Version 2.6

Contributed to Cookie Central by David Whalen

• A Note from the Author
  
  
• 1. INTRODUCTION
  • 1.1 What is a cookie?
  • 1.2 Where did the term cookies come from?
  • 1.3 Why do sites use cookies?
  • 1.4 Where can I get more information?
• 2. GENERAL QUESTIONS/MISCELLANEOUS
  • 2.1 Introduction
  • 2.2 Can I delete cookies?
  • 2.3 How do I set my browser to reject cookies?
  • 2.4 Are cookies dangerous to my computer?
  • 2.5 Will cookies fill up my hard drive?
  • 2.6 Are cookies a threat to my privacy?
  • 2.7 Sites are telling me I need to turn on cookies, but they are on. What's wrong?
  • 2.8 I deleted my cookies, and I can't log-on to my favorite site anymore. What can I do?
  • 2.9 How did I get a cookie from doubleclick? I've never been there!
  • 2.10 I looked at my Internet Explorer cookies, and they had my username on them! Can servers see my username?
  • 2.11 There are two extra files in my Cookies folder called Mm256.dat and Mm2048.dat. What are they?
• 3. COOKIE FUNDAMENTALS
  • 3.1 Introduction
  • 3.2 How does a cookie really work?
  • 3.3 Breakdown of Cookie Parameters
  • 3.4 How do cookies end up on my hard drive?
  • 3.5 What are all those entries in my cookies.txt file?
  • 3.6 Where does MSIE keep its cookies?
  • 3.7 Are cookies Year 2000 Compliant?
• 4. ADVANCED TOPICS
  • 4.1 Introduction
  • 4.2 Creating a cookie value
  • 4.3 Retrieving a cookie value
  • 4.4 Clearing a cookie value
  • 4.5 Detecting if cookies are accepted
  • 4.6 Compact Privacy Policies and IE6
• About the Author
• Important Disclaimer and Copyright Information

(Version 2.6)
Whew! After a long delay, I am returning to the FAQ. A lot of information has been out-of-date for some time, so I felt I had to at least get us back in place.

I have noticed a growing number of sites out there that have copied all or part of my FAQ. While I should be angered at what is clearly a violation of copyright, I'll choose to be understanding and a little flattered. Honestly, people, you could at least mention that it's my work, you know. Or, you could at least link back to Cookie Central, who provides space for the FAQ free of charge.

It's things like this, and the waning interest in the cookie issue (which I feel I've done my part to help), that leads me to think that this will be the last update to the FAQ. I nearly pulled it down a year ago, but the owner of Cookie Central prevailed upon me to keep it live.

In November of 1996, I wrote the very first draft of the Cookie FAQ. It was a short treatise that ended up getting posted on Netscape's JavaScript newsgroup. The FAQ led directly to my associations with Netscape, becoming the JavaScript DevEdge Champion, and presenting an impromptu talk on cookies at DevCon. Truly a high-point in my career.

In the past five-and-a-half years, I've been able to watch cookies grow from a fledgling concept to the most important part of the internet experience. Cookies have brought personalization, commerce, and convenience to the internet, legitimizing it as a place for business, not just for science or personal home pages.

I've seen the FAQ grow as well over that time. The first draft of the FAQ was written to target developers who were trying to understand what cookies were and how they could use them. To this day, I see my examples put to use all over the internet. But the FAQ served better by informing everyday people about cookies and how they really work. I hope I've done my part to cure the hysteria.

So, I bid the FAQ good-bye. Thanks to all of you in the past who have contributed ideas, feedback, questions, and complaints. You all made a difference.

And thanks for stopping by!

-- DW


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1. INTRODUCTION

Cookies are a very important method for maintaining state on the Web. "State" in this case refers to an application's ability to work interactively with a user, remembering all data since the application started, and differentiating between users and their individual data sets.

An analogy I like to use is a laundry cleaner's shop. You drop something off, and get a ticket. When you return with the ticket, you get your clothes back. If you don't have the ticket, then the laundry man doesn't know which clothes are yours. In fact, he won't be able to tell whether you are there to pick up clothes, or a brand new customer. As such, the ticket is critical to maintaining state between you and the laundry man.

