_ok(), client_only_, SelfId, adnl_,
+                                                          std::move(promise))
+                                                          .release();
+                                                    });
+                          });
+}
+
+void DhtMemberImpl::request_reverse_ping(adnl::AdnlNode target, adnl::AdnlNodeIdShort client,
+                                         td::Promise<td::Unit> promise) {
+  auto pubkey_hash = target.compute_short_id().pubkey_hash();
+  td::BufferSlice to_sign = serialize_tl_object(target.tl(), true);
+  td::actor::send_closure(keyring_, &keyring::Keyring::sign_message, pubkey_hash, std::move(to_sign),
+                          [SelfId = actor_id(this), promise = std::move(promise), target = std::move(target),
+                           client](td::Result<td::BufferSlice> R) mutable {
+                            TRY_RESULT_PROMISE(promise, signature, std::move(R));
+                            td::actor::send_closure(SelfId, &DhtMemberImpl::request_reverse_ping_cont,
+                                                    std::move(target), std::move(signature), client,
+                                                    std::move(promise));
+                          });
+}
+
+void DhtMemberImpl::request_reverse_ping_cont(adnl::AdnlNode target, td::BufferSlice signature,
+                                              adnl::AdnlNodeIdShort client, td::Promise<td::Unit> promise) {
+  auto it = reverse_connections_.find(client);
+  if (it != reverse_connections_.end()) {
+    if (it->second.ttl_.is_in_past()) {
+      reverse_connections_.erase(it);
+    } else {
+      td::actor::send_closure(adnl_, &adnl::Adnl::send_message, id_, it->second.dht_node_,
+                              create_serialize_tl_object<ton_api::dht_requestReversePingCont>(
+                                  target.tl(), std::move(signature), client.bits256_value()));
+      promise.set_result(td::Unit());
+      return;
+    }
+  }
+  auto key_id = get_reverse_connection_key(client).compute_key_id();
+  get_self_node([=, target = std::move(target), signature = std::move(signature), promise = std::move(promise),
+                 SelfId = actor_id(this), print_id = print_id(), list = get_nearest_nodes(key_id, k_),
+                 client_only = client_only_](td::Result<DhtNode> R) mutable {
+    R.ensure();
+    td::actor::create_actor<DhtQueryRequestReversePing>("RequestReversePing", client, std::move(target),
+                                                        std::move(signature), print_id, id_, std::move(list), k_, a_,
+                                                        R.move_as_ok(), client_only, SelfId, adnl_, std::move(promise))
+        .release();
+  });
+}
+
 void DhtMemberImpl::check() {
   VLOG(DHT_INFO) << this << ": ping=" << ping_queries_ << " fnode=" << find_node_queries_
                  << " fvalue=" << find_value_queries_ << " store=" << store_queries_
@@ -454,6 +600,16 @@ void DhtMemberImpl::check() {
       }
     }
   }
+  if (reverse_connections_.size() > 0) {
+    auto it = reverse_connections_.upper_bound(last_check_reverse_conn_);
+    if (it == reverse_connections_.end()) {
+      it = reverse_connections_.begin();
+    }
+    last_check_reverse_conn_ = it->first;
+    if (it->second.ttl_.is_in_past()) {
+      reverse_connections_.erase(it);
+    }
+  }
 
   if (republish_att_.is_in_past()) {
     auto it = our_values_.lower_bound(last_republish_key_);

dht/dht.h

@@ -54,6 +54,10 @@ class Dht : public td::actor::Actor {
   virtual void set_value(DhtValue key_value, td::Promise<td::Unit> result) = 0;
   virtual void get_value(DhtKey key, td::Promise<DhtValue> result) = 0;
 
+  virtual void register_reverse_connection(adnl::AdnlNodeIdFull client, td::Promise<td::Unit> promise) = 0;
+  virtual void request_reverse_ping(adnl::AdnlNode target, adnl::AdnlNodeIdShort client,
+                                    td::Promise<td::Unit> promise) = 0;
+
   virtual void dump(td::StringBuilder &sb) const = 0;
 
   virtual ~Dht() = default;

dht/dht.hpp

@@ -101,6 +101,10 @@ class DhtMember : public Dht {
   virtual void get_self_node(td::Promise<DhtNode> promise) = 0;
 
   virtual PrintId print_id() const = 0;
+
+  static DhtKey get_reverse_connection_key(adnl::AdnlNodeIdShort node) {
+    return DhtKey{node.pubkey_hash(), "address", 0};
+  }
 };
 
 inline td::StringBuilder &operator<<(td::StringBuilder &sb, const DhtMember::PrintId &id) {

tl/generate/scheme/ton_api.tl

@@ -68,6 +68,7 @@ adnl.address.udp6 ip:int128 port:int = adnl.Address;
 //adnl.address.tcp6 ip:int128 port:int = adnl.Address;
 
 adnl.address.tunnel to:int256 pubkey:PublicKey = adnl.Address; 
+adnl.address.reverse = adnl.Address;
 
 adnl.addressList addrs:(vector adnl.Address) version:int reinit_date:int priority:int expire_at:int = adnl.AddressList;
 
@@ -183,8 +184,12 @@ dht.pong random_id:long = dht.Pong;
 dht.valueNotFound nodes:dht.nodes = dht.ValueResult;
 dht.valueFound value:dht.Value = dht.ValueResult;
 
+dht.clientNotFound nodes:dht.nodes = dht.ReversePingResult;
+dht.reversePingOk = dht.ReversePingResult;
+
 dht.stored = dht.Stored;
 dht.message node:dht.node = dht.Message;
+dht.requestReversePingCont target:adnl.Node signature:bytes client:int256 = dht.RequestReversePingCont;
 
 dht.db.bucket nodes:dht.nodes = dht.db.Bucket;
 dht.db.key.bucket id:int = dht.db.Key;
@@ -196,6 +201,8 @@ dht.store value:dht.value = dht.Stored;
 dht.findNode key:int256 k:int = dht.Nodes; 
 dht.findValue key:int256 k:int = dht.ValueResult; 
 dht.getSignedAddressList = dht.Node;
+dht.registerReverseConnection node:PublicKey ttl:int signature:bytes = dht.Stored;
+dht.requestReversePing target:adnl.Node signature:bytes client:int256 k:int = dht.ReversePingResult;
 
 dht.query node:dht.node = True;