What Is IP Filtering?

IP filtering is simply a mechanism that decides which types of IP packets will be processed normally and which will be dropped or rejected. By dropped we mean that the packet is deleted and completely ignored, as if it had never been received. By rejected we mean that the firewall sends an ICMP response to the sender indicating a reason why the packet was rejected. You can apply many different sorts of criteria to determine which packets you wish to filter. Some examples of these are:

• Protocol type: TCP, UDP, ICMP, etc.
• Port number (for TCP/UPD)
• Packet type: SYN/ACK, data, ICMP Echo Request, etc.
• Packet source address: where it came from
• Packet destination address: where it is going to

It is important to understand at this point that IP filtering is a network layer facility. This means that it doesn't understand anything about the application using the network connections, only about the connections themselves. For example, you may deny users access to your internal network on the default Telnet port, but if you rely on IP filtering alone, you can't stop them from using the Telnet program with a port that you do allow to pass through your firewall. You can prevent this sort of problem by using proxy servers for each service that you allow across your firewall. The proxy servers understand the application that they were designed to proxy and can therefore prevent abuses, such as using the Telnet program to get past a firewall by using the World Wide Web port. If your firewall supports a World Wide Web proxy, outbound Telnet connections on the HTTP port will always be answered by the proxy and will allow only HTTP requests to pass. 

The IP filtering rule set is made up of many combinations of the criteria listed previously. For example, let's imagine that you wanted to allow World Wide Web users within the Virtual Brewery network to have no access to the Internet except to use other sites' web servers. You would configure your firewall to allow forwarding of the following:

• Packets with a source address on Virtual Brewery network, a destination address of anywhere, and with a destination port of 80 (WWW)
• Packets with a destination address of Virtual Brewery network and a source port of 80 (WWW) from a source address of anywhere

Note that we've used two rules here. We have to allow our data to go out, but also the corresponding reply data to come back in. In practice, as we'll see in the chapter on IP masquerade and Network Address Translation

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What is data management?

The control of data from acquisition and input through processing, output, and storage. In microcomputers, hardware manages data by gathering it, moving it, and following instructions to process it. The operating system manages the hardware and ensures that the parts of the system work in harmony so that data is stored safely and accurately. Application programs manage data by receiving and processing input according to the user’s commands, and sending results to an output device or to disk storage. The user also is responsible for data management by acquiring data, labeling and organizing disks, backing up data, archiving files, and removing unneeded material from the hard disk.

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HTTP status codes

Three-digit codes sent by an HTTP server that indicate the results of a request for data. Codes beginning with 1 respond to requests that the client may not have finished sending; with 2, successful requests; with 3, further action that the client must take; with 4, requests that failed because of client error; and with 5, requests that failed because of server error.
400, 401, 402, 403, 404, HTTP

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.NET Framework

A platform for building, deploying, and running XML Web services and applications. It provides a highly productive, standards-based, multilanguage environment for integrating existing investments with next generation applications and services, as well as the agility to solve the challenges of deployment and operation of Internet-scale applications. The .NET Framework consists of three main parts: the common language runtime, a hierarchical set of unified class libraries, and a componentized version of ASP called ASP.NET. See also ASP.NET, common language runtime, .NET Framework class library. 

NET Framework class library  
 A Common Language Specification (CLS)–compliant library of classes, interfaces, and value types that are included in the Microsoft .NET Framework SDK. This library provides access to system functionality and is designed to be the foundation on which .NET Framework applications, components, and controls are built.

.NET Framework data provider :- A component of ADO.NET that provides access to data from a relational data source. A .NET Framework data provider contains classes to connect to a data source, execute commands at the data source, and return query results from the data source, including the ability to execute commands within transactions. A .NET Framework data provider also contains classes to populate a DataSet with results from a data source and propagate changes in a DataSet back to the data source.

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What is GPS receiver?

