# phpman > man > IO::Socket::SSL

## NAME
    [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) - SSL sockets with [IO::Socket](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket/markdown) interface

## SYNOPSIS
        use strict;
        use [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown);

        # simple client
        my $cl = [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->new('www.google.com:443');
        print $cl "GET / HTTP/1.0\r\n\r\n";
        print <$cl>;

        # simple server
        my $srv = [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->new(
            LocalAddr => '0.0.0.0:1234',
            Listen => 10,
            SSL_cert_file => 'server-cert.pem',
            SSL_key_file => 'server-key.pem',
        );
        $srv->accept;

## DESCRIPTION
    [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) makes using SSL/TLS much easier by wrapping the necessary functionality into the
    familiar [IO::Socket](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket/markdown) interface and providing secure defaults whenever possible. This way,
    existing applications can be made SSL-aware without much effort, at least if you do blocking I/O
    and don't use select or poll.

    But, under the hood, SSL is a complex beast. So there are lots of methods to make it do what you
    need if the default behavior is not adequate. Because it is easy to inadvertently introduce
    critical security bugs or just hard to debug problems, I would recommend studying the following
    documentation carefully.

    The documentation consists of the following parts:

    *   "Essential Information About SSL/TLS"

    *   "Basic SSL Client"

    *   "Basic SSL Server"

    *   "Common Usage Errors"

    *   "Common Problems with SSL"

    *   "Using Non-Blocking Sockets"

    *   "Advanced Usage"

    *   "Integration Into Own Modules"

    *   "Description Of Methods"

    Additional documentation can be found in

    *   [IO::Socket::SSL::Intercept](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3AIntercept/markdown) - Doing Man-In-The-Middle with SSL

    *   [IO::Socket::SSL::Utils](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3AUtils/markdown) - Useful functions for certificates etc

## Essential Information About SSL/TLS
    SSL (Secure Socket Layer) or its successor TLS (Transport Layer Security) are protocols to
    facilitate end-to-end security. These protocols are used when accessing web sites (https),
    delivering or retrieving email, and in lots of other use cases. In the following documentation
    we will refer to both SSL and TLS as simply 'SSL'.

    SSL enables end-to-end security by providing two essential functions:

    Encryption
        This part encrypts the data for transit between the communicating parties, so that nobody in
        between can read them. It also provides tamper resistance so that nobody in between can
        manipulate the data.

    Identification
        This part makes sure that you talk to the right peer. If the identification is done
        incorrectly it is easy to mount man-in-the-middle attacks, e.g. if Alice wants to talk to
        Bob it would be possible for Mallory to put itself in the middle, so that Alice talks to
        Mallory and Mallory to Bob. All the data would still be encrypted, but not end-to-end
        between Alice and Bob, but only between Alice and Mallory and then between Mallory and Bob.
        Thus Mallory would be able to read and modify all traffic between Alice and Bob.

    Identification is the part which is the hardest to understand and the easiest to get wrong.

    With SSL, the Identification is usually done with certificates inside a PKI (Public Key
    Infrastructure). These Certificates are comparable to an identity card, which contains
    information about the owner of the card. The card then is somehow signed by the issuer of the
    card, the CA (Certificate Agency).

    To verify the identity of the peer the following must be done inside SSL:

    *   Get the certificate from the peer. If the peer does not present a certificate we cannot
        verify it.

    *   Check if we trust the certificate, e.g. make sure it's not a forgery.

        We believe that a certificate is not a fake if we either know the certificate already or if
        we trust the issuer (the CA) and can verify the issuers signature on the certificate. In
        reality there is often a hierarchy of certificate agencies and we only directly trust the
        root of this hierarchy. In this case the peer not only sends his own certificate, but also
        all intermediate certificates. Verification will be done by building a trust path from the
        trusted root up to the peers certificate and checking in each step if the we can verify the
        issuer's signature.

        This step often causes problems because the client does not know the necessary trusted root
        certificates. These are usually stored in a system dependent CA store, but often the
        browsers have their own CA store.

    *   Check if the certificate is still valid. Each certificate has a lifetime and should not be
        used after that time because it might be compromised or the underlying cryptography got
        broken in the mean time.

    *   Check if the subject of the certificate matches the peer. This is like comparing the picture
        on the identity card against the person representing the identity card.

        When connecting to a server this is usually done by comparing the hostname used for
        connecting against the names represented in the certificate. A certificate might contain
        multiple names or wildcards, so that it can be used for multiple hosts (e.g. *.example.com
        and *.example.org).

        Although nobody sane would accept an identity card where the picture does not match the
        person we see, it is a common implementation error with SSL to omit this check or get it
        wrong.

    *   Check if the certificate was revoked by the issuer. This might be the case if the
        certificate was compromised somehow and now somebody else might use it to claim the wrong
        identity. Such revocations happened a lot after the heartbleed attack.

        For SSL there are two ways to verify a revocation, CRL and OCSP. With CRLs (Certificate
        Revocation List) the CA provides a list of serial numbers for revoked certificates. The
        client somehow has to download the list (which can be huge) and keep it up to date. With
        OCSP (Online Certificate Status Protocol) the client can check a single certificate directly
        by asking the issuer.

        Revocation is the hardest part of the verification and none of today's browsers get it fully
        correct. But, they are still better than most other implementations which don't implement
        revocation checks or leave the hard parts to the developer.

    When accessing a web site with SSL or delivering mail in a secure way the identity is usually
    only checked one way, e.g. the client wants to make sure it talks to the right server, but the
    server usually does not care which client it talks to. But, sometimes the server wants to
    identify the client too and will request a certificate from the client which the server must
    verify in a similar way.

## Basic SSL Client
    A basic SSL client is simple:

        my $client = [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->new('www.example.com:443')
            or die "error=$!, ssl_error=$SSL_ERROR";

    This will take the OpenSSL default CA store as the store for the trusted CA. This usually works
    on UNIX systems. If there are no certificates in the store it will try use [Mozilla::CA](https://www.chedong.com/phpMan.php/perldoc/Mozilla%3A%3ACA/markdown) which
    provides the default CAs of Firefox.

    In the default settings, [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) will use a safer cipher set and SSL version, do a
    proper hostname check against the certificate, and use SNI (server name indication) to send the
    hostname inside the SSL handshake. This is necessary to work with servers which have different
    certificates behind the same IP address. It will also check the revocation of the certificate
    with OCSP, but currently only if the server provides OCSP stapling (for deeper checks see
    "ocsp_resolver" method).

    Lots of options can be used to change ciphers, SSL version, location of CA and much more. See
    documentation of methods for details.

    With protocols like SMTP it is necessary to upgrade an existing socket to SSL. This can be done
    like this:

        my $client = [IO::Socket::INET](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AINET/markdown)->new('mx.example.com:25') or die $!;
        # .. read greeting from server
        # .. send EHLO and read response
        # .. send STARTTLS command and read response
        # .. if response was successful we can upgrade the socket to SSL now:
        [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->start_SSL($client,
            # explicitly set hostname we should use for SNI
            SSL_hostname => 'mx.example.com'
        ) or die $SSL_ERROR;

    A more complete example for a simple HTTP client:

        my $client = [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->new(
            # where to connect
            PeerHost => "www.example.com",
            PeerPort => "https",

            # certificate verification - VERIFY_PEER is default
            SSL_verify_mode => SSL_VERIFY_PEER,

            # location of CA store
            # need only be given if default store should not be used
            SSL_ca_path => '/etc/ssl/certs', # typical CA path on Linux
            SSL_ca_file => '/etc/ssl/cert.pem', # typical CA file on BSD

            # or just use default path on system:
            [IO::Socket::SSL::default_ca](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3Adefaultca/markdown)(), # either explicitly
            # or implicitly by not giving SSL_ca_*

            # easy hostname verification
            # It will use PeerHost as default name a verification
            # scheme as default, which is safe enough for most purposes.
            SSL_verifycn_name => 'foo.bar',
            SSL_verifycn_scheme => 'http',

            # SNI support - defaults to PeerHost
            SSL_hostname => 'foo.bar',

        ) or die "failed connect or ssl handshake: $!,$SSL_ERROR";

        # send and receive over SSL connection
        print $client "GET / HTTP/1.0\r\n\r\n";
        print <$client>;

    And to do revocation checks with OCSP (only available with OpenSSL 1.0.0 or higher and
    [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) 1.59 or higher):

        # default will try OCSP stapling and check only leaf certificate
        my $client = [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->new($dst);

        # better yet: require checking of full chain
        my $client = [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->new(
            PeerAddr => $dst,
            SSL_ocsp_mode => SSL_OCSP_FULL_CHAIN,
        );

        # even better: make OCSP errors fatal
        # (this will probably fail with lots of sites because of bad OCSP setups)
        # also use common OCSP response cache
        my $ocsp_cache = [IO::Socket::SSL::OCSP_Cache](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3AOCSPCache/markdown)->new;
        my $client = [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->new(
            PeerAddr => $dst,
            SSL_ocsp_mode => SSL_OCSP_FULL_CHAIN|SSL_OCSP_FAIL_HARD,
            SSL_ocsp_cache => $ocsp_cache,
        );

        # disable OCSP stapling in case server has problems with it
        my $client = [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->new(
            PeerAddr => $dst,
            SSL_ocsp_mode => SSL_OCSP_NO_STAPLE,
        );

        # check any certificates which are not yet checked by OCSP stapling or
        # where we have already cached results. For your own resolving combine
        # $ocsp->requests with $ocsp->add_response(uri,response).
        my $ocsp = $client->ocsp_resolver();
        my $errors = $ocsp->resolve_blocking();
        if ($errors) {
            warn "OCSP verification failed: $errors";
            close($client);
        }

## Basic SSL Server
    A basic SSL server looks similar to other [IO::Socket](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket/markdown) servers, only that it also contains
    settings for certificate and key:

        # simple server
        my $server = [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->new(
            # where to listen
            LocalAddr => '127.0.0.1',
            LocalPort => 8080,
            Listen => 10,

            # which certificate to offer
            # with SNI support there can be different certificates per hostname
            SSL_cert_file => 'cert.pem',
            SSL_key_file => 'key.pem',
        ) or die "failed to listen: $!";

        # accept client
        my $client = $server->accept or die
            "failed to accept or ssl handshake: $!,$SSL_ERROR";

    This will automatically use a secure set of ciphers and SSL version and also supports Forward
    Secrecy with (Elliptic-Curve) Diffie-Hellmann Key Exchange.

