custom_ext_free_cb

       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,

       "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.
       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 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).

       <https://www.openssl.org/source/license.html>.

1.1.1c                            2019-05-31     SSL_EXTENSION_SUPPORTED(3SSL)
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