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
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, unsigned 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 run time. 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 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.
ssl(7),
SSL_CTX_set_tlsext_servername_callback(3),
SSL_bytes_to_cipher_list(3)
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.
Copyright 2017-2021 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (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
<
https://www.openssl.org/source/license.html>.