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107 lines
4.2 KiB
Plaintext
107 lines
4.2 KiB
Plaintext
=pod
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=head1 NAME
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SSL_CTX_set_cert_verify_callback - set peer certificate verification procedure
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=head1 SYNOPSIS
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#include <openssl/ssl.h>
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void SSL_CTX_set_cert_verify_callback(SSL_CTX *ctx,
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int (*callback)(X509_STORE_CTX *, void *),
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void *arg);
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=head1 DESCRIPTION
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SSL_CTX_set_cert_verify_callback() sets the verification callback function for
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I<ctx>. SSL objects that are created from I<ctx> inherit the setting valid at
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the time when L<SSL_new(3)> is called.
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=head1 NOTES
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When a peer certificate has been received during a SSL/TLS handshake,
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a verification function is called regardless of the verification mode.
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If the application does not explicitly specify a verification callback function,
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the built-in verification function is used.
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If a verification callback I<callback> is specified via
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SSL_CTX_set_cert_verify_callback(), the supplied callback function is called
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instead with the arguments callback(X509_STORE_CTX *x509_store_ctx, void *arg).
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The argument I<arg> is specified by the application when setting I<callback>.
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By setting I<callback> to NULL, the default behaviour is restored.
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I<callback> should return 1 to indicate verification success
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and 0 to indicate verification failure.
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In server mode, a return value of 0 leads to handshake failure.
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In client mode, the behaviour is as follows.
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All values, including 0, are ignored
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if the verification mode is B<SSL_VERIFY_NONE>.
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Otherwise, when the return value is less than or equal to 0, the handshake will
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fail.
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In client mode I<callback> may also call the L<SSL_set_retry_verify(3)>
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function on the B<SSL> object set in the I<x509_store_ctx> ex data (see
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L<SSL_get_ex_data_X509_STORE_CTX_idx(3)>) and return 1. This would be
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typically done in case the certificate verification was not yet able
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to succeed. This makes the handshake suspend and return control to the
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calling application with B<SSL_ERROR_WANT_RETRY_VERIFY>. The app can for
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instance fetch further certificates or cert status information needed for
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the verification. Calling L<SSL_connect(3)> again resumes the connection
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attempt by retrying the server certificate verification step.
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This process may even be repeated if need be.
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In any case a viable verification result value must be reflected
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in the B<error> member of I<x509_store_ctx>,
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which can be done using L<X509_STORE_CTX_set_error(3)>.
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This is particularly important in case
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the I<callback> allows the connection to continue (by returning 1).
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Note that the verification status in the store context is a possibly durable
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indication of the chain's validity!
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This gets recorded in the SSL session (and thus also in session tickets)
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and the validity of the originally presented chain is then visible
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on resumption, even though no chain is presented int that case.
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Moreover, the calling application will be informed about the detailed result of
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the verification procedure and may elect to base further decisions on it.
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Within I<x509_store_ctx>, I<callback> has access to the I<verify_callback>
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function set using L<SSL_CTX_set_verify(3)>.
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=head1 RETURN VALUES
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SSL_CTX_set_cert_verify_callback() does not return a value.
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=head1 WARNINGS
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Do not mix the verification callback described in this function with the
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B<verify_callback> function called during the verification process. The
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latter is set using the L<SSL_CTX_set_verify(3)>
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family of functions.
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Providing a complete verification procedure including certificate purpose
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settings etc is a complex task. The built-in procedure is quite powerful
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and in most cases it should be sufficient to modify its behaviour using
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the B<verify_callback> function.
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=head1 BUGS
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SSL_CTX_set_cert_verify_callback() does not provide diagnostic information.
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=head1 SEE ALSO
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L<ssl(7)>, L<SSL_CTX_set_verify(3)>,
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L<X509_STORE_CTX_set_error(3)>,
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L<SSL_get_verify_result(3)>,
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L<SSL_set_retry_verify(3)>,
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L<SSL_CTX_load_verify_locations(3)>
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=head1 COPYRIGHT
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Copyright 2001-2022 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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