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<title>TLS (SSL) Tcl Commands</title>
</head>

<body bgcolor="#FFFFFF">

<dl>
    <dd><a href="#NAME">NAME</a> <dl>
            <dd><strong>tls</strong> - binding to <strong>OpenSSL</strong>
                toolkit.</dd>
        </dl>
        <dd><b>tls</b> - binding to <b>OpenSSL</b> toolkit.</dd>
    </dl>
    </dd>
    <dd><a href="#SYNOPSIS">SYNOPSIS</a> </dd>
    <dd><dl>
            <dd><b>package require Tcl </b><em>?8.4?</em></dd>
            <dd><b>package require tls </b><em>?@@VERS@@?</em></dd>
            <dd><b>package require Tcl</b> <em>?8.4?</em></dd>
            <dd><b>package require tls</b></dd>
            <dt>&nbsp;</dt>
            <dd><b>tls::init </b><i>?options?</i> </dd>
            <dd><b>tls::socket </b><em>?options? host port</em></dd>
            <dd><b>tls::init</b> <em>?options?</em> </dd>
            <dd><b>tls::socket</b> <em>?options? host port</em></dd>
            <dd><b>tls::socket</b><em> ?-server command?
                ?options? port</em></dd>
            <dd><b>tls::handshake</b><em> channel</em></dd>
            <dd><b>tls::status </b><em>?-local? channel</em></dd>
            <dd><b>tls::import</b><em> channel ?options?</em></dd>
            <dd><b>tls::unimport</b><em> channel</em></dd>
            <dd><b>tls::ciphers </b><em>protocol ?verbose?</em></dd>
            <dd><b>tls::socket</b> <em> ?-server command? ?options? port</em></dd>
            <dd><b>tls::handshake</b> <em> channel</em></dd>
            <dd><b>tls::status </b> <em>?-local? channel</em></dd>
            <dd><b>tls::connection </b> <em>channel</em></dd>
            <dd><b>tls::import</b> <em>channel ?options?</em></dd>
            <dd><b>tls::unimport</b> <em>channel</em></dd>
            <dt>&nbsp;</dt>
            <dd><b>tls::ciphers </b> <em>protocol ?verbose? ?supported?</em></dd>
            <dd><b>tls::protocols</b></dd>
            <dd><b>tls::version</b></dd>
        </dl>
    </dd>
    <dd><a href="#COMMANDS">COMMANDS</a></dd>
    <dd><a href="#CALLBACK OPTIONS">CALLBACK OPTIONS</a></dd>
    <dd><a href="#HTTPS EXAMPLE">HTTPS EXAMPLE</a></dd>
    <dd><a href="#SEE ALSO">SPECIAL CONSIDERATIONS</a></dd>
    <dd><a href="#SEE ALSO">SEE ALSO</a></dd>
</dl>

