/usr/man/cat.3/SSL_CTX_add_client_custom_ext.3

SSL_CTX_set_custom_cli_ext(3)OpenSSLSSL_CTX_set_custom_cli_ext(3)

NAME

     SSL_CTX_add_client_custom_ext, SSL_CTX_add_server_custom_ext
     - custom TLS extension handling

SYNOPSIS

      #include <openssl/ssl.h>

      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);

      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);

DESCRIPTION

     SSL_CTX_add_client_custom_ext() adds a custom extension for
     a TLS client with extension type ext_type and callbacks
     add_cb, free_cb and parse_cb.

     SSL_CTX_add_server_custom_ext() adds a custom extension for
     a TLS server with extension type ext_type and callbacks
     add_cb, free_cb and parse_cb.

     In both 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

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SSL_CTX_set_custom_cli_ext(3)OpenSSLSSL_CTX_set_custom_cli_ext(3)

     to be included in ClientHello for TLS clients or ServerHello
     for servers. 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.

     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.

     For clients (but not servers) if add_cb is set to NULL a
     zero length extension is added for ext_type.

     For clients every registered add_cb is always called to see
     if the application wishes to add an extension to
     ClientHello.

     For servers every registered add_cb is called once if and
     only if the corresponding extension was received in
     ClientHello to see if the application wishes to add the
     extension to ServerHello. That is, if no corresponding
     extension was received in 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. For
     TLS clients the extension data will come from ServerHello
     and for TLS servers it will come from ClientHello.

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

     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.

1.0.2t               Last change: 2019-09-10                    2

SSL_CTX_set_custom_cli_ext(3)OpenSSLSSL_CTX_set_custom_cli_ext(3)

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.

     The ext_type parameter corresponds to the extension_type
     field of RFC5246 et al. It is not a NID.

     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 ServerHello which was
     not sent in ClientHello a fatal unsupported_extension alert
     is sent and the handshake is aborted. The ServerHello add_cb
     callback is only called if the corresponding extension was
     received in 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_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.

1.0.2t               Last change: 2019-09-10                    3

See also SSL_CTX_add_server_custom_ext(3)
See also SSL_CTX_set_custom_cli_ext(3)