diff --git a/doc/03-cli.md b/doc/03-cli.md
index a78c5ed93..7a0c5af2a 100644
--- a/doc/03-cli.md
+++ b/doc/03-cli.md
@@ -404,6 +404,12 @@ some tools like git or curl will only use the lower-cased `http_proxy` version.
 Alternatively you can also define the git proxy using
 `git config --global http.proxy <proxy url>`.
 
+### HTTP_PROXY_REQUEST_FULLURI
+
+If you use a proxy but it does not support the request_fulluri flag, then you
+should set this env var to `false` or `0` to prevent composer from setting the
+request_fulluri option.
+
 ### COMPOSER_HOME
 
 The `COMPOSER_HOME` var allows you to change the composer home directory. This
diff --git a/src/Composer/Command/DiagnoseCommand.php b/src/Composer/Command/DiagnoseCommand.php
index 3a17960ac..07600e0e5 100644
--- a/src/Composer/Command/DiagnoseCommand.php
+++ b/src/Composer/Command/DiagnoseCommand.php
@@ -56,7 +56,7 @@ EOT
         if (!empty($opts['http']['proxy'])) {
             $output->write('Checking HTTP proxy: ');
             $this->outputResult($output, $this->checkHttpProxy());
-            $output->write('Checking HTTPS proxy fullrequest_uri: ');
+            $output->write('Checking HTTPS proxy support for request_fulluri: ');
             $this->outputResult($output, $this->checkHttpsProxyFullUriRequestParam());
         }
 
@@ -145,31 +145,29 @@ EOT
      * Due to various proxy servers configurations, some servers cant handle non-standard HTTP "http_proxy_request_fulluri" parameter,
      * and will return error 500/501 (as not implemented), see discussion @ https://github.com/composer/composer/pull/1825.
      * This method will test, if you need to disable this parameter via setting extra environment variable in your system.
+     *
      * @return bool|string
      */
     private function checkHttpsProxyFullUriRequestParam()
     {
-        $protocol = 'https';
-        $resultMessage = true;
-
-        /* i've found no better file to test with so far :( */
-        $filePath = '://api.github.com/repos/Seldaek/jsonlint/zipball/1.0.0 ';
-        try
-        {
-            $rfcResult = $this->rfs->getContents('api.github.com', $protocol . $filePath, false);
-        }
-        catch (TransportException $e)
-        {
-            if (!extension_loaded('openssl'))
-            {
-                return "Sorry, but you need openssl extension installed for this check - please enable/recompile\n";
+        $url = 'https://api.github.com/repos/Seldaek/jsonlint/zipball/1.0.0 ';
+        try {
+            $rfcResult = $this->rfs->getContents('api.github.com', $url, false);
+        } catch (TransportException $e) {
+            if (!extension_loaded('openssl')) {
+                return 'You need the openssl extension installed for this check';
             }
 
-            $this->rfs->getContents('api.github.com', $protocol . $filePath, false, array('http' => array('request_fulluri' => false)));
-            $resultMessage = "Seems there is a problem with your proxy server, try setting value of your environment variable \"http_proxy_request_fulluri\" to false, and run diagnostics again\n";
+            try {
+                $this->rfs->getContents('api.github.com', $url, false, array('http' => array('request_fulluri' => false)));
+            } catch (TransportException $e) {
+                return 'Unable to assert the situation, maybe github is down ('.$e->getMessage().')';
+            }
+
+            return 'It seems there is a problem with your proxy server, try setting the "HTTP_PROXY_REQUEST_FULLURI" environment variable to "false"';
         }
 
-        return $resultMessage;
+        return true;
     }
 
     private function checkGithubOauth($domain, $token)
diff --git a/src/Composer/Util/StreamContextFactory.php b/src/Composer/Util/StreamContextFactory.php
index 11f3ea283..d89b115f6 100644
--- a/src/Composer/Util/StreamContextFactory.php
+++ b/src/Composer/Util/StreamContextFactory.php
@@ -62,13 +62,11 @@ final class StreamContextFactory
             }
 
             $options['http']['proxy'] = $proxyURL;
-            $options['http']['request_fulluri'] = true;
 
-            if ( strtolower( getenv('http_proxy_request_fulluri') ) == 'false' ||
-                 strtolower( getenv('HTTP_PROXY_REQUEST_FULLURI') ) == 'false'
-            )
-            {
-                $options['http']['request_fulluri'] = false;
+            // enabled request_fulluri unless it is explicitly disabled
+            $reqFullUriEnv = getenv('HTTP_PROXY_REQUEST_FULLURI');
+            if ($reqFullUriEnv === false || $reqFullUriEnv === '' || (strtolower($reqFullUriEnv) !== 'false' && (bool) $reqFullUriEnv)) {
+                $options['http']['request_fulluri'] = true;
             }
 
             if (isset($proxy['user'])) {