Lab #3: Introduction to Java Sockets - PoliformaT - UPV

P3­4 Computer Networks Laboratory3.How to obtain information about the establishedconnection?The Socket class has several methods that allow to obtain data onthe connection established between the client and the server. Among themwe can mention getLocalAddress() and getInetAddress(), thatreturns the IP local and remote address, respectively, and getLocalPort()and getPort(), that returns the local and remote ports, respectively. Next,the mentioned methods are detailed briefly. You can consult moreinformation of them in the Sun Webpage, that is referenced at the beginningof the practice• public int getPort(): it returns the remote port to whichSocket is connected.• public InetAddress getInetAddress(): it returns remote IPaddress of which Socket is connected.• public int getLocalPort(): it returns the local port to whichthe Socket is associated.• public InetAddress getLocalAddress(): It returns the localIP address local to which the Socket is associated.Exercise 3:Modify the client “ClientTCP2.java” of the previous exercise in order toshow the information in the screen of the client on the established connection(local and remote IP address and port numbers).Test it four times in a row, connect with the “zoltar.redes.upv.es” server inport 25 and verify which values are modified. Interpret the obtained result.

Lab #3: Introduction to Java Sockets P3­54.How read the received data trough the connection?In order to read the data that are being received through the Socketwe will use the method getInputStream of the Socket class. Thismethod returns an object of the InputStream type (flow of input bytes),which adjusts correctly to the TCP philosophy of transmission oriented tocontinuous flow of data. However applications usually need to exchangelines of text.The Scanner class is a “bridge” from the flows of bytes to those ofcharacter lines: read bytes and encodes them into characters. Data can thenbe read using several methods that make programmers life easier like:Scanner input = new Scanner(s.getInputStream());where “s” is the Socket object defined previously.When a program reads from a Scanner, the text is extracted from abuffer instead of acessing directly to the input stream. When the bufferbecomes empty, it is filled again with as much text as it is possible, eventhough not everything be needed. This will cause that the future readings becarried out more quickly.In our case, using the method nextLine() of the Scanner class wewill be able to read the answers of the server as complete text lines. Thismethod reads a line of text and it returns it as a String.Exercise 4:Rename the client “ClientTCP2.java” of the previous exercise as“ClientSMTP.java”. Modify it in such a way it is always connect to theport 25 and shows the first line of text received from the server.5.How to send data through the connectionTo write through a Socket also is more efficient to use a class thatprovides certain capacity of storage (buffering). For this reason, to writethrough a Socket, we will use an object of the classjava.io.PrintWriter connected to the output flow of the Socket. This

Lab #3: Introduction to Java Sockets P3­7the efficiency, output stream tries to fill its buffer as much as it is possiblebefore sending the data, but as the client does not have more data to send (atthe moment) its request will never be sent.The solution to this problem comes from the method flush() of thePrintWriter class. This method forces to the data be sent although thebuffer is still not full. In case of doubt about if it is or not necessary to use it,it is better to invoke it, since doing a unnecessary flush consumes fewresources, but to not use it when it is needed can produce blocking situationsin the program.Exercise 6:Modify the client SMTP so it uses the method flush and verify that now itworks correctly.We can empty the buffer in an automatic manner when writing on it(without having to use explicitly the method flush). For it we needed twothings:• The constructor of the PrintWriter class must be used as it hasbeen used before, with a second additional parameter equal to true.• The writing must be done by the method println(), instead of themethod print. In addition the method println adds the end ofline then we will not need to write it as part of the order to be send tothe client (unlike as we have done in the previous program).We are going to modify our client to verify this operation.Ejercicio 7:Modify the SMTP client to use the method println instead of print(remove what is not needed, like the sequence \r\n and flush). Verify thatit works returning to establish connection with the zoltar server in port 25.

Lab #3: Introduction to Java Sockets P3­9allows us to connect to us, means that there is a server listening in it. If itrejects the connection, then the port is closed. In this case of UPD, theexploration is not too easy, because it is a non connected service, thereforeif we don't receive an answer we will don't what had happened with ourmessage.In order to create the explorer of ports we are going use the Socketclass, that already we have already used in the previous exercises. Since wehave seen, its constructor can send two different exceptions:• UnknownHostException, is generated when the name of thecomputer with that we want to create the connection, cannot be solved toan IP address.• IOException, is generated when the connection cannot be establishedby any other reason, like for example that there is not a server listeningin the specified port in the parameters of Socket.We will use this second exception to create the explorer of ports, so ifwhile we sweep the ports of our equipment generates this exception, we willknow that the port is closed. If it is not generated, then there is a server inthat port. As the habitual thing is that most of the ports be closed, ourexplorer must show on the screen a message only when an open port isfound. On the contrary, usually it leaves so many messages then it will bedifficult to find out all the open ports.Exercise 8:Type and complete the following explorer of ports. Test it. Which ports areopen? Which services corresponds to those ports? (in /etc/services thereis a detailed listing of the services ordered by number of port).import java.net.*;import java.io.*;public class LowPortScanner {public static void main(String[] args) {String host = "localhost";for (int port = 1; port < 1024; port ++) {try {// COMPLETE THE CODE}

P3­10 Computer Networks Laboratorycatch (UnknownHostException e) {// COMPLETE THE CODE}catch (IOException e) {// COMPLETE THE CODE}} // end for} // end main} // end PortScannerExercise 9:Remove from your computer all the files you have created during thispractice.