SSL_EXTENSION_SUPPORTED(3SSL) OpenSSL SSL_EXTENSION_SUPPORTED(3SSL)
NAME
SSL_extension_supported, SSL_custom_ext_add_cb_ex,
SSL_custom_ext_free_cb_ex, SSL_custom_ext_parse_cb_ex,
SSL_CTX_add_custom_ext, SSL_CTX_add_client_custom_ext,
SSL_CTX_add_server_custom_ext, custom_ext_add_cb, custom_ext_free_cb,
custom_ext_parse_cb - custom TLS extension handling
SYNOPSIS
#include <openssl/ssl.h>
typedef int (*SSL_custom_ext_add_cb_ex)(SSL *s, unsigned int ext_type,
unsigned int context,
const unsigned char **out,
size_t *outlen, X509 *x,
size_t chainidx, int *al,
void *add_arg);
typedef void (*SSL_custom_ext_free_cb_ex)(SSL *s, unsigned int ext_type,
unsigned int context,
const unsigned char *out,
void *add_arg);
typedef int (*SSL_custom_ext_parse_cb_ex)(SSL *s, unsigned int ext_type,
unsigned int context,
const unsigned char *in,
size_t inlen, X509 *x,
size_t chainidx, int *al,
void *parse_arg);
int SSL_CTX_add_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
unsigned int context,
SSL_custom_ext_add_cb_ex add_cb,
SSL_custom_ext_free_cb_ex free_cb,
void *add_arg,
SSL_custom_ext_parse_cb_ex parse_cb,
void *parse_arg);
typedef int (*custom_ext_add_cb)(SSL *s, unsigned int ext_type,
const unsigned char **out,
size_t *outlen, int *al,
void *add_arg);
typedef void (*custom_ext_free_cb)(SSL *s, unsigned int ext_type,
const unsigned char *out,
void *add_arg);
typedef int (*custom_ext_parse_cb)(SSL *s, unsigned int ext_type,
const unsigned char *in,
size_t inlen, int *al,
void *parse_arg);
int SSL_CTX_add_client_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
custom_ext_add_cb add_cb,
custom_ext_free_cb free_cb, void *add_arg,
custom_ext_parse_cb parse_cb,
void *parse_arg);
int SSL_CTX_add_server_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
custom_ext_add_cb add_cb,
custom_ext_free_cb free_cb, void *add_arg,
custom_ext_parse_cb parse_cb,
void *parse_arg);
int SSL_extension_supported(unsigned int ext_type);
DESCRIPTION
SSL_CTX_add_custom_ext() adds a custom extension for a TLS/DTLS client
or server for all supported protocol versions with extension type
ext_type and callbacks add_cb, free_cb and parse_cb (see the "EXTENSION
CALLBACKS" section below). The context value determines which messages
and under what conditions the extension will be added/parsed (see the
"EXTENSION CONTEXTS" section below).
SSL_CTX_add_client_custom_ext() adds a custom extension for a TLS/DTLS
client with extension type ext_type and callbacks add_cb, free_cb and
parse_cb. This function is similar to SSL_CTX_add_custom_ext() except it
only applies to clients, uses the older style of callbacks, and
implicitly sets the context value to:
SSL_EXT_TLS1_2_AND_BELOW_ONLY | SSL_EXT_CLIENT_HELLO
| SSL_EXT_TLS1_2_SERVER_HELLO | SSL_EXT_IGNORE_ON_RESUMPTION
SSL_CTX_add_server_custom_ext() adds a custom extension for a TLS/DTLS
server with extension type ext_type and callbacks add_cb, free_cb and
parse_cb. This function is similar to SSL_CTX_add_custom_ext() except it
only applies to servers, uses the older style of callbacks, and
implicitly sets the context value to the same as for
SSL_CTX_add_client_custom_ext() above.
The ext_type parameter corresponds to the extension_type field of
RFC5246 et al. It is not a NID. In all cases the extension type must not
be handled by OpenSSL internally or an error occurs.
SSL_extension_supported() returns 1 if the extension ext_type is handled
internally by OpenSSL and 0 otherwise.
EXTENSION CALLBACKS
The callback add_cb is called to send custom extension data to be
included in various TLS messages. The ext_type parameter is set to the
extension type which will be added and add_arg to the value set when the
extension handler was added. When using the new style callbacks the
context parameter will indicate which message is currently being
constructed e.g. for the ClientHello it will be set to
SSL_EXT_CLIENT_HELLO.
If the application wishes to include the extension ext_type it should
set *out to the extension data, set *outlen to the length of the
extension data and return 1.
If the add_cb does not wish to include the extension it must return 0.
If add_cb returns -1 a fatal handshake error occurs using the TLS alert
value specified in *al.
When constructing the ClientHello, if add_cb is set to NULL a zero
length extension is added for ext_type. For all other messages if add_cb
is set to NULL then no extension is added.
When constructing a Certificate message the callback will be called for
each certificate in the message. The x parameter will indicate the
current certificate and the chainidx parameter will indicate the
position of the certificate in the message. The first certificate is
always the end entity certificate and has a chainidx value of 0. The
certificates are in the order that they were received in the Certificate
message.
