Recognising visitors between domains

Question

Is it at all possible to retrieve user information that can be used as a unique identifier between domains?

As a quick example of what I am trying to do (not exactly this but the theory is the same) say you had a main website at UK-news.com. You also had three other sites - England-news.com, Scotland-news.com and Wales-news.com, all hosted on the same server.

All 4 sites will share the same database and each would just pull the relevant info out of it. If a user becomes a member of one of the sites, they will also be given the option to become a member of any or all of the others. If a user signs-in to one of the sites he is a member of, and then goes to another how can I get that site to recognise him from the one he signed-in on so he is automatically logged in?

My theory was to store some user information (IP, USER_AGENT, browser, screen resolution, computer name, OS) in the database via PhP and then check against all of those as the user moves between sites. however, even checking against all of these, I am sure it will be possible for two different people to have exactly the same details.

Are there any truly unique identifiers that will guarantee that a person is recognised between domains?

Thanks Steve

Answer 1

I dont know what is the configuration of your server, but. If one site is under something.domain.com, and another something2.domain.com, and the domains England-news.com and Scotland-news.com are only links to those sites, you can use url overwrite, and cookies sharing over subdomains option in php. But I think that this is not the case. So...

There is no 100% sure user recognition. And this is great, imagine what will happen if there would be. You can NEVER trust user data, and headers data, while sometime you can not even trust $_SERVER array. So there is no option to recognize the same user over few domains.

1) The only answer that is useful is to suggest you to share the user mysql table, and make all the logins and passwords same for each site. IN that case someone can login into another site using the same data.

2) You can try to rely on second hand services like google acount or facebook acount to verify users on your site. But you must remember that there are people without gmail and facebook, and availability of such a site will be reduced.

3) Use a serrvice like forever cookie, or something like that, but this is also not 100% sure. It is using html5 storage, flash objects, and everything to verify if this is the same user. But as far as I know, everything can be ommited, if you are patient enough.

Best regards!