Contributed to Cookie Central by David Whalen
A Note from the Author
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has been out-of-date for some time, so I felt I had to at least get us back in place.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
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!
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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
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.'"
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
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?
[Back to Top]
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
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
2.3 How do I set my browser to reject
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 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.
Once a cookie is rejected, it is thrown out and not saved to
memory or disk. Don't forget, though, that servers will keep looking
for the cookie even if you have discarded it and may try to replace
it as you surf around.
This fact is almost comical in nature. Essentially, by removing
the way to tell the server to not send cookies, it can't remember to not
send you any cookies the next time!
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
Both Netscape and Microsoft have measures in place
that limit the number of cookies that will be saved on your hard drive at
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
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
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?
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
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?
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
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
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,
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.
2.10 I looked at my Internet Explorer
cookies, and they had my username on them! Can servers see my
Because Windows systems allow more than one user
to login and use programs, Microsoft had to come up with a way to keep
each user's cookies separate on a given machine. This can be common in
workplaces, where a single machine is shared by many employees.
This is accomplished by appending the username to the cookie file
name. This way, both Jane Doe and Joe Smith can get cookies from
coolsite.com and they don't get over-written. Also, this stop's Jane from
using Joe's cookies while she's surfing, since the browser will only use
her cookies when she is logged in. That is, the cookie file:
Doe's cookie for coolsite.com. If anyone else logs-in, then this cookie is
This is the only reason that the username is part of the
cookie file name. The username does not get sent to the server with
the cookie data.
2.11 There are two extra files in
my Cookies folder called Mm256.dat and Mm2048.dat. What are they?
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You can read more about this on Microsoft's Knowledge Base.
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
3.2 How does a cookie really
Understanding how cookies really work requires an
understanding of how HTTP works. Cookies transport from Server to Client
and back as an HTTP header. The specifications for this header are
explicitly laid out in RFC 2109.
When a cookie is sent from the server to the browser, an
additional line is added to the HTTP headers (example):
Set-Cookie: foo=bar; path=/; expires Mon, 09-Dec-2002 13:46:00 GMT
entry would result in a cookie named foo. The value of foo
is bar. In addition, this cookie has a path of /, meaning
that it is valid for the entire site, and it has an expiration date of
Dec 9, 2002 at 1:46pm Greenwich Mean Time (or Universal Time).
Provided the browser can understand this header, the cookie will be set.
When a cookie is sent from the browser to the server, the cookie
header is changed slightly:
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.
Two of these are mandatory (its name and its value). The
other four can be set manually or automatically. Each parameter is
separated by a semicolon when set explicitly. Here is a detailed
description of each.
The name of
a cookie and its value are set simply by pairing them together:
... foo=bar ...
The value of a cookie
can also be null, for the purpose of clearing the cookie value:
... foo= ...
The expires parameter lets you
determine the lifetime of the cookie.
... expires=Mon, 01-Jan-2001 00:00:00 GMT ...
Expires is not set explicitly, then it defaults to end-of-session.
The length of a session can vary depending on browsers and servers, but
generally a session is the length of time that the browser is open for
(even if the user is no longer at that site).
The path parameter is potentially the most useful of the 4
optional cookie settings. It sets the URL path the cookie is valid within.
Pages outside of that path cannot read or use the cookie.
... path=/promo ...
is not set explicitly, then it defaults to the URL path of the document
creating the cookie.
Netscape has identified a bug for VERY old
versions of Navigator where the path must be specified if an
expiration is specified. Furthermore, this path must be set to "/". For
more information, browse Netscape's Cookie Spec at:
The domain parameter takes the
flexibility of the path parameter one step further. If a site uses
multiple servers within a domain the it is important to make the cookie
accessible to pages on any of these servers.
... domain=www.myserver.com ...
Cookies can be
assigned to individual machines, or to an entire Internet domain. The only
restrictions on this value is that it must contain at least two dots
(.myserver.com, not myserver.com) for the normal top-level domains,
or three dots for the "extended" domains (.myserver.ny.us, not
IMPORTANT: The server issuing the cookie
must be a member of the domain that it tries to set in the cookie. That
is, a server called www.myserver.com cannot set a cookie for the domain
www.yourserver.com. The security implications should be obvious.
If Domain is not set explicitly, then it defaults to the
full domain of the document creating the cookie.
The secure parameter is a flag
indicating that a cookie should only be used under a secure server
condition, such as SSL. Since most sites do not require secure
connections, this defaults to FALSE.
3.4 How do cookies end up on my hard
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.
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.
3.5 What are all those entries in my
The layout of Netscape's
cookies.txt file is such that each line contains one name-value
pair. An example cookies.txt file may have an entry that looks like this:
.netscape.com TRUE / FALSE 946684799 NETSCAPE_ID 100103
represents a single piece of stored information. A tab is inserted between
each of the fields.
From left-to-right, here is what each field
domain - The domain that created AND that can
read the variable.
flag - A TRUE/FALSE value indicating if all
machines within a given domain can access the variable. This value is set
automatically by the browser, depending on the value you set for
path - The path within the domain that the
variable is valid for.
secure - A TRUE/FALSE value indicating
if a secure connection with the domain is needed to access the variable.
expiration - The UNIX time that the variable will expire on.
UNIX time is defined as the number of seconds since Jan 1, 1970 00:00:00
name - The name of the variable.
value - The
value of the variable.
3.6 Where does MSIE keep its
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@example.com.
Note that the
username is not sent with the cookie. See Section 2.10 for more
3.7 Are cookies Year 2000 Compliant?
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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. ADVANCED TOPICS
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
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:
- 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.
- 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
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
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
The two main steps to clearing a cookie you
have created are:
- Set the cookie's value to null.
- 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
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
If you use client-side languages to set a cookie, you can
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
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.
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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
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
Whalen is a Senior Internet Engineer. David writes Web-based applications
over 6 years. David is a Microsoft Certified Professional. In addition, he
was an inaugural Netscape DevEdge Champion, where he provided third-party
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
A Belorussian translation of this FAQ is available, provided by PC.
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