1=pod 2 3=head1 NAME 4 5SSL_extension_supported, 6SSL_CTX_add_custom_ext, 7SSL_CTX_add_client_custom_ext, SSL_CTX_add_server_custom_ext, 8custom_ext_add_cb, custom_ext_free_cb, custom_ext_parse_cb 9- custom TLS extension handling 10 11=head1 SYNOPSIS 12 13 #include <openssl/ssl.h> 14 15 typedef int (*SSL_custom_ext_add_cb_ex) (SSL *s, unsigned int ext_type, 16 unsigned int context, 17 const unsigned char **out, 18 size_t *outlen, X509 *x, 19 size_t chainidx, int *al, 20 void *add_arg); 21 22 typedef void (*SSL_custom_ext_free_cb_ex) (SSL *s, unsigned int ext_type, 23 unsigned int context, 24 const unsigned char *out, 25 void *add_arg); 26 27 typedef int (*SSL_custom_ext_parse_cb_ex) (SSL *s, unsigned int ext_type, 28 unsigned int context, 29 const unsigned char *in, 30 size_t inlen, X509 *x, 31 size_t chainidx, int *al, 32 void *parse_arg); 33 34 int SSL_CTX_add_custom_ext(SSL_CTX *ctx, unsigned int ext_type, 35 unsigned int context, 36 SSL_custom_ext_add_cb_ex add_cb, 37 SSL_custom_ext_free_cb_ex free_cb, 38 void *add_arg, 39 SSL_custom_ext_parse_cb_ex parse_cb, 40 void *parse_arg); 41 42 typedef int (*custom_ext_add_cb)(SSL *s, unsigned int ext_type, 43 const unsigned char **out, 44 size_t *outlen, int *al, 45 void *add_arg); 46 47 typedef void (*custom_ext_free_cb)(SSL *s, unsigned int ext_type, 48 const unsigned char *out, 49 void *add_arg); 50 51 typedef int (*custom_ext_parse_cb)(SSL *s, unsigned int ext_type, 52 const unsigned char *in, 53 size_t inlen, int *al, 54 void *parse_arg); 55 56 int SSL_CTX_add_client_custom_ext(SSL_CTX *ctx, unsigned int ext_type, 57 custom_ext_add_cb add_cb, 58 custom_ext_free_cb free_cb, void *add_arg, 59 custom_ext_parse_cb parse_cb, 60 void *parse_arg); 61 62 int SSL_CTX_add_server_custom_ext(SSL_CTX *ctx, unsigned int ext_type, 63 custom_ext_add_cb add_cb, 64 custom_ext_free_cb free_cb, void *add_arg, 65 custom_ext_parse_cb parse_cb, 66 void *parse_arg); 67 68 int SSL_extension_supported(unsigned int ext_type); 69 70=head1 DESCRIPTION 71 72SSL_CTX_add_custom_ext() adds a custom extension for a TLS/DTLS client or server 73for all supported protocol versions with extension type B<ext_type> and 74callbacks B<add_cb>, B<free_cb> and B<parse_cb> (see the 75L</EXTENSION CALLBACKS> section below). The B<context> value determines 76which messages and under what conditions the extension will be added/parsed (see 77the L</EXTENSION CONTEXTS> section below). 78 79SSL_CTX_add_client_custom_ext() adds a custom extension for a TLS/DTLS client 80with extension type B<ext_type> and callbacks B<add_cb>, B<free_cb> and 81B<parse_cb>. This function is similar to SSL_CTX_add_custom_ext() except it only 82applies to clients, uses the older style of callbacks, and implicitly sets the 83B<context> value to: 84 85 SSL_EXT_TLS1_2_AND_BELOW_ONLY | SSL_EXT_CLIENT_HELLO 86 | SSL_EXT_TLS1_2_SERVER_HELLO | SSL_EXT_IGNORE_ON_RESUMPTION 87 88SSL_CTX_add_server_custom_ext() adds a custom extension for a TLS/DTLS server 89with extension type B<ext_type> and callbacks B<add_cb>, B<free_cb> and 90B<parse_cb>. This function is similar to SSL_CTX_add_custom_ext() except it 91only applies to servers, uses the older style of callbacks, and implicitly sets 92the B<context> value to the same as for SSL_CTX_add_client_custom_ext() above. 93 94The B<ext_type> parameter corresponds to the B<extension_type> field of 95RFC5246 et al. It is B<not> a NID. In all cases the extension type must not be 96handled by OpenSSL internally or an error occurs. 97 98SSL_extension_supported() returns 1 if the extension B<ext_type> is handled 99internally by OpenSSL and 0 otherwise. 100 101=head1 EXTENSION CALLBACKS 102 103The callback B<add_cb> is called to send custom extension data to be 104included in various TLS messages. The B<ext_type> parameter is set to the 105extension type which will be added and B<add_arg> to the value set when the 106extension handler was added. When using the new style callbacks the B<context> 107parameter will indicate which message is currently being constructed e.g. for 108the ClientHello it will be set to B<SSL_EXT_CLIENT_HELLO>. 