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	diff --git a/.gitignore b/.gitignore
	--- a/.gitignore
	+++ b/.gitignore
	@@ -33,6 +33,9 @@
	# User-accessible hook for adhoc debugging scripts
	/support/debug.php

	+# User-accessible hook for adhoc startup code
	+/support/preamble.php
	+
	# Users can link binaries here
	/support/bin/*

	diff --git a/src/docs/user/configuration/configuration_guide.diviner b/src/docs/user/configuration/configuration_guide.diviner
	--- a/src/docs/user/configuration/configuration_guide.diviner
	+++ b/src/docs/user/configuration/configuration_guide.diviner
	@@ -197,6 +197,9 @@
	@{article:Configuring Accounts and Registration}; or
	- understanding advanced configuration topics with
	@{article:Configuration User Guide: Advanced Configuration}; or
	+ - configuring a preamble script to set up the environment properly behind a
	+ load balancer, or adjust rate limiting with
	+ @{article:Configuring a Preamble Script}; or
	- configuring where uploaded files and attachments will be stored with
	@{article:Configuring File Storage}; or
	- configuring Phabricator so it can send mail with
	diff --git a/src/docs/user/configuration/configuring_preamble.diviner b/src/docs/user/configuration/configuring_preamble.diviner
	new file mode 100644
	--- /dev/null
	+++ b/src/docs/user/configuration/configuring_preamble.diviner
	@@ -0,0 +1,114 @@
	+@title Configuring a Preamble Script
	+@group config
	+
	+Adjust environmental settings (SSL, remote IP, rate limiting) using a preamble
	+script.
	+
	+= Overview =
	+
	+If Phabricator is deployed in an environment where HTTP headers behave oddly
	+(usually, because it is behind a load balancer), it may not be able to detect
	+some environmental features (like the client's IP, or the presence of SSL)
	+correctly.
	+
	+You can use a special preamble script to make arbitrary adjustments to the
	+environment and some parts of Phabricator's configuration in order to fix these
	+problems and set up the environment which Phabricator expects.
	+
	+NOTE: This is an advanced feature. Most installs should not need to configure
	+a preamble script.
	+
	+= Creating a Preamble Script =
	+
	+To create a preamble script, write a file to:
	+
	+ phabricator/support/preamble.php
	+
	+(This file is in Phabricator's `.gitignore`, so you do not need to worry about
	+colliding with `git` or interacting with updates.)
	+
	+This file should be a valid PHP script. If you aren't very familiar with PHP,
	+you can check for syntax errors with `php -l`:
	+
	+ phabricator/ $ php -l support/preamble.php
	+ No syntax errors detected in support/preamble.php
	+
	+If present, this script will be executed at the very beginning of each web
	+request, allowing you to adjust the environment. For common adjustments and
	+examples, see the next sections.
	+
	+= Adjusting Client IPs =
	+
	+If your install is behind a load balancer, Phabricator may incorrectly detect
	+all requests as originating from the load balancer, rather than from the correct
	+client IPs. If this is the case and some other header (like `X-Forwarded-For`)
	+is known to be trustworthy, you can overwrite the `REMOTE_ADDR` setting so
	+Phabricator can figure out the client IP correctly:
	+
	+```
	+name=Overwrite REMOTE_ADDR with X-Forwarded-For
	+<?php
	+
	+$_SERVER['REMOTE_ADDR'] = $_SERVER['HTTP_X_FORWARDED_FOR'];
	+```
	+
	+You should do this //only// if the `X-Forwarded-For` header is always
	+trustworthy. In particular, if users can make requests to the web server
	+directly, they can provide an arbitrary `X-Forwarded-For` header, and thereby
	+spoof an arbitrary client IP.
	+
	+= Adjusting SSL =
	+
	+If your install is behind an SSL terminating load balancer, Phabricator may
	+detect requests as HTTP when the client sees them as HTTPS. This can cause
	+Phabricator to generate links with the wrong protocol, issue cookies without
	+the SSL-only flag, or reject requests outright.
	+
	+To fix this, you can set `$_SERVER['HTTPS']` explicitly:
	+
	+```
	+name=Explicitly Configure SSL Availability
	+<?php
	+
	+$_SERVER['HTTPS'] = true;
	+```
	+
	+You can also set this value to `false` to explicitly tell Phabricator that a
	+request is not an SSL request.
	+
	+= Adjusting Rate Limiting =
	+
	+Phabricator performs coarse, IP-based rate limiting by default. In most
	+situations the default settings should be reasonable: they are set fairly high,
	+and intended to prevent only significantly abusive behavior.
	+
	+However, if legitimate traffic is being rate limited (or you want to make the
	+limits more strict) you can adjust the limits in the preamble script.
	+
	+```
	+name=Adjust Rate Limiting Behavior
	+<?php
	+
	+// The default is 1000, so a value of 2000 increases the limit by a factor
	+// of 2: users will be able to make twice as many requests before being
	+// rate limited.
	+
	+// You can set the limit to 0 to disable rate limiting.
	+
	+PhabricatorStartup::setMaximumRate(2000);
	+```
	+
	+By examining `$_SERVER['REMOTE_ADDR']` or similar parameters, you could also
	+adjust the rate limit dynamically: for example, remove it for requests from an
	+internal network, but impose a strict limit for external requests.
	+
	+Rate limiting needs to be configured in this way in order to make it as cheap as
	+possible to activate after a client is rate limited. The limiting checks execute
	+before any libraries or configuration are loaded, and can emit a response within
	+a few milliseconds.
	+
	+= Next Steps =
	+
	+Continue by:
	+
	+ - returning to the @{article:Configuration Guide}.
	diff --git a/support/PhabricatorStartup.php b/support/PhabricatorStartup.php
	--- a/support/PhabricatorStartup.php
	+++ b/support/PhabricatorStartup.php
	@@ -8,10 +8,32 @@
	* NOTE: This class MUST NOT have any dependencies. It runs before libraries
	* load.
	*
	+ * Rate Limiting
	+ * =============
	+ *
	+ * Phabricator limits the rate at which clients can request pages, and issues
	+ * HTTP 429 "Too Many Requests" responses if clients request too many pages too
	+ * quickly. Although this is not a complete defense against high-volume attacks,
	+ * it can protect an install against aggressive crawlers, security scanners,
	+ * and some types of malicious activity.
	+ *
	+ * To perform rate limiting, each page increments a score counter for the
	+ * requesting user's IP. The page can give the IP more points for an expensive
	+ * request, or fewer for an authetnicated request.
	+ *
	+ * Score counters are kept in buckets, and writes move to a new bucket every
	+ * minute. After a few minutes (defined by @{method:getRateLimitBucketCount}),
	+ * the oldest bucket is discarded. This provides a simple mechanism for keeping
	+ * track of scores without needing to store, access, or read very much data.
	+ *
	+ * Users are allowed to accumulate up to 1000 points per minute, averaged across
	+ * all of the tracked buckets.
	+ *
	* @task info Accessing Request Information
	* @task hook Startup Hooks
	* @task apocalypse In Case Of Apocalypse
	* @task validation Validation
	+ * @task ratelimit Rate Limiting
	*/
	final class PhabricatorStartup {

