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	diff --git a/support/startup/__tests__/PreambleUtilsTestCase.php b/support/startup/__tests__/PreambleUtilsTestCase.php
	new file mode 100755
	index 0000000000..25062e3701
	--- /dev/null
	+++ b/support/startup/__tests__/PreambleUtilsTestCase.php
	@@ -0,0 +1,123 @@
	+#!/usr/bin/env php
	+<?php
	+
	+/**
	+ * /startup/ is not a Phutil library, so it can't use the phutil test fixture.
	+ * This script will just run the tests directly.
	+ *
	+ * NOTE: This test file will not run as part of `arc unit` run!
	+ */
	+
	+
	+final class PreambleUtilsTestCase {
	+
	+
	+ public function testTrustXForwardValues() {
	+ // For specific values of `$_SERVER['HTTP_X_FORWARDED_FOR']`,
	+ // `$_SERVER['REMOTE_ADDR']` will be updated with the result.
	+
	+
	+ $undefined = 'SPECIAL::UNDEFINED';
	+ $null_value = 'SPECIAL::NULL';
	+
	+ $test_cases = array(
	+ 'abc' => 'abc',
	+ $null_value => $undefined,
	+ '' => $undefined,
	+
	+ // Strange, unexpected cases:
	+ 144 => '144',
	+ );
	+
	+ foreach ($test_cases as $input => $expected) {
	+ switch ($input) {
	+ case $undefined:
	+ unset($_SERVER['HTTP_X_FORWARDED_FOR']);
	+ break;
	+
	+ case $null_value:
	+ $_SERVER['HTTP_X_FORWARDED_FOR'] = null;
	+ break;
	+
	+ default:
	+ $_SERVER['HTTP_X_FORWARDED_FOR'] = $input;
	+ break;
	+ }
	+
	+ unset($_SERVER['REMOTE_ADDR']);
	+
	+ preamble_trust_x_forwarded_for_header();
	+
	+ if (!isset($_SERVER['REMOTE_ADDR'])) {
	+ if ($expected === $undefined) {
	+ // test pass
	+ continue;
	+ } else {
	+ $this->failTest("Failed for input {$input} - result is not defined!");
	+ }
	+ }
	+
	+ $actual = $_SERVER['REMOTE_ADDR'];
	+
	+ if ($actual !== $expected) {
	+ var_dump($actual);
	+
	+ $this->failTest(
	+ "Failed for input {$input} - actual output is {$actual}");
	+ }
	+
	+ }
	+
	+ }
	+
	+
	+ private function failTest($message = null) {
	+ echo $message;
	+ echo "\n";
	+ throw new Exception();
	+ }
	+
	+ /**
	+ * Run all tests in this class.
	+ *
	+ * Return: True if all tests passed; False if any test failed.
	+ */
	+ final public function run() {
	+ $reflection = new ReflectionClass($this);
	+ $methods = $reflection->getMethods();
	+
	+ $any_fail = false;
	+
	+ // Try to ensure that poorly-written tests which depend on execution order
	+ // (and are thus not properly isolated) will fail.
	+ shuffle($methods);
	+
	+ foreach ($methods as $method) {
	+ $name = $method->getName();
	+ if (!preg_match('/^test/', $name)) {
	+ continue;
	+ }
	+
	+ try {
	+ call_user_func_array(
	+ array($this, $name),
	+ array());
	+ echo "Test passed: {$name}\n";
	+ } catch (Throwable $ex) {
	+ $any_fail = true;
	+ echo "Failed test: {$name}\n";
	+ }
	+ }
	+ return !$any_fail;
	+ }
	+
	+}
	+
	+require_once dirname(__DIR__).'/preamble-utils.php';
	+
	+$test_case = new PreambleUtilsTestCase();
	+$good = $test_case->run();
	+
	+if (!$good) {
	+ exit(3);
	+}
	diff --git a/support/startup/preamble-utils.php b/support/startup/preamble-utils.php
	index dfb5619a43..8dd3b502d6 100644
	--- a/support/startup/preamble-utils.php
	+++ b/support/startup/preamble-utils.php
	@@ -1,77 +1,77 @@
	<?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 = $_SERVER['HTTP_X_FORWARDED_FOR'];
	- if (!phutil_nonempty_string($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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