A device that includes an antenna, a radio receiver, and a processor for use with the worldwide GPS -Global Positioning System. A GPS receiver uses position and time information from four GPS satellites to calculate precise information about its current location, its speed of travel, and the current time. A portable GPS receiver may be a stand-alone device or a plug-in unit for use with a portable computer. GPS receivers are used for scientific work, such as surveying, mapping, and studies of volcanoes, as well as for land, sea, and air navigation. On the consumer front, they are used in outdoor activities such as hiking and sailing and in cars to provide location, destination, and traffic information.

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What is ini file?

ini file:-Short for initialization file, a text file containing information about the initial configuration of Windows and Windows-based applications, such as default settings for fonts, margins, and line spacing. Two ini files, win.ini and system.ini, are required to run the Windows operating system through version 3.1. In later versions of Windows, ini files are replaced by a database known as the registry. In addition to Windows itself, many older applications create their own ini files. Because they are composed only of text, ini files can be edited in any text editor or word processor to change information about the application or user preferences. All initialization files bear the extension .ini. 

MS-DOS and Windows 3.x, the file extension that identifies an initialization file, which contains user preferences and startup information about an application program

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what is three-tier client/server?

A client/server architecture in which software systems are structured into three tiers or layers: the user interface layer, the business logic layer, and the database layer. Layers may have one or more components. For example, there can be one or more user interfaces in the top tier, each user interface may communicate with more than one application in the middle tier at the same time, and the applications in the middle tier may use more than one database at a time. Components in a tier may run on a computer that is separate from the other tiers, communicating with the other components over a network.

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Whst is Password Authentication Protocol

Acronym for Password Authentication Protocol. A method for verifying the identity of a user attempting to log on to a Point-to-Point Protocol (PPP) server. PAP is used if a more rigorous method, such as the Challenge Handshake Authentication Protocol (CHAP), is not available or if the user name and password that the user submitted to PAP must be sent to another program without encryption. 2. Acronym for Printer Access Protocol. The protocol in AppleTalk networks that governs communication between computers and printers.

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Javascript Array

Creating an Object

We have already seen an example of an Array object being created. To create an Array object, we used the JavaScript statement

var myArray = new Array();

So how is this statement made up?

The first half of the statement is familiar to us. We use the var keyword to define a variable called myArray. This variable is initialized, using the assignment operator (=), to the right-hand side of the statement.

The right-hand side of the statement consists of two parts. First we have the keyword new. This tells JavaScript that we want to create a new object. Next we have Array(). This is the constructor for an Array object. It tells JavaScript what type of object we want to create. Most objects have constructors like this. For example, the Date object has the Date() constructor.

we can pass parameters to the constructor Array() to add data to our object. For example, to create an Array object that has three elements containing the data "Paul", "Paula", and "Pauline", we use

var myArray = new Array("Paul", "Paula", "Pauline");

Let's see some more examples, this time using the Date object. The simplest way of creating a Date object is

var myDate = new Date();

This will create a Date object containing the date and time that it was created. However,

var myDate = new Date("1 Jan 2000");

will create a Date object containing the date 1 January 2000.

How object data is stored in variables differs from how primitive data, such as text and numbers, is stored. (Primitive data is the most basic data possible in JavaScript.) With primitive data, the variable holds the data's actual value. For example

var myNumber = 23;

means that the variable myNumber will hold the data 23. However, variables assigned to objects don't hold the actual data, but rather a reference to the memory address where the data can be found. This doesn't mean we can get hold of the memory address—this is something only JavaScript has details of and keeps to itself in the background. All you need to remember is that when we say that a variable references an object, this is what we mean. We show this in the following example:

var myArrayRef = new Array(0, 1, 2);
var mySecondArrayRef = myArrayRef;
myArrayRef[0] = 100;
alert(mySecondArrayRef[0]);

