Configuring a Preamble Script

Phabricator User Documentation (Configuration)

Adjust environmental settings (SSL, remote IPs) 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.

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.

In common cases where networks are configured like this, the X-Forwarded-For header will have trustworthy information about the real client IP. You can use the function preamble_trust_x_forwarded_for_header() in your preamble to tell Phabricator that you expect to receive requests from a load balancer or proxy which modifies this header:

preamble_trust_x_forwarded_for_header();

You should do this only if the X-Forwarded-For header is known to be 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.

The X-Forwarded-For header may also contain a list of addresses if a request has been forwarded through multiple load balancers. If you know that requests on your network are routed through N trustworthy devices, you can specify that N to tell the function how many layers of X-Forwarded-For to discard:

preamble_trust_x_forwarded_for_header(3);

If you have an unusual network configuration (for example, the number of trustworthy devices depends on the network path) you can also implement your own logic.

Note that this is very odd, advanced, and easy to get wrong. If you get it wrong, users will most likely be able to spoof any client address.

if (isset($_SERVER['HTTP_X_FORWARDED_FOR'])) {
  $raw_header = $_SERVER['X_FORWARDED_FOR'];

  $real_address = your_custom_parsing_function($raw_header);

  $_SERVER['REMOTE_ADDR'] = $real_address;
}

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:

<?php

$_SERVER['HTTPS'] = true;

You can also set this value to false to explicitly tell Phabricator that a request is not an SSL request.

Next Steps

Continue by:

• returning to the Configuration Guide.