109 110If the application wishes to include the extension B<ext_type> it should 111set B<*out> to the extension data, set B<*outlen> to the length of the 112extension data and return 1. 113 114If the B<add_cb> does not wish to include the extension it must return 0. 115 116If B<add_cb> returns -1 a fatal handshake error occurs using the TLS 117alert value specified in B<*al>. 118 119When constructing the ClientHello, if B<add_cb> is set to NULL a zero length 120extension is added for B<ext_type>. For all other messages if B<add_cb> is set 121to NULL then no extension is added. 122 123When constructing a Certificate message the callback will be called for each 124certificate in the message. The B<x> parameter will indicate the 125current certificate and the B<chainidx> parameter will indicate the position 126of the certificate in the message. The first certificate is always the end 127entity certificate and has a B<chainidx> value of 0. The certificates are in the 128order that they were received in the Certificate message. 129 130For all messages except the ServerHello and EncryptedExtensions every 131registered B<add_cb> is always called to see if the application wishes to add an 132extension (as long as all requirements of the specified B<context> are met). 133 134For the ServerHello and EncryptedExtension messages every registered B<add_cb> 135is called once if and only if the requirements of the specified B<context> are 136met and the corresponding extension was received in the ClientHello. That is, if 137no corresponding extension was received in the ClientHello then B<add_cb> will 138not be called. 139 140If an extension is added (that is B<add_cb> returns 1) B<free_cb> is called 141(if it is set) with the value of B<out> set by the add callback. It can be 142used to free up any dynamic extension data set by B<add_cb>. Since B<out> is 143constant (to permit use of constant data in B<add_cb>) applications may need to 144cast away const to free the data. 145 146The callback B<parse_cb> receives data for TLS extensions. The callback is only 147called if the extension is present and relevant for the context (see 148L</EXTENSION CONTEXTS> below). 149 150The extension data consists of B<inlen> bytes in the buffer B<in> for the 151extension B<ext_type>. 152 153If the message being parsed is a TLSv1.3 compatible Certificate message then 154B<parse_cb> will be called for each certificate contained within the message. 155The B<x> parameter will indicate the current certificate and the B<chainidx> 156parameter will indicate the position of the certificate in the message. The 157first certificate is always the end entity certificate and has a B<chainidx> 158value of 0. 159 160If the B<parse_cb> considers the extension data acceptable it must return 1611. If it returns 0 or a negative value a fatal handshake error occurs 162using the TLS alert value specified in B<*al>. 163 164The buffer B<in> is a temporary internal buffer which will not be valid after 165the callback returns. 166 167=head1 EXTENSION CONTEXTS 168 169An extension context defines which messages and under which conditions an 170extension should be added or expected. The context is built up by performing 171a bitwise OR of multiple pre-defined values together. The valid context values 172are: 173 174=over 4 175 176=item SSL_EXT_TLS_ONLY 177 178The extension is only allowed in TLS 179 180=item SSL_EXT_DTLS_ONLY 181 182The extension is only allowed in DTLS 183 184=item SSL_EXT_TLS_IMPLEMENTATION_ONLY 185 186The extension is allowed in DTLS, but there is only a TLS implementation 187available (so it is ignored in DTLS). 