doc/man3/SSL_CTX_set_tlsext_servername_callback.pod - third_party/openssl - Git at Google

 =pod

 =head1 NAME

 SSL_CTX_set_tlsext_servername_callback, SSL_CTX_set_tlsext_servername_arg,
 SSL_get_servername_type, SSL_get_servername,
 SSL_set_tlsext_host_name - handle server name indication (SNI)

 =head1 SYNOPSIS

  #include <openssl/ssl.h>

  long SSL_CTX_set_tlsext_servername_callback(SSL_CTX *ctx,
                                    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);
  int SSL_get_servername_type(const SSL *s);

  int SSL_set_tlsext_host_name(const SSL *s, const char *name);

 =head1 DESCRIPTION

 The functionality provided by the servername callback is mostly superseded by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 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 (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.

 =item On the client, during or after the handshake and a TLSv1.2 (or below)
 resumption occurred

 If the session from the original 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
 B<TLSEXT_NAMETYPE_host_name>.

 SSL_set_tlsext_host_name() sets the server name indication ClientHello extension
 to contain the value B<name>. The type of server name indication extension is set
 to B<TLSEXT_NAMETYPE_host_name> (defined in RFC3546).

 =head1 NOTES

 Several callbacks are executed during ClientHello processing, including
 the ClientHello, ALPN, and servername callbacks.  The ClientHello callback is
 executed first, then the servername callback, followed by the ALPN callback.

 The SSL_set_tlsext_host_name() function should only be called on SSL objects
 that will act as clients; otherwise the configured B<name> will be ignored.

 =head1 RETURN VALUES

 SSL_CTX_set_tlsext_servername_callback() and
 SSL_CTX_set_tlsext_servername_arg() both always return 1 indicating success.
 SSL_set_tlsext_host_name() returns 1 on success, 0 in case of error.

 =head1 SEE ALSO

 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 explicit 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 occurred 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 2017-2020 The OpenSSL Project Authors. All Rights Reserved.

 Licensed under the Apache License 2.0 (the "License").  You may not use
 this file except in compliance with the License.  You can obtain a copy
 in the file LICENSE in the source distribution or at
 L<https://www.openssl.org/source/license.html>.

 =cut
	=pod

	=head1 NAME

	SSL_CTX_set_tlsext_servername_callback, SSL_CTX_set_tlsext_servername_arg,
	SSL_get_servername_type, SSL_get_servername,
	SSL_set_tlsext_host_name - handle server name indication (SNI)

	=head1 SYNOPSIS

	#include <openssl/ssl.h>

	long SSL_CTX_set_tlsext_servername_callback(SSL_CTX *ctx,
	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);
	int SSL_get_servername_type(const SSL *s);

	int SSL_set_tlsext_host_name(const SSL s, const char name);

	=head1 DESCRIPTION

	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 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 (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.

	=item On the client, during or after the handshake and a TLSv1.2 (or below)
	resumption occurred

	If the session from the original 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
	B<TLSEXT_NAMETYPE_host_name>.

	SSL_set_tlsext_host_name() sets the server name indication ClientHello extension
	to contain the value B<name>. The type of server name indication extension is set
	to B<TLSEXT_NAMETYPE_host_name> (defined in RFC3546).

	=head1 NOTES

	Several callbacks are executed during ClientHello processing, including
	the ClientHello, ALPN, and servername callbacks. The ClientHello callback is
	executed first, then the servername callback, followed by the ALPN callback.

	The SSL_set_tlsext_host_name() function should only be called on SSL objects
	that will act as clients; otherwise the configured B<name> will be ignored.

	=head1 RETURN VALUES

	SSL_CTX_set_tlsext_servername_callback() and
	SSL_CTX_set_tlsext_servername_arg() both always return 1 indicating success.
	SSL_set_tlsext_host_name() returns 1 on success, 0 in case of error.

	=head1 SEE ALSO

	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 explicit 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 occurred 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 2017-2020 The OpenSSL Project Authors. All Rights Reserved.

	Licensed under the Apache License 2.0 (the "License"). You may not use
	this file except in compliance with the License. You can obtain a copy
	in the file LICENSE in the source distribution or at
	L<https://www.openssl.org/source/license.html>.

	=cut