    If you are doing a forking or threading server, we recommend that you do the SSL handshake
    inside the new process/thread so that the master is free for new connections. We recommend this
    because a client with improper or slow SSL handshake could make the server block in the
    handshake which would be bad to do on the listening socket:

        # inet server
        my $server = [IO::Socket::INET](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AINET/markdown)->new(
            # where to listen
            LocalAddr => '127.0.0.1',
            LocalPort => 8080,
            Listen => 10,
        );

        # accept client
        my $client = $server->accept or die;

        # SSL upgrade client (in new process/thread)
        [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->start_SSL($client,
            SSL_server => 1,
            SSL_cert_file => 'cert.pem',
            SSL_key_file => 'key.pem',
        ) or die "failed to ssl handshake: $SSL_ERROR";

    Like with normal sockets, neither forking nor threading servers scale well. It is recommended to
    use non-blocking sockets instead, see "Using Non-Blocking Sockets"

## Common Usage Errors
    This is a list of typical errors seen with the use of [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown):

    *   Disabling verification with "SSL_verify_mode".

        As described in "Essential Information About SSL/TLS", a proper identification of the peer
        is essential and failing to verify makes Man-In-The-Middle attacks possible.

        Nevertheless, lots of scripts and even public modules or applications disable verification,
        because it is probably the easiest way to make the thing work and usually nobody notices any
        security problems anyway.

        If the verification does not succeed with the default settings, one can do the following:

        *       Make sure the needed CAs are in the store, maybe use "SSL_ca_file" or "SSL_ca_path"
                to specify a different CA store.

        *       If the validation fails because the certificate is self-signed and that's what you
                expect, you can use the "SSL_fingerprint" option to accept specific leaf
                certificates by their certificate or pubkey fingerprint.

        *       If the validation failed because the hostname does not match and you cannot access
                the host with the name given in the certificate, you can use "SSL_verifycn_name" to
                specify the hostname you expect in the certificate.

        A common error pattern is also to disable verification if they found no CA store (different
        modules look at different "default" places). Because [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) is now able to provide
        a usable CA store on most platforms (UNIX, Mac OSX and Windows) it is better to use the
        defaults provided by [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown). If necessary these can be checked with the
        "default_ca" method.

    *   Polling of SSL sockets (e.g. select, poll and other event loops).

        If you sysread one byte on a normal socket it will result in a syscall to read one byte.
        Thus, if more than one byte is available on the socket it will be kept in the network stack
        of your OS and the next select or poll call will return the socket as readable. But, with
        SSL you don't deliver single bytes. Multiple data bytes are packaged and encrypted together
        in an SSL frame. Decryption can only be done on the whole frame, so a sysread for one byte
        actually reads the complete SSL frame from the socket, decrypts it and returns the first
        decrypted byte. Further sysreads will return more bytes from the same frame until all bytes
        are returned and the next SSL frame will be read from the socket.

        Thus, in order to decide if you can read more data (e.g. if sysread will block) you must
        check if there are still data in the current SSL frame by calling "pending" and if there are
        no data pending you might check the underlying socket with select or poll. Another way might
        be if you try to sysread at least 16kByte all the time. 16kByte is the maximum size of an
        SSL frame and because sysread returns data from only a single SSL frame you can guarantee
        that there are no pending data.

        Additionally, contrary to plain sockets the data delivered on the socket are not necessarily
        application payload. It might be a TLS handshake, it might just be the beginning of a TLS
        record or it might be TLS session tickets which are send after the TLS handshake in TLS 1.3.
        In such situations select will return that data are available for read since it only looks
        at the plain socket. A sysread on the [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) socket will not return any data though
        since it is an abstraction which only returns application data. This causes the sysread to
        hang in case the socket was blocking or to return an error with EAGAIN on non-blocking
        sockets. Applications using select or similar should therefore set the socket to
        non-blocking and also expect that the sysread might temporarily fail with EAGAIN.

        See also "Using Non-Blocking Sockets".

    *   Expecting exactly the same behavior as plain sockets.

        [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) tries to emulate the usual socket behavior as good as possible, but full
        emulation can not be done. Specifically a read on the SSL socket might also result in a
        write on the TCP socket or a write on the SSL socket might result in a read on the TCP
        socket. Also "accept" and close on the SSL socket will result in writing and reading data to
        the TCP socket too.

        Especially the hidden writes might result in a connection reset if the underlying TCP socket
        is already closed by the peer. Unless signal PIPE is explicitly handled by the application
        this will usually result in the application crashing. It is thus recommended to explicitly
        IGNORE signal PIPE so that the errors get propagated as EPIPE instead of causing a crash of
        the application.

    *   Set 'SSL_version' or 'SSL_cipher_list' to a "better" value.

        [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) tries to set these values to reasonable, secure values which are compatible
        with the rest of the world. But, there are some scripts or modules out there which tried to
        be smart and get more secure or compatible settings. Unfortunately, they did this years ago
        and never updated these values, so they are still forced to do only 'TLSv1' (instead of also
        using TLSv12 or TLSv11). Or they set 'HIGH' as the cipher list and thought they were secure,
        but did not notice that 'HIGH' includes anonymous ciphers, e.g. without identification of
        the peer.

        So it is recommended to leave the settings at the secure defaults which [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) sets
        and which get updated from time to time to better fit the real world.

    *   Make SSL settings inaccessible by the user, together with bad builtin settings.

        Some modules use [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown), but don't make the SSL settings available to the user.
        This is often combined with bad builtin settings or defaults (like switching verification
        off).

        Thus the user needs to hack around these restrictions by using "set_args_filter_hack" or
        similar.

    *   Use of constants as strings.

        Constants like "SSL_VERIFY_PEER" or "SSL_WANT_READ" should be used as constants and not be
        put inside quotes, because they represent numerical values.

    *   Forking and handling the socket in parent and child.

        A fork of the process will duplicate the internal user space SSL state of the socket. If
        both master and child interact with the socket by using their own SSL state strange error
        messages will happen. Such interaction includes explicit or implicit close of the SSL
        socket. To avoid this the socket should be explicitly closed with SSL_no_shutdown.

    *   Forking and executing a new process.

        Since the SSL state is stored in user space it will be duplicated by a fork but it will be
        lost when doing exec. This means it is not possible to simply redirect stdin and stdout for
        the new process to the SSL socket by duplicating the relevant file handles. Instead
        explicitly exchanging plain data between child-process and SSL socket are needed.

## Common Problems with SSL
    SSL is a complex protocol with multiple implementations and each of these has their own quirks.
    While most of these implementations work together, it often gets problematic with older
    versions, minimal versions in load balancers, or plain wrong setups.

    Unfortunately these problems are hard to debug. Helpful for debugging are a knowledge of SSL
    internals, wireshark and the use of the debug settings of [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) and [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown), which
    can both be set with $[IO::Socket::SSL::DEBUG](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3ADEBUG/markdown). The following debugs levels are defined, but used
    not in any consistent way:

    *   0 - No debugging (default).

    *   1 - Print out errors from [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) and ciphers from [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown).

    *   2 - Print also information about call flow from [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) and progress information
        from [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown).

    *   3 - Print also some data dumps from [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) and from [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown).

    Also, "analyze-ssl.pl" from the ssl-tools repository at <<https://github.com/noxxi/p5-ssl-tools>>
    might be a helpful tool when debugging SSL problems, as do the "openssl" command line tool and a
    check with a different SSL implementation (e.g. a web browser).

    The following problems are not uncommon:

    *   Bad server setup: missing intermediate certificates.

        It is a regular problem that administrators fail to include all necessary certificates into
        their server setup, e.g. everything needed to build the trust chain from the trusted root.
        If they check the setup with the browser everything looks ok, because browsers work around
        these problems by caching any intermediate certificates and apply them to new connections if
        certificates are missing.

        But, fresh browser profiles which have never seen these intermediates cannot fill in the
        missing certificates and fail to verify; the same is true with [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown).

    *   Old versions of servers or load balancers which do not understand specific TLS versions or
        croak on specific data.

        From time to time one encounters an SSL peer, which just closes the connection inside the
        SSL handshake. This can usually be worked around by downgrading the SSL version, e.g. by
        setting "SSL_version". Modern Browsers usually deal with such servers by automatically
        downgrading the SSL version and repeat the connection attempt until they succeed.

        Worse servers do not close the underlying TCP connection but instead just drop the relevant
        packet. This is harder to detect because it looks like a stalled connection. But downgrading
        the SSL version often works here too.

        A cause of such problems are often load balancers or security devices, which have hardware
        acceleration and only a minimal (and less robust) SSL stack. They can often be detected
        because they support much fewer ciphers than other implementations.

    *   Bad or old OpenSSL versions.

        [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) uses OpenSSL with the help of the [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) library. It is recommend to
        have a recent version of this library, because it has more features and usually fewer known
        bugs.

    *   Validation of client certificates fail.

        Make sure that the purpose of the certificate allows use as ssl client (check with "openssl
        x509 -purpose", that the necessary root certificate is in the path specified by "SSL_ca*"
        (or the default path) and that any intermediate certificates needed to build the trust chain
        are sent by the client.

    *   Validation of self-signed certificate fails even if it is given with "SSL_ca*" argument.

        The "SSL_ca*" arguments do not give a general trust store for arbitrary certificates but
        only specify a store for CA certificates which then can be used to verify other
        certificates. This especially means that certificates which are not a CA get simply ignored,
        notably self-signed certificates which do not also have the CA-flag set.