Unfortunately, HTTP is a "stateless" protocol. This means that each visit to a site (or even clicks within a site) is seen by the server as the first visit by the user. In essence, the server "forgets" everything after each request, unless it can somehow mark a visitor (that is, hand him a "laundry ticket") to help it remember. Cookies can accomplish this.

1.1 What is a Cookie?

A cookie is a text-only string that gets entered into the memory of your browser. This value of a variable that a website sets. If the lifetime of this value is set to be longer than the time you spend at that site, then this string is saved to file for future reference.

1.2 Where did the term cookies come from?

According to an article written by Paul Bonner for Builder.Com on 11/18/1997:

"Lou Montulli, currently the protocols manager in Netscape's client product division, wrote the cookies specification for Navigator 1.0, the first browser to use the technology. Montulli says there's nothing particularly amusing about the origin of the name: 'A cookie is a well-known computer science term that is used when describing an opaque piece of data held by an intermediary. The term fits the usage precisely; it's just not a well-known term outside of computer science circles.'"

1.3 Why do sites use Cookies?

There are many reasons a given site would wish to use cookies. These range from the ability to personalize information (like on My Yahoo or Excite), or to help with on-line sales/services (like on Amazon Books or eBay), or simply for the purposes of collecting demographic information (like DoubleClick). Cookies also provide programmers with a quick and convenient means of keeping site content fresh and relevant to the user's interests. The newest servers use cookies to help with back-end interaction as well, which can improve the utility of a site by being able to securely store any personal data that the user has shared with a site (to help with quick logins on your favorite sites, for example).

1.4 Where Can I Get More Information?

Cookie Central is dedicated to answering questions about cookies. Feel free to look around.

There's a great article concerning cookies on Marshall Brain's "How Stuff Works". It goes even deeper than this FAQ does, especially in the realm of public opinion. Worth a look!

The World Wide Web Consortium has an excellent FAQ to answer the majority of Internet and Web-related questions. You can read their topic: "Do 'Cookies' Pose any Security Risks?"

In addition, there are an abundance of resources on the Internet that can help you find answers to your cookie questions. Conveniently, Yahoo has a great listing of them. I encourage you to stop by and check the list out!

2. GENERAL QUESTIONS/MISCELLANEOUS

2.1 Introduction

This section is devoted to general questions on cookies and their usage.

2.2 Can I delete cookies?

Yes. Whether you use Internet Explorer or Netscape, your cookies are saved to a simple text file that you can delete as you please.

In order to do this properly, remember to close your browser first. This is because all your cookies are held in memory until you close your browser. So, if you delete the file with your browser open, it will make a new file when you close it, and your cookies will be back.

Remember that deleting your cookie file entirely will cause you to "start from scratch" with every web site you usually visit. So, it may be preferable to open the cookies.txt file (in the case of Netscape) and remove only the entries you don't like, or go to the cookies folder (in the case of IE) and delete the files matching servers you don't want.

2.3 How do I set my browser to reject cookies?

Both Internet Explorer and Netscape allow some level of cookie verification. They both have menu options that allow you to accept all, some, or none of your incoming cookies. In addition, the "warn before accepting" feature is present in both, if you want to screen your incoming cookies.

In Netscape, go to the Edit/Preferences/Advanced menu. Your cookie choices can be changed there.

Microsoft has changed their approach to cookies over the last 3 versions of their browser. This is a reflection of how cookies have been thrust into the limelight of privacy on the Internet:

• In IE 6.0, go to the Tools/Internet Options/Privacy menu. This menu allows you to select how discriminating the browser will be when accepting cookies, based on two factors -- (1) the source of the cookie, and (2) whether the source has a "privacy policy." There are also features for the advanced user, if you'd like to have a greater control over cookies. [more info]
• In IE 5.0, go to the Tools/Internet Options/Security menu. In there, you can choose the security level for 4 different browsing conditions: Internet Sites, Local Sites, "Trusted" Sites, and Restricted Sites. If you select "Internet," and click on Custom Level, you'll get a dialog box where you can accept all, warn before accepting, or reject all cookies. [more info]
• In IE 4.0, go to the View/Internet Options/Advanced menu. There you can accept all, warn before accepting, or reject all.

