September 4th, 2003, 12:48 AM
[By Abhishek Bhuyan]
What is a “Cookie”?
A “cookie” is a small piece of information sent by a web server to be stored on a web browser so that it can later be read back from that browser. This becomes useful for having the browser remember specific information about a visitor to a particular website. The cookie is a text file that is saved in the browser’s directory and is stored in RAM while the browser is running. The cookie may also be stored on the computer’s hard drive once a user logs off from that website or web server.
Cookies are a very important method for maintaining ‘state’ on the Web. What does that mean? ‘State’ refers to the application ability to work interactively with the user. For example, when you book yourself for a train/bus you get a ticket. On the date of journey, when you show this ticket, you will be allowed to enter the train/bus else the ticket collector will not know if you are the right person or a new customer. Here ticket is critical to maintain state between you and ticket collector.
HTTP is a ‘stateless’ protocol. This means that each visit to a site (or clicks within a site) is seen by the server as the first visit by the user. That means the server forgets everything after each request, unless it can somehow mark a visitor ( i.e ‘Yes he is the right traveler’) to help it remember. Cookies do this job.
Cookies can only tell a web server if you have been there before and can pass short bits of information (such as a user number) from the web server back to itself the next time you visit. Most cookies last only until you quit your browser and then are destroyed. A second type of cookie known as a persistent cookie has an expiration date and is stored on your disk until that date. A persistent cookie can be used to track a user's browsing habits by identifying him whenever he returns to a site. Information about where you come from and what web pages you visit already exists in a web server's log files and could also be used to track users browsing habits, cookies just make it easier.
How do I examine Persistent Cookies already my own System?
Persistent cookies are stored in different places on your system depending on which web browser and browser version you are using. Netscape stores all its persistent cookies in a single file named cookies.txt on the PC . Both files are in the Netscape directory. You can open and edit this file with a text editor and delete any cookies that you don't want to keep or delete the file itself to get rid of all of your cookies. Internet Explorer stores persistent cookies in separate files named with the user's name and the domain name of the site that sent the cookie. For example: email@example.com. The cookie files are stored in /Windows/cookies or in /Windows/profiles/cookies directories, where ‘yourname’ is replaced with the user's login name. If your operating system directory is not named Windows (such as Winnt for Windows NT) then look in that directory instead of the Windows directory. You can delete any of these files you do not want to keep. You can open these files to see where they came from and what information they contain. For example, the following are the contents of an Internet Explorer cookie file.
This particular cookie file was named abhishek@<a rel="nofollow" href="htt....bazee.txt</a> (abhishek is my user name, I logged in). Cookie may contain different information; it depends on cookie to cookie. Here my IP address is stored(188.8.131.52) . We will not go into details now.
What Are Cookies Used For?
Cookies have some beneficial things. Site personalization is one of the most beneficial uses for cookies. For example, a person comes to the CNN or even Yahoo!(My Yahoo) site, but does not want to see any business news. The site allows the person to select this choice as an option. >From then on (or until the cookie expires), the person would not see business news when they access the CNN web pages. You must have also seen in some websites that when you log in (using a User ID & Password), there is an option for ‘remember me when I visit next time’; that’s possible because it stored your password and id on your machine in a cookie.
Some visitors feel it is an invasion of privacy for a website to track their progress on a site. It helps to get you the information or services you seek as quickly as possible and allow you to get back to work without delay. Site navigation statistics are critical to the continuing redesign of the site. Site administrator might need to know if 100 different people visited his site or if one person (or robot) continuously hit the reload button 100 times.
Cookies also have some demerits. Let me give you a example(real life). The DoubleClick Network is a system created by the DoubleClick Corporation to create profiles of individuals using the World Wide Web and to present them with advertising banners customized to their interests. DoubleClick's primary customers are Web sites looking to advertise their services. Each member of the DoubleClick Network becomes a host for the advertising of other members of the network. When a Web site joins DoubleClick it creates advertisements for its services and submits them to DoubleClick's server. The Web site then modifies its HTML pages to include an <IMG> graphic that points to DoubleClick. When a user goes to view one of these modified HTML pages, her browser makes a call to DoubleClick's server to retrieve the graphic. The server chooses one of its member's advertisements and returns it to the browser. If the user reloads the page, a different advertisement appears. If the user clicks on the graphic, her browser jumps to the advertised site. Currently many hundreds of sites belong to DoubleClick.
