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<a name="TLS-handshake"></a>
<div class="header">
<p>
Next: <a href="Data-transfer-and-termination.html#Data-transfer-and-termination" accesskey="n" rel="next">Data transfer and termination</a>, Previous: <a href="Setting-up-the-transport-layer.html#Setting-up-the-transport-layer" accesskey="p" rel="prev">Setting up the transport layer</a>, Up: <a href="How-to-use-GnuTLS-in-applications.html#How-to-use-GnuTLS-in-applications" accesskey="u" rel="up">How to use GnuTLS in applications</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="TLS-handshake-1"></a>
<h3 class="section">6.6 TLS handshake</h3>
<p>Once a session has been initialized and a network
connection has been set up, TLS and DTLS protocols
perform a handshake. The handshake is the actual key
exchange.
</p>
<dl>
<dt><a name="index-gnutls_005fhandshake"></a>Function: <em>int</em> <strong>gnutls_handshake</strong> <em>(gnutls_session_t <var>session</var>)</em></dt>
<dd><p><var>session</var>: is a <code>gnutls_session_t</code> type.
</p>
<p>This function does the handshake of the TLS/SSL protocol, and
initializes the TLS connection.
</p>
<p>This function will fail if any problem is encountered, and will
return a negative error code. In case of a client, if the client
has asked to resume a session, but the server couldn’t, then a
full handshake will be performed.
</p>
<p>The non-fatal errors expected by this function are:
<code>GNUTLS_E_INTERRUPTED</code> , <code>GNUTLS_E_AGAIN</code> ,
<code>GNUTLS_E_WARNING_ALERT_RECEIVED</code> , and <code>GNUTLS_E_GOT_APPLICATION_DATA</code> ,
the latter only in a case of rehandshake.
</p>
<p>The former two interrupt the handshake procedure due to the lower
layer being interrupted, and the latter because of an alert that
may be sent by a server (it is always a good idea to check any
received alerts). On these errors call this function again, until it
returns 0; cf. <code>gnutls_record_get_direction()</code> and
<code>gnutls_error_is_fatal()</code> . In DTLS sessions the non-fatal error
<code>GNUTLS_E_LARGE_PACKET</code> is also possible, and indicates that
the MTU should be adjusted.
</p>
<p>If this function is called by a server after a rehandshake request
then <code>GNUTLS_E_GOT_APPLICATION_DATA</code> or
<code>GNUTLS_E_WARNING_ALERT_RECEIVED</code> may be returned. Note that these
are non fatal errors, only in the specific case of a rehandshake.
Their meaning is that the client rejected the rehandshake request or
in the case of <code>GNUTLS_E_GOT_APPLICATION_DATA</code> it could also mean that
some data were pending. A client may receive that error code if
it initiates the handshake and the server doesn’t agreed.
</p>
<p><strong>Returns:</strong> <code>GNUTLS_E_SUCCESS</code> on success, otherwise a negative error code.
</p></dd></dl>
<dl>
<dt><a name="index-gnutls_005fhandshake_005fset_005ftimeout"></a>Function: <em>void</em> <strong>gnutls_handshake_set_timeout</strong> <em>(gnutls_session_t <var>session</var>, unsigned int <var>ms</var>)</em></dt>
<dd><p><var>session</var>: is a <code>gnutls_session_t</code> type.
</p>
<p><var>ms</var>: is a timeout value in milliseconds
</p>
<p>This function sets the timeout for the TLS handshake process
to the provided value. Use an <code>ms</code> value of zero to disable
timeout, or <code>GNUTLS_DEFAULT_HANDSHAKE_TIMEOUT</code> for a reasonable
default value. For the DTLS protocol, the more detailed
<code>gnutls_dtls_set_timeouts()</code> is provided.
</p>
<p>This function requires to set a pull timeout callback. See
<code>gnutls_transport_set_pull_timeout_function()</code> .
</p>
<p><strong>Since:</strong> 3.1.0
</p></dd></dl>
<p>In GnuTLS 3.5.0 and later it is recommended to use <a href="Core-TLS-API.html#gnutls_005fsession_005fset_005fverify_005fcert">gnutls_session_set_verify_cert</a>
for the handshake process to ensure the verification of the peer’s identity.
</p>
<p>In older GnuTLS versions it is required to manually verify the peer’s certificate
during the handshake by using <a href="Core-TLS-API.html#gnutls_005fcertificate_005fset_005fverify_005ffunction">gnutls_certificate_set_verify_function</a>, and
<a href="Core-TLS-API.html#gnutls_005fcertificate_005fverify_005fpeers2">gnutls_certificate_verify_peers2</a>. See <a href="Certificate-authentication.html#Certificate-authentication">Certificate authentication</a>
for more information.
</p>
<dl compact="compact">
<dt><code><var>void</var> <a href="Core-TLS-API.html#gnutls_005fsession_005fset_005fverify_005fcert">gnutls_session_set_verify_cert</a> (gnutls_session_t <var>session</var>, const char * <var>hostname</var>, unsigned <var>flags</var>)</code></dt>
<dt><code><var>int</var> <a href="Core-TLS-API.html#gnutls_005fcertificate_005fverify_005fpeers2">gnutls_certificate_verify_peers2</a> (gnutls_session_t <var>session</var>, unsigned int * <var>status</var>)</code></dt>
</dl>
<hr>
<div class="header">
<p>
Next: <a href="Data-transfer-and-termination.html#Data-transfer-and-termination" accesskey="n" rel="next">Data transfer and termination</a>, Previous: <a href="Setting-up-the-transport-layer.html#Setting-up-the-transport-layer" accesskey="p" rel="prev">Setting up the transport layer</a>, Up: <a href="How-to-use-GnuTLS-in-applications.html#How-to-use-GnuTLS-in-applications" accesskey="u" rel="up">How to use GnuTLS in applications</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
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