On server arduino, how to detect if client disconnects

Using Arduino Networking, Protocols, and Devices
1

Hello,
I am making a telnet port 23 server on an adafruit grandcentral with wiznet w5500 ethernet shield. I can detect when I initially connect with a putty terminal to the server. The server echoes back characters from the client.

I can't find out how can the arduino detect if the putty terminal client closes the connection.

Or alternatively, how do I detect that a new connection is initiated after a previous one was closed so that I can run again ''telnetClient.println("Hello, telnet client!\n\r ");'' Advice please.

Here's my server code. I won't include the setup() part that works. I run within the loop() the telnetProcess() function.

EthernetServer telnetServer(23);
boolean isTelnetAlreadyConnected = false;
void telnetProcess() {
	 
	// wait for a new client:
	EthernetClient telnetClient = telnetServer.available();

	// when the client sends the first byte, say hello:
	if (telnetClient) {
		if (telnetClient.connected() )		{
			digitalWrite(PIN_LED,			true);
		}else{
			//digitalWrite(PIN_LED,			false);//this does not detect that a client disconnected
		}
		if (!isTelnetAlreadyConnected) {
			// clean out the input buffer:
			telnetClient.flush();
			 
			Serial.print("telnetProcess() We have a new client");
			telnetClient.println("Hello, telnet client!\n\r ");
			isTelnetAlreadyConnected = true;
		}

		if (telnetClient.available() > 0) {
			// read the bytes incoming from the client:
			char thisChar = telnetClient.read();
			 
			// echo the bytes back to the client:
			telnetServer.write(thisChar);
			Serial.print(thisChar);
		}
	}else{
		//digitalWrite(PIN_LED,			false); this always runs if the connection is a live, but no characters are available
		if (telnetClient.connected() ==false)	{
			//digitalWrite(PIN_LED,			false); this always runs if the connection is a live, but no characters are available
		}
	}
	 
}
2

see the AdvancedChatServer example

3

The AdvancedCharServer does a good job at detecting and closing sessions where the client disconnects. I adapted portions of it in my code. Thank you so much!