>From the user's point of view DoubleClick's graphics appear no different from any other Web advertisement, and there's no visible indication of anything special about the graphic. However, there is an important difference. When a user first connects to the DoubleClick server to retrieve a graphic, the server assigns the browser a cookie that contains a unique identification number. From that time forward whenever the user connects to any Web site that subscribes to the DoubleClick Network, her browser returns the identification number to DoubleClick's server, allowing the server to recognize her. Over a period of time DoubleClick compiles a list of which member sites the user has visited and revisited, using this information to create a profile of the user's tastes and interests. With this profile in hand the DoubleClick server can select advertising that is likely to be of interest to the user. It can also use this information to compile valuable feedback for its member Web sites, such as providing them with audience profiles and rating the effectiveness of the advertisements.
So how do I know that I have been tracked by DoubleClick ? Well to find out whether you have been tracked by DoubleClick, examine your browser's cookies file in cookies directory . There will be something like this
ad.doubleclick.net FALSE / FALSE 942195440 IAA d2bbd5
How Do These Cookies Work?
A command line in the HTML code of a document tells the browser to set a cookie of a certain name or value. The following is a general example of a script used to set a cookie.
Set-Cookie: name = VALUE;
expires = DATE;
path = PATH;
domain = DOMAIN_NAME; secure
Lets go a bit detail of all these attributes….
This string is a sequence of characters excluding semi-colon, comma and white space. If there is a need to place such data in the name or value, some encoding method such as URL style %XX encoding is recommended, though no encoding is defined or required.
This is the only required attribute on the Set-Cookie header.
expires = DATE
The expires attribute specifies a date string that defines the valid life time of that cookie. Once the expiration date has been reached, the cookie will no longer be stored or given out.
The date string is formatted as:
Wdy, DD-Mon-YYYY HH:MM:SS GMT
expires is an optional attribute. If not specified, the cookie will expire when the user's session ends.
domain = DOMAIN_NAME
When searching the cookie list for valid cookies, a comparison of the domain attributes of the cookie is made with the Internet domain name of the host from which the URL will be fetched. If there is a tail match, then the cookie will go through path matching to see if it should be sent. "Tail matching" means that domain attribute is matched against the tail of the fully qualified domain name of the host. A domain attribute of "internet.com" would match host names "people.internet.com" as well as "shipping.computer.internet.com".
Only hosts within the specified domain can set a cookie for a domain and domains must have at least two (2) or three (3) periods in them to prevent domains of the form: ".com", ".edu", and "lu.in". Any domain that fails within one of the seven special top level domains listed below only require two periods. Any other domain requires at least three. The seven special top level domains are: "COM", "EDU", "NET", "ORG", "GOV", "MIL", and "INT".
The default value of domain is the host name of the server which generated the cookie response.
Path = PATH
The path attribute is used to specify the subset of URLs in a domain for which the cookie is valid. If a cookie has already passed domain matching, then the pathname component of the URL is compared with the path attribute, and if there is a match, the cookie is considered valid and is sent along with the URL request. The path "/foo" would match "/foobar" and "/foo/bar.html". The path "/" is the most general path.
If the path is not specified, it as assumed to be the same path as the document being described by the header which contains the cookie.
If a cookie is marked secure, it will only be transmitted if the communications channel with the host is a secure one. Currently this means that secure cookies will only be sent to HTTPS (HTTP over SSL) servers.
If secure is not specified, a cookie is considered safe to be sent in the clear over unsecured channels.
I hope now you understand the pros and cons of Cookies. This is not over yet. In my next article I’ll be explaining more details about cookies, how to hack those, cookie hijacking , using those how to spoof, & countermeasures.
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September 4th, 2003, 01:29 AM
Maybe you should consider writing your own tutorials. I mean it is great and all that you give proper sources to astalavista , but still don't you want to know that you wrote a tutorial yourself ?
"Serenity is not the absence of conflict, but the ability to cope with it."
September 12th, 2003, 09:30 PM
half from windows help file and the other half from altavista, what next?? linux help files!!
September 13th, 2003, 04:21 PM
Szafran: first of all please only contribute origanal tutorials and place them in the Tutorials forum. Secondly when posting work that is not your own simply give a link to the website and recommend the tutorial, it is easier especially for Neg who is in the process of removing all non-origanal tutorial from his forum.