<hr>

<h3><a name="NAME">NAME</a></h3>

<p><strong>tls</strong> - binding to <strong>OpenSSL</strong>
toolkit.</p>

<h3><a name="SYNOPSIS">SYNOPSIS</a></h3>

<p><b>package require Tcl 8.4</b><br>
<b>package require tls @@VERS@@</b><br>
<b>package require tls</b><br>
<br>
<a href="#tls::init"><b>tls::init </b><i>?options?</i><br>
</a><a href="#tls::socket"><b>tls::socket </b><em>?options? host
<a href="#tls::init"><b>tls::init</b> <i>?options?</i></a><br>
<a href="#tls::socket"><b>tls::socket</b> <i>?options? host port</i><br>
port</em><br>
<b>tls::socket</b><em> ?-server command? ?options? port</em><br>
</a><a href="#tls::status"><b>tls::status </b><em>?-local? channel</em><br>
</a><a href="#tls::handshake"><b>tls::handshake</b><em> channel</em></a><br>
<a href="#tls::socket"><b>tls::socket</b> <i>?-server command? ?options? port</i></a><br>
<a href="#tls::status"><b>tls::status</b> <i>?-local? channel</i></a><br>
<a href="#tls::connection"><b>tls::connection</b> <i>channel</i></a><br>
<a href="#tls::handshake"><b>tls::handshake</b> <i>channel</i></a><br>
<br>
<a href="#tls::import"><b>tls::import </b><i>channel ?options?</i></a><br>
<a href="#tls::unimport"><b>tls::unimport </b><i>channel</i></a><br>
<a href="#tls::ciphers protocol ?verbose?"><strong>tls::ciphers</strong>
<em>protocol ?verbose?</em></a><br>
<a href="#tls::import"><b>tls::import</b> <i>channel ?options?</i></a><br>
<a href="#tls::unimport"><b>tls::unimport</b> <i>channel</i></a><br>
<br>
<a href="#tls::ciphers"><b>tls::ciphers</b> <i>protocol ?verbose? ?supported?</i></a><br>
<a href="#tls::protocols"><b>tls::protocols</b></a>
<a href="#tls::version"><b>tls::version</b></a>
</p>

<h3><a name="DESCRIPTION">DESCRIPTION</a></h3>

<p>This extension provides a generic binding to <a
href="http://www.openssl.org/">OpenSSL</a>, utilizing the
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<p>Typically one would use the <strong>tls::socket </strong>command
which provides compatibility with the native Tcl <strong>socket</strong>
command. In such cases <strong>tls::import</strong> should not be
used directly.</p>

<dl>
    <dt><a name="tls::init"><b>tls::init </b><i>?options?</i></a></dt>
    <dd>This routine sets the default options used by <strong>tls::socket</strong>
        and is <em>optional</em>. If you call <strong>tls::import</strong>
    <dd>Optional function to set the default options used by
	<strong>tls::socket</strong>. If you call <strong>tls::import</strong>
        directly this routine has no effect. Any of the options
        that <strong>tls::socket</strong> accepts can be set
        using this command, though you should limit your options
        to only TLS related ones.</dd>
    <dt>&nbsp;</dt>
    <dt><a name="tls::socket"><b>tls::socket </b><em>?options?
        host port</em></a></dt>
    <dt><b>tls::socket</b><em> ?-server command? ?options? port</em></dt>
    <dd>This is a helper function that utilizes the underlying
        commands (<strong>tls::import</strong>). It behaves
        exactly the same as the native Tcl <strong>socket</strong>
        command except that the options can include any of the
        applicable <a href="#tls::import"><strong>tls:import</strong></a>
        options with one additional option:
<blockquote>
    <dl>
        <dt><strong>-autoservername</strong> <em>bool</em></dt>
        <dd>Automatically send the -servername as the <em>host</em> argument
            (<strong>default</strong>: <em>false</em>)</dd>
            (default is <em>false</em>)</dd>
    </dl>
</blockquote>

    <dt><a name="tls::import"><b>tls::import </b><i>channel
        ?options?</i></a></dt>
    <dd>SSL-enable a regular Tcl channel - it need not be a
        socket, but must provide bi-directional flow. Also
        setting session parameters for SSL handshake.</dd>

