Tracking Users By IP Address

[Nick Craig-Wood]

Paul McNett <p at ulmcnett.com> wrote:
> > Some (many? all? negligibly few? I don't know) proxy
> > firewalls will tell you which internal address the request
> > came from, so you can combine the two to get a unique address
> > for that user.
> 
>  I'd say "negligibly few", since there really isn't good reason 
>  to give the world your internal ip address. Anyway, you can't 
>  count on it, and you wouldn't have a basis to know what 
>  percentage weren't unique.

An awful lot of proxies do do exactly that (I used this technique
before).

HTTP_X_FORWARDED_FOR and REMOTE_ADDR are the headers you want

Its not safe to use this as a unique address for a user but if you are
interested in exactly where your users are (for fraud checking
purposes) its a useful technique.

-- 
Nick Craig-Wood <nick at craig-wood.com> -- http://www.craig-wood.com/nick