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IO::Socket::SSL(3)    User Contributed Perl Documentation   IO::Socket::SSL(3)




NAME

       IO::Socket::SSL - SSL sockets with IO::Socket interface


SYNOPSIS

           use strict;
           use IO::Socket::SSL;

           # simple client
           my $cl = IO::Socket::SSL->new('www.google.com:443');
           print $cl "GET / HTTP/1.0\r\n\r\n";
           print <$cl>;

           # simple server
           my $srv = IO::Socket::SSL->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 makes using SSL/TLS much easier by wrapping the
       necessary functionality into the familiar IO::Socket 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:

       o   "Essential Information About SSL/TLS"

       o   "Basic SSL Client"

       o   "Basic SSL Server"

       o   "Common Usage Errors"

       o   "Common Problems with SSL"

       o   "Using Non-Blocking Sockets"

       o   "Advanced Usage"

       o   "Integration Into Own Modules"

       o   "Description Of Methods"

       Additional documentation can be found in

       o   IO::Socket::SSL::Intercept - Doing Man-In-The-Middle with SSL

       o   IO::Socket::SSL::Utils - 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:

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

       o   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.

       o   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.

       o   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.

       o   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 $client = IO::Socket::SSL->new('www.example.com:443')
               or die "error=$!, 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 which provides
       the default CAs of Firefox.

       In the default settings, IO::Socket::SSL 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->new('mx.example.com:25') or die $!;
           # .. read greeting from server
           # .. send EHLO and read response
           # .. send STARTTLS command and read response
           # .. if response was successful we can upgrade the socket to SSL now:
           IO::Socket::SSL->start_SSL($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->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(), # 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 1.59 or higher):

           # default will try OCSP stapling and check only leaf certificate
           my $client = IO::Socket::SSL->new($dst);

           # better yet: require checking of full chain
           my $client = IO::Socket::SSL->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->new;
           my $client = IO::Socket::SSL->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->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 servers, only that
       it also contains settings for certificate and key:

           # simple server
           my $server = IO::Socket::SSL->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->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->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:

       o   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:

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

           o       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.

           o       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 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.  If necessary
           these can be checked with the "default_ca" method.

       o   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.

           See also "Using Non-Blocking Sockets".

       o   Expecting exactly the same behavior as plain sockets.

           IO::Socket::SSL 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
           ususally 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.

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

           IO::Socket::SSL 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 sets and which get updated from time to time
           to better fit the real world.

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

           Some modules use IO::Socket::SSL, 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.

       o   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.

       o   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.

       o   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 and Net::SSLeay, which can both be set with
       $IO::Socket::SSL::DEBUG.  The following debugs levels are defined, but
       used not in any consistent way:

       o   0 - No debugging (default).

       o   1 - Print out errors from IO::Socket::SSL and ciphers from
           Net::SSLeay.

       o   2 - Print also information about call flow from IO::Socket::SSL and
           progress information from Net::SSLeay.

       o   3 - Print also some data dumps from IO::Socket::SSL and from
           Net::SSLeay.

       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:

       o   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.

       o   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.

       o   Bad or old OpenSSL versions.

           IO::Socket::SSL uses OpenSSL with the help of the Net::SSLeay
           library. It is recommend to have a recent version of this library,
           because it has more features and usually fewer known bugs.

       o   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.

       o   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 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, IO::Socket::INET..) 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" 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 $cl = IO::Socket::SSL->new($dst);
           $cl->blocking(0);
           my $sel = IO::Select->new($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.  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:: 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->can_client_sni()".

       On the server side, earlier versions of OpenSSL are supported, but only
       together with Net::SSLeay version >= 1.50.  To check for support you
       might call "IO::Socket::SSL->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->new(...) or die $!;
           ... exchange plain data on $sock until starttls command ...
           IO::Socket::SSL->start_SSL($sock,%sslargs) or die $SSL_ERROR;
           ... now $sock is an IO::Socket::SSL 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 object ...