<blockquote>
    <dl>
        <dt><strong>-alpn</strong> <em>list</em></dt>
        <dd>List of protocols to offer during Application-Layer
	    Protocol Negotiation (ALPN). For example: h2, http/1.1, etc.</dd>
        <dt><strong>-cadir</strong> <em>dir</em></dt>
        <dd>Specify the directory containing the CA certificates. The
	    default directory is platform specific and can be set at
	    compile time. This can be overridden via the <b>SSL_CERT_DIR</b>
	    environment variable.</dd>
        <dt><strong>-cafile </strong><em>filename</em></dt>
        <dd>Specify the certificate authority (CA) file to use.</dd>
        <dt><strong>-certfile</strong> <em>filename</em></dt>
        <dd>Specify the filename containing the certificate to use. The
	    default name is <b>cert.pem</b>. This can be overridden via
	    the <b>SSL_CERT_FILE</b> environment variable.</dd>
        <dt><strong>-cert</strong> <em>filename</em></dt>
        <dd>Specify the contents of a certificate to use, as a DER
	    encoded binary value (X.509 DER).</dd>
        <dt><strong>-cipher</strong> <em>string</em></dt>
        <dd>List of ciphers to use. String is a colon (":") separated list
	    of ciphers or cipher suites. Cipher suites can be combined
	    using the <b>+</b> character. Prefixes can be used to permanently
	    remove ("!"), delete ("-"), or move a cypher to the end of
	    the list ("+"). Keywords <b>@STRENGTH</b> (sort by algorithm
	    key length), <b>@SECLEVEL=</b><i>n</i> (set security level to
	    n), and <b>DEFAULT</b> (use default cipher list, at start only)
	    can also be specified. See OpenSSL documentation for the full
	    list of valid values. (TLS 1.2 and earlier only)</dd>
        <dt><strong>-ciphersuites</strong> <em>string</em></dt>
        <dd>List of cipher suites to use. String is a colon (":")
	    separated list of cipher suite names. (TLS 1.3 only)</dd>
        <dt><strong>-command</strong> <em>callback</em></dt>
        <dd>Callback to invoke at several points during the handshake.
	    This is used to pass errors and tracing information, and
	    it can allow Tcl scripts to perform their own certificate
	    validation in place of the default validation provided by
	    OpenSSL. See <a href="#CALLBACK OPTIONS">CALLBACK OPTIONS</a>
	    for further discussion.</dd>
        <dt><strong>-dhparams </strong><em>filename</em></dt>
        <dd>Specify the Diffie-Hellman parameters file.</dd>
        <dt><strong>-keyfile</strong> <em>filename</em></dt>
        <dd>Specify the private key file. (default is
            value of -certfile)</dd>
        <dt><strong>-key</strong> <em>filename</em></dt>
        <dd>Specify the private key to use as a DER encoded value (PKCS#1 DER)</dd>
        <dt><strong>-model</strong> <em>channel</em></dt>
        <dd>Force this channel to share the same <em><strong>SSL_CTX</strong></em>
            structure as the specified <em>channel</em>, and
            therefore share callbacks etc.</dd>
        <dt><strong>-password</strong> <em>callback</em></dt>
        <dd>Callback to invoke when OpenSSL needs to obtain a password,
	    typically to unlock the private key of a certificate. The
            callback should return a string which represents the password
            to be used. See <a href="#CALLBACK OPTIONS">CALLBACK OPTIONS</a>
	    for further discussion.</dd>
        <dt><strong>-request </strong><em>bool</em></dt>
        <dd>Request a certificate from peer during SSL handshake.
            (default is <em>true</em>)</dd>
        <dt><strong>-require</strong> <em>bool</em></dt>
        <dd>Require a valid certificate from peer during SSL handshake.
	    If this is set to true, then <strong>-request</strong> must
            also be set to true. (default is <em>false</em>)</dd>
        <dt><strong>-securitylevel</strong> <em>integer</em></dt>
        <dd>Set security level. Must be 0 to 5. The security level affects
	    cipher suite encryption algorithms, supported ECC curves,
	    supported signature algorithms, DH parameter sizes, certificate
	    key sizes and signature algorithms. The default is 1.
	    Level 3 and higher disable support for session tickets and only
	    accept cipher suites that provide forward secrecy.</dd>
        <dt><strong>-server</strong> <em>bool</em></dt>
        <dd>Handshake as server if true, else handshake as
            client. (default is <em>false</em>)</dd>
        <dt><strong>-servername</strong> <em>host</em></dt>
        <dd>Specify server hostname. Only available if the OpenSSL library
	    the package is linked against supports the TLS hostname extension
	    for 'Server Name Indication' (SNI). Use to name the logical host
	    we are talking to and expecting a certificate for.</dd>
        <dt><strong>-session_id</strong> <em>string</em></dt>
        <dd>Session id to resume session.</dd>
        <dt><strong>-ssl2</strong> <em>bool</em></dt>
        <dd>Enable use of SSL v2. (default is <em>false</em>)</dd>
        <dt><strong>-ssl3 </strong><em>bool</em></dt>
        <dd>Enable use of SSL v3. (default is <em>false</em>)</dd>
        <dt>-<strong>tls1</strong> <em>bool</em></dt>
        <dd>Enable use of TLS v1. (default is <em>true</em>)</dd>
        <dt>-<strong>tls1.1</strong> <em>bool</em></dt>
        <dd>Enable use of TLS v1.1 (default is <em>true</em>)</dd>
        <dt>-<strong>tls1.2</strong> <em>bool</em></dt>
        <dd>Enable use of TLS v1.2 (default is <em>true</em>)</dd>
        <dt>-<strong>tls1.3</strong> <em>bool</em></dt>
        <dd>Enable use of TLS v1.3 (default is <em>true</em>)</dd>
    </dl>
</blockquote>

