1SSL_EXTENSION_SUPPORTED(3)          OpenSSL         SSL_EXTENSION_SUPPORTED(3)
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NAME

6       SSL_extension_supported, SSL_CTX_add_custom_ext,
7       SSL_CTX_add_client_custom_ext, SSL_CTX_add_server_custom_ext,
8       custom_ext_add_cb, custom_ext_free_cb, custom_ext_parse_cb - custom TLS
9       extension handling
10

SYNOPSIS

12        #include <openssl/ssl.h>
13
14        typedef int (*SSL_custom_ext_add_cb_ex) (SSL *s, unsigned int ext_type,
15                                                 unsigned int context,
16                                                 const unsigned char **out,
17                                                 size_t *outlen, X509 *x,
18                                                 size_t chainidx, int *al,
19                                                 void *add_arg);
20
21        typedef void (*SSL_custom_ext_free_cb_ex) (SSL *s, unsigned int ext_type,
22                                                   unsigned int context,
23                                                   const unsigned char *out,
24                                                   void *add_arg);
25
26        typedef int (*SSL_custom_ext_parse_cb_ex) (SSL *s, unsigned int ext_type,
27                                                   unsigned int context,
28                                                   const unsigned char *in,
29                                                   size_t inlen, X509 *x,
30                                                   size_t chainidx, int *al,
31                                                   void *parse_arg);
32
33        int SSL_CTX_add_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
34                                   unsigned int context,
35                                   SSL_custom_ext_add_cb_ex add_cb,
36                                   SSL_custom_ext_free_cb_ex free_cb,
37                                   void *add_arg,
38                                   SSL_custom_ext_parse_cb_ex parse_cb,
39                                   void *parse_arg);
40
41        typedef int (*custom_ext_add_cb)(SSL *s, unsigned int ext_type,
42                                         const unsigned char **out,
43                                         size_t *outlen, int *al,
44                                         void *add_arg);
45
46        typedef void (*custom_ext_free_cb)(SSL *s, unsigned int ext_type,
47                                           const unsigned char *out,
48                                           void *add_arg);
49
50        typedef int (*custom_ext_parse_cb)(SSL *s, unsigned int ext_type,
51                                           const unsigned char *in,
52                                           size_t inlen, int *al,
53                                           void *parse_arg);
54
55        int SSL_CTX_add_client_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
56                                          custom_ext_add_cb add_cb,
57                                          custom_ext_free_cb free_cb, void *add_arg,
58                                          custom_ext_parse_cb parse_cb,
59                                          void *parse_arg);
60
61        int SSL_CTX_add_server_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
62                                          custom_ext_add_cb add_cb,
63                                          custom_ext_free_cb free_cb, void *add_arg,
64                                          custom_ext_parse_cb parse_cb,
65                                          void *parse_arg);
66
67        int SSL_extension_supported(unsigned int ext_type);
68

DESCRIPTION

70       SSL_CTX_add_custom_ext() adds a custom extension for a TLS/DTLS client
71       or server for all supported protocol versions with extension type
72       ext_type and callbacks add_cb, free_cb and parse_cb (see the "EXTENSION
73       CALLBACKS" section below). The context value determines which messages
74       and under what conditions the extension will be added/parsed (see the
75       "EXTENSION CONTEXTS" section below).
76
77       SSL_CTX_add_client_custom_ext() adds a custom extension for a TLS/DTLS
78       client with extension type ext_type and callbacks add_cb, free_cb and
79       parse_cb. This function is similar to SSL_CTX_add_custom_ext() except
80       it only applies to clients, uses the older style of callbacks, and
81       implicitly sets the context value to:
82
83        SSL_EXT_TLS1_2_AND_BELOW_ONLY | SSL_EXT_CLIENT_HELLO
84        | SSL_EXT_TLS1_2_SERVER_HELLO | SSL_EXT_IGNORE_ON_RESUMPTION
85
86       SSL_CTX_add_server_custom_ext() adds a custom extension for a TLS/DTLS
87       server with extension type ext_type and callbacks add_cb, free_cb and
88       parse_cb. This function is similar to SSL_CTX_add_custom_ext() except
89       it only applies to servers, uses the older style of callbacks, and
90       implicitly sets the context value to the same as for
91       SSL_CTX_add_client_custom_ext() above.
92
93       The ext_type parameter corresponds to the extension_type field of
94       RFC5246 et al. It is not a NID. In all cases the extension type must
95       not be handled by OpenSSL internally or an error occurs.
96
97       SSL_extension_supported() returns 1 if the extension ext_type is
98       handled internally by OpenSSL and 0 otherwise.
99