	@@ -19,6 +41,7 @@
	private static $globals = array();
	private static $capturingOutput;
	private static $rawInput;
	+ private static $maximumRate = 1000;


	/* -( Accessing Request Information )-------------------------------------- */
	@@ -93,6 +116,10 @@
	self::setupPHP();
	self::verifyPHP();

	+ if (isset($_SERVER['REMOTE_ADDR'])) {
	+ self::rateLimitRequest($_SERVER['REMOTE_ADDR']);
	+ }
	+
	self::normalizeInput();

	self::verifyRewriteRules();
	@@ -521,4 +548,229 @@
	"'post_max_size' is set to '{$config}'.");
	}

	+
	+/* -( Rate Limiting )------------------------------------------------------ */
	+
	+
	+ /**
	+ * Adjust the permissible rate limit score.
	+ *
	+ * By default, the limit is `1000`. You can use this method to set it to
	+ * a larger or smaller value. If you set it to `2000`, users may make twice
	+ * as many requests before rate limiting.
	+ *
	+ * @param int Maximum score before rate limiting.
	+ * @return void
	+ * @task ratelimit
	+ */
	+ public static function setMaximumRate($rate) {
	+ self::$maximumRate = $rate;
	+ }
	+
	+
	+ /**
	+ * Check if the user (identified by `$user_identity`) has issued too many
	+ * requests recently. If they have, end the request with a 429 error code.
	+ *
	+ * The key just needs to identify the user. Phabricator uses both user PHIDs
	+ * and user IPs as keys, tracking logged-in and logged-out users separately
	+ * and enforcing different limits.
	+ *
	+ * @param string Some key which identifies the user making the request.
	+ * @return void If the user has exceeded the rate limit, this method
	+ * does not return.
	+ * @task ratelimit
	+ */
	+ public static function rateLimitRequest($user_identity) {
	+ if (!self::canRateLimit()) {
	+ return;
	+ }
	+
	+ $score = self::getRateLimitScore($user_identity);
	+ if ($score > (self::$maximumRate * self::getRateLimitBucketCount())) {
	+ // Give the user some bonus points for getting rate limited. This keeps
	+ // bad actors who keep slamming the 429 page locked out completely,
	+ // instead of letting them get a burst of requests through every minute
	+ // after a bucket expires.
	+ self::addRateLimitScore($user_identity, 50);
	+ self::didRateLimit($user_identity);
	+ }
	+ }
	+
	+
	+ /**
	+ * Add points to the rate limit score for some user.
	+ *
	+ * If users have earned more than 1000 points per minute across all the
	+ * buckets they'll be locked out of the application, so awarding 1 point per
	+ * request roughly corresponds to allowing 1000 requests per second, while
	+ * awarding 50 points roughly corresponds to allowing 20 requests per second.
	+ *
	+ * @param string Some key which identifies the user making the request.
	+ * @param float The cost for this request; more points pushes them toward
	+ * the limit faster.
	+ * @return void
	+ * @task ratelimit
	+ */
	+ public static function addRateLimitScore($user_identity, $score) {
	+ if (!self::canRateLimit()) {
	+ return;
	+ }
	+
	+ $current = self::getRateLimitBucket();
	+
	+ // There's a bit of a race here, if a second process reads the bucket before
	+ // this one writes it, but it's fine if we occasionally fail to record a
	+ // user's score. If they're making requests fast enough to hit rate
	+ // limiting, we'll get them soon.
	+
	+ $bucket_key = self::getRateLimitBucketKey($current);
	+ $bucket = apc_fetch($bucket_key);
	+ if (!is_array($bucket)) {
	+ $bucket = array();
	+ }
	+
	+ if (empty($bucket[$user_identity])) {
	+ $bucket[$user_identity] = 0;
	+ }
	+