    <dt><a name="tls::unimport"><b>tls::unimport </b><i>channel</i></a></dt>
    <dd>Provided for symmetry to <strong>tls::import</strong>, this
      unstacks the SSL-enabling of a regular Tcl channel.  An error
      is thrown if TLS is not the top stacked channel type.</dd>
    <dt>&nbsp;</dt>
    <dt><a name="tls::handshake"><strong>tls::handshake</strong> <em>channel</em></a></dt>
    <dd>Forces handshake to take place, and returns 0 if
        handshake is still in progress (non-blocking), or 1 if
        the handshake was successful. If the handshake failed
        this routine will throw an error.</dd>
    <dt>&nbsp;</dt>
    <dt><a name="tls::status"><strong>tls::status</strong>
    <em>?-local? channel</em></a></dt>
    <dd>Returns the current security status of an SSL channel. The
    <dd>Returns the current certificate status of an SSL channel. The
        result is a list of key-value pairs describing the
        connected peer. If the result is an empty list then the
        SSL handshake has not yet completed.
        If <em>-local</em> is given, then the certificate information
        is the one used locally.</dd>
</dl>

<blockquote>
    <dl>
        <dt><strong>issuer</strong> <em>dn</em></dt>
        <dd>The distinguished name (DN) of the certificate
            issuer.</dd>
        <dt><strong>subject</strong> <em>dn</em></dt>
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        <dd>The number of bits used for the session key.</dd>
        <dt><strong>certificate</strong> <em>cert</em></dt>
        <dd>The PEM encoded certificate.</dd>
        <dt><strong>sha1_hash</strong> <em>hash</em></dt>
        <dd>The SHA1 hash of the certificate.</dd>
        <dt><strong>sha256_hash</strong> <em>hash</em></dt>
        <dd>The SHA256 hash of the certificate.</dd>
        <dt><strong>validation</strong> <em>result</em></dt>
        <dd>Certificate validation result.</dd>
        <dt><strong>alpn</strong> <em>protocol</em></dt>
        <dd>The protocol selected after Application-Layer Protocol
	    Negotiation (ALPN).</dd>
        <dt><strong>version</strong> <em>value</em></dt>
        <dd>The protocol version used for the connection:
	  SSLv2, SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3, unknown</dd>
	  SSLv2, SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3, or unknown</dd>
    </dl>
</blockquote>

<dl>
    <dt><a name="tls::import"><b>tls::import </b><i>channel
    <dt><a name="tls::connection"><strong>tls::connection</strong>
        ?options?</i></a></dt>
    <dd>SSL-enable a regular Tcl channel - it need not be a
    <em>channel</em></a></dt>
        socket, but must provide bi-directional flow. Also
        setting session parameters for SSL handshake.</dd>
</dl>
    <dd>Returns the current connection status of an SSL channel. The
        result is a list of key-value pairs describing the
        connected peer.</dd>

