http, mod_ssl: Introduce and return the 421 (Misdirected Request) status code

for clients requesting a hostname on a reused connection whose SNI (from the
TLS handshake) does not match.
PR 5802.

This allows HTTP/2 clients to fall back to a new connection as per:
https://tools.ietf.org/html/rfc7540#section-9.1.2

Proposed by: Stefan Eissing <stefan eissing.org>
Reviewed by: ylavic


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1685069 13f79535-47bb-0310-9956-ffa450edef68

modules/http/http_protocol.c

@@ -135,7 +135,7 @@ static const char * const status_lines[RESPONSE_CODES] =
     "418 I'm A Teapot",
     NULL, /* 419 */
     NULL, /* 420 */
-    NULL, /* 421 */
+    "421 Misdirected Request",
     "422 Unprocessable Entity",
     "423 Locked",
     "424 Failed Dependency",
@@ -1308,6 +1308,11 @@ static const char *get_canned_error_string(int status,
     case HTTP_IM_A_TEAPOT:
         return("<p>The resulting entity body MAY be short and\n"
                 "stout.</p>\n");
+    case HTTP_MISDIRECTED_REQUEST:
+        return("<p>The client needs a new connection for this\n"
+               "request as the requested host name does not match\n"
+               "the Server Name Indication (SNI) in use for this\n"
+               "connection.</p>\n");
     default:                    /* HTTP_INTERNAL_SERVER_ERROR */
         /*
          * This comparison to expose error-notes could be modified to