        This behavior of OpenSSL differs from the more general trust-store concept which can be
        found in browsers and where it is possible to simply added arbitrary certificates (CA or
        not) as trusted.

## Using Non-Blocking Sockets
    If you have a non-blocking socket, the expected behavior on read, write, accept or connect is to
    set $! to EWOULDBLOCK if the operation cannot be completed immediately. Note that EWOULDBLOCK is
    the same as EAGAIN on UNIX systems, but is different on Windows.

    With SSL, handshakes might occur at any time, even within an established connection. In these
    cases it is necessary to finish the handshake before you can read or write data. This might
    result in situations where you want to read but must first finish the write of a handshake or
    where you want to write but must first finish a read. In these cases $! is set to EAGAIN like
    expected, and additionally $SSL_ERROR is set to either SSL_WANT_READ or SSL_WANT_WRITE. Thus if
    you get EWOULDBLOCK on a SSL socket you must check $SSL_ERROR for SSL_WANT_* and adapt your
    event mask accordingly.

    Using readline on non-blocking sockets does not make much sense and I would advise against using
    it. And, while the behavior is not documented for other [IO::Socket](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket/markdown) classes, it will try to
    emulate the behavior seen there, e.g. to return the received data instead of blocking, even if
    the line is not complete. If an unrecoverable error occurs it will return nothing, even if it
    already received some data.

    Also, I would advise against using "accept" with a non-blocking SSL object because it might
    block and this is not what most would expect. The reason for this is that "accept" on a
    non-blocking TCP socket (e.g. [IO::Socket::IP](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AIP/markdown), [IO::Socket::INET](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AINET/markdown)..) results in a new TCP socket
    which does not inherit the non-blocking behavior of the master socket. And thus, the initial SSL
    handshake on the new socket inside "[IO::Socket::SSL::accept](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3Aaccept/markdown)" will be done in a blocking way. To
    work around this you are safer by doing a TCP accept and later upgrade the TCP socket in a
    non-blocking way with "start_SSL" and "accept_SSL".

        my $cl = [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->new($dst);
        $cl->[blocking(0)](https://www.chedong.com/phpMan.php/man/blocking/0/markdown);
        my $sel = [IO::Select](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASelect/markdown)->new($cl);
        while (1) {
            # with SSL a call for reading n bytes does not result in reading of n
            # bytes from the socket, but instead it must read at least one full SSL
            # frame. If the socket has no new bytes, but there are unprocessed data
            # from the SSL frame can_read will block!

            # wait for data on socket
            $sel->can_read();

            # new data on socket or eof
            READ:
            # this does not read only 1 byte from socket, but reads the complete SSL
            # frame and then just returns one byte. On subsequent calls it than
            # returns more byte of the same SSL frame until it needs to read the
            # next frame.
            my $n = sysread( $cl,my $buf,1);
            if ( ! defined $n ) {
                die $! if not $!{EWOULDBLOCK};
                next if $SSL_ERROR == SSL_WANT_READ;
                if ( $SSL_ERROR == SSL_WANT_WRITE ) {
                    # need to write data on renegotiation
                    $sel->can_write;
                    next;
                }
                die "something went wrong: $SSL_ERROR";
            } elsif ( ! $n ) {
                last; # eof
            } else {
                # read next bytes
                # we might have still data within the current SSL frame
                # thus first process these data instead of waiting on the underlying
                # socket object
                goto READ if $cl->pending;    # goto sysread
                next;                         # goto $sel->can_read
            }
        }

    Additionally there are differences to plain sockets when using select, poll, kqueue or similar
    technologies to get notified if data are available. Relying only on these calls is not
    sufficient in all cases since unread data might be internally buffered in the SSL stack. To
    detect such buffering pending() need to be used. Alternatively the buffering can be avoided by
    using sysread with the maximum size of an SSL frame. See "Common Usage Errors" for details.

## Advanced Usage
  SNI Support
    Newer extensions to SSL can distinguish between multiple hostnames on the same IP address using
    Server Name Indication (SNI).

    Support for SNI on the client side was added somewhere in the OpenSSL 0.9.8 series, but with 1.0
    a bug was fixed when the server could not decide about its hostname. Therefore client side SNI
    is only supported with OpenSSL 1.0 or higher in [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown). With a supported version, SNI
    is used automatically on the client side, if it can determine the hostname from "PeerAddr" or
    "PeerHost" (which are synonyms in the underlying [IO::Socket](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket/markdown):: classes and thus should never be
    set both or at least not to different values). On unsupported OpenSSL versions it will silently
    not use SNI. The hostname can also be given explicitly given with "SSL_hostname", but in this
    case it will throw in error, if SNI is not supported. To check for support you might call
    "[IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->can_client_sni()".

    On the server side, earlier versions of OpenSSL are supported, but only together with
    [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) version >= 1.50. To check for support you might call
    "[IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->can_server_sni()". If server side SNI is supported, you might specify
    different certificates per host with "SSL_cert*" and "SSL_key*", and check the requested name
    using "get_servername".

### Talk Plain and SSL With The Same Socket
    It is often required to first exchange some plain data and then upgrade the socket to SSL after
    some kind of STARTTLS command. Protocols like FTPS even need a way to downgrade the socket again
    back to plain.

    The common way to do this would be to create a normal socket and use "start_SSL" to upgrade and
    stop_SSL to downgrade:

        my $sock = [IO::Socket::INET](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AINET/markdown)->new(...) or die $!;
        ... exchange plain data on $sock until starttls command ...
        [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->start_SSL($sock,%sslargs) or die $SSL_ERROR;
        ... now $sock is an [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) object ...
        ... exchange data with SSL on $sock until stoptls command ...
        $sock->stop_SSL or die $SSL_ERROR;
        ... now $sock is again an [IO::Socket::INET](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AINET/markdown) object ...

    But, lots of modules just derive directly from [IO::Socket::INET](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AINET/markdown). While this base class can be
    replaced with [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown), these modules cannot easily support different base classes for
    SSL and plain data and switch between these classes on a starttls command.

    To help in this case, [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) can be reduced to a plain socket on startup, and
    connect_SSL/accept_SSL/start_SSL can be used to enable SSL and "stop_SSL" to talk plain again:

        my $sock = [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->new(
            PeerAddr => ...
            SSL_startHandshake => 0,
            %sslargs
        ) or die $!;
        ... exchange plain data on $sock until starttls command ...
        $sock->connect_SSL or die $SSL_ERROR;
        ... now $sock is an [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) object ...
        ... exchange data with SSL on $sock until stoptls command ...
        $sock->stop_SSL or die $SSL_ERROR;
        ... $sock is still an [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) object ...
        ... but data exchanged again in plain ...

## Integration Into Own Modules
    [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) behaves similarly to other [IO::Socket](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket/markdown) modules and thus could be integrated in
    the same way, but you have to take special care when using non-blocking I/O (like for handling
    timeouts) or using select or poll. Please study the documentation on how to deal with these
    differences.

    Also, it is recommended to not set or touch most of the "SSL_*" options, so that they keep their
    secure defaults. It is also recommended to let the user override these SSL specific settings
    without the need of global settings or hacks like "set_args_filter_hack".

    The notable exception is "SSL_verifycn_scheme". This should be set to the hostname verification
    scheme required by the module or protocol.

## Description Of Methods
    [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) inherits from another [IO::Socket](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket/markdown) module. The choice of the super class depends
    on the installed modules:

    *   If [IO::Socket::IP](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AIP/markdown) with at least version 0.20 is installed it will use this module as super
        class, transparently providing IPv6 and IPv4 support.

    *   If [IO::Socket::INET6](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AINET6/markdown) is installed it will use this module as super class, transparently
        providing IPv6 and IPv4 support.

    *   Otherwise it will fall back to [IO::Socket::INET](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AINET/markdown), which is a perl core module. With
        [IO::Socket::INET](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AINET/markdown) you only get IPv4 support.

    Please be aware that with the IPv6 capable super classes, it will look first for the IPv6
    address of a given hostname. If the resolver provides an IPv6 address, but the host cannot be
    reached by IPv6, there will be no automatic fallback to IPv4. To avoid these problems you can
    enforce IPv4 for a specific socket by using the "Domain" or "Family" option with the value
    AF_INET as described in [IO::Socket::IP](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AIP/markdown). Alternatively you can enforce IPv4 globally by loading
    [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) with the option 'inet4', in which case it will use the IPv4 only class
    [IO::Socket::INET](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AINET/markdown) as the super class.

    [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) will provide all of the methods of its super class, but sometimes it will
    override them to match the behavior expected from SSL or to provide additional arguments.

    The new or changed methods are described below, but please also read the section about SSL
    specific error handling.

    Error Handling
        If an SSL specific error occurs, the global variable $SSL_ERROR will be set. If the error
        occurred on an existing SSL socket, the method "errstr" will give access to the latest
        socket specific error. Both $SSL_ERROR and the "errstr" method give a dualvar similar to $!,
        e.g. providing an error number in numeric context or an error description in string context.

### new
        Creates a new [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) object. You may use all the friendly options that came bundled
        with the super class (e.g. [IO::Socket::IP](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AIP/markdown), [IO::Socket::INET](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AINET/markdown), ...) plus (optionally) the ones
        described below. If you don't specify any SSL related options it will do its best in using
        secure defaults, e.g. choosing good ciphers, enabling proper verification, etc.

        SSL_server
          Set this option to a true value if the socket should be used as a server. If this is not
          explicitly set it is assumed if the "Listen" parameter is given when creating the socket.

        SSL_hostname
          This can be given to specify the hostname used for SNI, which is needed if you have
          multiple SSL hostnames on the same IP address. If not given it will try to determine the
          hostname from "PeerAddr", which will fail if only an IP was given or if this argument is
          used within "start_SSL".

          If you want to disable SNI, set this argument to ''.

