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.
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.
Introduction
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:
sesssion:
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:
For logged in users, you can always generate the same session id from the users username / password / email.
You can still track guest users using sessionID.
Even if several people use the same computer (ie cybercafe) you can track them separately if they log in.
Disadvantages of session based tracking:
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.
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.
Implementation
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.
Basics
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).
Legal
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:
No database lookup / overhead / load / latency / space / etc.
User can delete their data whenever they want without the need to write me annoying emails.
and disadvantages:
Data has to be encrypted / decrypted and signed / verified which creates cpu overhead on client (not so bad) and server (bah!).
Data is deleted when user deletes their cookies and cache. (this is what I want really)
Data is unavailable for analytics when users go off-line. (analytics for currently browsing users only)
UUIDS
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
Mechanism
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:
Deletes history of visited websites.
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)
example.com/assets/js/tracking.js (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:
setHeaders({
"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.
example.com/assets/js/tracking.php
$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($sid) ) {
continueSession($sid);
send304();
} else {
startSession($sid);
send304();
}
} else if( $ifModifiedSince ) {
send304();
} else {
startSession();
send200();
}
Now every time the browser requests tracking.js our server will respond with a 304 Not Modified result and force an execute of the local copy of tracking.js.
I still don't understand. Explain it to me
Lets suppose the user clears their browsing history and refreshes the page. The only thing left on the users computer is a copy of tracking.js in browser cache. When the browser requests tracking.js it recieves a 304 Not Modified response which causes it to execute the 1st version of tracking.js it recieved. tracking.js executes and restores the SessionID that was deleted.
Validation
Suppose Haxor X steals our customers cookies while they are still logged in. How do we protect them? Cryptography and Browser fingerprinting to the rescue. Remember our original definition for SessionID was:
BrowserID|ComputerID|randomBytes(256)
We can change this to:
Timestamp|BrowserID|ComputerID|encrypt(randomBytes(256), hk)|sign(Timestamp|BrowserID|ComputerID|randomBytes(256), hk)
Where hk = sign(Timestamp|BrowserID|ComputerID, serverKey).
Now we can validate our SessionID using the following algorithm:
if( getTimestamp($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), $hk) ) return false;
return true;
Now in order for Haxor's attack to work they must:
Have same ComputerID. That means they have to have the same ISP provider as victim (Tricky). This will give our victim the opportunity to take legal action in their own country. Haxor must also obtain HTTPS session key from victim (Hard).
Have same BrowserID. Anyone can spoof User-Agent string (Annoying).
Be able to create their own fake SessionID (Very Hard). Volume atacks won't work because we use a time-stamp to generate encryption / signing key so basically its like generating a new key for each session. On top of that we encrypt random bytes so a simple dictionary attack is also out of the question.
We can improve validation by forwarding GoogleID and FingerprintID (via ajax or hidden fields) and matching against those.
if( GoogleID != getStoredGoodleID($sid) ) return false;
if( byte_difference(FingerPrintID, getStoredFingerprint($sid) > 10%) return false;
These people have developed a fingerprinting method for recognising a user with a high level of accuracy:
https://panopticlick.eff.org/static/browser-uniqueness.pdf
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,
panopticlick.eff.org. 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.
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.
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:
Pixel Perfect: Fingerprinting Canvas in HTML5
https://en.wikipedia.org/wiki/Canvas_fingerprinting
You may want to try setting a unique ID in an evercookie (it will work cross browser, see their FAQs):
http://samy.pl/evercookie/
There is also a company called ThreatMetrix that is used by a lot of big companies to solve this problem:
http://threatmetrix.com/our-solutions/solutions-by-product/trustdefender-id/
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:
https://github.com/carlo/jquery-browser-fingerprint
It looks pretty half baked right now but could be expanded upon.
Hope it helps!
There is only a small amount of information that you can get via an HTTP connection.
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.
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.
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.
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 example-isp.com. 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 example-isp.com.
In the future, it won't ask for extra authentication (beyond your password) when you're accessing the site via example-isp.com. If you try to access the bank via another-isp.com, 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.
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
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. http://www.the41.com/products.asp.
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.
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.
I will give my ideas starting from simpler to more complex.
In all the above you can create sessions and the problem essentialy translates to match session with request.
a) (difficulty: easy) use client hardware to store explicitely a session id/hash of some sort (there are quite some privace/security issues so make sure you hash anything you store ), solutions include:
cookies storage
browser storage/webDB/ (more exotic browser solutions )
extensions with permission to store things in files.
The above suffer from the fact the the user can just empty his cache in case he doesn want.
b) (difficulty: medium) Login based authentication.
Most modern web frameworks provide such solution the core idea is you let the user voluntarily identify himself, quite straghtforward but adds complexity in the architecture.
