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adc2002d28
Summary: Fixes T13392. If you have 17 load balancers in sequence, Phabricator will receive requests with at least 17 "X-Forwarded-For" components in the header. We want to select the 17th-from-last element, since prior elements are not trustworthy. This currently isn't very easy/obvious, and you have to add a kind of sketchy piece of custom code to `preamble.php` to do any "X-Forwarded-For" parsing. Make handling this correctly easier. Test Plan: - Ran unit tests. - Configured my local `preamble.php` to call `preamble_trust_x_forwarded_for_header(4)`, then made `/debug/` dump the header and the final value of `REMOTE_ADDR`. ``` $ curl http://local.phacility.com/debug/ <pre> HTTP_X_FORWARDED_FOR = FINAL REMOTE_ADDR = 127.0.0.1 </pre> ``` ``` $ curl -H 'X-Forwarded-For: 1.1.1.1, 2.2.2.2, 3.3.3.3, 4.4.4.4, 5.5.5.5, 6.6.6.6' http://local.phacility.com/debug/ <pre> HTTP_X_FORWARDED_FOR = 1.1.1.1, 2.2.2.2, 3.3.3.3, 4.4.4.4, 5.5.5.5, 6.6.6.6 FINAL REMOTE_ADDR = 3.3.3.3 </pre> ``` ``` $ curl -H 'X-Forwarded-For: 5.5.5.5, 6.6.6.6' http://local.phacility.com/debug/ <pre> HTTP_X_FORWARDED_FOR = 5.5.5.5, 6.6.6.6 FINAL REMOTE_ADDR = 5.5.5.5 </pre> ``` Maniphest Tasks: T13392 Differential Revision: https://secure.phabricator.com/D20785
77 lines
2.6 KiB
PHP
77 lines
2.6 KiB
PHP
<?php
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/**
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* Parse the "X_FORWARDED_FOR" HTTP header to determine the original client
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* address.
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*
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* @param int Number of devices to trust.
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* @return void
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*/
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function preamble_trust_x_forwarded_for_header($layers = 1) {
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if (!is_int($layers) || ($layers < 1)) {
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echo
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'preamble_trust_x_forwarded_for_header(<layers>): '.
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'"layers" parameter must an integer larger than 0.'."\n";
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echo "\n";
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exit(1);
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}
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if (!isset($_SERVER['HTTP_X_FORWARDED_FOR'])) {
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return;
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}
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$forwarded_for = $_SERVER['HTTP_X_FORWARDED_FOR'];
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if (!strlen($forwarded_for)) {
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return;
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}
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$address = preamble_get_x_forwarded_for_address($forwarded_for, $layers);
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$_SERVER['REMOTE_ADDR'] = $address;
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}
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function preamble_get_x_forwarded_for_address($raw_header, $layers) {
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// The raw header may be a list of IPs, like "1.2.3.4, 4.5.6.7", if the
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// request the load balancer received also had this header. In particular,
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// this happens routinely with requests received through a CDN, but can also
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// happen illegitimately if the client just makes up an "X-Forwarded-For"
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// header full of lies.
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// We can only trust the N elements at the end of the list which correspond
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// to network-adjacent devices we control. Usually, we're behind a single
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// load balancer and "N" is 1, so we want to take the last element in the
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// list.
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// In some cases, "N" may be more than 1, if the network is configured so
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// that that requests are routed through multiple layers of load balancers
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// and proxies. In this case, we want to take the Nth-to-last element of
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// the list.
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$addresses = explode(',', $raw_header);
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// If we have more than one trustworthy device on the network path, discard
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// corresponding elements from the list. For example, if we have 7 devices,
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// we want to discard the last 6 elements of the list.
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// The final device address does not appear in the list, since devices do
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// not append their own addresses to "X-Forwarded-For".
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$discard_addresses = ($layers - 1);
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// However, we don't want to throw away all of the addresses. Some requests
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// may originate from within the network, and may thus not have as many
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// addresses as we expect. If we have fewer addresses than trustworthy
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// devices, discard all but one address.
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$max_discard = (count($addresses) - 1);
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$discard_count = min($discard_addresses, $max_discard);
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if ($discard_count) {
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$addresses = array_slice($addresses, 0, -$discard_count);
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}
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$original_address = end($addresses);
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$original_address = trim($original_address);
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return $original_address;
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}
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