          Currently only supported for the client side and will be ignored for the server side.

          See section "SNI Support" for details of SNI the support.

        SSL_startHandshake
          If this option is set to false (defaults to true) it will not start the SSL handshake yet.
          This has to be done later with "accept_SSL" or "connect_SSL". Before the handshake is
          started read/write/etc. can be used to exchange plain data.

        SSL_keepSocketOnError
          If this option is set to true (defaults to false) it will not close the underlying TCP
          socket on errors. In most cases there is no real use for this behavior since both sides of
          the TCP connection will probably have a different idea of the current state of the
          connection.

        SSL_ca | SSL_ca_file | SSL_ca_path
          Usually you want to verify that the peer certificate has been signed by a trusted
          certificate authority. In this case you should use this option to specify the file
          ("SSL_ca_file") or directory ("SSL_ca_path") containing the certificate(s) of the trusted
          certificate authorities.

          "SSL_ca_path" can also be an array or a string containing multiple path, where the path
          are separated by the platform specific separator. This separator is ";" on DOS, Windows,
          Netware, "," on VMS and ":" for all the other systems. If multiple path are given at least
          one of these must be accessible.

          You can also give a list of X509* certificate handles (like you get from [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) or
          [IO::Socket::SSL::Utils::PEM_xxx2cert](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3AUtils%3A%3APEMxxx2cert/markdown)) with "SSL_ca". These will be added to the CA store
          before path and file and thus take precedence. If neither SSL_ca, nor SSL_ca_file or
          SSL_ca_path are set it will use "default_ca()" to determine the user-set or system
          defaults. If you really don't want to set a CA set SSL_ca_file or SSL_ca_path to "\undef"
          or SSL_ca to an empty list. (unfortunately '' is used by some modules using
          [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) when CA is not explicitly given).

        SSL_client_ca | SSL_client_ca_file
          If verify_mode is VERIFY_PEER on the server side these options can be used to set the list
          of acceptable CAs for the client. This way the client can select they required certificate
          from a list of certificates. The value for these options is similar to "SSL_ca" and
          "SSL_ca_file".

        SSL_fingerprint
          Sometimes you have a self-signed certificate or a certificate issued by an unknown CA and
          you really want to accept it, but don't want to disable verification at all. In this case
          you can specify the fingerprint of the certificate as 'algo$hex_fingerprint'. "algo" is a
          fingerprint algorithm supported by OpenSSL, e.g. 'sha1','sha256'... and "hex_fingerprint"
          is the hexadecimal representation of the binary fingerprint. Any colons inside the hex
          string will be ignored.

          If you want to use the fingerprint of the pubkey inside the certificate instead of the
          certificate use the syntax 'algo$pub$hex_fingerprint' instead. To get the fingerprint of
          an established connection you can use "get_fingerprint".

          It is also possible to skip "algo$", i.e. only specify the fingerprint. In this case the
          likely algorithms will be automatically detected based on the length of the digest string.

          You can specify a list of fingerprints in case you have several acceptable certificates.
          If a fingerprint matches the topmost (i.e. leaf) certificate no additional validations can
          make the verification fail.

        SSL_cert_file | SSL_cert | SSL_key_file | SSL_key
          If you create a server you usually need to specify a server certificate which should be
          verified by the client. Same is true for client certificates, which should be verified by
          the server. The certificate can be given as a file with SSL_cert_file or as an internal
          representation of an X509* object (like you get from [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) or
          [IO::Socket::SSL::Utils::PEM_xxx2cert](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3AUtils%3A%3APEMxxx2cert/markdown)) with SSL_cert. If given as a file it will
          automatically detect the format. Supported file formats are PEM, DER and PKCS#12, where
          PEM and PKCS#12 can contain the certificate and the chain to use, while DER can only
          contain a single certificate.

          If given as a list of X509* please note, that the all the chain certificates (e.g. all
          except the first) will be "consumed" by openssl and will be freed if the SSL context gets
          destroyed - so you should never free them yourself. But the servers certificate (e.g. the
          first) will not be consumed by openssl and thus must be freed by the application.

          For each certificate a key is need, which can either be given as a file with SSL_key_file
          or as an internal representation of an EVP_PKEY* object with SSL_key (like you get from
          [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) or [IO::Socket::SSL::Utils::PEM_xxx2key](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3AUtils%3A%3APEMxxx2key/markdown)). If a key was already given within the
          PKCS#12 file specified by SSL_cert_file it will ignore any SSL_key or SSL_key_file. If no
          SSL_key or SSL_key_file was given it will try to use the PEM file given with SSL_cert_file
          again, maybe it contains the key too.

          If your SSL server should be able to use different certificates on the same IP address,
          depending on the name given by SNI, you can use a hash reference instead of a file with
          "<hostname =" cert_file>>.

          If your SSL server should be able to use both RSA and ECDSA certificates for the same
          domain/IP a similar hash reference like with SNI is given. The domain names used to
          specify the additional certificates should be "hostname%whatever", i.e. "hostname%ecc" or
          similar. This needs at least OpenSSL 1.0.2. To let the server pick the certificate based
          on the clients cipher preference "SSL_honor_cipher_order" should be set to false.

          In case certs and keys are needed but not given it might fall back to builtin defaults,
          see "Defaults for Cert, Key and CA".

          Examples:

           SSL_cert_file => 'mycert.pem',
           SSL_key_file => 'mykey.pem',

           SSL_cert_file => {
              "foo.example.org" => 'foo-cert.pem',
              "foo.example.org%ecc" => 'foo-ecc-cert.pem',
              "bar.example.org" => 'bar-cert.pem',
              # used when nothing matches or client does not support SNI
              '' => 'default-cert.pem',
              '%ecc' => 'default-ecc-cert.pem',
           },
           SSL_key_file => {
              "foo.example.org" => 'foo-key.pem',
              "foo.example.org%ecc" => 'foo-ecc-key.pem',
              "bar.example.org" => 'bar-key.pem',
              # used when nothing matches or client does not support SNI
              '' => 'default-key.pem',
              '%ecc' => 'default-ecc-key.pem',
           }

        SSL_passwd_cb
          If your private key is encrypted, you might not want the default password prompt from
          [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown). This option takes a reference to a subroutine that should return the password
          required to decrypt your private key.

        SSL_use_cert
          If this is true, it forces [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) to use a certificate and key, even if you are
          setting up an SSL client. If this is set to 0 (the default), then you will only need a
          certificate and key if you are setting up a server.

          SSL_use_cert will implicitly be set if SSL_server is set. For convenience it is also set
          if it was not given but a cert was given for use (SSL_cert_file or similar).

        SSL_version
          Sets the version of the SSL protocol used to transmit data. 'SSLv23' uses a handshake
          compatible with SSL2.0, SSL3.0 and TLS1.x, while 'SSLv2', 'SSLv3', 'TLSv1', 'TLSv1_1',
          'TLSv1_2', or 'TLSv1_3' restrict handshake and protocol to the specified version. All
          values are case-insensitive. Instead of 'TLSv1_1', 'TLSv1_2', and 'TLSv1_3' one can also
          use 'TLSv11', 'TLSv12', and 'TLSv13'. Support for 'TLSv1_1', 'TLSv1_2', and 'TLSv1_3'
          requires recent versions of [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) and openssl.

          Independent from the handshake format you can limit to set of accepted SSL versions by
          adding !version separated by ':'.

          The default SSL_version is 'SSLv23:!SSLv3:!SSLv2' which means, that the handshake format
          is compatible to SSL2.0 and higher, but that the successful handshake is limited to TLS1.0
          and higher, that is no SSL2.0 or SSL3.0 because both of these versions have serious
          security issues and should not be used anymore. You can also use !TLSv1_1 and !TLSv1_2 to
          disable TLS versions 1.1 and 1.2 while still allowing TLS version 1.0.

          Setting the version instead to 'TLSv1' might break interaction with older clients, which
          need and SSL2.0 compatible handshake. On the other side some clients just close the
          connection when they receive a TLS version 1.1 request. In this case setting the version
          to 'SSLv23:!SSLv2:!SSLv3:!TLSv1_1:!TLSv1_2' might help.

        SSL_cipher_list
          If this option is set the cipher list for the connection will be set to the given value,
          e.g. something like 'ALL:!LOW:!EXP:!aNULL'. This will only affect ciphers for TLS 1.2 and
          lower. See the OpenSSL documentation
          (<<https://www.openssl.org/docs/manmaster/man1/openssl-ciphers.html#CIPHER-STRINGS>>) for
          more details.

          Unless you fail to contact your peer because of no shared ciphers it is recommended to
          leave this option at the default setting, which uses the system default but disables some
          insecure ciphers which might still be enabled on older systems.

          In case different cipher lists are needed for different SNI hosts a hash can be given with
          the host as key and the cipher suite as value, similar to SSL_cert*.

        SSL_ciphersuites
          If this option is set the TLS 1.3 ciphersuites for the connection will be set to the given
          value. This is similar to SSL_cipher_list, but only for TLS 1.3 ciphers. See argument
          "-ciphersuits" in the OpenSSL documentation
          (<<https://www.openssl.org/docs/manmaster/man1/openssl-ciphers.html>>) for details.

          Unless you fail to contact your peer because of no shared ciphers it is recommended to
          leave this option at the default setting, which uses the system default.

          In case different cipher lists are needed for different SNI hosts a hash can be given with
          the host as key and the cipher suite as value, similar to SSL_cert*.

        SSL_honor_cipher_order
          If this option is true the cipher order the server specified is used instead of the order
          proposed by the client. This option defaults to true to make use of our secure cipher list
          setting.

        SSL_dh_file
          To create a server which provides forward secrecy you need to either give the DH
          parameters or (better, because faster) the ECDH curve. This setting cares about DH
          parameters.

          To support non-elliptic Diffie-Hellman key exchange a suitable file needs to be given here
          or the SSL_dh should be used with an appropriate value. See dhparam command in openssl for
          more information.