2.4 Are Cookies Dangerous to My Computer?

NO. A cookie is a simple piece of text. It is not a program, or a plug-in. It cannot be used as a virus, and it cannot access your hard drive. Your browser (not a programmer) can save cookie values to your hard disk if it needs to, but that is the limit of the effect on your system.

2.5 Will cookies fill up my hard drive?

Both Netscape and Microsoft have measures in place that limit the number of cookies that will be saved on your hard drive at one time.

Both Internet Explorer and Netscape conform to the RFC 2109 limitations on your total cookie count to 300 (this includes a limit of 20 cookies per individual domain). If you exceed this, the browser will discard your least-used cookies to make room for the new ones.

Microsoft saves cookies into the "Temporary Internet Files" folder, a system folder that you can set the maximum size of (the default is 2% of your hard drive).

In any event, remember that most cookie files are 4KB or smaller, so you would need about a million cookies to fill up a 4GB drive. This is incredibly unlikely.

2.6 Are Cookies a Threat to My Privacy?

The sad truth is that revealing any kind of personal information opens the door for that information to be spread.

Consider the growing trend of technology conveniences in our lives. We use "frequent buyer" cards at supermarkets and gas stations. We place electronic tags on our cars to pay tolls faster and easier. We let banks pay our bills for us automatically each month without checks.

While each of these technologies (and others like them) have made our lives more convenient, each time we use them exposes us to a loss of privacy. Stores know what foods you eat. Gas stations know how much you spend on gas per fill-up. Turnpike operators know how fast you drive on their highways. Banks know how you spend your money each month.

It's the same with cookies. In fact, one may argue that cookies in the long-run will be less damaging to privacy efforts than those technologies described above. If you're going to single-out cookies as your sole vulnerability to personal privacy, you should re-examine how you live your daily life.

The never-ending ethical debate associated with these facts shall be left to other forums. However, it is wise to consider carefully the information you collect and share over the Internet.

2.7 Sites are telling me I need to turn on cookies, but they are on. What's wrong?

There are three likely possibilities for problems like this. Firstly, the site you are visiting may be detecting cookies improperly. As a result, it may appear to the site that you are rejecting cookies when in fact you are not.

Another possibility is that you may be running software that interferes with cookie usage. There are many filtering and blocking software packages available for Internet users these days, and many of them also filter cookies. If you are running software like this, then your computer may not receive or send cookies. This will cause sites you visit to assume you are not accepting cookies.

Finally, your machine may be behind a firewall or proxy server that prevents cookie transmission. This is most likely in a corporate environment. So, regardless of how your browser is set, cookies won't be sent or received by your browser. Since the cookies aren't making it through to your browser, the Web Site will assume you personally aren't accepting them.

2.8 I deleted my cookies, and I can't log-on to my favorite site anymore. What can I do?

Many sites use a cookie to keep track of your settings on their servers, and to help you log in to their site. If you lose your cookie, that site cannot recall your settings for you to use.

If this happens to you, the best thing you can do is contact that site's webmaster or customer service department.

2.9 How did I get a cookie from doubleclick? I've never been there!

In section 3.3, we'll see that a server cannot set a cookie for a domain that it isn't a member of. However, almost every Web user has gotten a cookie from "ad.doubleclick.net" at one time or another, without ever visiting there. DoubleClick and other advertisers have employed a clever solution that enables them to track users and serve media content without violating this rule.

Most sites on the Internet do not keep their advertisements locally. Rather, they subscribe to a media service that places those ads for them. This is accomplished via a simple HTML call to the media service. When a page is requested, it is assembled through many HTTP requests by the browser. First, there is a request for the HTML itself. Then, everything the HTML needs is requested, including images, sounds, and plug-ins.