First we set variable myArrayRef reference to the new array object, and then we set mySecondArrayRef to the same reference—for example, now mySecondArrayRef is set to reference the same array object. So when we set the first element of the array to 100 as shown here:

myArrayRef [0] = 100;

and display the contents of the first element of the array referenced in mySecondArrayRef as follows:

alert(mySecondArrayRef[0])

we'll see it also magically has changed to 100! However, as we now know, it's not magic; it's because both variables referenced the same array object because when it comes to objects, it's a reference to the object and not the object stored in a variable. When we did the assignment, it didn't make a copy of the array object, it simply copied the reference. Contrast that with the following:

var myVariable = "ABC";
var mySecondVariable = myVariable;
myVariable = "DEF";
alert(mySecondVariable);

In this case we're dealing with a string, which is primitive data type, as are numbers. This time it's the actual data that's stored in the variable, so when we do this:

var mySecondVariable = myVariable;

mySecondVariable gets its own separate copy of the data in myVariable. So the alert at the end will still show mySecondVariable as holding "ABC."

To summarize this section, we create a JavaScript object using the following basic syntax:

var myVariable = new ObjectName(optional parameters);

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Creating a Cookie-Javascript

To make life easier for ourselves, we'll write a function that allows us to create a new cookie and set certain of its attributes with more ease. We'll look at the code first and create an example using it shortly.

function setCookie(cookieName, cookieValue, cookiePath, cookieExpires)
{
   cookieValue = escape(cookieValue);
   if (cookieExpires == "")
   {
      var nowDate = new Date();
      nowDate.setMonth(nowDate.getMonth() + 6);
      cookieExpires = nowDate.toGMTString();
   }
   if (cookiePath != "")
   {
      cookiePath = ";Path=" + cookiePath;
   }
   document.cookie = cookieName + "=" + cookieValue +
      ";expires=" + cookieExpires + cookiePath;
}

The secure and domain parts of the cookie string are unlikely to be needed, so we just allow the name, value, expires, and path parts of a cookie to be set by the function. If we don't want to set a path or expiration date, we just pass empty strings for those parameters. If no path is specified, the current directory and its subdirectories will be the path. If no expiration date is set, we just assume a date six months from now.

The first line of the function introduces the escape() function, which we've not seen before.

cookieValue = escape(cookieValue);

When we talked about setting the value of a cookie, we mentioned that certain characters cannot be used directly, such as a semicolon. (This also applies to the name of the cookie.) To get around this problem, we can use the built-in escape() and unescape() functions.

alert(escape("2001 a space odyssey;"));

We can see that the spaces have been converted to %20, % indicating that they represent an escape or special character rather than an actual character, and that 20 is the ASCII value of the actual character. The semicolon has been converted to %3B, as we'd expect.

As we'll see later, when retrieving cookie values we can use the unescape() function to convert from the encoded version to plain text.

Back to our function; we next have an if statement.

   if (cookieExpires == "")
   {
      var nowDate = new Date();
      nowDate.setMonth(nowDate.getMonth() + 6);
      cookieExpires = nowDate.toGMTString();
   }

This deals with the situation where an empty string, "", has been passed for the cookieExpires parameter of the function. Because most of the time we want a cookie to last longer than the session it's created in, we set a default value for expires that is six months after the current date.

Next, if a value other than an empty string ("") has been passed to the function for the cookiePath parameter, we need to add that value when we create the cookie. We simply put "path=" in front of any value that has been passed in the cookiePath parameter.

   if (cookiePath != "")
   {
      cookiePath = ";Path=" + cookiePath;
   }

Finally on the last line we actually create the cookie, putting together the cookieName, cookieValue, cookieExpires, and cookiePath parts of the string.

   document.cookie = cookieName + "=" + cookieValue +
      ";expires=" + cookieExpires + cookiePath;

We'll be using the setCookie() function whenever we want to create a new cookie because it makes setting a cookie slightly easier than having to remember all the parts we want to set. More importantly, it can be used to set the expiration date to a date six months ahead of the current date.

For example, to use the function and set a cookie with default values for expires and path we just type the following:

setCookie("cookieName","cookieValue","","")

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