HTTP-Headers
HTTP message-headers are used by HTTP, the Hypertext-Transfer Protocol, to control connections or supply additional information like the browser's version string, the system language or even dynamic content such as cookies. A HTTP message-header as defined by RFC2616 looks like this:
message-header = field-name ":" [ field-value ]
field-name = token
field-value = *( field-content | LWS )
field-content = <the OCTETs making up the field-value
and consisting of either *TEXT or combinations
of token, separators, and quoted-string>
This shows us that a HTTP header consists of several fields, each containing its name and value, separated by a ":".
HTTP-Headers and the $_SERVER Variable
The $_SERVER variable is the preferred method of retrieving external variables. It provides a PHP programmer with various information related to the server, provided by the webserver, and clients, transmitted via the HTTP headers which are the "HTTP_"-prefixed elements in $_SERVER. Now to get to the point: A connected clients IP address is stored in the REMOTE_ADDR element of the array $_SERVER. If proxies are used to open a connection the value of this element is changed to the IP of the proxy, which breaks any reference a script may try to establish using just this field of the header.
Proxies and HTTP Headers
As proxies introduce additional functionality, they also introduce new HTTP message-header fields. One of these additional fields is stored inside the HTTP_X_FORWARD element and is used by various proxy servers to submit the IP of the client using it. Similar to this the HTTP_X_FORWARDED_FOR field is used and may be seen as an indication of a relayed connection. Using just these two elements of the HTTP header one might find references to users trying to avoid detection schemes. On the other hand, a user trying to circumvent such a measure may be advised to check for these header fields being used by his chosen proxy.
Using Cookies to Detect Relayed Communication
Since the possibility of detecting a stealth connection by the means of HTTP message-headers is limited by the fact that the proxy is responsible for sending the clients information in order to process them; so, if the proxy is completely anonymous, not relaying any client information whatsoever, the only way left is to make the client tell you who he is. To make this more understandable here is a piece of code I use to detect various users using "stealth-proxies":
$r_ip=$_SERVER['REMOTE_ADDR'];
$seed="8hasdv9fjadzf8jzssadf0a7shi";
$e_id=query_base_entry($r_ip);
if($e_id!=0) {
$cookiename="myPROXYdetectionCOOKIE";
$olcookie=$_COOKIE[$cookiename]; // old visit's cookie
$thiscookie=md5($host.$seed); // this visits cookie
if($olcookie&&$thiscookie!=$olcookie) {
if(!islogcookie($e_id,$olcookie)) add_logcookie(get_bidbylogcookie($olcookie),$thiscookie);
if(!islogcookie($e_id,$thiscookie)) add_logcookie($e_bid,$thiscookie);
}
if(!$olcookie) {
setcookie($cookiename,$thiscookie); // send user his cookie (he aint got none)
if(!$cthiscookie=get_logcookie($e_id,$thiscookie)) { // if a reference between the cookie and $e_id is not saved yet, INSERT one..
add_logcookie($e_id,$thiscookie);
}
} elseif($olcookie&&$thiscookie==$olcookie) {
// we loged this user, and he gots his cookie
// yas # yet another sheep
}
I use a table of unique IP's that are referenced to timestamps, information on individual clicks, cookies, proxies, etc. to implement integer ID's for any host that connected to one of my sites. I will not publish all the functions used in this piece of code since it would fail the subject of this article. Still, if you are interested in this project of mine, I might publish it if you ask me to do so :) Nevertheless, to ensure a sane understanding of what's going on, here a short explanation on what these functions do:
query_base_entry($r_ip);--returns the unique ID of an IP, $r_ip as referenced to by the database
islogcookie($id,$cookie);--queries a database of ip-addresses if a certain cookie is already referenced to IP(s)
add_logcookie($id,$cookie);--adds a reference between a $cookie and an IP address as referenced to by $id
get_bidbylogcookie($cookie);--returns the unique id of a users IP that has been given a cookie which has now been found to be used by a client with a different ip. This indicates a change of location, like a laptop, or the use of a stealth proxy
What the above code does is lookup a database for a unique ID linked to the connected user's IP address. If it finds one, it looks if a cookie is set that is also referenced to a different IP address by a local database. If such a cookie is found, the script has detected a user that changed his IP-address or is using a stealth proxy.
Conclusion
Programmers
A client can run, but hiding is a whole different game where rules have to be learned first . . .
Users
Using a proxy does not automatically make you anonymous on the web. You need to stealth or erase every reference to you, HTTP headers, and your previous activities cookies to make sure not to be detected.
References/Lookups
PHP-Docs Predefined Variables/$_SERVER
PHP-Docs setcookie()
RFC2616 Hypertext Transfer Protocol--HTTP/1.1 |
Copyright Open Source Institute, 2006 