For all messages except the ServerHello and EncryptedExtensions every
registered add_cb is always called to see if the application wishes to
add an extension (as long as all requirements of the specified context
are met).
For the ServerHello and EncryptedExtension messages every registered
add_cb is called once if and only if the requirements of the specified
context are met and the corresponding extension was received in the
ClientHello. That is, if no corresponding extension was received in the
ClientHello then add_cb will not be called.
If an extension is added (that is add_cb returns 1) free_cb is called
(if it is set) with the value of out set by the add callback. It can be
used to free up any dynamic extension data set by add_cb. Since out is
constant (to permit use of constant data in add_cb) applications may
need to cast away const to free the data.
The callback parse_cb receives data for TLS extensions. The callback is
only called if the extension is present and relevant for the context
(see "EXTENSION CONTEXTS" below).
The extension data consists of inlen bytes in the buffer in for the
extension ext_type.
If the message being parsed is a TLSv1.3 compatible Certificate message
then parse_cb will be called for each certificate contained within the
message. The x parameter will indicate the current certificate and the
chainidx parameter will indicate the position of the certificate in the
message. The first certificate is always the end entity certificate and
has a chainidx value of 0.
If the parse_cb considers the extension data acceptable it must return
1. If it returns 0 or a negative value a fatal handshake error occurs
using the TLS alert value specified in *al.
The buffer in is a temporary internal buffer which will not be valid
after the callback returns.
EXTENSION CONTEXTS
An extension context defines which messages and under which conditions
an extension should be added or expected. The context is built up by
performing a bitwise OR of multiple pre-defined values together. The
valid context values are:
SSL_EXT_TLS_ONLY
The extension is only allowed in TLS
SSL_EXT_DTLS_ONLY
The extension is only allowed in DTLS
SSL_EXT_TLS_IMPLEMENTATION_ONLY
The extension is allowed in DTLS, but there is only a TLS
implementation available (so it is ignored in DTLS).
SSL_EXT_SSL3_ALLOWED
Extensions are not typically defined for SSLv3. Setting this value
will allow the extension in SSLv3. Applications will not typically
need to use this.
SSL_EXT_TLS1_2_AND_BELOW_ONLY
The extension is only defined for TLSv1.2/DTLSv1.2 and below.
Servers will ignore this extension if it is present in the
ClientHello and TLSv1.3 is negotiated.
SSL_EXT_TLS1_3_ONLY
The extension is only defined for TLS1.3 and above. Servers will
ignore this extension if it is present in the ClientHello and
TLSv1.2 or below is negotiated.
SSL_EXT_IGNORE_ON_RESUMPTION
The extension will be ignored during parsing if a previous session
is being successfully resumed.
SSL_EXT_CLIENT_HELLO
The extension may be present in the ClientHello message.
SSL_EXT_TLS1_2_SERVER_HELLO
The extension may be present in a TLSv1.2 or below compatible
ServerHello message.
SSL_EXT_TLS1_3_SERVER_HELLO
The extension may be present in a TLSv1.3 compatible ServerHello
message.
SSL_EXT_TLS1_3_ENCRYPTED_EXTENSIONS
The extension may be present in an EncryptedExtensions message.
SSL_EXT_TLS1_3_HELLO_RETRY_REQUEST
The extension may be present in a HelloRetryRequest message.
SSL_EXT_TLS1_3_CERTIFICATE
The extension may be present in a TLSv1.3 compatible Certificate
message.
SSL_EXT_TLS1_3_NEW_SESSION_TICKET
The extension may be present in a TLSv1.3 compatible
NewSessionTicket message.
SSL_EXT_TLS1_3_CERTIFICATE_REQUEST
The extension may be present in a TLSv1.3 compatible
CertificateRequest message.
The context must include at least one message value (otherwise the
extension will never be used).
NOTES
The add_arg and parse_arg parameters can be set to arbitrary values
which will be passed to the corresponding callbacks. They can, for
example, be used to store the extension data received in a convenient
structure or pass the extension data to be added or freed when adding
extensions.
If the same custom extension type is received multiple times a fatal
decode_error alert is sent and the handshake aborts. If a custom
extension is received in a ServerHello/EncryptedExtensions message which
was not sent in the ClientHello a fatal unsupported_extension alert is
sent and the handshake is aborted. The ServerHello/EncryptedExtensions
add_cb callback is only called if the corresponding extension was
received in the ClientHello. This is compliant with the TLS
specifications. This behaviour ensures that each callback is called at
most once and that an application can never send unsolicited extensions.
RETURN VALUES
SSL_CTX_add_custom_ext(), SSL_CTX_add_client_custom_ext() and
SSL_CTX_add_server_custom_ext() return 1 for success and 0 for failure.
A failure can occur if an attempt is made to add the same ext_type more
than once, if an attempt is made to use an extension type handled
internally by OpenSSL or if an internal error occurs (for example a
memory allocation failure).
SSL_extension_supported() returns 1 if the extension ext_type is handled
internally by OpenSSL and 0 otherwise.
SEE ALSO
ssl(7)
HISTORY
The SSL_CTX_add_custom_ext() function was added in OpenSSL 1.1.1.
COPYRIGHT
Copyright 2014-2020 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>.
3.5.7 2026-06-09 SSL_EXTENSION_SUPPORTED(3SSL)
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