关于[HTTP_X_FORWARDED_FOR]的安全问题，我应该使用它而不是[REMOTE_ADDR]吗？

Question

I'm trying to create a php counter, and in order not to count repeated visits from the same visitor, I've been thinking about saving the visitor's IP address into the database, and I should turn to $_SERVER

I've read this sample funtion by @Dusza that seems nice and convenient:

<?php
function get_IP() {

// ADDRESS IP
   if     (getenv('HTTP_CLIENT_IP'))       $ipaddress = getenv('HTTP_CLIENT_IP');
   else if(getenv('HTTP_X_FORWARDED_FOR')) $ipaddress = getenv('HTTP_X_FORWARDED_FOR');
   else if(getenv('HTTP_X_FORWARDED'))     $ipaddress = getenv('HTTP_X_FORWARDED');
   else if(getenv('HTTP_FORWARDED_FOR'))   $ipaddress = getenv('HTTP_FORWARDED_FOR');
   else if(getenv('HTTP_FORWARDED'))       $ipaddress = getenv('HTTP_FORWARDED');
   else if(getenv('REMOTE_ADDR'))          $ipaddress = getenv('REMOTE_ADDR');
   else                                    $ipaddress = 'UNKNOWN';
//return $ipaddress;
    }
?>

But I've done some research here, and found that there's a security hole in that because the user can spoof all values except REMOTE_ADDR, which can be modified by a proxy.

So I guess that when they say that there's a security hole, it means that I should sanitize the user's input when I insert it into the database doing some bindings.

Is there any other precaution?

Given that all other values are unreliable I should avoid using them altogether?

But what about the un-spoffing value of REMOTE_ADDR? That can be modified by a proxy.

Any suggestions on what path should I take?

If you want to downvote, or vote the question to be closed or deleted, please leave me a comment about why, so I can improve my questions. Thanks.

Answer 1

REMOTE_ADDR is the IP address established through a 3-way TCP/IP handshake. It is the IP the response will be sent back to. It is the only thing that your server has verified. Everything else is just arbitrary HTTP headers anyone could set.

Now, if you know that your server is running behind a proxy (e.g. a load balancer) which would mask the visitor's IP address (your server would only see the proxy's IP), but you know that the proxy is helpfully forwarding you the visitor's IP in an HTTP header (as workaround for this situation so your server can still see the visitor's IP), then and only then may you use one of these HTTP headers and only the one that you know your proxy is setting. If your server is not behind a proxy, use REMOTE_ADDR exclusively. Otherwise, consult your proxy's manual and implement according to the situation.