The call to the media service is an HTTP request for an image. Once the request is made to the media service, it can return more than just an ad. It can also return a cookie. Or, if is has given the user a cookie previously, it can read that first, and check to see what ad to send. The net result is that the user gets a cookie from the media service without ever having visited it.

This usage of cookies is the most controversial, and has led to the polarized opinions on cookies, privacy, and the Internet.

2.11 There are two extra files in my Cookies folder called Mm256.dat and Mm2048.dat. What are they?

You can read more about this on Microsoft's Knowledge Base.

3. COOKIE FUNDAMENTALS

3.1 Introduction

Whether you're a programmer or just a web user looking for answers, a big part of understanding cookies is to go into the gory details. This section does just that.

Here, the server is made aware of a cookie called foo, whose value is bar.

3.3 Breakdown of Cookie Parameters

As we have just seen, a cookie contains more than simply a name and a value. In fact, a cookie has 6 parameters that can be passed to it:

• The name of the cookie,
• The value of the cookie,
• The expiration date of the cookie,
• The path the cookie is valid for,
• The domain the cookie is valid for,
• The need for a secure connection to exist to use the cookie.

3.4 How do cookies end up on my hard drive?

After a cookie is transmitted through an HTTP header, it is stored in the memory of your browser. This way the information is quickly and readily available without re-transmission. As we have seen, however, it is possible for the lifetime of a cookie to greatly exceed the amount of time the browser will be open.

In such cases, the browser must have a way of saving the cookie when you are not browsing, or when your computer is shut off. The only way the browser can do this is to move the cookies from memory into the hard drive. This way, when you start your browser a few days later, you still have the cookies you had previously.

The browser is constantly performing maintenance on its cookies. Every time you open your browser, your cookies are read in from disk, and every time you close your browser, your cookies are re-saved to disk. As a cookie expires, it is discarded from memory and it is no longer saved to the hard drive.

3.6 Where does MSIE keep its cookies?

Microsoft keeps its cookies in different locations, depending on the version of explorer and Windows you are using. The best way to find it is to use the Windows "Search" feature an look for the "Cookies" folder. More information can be found here.

Although the location may be different, the format is the same. Each individual domain's cookies are stored in their own file, along with the username that accessed the site. For example, if I went to Yahoo.com, I would get a cookie that is stored in the file [email protected].

Note that the username is not sent with the cookie. See Section 2.10 for more information.

3.7 Are cookies Year 2000 Compliant?

There is no date-specific restriction on the HTTP header used to transmit cookies. In fact, the only determining factor in whether a cookie will be accepted is the programming of the client receiving the cookie.

The major browsers do not have an issue with this. Cookies with expiration dates set with 2-digit or 4-digit years are understood properly. Naturally, it's always advisable to use 4-digit years when setting cookies.

4.1 Introduction

This section covers topics beyond those discussed above.

4.2 Creating a Cookie Value

Creating a cookie generally involves duplicating the HTTP cookie header in some fashion so that the browser will store the name-value pair in memory. Some languages expect an exact HTTP header to be sent, while others will use built-in functions to help you speed the process along.

Cookies can be set from the browser-side or from the server-side. The determining factor will be the language you use to create the cookie. Once the cookie is created, it should flow easily from server to client and back via the HTTP headers.

There are limits on the contents of both the cookie string and the cookie file. These limits are partially imposed by HTTP and partially by arbitrary choice. They are as follows:

1. You CANNOT set Cookies for domains other than those that your response originates from. That is, a page on www.myserver.com can set a Cookie for myserver.com and www.myserver.com, but NOT www.yourserver.com.
2. The cookie HTTP header must be no more than 4K in size.

Note that this applies to cookies while they are in memory or stored in the cookies.txt file.

4.3 Retrieving a Cookie Value

For the most part, retrieving cookies does not require reading the HTTP Cookie: header. Most languages read this header for you and make it accessible through a variable or object.

Cookies can be read on the browser side or the server side. Again, the determining factor is the language used.

