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SSL_EXTENSION_SUPPORTED(3)	    OpenSSL	    SSL_EXTENSION_SUPPORTED(3)

NAME
       SSL_extension_supported,	SSL_CTX_add_custom_ext,
       SSL_CTX_add_client_custom_ext, SSL_CTX_add_server_custom_ext,
       custom_ext_add_cb, custom_ext_free_cb, custom_ext_parse_cb - custom TLS
       extension handling

SYNOPSIS
	#include <openssl/ssl.h>

	typedef	int (*SSL_custom_ext_add_cb_ex)	(SSL *s, unsigned int ext_type,
						 unsigned int context,
						 const unsigned	char **out,
						 size_t	*outlen, X509 *x,
						 size_t	chainidx, int *al,
						 void *add_arg);

	typedef	void (*SSL_custom_ext_free_cb_ex) (SSL *s, unsigned int	ext_type,
						   unsigned int	context,
						   const unsigned char *out,
						   void	*add_arg);

	typedef	int (*SSL_custom_ext_parse_cb_ex) (SSL *s, unsigned int	ext_type,
						   unsigned int	context,
						   const unsigned char *in,
						   size_t inlen, X509 *x,
						   size_t chainidx, int	*al,
						   void	*parse_arg);

	int SSL_CTX_add_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
				   unsigned int	context,
				   SSL_custom_ext_add_cb_ex add_cb,
				   SSL_custom_ext_free_cb_ex free_cb,
				   void	*add_arg,
				   SSL_custom_ext_parse_cb_ex parse_cb,
				   void	*parse_arg);

	typedef	int (*custom_ext_add_cb)(SSL *s, unsigned int ext_type,
					 const unsigned	char **out,
					 size_t	*outlen, int *al,
					 void *add_arg);

	typedef	void (*custom_ext_free_cb)(SSL *s, unsigned int	ext_type,
					   const unsigned char *out,
					   void	*add_arg);

	typedef	int (*custom_ext_parse_cb)(SSL *s, unsigned int	ext_type,
					   const unsigned char *in,
					   size_t inlen, int *al,
					   void	*parse_arg);

	int SSL_CTX_add_client_custom_ext(SSL_CTX *ctx,	unsigned int ext_type,
					  custom_ext_add_cb add_cb,
					  custom_ext_free_cb free_cb, void *add_arg,
					  custom_ext_parse_cb parse_cb,
					  void *parse_arg);

	int SSL_CTX_add_server_custom_ext(SSL_CTX *ctx,	unsigned int ext_type,
					  custom_ext_add_cb add_cb,
					  custom_ext_free_cb free_cb, void *add_arg,
					  custom_ext_parse_cb parse_cb,
					  void *parse_arg);

	int SSL_extension_supported(unsigned int ext_type);

DESCRIPTION
       SSL_CTX_add_custom_ext()	adds a custom extension	for a TLS/DTLS client
       or server for all supported protocol versions with extension type
       ext_type	and callbacks add_cb, free_cb and parse_cb (see	the "EXTENSION
       CALLBACKS" section below). The context value determines which messages
       and under what conditions the extension will be added/parsed (see the
       "EXTENSION CONTEXTS" section below).

       SSL_CTX_add_client_custom_ext() adds a custom extension for a TLS/DTLS
       client with extension type ext_type and callbacks add_cb, free_cb and
       parse_cb. This function is similar to SSL_CTX_add_custom_ext() except
       it only applies to clients, uses	the older style	of callbacks, and
       implicitly sets the context value to:

	SSL_EXT_TLS1_2_AND_BELOW_ONLY |	SSL_EXT_CLIENT_HELLO
	| SSL_EXT_TLS1_2_SERVER_HELLO |	SSL_EXT_IGNORE_ON_RESUMPTION

       SSL_CTX_add_server_custom_ext() adds a custom extension for a TLS/DTLS
       server with extension type ext_type and callbacks add_cb, free_cb and
       parse_cb. This function is similar to SSL_CTX_add_custom_ext() except
       it only applies to servers, uses	the older style	of callbacks, and
       implicitly sets the context value to the	same as	for
       SSL_CTX_add_client_custom_ext() above.