	+ $bucket[$user_identity] += $score;
	+ apc_store($bucket_key, $bucket);
	+ }
	+
	+
	+ /**
	+ * Determine if rate limiting is available.
	+ *
	+ * Rate limiting depends on APC, and isn't available unless the APC user
	+ * cache is available.
	+ *
	+ * @return bool True if rate limiting is available.
	+ * @task ratelimit
	+ */
	+ private static function canRateLimit() {
	+ if (!self::$maximumRate) {
	+ return false;
	+ }
	+
	+ if (!function_exists('apc_fetch')) {
	+ return false;
	+ }
	+
	+ return true;
	+ }
	+
	+
	+ /**
	+ * Get the current bucket for storing rate limit scores.
	+ *
	+ * @return int The current bucket.
	+ * @task ratelimit
	+ */
	+ private static function getRateLimitBucket() {
	+ return (int)(time() / 60);
	+ }
	+
	+
	+ /**
	+ * Get the total number of rate limit buckets to retain.
	+ *
	+ * @return int Total number of rate limit buckets to retain.
	+ * @task ratelimit
	+ */
	+ private static function getRateLimitBucketCount() {
	+ return 5;
	+ }
	+
	+
	+ /**
	+ * Get the APC key for a given bucket.
	+ *
	+ * @param int Bucket to get the key for.
	+ * @return string APC key for the bucket.
	+ * @task ratelimit
	+ */
	+ private static function getRateLimitBucketKey($bucket) {
	+ return 'rate:bucket:'.$bucket;
	+ }
	+
	+
	+ /**
	+ * Get the APC key for the smallest stored bucket.
	+ *
	+ * @return string APC key for the smallest stored bucket.
	+ * @task ratelimit
	+ */
	+ private static function getRateLimitMinKey() {
	+ return 'rate:min';
	+ }
	+
	+
	+ /**
	+ * Get the current rate limit score for a given user.
	+ *
	+ * @param string Unique key identifying the user.
	+ * @return float The user's current score.
	+ * @task ratelimit
	+ */
	+ private static function getRateLimitScore($user_identity) {
	+ $min_key = self::getRateLimitMinKey();
	+
	+ // Identify the oldest bucket stored in APC.
	+ $cur = self::getRateLimitBucket();
	+ $min = apc_fetch($min_key);
	+
	+ // If we don't have any buckets stored yet, store the current bucket as
	+ // the oldest bucket.
	+ if (!$min) {
	+ apc_store($min_key, $cur);
	+ $min = $cur;
	+ }
	+
	+ // Destroy any buckets that are older than the minimum bucket we're keeping
	+ // track of. Under load this normally shouldn't do anything, but will clean
	+ // up an old bucket once per minute.
	+ $count = self::getRateLimitBucketCount();
	+ for ($cursor = $min; $cursor < ($cur - $count); $cursor++) {
	+ apc_delete(self::getRateLimitBucketKey($cursor));
	+ apc_store($min_key, $cursor + 1);
	+ }
	+
	+ // Now, sum up the user's scores in all of the active buckets.
	+ $score = 0;
	+ for (; $cursor <= $cur; $cursor++) {
	+ $bucket = apc_fetch(self::getRateLimitBucketKey($cursor));
	+ if (isset($bucket[$user_identity])) {
	+ $score += $bucket[$user_identity];
	+ }
	+ }
	+
	+ return $score;
	+ }
	+
	+
	+ /**
	+ * Emit an HTTP 429 "Too Many Requests" response (indicating that the user
	+ * has exceeded application rate limits) and exit.
	+ *
	+ * @return exit This method does not return.
	+ * @task ratelimit
	+ */
	+ private static function didRateLimit() {
	+ $message =
	+ "TOO MANY REQUESTS\n".
	+ "You are issuing too many requests too quickly.\n".
	+ "To adjust limits, see \"Configuring a Preamble Script\" in the ".
	+ "documentation.";
	+
	+ header(
	+ 'Content-Type: text/plain; charset=utf-8',
	+ $replace = true,
	+ $http_error = 429);
	+
	+ echo $message;
	+
	+ exit(1);
	+ }
	+
	}
	diff --git a/webroot/index.php b/webroot/index.php
	--- a/webroot/index.php
	+++ b/webroot/index.php
	@@ -1,6 +1,14 @@
	<?php

