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