1SSL_CTX_SET_CLIENT_HELLO_CB(3) OpenSSL SSL_CTX_SET_CLIENT_HELLO_CB(3)
2
6 SSL_CTX_set_client_hello_cb, SSL_client_hello_cb_fn,
7 SSL_client_hello_isv2, SSL_client_hello_get0_legacy_version,
8 SSL_client_hello_get0_random, SSL_client_hello_get0_session_id,
9 SSL_client_hello_get0_ciphers,
10 SSL_client_hello_get0_compression_methods,
11 SSL_client_hello_get1_extensions_present, SSL_client_hello_get0_ext -
12 callback functions for early server-side ClientHello processing
13
15 typedef int (*SSL_client_hello_cb_fn)(SSL *s, int *al, void *arg);
16 void SSL_CTX_set_client_hello_cb(SSL_CTX *c, SSL_client_hello_cb_fn *f,
17 void *arg);
18 int SSL_client_hello_isv2(SSL *s);
19 unsigned int SSL_client_hello_get0_legacy_version(SSL *s);
20 size_t SSL_client_hello_get0_random(SSL *s, const unsigned char **out);
21 size_t SSL_client_hello_get0_session_id(SSL *s, const unsigned char **out);
22 size_t SSL_client_hello_get0_ciphers(SSL *s, const unsigned char **out);
23 size_t SSL_client_hello_get0_compression_methods(SSL *s,
24 const unsigned char **out);
25 int SSL_client_hello_get1_extensions_present(SSL *s, int **out,
26 size_t *outlen);
27 int SSL_client_hello_get0_ext(SSL *s, int type, const unsigned char **out,
28 size_t *outlen);
29
31 SSL_CTX_set_client_hello_cb() sets the callback function, which is
32 automatically called during the early stages of ClientHello processing
33 on the server. The argument supplied when setting the callback is
34 passed back to the callback at runtime. A callback that returns
35 failure (0) will cause the connection to terminate, and callbacks
36 returning failure should indicate what alert value is to be sent in the
37 al parameter. A callback may also return a negative value to suspend
38 the handshake, and the handshake function will return immediately.
39 SSL_get_error(3) will return SSL_ERROR_WANT_CLIENT_HELLO_CB to indicate
40 that the handshake was suspended. It is the job of the ClientHello
41 callback to store information about the state of the last call if
42 needed to continue. On the next call into the handshake function, the
43 ClientHello callback will be called again, and, if it returns success,
44 normal handshake processing will continue from that point.
45 SSL_client_hello_isv2() indicates whether the ClientHello was carried
47 in a SSLv2 record and is in the SSLv2 format. The SSLv2 format has
48 substantial differences from the normal SSLv3 format, including using
49 three bytes per cipher suite, and not allowing extensions.
50 Additionally, the SSLv2 format 'challenge' field is exposed via
51 SSL_client_hello_get0_random(), padded to SSL3_RANDOM_SIZE bytes with
52 zeros if needed. For SSLv2 format ClientHellos,
53 SSL_client_hello_get0_compression_methods() returns a dummy list that
54 only includes the null compression method, since the SSLv2 format does
55 not include a mechanism by which to negotiate compression.
56 SSL_client_hello_get0_random(), SSL_client_hello_get0_session_id(),
58 SSL_client_hello_get0_ciphers(), and
59 SSL_client_hello_get0_compression_methods() provide access to the
60 corresponding ClientHello fields, returning the field length and
61 optionally setting an out pointer to the octets of that field.
62 Similarly, SSL_client_hello_get0_ext() provides access to individual
64 extensions from the ClientHello on a per-extension basis. For the
65 provided wire protocol extension type value, the extension value and
66 length are returned in the output parameters (if present).
67 SSL_client_hello_get1_extensions_present() can be used prior to
69 SSL_client_hello_get0_ext(), to determine which extensions are present
70 in the ClientHello before querying for them. The out and outlen
71 parameters are both required, and on success the caller must release
72 the storage allocated for *out using OPENSSL_free(). The contents of
73 *out is an array of integers holding the numerical value of the TLS
74 extension types in the order they appear in the ClientHello. *outlen
75 contains the number of elements in the array. In situations when the
76 ClientHello has no extensions, the function will return success with
77 *out set to NULL and *outlen set to 0.
78
80 The ClientHello callback provides a vast window of possibilities for
81 application code to affect the TLS handshake. A primary use of the
82 callback is to allow the server to examine the server name indication
83 extension provided by the client in order to select an appropriate
84 certificate to present, and make other configuration adjustments
85 relevant to that server name and its configuration. Such configuration
86 changes can include swapping out the associated SSL_CTX pointer,
87 modifying the server's list of permitted TLS versions, changing the
88 server's cipher list in response to the client's cipher list, etc.
89 It is also recommended that applications utilize a ClientHello callback
91 and not use a servername callback, in order to avoid unexpected
92 behavior that occurs due to the relative order of processing between
93 things like session resumption and the historical servername callback.
94 The SSL_client_hello_* family of functions may only be called from code
96 executing within a ClientHello callback.
97
99 The application's supplied ClientHello callback returns
100 SSL_CLIENT_HELLO_SUCCESS on success, SSL_CLIENT_HELLO_ERROR on failure,
101 and SSL_CLIENT_HELLO_RETRY to suspend processing.
102 SSL_client_hello_isv2() returns 1 for SSLv2-format ClientHellos and 0
104 otherwise.
105 SSL_client_hello_get0_random(), SSL_client_hello_get0_session_id(),
107 SSL_client_hello_get0_ciphers(), and
108 SSL_client_hello_get0_compression_methods() return the length of the
109 corresponding ClientHello fields. If zero is returned, the output
110 pointer should not be assumed to be valid.
111 SSL_client_hello_get0_ext() returns 1 if the extension of type 'type'
113 is present, and 0 otherwise.
114 SSL_client_hello_get1_extensions_present() returns 1 on success and 0
116 on failure.
117
119 ssl(7), SSL_CTX_set_tlsext_servername_callback(3),
120 SSL_bytes_to_cipher_list
121
123 The SSL ClientHello callback, SSL_client_hello_isv2(),
124 SSL_client_hello_get0_random(), SSL_client_hello_get0_session_id(),
125 SSL_client_hello_get0_ciphers(),
126 SSL_client_hello_get0_compression_methods(),
127 SSL_client_hello_get0_ext(), and
128 SSL_client_hello_get1_extensions_present() were added in OpenSSL 1.1.1.
129
131 Copyright 2017-2019 The OpenSSL Project Authors. All Rights Reserved.
132 Licensed under the OpenSSL license (the "License"). You may not use
134 this file except in compliance with the License. You can obtain a copy
135 in the file LICENSE in the source distribution or at
136 <https://www.openssl.org/source/license.html>.
1371.1.1i 2021-07-22 SSL_CTX_SET_CLIENT_HELLO_CB(3)