How do I uniquely identify computers visiting my web site?

I need to figure out a way uniquely identify each computer which visits the web site I am creating. Does anybody have any advice on how to achieve this?

Because i want the solution to work on all machines and all browsers (within reason) I am trying to create a solution using javascript.

I appreciate the help. Thanks.


Cookies will not do.

I need the ability to basically create a guid which is unique to a computer and repeatable, assuming no hardware changes have happened to the computer. Directions i am thinking of are getting the MAC of the network card and other information of this nature which will id the machine visiting the web site.



These people have developed a fingerprinting method for recognising a user with a high level of accuracy:

We investigate the degree to which modern web browsers are subject to “device fingerprinting” via the version and configuration information that they will transmit to websites upon request. We implemented one possible fingerprinting algorithm, and collected these fingerprints from a large sample of browsers that visited our test side, We observe that the distribution of our finger- print contains at least 18.1 bits of entropy, meaning that if we pick a browser at random, at best we expect that only one in 286,777 other browsers will share its fingerprint. Among browsers that support Flash or Java, the situation is worse, with the average browser carrying at least 18.8 bits of identifying information. 94.2% of browsers with Flash or Java were unique in our sample.

By observing returning visitors, we estimate how rapidly browser fingerprints might change over time. In our sample, fingerprints changed quite rapidly, but even a simple heuristic was usually able to guess when a fingerprint was an “upgraded” version of a previously observed browser’s fingerprint, with 99.1% of guesses correct and a false positive rate of only 0.86%.

We discuss what privacy threat browser fingerprinting poses in practice, and what countermeasures may be appropriate to prevent it. There is a tradeoff between protection against fingerprintability and certain kinds of debuggability, which in current browsers is weighted heavily against privacy. Paradoxically, anti-fingerprinting privacy technologies can be self- defeating if they are not used by a sufficient number of people; we show that some privacy measures currently fall victim to this paradox, but others do not...



I don't know if there is or ever will be a way to uniquely identify machines using a browser alone. The main reasons are:

  • You will need to save data on the users computer. This data can be deleted by the user any time. Unless you have a way to recreate this data which is unique for each and every machine then your stuck.
  • Validation. You need to guard against spoofing, session hijacking, etc.

Even if there are ways to track a computer without using cookies there will always be a way to bypass it and software that will do this automatically. If you really need to track something based on a computer you will have to write a native application (Apple Store / Android Store / Windows Program / etc).

I might not be able to give you an answer to the question you asked but I can show you how to implement session tracking. With session tracking you try to track the browsing session instead of the computer visiting your site. By tracking the session, your database schema will look like this:

  sessionID: string
  // Global session data goes here

  computers: [{
     BrowserID: string
     ComputerID: string
     FingerprintID: string
     userID: string
     authToken: string
     ipAddresses: ["203.525....", "203.525...", ...]
     // Computer session data goes here
  }, ...]

Advantages of session based tracking:

  1. For logged in users, you can always generate the same session id from the users username / password / email.
  2. You can still track guest users using sessionID.
  3. Even if several people use the same computer (ie cybercafe) you can track them separately if they log in.

Disadvantages of session based tracking:

  1. Sessions are browser based and not computer based. If a user uses 2 different browsers it will result in 2 different sessions. If this is a problem you can stop reading here.
  2. Sessions expire if user is not logged in. If a user is not logged in, then they will use a guest session which will be invalidated if user deletes cookies and browser cache.


There are many ways of implementing this. I don't think I can cover them all I'll just list my favorite which would make this an opinionated answer. Bear that in mind.


I will track the session by using what is known as a forever cookie. This is data which will automagically recreate itself even if the user deletes his cookies or updates his browser. It will not however survive the user deleting both their cookies and their browsing cache.

To implement this I will use the browsers caching mechanism (RFC), WebStorage API (MDN) and browser cookies (RFC, Google Analytics).


