Cookies
Let's discuss another key concept of computer networking, cookies!
Introduction
You might have heard of the term ‘cookie’ used a lot in the context of computer networks and privacy. Let’s have a closer look at what they are.
HTTP is a stateless protocol, but we often see websites where session state is needed. For instance, imagine you are browsing for products on an e-commerce website. How does the server know if you are logged in or not, or if the protocol is stateless? How does the server know what’s in your shopping cart when checking out if the protocol is stateless? Cookies allow the server to keep track of this sort of information.
How Cookies Work
- Cookies are unique string identifiers that can be stored on the client’s browser.
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These identifiers are set by the server through HTTP headers when the client first navigates to the website.
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After the cookie is set, it’s sent along with subsequent HTTP requests to the same server. This allows the server to know who is contacting it and hence serve content accordingly.
So the HTTP request, the HTTP response, the cookie file on the client’s browser, and a database of cookie-user values on the server’s end are all involved in the process of setting and using cookies.
Set-cookie Header
Let’s look at how cookies work in a bit more detail. When a server wants to set a cookie on the client-side, it includes the header Set-cookie: value in the HTTP response. This value is then appended to a special cookie file stored on your browser. The cookie file contains:
- The website’s domain
- The string value of the cookie
- The date that the cookie expires (yes, much like actual cookies, they do expire)
Have a look at the following slides to see how cookies work in practice.
The Dangers of Cookies
While cookies seem like a great idea to make HTTP persistent when needed, cookies have been severely abused in the past.
If a website has stored a cookie on your browser, it knows exactly when you visit it, what pages you visit and in what order. This itself makes some people uncomfortable.
Third-party Cookies
Also, websites may not necessarily know personally identifiable information about you such as your name (by the way, websites that require you to sign-up do know your name), and they may only know the value of your cookie. But what if websites can track what you do on other websites? Well, they can. Welcome to the concept of third-party cookies.
While we can’t go into too much detail, it suffices to know that third-party cookies are cookies set for domains that are not being visited.
Example
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A user visits amazon.com.
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A cookie for free-stats.com is subsequently set on their browser because free-stats has placed an advertisement on Amazon. Notice that this is a third-party cookie!
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Suppose, the user visits ebay.com, and eBay also has placed an advertisement for free-stats.com.
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The same cookie set on the Amazon site will be reused and sent to free-stats along in an HTTP request with the name of the host that the user is on.
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Free-stats can in this way track every website the user visits that they are advertising on and create more targeted ads in order to generate greater revenue.
Also, the public has largely considered third-party cookies to be a breach of privacy and so rejected them. Most modern browsers come with the in-built option to block third-party cookies.
Blocking Third-Party Cookies Is Not Enough!
However, firms have come up with several workarounds including but not limited to:
Please attempt the question in the widget below. Our AI will evaluate your answer.
Quick Quiz!
What is a cookie?
An identifier that’s stored in client browsers. It’s used by websites to serve customized content.
An http header
Only a way to track people across websites
They are http headers that make http stateless
Now that we know the basics of cookies, let’s look at them in practice with a quick exercise!
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