          If neither "SSL_dh_file" nor "SSL_dh" are set a builtin DH parameter with a length of 2048
          bit is used to offer DH key exchange by default. If you don't want this (e.g. disable DH
          key exchange) explicitly set this or the "SSL_dh" parameter to undef.

        SSL_dh
          Like SSL_dh_file, but instead of giving a file you use a preloaded or generated DH*.

        SSL_ecdh_curve
          To create a server which provides forward secrecy you need to either give the DH
          parameters or (better, because faster) the ECDH curve. This setting cares about the ECDH
          curve(s).

          To support Elliptic Curve Diffie-Hellmann key exchange the OID or NID of at least one
          suitable curve needs to be provided here.

          With OpenSSL 1.1.0+ this parameter defaults to "auto", which means that it lets OpenSSL
          pick the best settings. If support for CTX_set_ecdh_auto is implemented in [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown)
          (needs at least version 1.86) it will use this to implement the same default. Otherwise it
          will default to "prime256v1" (builtin of OpenSSL) in order to offer ECDH key exchange by
          default.

          If setting groups or curves is supported by [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) (needs at least version 1.86) then
          multiple curves can be given here in the order of the preference, i.e.
          "P-521:P-384:P-256". When used at the client side this will include the supported curves
          as extension in the TLS handshake.

          If you don't want to have ECDH key exchange this could be set to undef or set
          "SSL_ciphers" to exclude all of these ciphers.

          You can check if ECDH support is available by calling "[IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->can_ecdh".

        SSL_verify_mode
          This option sets the verification mode for the peer certificate. You may combine
          SSL_VERIFY_PEER (verify_peer), SSL_VERIFY_FAIL_IF_NO_PEER_CERT (fail verification if no
          peer certificate exists; ignored for clients), SSL_VERIFY_CLIENT_ONCE (verify client once;
          ignored for clients). See OpenSSL man page for SSL_CTX_set_verify for more information.

          The default is SSL_VERIFY_NONE for server (e.g. no check for client certificate) and
          SSL_VERIFY_PEER for client (check server certificate).

        SSL_verify_callback
          If you want to verify certificates yourself, you can pass a sub reference along with this
          parameter to do so. When the callback is called, it will be passed:

          1. a true/false value that indicates what OpenSSL thinks of the certificate,
          2. a C-style memory address of the certificate store,
          3. a string containing the certificate's issuer attributes and owner attributes, and
          4. a string containing any errors encountered (0 if no errors).
          5. a C-style memory address of the peer's own certificate (convertible to PEM form with
          [Net::SSLeay::PEM_get_string_X509](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay%3A%3APEMgetstringX509/markdown)()).
          6. The depth of the certificate in the chain. Depth 0 is the leaf certificate.

          The function should return 1 or 0, depending on whether it thinks the certificate is valid
          or invalid. The default is to let OpenSSL do all of the busy work.

          The callback will be called for each element in the certificate chain.

          See the OpenSSL documentation for SSL_CTX_set_verify for more information.

        SSL_verifycn_scheme
          The scheme is used to correctly verify the identity inside the certificate by using the
          hostname of the peer. See the information about the verification schemes in
          verify_hostname.

          If you don't specify a scheme it will use 'default', but only complain loudly if the name
          verification fails instead of letting the whole certificate verification fail. THIS WILL
          CHANGE, e.g. it will let the certificate verification fail in the future if the hostname
          does not match the certificate !!!! To override the name used in verification use
          SSL_verifycn_name.

          The scheme 'default' is a superset of the usual schemes, which will accept the hostname in
          common name and subjectAltName and allow wildcards everywhere. While using this scheme is
          way more secure than no name verification at all you better should use the scheme specific
          to your application protocol, e.g. 'http', 'ftp'...

          If you are really sure, that you don't want to verify the identity using the hostname you
          can use 'none' as a scheme. In this case you'd better have alternative forms of
          verification, like a certificate fingerprint or do a manual verification later by calling
          verify_hostname yourself.

        SSL_verifycn_publicsuffix
          This option is used to specify the behavior when checking wildcards certificates for
          public suffixes, e.g. no wildcard certificates for *.com or *.co.uk should be accepted,
          while *.example.com or *.example.co.uk is ok.

          If not specified it will simply use the builtin default of [IO::Socket::SSL::PublicSuffix](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3APublicSuffix/markdown),
          you can create another object with from_string or from_file of this module.

          To disable verification of public suffix set this option to ''.

        SSL_verifycn_name
          Set the name which is used in verification of hostname. If SSL_verifycn_scheme is set and
          no SSL_verifycn_name is given it will try to use SSL_hostname or PeerHost and PeerAddr
          settings and fail if no name can be determined. If SSL_verifycn_scheme is not set it will
          use a default scheme and warn if it cannot determine a hostname, but it will not fail.

          Using PeerHost or PeerAddr works only if you create the connection directly with
          "[IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->new", if an [IO::Socket::INET](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AINET/markdown) object is upgraded with start_SSL the name
          has to be given in SSL_verifycn_name or SSL_hostname.

        SSL_check_crl
          If you want to verify that the peer certificate has not been revoked by the signing
          authority, set this value to true. OpenSSL will search for the CRL in your SSL_ca_path, or
          use the file specified by SSL_crl_file. See the [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) documentation for more
          details. Note that this functionality appears to be broken with OpenSSL < v0.9.7b, so its
          use with lower versions will result in an error.

        SSL_crl_file
          If you want to specify the CRL file to be used, set this value to the pathname to be used.
          This must be used in addition to setting SSL_check_crl.

        SSL_ocsp_mode
          Defines how certificate revocation is done using OCSP (Online Status Revocation Protocol).
          The default is to send a request for OCSP stapling to the server and if the server sends
          an OCSP response back the result will be used.

          Any other OCSP checking needs to be done manually with "ocsp_resolver".

          The following flags can be combined with "|":

          SSL_OCSP_NO_STAPLE
                  Don't ask for OCSP stapling. This is the default if SSL_verify_mode is
                  VERIFY_NONE.

          SSL_OCSP_TRY_STAPLE
                  Try OCSP stapling, but don't complain if it gets no stapled response back. This is
                  the default if SSL_verify_mode is VERIFY_PEER (the default).

          SSL_OCSP_MUST_STAPLE
                  Consider it a hard error, if the server does not send a stapled OCSP response
                  back. Most servers currently send no stapled OCSP response back.

          SSL_OCSP_FAIL_HARD
                  Fail hard on response errors, default is to fail soft like the browsers do. Soft
                  errors mean, that the OCSP response is not usable, e.g. no response, error
                  response, no valid signature etc. Certificate revocations inside a verified
                  response are considered hard errors in any case.

                  Soft errors inside a stapled response are never considered hard, e.g. it is
                  expected that in this case an OCSP request will be send to the responsible OCSP
                  responder.

          SSL_OCSP_FULL_CHAIN
                  This will set up the "ocsp_resolver" so that all certificates from the peer chain
                  will be checked, otherwise only the leaf certificate will be checked against
                  revocation.

        SSL_ocsp_staple_callback
          If this callback is defined, it will be called with the SSL object and the OCSP response
          handle obtained from the peer, e.g. "<$cb-"($ssl,$resp)>>. If the peer did not provide a
          stapled OCSP response the function will be called with "$resp=undef". Because the OCSP
          response handle is no longer valid after leaving this function it should not by copied or
          freed. If access to the response is necessary after leaving this function it can be
          serialized with "[Net::SSLeay::i2d_OCSP_RESPONSE](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay%3A%3Ai2dOCSPRESPONSE/markdown)".

          If no such callback is provided, it will use the default one, which verifies the response
          and uses it to check if the certificate(s) of the connection got revoked.

        SSL_ocsp_cache
          With this option a cache can be given for caching OCSP responses, which could be shared
          between different SSL contexts. If not given a cache specific to the SSL context only will
          be used.

          You can either create a new cache with "[IO::Socket::SSL::OCSP_Cache](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3AOCSPCache/markdown)->new([size])" or
          implement your own cache, which needs to have methods "put($key,\%entry)" and "get($key)"
          (returning "\%entry") where entry is the hash representation of the OCSP response with
          fields like "nextUpdate". The default implementation of the cache will consider responses
          valid as long as "nextUpdate" is less then the current time.

        SSL_reuse_ctx
          If you have already set the above options for a previous instance of [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown), then
          you can reuse the SSL context of that instance by passing it as the value for the
          SSL_reuse_ctx parameter. You may also create a new instance of the
          [IO::Socket::SSL::SSL_Context](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3ASSLContext/markdown) class, using any context options that you desire without
          specifying connection options, and pass that here instead.

          If you use this option, all other context-related options that you pass in the same call
          to new() will be ignored unless the context supplied was invalid. Note that, contrary to
          versions of [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) below v0.90, a global SSL context will not be implicitly used
          unless you use the set_default_context() function.

        SSL_create_ctx_callback
          With this callback you can make individual settings to the context after it got created
          and the default setup was done. The callback will be called with the CTX object from
          [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) as the single argument.

          Example for limiting the server session cache size:

            SSL_create_ctx_callback => sub {
                my $ctx = shift;
                [Net::SSLeay::CTX_sess_set_cache_size](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay%3A%3ACTXsesssetcachesize/markdown)($ctx,128);
            }

        SSL_session_cache_size
          If you make repeated connections to the same host/port and the SSL renegotiation time is
          an issue, you can turn on client-side session caching with this option by specifying a
          positive cache size. For successive connections, pass the SSL_reuse_ctx option to the
          new() calls (or use set_default_context()) to make use of the cached sessions. The session
          cache size refers to the number of unique host/port pairs that can be stored at one time;
          the oldest sessions in the cache will be removed if new ones are added.

          This option does not effect the session cache a server has for it's clients, e.g. it does
          not affect SSL objects with SSL_server set.

          Note that session caching with TLS 1.3 needs at least [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) 1.86.