<blockquote>
    <dl>
        <dt><strong>-alpn</strong> <em>list</em></dt>
        <dd>List of protocols to offer during Application-Layer
        <dt><strong>state</strong> <em>state</em></dt>
        <dd>State of the connection: initializing, handshake, established</dd>
	    Protocol Negotiation (ALPN). For example: h2, http/1.1, etc.</dd>
        <dt><strong>-cadir</strong> <em>dir</em></dt>
        <dt><strong>servername</strong> <em>name</em></dt>
        <dd>Provide the directory containing the CA certificates. The
	default directory is platform specific and can be set at
	compile time. This can be overridden via the <b>SSL_CERT_DIR</b>
	environment variable.</dd>
        <dt><strong>-cafile </strong><em>filename</em></dt>
        <dd>Provide the CA file.</dd>
        <dt><strong>-certfile</strong> <em>filename</em></dt>
        <dd>The name of the connected to server.</dd>
        <dt><strong>protocol</strong> <em>version</em></dt>
        <dd>The protocol version used for the connection:
	    SSL2, SSL3, TLS1, TLS1.1, TLS1.2, TLS1.3, or unknown.</dd>
        <dt><strong>renegotiation</strong> <em>state</em></dt>
        <dd>Provide the name of a file containing certificate to use.
	The default name is cert.pem. This can be overridden via the
	<b>SSL_CERT_FILE</b> environment variable.</dd>
        <dt><strong>-cert</strong> <em>filename</em></dt>
        <dd>Whether protocol renegotiation is allowed or disallowed.</dd>
        <dt><strong>alpn</strong> <em>protocol</em></dt>
        <dd>Provide the contents of a certificate to use, as a DER encoded binary value (X.509 DER).</dd>
        <dt><strong>-cipher </strong><em>string</em></dt>
        <dd>Provide the cipher suites to use. Syntax is as per
            OpenSSL.</dd>
        <dt><strong>-command</strong> <em>callback</em></dt>
        <dd>The protocol selected after Application-Layer Protocol
	    Negotiation (ALPN).</dd>
        <dt><strong>securitylevel</strong> <em>level</em></dt>
        <dd>If specified, this callback will be invoked at several points
            during the OpenSSL handshake.  It can pass errors and tracing
            information, and it can allow Tcl scripts to perform
            their own validation of the certificate in place of the
            default validation provided by OpenSSL.
            <br>
            See <a href="#CALLBACK OPTIONS">CALLBACK OPTIONS</a> for
            further discussion.</dd>
        <dt><strong>-dhparams </strong><em>filename</em></dt>
        <dd>Provide a Diffie-Hellman parameters file.</dd>
        <dt><strong>-keyfile</strong> <em>filename</em></dt>
        <dd>The security level used for selection of ciphers, key size, etc.</dd>
        <dt><strong>cipher</strong> <em>cipher</em></dt>
        <dd>The current cipher in use for the connection.</dd>
        <dt><strong>standard_name</strong> <em>name</em></dt>
        <dd>Provide the private key file. (<strong>default</strong>:
            value of -certfile)</dd>
        <dt><strong>-key</strong> <em>filename</em></dt>
        <dd>The standard RFC name of cipher.</dd>
        <dt><strong>bits</strong> <em>n</em></dt>
        <dd>Provide the private key to use as a DER encoded value (PKCS#1 DER)</dd>
        <dt><strong>-model</strong> <em>channel</em></dt>
        <dd>This will force this channel to share the same <em><strong>SSL_CTX</strong></em>
        <dd>The number of processed bits used for cipher.</dd>
            structure as the specified <em>channel</em>, and
