98 lines
3.4 KiB
Plaintext
98 lines
3.4 KiB
Plaintext
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=pod
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=head1 NAME
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SSL_connect - initiate the TLS/SSL handshake with an TLS/SSL server
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=head1 SYNOPSIS
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#include <openssl/ssl.h>
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int SSL_connect(SSL *ssl);
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=head1 DESCRIPTION
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SSL_connect() initiates the TLS/SSL handshake with a server. The communication
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channel must already have been set and assigned to the B<ssl> by setting an
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underlying B<BIO>.
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=head1 NOTES
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The behaviour of SSL_connect() depends on the underlying BIO.
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If the underlying BIO is B<blocking>, SSL_connect() will only return once the
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handshake has been finished or an error occurred.
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If the underlying BIO is B<nonblocking>, SSL_connect() will also return
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when the underlying BIO could not satisfy the needs of SSL_connect()
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to continue the handshake, indicating the problem by the return value -1.
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In this case a call to SSL_get_error() with the
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return value of SSL_connect() will yield B<SSL_ERROR_WANT_READ> or
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B<SSL_ERROR_WANT_WRITE>. The calling process then must repeat the call after
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taking appropriate action to satisfy the needs of SSL_connect().
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The action depends on the underlying BIO. When using a nonblocking socket,
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nothing is to be done, but select() can be used to check for the required
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condition. When using a buffering BIO, like a BIO pair, data must be written
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into or retrieved out of the BIO before being able to continue.
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Many systems implement Nagle's algorithm by default which means that it will
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buffer outgoing TCP data if a TCP packet has already been sent for which no
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corresponding ACK has been received yet from the peer. This can have performance
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impacts after a successful TLSv1.3 handshake or a successful TLSv1.2 (or below)
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resumption handshake, because the last peer to communicate in the handshake is
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the client. If the client is also the first to send application data (as is
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typical for many protocols) then this data could be buffered until an ACK has
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been received for the final handshake message.
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The B<TCP_NODELAY> socket option is often available to disable Nagle's
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algorithm. If an application opts to disable Nagle's algorithm consideration
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should be given to turning it back on again later if appropriate. The helper
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function BIO_set_tcp_ndelay() can be used to turn on or off the B<TCP_NODELAY>
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option.
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=head1 RETURN VALUES
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The following return values can occur:
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=over 4
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=item Z<>0
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The TLS/SSL handshake was not successful but was shut down controlled and
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by the specifications of the TLS/SSL protocol. Call SSL_get_error() with the
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return value B<ret> to find out the reason.
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=item Z<>1
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The TLS/SSL handshake was successfully completed, a TLS/SSL connection has been
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established.
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=item E<lt>0
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The TLS/SSL handshake was not successful, because a fatal error occurred either
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at the protocol level or a connection failure occurred. The shutdown was
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not clean. It can also occur if action is needed to continue the operation
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for nonblocking BIOs. Call SSL_get_error() with the return value B<ret>
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to find out the reason.
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=back
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=head1 SEE ALSO
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L<SSL_get_error(3)>, L<SSL_accept(3)>,
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L<SSL_shutdown(3)>, L<ssl(7)>, L<bio(7)>,
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L<SSL_set_connect_state(3)>,
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L<SSL_do_handshake(3)>,
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L<SSL_CTX_new(3)>
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=head1 COPYRIGHT
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Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the OpenSSL license (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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