HTML codes

http://www.quackit.com

Windows commands

2.3.2 Commands and tools (Windows) :-

Commands
1)date............ Display or set the date of the system
2)time............ Display or set the time of the system
3)ver............. Display the MS-DOS version that is being used
4)dir.............. Display the list of subdirectories and files of a directory
5)cls.............. Clear the screen
6)mkdir,
md directory..Make a directory with the name “directory”
Example: md tools
7)chdir, cd directory.. Display the name or change the current directory to “directory”
Example: cd tools
8)rmdir, rd directory.. Delete the directory with the name “directory”
Example: rd tools
9)tree directory....... Display the structure of folders of a path in text-graphic format
Example: tree c:\tools
10)chkdsk......... Check a disk and show a status report
11)mem........... Show the amount of memory used and free in the system
12)rename,
ren source dest
.......Change the name of files
Example: ren oldname newname
13)copy source dest....... Copy one or more files to another location
Example: copy c:\tools\myfile.txt c:\tmp
14)move source dest....... Move files and change the name of files and directories
Example: move c:\tools c:\tmp
15)type file............. Type the content of one or more text files
Example: type c:\tools\myfile.txt
16)more file........... Display the information screen by screen
Example: more c:\tools\myfile.txt
17)delete, del file....... Delete one or more files
Example: del c:\tools\myfile.txt

you can check many commands from net

Note: The words in italics are not commands, and must be replaced by the desired
values. Some of the commands can be used by typing either their long version or short
version; for example, "delete" and "del‚" are the same command.

Tools :-

1)ping host Verify contact with the machine “host”
The command ping sends "packets" using ICMP (Internet Control
Message Protocol) to another computer, to learn whether it is
accessible through the network. In addition, it shows a statistical
summary about the percentage of packets that have not been
answered and the response time. The name of the machine can
be used directly or its IP address.
Examples: ping http://www.google.com
ping 193.145.85.2
Some options are:
- n N: send N packets
- t: ping the specified host until stopped (press CTRL+C to end)
To see more options: ping /h

2)tracert host Show the route that packets follow to reach the machine “host”
The command tracert is the abbreviation of trace route, which
allows you to learn the route that a packet follows from the origin,
(your machine) to the destination machine. It can also tell you
the time it takes to make each jump. At the most, 30 jumps will be
listed. It is sometimes interesting to observe the names of the
machines through which the packets travel.

Examples: tracert http://www.google.com

tracert 193.145.85.2
Some options are:
- h N: to specify N, at the most, jumps.
- d: to not show the names of the machines.
To see more options: tracert

3)ipconfig Display information on the active interfaces (ethernet, ppp, etc.)
in the computer.
Some options:
/all: to show more details
/renew name: renews connection with “name” when automatic
configuration with DHCP is used.
/release name: deactivates all matching connections when
automatic configuration with DHCP is used.
To see more options: ipconfig /?

4)route print Display the routing table
The command route serves to define static routes, to erase routes
or simply to see the state of the routes.
Some options:
print: to show the list of routes.
delete: to delete a route.
add: to add a route.
To see more options: route/?

5)netstat Displays information on the status of the network and established
connections with remote machines.
Some options:
-a: To sample all the connections and listening ports
-n: to display addresses and port numbers in numeric form
-e: to sample Ethernet statistics
For example: netstat - an
To see more options: netstat/?

Codepen for Web development

http://codepen.io/rssatnam/pen/LHluy

Erlang calculator

http://www.wirelesscommunication.nl/reference/chaptr04/cellplan/reuse.htm

Cloud computing

Cloud computing

Introduction

Cloud computing was and is still a much discussed subject in the field of computing and the internet. This happens because this technology has a lot of strong advantages but disadvantages as well. This technology enables centralized communication with computer resources and data over the network. These resources including computers, servers, printers, video game consoles, etc are connected to the wider network infrastructure. Data is usually stored in servers located in massive data centres. It is not a new concept, as its general idea is based on an older computing model known as “Timesharing”. It is the result of more than forty years of evolution.  Cloud computing nowadays, is evolving faster than ever before, and day by day, the number of companies using is increasing. In this article we are going to present both some of its strengths and weaknesses and how it works.

Cloud structure

The principal structure of Cloud computing involves two parts which are connected to each other via a network:

     The Front end

In a “Front end” example, a computer acts as a client and includes the application needed to gain access to the back end (Cloud service). Such applications are known as APIs (Application Programming Interface). APIs are protocols used as user interfaces for providing various functions of software communication.

