Added support for reverse telnet (#1920)

* Added support for reverse telnet

* Fix issues with first command being ignored

* Fix incorrect client creation

* Integrate PR 1927

code/espurna/config/general.h

@@ -130,6 +130,13 @@
 #define TELNET_SERVER           TELNET_SERVER_ASYNC // Can be either TELNET_SERVER_ASYNC (using ESPAsyncTCP) or TELNET_SERVER_WIFISERVER (using WiFiServer)
 #endif
 
+// Enable this flag to add support for reverse telnet (+800 bytes)
+// This is useful to telnet to a device behind a NAT or firewall
+// To use this feature, start a listen server on a publicly reachable host with e.g. "ncat -vlp <port>" and use the MQTT reverse telnet command to connect
+#ifndef TELNET_REVERSE_SUPPORT
+#define TELNET_REVERSE_SUPPORT  0
+#endif
+
 //------------------------------------------------------------------------------
 // TERMINAL
 //------------------------------------------------------------------------------
@@ -1068,6 +1075,7 @@
 #define MQTT_TOPIC_IRIN             "irin"
 #define MQTT_TOPIC_IROUT            "irout"
 #define MQTT_TOPIC_OTA              "ota"
+#define MQTT_TOPIC_TELNET_REVERSE   "telnet_reverse"
 
 // Light module
 #define MQTT_TOPIC_CHANNEL          "channel"

code/espurna/telnet.ino

@@ -43,6 +43,58 @@ void _telnetWebSocketOnConnected(JsonObject& root) {
 
 #endif
 
+#if TELNET_REVERSE_SUPPORT
+    void _telnetReverse(const char * host, uint16_t port) {
+        DEBUG_MSG_P(PSTR("[TELNET] Connecting to reverse telnet on %s:%d\n"), host, port);
+
+        unsigned char i;
+        for (i = 0; i < TELNET_MAX_CLIENTS; i++) {
+            if (!_telnetClients[i] || !_telnetClients[i]->connected()) {
+                #if TELNET_SERVER == TELNET_SERVER_WIFISERVER
+                    _telnetClients[i] = std::make_unique<WiFiClient>();
+                #else // TELNET_SERVER_ASYNC
+                    _telnetClients[i] = std::make_unique<AsyncClient>();
+                #endif
+
+                if (_telnetClients[i]->connect(host, port)) {
+                    return _telnetNotifyConnected(i);
+                } else {
+                    DEBUG_MSG_P(PSTR("[TELNET] Error connecting reverse telnet\n"));
+                    return _telnetDisconnect(i);
+                }
+            }
+        }
+
+        //no free/disconnected spot so reject
+        if (i == TELNET_MAX_CLIENTS) {
+            DEBUG_MSG_P(PSTR("[TELNET] Failed too connect - too many clients connected\n"));
+        }
+    }
+
+    #if MQTT_SUPPORT
+    void _telnetReverseMQTTCallback(unsigned int type, const char * topic, const char * payload) {
+        if (type == MQTT_CONNECT_EVENT) {
+            mqttSubscribe(MQTT_TOPIC_TELNET_REVERSE);
+        } else if (type == MQTT_MESSAGE_EVENT) {
+            String t = mqttMagnitude((char *) topic);
+
+            if (t.equals(MQTT_TOPIC_TELNET_REVERSE)) {
+                String pl = String(payload);
+                int col = pl.indexOf(':');
+                if (col != -1) {
+                    String host = pl.substring(0, col);
+                    uint16_t port = pl.substring(col + 1).toInt();
+
+                    _telnetReverse(host.c_str(), port);
+                } else {
+                    DEBUG_MSG_P(PSTR("[TELNET] Incorrect reverse telnet value given, use the form \"host:ip\""));
+                }
+            }
+        }
+    }
+    #endif
+#endif
+
 void _telnetDisconnect(unsigned char clientId) {
     // ref: we are called from onDisconnect, async is already stopped
     #if TELNET_SERVER == TELNET_SERVER_WIFISERVER
@@ -127,6 +179,28 @@ void _telnetNotifyConnected(unsigned char i) {
 