       The ext_type parameter corresponds to the extension_type	field of
       RFC5246 et al. It is not	a NID. In all cases the	extension type must
       not be handled by OpenSSL internally or an error	occurs.

       SSL_extension_supported() returns 1 if the extension ext_type is
       handled internally by OpenSSL and 0 otherwise.

EXTENSION CALLBACKS
       The callback add_cb is called to	send custom extension data to be
       included	in various TLS messages. The ext_type parameter	is set to the
       extension type which will be added and add_arg to the value set when
       the extension handler was added.	When using the new style callbacks the
       context parameter will indicate which message is	currently being
       constructed e.g.	for the	ClientHello it will be set to
       SSL_EXT_CLIENT_HELLO.

       If the application wishes to include the	extension ext_type it should
       set *out	to the extension data, set *outlen to the length of the
       extension data and return 1.

       If the add_cb does not wish to include the extension it must return 0.

       If add_cb returns -1 a fatal handshake error occurs using the TLS alert
       value specified in *al.

       When constructing the ClientHello, if add_cb is set to NULL a zero
       length extension	is added for ext_type. For all other messages if
       add_cb is set to	NULL then no extension is added.

       When constructing a Certificate message the callback will be called for
       each certificate	in the message.	The x parameter	will indicate the
       current certificate and the chainidx parameter will indicate the
       position	of the certificate in the message. The first certificate is
       always the end entity certificate and has a chainidx value of 0.	The
       certificates are	in the order that they were received in	the
       Certificate message.

       For all messages	except the ServerHello and EncryptedExtensions every
       registered add_cb is always called to see if the	application wishes to
       add an extension	(as long as all	requirements of	the specified context
       are met).

       For the ServerHello and EncryptedExtension messages every registered
       add_cb is called	once if	and only if the	requirements of	the specified
       context are met and the corresponding extension was received in the
       ClientHello. That is, if	no corresponding extension was received	in the
       ClientHello then	add_cb will not	be called.

       If an extension is added	(that is add_cb	returns	1) free_cb is called
       (if it is set) with the value of	out set	by the add callback. It	can be
       used to free up any dynamic extension data set by add_cb. Since out is
       constant	(to permit use of constant data	in add_cb) applications	may
       need to cast away const to free the data.

       The callback parse_cb receives data for TLS extensions. The callback is
       only called if the extension is present and relevant for	the context
       (see "EXTENSION CONTEXTS" below).

       The extension data consists of inlen bytes in the buffer	in for the
       extension ext_type.

       If the message being parsed is a	TLSv1.3	compatible Certificate message
       then parse_cb will be called for	each certificate contained within the
       message.	 The x parameter will indicate the current certificate and the
       chainidx	parameter will indicate	the position of	the certificate	in the
       message.	The first certificate is always	the end	entity certificate and
       has a chainidx value of 0.

       If the parse_cb considers the extension data acceptable it must return
       1. If it	returns	0 or a negative	value a	fatal handshake	error occurs
       using the TLS alert value specified in *al.

       The buffer in is	a temporary internal buffer which will not be valid
       after the callback returns.

EXTENSION CONTEXTS
       An extension context defines which messages and under which conditions
       an extension should be added or expected. The context is	built up by
       performing a bitwise OR of multiple pre-defined values together.	The
       valid context values are:

       SSL_EXT_TLS_ONLY
	   The extension is only allowed in TLS

       SSL_EXT_DTLS_ONLY
	   The extension is only allowed in DTLS

       SSL_EXT_TLS_IMPLEMENTATION_ONLY
	   The extension is allowed in DTLS, but there is only a TLS
	   implementation available (so	it is ignored in DTLS).

