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

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:

       ·   "Essential Information About SSL/TLS"

       ·   "Basic SSL Client"

       ·   "Basic SSL Server"

       ·   "Common Usage Errors"

       ·   "Common Problems with SSL"

       ·   "Using Non-Blocking Sockets"

       ·   "Advanced Usage"

       ·   "Integration Into Own Modules"

       ·   "Description Of Methods"

       Additional documentation can be found in

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

       ·   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:

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

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

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

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

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

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

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

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

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

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

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

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

Basic SSL Client
       A basic SSL client is simple:

	   my $client = IO::Socket::SSL->new('www.example.com:443')
	       or die "error=$!, 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:

       ·   Disabling verification with "SSL_verify_mode".

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

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

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

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

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

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

       ·   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".

       ·   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.

       ·   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.

       ·   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.

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:

       ·   0 - No debugging (default).

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

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

       ·   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:

       ·   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.

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

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

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

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

       ·   Bad or old OpenSSL versions.

	   IO::Socket::SSL 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.

       ·   Validation of client certificates fail.

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

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

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

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

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

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

       Using readline on non-blocking sockets does not make much sense and I
       would advise against using it.  And, while the behavior is not
       documented for other IO::Socket 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
	       }
	   }

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:

       ·   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.

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

       ·   Otherwise it will fall back to IO::Socket::INET, 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
	     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 contextes. 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+.	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(...)
	   There are a number of nasty traps that lie in wait if you are not
	   careful about using close().	 The first of these will bite you if
	   you have been using shutdown() on your sockets.  Since the SSL
	   protocol mandates that a SSL "close notify" message be sent before
	   the socket is closed, a shutdown() that closes the socket's write
	   channel will cause the close() call to hang.	 For a similar reason,
	   if you try to close a copy of a socket (as in a forking server) you
	   will affect the original socket as well.  To get around these
	   problems, call close with an object-oriented syntax (e.g.
	   $socket->close(SSL_no_shutdown => 1)) and one or more of the
	   following parameters:

	   SSL_no_shutdown
	     If set to a true value, this option will make close() not use the
	     SSL_shutdown() call on the socket in question so that the close
	     operation can complete without problems if you have used
	     shutdown() or are working on a copy of a socket.

	     Not using a real ssl shutdown on a socket will make session
	     caching unusable.

	   SSL_fast_shutdown
	     If set to true only a unidirectional shutdown will be done, e.g.
	     only the close_notify (see SSL_shutdown(3)) will be sent.
	     Otherwise a bidirectional shutdown will be done where it waits
	     for the close_notify of the peer too.

	     Because a unidirectional shutdown is enough to keep session cache
	     working it defaults to fast shutdown inside close.

	   SSL_ctx_free
	     If you want to make sure that the SSL context of the socket is
	     destroyed when you close it, set this option to a true value.

       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.

       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.

SEE ALSO
       IO::Socket::INET, IO::Socket::INET6, IO::Socket::IP, Net::SSLeay.

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.

perl v5.26.0			  2017-10-27		  IO::Socket::SSL(3pm)
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