Nginx default hostname "_" forwarded to SSO server

Default nginx config has a host name of "_"
Diff to support this is:

--- a/source/CAS/Client.php
+++ b/source/CAS/Client.php
@@ -3970,7 +3970,7 @@ class CAS_Client
         } else if (!empty($_SERVER['HTTP_X_FORWARDED_SERVER'])) {
             $server_url = $_SERVER['HTTP_X_FORWARDED_SERVER'];
         } else {
-            if (empty($_SERVER['SERVER_NAME'])) {
+            if (empty($_SERVER['SERVER_NAME']) || $_SERVER['SERVER_NAME'] == '_') {
                 $server_url = $_SERVER['HTTP_HOST'];
             } else {
                 $server_url = $_SERVER['SERVER_NAME'];

Obviously you need to fix the config sample in the examples folder as well:

--- a/docs/examples/config.example.php
+++ b/docs/examples/config.example.php
@@ -63,11 +63,13 @@ $driver_options = '';
 ///////////////////////////////////////////

 // Generating the URLS for the local cas example services for proxy testing
+$sname = (empty($_SERVER['SERVER_NAME']) || $_SERVER['SERVER_NAME']=='_' )?$_SERVER['HOST_NAME']:$_SERVER['SERVER_NAME'];
 if (isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] == 'on') {
-    $curbase = 'https://' . $_SERVER['SERVER_NAME'];
+    $curbase = 'https://' . $sname;
 } else {
-    $curbase = 'http://' . $_SERVER['SERVER_NAME'];
+    $curbase = 'http://' . $sname;
 }
+unset($sname);
 if ($_SERVER['SERVER_PORT'] != 80 && $_SERVER['SERVER_PORT'] != 443) {
     $curbase .= ':' . $_SERVER['SERVER_PORT'];
 }

Start by reading source/CAS/Client.php around the server URL selection and docs/examples/config.example.php around the local CAS example URL setup. Compare both paths with the reported nginx hostname value and verify that the generated SSO and example-service URLs use the forwarded host rather than "_"; the issue is done when both affected configurations behave correctly.