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preamble-utils.php
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<?php
/**
* Parse the "X_FORWARDED_FOR" HTTP header to determine the original client
* address.
*
* @param int Number of devices to trust.
* @return void
*/
function preamble_trust_x_forwarded_for_header($layers = 1) {
if (!is_int($layers) || ($layers < 1)) {
echo
'preamble_trust_x_forwarded_for_header(<layers>): '.
'"layers" parameter must an integer larger than 0.'."\n";
echo "\n";
exit(1);
}
if (!isset($_SERVER['HTTP_X_FORWARDED_FOR'])) {
return;
}
$forwarded_for = $_SERVER['HTTP_X_FORWARDED_FOR'];
if (!strlen($forwarded_for)) {
return;
}
$address = preamble_get_x_forwarded_for_address($forwarded_for, $layers);
$_SERVER['REMOTE_ADDR'] = $address;
}
function preamble_get_x_forwarded_for_address($raw_header, $layers) {
// The raw header may be a list of IPs, like "1.2.3.4, 4.5.6.7", if the
// request the load balancer received also had this header. In particular,
// this happens routinely with requests received through a CDN, but can also
// happen illegitimately if the client just makes up an "X-Forwarded-For"
// header full of lies.
// We can only trust the N elements at the end of the list which correspond
// to network-adjacent devices we control. Usually, we're behind a single
// load balancer and "N" is 1, so we want to take the last element in the
// list.
// In some cases, "N" may be more than 1, if the network is configured so
// that that requests are routed through multiple layers of load balancers
// and proxies. In this case, we want to take the Nth-to-last element of
// the list.
$addresses = explode(',', $raw_header);
// If we have more than one trustworthy device on the network path, discard
// corresponding elements from the list. For example, if we have 7 devices,
// we want to discard the last 6 elements of the list.
// The final device address does not appear in the list, since devices do
// not append their own addresses to "X-Forwarded-For".
$discard_addresses = ($layers - 1);
// However, we don't want to throw away all of the addresses. Some requests
// may originate from within the network, and may thus not have as many
// addresses as we expect. If we have fewer addresses than trustworthy
// devices, discard all but one address.
$max_discard = (count($addresses) - 1);
$discard_count = min($discard_addresses, $max_discard);
if ($discard_count) {
$addresses = array_slice($addresses, 0, -$discard_count);
}
$original_address = end($addresses);
$original_address = trim($original_address);
return $original_address;
}

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Content licensed under Creative Commons Attribution-ShareAlike 4.0 (CC-BY-SA) unless otherwise noted; code licensed under Apache 2.0 or other open source licenses. · CC BY-SA 4.0 · Apache 2.0