In order to utilize tracking ids you need to add them to both your privacy policy and your terms of use preferably under the sub-heading Tracking. We will use the following keys on both document.cookie and window.localStorage:

  • _ga: Google Analytics data
  • __utma: Google Analytics tracking cookie
  • sid: SessionID

Make sure you include links to your Privacy policy and terms of use on all pages that use tracking.

Where do I store my session data?

You can either store your session data in your website database or on the users computer. Since I normally work on smaller sites (let than 10 thousand continuous connections) that use 3rd party applications (Google Analytics / Clicky / etc) it's best for me to store data on clients computer. This has the following advantages:

  1. No database lookup / overhead / load / latency / space / etc.
  2. User can delete their data whenever they want without the need to write me annoying emails.

and disadvantages:

  1. Data has to be encrypted / decrypted and signed / verified which creates cpu overhead on client (not so bad) and server (bah!).
  2. Data is deleted when user deletes their cookies and cache. (this is what I want really)
  3. Data is unavailable for analytics when users go off-line. (analytics for currently browsing users only)


  • BrowserID: Unique id generated from the browsers user agent string. Browser|BrowserVersion|OS|OSVersion|Processor|MozzilaMajorVersion|GeckoMajorVersion
  • ComputerID: Generated from users IP Address and HTTPS session key. getISP(requestIP)|getHTTPSClientKey()
  • FingerPrintID: JavaScript based fingerprinting based on a modified fingerprint.js. FingerPrint.get()
  • SessionID: Random key generated when user 1st visits site. BrowserID|ComputerID|randombytes(256)
  • GoogleID: Generated from __utma cookie. getCookie(__utma).uniqueid


The other day I was watching the wendy williams show with my girlfriend and was completely horrified when the host advised her viewers to delete their browser history at least once a month. Deleting browser history normally has the following effects:

  1. Deletes history of visited websites.
  2. Deletes cookies and window.localStorage (aww man).

Most modern browsers make this option readily available but fear not friends. For there is a solution. The browser has a caching mechanism to store scripts / images and other things. Usually even if we delete our history, this browser cache still remains. All we need is a way to store our data here. There are 2 methods of doing this. The better one is to use a SVG image and store our data inside its tags. This way data can still be extracted even if JavaScript is disabled using flash. However since that is a bit complicated I will demonstrate the other approach which uses JSONP (Wikipedia) (actually tracking.php)

var now = new Date();
var window.__sid = "SessionID"; // Server generated

setCookie("sid", window.__sid, now.setFullYear(now.getFullYear() + 1, now.getMonth(), now.getDate() - 1));

if( "localStorage" in window ) {
  window.localStorage.setItem("sid", window.__sid);

Now we can get our session key any time:

window.__sid || window.localStorage.getItem("sid") || getCookie("sid") || ""

How do I make tracking.js stick in browser?

We can achieve this using Cache-Control, Last-Modified and ETag HTTP headers. We can use the SessionID as value for etag header:

  "ETag": SessionID,
  "Last-Modified": new Date(0).toUTCString(),
  "Cache-Control": "private, max-age=31536000, s-max-age=31536000, must-revalidate"

Last-Modified header tells the browser that this file is basically never modified. Cache-Control tells proxies and gateways not to cache the document but tells the browser to cache it for 1 year.

The next time the browser requests the document, it will send If-Modified-Since and If-None-Match headers. We can use these to return a 304 Not Modified response.

$sid = getHeader("If-None-Match") ?: getHeader("if-none-match") ?: getHeader("IF-NONE-MATCH") ?: ""; 
$ifModifiedSince = hasHeader("If-Modified-Since") ?: hasHeader("if-modified-since") ?: hasHeader("IF-MODIFIED-SINCE");

if( validateSession($sid) ) {
  if( sessionExists($sid) ) {
  } else {
} else if( $ifModifiedSince ) {
} else {

Now every time the browser requests tracking.js our server will respond with a 304 Not Modified result and force an execute of the local copy of tracking.js.

