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=head1 NAME

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_get0_ciphers, SSL_client_hello_get0_compression_methods, SSL_client_hello_get1_extensions_present, SSL_client_hello_get0_ext - callback functions for early server-side ClientHello processing

=head1 SYNOPSIS

 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);

=head1 DESCRIPTION

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 B<al> parameter.  A callback may

also return a negative value to suspend the handshake, and the handshake

function will return immediately.  L<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 B<out> and B<outlen> parameters are

both required, and on success the caller must release the storage allocated for

B<*out> using OPENSSL_free().  The contents of B<*out> is an array of integers

holding the numerical value of the TLS extension types in the order they appear

in the ClientHello.  B<*outlen> contains the number of elements in the array.

In situations when the ClientHello has no extensions, the function will return

success with B<*out> set to NULL and B<*outlen> set to 0.

=head1 NOTES

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.

=head1 RETURN VALUES

The application's supplied ClientHello callback returns

SSL_CLIENT_HELLO_SUCCESS on success, SSL_CLIENT_HELLO_ERROR on failure, and

SSL_CLIENT_HELLO_RETRY to suspend processing.

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

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.

=head1 SEE ALSO

L<ssl(7)>, L<SSL_CTX_set_tlsext_servername_callback(3)>,

L<SSL_bytes_to_cipher_list>

=head1 HISTORY

The SSL ClientHello callback, SSL_client_hello_isv2(),

SSL_client_hello_get0_random(), SSL_client_hello_get0_session_id(),

SSL_client_hello_get0_ciphers(), SSL_client_hello_get0_compression_methods(),

SSL_client_hello_get0_ext(), and SSL_client_hello_get1_extensions_present()

were added in OpenSSL 1.1.1.

=head1 COPYRIGHT

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

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

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