The main limit on retrieving a cookie is that you can only retrieve cookies that are valid for the document your script resides in. That is, a script on www.myserver.com cannot read cookies from www.yourserver.com. This is also true for subdirectories within your site. A cookie valid for /dirOne cannot be read by a script in /dirTwo. This is mainly governed on the browser side, as browsers know the URL that they are accessing, and only transmit cookies for that server across the connection.

4.4 Clearing a Cookie Value

When programming a Web site, there are many reasons that you may need to erase a cookie you have created. Often it is because the cookie is no longer needed, or the scheme of your cookie has been altered, and requires resetting.

The two main steps to clearing a cookie you have created are:

1. Set the cookie's value to null.
2. Set the cookie's expiration date to some time in the past.

The reason you must do both is that simply setting the expiration to a past time will not change it's value until the browser is closed. That is, all cookie names, values, expirations, etc are resolved once the browser program has been closed. Setting the cookie to null allows you to properly test for the cookie until that resolution.

4.5 Detecting if cookies are accepted

To properly detect if a cookie is being accepted via the server, the cookie needs to be set on one HTTP request and read back in another. This cannot be accomplished within 1 request. When using PERL or ASP, try to funnel your visitors through a common page where you can set a test cookie. Then, when the time comes to detect, check for that cookie.

If you use client-side languages to set a cookie, you can set and read on the same page. Cookies set by JavaScript or VBScript reside in the browser's memory already, so you will know if they have been accepted right away. Check by setting a test value, and then try to read that value back out of the cookie. If the value still exists, the cookie was accepted.

4.6 Compact Privacy Policies and IE6

In 1998, the W3C started drafting a proposal for a Platform for Privacy Preferences (P3P). P3P has 3 main goals (courtesy of the W3C):

• To inform a user agent of a site's data collection and privacy practices.
• To allow a user agent and service to automatically negotiate and to come to an agreement satisfactory to both parties; alternatively, for the user agent to notify the user and take instruction concerning proposed data exchanges from the user.
• To exchange data when such exchange is authorized by the user and consistent with a user's preferences and any outstanding agreement.

Now an official specification, P3Ps use an XML file to describe in as much detail as possible how a web site uses personal data during and after a user's session. This can include the intended usage of cookies to hold or refer to such information.

Alternatively, a site can create a P3P policy that refers solely to its cookie use. These Compact Privacy Policies are a focal point in Microsoft's new strategy in addressing the cookie "problem."

Users of Internet Explorer 6 can set their Privacy preferences based upon whether the target site has a Privacy Policy or not. If a site does not have a policy, its cookies may be automatically rejected by IE, and the user will see an icon on the status bar indicating a conflict with the user's privacy preferences.

P3P may have a broad impact on cookies and their future use. Especially in the context of advertising and commerce. Even though compact policies are essentially straightforward to create, users still stand to regain a great deal of control over their browser's communications.

About the Author

David Whalen is a Senior Internet Engineer. David writes Web-based applications in ASP, VB, C#, PERL, Java, and JavaScript. He has worked in the Web industry for over 6 years. David is a Microsoft Certified Professional. In addition, he was an inaugural Netscape DevEdge Champion, where he provided third-party support to developers in JavaScript and Netscape's LiveWire.

David holds a Masters Degree in Astrophysics. His personal interests include traveling, photography, gaming, animation, and sports. When not programming, he can be found either playing Quake or out on the golf course.

A Belorussian translation of this FAQ is available, provided by PC.

Copyright �1997-2024 David Whalen. This document is provided "as is" without any guarantees or warranty. Although the author has attempted to find and correct any errors or mistakes he, and everyone who contributed to it, are not responsible for any damage or losses of any kind caused by the use or misuse of information in this FAQ. The author is under no obligation to provide service, corrections, or upgrades to this FAQ.

The following is legal information and refers to all the information in this document. This information pertains to all use of the FAQ worldwide. All specific names included in the package are registered trademarks and are hereby acknowledged. Any other trademarks not mentioned in the FAQ are still hypothetically acknowledged.