openssl/doc/man3/SSL_CTX_set_early_cb.pod

=pod

=head1 NAME

SSL_CTX_set_early_cb, SSL_early_cb_fn, SSL_early_isv2, SSL_early_get0_legacy_version, SSL_early_get0_random, SSL_early_get0_session_id, SSL_early_get0_ciphers, SSL_early_get0_compression_methods, SSL_early_get1_extensions_present, SSL_early_get0_ext - callback functions for early server-side ClientHello processing

=head1 SYNOPSIS

 typedef int (*SSL_early_cb_fn)(SSL *s, int *al, void *arg);
 void SSL_CTX_set_early_cb(SSL_CTX *c, SSL_early_cb_fn *f, void *arg);
 int SSL_early_isv2(SSL *s);
 unsigned int SSL_early_get0_legacy_version(SSL *s);
 size_t SSL_early_get0_random(SSL *s, const unsigned char **out);
 size_t SSL_early_get0_session_id(SSL *s, const unsigned char **out);
 size_t SSL_early_get0_ciphers(SSL *s, const unsigned char **out);
 size_t SSL_early_get0_compression_methods(SSL *s, const unsigned char **out);
 int SSL_early_get1_extensions_present(SSL *s, int **out, size_t *outlen);
 int SSL_early_get0_ext(SSL *s, int type, const unsigned char **out,
                        size_t *outlen);

=head1 DESCRIPTION

SSL_CTX_set_early_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_EARLY to indicate that the handshake was suspended.
It is the job of the early callback to store information about the state
of the last call if needed to continue.  On the next call into the handshake
function, the early callback will be called again, and, if it returns
success, normal handshake processing will continue from that point.

SSL_early_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_early_get0_random(), padded to
SSL3_RANDOM_SIZE bytes with zeros if needed.  For SSLv2 format ClientHellos,
SSL_early_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_early_get0_random(), SSL_early_get0_session_id(), SSL_early_get0_ciphers(),
and SSL_early_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_early_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_early_get1_extensions_present() can be used prior to SSL_early_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.

=head1 NOTES

The early 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 an early 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_early_* family of functions may only be called from code executing
within an early callback.

=head1 RETURN VALUES

The application's supplied early callback returns 1 on success, 0 on failure,
and a negative value to suspend processing.

SSL_early_isv2() returns 1 for SSLv2-format ClientHellos and 0 otherwise.

SSL_early_get0_random(), SSL_early_get0_session_id(), SSL_early_get0_ciphers(),
and SSL_early_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_early_get0_ext() returns 1 if the extension of type 'type' is present, and
0 otherwise.

SSL_early_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 early callback, SSL_early_isv2(), SSL_early_get0_random(),
SSL_early_get0_session_id(), SSL_early_get0_ciphers(),
SSL_early_get0_compression_methods(), SSL_early_get0_ext(), and
SSL_early_get1_extensions_present() were added in OpenSSL 1.1.1.

=head1 COPYRIGHT

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

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