mirror of https://github.com/openssl/openssl.git
153 lines
7.7 KiB
Plaintext
153 lines
7.7 KiB
Plaintext
=pod
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=head1 NAME
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SSL_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_get_extension_order, SSL_client_hello_get0_ext - callback functions for early server-side ClientHello processing
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=head1 SYNOPSIS
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typedef int (*SSL_client_hello_cb_fn)(SSL *s, int *al, void *arg);
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void SSL_CTX_set_client_hello_cb(SSL_CTX *c, SSL_client_hello_cb_fn *f,
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void *arg);
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int SSL_client_hello_isv2(SSL *s);
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unsigned int SSL_client_hello_get0_legacy_version(SSL *s);
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size_t SSL_client_hello_get0_random(SSL *s, const unsigned char **out);
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size_t SSL_client_hello_get0_session_id(SSL *s, const unsigned char **out);
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size_t SSL_client_hello_get0_ciphers(SSL *s, const unsigned char **out);
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size_t SSL_client_hello_get0_compression_methods(SSL *s,
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const unsigned char **out);
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int SSL_client_hello_get1_extensions_present(SSL *s, int **out,
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size_t *outlen);
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int SSL_client_hello_get_extension_order(SSL *s, uint16_t *exts,
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size_t *num_exts);
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int SSL_client_hello_get0_ext(SSL *s, unsigned int type, const unsigned char **out,
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size_t *outlen);
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=head1 DESCRIPTION
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SSL_CTX_set_client_hello_cb() sets the callback function, which is automatically
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called during the early stages of ClientHello processing on the server.
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The argument supplied when setting the callback is passed back to the
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callback at run time. A callback that returns failure (0) will cause the
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connection to terminate, and callbacks returning failure should indicate
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what alert value is to be sent in the B<al> parameter. A callback may
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also return a negative value to suspend the handshake, and the handshake
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function will return immediately. L<SSL_get_error(3)> will return
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SSL_ERROR_WANT_CLIENT_HELLO_CB to indicate that the handshake was suspended.
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It is the job of the ClientHello callback to store information about the state
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of the last call if needed to continue. On the next call into the handshake
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function, the ClientHello callback will be called again, and, if it returns
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success, normal handshake processing will continue from that point.
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SSL_client_hello_isv2() indicates whether the ClientHello was carried in a
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SSLv2 record and is in the SSLv2 format. The SSLv2 format has substantial
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differences from the normal SSLv3 format, including using three bytes per
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cipher suite, and not allowing extensions. Additionally, the SSLv2 format
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'challenge' field is exposed via SSL_client_hello_get0_random(), padded to
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SSL3_RANDOM_SIZE bytes with zeros if needed. For SSLv2 format ClientHellos,
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SSL_client_hello_get0_compression_methods() returns a dummy list that only includes
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the null compression method, since the SSLv2 format does not include a
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mechanism by which to negotiate compression.
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SSL_client_hello_get0_random(), SSL_client_hello_get0_session_id(),
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SSL_client_hello_get0_ciphers(), and
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SSL_client_hello_get0_compression_methods() provide access to the corresponding
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ClientHello fields, returning the field length and optionally setting an out
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pointer to the octets of that field.
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Similarly, SSL_client_hello_get0_ext() provides access to individual extensions
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from the ClientHello on a per-extension basis. For the provided wire
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protocol extension type value, the extension value and length are returned
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in the output parameters (if present).
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SSL_client_hello_get1_extensions_present() can be used prior to
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SSL_client_hello_get0_ext(), to determine which extensions are present in the
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ClientHello before querying for them. The B<out> and B<outlen> parameters are
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both required, and on success the caller must release the storage allocated for
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B<*out> using OPENSSL_free(). The contents of B<*out> is an array of integers
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holding the numerical value of the TLS extension types in the order they appear
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in the ClientHello. B<*outlen> contains the number of elements in the array.
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In situations when the ClientHello has no extensions, the function will return
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success with B<*out> set to NULL and B<*outlen> set to 0.
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SSL_client_hello_get_extension_order() is similar to
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SSL_client_hello_get1_extensions_present(), without internal memory allocation.
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When called with B<exts> set to NULL, returns the number of extensions
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(e.g., to allocate storage for a subsequent call). Otherwise, B<*exts> is populated
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with the ExtensionType values in the order that the corresponding extensions
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appeared in the ClientHello. B<*num_exts> is an input/output parameter, used
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as input to supply the size of storage allocated by the caller, and as output to
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indicate how many ExtensionType values were written. If the input B<*num_exts>
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is smaller then the number of extensions in question, that is treated as an error.
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A subsequent call with B<exts> set to NULL can retrieve the size of storage needed.
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A ClientHello that contained no extensions is treated as success, with B<*num_exts>
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set to 0.
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=head1 NOTES
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The ClientHello callback provides a vast window of possibilities for application
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code to affect the TLS handshake. A primary use of the callback is to
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allow the server to examine the server name indication extension provided
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by the client in order to select an appropriate certificate to present,
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and make other configuration adjustments relevant to that server name
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and its configuration. Such configuration changes can include swapping out
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the associated SSL_CTX pointer, modifying the server's list of permitted TLS
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versions, changing the server's cipher list in response to the client's
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cipher list, etc.
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It is also recommended that applications utilize a ClientHello callback and
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not use a servername callback, in order to avoid unexpected behavior that
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occurs due to the relative order of processing between things like session
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resumption and the historical servername callback.
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The SSL_client_hello_* family of functions may only be called from code executing
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within a ClientHello callback.
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=head1 RETURN VALUES
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The application's supplied ClientHello callback returns
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SSL_CLIENT_HELLO_SUCCESS on success, SSL_CLIENT_HELLO_ERROR on failure, and
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SSL_CLIENT_HELLO_RETRY to suspend processing.
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SSL_client_hello_isv2() returns 1 for SSLv2-format ClientHellos and 0 otherwise.
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SSL_client_hello_get0_random(), SSL_client_hello_get0_session_id(),
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SSL_client_hello_get0_ciphers(), and
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SSL_client_hello_get0_compression_methods() return the length of the
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corresponding ClientHello fields. If zero is returned, the output pointer
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should not be assumed to be valid.
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SSL_client_hello_get0_ext() returns 1 if the extension of type 'type' is present, and
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0 otherwise.
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SSL_client_hello_get1_extensions_present() returns 1 on success and 0 on failure.
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SSL_client_hello_get_extension_order() returns 1 on success and 0 on failure.
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=head1 SEE ALSO
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L<ssl(7)>, L<SSL_CTX_set_tlsext_servername_callback(3)>,
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L<SSL_bytes_to_cipher_list(3)>
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=head1 HISTORY
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The SSL ClientHello callback, SSL_client_hello_isv2(),
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SSL_client_hello_get0_random(), SSL_client_hello_get0_session_id(),
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SSL_client_hello_get0_ciphers(), SSL_client_hello_get0_compression_methods(),
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SSL_client_hello_get0_ext(), and SSL_client_hello_get1_extensions_present()
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were added in OpenSSL 1.1.1.
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SSL_client_hello_get_extension_order()
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was added in OpenSSL 3.2.0.
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=head1 COPYRIGHT
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Copyright 2017-2022 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the Apache License 2.0 (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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