#include <openssl/ssl.h>
long SSL_CTX_set_tlsext_servername_callback(SSL_CTX *ctx,
- int (*cb)(SSL *, int *, void *));
+ int (*cb)(SSL *s, int *al, void *arg));
long SSL_CTX_set_tlsext_servername_arg(SSL_CTX *ctx, void *arg);
const char *SSL_get_servername(const SSL *s, const int type);
=head1 DESCRIPTION
-The functionality provided by the servername callback is superseded by the
-ClientHello callback, which can be set using SSL_CTX_set_client_hello_cb().
-The servername callback is retained for historical compatibility.
+The functionality provided by the servername callback is mostly superseded by
+the ClientHello callback, which can be set using SSL_CTX_set_client_hello_cb().
+However, even where the ClientHello callback is used, the servername callback is
+still necessary in order to acknowledge the servername requested by the client.
SSL_CTX_set_tlsext_servername_callback() sets the application callback B<cb>
used by a server to perform any actions or configuration required based on
the servername extension received in the incoming connection. When B<cb>
-is NULL, SNI is not used. The B<arg> value is a pointer which is passed to
-the application callback.
+is NULL, SNI is not used.
+
+The servername callback should return one of the following values:
+
+=over 4
+
+=item SSL_TLSEXT_ERR_OK
+
+This is used to indicate that the servername requested by the client has been
+accepted. Typically a server will call SSL_set_SSL_CTX() in the callback to set
+up a different configuration for the selected servername in this case.
+
+=item SSL_TLSEXT_ERR_ALERT_FATAL
+
+In this case the servername requested by the client is not accepted and the
+handshake will be aborted. The value of the alert to be used should be stored in
+the location pointed to by the B<al> parameter to the callback. By default this
+value is initialised to SSL_AD_UNRECOGNIZED_NAME.
+
+=item SSL_TLSEXT_ERR_ALERT_WARNING
+
+If this value is returned then the servername is not accepted by the server.
+However the handshake will continue and send a warning alert instead. The value
+of the alert should be stored in the location pointed to by the B<al> parameter
+as for SSL_TLSEXT_ERR_ALERT_FATAL above. Note that TLSv1.3 does not support
+warning alerts, so if TLSv1.3 has been negotiated then this return value is
+treated the same way as SSL_TLSEXT_ERR_NOACK.
+
+=item SSL_TLSEXT_ERR_NOACK
+
+This return value indicates that the servername is not accepted by the server.
+No alerts are sent and the server will not acknowledge the requested servername.
+
+=back
SSL_CTX_set_tlsext_servername_arg() sets a context-specific argument to be
-passed into the callback for this B<SSL_CTX>.
+passed into the callback (via the B<arg> parameter) for this B<SSL_CTX>.
+
+The behaviour of SSL_get_servername() depends on a number of different factors.
+In particular note that in TLSv1.3 the servername is negotiated in every
+handshake. In TLSv1.2 the servername is only negotiated on initial handshakes
+and not on resumption handshakes.
+
+=over 4
+
+=item On the client, before the handshake
+
+If a servername has been set via a call to SSL_set_tlsext_host_name() then it
+will return that servername.
+
+If one has not been set, but a TLSv1.2 resumption is being attempted and the
+session from the original handshake had a servername accepted by the server then
+it will return that servername.
+
+Otherwise it returns NULL.
-SSL_get_servername() returns a servername extension value of the specified
-type if provided in the Client Hello or NULL.
+=item On the client, during or after the handshake and a TLSv1.2 (or below)
+resumption occurred
+
+If the session from the orignal handshake had a servername accepted by the
+server then it will return that servername.
+
+Otherwise it returns the servername set via SSL_set_tlsext_host_name() or NULL
+if it was not called.
+
+=item On the client, during or after the handshake and a TLSv1.2 (or below)
+resumption did not occur
+
+It will return the servername set via SSL_set_tlsext_host_name() or NULL if it
+was not called.
+
+=item On the server, before the handshake
+
+The function will always return NULL before the handshake
+
+=item On the server, after the servername extension has been processed and a
+TLSv1.2 (or below) resumption occurred
+
+If a servername was accepted by the server in the original handshake then it
+will return that servername, or NULL otherwise.
+
+=item On the server, after the servername extension has been processed and a
+TLSv1.2 (or below) resumption did not occur
+
+The function will return the servername requested by the client in this
+handshake or NULL if none was requested.
+
+=back
+
+Note that the ClientHello callback occurs before a servername extension from the
+client is processed. The servername, certificate and ALPN callbacks occur after
+a servername extension from the client is processed.
SSL_get_servername_type() returns the servername type or -1 if no servername
is present. Currently the only supported type (defined in RFC3546) is
L<ssl(7)>, L<SSL_CTX_set_alpn_select_cb(3)>,
L<SSL_get0_alpn_selected(3)>, L<SSL_CTX_set_client_hello_cb(3)>
+=head1 HISTORY
+
+SSL_get_servername() historically provided some unexpected results in certain
+corner cases. This has been fixed from OpenSSL 1.1.1e.
+
+Prior to 1.1.1e, when the client requested a servername in an initial TLSv1.2
+handshake, the server accepted it, and then the client successfully resumed but
+set a different explict servername in the second handshake then when called by
+the client it returned the servername from the second handshake. This has now
+been changed to return the servername requested in the original handshake.
+
+Also prior to 1.1.1e, if the client sent a servername in the first handshake but
+the server did not accept it, and then a second handshake occured where TLSv1.2
+resumption was successful then when called by the server it returned the
+servername requested in the original handshake. This has now been changed to
+NULL.
+
=head1 COPYRIGHT
Copyright 2017 The OpenSSL Project Authors. All Rights Reserved.
projects / openssl.git / commitdiff