The best way to think about cloud computing is “Data Centre with API”. [3]

Different applications use different interfaces. Some applications enable network connection directly through the web (e.g. e-mail) and some others through specifically designed network interfaces.

The Back end

In a “Back end” example, the cloud service is communicating with a client’s computer. A cloud contains servers, computers, network routers, database storage machines and more. Also there are applications stored there, such as video editing, video gaming, word processing and anything that can fit in software. These applications can be accessed by a client computer, via communicating with the corresponding application’s servers as shown in Figure1.

A control node, typically a central server is used for administrating and monitoring the whole network’s operation.

Most of services provided, are transparently delivered, without the physical server knowing much about where the data is actually stored. This concept is similar to virtualization, where multiple resources seem like a single one.

In server environments, virtualization has become the norm with hypervisors providing for increased flexibility and improved utilization of physical assets [4].

Storage

Storing of the data takes place in massive data centers (Figure 2). Data centers house server machines, which act as database or application storage mediums.

As we know, servers often require maintenance or repair, and thus it is important to store copies of the data in multiple places. This is done or should be done to achieve redundancy of data. Without it, a cloud service provider cannot ensure its clients anywhere and anytime access of data. Also data is stored in servers using different power supplies or located in different places, to overcome the sub-sequence of a power failure event.

As we noted earlier, servers can store both data and applications. Some applications are web-based and can access the cloud through an internet browser. Such applications include:

- Web mail

·         - Social networking

·         - Multimedia albums

·         - Video hosting

·         - Online gaming

·         - Online documents

In the other hand, some applications require the use of specific network enabled interfaces. Such applications include:

·         - Antivirus software

·         - Network monitoring tools

·         - Operating system/software updating

·         - Data sharing (Drop box)

Advantages of the Cloud

Cost

The most important advantage is that it can reduce the cost of the infrastructure, of a company using it. It is easier to maintain and upgrade, because network infrastructure is centrally located and can be accessed both remotely and locally.

Storage

Storage space in a cloud can be continuously expanded, as there is no limit in disk sizes. Data is hosted in third party servers and it is the host’s responsibility to take care of such issues. Such data can be accessed via a web-based interface, e.g. Google’s “drive” feature, which enables saving files on the cloud.

Backing up and recovery of data is also easier to be implemented. Most third parties will keep copies of the data, and thus it will be easier for it, to be recovered. Furthermore, data is stored in a safer place than a local physical device, in terms of objective loss or destruction of data. This reliable and redundant storage of data can provide strong “Business continuity” and “Disaster recovery”. A minor concern about storage, is that despite that most cloud services try to issue storage concerns via redundancy, there's still the possibility that an entire system could be destroyed (e.g. fire, flooding), and data will be lost completely. So it is important not only to store data in different logical places but in physical too.

Subjective loss, theft and privacy of this data issues will be discussed later.

Remote access

Data stored in the cloud, can be accessed from anywhere at any given time, given that a network connection is provided. This enables geographical and time independence. An employer of a company can retrieve his office work from whenever he wants and thus improve his productivity. Carriage of physical storage devices is not needed, and thus possibility of physically losing it is not present. Other people can also be enabled to access your data remotely.

Disadvantages of the Cloud

Security

Cloud computing means Internet computing [5].

The biggest disadvantage of the internet today, is security. And the same happens for the cloud services, as those reside on the internet. Cloud data, is stored in third party servers and thus is vulnerable to loss or theft. Some hosting providers can be more reliable than others and hence it is important to choose the best one. As cloud service’s use increase, more and more hackers will try to break into the system. This reveals that strong encryption, decryption and authentication schemes need to be used. RADIUS protocol is an example of this discipline applied to a server infrastructure, and which provides a decent level of data protection.

Technical issues

Cloud computing need more resources, smarter computers and thus more capable processor units. There is also the need of excellent, 24/7 internet connection. If for any reason connections fail, the ability to access and upload data will be hugely affected. Also there will be times where systems will face hardware or software malfunctions, and if not back-up plans are deployed a serious outage will be caused. A company served by a cloud service provider, should also keep back-ups of its data locally too. This again reveals the lack of reliability of the cloud. If so many measures need to be taken, then what is for?

A serious service outage who I experienced myself too, was the Playstation network’s one, which lasted for 25 days and affected millions of users. In this case hackers attacked user accounts and the service had to face major changes in security [6]. Users pay a lot of money to such services, and are expecting their information to be kept securely and have 24/7 service available.