            therefore share callbacks etc.</dd>
        <dt><strong>-password</strong> <em>callback</em></dt>
        <dt><strong>secret_bits</strong> <em>n</em></dt>
        <dd>If supplied, this callback will be invoked when OpenSSL needs
            to obtain a password, typically to unlock the private key of
	    a certificate.
            The callback should return a string which represents the
            password to be used.
            <br>
            See <a href="#CALLBACK OPTIONS">CALLBACK OPTIONS</a> for
            further discussion.</dd>
        <dt><strong>-request </strong><em>bool</em></dt>
        <dd>The number of secret bits used for cipher.</dd>
        <dt><strong>min_version</strong> <em>version</em></dt>
        <dd>Request a certificate from peer during SSL handshake.
            (<strong>default</strong>: <em>true</em>)</dd>
        <dt><strong>-require</strong> <em>bool</em></dt>
        <dd>The minimum protocol version for cipher.</dd>
        <dt><strong>description</strong> <em>string</em></dt>
        <dd>Require a valid certificate from peer during SSL
            handshake. If this is set to true then <strong>-request</strong>
            must also be set to true. (<strong>default</strong>: <em>false</em>)</dd>
        <dt><strong>-server</strong> <em>bool</em></dt>
        <dd>A text description of the cipher.</dd>
        <dt><strong>session_reused</strong> <em>boolean</em></dt>
        <dd>Handshake as server if true, else handshake as
            client.(<strong>default</strong>: <em>false</em>)</dd>
        <dt><strong>-servername</strong> <em>host</em></dt>
        <dd>Whether the session has been reused or not.</dd>
        <dt><strong>session_id</strong> <em>string</em></dt>
        <dd>Only available if the OpenSSL library the package is linked
	    against supports the TLS hostname extension for 'Server Name
	    Indication' (SNI). Use to name the logical host we are talking
	    to and expecting a certificate for</dd>
        <dt><strong>-ssl2</strong> <em>bool</em></dt>
        <dd>Unique session id for use in resuming the session.</dd>
        <dt><strong>session_ticket</strong> <em>string</em></dt>
        <dd>Enable use of SSL v2. (<strong>default</strong>: <em>false</em>)</dd>
        <dt><strong>-ssl3 </strong><em>bool</em></dt>
        <dd>Enable use of SSL v3. (<strong>default</strong>: <em>false</em>)</dd>
        <dt>-<strong>tls1</strong> <em>bool</em></dt>
        <dd>Unique session ticket for use in resuming the session.</dd>
        <dt><strong>resumable</strong> <em>boolean</em></dt>
        <dd>Enable use of TLS v1. (<strong>default</strong>: <em>true</em>)</dd>
        <dt>-<strong>tls1.1</strong> <em>bool</em></dt>
        <dd>Can the session be resumed or not.</dd>
        <dt><strong>start_time</strong> <em>seconds</em></dt>
        <dd>Enable use of TLS v1.1 (<strong>default</strong>: <em>true</em>)</dd>
        <dt>-<strong>tls1.2</strong> <em>bool</em></dt>
        <dd>Time since session started in seconds since epoch.</dd>
        <dt><strong>timeout</strong> <em>seconds</em></dt>
        <dd>Enable use of TLS v1.2 (<strong>default</strong>: <em>true</em>)</dd>
        <dt>-<strong>tls1.3</strong> <em>bool</em></dt>
        <dd>Max duration of session in seconds before time-out.</dd>
        <dt><strong>session_cache_mode</strong> <em>mode</em></dt>
        <dd>Enable use of TLS v1.3 (<strong>default</strong>: <em>true</em>)</dd>
        <dd>Server cache mode (client, server, or both).</dd>
    </dl>
</blockquote>