        SSL_session_cache
          Specifies session cache object which should be used instead of creating a new. Overrules
          SSL_session_cache_size. This option is useful if you want to reuse the cache, but not the
          rest of the context.

          A session cache object can be created using "[IO::Socket::SSL::Session_Cache](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3ASessionCache/markdown)->new(
          cachesize )".

          Use set_default_session_cache() to set a global cache object.

        SSL_session_key
          Specifies a key to use for lookups and inserts into client-side session cache. Per default
          ip:port of destination will be used, but sometimes you want to share the same session over
          multiple ports on the same server (like with FTPS).

        SSL_session_id_context
          This gives an id for the servers session cache. It's necessary if you want clients to
          connect with a client certificate. If not given but SSL_verify_mode specifies the need for
          client certificate a context unique id will be picked.

        SSL_error_trap
          When using the accept() or connect() methods, it may be the case that the actual socket
          connection works but the SSL negotiation fails, as in the case of an HTTP client
          connecting to an HTTPS server. Passing a subroutine ref attached to this parameter allows
          you to gain control of the orphaned socket instead of having it be closed forcibly. The
          subroutine, if called, will be passed two parameters: a reference to the socket on which
          the SSL negotiation failed and the full text of the error message.

        SSL_npn_protocols
          If used on the server side it specifies list of protocols advertised by SSL server as an
          array ref, e.g. ['spdy/2','http1.1']. On the client side it specifies the protocols
          offered by the client for NPN as an array ref. See also method "next_proto_negotiated".

          Next Protocol Negotiation (NPN) is available with [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) 1.46+ and openssl-1.0.1+.
          NPN is unavailable in TLSv1.3 protocol. To check support you might call
          "[IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->can_npn()". If you use this option with an unsupported
          [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown)/OpenSSL it will throw an error.

        SSL_alpn_protocols
          If used on the server side it specifies list of protocols supported by the SSL server as
          an array ref, e.g. ['http/2.0', 'spdy/3.1','http/1.1']. On the client side it specifies
          the protocols advertised by the client for ALPN as an array ref. See also method
          "alpn_selected".

          Application-Layer Protocol Negotiation (ALPN) is available with [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) 1.56+ and
          openssl-1.0.2+. More details about the extension are in RFC7301. To check support you
          might call "[IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->can_alpn()". If you use this option with an unsupported
          [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown)/OpenSSL it will throw an error.

          Note that some client implementations may encounter problems if both NPN and ALPN are
          specified. Since ALPN is intended as a replacement for NPN, try providing ALPN protocols
          then fall back to NPN if that fails.

        SSL_ticket_keycb => [$sub,$data] | $sub
          This is a callback used for stateless session reuse (Session Tickets, RFC 5077).

          This callback will be called as "$sub->($data,[$key_name])" where $data is the argument
          given to SSL_ticket_keycb (or undef) and $key_name depends on the mode:

          encrypt ticket
                  If a ticket needs to be encrypted the callback will be called without $key_name.
                  In this case it should return "($current_key,$current_key_name") where
                  $current_key is the current key (32 byte random data) and $current_key_name the
                  name associated with this key (exactly 16 byte). This $current_key_name will be
                  incorporated into the ticket.

          decrypt ticket
                  If a ticket needs to be decrypted the callback will be called with $key_name as
                  found in the ticket. It should return "($key,$current_key_name") where $key is the
                  key associated with the given $key_name and $current_key_name the name associated
                  with the currently active key. If $current_key_name is different from the given
                  $key_name the callback will be called again to re-encrypt the ticket with the
                  currently active key.

                  If no key can be found which matches the given $key_name then this function should
                  return nothing (empty list).

                  This mechanism should be used to limit the life time for each key encrypting the
                  ticket. Compromise of a ticket encryption key might lead to decryption of SSL
                  sessions which used session tickets protected by this key.

          Example:

              [Net::SSLeay::RAND_bytes](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay%3A%3ARANDbytes/markdown)(my $oldkey,32);
              [Net::SSLeay::RAND_bytes](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay%3A%3ARANDbytes/markdown)(my $newkey,32);
              my $oldkey_name = pack("a16",'oldsecret');
              my $newkey_name = pack("a16",'newsecret');

              my @keys = (
                 [ $newkey_name, $newkey ], # current active key
                 [ $oldkey_name, $oldkey ], # already expired
              );

              my $keycb = [ sub {
                 my ($mykeys,$name) = @_;

                 # return (current_key, current_key_name) if no name given
                 return ($mykeys->[0][1],$mykeys->[0][0]) if ! $name;

                 # return (matching_key, current_key_name) if we find a key matching
                 # the given name
                 for(my $i = 0; $i<@$mykeys; $i++) {
                     next if $name ne $mykeys->[$i][0];
                     return ($mykeys->[$i][1],$mykeys->[0][0]);
                 }

                 # no matching key found
                 return;
              },\@keys ];

              my $srv = [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->new(..., SSL_ticket_keycb => $keycb);

        SSL_mode_release_buffers 1|0
          This enables or disables the SSL_MODE_RELEASE_BUFFERS option on the SSL object. With this
          option the read buffer will be released after each SSL_read but will need to be
          reallocated for each new SSL_read. If memory usage is a concern this might save lots of
          memory in the mean time though, about 34k per idle SSL connection according to the
          documentation in [SSL_CTX_set_mode(3ssl)](https://www.chedong.com/phpMan.php/man/SSLCTXsetmode/3ssl/markdown).

    accept
        This behaves similar to the accept function of the underlying socket class, but additionally
        does the initial SSL handshake. But because the underlying socket class does return a
        blocking file handle even when accept is called on a non-blocking socket, the SSL handshake
        on the new file object will be done in a blocking way. Please see the section about
        non-blocking I/O for details. If you don't like this behavior you should do accept on the
        TCP socket and then upgrade it with "start_SSL" later.

### connect
        This behaves similar to the connect function but also does an SSL handshake. Because you
        cannot give SSL specific arguments to this function, you should better either use "new" to
        create a connect SSL socket or "start_SSL" to upgrade an established TCP socket to SSL.

### close
        Contrary to a close for a simple INET socket a close in SSL also mandates a proper shutdown
        of the SSL part. This is done by sending a close notify message by both peers.

        A naive implementation would thus wait until it receives the close notify message from the
        peer - which conflicts with the commonly expected semantic that a close will not block. The
        default behavior is thus to only send a close notify but not wait for the close notify of
        the peer. If this is required "SSL_fast_shutdown" need to be explicitly set to false.

        There are also cases where a SSL shutdown should not be done at all. This is true for
        example when forking to let a child deal with the socket and closing the socket in the
        parent process. A naive explicit "close" or an implicit close when destroying the socket in
        the parent would send a close notify to the peer which would make the SSL socket in the
        client process unusable. In this case an explicit "close" with "SSL_no_shutdown" set to true
        should be done in the parent process.

        For more details and other arguments see "stop_SSL" which gets called from "close" to
        shutdown the SSL state of the socket.

### sysread
        This function behaves from the outside the same as sysread in other [IO::Socket](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket/markdown) objects, e.g.
        it returns at most LEN bytes of data. But in reality it reads not only LEN bytes from the
        underlying socket, but at a single SSL frame. It then returns up to LEN bytes it decrypted
        from this SSL frame. If the frame contained more data than requested it will return only LEN
        data, buffer the rest and return it on further read calls. This means, that it might be
        possible to read data, even if the underlying socket is not readable, so using poll or
        select might not be sufficient.

        sysread will only return data from a single SSL frame, e.g. either the pending data from the
        already buffered frame or it will read a frame from the underlying socket and return the
        decrypted data. It will not return data spanning several SSL frames in a single call.

        Also, calls to sysread might fail, because it must first finish an SSL handshake.

        To understand these behaviors is essential, if you write applications which use event loops
        and/or non-blocking sockets. Please read the specific sections in this documentation.

### syswrite
        This functions behaves from the outside the same as syswrite in other [IO::Socket](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket/markdown) objects,
        e.g. it will write at most LEN bytes to the socket, but there is no guarantee, that all LEN
        bytes are written. It will return the number of bytes written. Because it basically just
        calls SSL_write from OpenSSL syswrite will write at most a single SSL frame. This means,
        that no more than 16.384 bytes, which is the maximum size of an SSL frame, will be written
        at once.

        For non-blocking sockets SSL specific behavior applies. Pease read the specific section in
        this documentation.

### peek
        This function has exactly the same syntax as sysread, and performs nearly the same task but
        will not advance the read position so that successive calls to peek() with the same
        arguments will return the same results. This function requires OpenSSL 0.9.6a or later to
        work.

### pending
        This function gives you the number of bytes available without reading from the underlying
        socket object. This function is essential if you work with event loops, please see the
        section about polling SSL sockets.

### get_fingerprint
        This methods returns the fingerprint of the given certificate in the form "algo$digest_hex",
        where "algo" is the used algorithm, default 'sha256'. If no certificate is given the peer
        certificate of the connection is used. If "pubkey" is true it will not return the
        fingerprint of the certificate but instead the fingerprint of the pubkey inside the
        certificate as "algo$pub$digest_hex".

### get_fingerprint_bin
        This methods returns the binary fingerprint of the given certificate by using the algorithm
        "algo", default 'sha256'. If no certificate is given the peer certificate of the connection
        is used. If "pubkey" is true it will not return the fingerprint of the certificate but
        instead the fingerprint of the pubkey inside the certificate.

### get_cipher
        Returns the string form of the cipher that the [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) object is using.

### get_sslversion
        Returns the string representation of the SSL version of an established connection.

### get_sslversion_int
        Returns the integer representation of the SSL version of an established connection.

### get_session_reused
        This returns true if the session got reused and false otherwise. Note that with a reused
        session no certificates are send within the handshake and no ciphers are offered and thus
        functions which rely on this might not work.

### dump_peer_certificate
        Returns a parsable string with select fields from the peer SSL certificate. This method
        directly returns the result of the dump_peer_certificate() method of [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown).

