SSL: ssl_reject_handshake directive (ticket #195).

In some cases it might be needed to reject SSL handshake based on SNI
server name provided, for example, to make sure an invalid certificate
is not returned to clients trying to contact a name-based virtual server
without SSL configured.  Previously, a "ssl_ciphers aNULL;" was used for
this.  This workaround, however, is not compatible with TLSv1.3, in
particular, when using BoringSSL, where it is not possible to configure
TLSv1.3 ciphers at all.

With this change, the ssl_reject_handshake directive is introduced,
which instructs nginx to reject SSL handshakes with an "unrecognized_name"
alert in a particular server block.

For example, to reject handshake with names other than example.com,
one can use the following configuration:

    server {
        listen 443 ssl;
        ssl_reject_handshake on;
    }

    server {
        listen 443 ssl;
        server_name example.com;
        ssl_certificate example.com.crt;
        ssl_certificate_key example.com.key;
    }

The following configuration can be used to reject all SSL handshakes
without SNI server name provided:

    server {
        listen 443 ssl;
        ssl_reject_handshake on;
    }

    server {
        listen 443 ssl;
        server_name ~^;
        ssl_certificate example.crt;
        ssl_certificate_key example.key;
    }

Additionally, the ssl_reject_handshake directive makes configuring
certificates for the default server block optional.  If no certificates
are configured in the default server for a given listening socket,
certificates must be defined in all non-default server blocks with
the listening socket in question.

2 files changed, 11 insertions, 2 deletions

diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
index dfb816055..2eef87e5e 100644
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -1793,6 +1793,13 @@ ngx_ssl_handshake(ngx_connection_t *c)
         return NGX_ERROR;
     }
 
+    if (c->ssl->handshake_rejected) {
+        ngx_connection_error(c, err, "handshake rejected");
+        ERR_clear_error();
+
+        return NGX_ERROR;
+    }
+
     c->read->error = 1;
 
     ngx_ssl_connection_error(c, sslerr, err, "SSL_do_handshake() failed");
@@ -3354,8 +3361,9 @@ ngx_ssl_session_id_context(ngx_ssl_t *ssl, ngx_str_t *sess_ctx,
         }
     }
 
-    if (SSL_CTX_get_ex_data(ssl->ctx, ngx_ssl_certificate_index) == NULL) {
-
+    if (SSL_CTX_get_ex_data(ssl->ctx, ngx_ssl_certificate_index) == NULL
+        && certificates != NULL)
+    {
         /*
          * If certificates are loaded dynamically, we use certificate
          * names as specified in the configuration (with variables).
diff --git a/src/event/ngx_event_openssl.h b/src/event/ngx_event_openssl.h
index 821bb13d1..329760d09 100644
--- a/src/event/ngx_event_openssl.h
+++ b/src/event/ngx_event_openssl.h
@@ -95,6 +95,7 @@ struct ngx_ssl_connection_s {
     u_char                      early_buf;
 
     unsigned                    handshaked:1;
+    unsigned                    handshake_rejected:1;
     unsigned                    renegotiation:1;
     unsigned                    buffer:1;
     unsigned                    no_wait_shutdown:1;