HTTP/2: the "421 Misdirected Request" response (closes #848). Since 4fbef397c753 nginx rejects with the 400 error any attempts of requesting different host over the same connection, if the relevant virtual server requires verification of a client certificate. While requesting hosts other than negotiated isn't something legal in HTTP/1.x, the HTTP/2 specification explicitly permits such requests for connection reuse and has introduced a special response code 421. According to RFC 7540 Section 9.1.2 this code can be sent by a server that is not configured to produce responses for the combination of scheme and authority that are included in the request URI. And the client may retry the request over a different connection. Now this code is used for requests that aren't authorized in current connection. After receiving the 421 response a client will be able to open a new connection, provide the required certificate and retry the request. Unfortunately, not all clients currently are able to handle it well. Notably Chrome just shows an error, while at least the latest version of Firefox retries the request over a new connection.

--- a/src/http/ngx_http_request.c
+++ b/src/http/ngx_http_request.c
@@ -2069,7 +2069,7 @@ ngx_http_set_virtual_server(ngx_http_req
             ngx_log_error(NGX_LOG_INFO, r->connection->log, 0,
                           "client attempted to request the server name "
                           "different from that one was negotiated");
-            ngx_http_finalize_request(r, NGX_HTTP_BAD_REQUEST);
+            ngx_http_finalize_request(r, NGX_HTTP_MISDIRECTED_REQUEST);
             return NGX_ERROR;
         }
     }