<dl>
    <dt><a name="tls::unimport"><b>tls::unimport </b><i>channel</i></a></dt>
    <dd>Provided for symmetry to <strong>tls::import</strong>, this
      unstacks the SSL-enabling of a regular Tcl channel.  An error
      is thrown if TLS is not the top stacked channel type.</dd>
</dl>

<dl>
    <dt><a name="tls::ciphers protocol ?verbose?"><strong>tls::ciphers</strong>
        <em>protocol ?verbose?</em></a></dt>
    <dd>Returns list of supported ciphers based on the <em>protocol</em>
        you supply, which must be one of <em>ssl2, ssl3, or tls1</em>.
        If <em>verbose</em> is specified as true then a verbose,
        semi-human readable list is returned providing additional
        information on the nature of the cipher support. In each
    <dt><a name="tls::ciphers"><strong>tls::ciphers</strong> 
    <em>protocol ?verbose? ?supported?</em></a></dt>
    <dd>Returns a list of supported ciphers available for <em>protocol</em>,
        where protocol must be one of <b>ssl2, ssl3, tls1, tls1.1,
	tls1.2,</b> or <b>tls1.3</b>. If <em>verbose</em> is specified as
	true then a verbose, human readable list is returned with
	additional information on the cipher. If <em>supported</em>
        case the result is a Tcl list.</dd>
</dl>
	is specified as true, then only the ciphers supported for protocol
	will be listed.</dd>

    <dt><a name="tls::protocols"><strong>tls::protocols</strong></a></dt>
    <dd>Returns a list of supported protocols. Valid values are:
	<b>ssl2</b>, <b>ssl3</b>, <b>tls1</b>, <b>tls1.1</b>, <b>tls1.2</b>,
	and <b>tls1.3</b>.</dd>
<dl>

    <dt><a name="tls::version"><strong>tls::version</strong></a></dt>
    <dd>Returns the version string defined by OpenSSL.</dd>
    <dd>Returns the OpenSSL version string.</dd>
</dl>

<h3><a name="CALLBACK OPTIONS">CALLBACK OPTIONS</a></h3>

<p>
As indicated above, individual channels can be given their own callbacks
to handle intermediate processing by the OpenSSL library, using the
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	Arguments appended to the script upon callback take one of the
	following forms:

	<br>
	<br>

	<dl>

	<dt>
	  <strong>alpn</strong> <em></em>
	</dt>
	<dd>
	  This form of callback is invoked when server selects the first
	  -alpn specified protocol common to the client and server. If none,
	  first client one is used.
	</dd>

	<br>

<!--	This form of callback is disabled.

	<dt>
	  <strong>error</strong> <em>channel message</em>
	</dt>
	<dd>
	  The <em>message</em> argument contains an error message generated
	  by the OpenSSL function
	  <code>ERR_reason_error_string()</code>.
	</dd>

	<br>
-->

	<dt>
	  <strong>hello</strong> <em></em>
	</dt>
	<dd>
	  This form of callback is invoked during client hello message processing.
	</dd>

	<br>

	<dt>
	  <strong>info</strong> <em>channel major minor message</em>
	</dt>
	<dd>
	  This form of callback is invoked by the OpenSSL function
	  <code>SSL_CTX_set_info_callback()</code>.
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	  The <em>message</em> argument is a descriptive string which may
	  be generated either by
	  <code>SSL_state_string_long()</code> or by
	  <code>SSL_alert_desc_string_long()</code>,
	  depending on context.
	</dd>

	<br>

	<dt>
	  <strong>session</strong> <em>session_id ticket lifetime</em>
	</dt>
	<dd>
	  This form of callback is invoked by the OpenSSL function
	  <code>SSL_CTX_sess_set_new_cb()</code>.
	  Where <em>session_id</em> is the current session identifier,
	  <em>ticket</em> is the session ticket info, and <em>lifetime</em>
	  is the the ticket lifetime in seconds.
	</dd>

	<br>

	<dt>
	  <strong>sni</strong> <em>servername</em>
	</dt>
	<dd>
	  This form of callback is invoked when the server receives the SNI
	  header from the client where <i>servername</i> is the client
	  specified servername. Used to allow multiple names for
	  same server so the right certificate can be used.
	</dd>

	<br>

	<dt>
	  <strong>verify</strong> <em>channel depth cert status error</em>
	</dt>
	<dd>
	  This form of callback is invoked by the OpenSSL function