       But, lots of modules just derive directly from IO::Socket::INET.  While
       this base class can be replaced with IO::Socket::SSL, 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 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->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 object ...
           ... exchange data with SSL on $sock until stoptls command ...
           $sock->stop_SSL or die $SSL_ERROR;
           ... $sock is still an IO::Socket::SSL object ...
           ... but data exchanged again in plain ...


Integration Into Own Modules

       IO::Socket::SSL behaves similarly to other IO::Socket 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 inherits from another IO::Socket module.  The choice of
       the super class depends on the installed modules:

       o   If IO::Socket::IP with at least version 0.20 is installed it will
           use this module as super class, transparently providing IPv6 and
           IPv4 support.

       o   If IO::Socket::INET6 is installed it will use this module as super
           class, transparently providing IPv6 and IPv4 support.

       o   Otherwise it will fall back to IO::Socket::INET, which is a perl
           core module.  With IO::Socket::INET 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.
       Alternatively you can enforce IPv4 globally by loading IO::Socket::SSL
       with the option 'inet4', in which case it will use the IPv4 only class
       IO::Socket::INET as the super class.

       IO::Socket::SSL 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 object.  You may use all the friendly
           options that came bundled with the super class (e.g.
           IO::Socket::IP, IO::Socket::INET, ...) 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 or IO::Socket::SSL::Utils::PEM_xxx2cert)
             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 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$hex_fingerprint'. "algo" is a fingerprint algorithm
             supported by OpenSSL, e.g. 'sha1','sha256'... and
             "hex_fingerprint" is the hexadecimal representation of the binary
             fingerprint.  If you want to use the fingerprint of the pubkey
             inside the certificate instead of the certificate use the syntax
             'algo$pub$hex_fingerprint' instead.  To get the fingerprint of an
             established connection you can use "get_fingerprint".

             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 or IO::Socket::SSL::Utils::PEM_xxx2cert) 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 or
             IO::Socket::SSL::Utils::PEM_xxx2key).  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>>.

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

           SSL_passwd_cb
             If your private key is encrypted, you might not want the default
             password prompt from Net::SSLeay.  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 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' or 'TLSv1_2'
             restrict handshake and protocol to the specified version.  All
             values are case-insensitive.  Instead of 'TLSv1_1' and 'TLSv1_2'
             one can also use 'TLSv11' and 'TLSv12'.  Support for 'TLSv1_1'
             and 'TLSv1_2' requires recent versions of Net::SSLeay 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'. Look into the OpenSSL documentation
             (<http://www.openssl.org/docs/apps/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.
             The default setting prefers ciphers with forward secrecy,
             disables anonymous authentication and disables known insecure
             ciphers like MD5, DES etc. This gives a grade A result at the
             tests of SSL Labs.  To use the less secure OpenSSL builtin
             default (whatever this is) set SSL_cipher_list to ''.

             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
             If you want Diffie-Hellman key exchange you need to supply a
             suitable file here or use the SSL_dh parameter. See dhparam
             command in openssl for more information.  To create a server
             which provides forward secrecy you need to either give the DH
             parameters or (better, because faster) the ECDH curve.

             If neither "SSL_dh_file" not "SSL_dh" is 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
             If you want Elliptic Curve Diffie-Hellmann key exchange you need
             to supply the OID or NID of a suitable curve (like 'prime256v1')
             here.  To create a server which provides forward secrecy you need
             to either give the DH parameters or (better, because faster) the
             ECDH curve.

             This parameter defaults to 'prime256v1' (builtin of OpenSSL) to
             offer ECDH key exchange by default. If you don't want this
             explicitly set it to undef.

             You can check if ECDH support is available by calling
             "IO::Socket::SSL->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()).
             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, 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->new", if an
             IO::Socket::INET 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 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".

             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->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, 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 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 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 as
             the single argument.

             Example for limiting the server session cache size:

               SSL_create_ctx_callback => sub {
                   my $ctx = shift;
                   Net::SSLeay::CTX_sess_set_cache_size($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.