	-require_once dirname(dirname(__FILE__)).'/support/PhabricatorStartup.php';
	+$phabricator_root = dirname(dirname(__FILE__));
	+require_once $phabricator_root.'/support/PhabricatorStartup.php';
	+
	+// If the preamble script exists, load it.
	+$preamble_path = $phabricator_root.'/support/preamble.php';
	+if (file_exists($preamble_path)) {
	+ require_once $preamble_path;
	+}
	+
	PhabricatorStartup::didStartup();

	$show_unexpected_traces = false;
	@@ -142,6 +150,23 @@
	));

	DarkConsoleXHProfPluginAPI::saveProfilerSample($access_log);
	+
	+ // Add points to the rate limits for this request.
	+ if (isset($_SERVER['REMOTE_ADDR'])) {
	+ $user_ip = $_SERVER['REMOTE_ADDR'];
	+
	+ // The base score for a request allows users to make 30 requests per
	+ // minute.
	+ $score = (1000 / 30);
	+
	+ // If the user was logged in, let them make more requests.
	+ if ($request->getUser() && $request->getUser()->getPHID()) {
	+ $score = $score / 5;
	+ }
	+
	+ PhabricatorStartup::addRateLimitScore($user_ip, $score);
	+ }
	+
	} catch (Exception $ex) {
	PhabricatorStartup::didEncounterFatalException(
	'Core Exception',

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