Privacy

Nowadays tons of user data resides on the internet. If somebody thinks that is not a cloud service user, then he should think twice. Facebook, twitter, gmail are only some of the web-applications that are keeping our personal information of all sorts of things. This data is circulated and stored in unknown third party storage places. Of course most of these provide service level agreement contracts that will cover various privacy concerns. Personal data should be processed in accordance to these contracts. Legal measures will be able to cover most of these concerns, but in some cases the laws will be applied only within the country that the company or the service resides. Legalisation of different areas of IT is very challenging and complicated, and will always struggle people and companies

Conclusion

Like most IT technologies, we saw that cloud computing too carries a lot of advantages and disadvantages. While the technology is totally evolutionary, it can also cause great harm to an amateur user, who do not understand and use it properly.

Most of people nowadays are unaware that the whole internet is dominated by cloud services. Therefore I believe that some computing privacy and security issues should be taught to younger people in schools, because by that way, they will possess a greater awareness of the dangers that surround the current and upcoming use of the internet.

From a business perspective, we saw that a very big opportunity is opened for smaller and bigger IT companies. The evolution of internet and computing speeds, gives the ability for efficient virtualisation and remote storage of data, with lots of benefits. It is up to the companies to fulfil user needs and eliminate the presence of some disadvantages. As internet security evolves and improves, so cloud computing will become better and more trusted.

Nikolas Georgiou

Prime number calculator

import java.util.Scanner;
//Start of class PrimeNumbers
public class PrimeNumbers {
/** Static boolean method that checks if an integer number is prime 
      * or not and returns true or false correspondingly
      */
static boolean checkNumber(int number){
     //for loop for checking integer numbers
        for(int i=2;i<number;i++){
            //if statement returning false if a number 
            //has not a remainder after division with number 2
            if(number%i==0) { 
                 return false; 
                }
             }
            return true;
          }
 
     public static void main(String[] args) {
            int num;
            Scanner scanNumber= new Scanner(System.in);
            System.out.print("Please enter a number: ");
            //Reads an integer value
            num=scanNumber.nextInt();
            //if-else statement to check if the entered number is negative 
            if (num<0){
                //if statements executed if entered number is negative
                System.err.println("Error!You inserted a negative number");
                System.err.println("The program will now exit");
                //Terminates the program
             System.exit(0);
            }
            //else statements executed if the entered number is not negative
             else {
                //Boolean variable for storing checkNumber method's variables
                // and checks if a number is prime or not
                boolean prime=checkNumber(num);
                //if checkNumber method returns true
                if (prime==true){
                    System.out.println("Is prime");
                }
                //if checkNumber method returns false
                 else{
                     System.out.println("Is not prime");
                     }
                System.out.println("Prime numbers <= entered number are:");
                 //for loop for checking all entered numbers
                 for(int i = 2; i <= num; i++ ){
//prime variable set to true
                    prime = true;
                    //for loop for checking and printing prime 
                    //numbers between 2 and the entered number
                    for(int j=2; j <= i/2; j++) {
                        //if statement that sets prime variable as false 
                        //if the number is not prime
                        if (i%j == 0){
                        prime = false;
                        }
                      }
                 //If statement that is executed not only if a number is prime
                 if (prime){
//Displays numbers seperated by a comma
                   System.out.print(i+",");
                   }
                } 
            }
        }

//End of class PrimeNumbers
} 

Multithreaded Web Server

import java.io.* ;
import java.net.* ;
import java.lang.*;
import java.util.* ;

//Start of class HttpRequest
final class HttpRequest implements Runnable
{
   final static String CRLF = "\r\n";
   public final static int SO_KEEPALIVE = 0x0008;
   //Set the default homepage to "index.html"
   private static final String DEFAULT_PAGE = "index.html";
 
   Socket socket;
   //Constructor
   public HttpRequest(Socket socket)throws Exception
   {
     this.socket = socket;
   }
 
   //Implement the run()method of the Runnable interface
   public void run()
       {
        try{
         processRequest();
 }
   catch (Exception e){
        System.out.println(e);
 }
   }
 
  //processRequest() method that processes individual HTTP requests

   private void processRequest() throws Exception
   {
 
String sentence;
//Get a reference to the socket's input and output streams
DataOutputStream os = new DataOutputStream (socket.getOutputStream());
//Set up input stream filters
BufferedReader bfReader = new BufferedReader (new   InputStreamReader(socket.getInputStream()));
sentence = bfReader.readLine() ;
//Get the request line of the HTTP request message.
String requestLine = sentence;