The above suffer from additional complexity and making essentially non public content.
c)(difficulty: hard -R&D) Identification based on metadata, (browser ip/language /browser / and other privace invasice stuff so make sure you let your users know or you miay get sued )
non perfect solution can get more complicated (a user typing with specific frequency or using mouse with specific patterns ? you even apply ML solutions ).
The claimed solutions
The most powerful since the user even without wanting explicitely he can be identified. It is straight invasion of privacy(see GDPR) and not perfect eg. ip can change .
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 www.supertorrents.org 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:
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.)
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.
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
I want to give a "like" option on my page for non-logged users.
The simpliest thing would be to detect user IP ( e.g. by $_SERVER['REMOTE_ADDR']).
More sophisticated would be detecting user's agent (e.g. by $_SERVER['HTTP_USER_AGENT']).
But I want to give like-posibility for "each PC in family" (real-life family) - this could also mean they all have not only the same IP, not only the same browser but also the same browser-version...
So how would I determinate whether it is a different PC? (without using cookies/session)
I want to store one "like" per PC and since cookies can be cleared I didn't want to use them :)
I wanted to abstract my particular interest from the whole problematics - so I did.
However you should never trust user input (as David pointed out) - so do not base your final like-count on just that ! At least put a likes/per IP limit and combine it with cookies/session.
Your only option to do this outside the simple methods of using cookies, logins, etc. is to do browser fingerprinting. This technique involves gather a variety of information that the browser outputs to the server/webpage and making a hash of it to create a unique ID for that client. It has a remarkably high accuracy and would work fairly well under the circumstances you are describing.
It is based on the idea that "no two browsers are exactly the same". In other words, you look at screen resolution, user agent strings, active plugins, etc. and create a "fingerprint" of those settings. There is almost always going to be a variance in some way.
There are available libraries that can help get you started. Here is one that is very easy to implement and understand... https://github.com/Valve/fingerprintjs
You can use sessions without using cookies. When the user logs in, they get a token, and this token is appended to every URL they visit. (In PHP you can see this if you disable cookies in the browser, you will get "PHPSESSIONID" in the URL). So, if you make users log in before voting / liking / whatever, then you can achieve this using sessions but not cookies.
If you are talking about public users without a login mechanism, then there really isn't any way to achieve this, unless you set a cookie recording the fact that this browser has voted.
Note however that not only can cookies be deleted, but it won't actually achieve what you want unless everyone in the family uses a different browser or has a separate login on their operating system. Otherwise they are effectively all the same user as far as you can tell. Also people can use multiple browsers so one person could vote / like more than once anyway.
Detecting the User Agent can easily be spoofed; so it isnt a reliable way. The best way to do this is sessions or cookies. Why do you not wish to use them?
Short answer: you can't.
Remember, each request to a web server is a new event. Cookies are the only way to persist data between calls. So if you rule them out you really can't differentiate them. This is a major reason why Google puts long life cookies on their site.
Can cookies be deleted? Sure. But they're really the only option you have.
You cannot give a single identity to a PC.
Cookies can be cleared.
User logins can be done from different computers.
$ip.$http_user_agent will not work.
User may restart the modem and ISP might assign a new IP.
Or use a different browser to change $http_user_agent.
Or another system on a LAN might have the same $http_user_agent.
What is the significance of giving one "like" per PC (provided you are able to even identify a PC correctly)?
What if two different users with different tastes use the same PC?
The reason I ask is b.c. of this scenario:
A Remember Me cookie is stolen. It is deleted from the stolen computer and placed on the attackers computer. All the models(this one got many upvotes) I have seen would simply treat the attackers computer like the hacked computer and log the person it.
How would one prevent this? I have two ways both unsuitable.
1.) Only allow only one Remember Me Computer.
2.) Track multiple computers and have the user monitor how many computers have him / her remembered.
Is there a way to track something hard on the computer like the Ethernet MAC address perhaps?
EDIT ANSWER:
Use a MAC address. Below link provides external component for IE and Firefox. Need an external component for Safari.
SO Solution
ADDED FOR CLARIFICATION:
Not IP Addresses (hard as in something that can not change)
Save in the cookie hash of the User Agent + something else (like resolution). Maybe not very secure but better than simple cookie with remember me information and would work for people with dynamic IPs.
Change the cookie on every login/visit. If the computer is stolen, you log in and the cookie changes. Stolen computer has old cookie gets logged out.
If the stolen computer logs in then he/she changes the cookie. The real user gets logged out, and then has to log back in which changes the cookie again.
Tie the cookie to an IP address, server side?
You can set the last remember me loged in ip into a database (or into the cookie)
$_SERVER["HTTP_CLIENT_IP"]
My approach to this would be a hybrid of various techniques. I'd use cookies containing information about the PC that had been logged in - Browser, Resolution, etc, as well as (by default) the IP. Obviously I could match these against the last values recorded in the database to ensure the machine with the cookie hadn't changed.