188 189=item SSL_EXT_SSL3_ALLOWED 190 191Extensions are not typically defined for SSLv3. Setting this value will allow 192the extension in SSLv3. Applications will not typically need to use this. 193 194=item SSL_EXT_TLS1_2_AND_BELOW_ONLY 195 196The extension is only defined for TLSv1.2/DTLSv1.2 and below. Servers will 197ignore this extension if it is present in the ClientHello and TLSv1.3 is 198negotiated. 199 200=item SSL_EXT_TLS1_3_ONLY 201 202The extension is only defined for TLS1.3 and above. Servers will ignore this 203extension if it is present in the ClientHello and TLSv1.2 or below is 204negotiated. 205 206=item SSL_EXT_IGNORE_ON_RESUMPTION 207 208The extension will be ignored during parsing if a previous session is being 209successfully resumed. 210 211=item SSL_EXT_CLIENT_HELLO 212 213The extension may be present in the ClientHello message. 214 215=item SSL_EXT_TLS1_2_SERVER_HELLO 216 217The extension may be present in a TLSv1.2 or below compatible ServerHello 218message. 219 220=item SSL_EXT_TLS1_3_SERVER_HELLO 221 222The extension may be present in a TLSv1.3 compatible ServerHello message. 223 224=item SSL_EXT_TLS1_3_ENCRYPTED_EXTENSIONS 225 226The extension may be present in an EncryptedExtensions message. 227 228=item SSL_EXT_TLS1_3_HELLO_RETRY_REQUEST 229 230The extension may be present in a HelloRetryRequest message. 231 232=item SSL_EXT_TLS1_3_CERTIFICATE 233 234The extension may be present in a TLSv1.3 compatible Certificate message. 235 236=item SSL_EXT_TLS1_3_NEW_SESSION_TICKET 237 238The extension may be present in a TLSv1.3 compatible NewSessionTicket message. 239 240=item SSL_EXT_TLS1_3_CERTIFICATE_REQUEST 241 242The extension may be present in a TLSv1.3 compatible CertificateRequest message. 243 244=back 245 246The context must include at least one message value (otherwise the extension 247will never be used). 248 249=head1 NOTES 250 251The B<add_arg> and B<parse_arg> parameters can be set to arbitrary values 252which will be passed to the corresponding callbacks. They can, for example, 253be used to store the extension data received in a convenient structure or 254pass the extension data to be added or freed when adding extensions. 255 256If the same custom extension type is received multiple times a fatal 257B<decode_error> alert is sent and the handshake aborts. If a custom extension 258is received in a ServerHello/EncryptedExtensions message which was not sent in 259the ClientHello a fatal B<unsupported_extension> alert is sent and the 260handshake is aborted. The ServerHello/EncryptedExtensions B<add_cb> callback is 261only called if the corresponding extension was received in the ClientHello. This 262is compliant with the TLS specifications. This behaviour ensures that each 263callback is called at most once and that an application can never send 264unsolicited extensions. 265 266=head1 RETURN VALUES 267 268SSL_CTX_add_custom_ext(), SSL_CTX_add_client_custom_ext() and 269SSL_CTX_add_server_custom_ext() return 1 for success and 0 for failure. A 270failure can occur if an attempt is made to add the same B<ext_type> more than 271once, if an attempt is made to use an extension type handled internally by 272OpenSSL or if an internal error occurs (for example a memory allocation 273failure). 274 275SSL_extension_supported() returns 1 if the extension B<ext_type> is handled 276internally by OpenSSL and 0 otherwise. 277 278=head1 HISTORY 279 280The SSL_CTX_add_custom_ext() function was added in OpenSSL 1.1.1. 281 282=head1 COPYRIGHT 283 284Copyright 2014-2017 The OpenSSL Project Authors. All Rights Reserved. 285 286Licensed under the OpenSSL license (the "License"). You may not use 287this file except in compliance with the License. You can obtain a copy 288in the file LICENSE in the source distribution or at 289L<https://www.openssl.org/source/license.html>. 290 291=cut 292