 //Display the request line in a structured way
 System.out.println();
 System.out.println("HTTP request message:");
 System.out.println("---------------------");
 System.out.println(requestLine);
 os.writeBytes("HTTP request message:"+CRLF);
 os.writeBytes("---------------------"+CRLF);
 os.writeBytes(requestLine+CRLF);

 //Get and display the header lines.
 String headerLine;
 while ((headerLine = bfReader.readLine()).length()!= 0){
    System.out.println(headerLine);
  }

    //Split the request line on white space
    String [] tokens=requestLine.split("\\s+" ,3);
    String fileName = tokens[1]; // second element is filename
    //Prepend a "." so that file request is within the current directory
    fileName = "."+fileName;
 //Filenames values is assigned with a file in the directory
 fileName = "homepage.html";

    FileInputStream fis = null;
 boolean fileExists=true;

          try{
              fis = new FileInputStream(fileName);
              }
             catch(FileNotFoundException e)
     {fileExists = false;}

try {

boolean append=true;
//Initialise  log string's value according to common log format
String log= InetAddress.getLocalHost()+" ["+(new Date()).toString()+"]" +" "+requestLine+" "+(new     Integer(fis.available())).toString() ;
//Write each web log generated to a text file
BufferedWriter out = new BufferedWriter(new FileWriter("webLogs.txt",append));
out.append(log);
//Writes each log line by line in the text file
out.newLine();
//closes the stream
out.close();
 
}
catch (IOException |NullPointerException e)
        {
            System.out.println("Log File Exception");    
        }

    //Construct the response message
    String statusLine = null;
    String contentTypeLine = null;
 String contentLengthLine =null;
    String entityBody="error";
 //New string hash for storing the MD5 hash's value
 String hash="";
 //Password is obtained from the URL's content
 String password =requestLine;
 //Username is obtained from the URL's content
 String user =requestLine;
 //URL is splitted in parts using the = symbol as delimiter
    String delimiter = "=";
    //Array of string to hold the split values of the URL
 String[] entry1;
 //Some parts of the URL are then thrown away
 user = user.replace("&pw","");
 entry1 = user.split(delimiter);
 // //Array of string to hold the split values of the URL
 String[] entry2;
 //Some parts of the URL are then thrown away
    password = password.replace("&action","");
    entry2 = password.split(delimiter);
       
 //If the filename is in the linked directory return the corresponding message
          if (fileExists){
              statusLine = "HTTP/1.0 200 OK";
              contentTypeLine = "Content-type:"  + contentType(fileName);
 //Return the size of the file in bytes
 contentLengthLine = "Content-length:" + (new Integer(fis.available())).toString();
          }
 //Else if the filename is not in the linked directory return the corresponding message
          else{
               statusLine = "HTTP/1.0 404 Not found";
               contentTypeLine="Content-type:text/html";
               entityBody ="<HTML>"+
                            "<HEAD><TITLE>404 Not Found</TITLE></HEAD>" +
                            "<BODY>404 Not Found"+"</BODY></HTML>" ;
  //Return the size of the file in bytes
  contentLengthLine="Content-length:" +(new Integer(entityBody.length()).toString());

       }
     //Try catch block for displaying authentication's output if carried out
       try{
 
  //Password string is assigned with the new extracted from the URL value
  password=entry2[2];
    hash=MD5(password);
//Display the username,password and MD5 hash code to the system
System.out.println("----------------------");
 System.out.println("Username:"+entry1[1]);
System.out.println("Password:"+entry2[2]);
System.out.println("MD5 hashcode:"+hash);
//If the URL's size is smaller than 48 bytes and credentials were not entered,
//an error that authentication was not carried out is displayed in the HTTP request.
if(requestLine.length()<48){
statusLine = "HTTP/1.0 403 Forbidden";
                    contentTypeLine="Content-type:text/html";
                    entityBody ="<HTML>" +
                            "<HEAD><TITLE>403 Forbidden</TITLE></HEAD>" +
                            "<BODY>NOT AUTHENTICATED" +                                            
                            "</BODY></HTML>" ;
       contentLengthLine="Content-length:" +(new Integer(entityBody.length()).toString());}

      }
 catch(ArrayIndexOutOfBoundsException|NullPointerException e)
            {hash="";}

//Send the Status line to server
    os.writeBytes(statusLine+CRLF);
os.writeBytes(contentTypeLine+CRLF);
  //Send the content length line
  os.writeBytes(contentLengthLine+CRLF);
 //Print a timestamp of the request
 os.writeBytes("Request Date:" + (new Date()).toString() + "\n");
    //Send a blank line to indicate the end of the header lines.
    os.writeBytes(CRLF);