       SSL_EXT_SSL3_ALLOWED
	   Extensions are not typically	defined	for SSLv3. Setting this	value
	   will	allow the extension in SSLv3. Applications will	not typically
	   need	to use this.

       SSL_EXT_TLS1_2_AND_BELOW_ONLY
	   The extension is only defined for TLSv1.2/DTLSv1.2 and below.
	   Servers will	ignore this extension if it is present in the
	   ClientHello and TLSv1.3 is negotiated.

       SSL_EXT_TLS1_3_ONLY
	   The extension is only defined for TLS1.3 and	above. Servers will
	   ignore this extension if it is present in the ClientHello and
	   TLSv1.2 or below is negotiated.

       SSL_EXT_IGNORE_ON_RESUMPTION
	   The extension will be ignored during	parsing	if a previous session
	   is being successfully resumed.

       SSL_EXT_CLIENT_HELLO
	   The extension may be	present	in the ClientHello message.

       SSL_EXT_TLS1_2_SERVER_HELLO
	   The extension may be	present	in a TLSv1.2 or	below compatible
	   ServerHello message.

       SSL_EXT_TLS1_3_SERVER_HELLO
	   The extension may be	present	in a TLSv1.3 compatible	ServerHello
	   message.

       SSL_EXT_TLS1_3_ENCRYPTED_EXTENSIONS
	   The extension may be	present	in an EncryptedExtensions message.

       SSL_EXT_TLS1_3_HELLO_RETRY_REQUEST
	   The extension may be	present	in a HelloRetryRequest message.

       SSL_EXT_TLS1_3_CERTIFICATE
	   The extension may be	present	in a TLSv1.3 compatible	Certificate
	   message.

       SSL_EXT_TLS1_3_NEW_SESSION_TICKET
	   The extension may be	present	in a TLSv1.3 compatible
	   NewSessionTicket message.

       SSL_EXT_TLS1_3_CERTIFICATE_REQUEST
	   The extension may be	present	in a TLSv1.3 compatible
	   CertificateRequest message.

       The context must	include	at least one message value (otherwise the
       extension will never be used).

NOTES
       The add_arg and parse_arg parameters can	be set to arbitrary values
       which will be passed to the corresponding callbacks. They can, for
       example,	be used	to store the extension data received in	a convenient
       structure or pass the extension data to be added	or freed when adding
       extensions.

       If the same custom extension type is received multiple times a fatal
       decode_error alert is sent and the handshake aborts. If a custom
       extension is received in	a ServerHello/EncryptedExtensions message
       which was not sent in the ClientHello a fatal unsupported_extension
       alert is	sent and the handshake is aborted. The
       ServerHello/EncryptedExtensions add_cb callback is only called if the
       corresponding extension was received in the ClientHello.	This is
       compliant with the TLS specifications. This behaviour ensures that each
       callback	is called at most once and that	an application can never send
       unsolicited extensions.

RETURN VALUES
       SSL_CTX_add_custom_ext(), SSL_CTX_add_client_custom_ext() and
       SSL_CTX_add_server_custom_ext() return 1	for success and	0 for failure.
       A failure can occur if an attempt is made to add	the same ext_type more
       than once, if an	attempt	is made	to use an extension type handled
       internally by OpenSSL or	if an internal error occurs (for example a
       memory allocation failure).

       SSL_extension_supported() returns 1 if the extension ext_type is
       handled internally by OpenSSL and 0 otherwise.

HISTORY
       The function SSL_CTX_add_custom_ext() was added in OpenSSL 1.1.1.

COPYRIGHT
       Copyright 2014-2017 The OpenSSL Project Authors.	All Rights Reserved.

       Licensed	under the OpenSSL license (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
       <https://www.openssl.org/source/license.html>.

1.1.1a				  2018-11-20	    SSL_EXTENSION_SUPPORTED(3)

NAME | SYNOPSIS | DESCRIPTION | EXTENSION CALLBACKS | EXTENSION CONTEXTS | NOTES | RETURN VALUES | HISTORY | COPYRIGHT

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