EXTENSION CALLBACKS

101       The callback add_cb is called to send custom extension data to be
102       included in various TLS messages. The ext_type parameter is set to the
103       extension type which will be added and add_arg to the value set when
104       the extension handler was added. When using the new style callbacks the
105       context parameter will indicate which message is currently being
106       constructed e.g. for the ClientHello it will be set to
107       SSL_EXT_CLIENT_HELLO.
108
109       If the application wishes to include the extension ext_type it should
110       set *out to the extension data, set *outlen to the length of the
111       extension data and return 1.
112
113       If the add_cb does not wish to include the extension it must return 0.
114
115       If add_cb returns -1 a fatal handshake error occurs using the TLS alert
116       value specified in *al.
117
118       When constructing the ClientHello, if add_cb is set to NULL a zero
119       length extension is added for ext_type. For all other messages if
120       add_cb is set to NULL then no extension is added.
121
122       When constructing a Certificate message the callback will be called for
123       each certificate in the message. The x parameter will indicate the
124       current certificate and the chainidx parameter will indicate the
125       position of the certificate in the message. The first certificate is
126       always the end entity certificate and has a chainidx value of 0. The
127       certificates are in the order that they were received in the
128       Certificate message.
129
130       For all messages except the ServerHello and EncryptedExtensions every
131       registered add_cb is always called to see if the application wishes to
132       add an extension (as long as all requirements of the specified context
133       are met).
134
135       For the ServerHello and EncryptedExtension messages every registered
136       add_cb is called once if and only if the requirements of the specified
137       context are met and the corresponding extension was received in the
138       ClientHello. That is, if no corresponding extension was received in the
139       ClientHello then add_cb will not be called.
140
141       If an extension is added (that is add_cb returns 1) free_cb is called
142       (if it is set) with the value of out set by the add callback. It can be
143       used to free up any dynamic extension data set by add_cb. Since out is
144       constant (to permit use of constant data in add_cb) applications may
145       need to cast away const to free the data.
146
147       The callback parse_cb receives data for TLS extensions. The callback is
148       only called if the extension is present and relevant for the context
149       (see "EXTENSION CONTEXTS" below).
150
151       The extension data consists of inlen bytes in the buffer in for the
152       extension ext_type.
153
154       If the message being parsed is a TLSv1.3 compatible Certificate message
155       then parse_cb will be called for each certificate contained within the
156       message.  The x parameter will indicate the current certificate and the
157       chainidx parameter will indicate the position of the certificate in the
158       message. The first certificate is always the end entity certificate and
159       has a chainidx value of 0.
160
161       If the parse_cb considers the extension data acceptable it must return
162       1. If it returns 0 or a negative value a fatal handshake error occurs
163       using the TLS alert value specified in *al.
164
165       The buffer in is a temporary internal buffer which will not be valid
166       after the callback returns.
167