### peer_certificate
        If a peer certificate exists, this function can retrieve values from it. If no field is
        given the internal representation of certificate from [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) is returned. If refresh is
        true it will not used a cached version, but check again in case the certificate of the
        connection has changed due to renegotiation.

        The following fields can be queried:

        authority (alias issuer)
                The certificate authority which signed the certificate.

        owner (alias subject)
                The owner of the certificate.

        commonName (alias cn) - only for [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) version >=1.30
                The common name, usually the server name for SSL certificates.

        subjectAltNames - only for [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) version >=1.33
                Alternative names for the subject, usually different names for the same server, like
                example.org, example.com, *.example.com.

                It returns a list of (typ,value) with typ GEN_DNS, GEN_IPADD etc (these constants
                are exported from [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)). See [Net::SSLeay::X509_get_subjectAltNames](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay%3A%3AX509getsubjectAltNames/markdown).

### sock_certificate
        This is similar to "peer_certificate" but will return the sites own certificate. The same
        arguments for $field can be used. If no $field is given the certificate handle from the
        underlying OpenSSL will be returned. This handle will only be valid as long as the SSL
        connection exists and if used afterwards it might result in strange crashes of the
        application.

    peer_certificates
        This returns all the certificates send by the peer, e.g. first the peers own certificate and
        then the rest of the chain. You might use CERT_asHash from [IO::Socket::SSL::Utils](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3AUtils/markdown) to inspect
        each of the certificates.

        This function depends on a version of [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) >= 1.58 .

    get_servername
        This gives the name requested by the client if Server Name Indication (SNI) was used.

### verify_hostname
        This verifies the given hostname against the peer certificate using the given scheme.
        Hostname is usually what you specify within the PeerAddr. See the
        "SSL_verifycn_publicsuffix" parameter for an explanation of suffix checking and for the
        possible values.

        Verification of hostname against a certificate is different between various applications and
        RFCs. Some scheme allow wildcards for hostnames, some only in subjectAltNames, and even
        their different wildcard schemes are possible. RFC 6125 provides a good overview.

        To ease the verification the following schemes are predefined (both protocol name and
        rfcXXXX name can be used):

        rfc2818, xmpp (rfc3920), ftp (rfc4217)
                Extended wildcards in subjectAltNames and common name are possible, e.g.
                *.example.org or even www*.example.org. The common name will be only checked if no
                DNS names are given in subjectAltNames.

        http (alias www)
                While name checking is defined in rfc2818 the current browsers usually accept also
                an IP address (w/o wildcards) within the common name as long as no subjectAltNames
                are defined. Thus this is rfc2818 extended with this feature.

        smtp (rfc2595), imap, pop3, acap (rfc4642), netconf (rfc5538), syslog (rfc5425), snmp
        (rfc5953)
                Simple wildcards in subjectAltNames are possible, e.g. *.example.org matches
                www.example.org but not lala.www.example.org. If nothing from subjectAltNames match
                it checks against the common name, where wildcards are also allowed to match the
                full leftmost label.

        ldap (rfc4513)
                Simple wildcards are allowed in subjectAltNames, but not in common name. Common name
                will be checked even if subjectAltNames exist.

        sip (rfc5922)
                No wildcards are allowed and common name is checked even if subjectAltNames exist.

        gist (rfc5971)
                Simple wildcards are allowed in subjectAltNames and common name, but common name
                will only be checked if their are no DNS names in subjectAltNames.

        default This is a superset of all the rules and is automatically used if no scheme is given
                but a hostname (instead of IP) is known. Extended wildcards are allowed in
                subjectAltNames and common name and common name is checked always.

        none    No verification will be done. Actually is does not make any sense to call
                verify_hostname in this case.

        The scheme can be given either by specifying the name for one of the above predefined
        schemes, or by using a hash which can have the following keys and values:

        check_cn: 0|'always'|'when_only'
                Determines if the common name gets checked. If 'always' it will always be checked
                (like in ldap), if 'when_only' it will only be checked if no names are given in
                subjectAltNames (like in http), for any other values the common name will not be
                checked.

        wildcards_in_alt: 0|'full_label'|'anywhere'
                Determines if and where wildcards in subjectAltNames are possible. If 'full_label'
                only cases like *.example.org will be possible (like in ldap), for 'anywhere'
                www*.example.org is possible too (like http), dangerous things like but www.*.org or
                even '*' will not be allowed. For compatibility with older versions 'leftmost' can
                be given instead of 'full_label'.

        wildcards_in_cn: 0|'full_label'|'anywhere'
                Similar to wildcards_in_alt, but checks the common name. There is no predefined
                scheme which allows wildcards in common names.

        ip_in_cn: 0|1|4|6
                Determines if an IP address is allowed in the common name (no wildcards are
                allowed). If set to 4 or 6 it only allows IPv4 or IPv6 addresses, any other true
                value allows both.

        callback: \&coderef
                If you give a subroutine for verification it will be called with the arguments
                ($hostname,$commonName,@subjectAltNames), where hostname is the name given for
                verification, commonName is the result from peer_certificate('cn') and
                subjectAltNames is the result from peer_certificate('subjectAltNames').

                All other arguments for the verification scheme will be ignored in this case.

### next_proto_negotiated
        This method returns the name of negotiated protocol - e.g. 'http/1.1'. It works for both
        client and server side of SSL connection.

        NPN support is available with [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) 1.46+ and openssl-1.0.1+. To check support you
        might call "[IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->can_npn()".

### alpn_selected
        Returns the protocol negotiated via ALPN as a string, e.g. 'http/1.1', 'http/2.0' or
        'spdy/3.1'.

        ALPN support is available with [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) 1.56+ and openssl-1.0.2+. To check support, use
        "[IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->can_alpn()".

### errstr
        Returns the last error (in string form) that occurred. If you do not have a real object to
        perform this method on, call [IO::Socket::SSL::errstr](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3Aerrstr/markdown)() instead.

        For read and write errors on non-blocking sockets, this method may include the string "SSL
        wants a read first!" or "SSL wants a write first!" meaning that the other side is expecting
        to read from or write to the socket and wants to be satisfied before you get to do anything.
        But with version 0.98 you are better comparing the global exported variable $SSL_ERROR
        against the exported symbols SSL_WANT_READ and SSL_WANT_WRITE.

### opened
        This returns false if the socket could not be opened, 1 if the socket could be opened and
        the SSL handshake was successful done and -1 if the underlying [IO::Handle](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3AHandle/markdown) is open, but the
        SSL handshake failed.

    [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->start_SSL($socket, ... )
        This will convert a glob reference or a socket that you provide to an [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)
        object. You may also pass parameters to specify context or connection options as with a call
        to new(). If you are using this function on an accept()ed socket, you must set the parameter
        "SSL_server" to 1, i.e. [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->start_SSL($socket, SSL_server => 1). If you have a
        class that inherits from [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) and you want the $socket to be blessed into your
        own class instead, use MyClass->start_SSL($socket) to achieve the desired effect.

        Note that if start_SSL() fails in SSL negotiation, $socket will remain blessed in its
        original class. For non-blocking sockets you better just upgrade the socket to
        [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) and call accept_SSL or connect_SSL and the upgraded object. To just upgrade
        the socket set SSL_startHandshake explicitly to 0. If you call start_SSL w/o this parameter
        it will revert to blocking behavior for accept_SSL and connect_SSL.

        If given the parameter "Timeout" it will stop if after the timeout no SSL connection was
        established. This parameter is only used for blocking sockets, if it is not given the
        default Timeout from the underlying [IO::Socket](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket/markdown) will be used.

### stop_SSL
        This is the opposite of start_SSL(), connect_SSL() and accept_SSL(), e.g. it will shutdown
        the SSL connection and return to the class before start_SSL(). It gets the same arguments as
        close(), in fact close() calls stop_SSL() (but without downgrading the class).

        Will return true if it succeeded and undef if failed. This might be the case for
        non-blocking sockets. In this case $! is set to EWOULDBLOCK and the ssl error to
        SSL_WANT_READ or SSL_WANT_WRITE. In this case the call should be retried again with the same
        arguments once the socket is ready.

        For calling from "stop_SSL" "SSL_fast_shutdown" default to false, e.g. it waits for the
        close_notify of the peer. This is necessary in case you want to downgrade the socket and
        continue to use it as a plain socket.

        After stop_SSL the socket can again be used to exchange plain data.

    connect_SSL, accept_SSL
        These functions should be used to do the relevant handshake, if the socket got created with
        "new" or upgraded with "start_SSL" and "SSL_startHandshake" was set to false. They will
        return undef until the handshake succeeded or an error got thrown. As long as the function
        returns undef and $! is set to EWOULDBLOCK one could retry the call after the socket got
        readable (SSL_WANT_READ) or writeable (SSL_WANT_WRITE).

    ocsp_resolver
        This will create an OCSP resolver object, which can be used to create OCSP requests for the
        certificates of the SSL connection. Which certificates are verified depends on the setting
        of "SSL_ocsp_mode": by default only the leaf certificate will be checked, but with
        SSL_OCSP_FULL_CHAIN all chain certificates will be checked.

        Because to create an OCSP request the certificate and its issuer certificate need to be
        known it is not possible to check certificates when the trust chain is incomplete or if the
        certificate is self-signed.

        The OCSP resolver gets created by calling "$ssl->ocsp_resolver" and provides the following
        methods:

        hard_error
                This returns the hard error when checking the OCSP response. Hard errors are
                certificate revocations. With the "SSL_ocsp_mode" of SSL_OCSP_FAIL_HARD any soft
                error (e.g. failures to get signed information about the certificates) will be
                considered a hard error too.

                The OCSP resolving will stop on the first hard error.