I still don't understand. Explain it to me

Lets suppose the user clears their browsing history and refreshes the page. The only thing left on the users computer is a copy of tracking.js in browser cache. When the browser requests tracking.js it recieves a 304 Not Modified response which causes it to execute the 1st version of tracking.js it recieved. tracking.js executes and restores the SessionID that was deleted.


Suppose Haxor X steals our customers cookies while they are still logged in. How do we protect them? Cryptography and Browser fingerprinting to the rescue. Remember our original definition for SessionID was:


We can change this to:

Timestamp|BrowserID|ComputerID|encrypt(randomBytes(256), hk)|sign(Timestamp|BrowserID|ComputerID|randomBytes(256), hk)

Where hk = sign(Timestamp|BrowserID|ComputerID, serverKey).

Now we can validate our SessionID using the following algorithm:

if( getTimestamp($sid) is older than 1 year ) return false;
if( getBrowserID($sid) !== createBrowserID($_Request, $_Server) ) return false;
if( getComputerID($sid) !== createComputerID($_Request, $_Server) return false;

$hk = sign(getTimestamp($sid) + getBrowserID($sid) + getComputerID($sid), $SERVER["key"]);

if( !verify(getTimestamp($sid) + getBrowserID($sid) + getComputerID($sid) + decrypt(getRandomBytes($sid), hk), getSignature($sid), $hk) ) return false;

return true; 

Now in order for Haxor's attack to work they must:

  1. Have same ComputerID. That means they have to have the same ISP provider as victim (Tricky). This will give our victim the opportunity to take legal action in their own country. Haxor must also obtain HTTPS session key from victim (Hard).
  2. Have same BrowserID. Anyone can spoof User-Agent string (Annoying).
  3. Be able to create their own fake SessionID (Very Hard). Volume atacks won't work because we use a time-stamp to generate encryption / signing key so basically its like generating a new key for each session. On top of that we encrypt random bytes so a simple dictionary attack is also out of the question.

We can improve validation by forwarding GoogleID and FingerprintID (via ajax or hidden fields) and matching against those.

if( GoogleID != getStoredGoodleID($sid) ) return false;
if( byte_difference(FingerPrintID, getStoredFingerprint($sid) > 10%) return false;

It's not possible to identify the computers accessing a web site without the cooperation of their owners. If they let you, however, you can store a cookie to identify the machine when it visits your site again. The key is, the visitor is in control; they can remove the cookie and appear as a new visitor any time they wish.


A possibility is using flash cookies:

  • Ubiquitous availability (95 percent of visitors will probably have flash)
  • You can store more data per cookie (up to 100 KB)
  • Shared across browsers, so more likely to uniquely identify a machine
  • Clearing the browser cookies does not remove the flash cookies.

You'll need to build a small (hidden) flash movie to read and write them.

Whatever route you pick, make sure your users opt IN to being tracked, otherwise you're invading their privacy and become one of the bad guys.


You may want to try setting a unique ID in an evercookie (it will work cross browser, see their FAQs):

There is also a company called ThreatMetrix that is used by a lot of big companies to solve this problem: They are quite expensive and some of their other products aren't very good, but their device id works well.

Finally, there is this open source jquery implementation of the panopticlick idea: It looks pretty half baked right now but could be expanded upon.

Hope it helps!


There is a popular method called canvas fingerprinting, described in this scientific article: The Web Never Forgets: Persistent Tracking Mechanisms in the Wild. Once you start looking for it, you'll be surprised how frequently it is used. The method creates a unique fingerprint, which is consistent for each browser/hardware combination.

The article also reviews other persistent tracking methods, like evercookies, respawning http and Flash cookies, and cookie syncing.

More info about canvas fingerprinting here:


There is only a small amount of information that you can get via an HTTP connection.

  1. IP - But as others have said, this is not fixed for many, if not most Internet users due to their ISP's dynamic allocation policies.

