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SSL_CTX_set_client_hello_cb(3)      OpenSSL     SSL_CTX_set_client_hello_cb(3)

       SSL_CTX_set_client_hello_cb, SSL_client_hello_cb_fn,
       SSL_client_hello_isv2, SSL_client_hello_get0_legacy_version,
       SSL_client_hello_get0_random, SSL_client_hello_get0_session_id,
       SSL_client_hello_get1_extensions_present, SSL_client_hello_get0_ext -
       callback functions for early server-side ClientHello processing

       libcrypto, -lcrypto

        typedef int (*SSL_client_hello_cb_fn)(SSL *s, int *al, void *arg);
        void SSL_CTX_set_client_hello_cb(SSL_CTX *c, SSL_client_hello_cb_fn *f,
                                         void *arg);
        int SSL_client_hello_isv2(SSL *s);
        unsigned int SSL_client_hello_get0_legacy_version(SSL *s);
        size_t SSL_client_hello_get0_random(SSL *s, const unsigned char **out);
        size_t SSL_client_hello_get0_session_id(SSL *s, const unsigned char **out);
        size_t SSL_client_hello_get0_ciphers(SSL *s, const unsigned char **out);
        size_t SSL_client_hello_get0_compression_methods(SSL *s,
                                                         const unsigned char **out);
        int SSL_client_hello_get1_extensions_present(SSL *s, int **out,
                                                     size_t *outlen);
        int SSL_client_hello_get0_ext(SSL *s, int type, const unsigned char **out,
                                      size_t *outlen);

       SSL_CTX_set_client_hello_cb() sets the callback function, which is
       automatically called during the early stages of ClientHello processing
       on the server.  The argument supplied when setting the callback is
       passed back to the callback at runtime.  A callback that returns
       failure (0) will cause the connection to terminate, and callbacks
       returning failure should indicate what alert value is to be sent in the
       al parameter.  A callback may also return a negative value to suspend
       the handshake, and the handshake function will return immediately.
       SSL_get_error(3) will return SSL_ERROR_WANT_CLIENT_HELLO_CB to indicate
       that the handshake was suspended.  It is the job of the ClientHello
       callback to store information about the state of the last call if
       needed to continue.  On the next call into the handshake function, the
       ClientHello callback will be called again, and, if it returns success,
       normal handshake processing will continue from that point.

       SSL_client_hello_isv2() indicates whether the ClientHello was carried
       in a SSLv2 record and is in the SSLv2 format.  The SSLv2 format has
       substantial differences from the normal SSLv3 format, including using
       three bytes per cipher suite, and not allowing extensions.
       Additionally, the SSLv2 format 'challenge' field is exposed via
       SSL_client_hello_get0_random(), padded to SSL3_RANDOM_SIZE bytes with
       zeros if needed.  For SSLv2 format ClientHellos,
       SSL_client_hello_get0_compression_methods() returns a dummy list that
       only includes the null compression method, since the SSLv2 format does
       not include a mechanism by which to negotiate compression.

       SSL_client_hello_get0_random(), SSL_client_hello_get0_session_id(),
       SSL_client_hello_get0_ciphers(), and
       SSL_client_hello_get0_compression_methods() provide access to the
       corresponding ClientHello fields, returning the field length and
       optionally setting an out pointer to the octets of that field.

       Similarly, SSL_client_hello_get0_ext() provides access to individual
       extensions from the ClientHello on a per-extension basis.  For the
       provided wire protocol extension type value, the extension value and
       length are returned in the output parameters (if present).

       SSL_client_hello_get1_extensions_present() can be used prior to
       SSL_client_hello_get0_ext(), to determine which extensions are present
       in the ClientHello before querying for them.  The out and outlen
       parameters are both required, and on success the caller must release
       the storage allocated for *out using OPENSSL_free().  The contents of
       *out is an array of integers holding the numerical value of the TLS
       extension types in the order they appear in the ClientHello.  *outlen
       contains the number of elements in the array.  In situations when the
       ClientHello has no extensions, the function will return success with
       *out set to NULL and *outlen set to 0.

       The ClientHello callback provides a vast window of possibilities for
       application code to affect the TLS handshake.  A primary use of the
       callback is to allow the server to examine the server name indication
       extension provided by the client in order to select an appropriate
       certificate to present, and make other configuration adjustments
       relevant to that server name and its configuration.  Such configuration
       changes can include swapping out the associated SSL_CTX pointer,
       modifying the server's list of permitted TLS versions, changing the
       server's cipher list in response to the client's cipher list, etc.

       It is also recommended that applications utilize a ClientHello callback
       and not use a servername callback, in order to avoid unexpected
       behavior that occurs due to the relative order of processing between
       things like session resumption and the historical servername callback.

       The SSL_client_hello_* family of functions may only be called from code
       executing within a ClientHello callback.

       The application's supplied ClientHello callback returns
       and SSL_CLIENT_HELLO_RETRY to suspend processing.

       SSL_client_hello_isv2() returns 1 for SSLv2-format ClientHellos and 0

       SSL_client_hello_get0_random(), SSL_client_hello_get0_session_id(),
       SSL_client_hello_get0_ciphers(), and
       SSL_client_hello_get0_compression_methods() return the length of the
       corresponding ClientHello fields.  If zero is returned, the output
       pointer should not be assumed to be valid.

       SSL_client_hello_get0_ext() returns 1 if the extension of type 'type'
       is present, and 0 otherwise.

       SSL_client_hello_get1_extensions_present() returns 1 on success and 0
       on failure.

       ssl(7), SSL_CTX_set_tlsext_servername_callback(3),

       The SSL ClientHello callback, SSL_client_hello_isv2(),
       SSL_client_hello_get0_random(), SSL_client_hello_get0_session_id(),
       SSL_client_hello_get0_ext(), and
       SSL_client_hello_get1_extensions_present() were added in OpenSSL 1.1.1.

       Copyright 2017-2019 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

1.1.1i                            2019-06-09    SSL_CTX_set_client_hello_cb(3)