1=pod
2
3=head1 NAME
4
5SSL_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
6
7=head1 SYNOPSIS
8
9 typedef int (*SSL_client_hello_cb_fn)(SSL *s, int *al, void *arg);
10 void SSL_CTX_set_client_hello_cb(SSL_CTX *c, SSL_client_hello_cb_fn *f,
11                                  void *arg);
12 int SSL_client_hello_isv2(SSL *s);
13 unsigned int SSL_client_hello_get0_legacy_version(SSL *s);
14 size_t SSL_client_hello_get0_random(SSL *s, const unsigned char **out);
15 size_t SSL_client_hello_get0_session_id(SSL *s, const unsigned char **out);
16 size_t SSL_client_hello_get0_ciphers(SSL *s, const unsigned char **out);
17 size_t SSL_client_hello_get0_compression_methods(SSL *s,
18                                                  const unsigned char **out);
19 int SSL_client_hello_get1_extensions_present(SSL *s, int **out,
20                                              size_t *outlen);
21 int SSL_client_hello_get0_ext(SSL *s, unsigned int type, const unsigned char **out,
22                               size_t *outlen);
23
24=head1 DESCRIPTION
25
26SSL_CTX_set_client_hello_cb() sets the callback function, which is automatically
27called during the early stages of ClientHello processing on the server.
28The argument supplied when setting the callback is passed back to the
29callback at run time.  A callback that returns failure (0) will cause the
30connection to terminate, and callbacks returning failure should indicate
31what alert value is to be sent in the B<al> parameter.  A callback may
32also return a negative value to suspend the handshake, and the handshake
33function will return immediately.  L<SSL_get_error(3)> will return
34SSL_ERROR_WANT_CLIENT_HELLO_CB to indicate that the handshake was suspended.
35It is the job of the ClientHello callback to store information about the state
36of the last call if needed to continue.  On the next call into the handshake
37function, the ClientHello callback will be called again, and, if it returns
38success, normal handshake processing will continue from that point.
39
40SSL_client_hello_isv2() indicates whether the ClientHello was carried in a
41SSLv2 record and is in the SSLv2 format.  The SSLv2 format has substantial
42differences from the normal SSLv3 format, including using three bytes per
43cipher suite, and not allowing extensions.  Additionally, the SSLv2 format
44'challenge' field is exposed via SSL_client_hello_get0_random(), padded to
45SSL3_RANDOM_SIZE bytes with zeros if needed.  For SSLv2 format ClientHellos,
46SSL_client_hello_get0_compression_methods() returns a dummy list that only includes
47the null compression method, since the SSLv2 format does not include a
48mechanism by which to negotiate compression.
49
50SSL_client_hello_get0_random(), SSL_client_hello_get0_session_id(),
51SSL_client_hello_get0_ciphers(), and
52SSL_client_hello_get0_compression_methods() provide access to the corresponding
53ClientHello fields, returning the field length and optionally setting an out
54pointer to the octets of that field.
55
56Similarly, SSL_client_hello_get0_ext() provides access to individual extensions
57from the ClientHello on a per-extension basis.  For the provided wire
58protocol extension type value, the extension value and length are returned
59in the output parameters (if present).
60
61SSL_client_hello_get1_extensions_present() can be used prior to
62SSL_client_hello_get0_ext(), to determine which extensions are present in the
63ClientHello before querying for them.  The B<out> and B<outlen> parameters are
64both required, and on success the caller must release the storage allocated for
65B<*out> using OPENSSL_free().  The contents of B<*out> is an array of integers
66holding the numerical value of the TLS extension types in the order they appear
67in the ClientHello.  B<*outlen> contains the number of elements in the array.
68In situations when the ClientHello has no extensions, the function will return
69success with B<*out> set to NULL and B<*outlen> set to 0.
70
71=head1 NOTES
72
73The ClientHello callback provides a vast window of possibilities for application
74code to affect the TLS handshake.  A primary use of the callback is to
75allow the server to examine the server name indication extension provided
76by the client in order to select an appropriate certificate to present,
77and make other configuration adjustments relevant to that server name
78and its configuration.  Such configuration changes can include swapping out
79the associated SSL_CTX pointer, modifying the server's list of permitted TLS
80versions, changing the server's cipher list in response to the client's
81cipher list, etc.
82
83It is also recommended that applications utilize a ClientHello callback and
84not use a servername callback, in order to avoid unexpected behavior that
85occurs due to the relative order of processing between things like session
86resumption and the historical servername callback.
87
88The SSL_client_hello_* family of functions may only be called from code executing
89within a ClientHello callback.
90
91=head1 RETURN VALUES
92
93The application's supplied ClientHello callback returns
94SSL_CLIENT_HELLO_SUCCESS on success, SSL_CLIENT_HELLO_ERROR on failure, and
95SSL_CLIENT_HELLO_RETRY to suspend processing.
96
97SSL_client_hello_isv2() returns 1 for SSLv2-format ClientHellos and 0 otherwise.
98
99SSL_client_hello_get0_random(), SSL_client_hello_get0_session_id(),
100SSL_client_hello_get0_ciphers(), and
101SSL_client_hello_get0_compression_methods() return the length of the
102corresponding ClientHello fields.  If zero is returned, the output pointer
103should not be assumed to be valid.
104
105SSL_client_hello_get0_ext() returns 1 if the extension of type 'type' is present, and
1060 otherwise.
107
108SSL_client_hello_get1_extensions_present() returns 1 on success and 0 on failure.
109
110=head1 SEE ALSO
111
112L<ssl(7)>, L<SSL_CTX_set_tlsext_servername_callback(3)>,
113L<SSL_bytes_to_cipher_list(3)>
114
115=head1 HISTORY
116
117The SSL ClientHello callback, SSL_client_hello_isv2(),
118SSL_client_hello_get0_random(), SSL_client_hello_get0_session_id(),
119SSL_client_hello_get0_ciphers(), SSL_client_hello_get0_compression_methods(),
120SSL_client_hello_get0_ext(), and SSL_client_hello_get1_extensions_present()
121were added in OpenSSL 1.1.1.
122
123=head1 COPYRIGHT
124
125Copyright 2017-2021 The OpenSSL Project Authors. All Rights Reserved.
126
127Licensed under the Apache License 2.0 (the "License").  You may not use
128this file except in compliance with the License.  You can obtain a copy
129in the file LICENSE in the source distribution or at
130L<https://www.openssl.org/source/license.html>.
131
132=cut
133