SSL_CTX_SET_CLIENT_HELLO_CB(3ossl) OpenSSL SSL_CTX_SET_CLIENT_HELLO_CB(3ossl)
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_get_extension_order, SSL_client_hello_get0_ext -
callback functions for early server-side ClientHello processing
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_get_extension_order(SSL *s, uint16_t *exts,
size_t *num_exts);
int SSL_client_hello_get0_ext(SSL *s, unsigned int type, const unsigned char **out,
size_t *outlen);
DESCRIPTION
SSL_CTX_set_client_hello_cb(3) 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.
SSL_client_hello_get_extension_order() is similar to
SSL_client_hello_get1_extensions_present(), without internal memory
allocation. When called with exts set to NULL, returns the number of
extensions (e.g., to allocate storage for a subsequent call).
Otherwise, *exts is populated with the ExtensionType values in the
order that the corresponding extensions appeared in the ClientHello.
*num_exts is an input/output parameter, used as input to supply the
size of storage allocated by the caller, and as output to indicate how
many ExtensionType values were written. If the input *num_exts is
smaller then the number of extensions in question, that is treated as
an error. A subsequent call with exts set to NULL can retrieve the
size of storage needed. A ClientHello that contained no extensions is
treated as success, with *num_exts set to 0.
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.
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.
SSL_client_hello_get_extension_order() returns 1 on success and 0 on
failure.
SEE ALSO
ssl(7), SSL_CTX_set_tlsext_servername_callback(3),
SSL_bytes_to_cipher_list(3)
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.
SSL_client_hello_get_extension_order() was added in OpenSSL 3.2.0.
COPYRIGHT
Copyright 2017-2022 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>.
3.2.0 2023-11-23
SSL_CTX_SET_CLIENT_HELLO_CB(3ossl)
openssl 3.2.0 - Generated Sat Dec 2 12:40:56 CST 2023