  2. Useragent String - Nearly all browsers send what kind of browser they are with every request. However, this can be set by the user in many browsers today.

  3. Collection of request fields - There are other fields sent with each request, such as supported encodings, etc. These, if used in the aggregate can help to ID a user's machine, but again are browser dependent and can be changed.

  4. Cookies - Setting a cookie is another way to identify a machine, or more specifically a browser on a machine, but as others have said, these can be deleted, or turned off by the users, and are only applicable on a browser, not a machine.

So, the correct response is that you cannot achieve what you would live via the HTTP over IP protocols alone. However, using a combination of cookies, as well as IP, and the fields in the HTTP request, you have a good chance at guessing, sort of, what machine it is. Users tend to use only one browser, and often from one machine, so this may be fairly relieable, but this will vary depending on the audience...techies are more likely to mess with this stuff, and use more machines/browsers. Additionally, this could even be coupled with some attempt to geo-locate the IP, and use that data as well. But in any case, there is no solution that will be correct all of the time.


There are flaws with both cookie and non-cookie approaches. But if you can forgive the shortcomings of the cookie approach, here's an idea.

If you're already using Google Analytics on your site, then you don't need to write code to track unique users yourself. Google Analytics does that for you via the __utma cookie value, as described in Google's documentation. And by reusing this value you're not creating additional cookie payload, which has efficiency benefits with page requests.

And you could write some code easily enough to access that value, or use this script's getUniqueId() function.


As with the previous solutions cookies are a good method, be aware that they identify browsers though. If I visited a website in Firefox and then in Internet Explorer cookies would be stored for both attempts seperately. Some users also disable cookies (but more people disable JavaScript).

Another method to consider would be I.P. and hostname identification (be aware these can vary for dial-up/non-static IP users, AOL also uses blanket IPs). However since this only identifies networks this might not work as well as cookies.


The suggestions to use cookies aside, the only comprehensive set of identifying attributes available to interrogate are contained in the HTTP request header. So it is possible to use some subset of these to create a pseudo-unique identifier for a user agent (i.e., browser). Further, most of this information is possibly already being logged in the so-called "access log" of your web server software by default and, if not, can be easily configured to do so. Then, a utlity could be developed that simply scans the content of this log, creating fingerprints of each request comprised of, say, the IP address and User Agent string, etc. The more data available, even including the contents of specific cookies, adds to the quality of the uniqueness of this fingerprint. Though, as many others have stated already, the HTTP protocol doesn't make this 100% foolproof - at best it can only be a fairly good indicator.


When i use a machine which has never visited my online banking web site i get asked for additional authentification. then, if i go back a second time to the online banking site i dont get asked the additional authentification...i deleted all cookies in IE and relogged onto my online banking site fully expecting to be asked the authentification questions again. to my surprise i was not asked. doesnt this lead one to believe the bank is doing some kind of pc tagging which doesnt involve cookies?

This is a pretty common type of authentication used by banks.

Say you're accessing your bank website via The first time you're there, you'll be asked for your password, as well as additional authentication. Once you've passed, the bank knows that user "thatisvaliant" is authenticated to access the site via

In the future, it won't ask for extra authentication (beyond your password) when you're accessing the site via If you try to access the bank via, the bank will go through the same routine again.

So to summarize, what the bank's identifying is your ISP and/or netblock, based on your IP address. Obviously not every user at your ISP is you, which is why the bank still asks you for your password.

Have you ever had a credit card company call to verify that things are OK when you use a credit card in a different country? Same concept.


Really, what you want to do cannot be done because the protocols do not allow for this. If static IPs were universally used then you might be able to do it. They are not, so you cannot.

If you really want to identify people, have them log in.

Since they will probably be moving around to different pages on your web site, you need a way to keep track of them as they move about.

So long as they are logged in, and you are tracking their session within your site via cookies/link-parameters/beacons/whatever, you can be pretty sure that they are using the same computer during that time.