EXTENSION CONTEXTS

169       An extension context defines which messages and under which conditions
170       an extension should be added or expected. The context is built up by
171       performing a bitwise OR of multiple pre-defined values together. The
172       valid context values are:
173
174       SSL_EXT_TLS_ONLY
175           The extension is only allowed in TLS
176
177       SSL_EXT_DTLS_ONLY
178           The extension is only allowed in DTLS
179
180       SSL_EXT_TLS_IMPLEMENTATION_ONLY
181           The extension is allowed in DTLS, but there is only a TLS
182           implementation available (so it is ignored in DTLS).
183
184       SSL_EXT_SSL3_ALLOWED
185           Extensions are not typically defined for SSLv3. Setting this value
186           will allow the extension in SSLv3. Applications will not typically
187           need to use this.
188
189       SSL_EXT_TLS1_2_AND_BELOW_ONLY
190           The extension is only defined for TLSv1.2/DTLSv1.2 and below.
191           Servers will ignore this extension if it is present in the
192           ClientHello and TLSv1.3 is negotiated.
193
194       SSL_EXT_TLS1_3_ONLY
195           The extension is only defined for TLS1.3 and above. Servers will
196           ignore this extension if it is present in the ClientHello and
197           TLSv1.2 or below is negotiated.
198
199       SSL_EXT_IGNORE_ON_RESUMPTION
200           The extension will be ignored during parsing if a previous session
201           is being successfully resumed.
202
203       SSL_EXT_CLIENT_HELLO
204           The extension may be present in the ClientHello message.
205
206       SSL_EXT_TLS1_2_SERVER_HELLO
207           The extension may be present in a TLSv1.2 or below compatible
208           ServerHello message.
209
210       SSL_EXT_TLS1_3_SERVER_HELLO
211           The extension may be present in a TLSv1.3 compatible ServerHello
212           message.
213
214       SSL_EXT_TLS1_3_ENCRYPTED_EXTENSIONS
215           The extension may be present in an EncryptedExtensions message.
216
217       SSL_EXT_TLS1_3_HELLO_RETRY_REQUEST
218           The extension may be present in a HelloRetryRequest message.
219
220       SSL_EXT_TLS1_3_CERTIFICATE
221           The extension may be present in a TLSv1.3 compatible Certificate
222           message.
223
224       SSL_EXT_TLS1_3_NEW_SESSION_TICKET
225           The extension may be present in a TLSv1.3 compatible
226           NewSessionTicket message.
227
228       SSL_EXT_TLS1_3_CERTIFICATE_REQUEST
229           The extension may be present in a TLSv1.3 compatible
230           CertificateRequest message.
231
232       The context must include at least one message value (otherwise the
233       extension will never be used).
234

NOTES

236       The add_arg and parse_arg parameters can be set to arbitrary values
237       which will be passed to the corresponding callbacks. They can, for
238       example, be used to store the extension data received in a convenient
239       structure or pass the extension data to be added or freed when adding
240       extensions.
241
242       If the same custom extension type is received multiple times a fatal
243       decode_error alert is sent and the handshake aborts. If a custom
244       extension is received in a ServerHello/EncryptedExtensions message
245       which was not sent in the ClientHello a fatal unsupported_extension
246       alert is sent and the handshake is aborted. The
247       ServerHello/EncryptedExtensions add_cb callback is only called if the
248       corresponding extension was received in the ClientHello. This is
249       compliant with the TLS specifications. This behaviour ensures that each
250       callback is called at most once and that an application can never send
251       unsolicited extensions.
252

RETURN VALUES

254       SSL_CTX_add_custom_ext(), SSL_CTX_add_client_custom_ext() and
255       SSL_CTX_add_server_custom_ext() return 1 for success and 0 for failure.
256       A failure can occur if an attempt is made to add the same ext_type more
257       than once, if an attempt is made to use an extension type handled
258       internally by OpenSSL or if an internal error occurs (for example a
259       memory allocation failure).
260
261       SSL_extension_supported() returns 1 if the extension ext_type is
262       handled internally by OpenSSL and 0 otherwise.
263

HISTORY

265       The SSL_CTX_add_custom_ext() function was added in OpenSSL 1.1.1.
266
268       Copyright 2014-2017 The OpenSSL Project Authors. All Rights Reserved.
269
270       Licensed under the OpenSSL license (the "License").  You may not use
271       this file except in compliance with the License.  You can obtain a copy
272       in the file LICENSE in the source distribution or at
273       <https://www.openssl.org/source/license.html>.
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2771.1.1q                            2022-07-21        SSL_EXTENSION_SUPPORTED(3)
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