Single login across multiple domains

[A Dead Puppie]

I've seen mods talking about a single login across subdomains and stuff, but what about full domains?

for example i have example.com, example2.com, example3.com and example4.com all running the same version of vBulletin with different themes/mods/purposes

is there a mod or a way to have it so your username/pass on example2.com will work across all of them, and vice versa?

[kh99]

Well, I think it would have to rely on cookies to identify that the same user is visiting each site. Since cookies are only sent to the domain they originate from, you can't just set a cookie to keep someone logged in on all domains. My understanding is that this problem is normally handled by having one domain handle the cookies, and when a user visits one of the other domains and they are not logged in, they would be redirected to the cookie-handling domain which would then redirect them back with some informaiton to log them in. It could be done in vbulletin of course but it would take some programming.