                The method will return undef as long as no hard errors occurred and still requests
                to be resolved. If all requests got resolved and no hard errors occurred the method
                will return ''.

        soft_error
                This returns the soft error(s) which occurred when asking the OCSP responders.

        requests
                This will return a hash consisting of "(url,request)"-tuples, e.g. which contain the
                OCSP request string and the URL where it should be sent too. The usual way to send
                such a request is as HTTP POST request with a content-type of
                "application/ocsp-request" or as a GET request with the base64 and url-encoded
                request is added to the path of the URL.

                After you've handled all these requests and added the response with "add_response"
                you should better call this method again to make sure, that no more requests are
                outstanding. [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) will combine multiple OCSP requests for the same server
                inside a single request, but some server don't give a response to all these
                requests, so that one has to ask again with the remaining requests.

        add_response($uri,$response)
                This method takes the HTTP body of the response which got received when sending the
                OCSP request to $uri. If no response was received or an error occurred one should
                either retry or consider $response as empty which will trigger a soft error.

                The method returns the current value of "hard_error", e.g. a defined value when no
                more requests need to be done.

        resolve_blocking(%args)
                This combines "requests" and "add_response" which [HTTP::Tiny](https://www.chedong.com/phpMan.php/perldoc/HTTP%3A%3ATiny/markdown) to do all necessary
                requests in a blocking way. %args will be given to [HTTP::Tiny](https://www.chedong.com/phpMan.php/perldoc/HTTP%3A%3ATiny/markdown) so that you can put
                proxy settings etc here. [HTTP::Tiny](https://www.chedong.com/phpMan.php/perldoc/HTTP%3A%3ATiny/markdown) will be called with "verify_SSL" of false,
                because the OCSP responses have their own signatures so no extra SSL verification is
                needed.

                If you don't want to use blocking requests you need to roll your own user agent with
                "requests" and "add_response".

    [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->new_from_fd($fd, [mode], %sslargs)
        This will convert a socket identified via a file descriptor into an SSL socket. Note that
        the argument list does not include a "MODE" argument; if you supply one, it will be
        thoughtfully ignored (for compatibility with [IO::Socket::INET](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AINET/markdown)). Instead, a mode of '+<' is
        assumed, and the file descriptor passed must be able to handle such I/O because the initial
        SSL handshake requires bidirectional communication.

        Internally the given $fd will be upgraded to a socket object using the "new_from_fd" method
        of the super class ([IO::Socket::INET](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AINET/markdown) or similar) and then "start_SSL" will be called using
        the given %sslargs. If $fd is already an [IO::Socket](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket/markdown) object you should better call
        "start_SSL" directly.

    [IO::Socket::SSL::default_ca](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3Adefaultca/markdown)([ path|dir| SSL_ca_file = ..., SSL_ca_path => ... ])>
        Determines or sets the default CA path. If existing path or dir or a hash is given it will
        set the default CA path to this value and never try to detect it automatically. If "undef"
        is given it will forget any stored defaults and continue with detection of system defaults.
        If no arguments are given it will start detection of system defaults, unless it has already
        stored user-set or previously detected values.

        The detection of system defaults works similar to OpenSSL, e.g. it will check the directory
        specified in environment variable SSL_CERT_DIR or the path OPENSSLDIR/certs (SSLCERTS: on
        VMS) and the file specified in environment variable SSL_CERT_FILE or the path
        OPENSSLDIR/cert.pem (SSLCERTS:cert.pem on VMS). Contrary to OpenSSL it will check if the
        SSL_ca_path contains PEM files with the hash as file name and if the SSL_ca_file looks like
        PEM. If no usable system default can be found it will try to load and use [Mozilla::CA](https://www.chedong.com/phpMan.php/perldoc/Mozilla%3A%3ACA/markdown) and if
        not available give up detection. The result of the detection will be saved to speed up
        future calls.

        The function returns the saved default CA as hash with SSL_ca_file and SSL_ca_path.

    [IO::Socket::SSL::set_default_context](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3Asetdefaultcontext/markdown)(...)
        You may use this to make [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) automatically re-use a given context (unless
        specifically overridden in a call to new()). It accepts one argument, which should be either
        an [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) object or an [IO::Socket::SSL::SSL_Context](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3ASSLContext/markdown) object. See the SSL_reuse_ctx
        option of new() for more details. Note that this sets the default context globally, so use
        with caution (esp. in mod_perl scripts).

    [IO::Socket::SSL::set_default_session_cache](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3Asetdefaultsessioncache/markdown)(...)
        You may use this to make [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) automatically re-use a given session cache (unless
        specifically overridden in a call to new()). It accepts one argument, which should be an
        [IO::Socket::SSL::Session_Cache](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3ASessionCache/markdown) object or similar (e.g. something which implements
        get_session, add_session and del_session like [IO::Socket::SSL::Session_Cache](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3ASessionCache/markdown) does). See the
        SSL_session_cache option of new() for more details. Note that this sets the default cache
        globally, so use with caution.

    [IO::Socket::SSL::set_defaults](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3Asetdefaults/markdown)(%args)
        With this function one can set defaults for all SSL_* parameter used for creation of the
        context, like the SSL_verify* parameter. Any SSL_* parameter can be given or the following
        short versions:

        mode - SSL_verify_mode
        callback - SSL_verify_callback
        scheme - SSL_verifycn_scheme
        name - SSL_verifycn_name

    [IO::Socket::SSL::set_client_defaults](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3Asetclientdefaults/markdown)(%args)
        Similar to "set_defaults", but only sets the defaults for client mode.

    [IO::Socket::SSL::set_server_defaults](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3Asetserverdefaults/markdown)(%args)
        Similar to "set_defaults", but only sets the defaults for server mode.

    [IO::Socket::SSL::set_args_filter_hack](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3Asetargsfilterhack/markdown)(\&code|'use_defaults')
        Sometimes one has to use code which uses unwanted or invalid arguments for SSL, typically
        disabling SSL verification or setting wrong ciphers or SSL versions. With this hack it is
        possible to override these settings and restore sanity. Example:

            [IO::Socket::SSL::set_args_filter_hack](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL%3A%3Asetargsfilterhack/markdown)( sub {
                my ($is_server,$args) = @_;
                if ( ! $is_server ) {
                    # client settings - enable verification with default CA
                    # and fallback hostname verification etc
                    delete @{$args}{qw(
                        SSL_verify_mode
                        SSL_ca_file
                        SSL_ca_path
                        SSL_verifycn_scheme
                        SSL_version
                    )};
                    # and add some fingerprints for known certs which are signed by
                    # unknown CAs or are self-signed
                    $args->{SSL_fingerprint} = ...
                }
            });

        With the short setting "set_args_filter_hack('use_defaults')" it will prefer the default
        settings in all cases. These default settings can be modified with "set_defaults",
        "set_client_defaults" and "set_server_defaults".

    The following methods are unsupported (not to mention futile!) and [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) will emit a
    large CROAK() if you are silly enough to use them:

    truncate
    stat
    ungetc
    setbuf
    setvbuf
    fdopen
    send/recv
        Note that send() and recv() cannot be reliably trapped by a tied filehandle (such as that
        used by [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)) and so may send unencrypted data over the socket. Object-oriented
        calls to these functions will fail, telling you to use the print/printf/syswrite and
        read/sysread families instead.

## DEPRECATIONS
    The following functions are deprecated and are only retained for compatibility:

### context_init
      use the SSL_reuse_ctx option if you want to re-use a context

### socketToSSL
      use [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown)->start_SSL() instead

### kill_socket
      use close() instead

### get_peer_certificate
      use the peer_certificate() function instead. Used to return X509_Certificate with methods
      subject_name and issuer_name. Now simply returns $self which has these methods (although
      deprecated).

### issuer_name
      use peer_certificate( 'issuer' ) instead

### subject_name
      use peer_certificate( 'subject' ) instead

## EXAMPLES
    See the 'example' directory, the tests in 't' and also the tools in 'util'.

## BUGS
    If you use [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) together with threads you should load it (e.g. use or require) inside
    the main thread before creating any other threads which use it. This way it is much faster
    because it will be initialized only once. Also there are reports that it might crash the other
    way.

    Creating an [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) object in one thread and closing it in another thread will not work.

    [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) does not work together with [Storable::fd_retrieve](https://www.chedong.com/phpMan.php/perldoc/Storable%3A%3Afdretrieve/markdown)/fd_store. See BUGS file for
    more information and how to work around the problem.

    Non-blocking and timeouts (which are based on non-blocking) are not supported on Win32, because
    the underlying [IO::Socket::INET](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AINET/markdown) does not support non-blocking on this platform.

    If you have a server and it looks like you have a memory leak you might check the size of your
    session cache. Default for [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) seems to be 20480, see the example for
    SSL_create_ctx_callback for how to limit it.

    TLS 1.3 support regarding session reuse is incomplete.

## SEE ALSO
    [IO::Socket::INET](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AINET/markdown), [IO::Socket::INET6](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AINET6/markdown), [IO::Socket::IP](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3AIP/markdown), [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown).

## THANKS
    Many thanks to all who added patches or reported bugs or helped [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown) another way.
    Please keep reporting bugs and help with patches, even if they just fix the documentation.

    Special thanks to the team of [Net::SSLeay](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ASSLeay/markdown) for the good cooperation.

## AUTHORS
    Steffen Ullrich, <sullr at cpan.org> is the current maintainer.

    Peter Behroozi, <behrooz at fas.harvard.edu> (Note the lack of an "i" at the end of "behrooz")

    Marko Asplund, <marko.asplund at kronodoc.fi>, was the original author of [IO::Socket::SSL](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3ASocket%3A%3ASSL/markdown).

    Patches incorporated from various people, see file Changes.

## COPYRIGHT
    The original versions of this module are Copyright (C) 1999-2002 Marko Asplund.

    The rewrite of this module is Copyright (C) 2002-2005 Peter Behroozi.

    Versions 0.98 and newer are Copyright (C) 2006-2014 Steffen Ullrich.

    This module is free software; you can redistribute it and/or modify it under the same terms as
    Perl itself.