     DEBUG_MSG_P(PSTR("[TELNET] Client #%u connected\n"), i);
 
+    #if TELNET_SERVER == TELNET_SERVER_ASYNC
+        _telnetClients[i]->onAck([i](void *s, AsyncClient *c, size_t len, uint32_t time) {
+        }, 0);
+
+        _telnetClients[i]->onData([i](void *s, AsyncClient *c, void *data, size_t len) {
+            _telnetData(i, data, len);
+        }, 0);
+
+        _telnetClients[i]->onDisconnect([i](void *s, AsyncClient *c) {
+            _telnetDisconnect(i);
+        }, 0);
+
+        _telnetClients[i]->onError([i](void *s, AsyncClient *c, int8_t error) {
+            DEBUG_MSG_P(PSTR("[TELNET] Error %s (%d) on client #%u\n"), c->errorToString(error), error, i);
+        }, 0);
+
+        _telnetClients[i]->onTimeout([i](void *s, AsyncClient *c, uint32_t time) {
+            DEBUG_MSG_P(PSTR("[TELNET] Timeout on client #%u at %lu\n"), i, time);
+            c->close();
+        }, 0);
+    #endif
+
     // If there is no terminal support automatically dump info and crash data
     #if TERMINAL_SUPPORT == 0
         info();
@@ -203,7 +277,7 @@ void _telnetLoop() {
                     char data[TERMINAL_BUFFER_SIZE];
                     size_t len = _telnetClients[i]->available();
                     unsigned int r = _telnetClients[i]->readBytes(data, min(sizeof(data), len));
-                    
+
                     _telnetData(i, data, r);
                 }
             }
@@ -238,26 +312,6 @@ void _telnetNewClient(AsyncClient* client) {
 
             _telnetClients[i] = std::unique_ptr<AsyncClient>(client);
 
-            _telnetClients[i]->onAck([i](void *s, AsyncClient *c, size_t len, uint32_t time) {
-            }, 0);
-
-            _telnetClients[i]->onData([i](void *s, AsyncClient *c, void *data, size_t len) {
-                _telnetData(i, data, len);
-            }, 0);
-
-            _telnetClients[i]->onDisconnect([i](void *s, AsyncClient *c) {
-                _telnetDisconnect(i);
-            }, 0);
-
-            _telnetClients[i]->onError([i](void *s, AsyncClient *c, int8_t error) {
-                DEBUG_MSG_P(PSTR("[TELNET] Error %s (%d) on client #%u\n"), c->errorToString(error), error, i);
-            }, 0);
-
-            _telnetClients[i]->onTimeout([i](void *s, AsyncClient *c, uint32_t time) {
-                DEBUG_MSG_P(PSTR("[TELNET] Timeout on client #%u at %lu\n"), i, time);
-                c->close();
-            }, 0);
-
             _telnetNotifyConnected(i);
             return;
         }
@@ -312,6 +366,27 @@ void telnetSetup() {
             .onKeyCheck(_telnetWebSocketOnKeyCheck);
     #endif
 
+    #if TELNET_REVERSE_SUPPORT
+        #if MQTT_SUPPORT
+            mqttRegister(_telnetReverseMQTTCallback);
+        #endif
+
+        #if TERMINAL_SUPPORT
+            terminalRegisterCommand(F("TELNET.REVERSE"), [](Embedis* e) {
+                if (e->argc < 3) {
+                    terminalError(F("Wrong arguments. Usage: TELNET.REVERSE <host> <port>"));
+                    return;
+                }
+
+                String host = String(e->argv[1]);
+                uint16_t port = String(e->argv[2]).toInt();
+
+                terminalOK();
+                _telnetReverse(host.c_str(), port);
+            });
+        #endif
+    #endif
+
     espurnaRegisterReload(_telnetConfigure);
     _telnetConfigure();