Ultimately, it is incorrect to say this tells you which computer they are using if your users are not using your own local network and do not have static IP addresses.

If what you want to do is being done with the cooperation of the users and there is only one user per cookie and they use a single web browser, just use a cookie.


Cookies won't be useful for determining unique visitors. A user could clear cookies and refresh the site - he then is classed as a new user again.

I think that the best way to go about doing this is to implement a server side solution (as you will need somewhere to store your data). Depending on the complexity of your needs for such data, you will need to determine what is classed as a unique visit. A sensible method would be to allow an IP address to return the following day and be given a unique visit. Several visits from one IP address in one day shouldn't be counted as uniques.

Using PHP, for example, it is trivial to get the IP address of a visitor, and store it in a text file (or a sql database).

A server side solution will work on all machines, because you are going to track the user when he first loads up your site. Don't use javascript, as that is meant for client side scripting, plus the user may have disabled it in any case.

Hope that helps.


Because i want the solution to work on all machines and all browsers (within reason) I am trying to create a solution using javascript.

Isn't that a really good reason not to use javascript?

As others have said - cookies are probably your best option - just be aware of the limitations.


I guess the verdict is i cannot programmatically uniquely identify a computer which is visiting my web site.

I have the following question. When i use a machine which has never visited my online banking web site i get asked for additional authentification. then, if i go back a second time to the online banking site i dont get asked the additional authentification. reading the answers to my question i decided it must be a cookie involved. therefore, i deleted all cookies in IE and relogged onto my online banking site fully expecting to be asked the authentification questions again. to my surprise i was not asked. doesnt this lead one to believe the bank is doing some kind of pc tagging which doesnt involve cookies?

further, after much googling today i found the following company who claims to sell a solution which does uniquely identify machines which visit a web site.

i appreciate all the good information if you could clarify further this conflicting information i found i would greatly appreciate it.


I would do this using a combination of cookies and flash cookies. Create a GUID and store it in a cookie. If the cookie doesn't exist, try to read it from the flash cookie. If it's still not found, create it and write it to the flash cookie. This way you can share the same GUID across browsers.


I think cookies might be what you are looking for; this is how most websites uniquely identify visitors.


Assuming you don't want the user to be in control, you can't. The web doesn't work like that, the best you can hope for is some heuristics.

If it is an option to force your visitor to install some software and use TCPA you may be able to pull something off.


My post might not be a solution, but I can provide an example, where this feature has been implemented.

If you visit the signup page of for the first time from you computer, it's fine. But if you refresh the page or open the page again, it identifies you've previously visited the page. The real beauty comes here - it identifies even if you re-install Windows or other OS.

I read somewhere that they store the CPU ID. Although I couldn't find how do they do it, I seriously doubt it, and they might use MAC Address to do it.

I'll definitely share if I find how to do it.


A Trick:

  1. Create 2 Registration Pages:

    First Registration Page: without any email or security check (just with username and password)

    Second Registration Page: with high security level (email verification request and security image and etc.)

  2. For customer satisfaction, and easy registration, default registration page should be the (First Registration Page) but in the (First Registration Page) there is a hidden restriction. It's IP Restriction. If an IP tried to register for second time, (for example less than 1 hour) instead of showing the block page. you can show the (Second Registration Page) automatically.

  3. in the (First Registration Page) you can set (for example: block 2 attempts from 1 ip for just 1 hour or 24 hours) and after (for example) 1 hour, you can open access from that ip automatically

Please note: (First Registration Page) and (Second Registration Page) should not be in separated pages. you make just 1 page. (for example: register.php) and make it smart to switch between First PHP Style and Second PHP Style


You can use fingerprintjs2

new Fingerprint2().get(function(result, components) {
  console.log(result) // a hash, representing your device fingerprint
  console.log(components) // an array of FP components
  //submit hash and JSON object to the server 

After that you can check all your users against existing and check JSON similarity, so even if their fingerprint mutates, you still can track them


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