I'd then provide a method for those with dynamic IP addresses to opt-out of IP address matching for the sessions, with plenty of appropriate warnings that doing so would significantly decrease the security protecting their account, of course.
I like to look at Windows Live Messenger in this instance, if I log into the desktop app as well as hotmail I get a notification that I'm logged into more than 1 place - it asks me if I want to log out of the other location.
What I like about this is it puts control into the users hands, what I don't like about this is an attacker can kick out a genuine user and completely hijack an account.
This whole topic to me looks like you're damned if you do and damned if you don't.
Lets just take a step back for a minute, who can be an attacker? Well anyone really - take for example your ex-husband or ex-wife who knows your usernames and passwords, he/she can easily attack your account because he/she knows all of your information to start with. You can never completely protect yourself, you change your password but he/she probably knows that too!
I ask you to look at your project and evaluate if a remember me system would actually deliver added value over any security risk it poses.
Now, the direct answers for your questions:
Generate a unique key for the user, insert it into your database and write the key to your cookie, when the cookie is sent to the server you evaluate it against the key in the database, if it is a match - bingo, you have access, if not run a logout script and destroy the cookie.
For this, each time you write a cookie store the information in your database against a user ID and status of the cookie (e.g. alive or dead), then each time you read the cookie check the status of it - if it is set to alive then simply log the event and move on but, if it is set to dead you kill the cookie and log that it was killed.
Next you can display this log information and give certain controls to the user, for example you can allow the user to set a cookie to dead then next time you read the cookie you kill it.
Working with hardware is easier said then done but it is doable. The easiest way I have found is to use a Java applet that is run locally, you can then detect hardware information and send it to your application for processing. For your average website I don't personally think it's worth it but it is doable.
Another alternative is to wrap your application in something like Adobe AIR that can detect the hardware information for you to work with.
I hope this helps you.
I need to make sure that every users accessing my web application can do that from one machine only, so 100 users would mean 100 machines. What would be the best solution? Is detecting and storing IP during first login good idea? I think IP might change even during lifetime of the session is that right? I was also thinking of storing cookie when user first logs in. Then assigning these cookie to the user, same as I do with password and username already, and every time when accessing application checking for presence of that cookie.
Please let me know what in your opinion would be the best solution. My backend is php/mysql if that matters.
EDIT: I need to clarify... This is in addition to normal session management. I need to restrict users to be able to login to web application from one specific machine only. So if user originally logged in from his computer at work and I stored its ip/cookie/etc., then client logs out (or even not), goes home and tries to login won't be able to do that. I agree its horrible idea but client insists :)
IP address might change in the case of mobile clients, or clients that switch between wired and wireless networks. Your best bet would probably be to provide a randomly-generated UID to each client when it first connects (if it doesn't already have the cookie). Then you can check that the same username isn't connecting using two different UIDs.
The trick is that you need to make sure to time this UID out, so that if the user goes to another computer they aren't locked out. Perhaps one change to the UID is okay, but they can't go back to a UID that's already been used?
You can limit to a single useragent by issuing the client with a client side SSL certificate created with the keygen element, this gets the browser to generate a key pair, keeping the private key in the user agent, then you receive an SPKAC, which you can use to openssl create a certificate, which you then send back to the user agent, it installs it and it can be used to identify the user in that specific browser only via HTTP+TLS from then on.
Anything else, simply won't work 100% - although you can hack ways that appear to work (until something goes wrong and it doesn't work) :)
Unfortunately, an IP is not machine-specific for multiple reasons:
The IP address could change during the session, with no notice (the user might not even be aware of it)
Most users have dynamic IP, so it most definitely will change at some point
For machines such as a laptop, tablet or cell phone, the IP address is based on the current service provider
All users behind a proxy would appear to you as a single IP, so you still wouldn't be able to detect if they moved from one machine to another
Instead, generate some kind of unique key for the session and track it in combination with the user name. Prevent them from logging in if the same user name is already in another active session. (You'll also want some way to automatically flush these, just in case you lose the session-end event.)
The best solution is already built into the web server depending on which one you are using. That's what the Sessions are for. In ASP.NET/IIS, usually there is a 20minutes per session timeout.
So if a user uses another computer to access your webapplication, then the session timeout will release connection from the machine that is idle.
UPDATE
You might want to consider restricting user by the MAC Address of their machines which are unique.
If it is a very internal application that will be used only inside a company, it might be possible to define an IP range because smaller companies which do not operate worldwide will probably have a certain amount of IPs from their internet access provider.
You could also think about using some info from $_SERVER to restrict users to a combnation of a single web browser (HTTP_USER_AGENT) and a single port (REMOTE_PORT) - as an additional way to differentiate machines.
But all these solutions are bad or worse, it's technically probably not possible to solve this problem (unless you will have guarantees from your client that all machines will keep a static IP in which case it is a trivial if else problem).
Don't do that. Many people will access your website from multiple computers, and they will complain if you block them.