           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->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
             1.46+ and openssl-1.0.1+. NPN is unavailable in TLSv1.3 protocol.
             To check support you might call "IO::Socket::SSL->can_npn()".  If
             you use this option with an unsupported Net::SSLeay/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 1.56+ and openssl-1.0.2+. More details about the
             extension are in RFC7301. To check support you might call
             "IO::Socket::SSL->can_alpn()". If you use this option with an
             unsupported Net::SSLeay/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(my $oldkey,32);
                 Net::SSLeay::RAND_bytes(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->new(..., SSL_ticket_keycb => $keycb);

       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( BUF, LEN, [ OFFSET ] )
           This function behaves from the outside the same as sysread in other
           IO::Socket 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( BUF, [ LEN, [ OFFSET ]] )
           This functions behaves from the outside the same as syswrite in
           other IO::Socket 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( BUF, LEN, [ OFFSET ])
           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([algo,certificate,pubkey])
           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([algo,certificate,pubkey])
           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
           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.

       peer_certificate($field;[$refresh])
           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 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 version >=1.30
                   The common name, usually the server name for SSL
                   certificates.

           subjectAltNames - only for Net::SSLeay 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).  See
                   Net::SSLeay::X509_get_subjectAltNames.

       sock_certificate($field)
           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 to inspect each of the
           certificates.

           This function depends on a version of Net::SSLeay >= 1.58 .

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

       verify_hostname($hostname,$scheme,$publicsuffix)
           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 1.46+ and openssl-1.0.1+.
           To check support you might call "IO::Socket::SSL->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 1.56+ and
           openssl-1.0.2+.  To check support, use
           "IO::Socket::SSL->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() 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 is open, but the SSL handshake
           failed.

       IO::Socket::SSL->start_SSL($socket, ... )
           This will convert a glob reference or a socket that you provide to
           an IO::Socket::SSL object.   You may also pass parameters to
           specify context or connection options as with a call to new().  If
           you are using this function on an accept()ed socket, you must set
           the parameter "SSL_server" to 1, i.e.
           IO::Socket::SSL->start_SSL($socket, SSL_server => 1).  If you have
           a class that inherits from IO::Socket::SSL 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 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 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 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 to do all necessary requests in a blocking way.
                   %args will be given to HTTP::Tiny so that you can put proxy
                   settings etc here. HTTP::Tiny 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->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).  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 or
           similar) and then "start_SSL" will be called using the given
           %sslargs.  If $fd is already an IO::Socket object you should better
           call "start_SSL" directly.

       IO::Socket::SSL::default_ca([ 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 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(...)
           You may use this to make IO::Socket::SSL 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
           object or an IO::Socket::SSL::SSL_Context 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(...)
           You may use this to make IO::Socket::SSL 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 object or similar (e.g. something
           which implements get_session, add_session and maybe del_session
           like IO::Socket::SSL::Session_Cache 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(%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(%args)
           Similar to "set_defaults", but only sets the defaults for client
           mode.

       IO::Socket::SSL::set_server_defaults(%args)
           Similar to "set_defaults", but only sets the defaults for server
           mode.

       IO::Socket::SSL::set_args_filter_hack(\&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( 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 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) 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() and socket_to_SSL()
         use IO::Socket::SSL->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 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 object in one thread and closing it in
       another thread will not work.

       IO::Socket::SSL does not work together with
       Storable::fd_retrieve/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 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 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(3), IO::Socket::INET6(3), IO::Socket::IP(3), 
       Net::SSLeay(3).


THANKS

       Many thanks to all who added patches or reported bugs or helped
       IO::Socket::SSL 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 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.

       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.


POD ERRORS

       Hey! The above document had some coding errors, which are explained
       below:

       Around line 1493:
           '=item' outside of any '=over'



perl v5.26.2                      2018-09-16                IO::Socket::SSL(3)

io-socket-ssl 2.60.0 - Generated Thu Sep 20 06:03:20 CDT 2018
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