        //Print the lines in the system output as well
      System.out.println("--------------------");
      System.out.println(statusLine);
      System.out.println(contentTypeLine);
      System.out.println(contentLengthLine);
      //Print a timestamp of the request
      System.out.println("Last Modified:" + (new Date()).toString());
      System.out.println("--------------------");
         //Flush the output
      System.out.flush();

          //Send the entity body
          if (fileExists){
              sendBytes(fis,os);
              fis.close();
          }
          else{
              os.writeBytes(entityBody);
       }

 //Close streams and sockets
 os.close();
 bfReader.close();
 socket.close();
 }
 
//Method to calculate an MD5 checksum of the entered password String in the HTML form
private static String MD5(String password) {
    try {
        //Create an object from java's security library
        java.security.MessageDigest md = java.security.MessageDigest.getInstance("MD5");
        byte[] array = md.digest(password.getBytes());
        StringBuilder sb = new StringBuilder();
//For loop for calculating each digit of the hash string
        for (int i = 0; i < array.length; ++i) {
          sb.append(Integer.toHexString((array[i] & 0xFF) | 0x100).substring(1,3));
       }
  //returns the hash string
        return sb.toString();

    } catch (java.security.NoSuchAlgorithmException e) {}
    return null;
}

   //ContentType() method that returns a String included in the content type line
   private static String contentType(String fileName)
   {  
   //if filename is of type html return the corresponding string
//and so on..
        if(fileName.endsWith(".htm")|| fileName.endsWith(".html"))
          { return "text/html";}
if(fileName.endsWith(".txt")|| fileName.endsWith(".text"))
          { return "text/plain";}
        if(fileName.endsWith(".gif")|| fileName.endsWith(".ico"))
          { return "image/gif";}
if(fileName.endsWith(".xml"))
          { return "text/xml";}
if(fileName.endsWith(".png"))
          { return "image/png";}
        if(fileName.endsWith(".jpeg")|| fileName.endsWith(".jpg"))
          { return "image/jpeg";}
       
//otherwise return this string
   return "application/octet-stream";
    }
     //Method sendBytes
     private static void sendBytes(FileInputStream fis, OutputStream os)
     throws Exception
     {
       //Construct a 1K buffer to hold bytes on their way to the socket.
       byte[] buffer = new byte[1024];
       int bytes = 0;
     
       //Copy requested file into the socket's output stream.
       while((bytes = fis.read(buffer))!= -1){
           os.write(buffer, 0, bytes);
       }
  //close the file input stream
  fis.close();
    }

 //End of class HttpRequest
 }

_________________________________________________________________________________

import java.io.* ;

import java.net.* ;

import java.lang.*;

import java.util.* ;

public final class WebServer

{

    public static void main(String argv[]) throws Exception

    {

         //set the port number

         int port = 4040;

 

    //Establish the listen socket

   ServerSocket welcomeSocket = new ServerSocket (4040) ;

//Prints a message to inform the user that the server is running and on which port

System.out.println("Web server is running on port " + port + "... (press CTRL-C to quit)");

//Prints a message about the date of the session's starting point

System.out.println("Session started on:" + (new Date()).toString());

System.out.println();

   //Process HTTP service requests in an infinite loop.

   while (true){

   //Listen for a TCP connection request

   Socket connectionSocket = welcomeSocket.accept() ;

   System.out.println();

//Prints out some information about the new connection

System.out.println("NEW CONNECTION ESTABLISHED:" +

   InetAddress.getLocalHost() + ":" + connectionSocket.getPort());

System.out.println("------------------------------------------------------------");

 

     try

       {

  //Set keep alive to true

connectionSocket.setKeepAlive(true);

//Set timeout to 5 minutes

  connectionSocket.setSoTimeout(300000);

//Prints some information about the new connection

System.out.format("Send Buffer Size:          %s\n",   connectionSocket.getSendBufferSize());

   System.out.format("Keep-Alive:                %s\n",   connectionSocket.getKeepAlive());

System.out.format("Socket Timeout:            %s\n",   connectionSocket.getSoTimeout()/60000+" minutes");

    System.out.println();

}

catch (Exception e)

          {e.printStackTrace();}

 

           try{

             //Construct an object to process the HTTP request message

             HttpRequest request = new HttpRequest(connectionSocket);

             //Create a new thread to process the request

             Thread thread = new Thread(request);

 

             //Start the thread.

             thread.start();

  }

  catch(Exception e) 

            { e.printStackTrace();

          }

        }

     }

//End of class WebServer   

}

Nikolas Georgiou