diff --git a/lib/moodlelib.php b/lib/moodlelib.php
index a53a8de8fb8..fd1f8167293 100644
--- a/lib/moodlelib.php
+++ b/lib/moodlelib.php
@@ -2817,6 +2817,11 @@ function get_login_url() {
 function require_login($courseorid = NULL, $autologinguest = true, $cm = NULL, $setwantsurltome = true, $preventredirect = false) {
     global $CFG, $SESSION, $USER, $PAGE, $SITE, $DB, $OUTPUT;
 
+    // Must not redirect when byteserving already started.
+    if (!empty($_SERVER['HTTP_RANGE'])) {
+        $preventredirect = true;
+    }
+
     // setup global $COURSE, themes, language and locale
     if (!empty($courseorid)) {
         if (is_object($courseorid)) {
diff --git a/lib/outputrenderers.php b/lib/outputrenderers.php
index 019f01982ea..1c014595e39 100644
--- a/lib/outputrenderers.php
+++ b/lib/outputrenderers.php
@@ -2317,12 +2317,16 @@ EOD;
             }
             error_reporting($CFG->debug);
 
-            // Header not yet printed
-            if (isset($_SERVER['SERVER_PROTOCOL'])) {
-                // server protocol should be always present, because this render
-                // can not be used from command line or when outputting custom XML
-                @header($_SERVER['SERVER_PROTOCOL'] . ' 404 Not Found');
+            // Output not yet started.
+            $protocol = (isset($_SERVER['SERVER_PROTOCOL']) ? $_SERVER['SERVER_PROTOCOL'] : 'HTTP/1.0');
+            if (empty($_SERVER['HTTP_RANGE'])) {
+                @header($protocol . ' 404 Not Found');
+            } else {
+                // Must stop byteserving attempts somehow,
+                // this is weird but Chrome PDF viewer can be stopped only with 407!
+                @header($protocol . ' 407 Proxy Authentication Required');
             }
+
             $this->page->set_context(null); // ugly hack - make sure page context is set to something, we do not want bogus warnings here
             $this->page->set_url('/'); // no url
             //$this->page->set_pagelayout('base'); //TODO: MDL-20676 blocks on error pages are weird, unfortunately it somehow detect the pagelayout from URL :-(
diff --git a/lib/setuplib.php b/lib/setuplib.php
index 129147a9b1d..e8258447f81 100644
--- a/lib/setuplib.php
+++ b/lib/setuplib.php
@@ -1547,12 +1547,9 @@ width: 80%; -moz-border-radius: 20px; padding: 15px">
         }
 
         // In the name of protocol correctness, monitoring and performance
-        // profiling, set the appropriate error headers for machine consumption
-        if (isset($_SERVER['SERVER_PROTOCOL'])) {
-            // Avoid it with cron.php. Note that we assume it's HTTP/1.x
-            // The 503 ode here means our Moodle does not work at all, the error happened too early
-            @header($_SERVER['SERVER_PROTOCOL'] . ' 503 Service Unavailable');
-        }
+        // profiling, set the appropriate error headers for machine consumption.
+        $protocol = (isset($_SERVER['SERVER_PROTOCOL']) ? $_SERVER['SERVER_PROTOCOL'] : 'HTTP/1.0');
+        @header($protocol . ' 503 Service Unavailable');
 
         // better disable any caching
         @header('Content-Type: text/html; charset=utf-8');