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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 HTTP client -----------------------------------------------
	   my $sock = IO::Socket::SSL->new(
	       # where to connect
	       PeerHost => "www.example.com",
	       PeerPort => "https",

	       # certificate verification
	       SSL_verify_mode => SSL_VERIFY_PEER,
	       SSL_ca_path => '/etc/ssl/certs', # typical CA path on Linux
	       # on OpenBSD instead: SSL_ca_file => '/etc/ssl/cert.pem'

	       # easy hostname verification
	       SSL_verifycn_name => 'foo.bar', # defaults to PeerHost
	       SSL_verifycn_schema => 'http',

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

	   ) 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>;

	   # 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";

	   # Upgrade existing socket to SSL ---------------------------------
	   my $sock = IO::Socket::INET->new('imap.example.com:imap');
	   # ... receive greeting, send STARTTLS, receive ok ...
	   IO::Socket::SSL->start_SSL($sock,
	       SSL_verify_mode => SSL_VERIFY_PEER,
	       SSL_ca_path => '/etc/ssl/certs',
	       ...
	   ) or die "failed to upgrade to SSL: $SSL_ERROR";

	   # manual name verification, could also be done in start_SSL with
	   # SSL_verifycn_name etc
	   $client->verify_hostname( 'imap.example.com','imap' )
	       or die "hostname verification failed";

	   # all data are now SSL encrypted
	   print $sock ....

DESCRIPTION
       This module provides an interface to SSL sockets, similar to other IO::Socket modules.
       Because of that, it can be used to make existing programs using IO::Socket::INET or
       similar modules to provide SSL encryption without much effort.  IO::Socket::SSL supports
       all the extra features that one needs to write a full-featured SSL client or server
       application: multiple SSL contexts, cipher selection, certificate verification, Server
       Name Indication (SNI), Next Protocol Negotiation (NPN), SSL version selection and more.

       If you have never used SSL before, you should read the appendix labelled 'Using SSL'
       before attempting to use this module.

       If you are trying to use it with threads see the BUGS section.

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 lookup 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 either force IPv4 by specifying and AF_INET as "Domain" of the socket or
       globally enforce IPv4 by loading IO::Socket::SSL with the option 'inet4'.

       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 read also the section about SSL
       specific error handling.

       new(...)
	   Creates a new IO::Socket::SSL object.  You may use all the friendly options that came
	   bundled with IO::Socket::INET, plus (optionally) the ones that follow:

	   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
	     hostname from PeerAddr, which will fail if only 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_version
	     Sets the version of the SSL protocol used to transmit data. 'SSLv23' auto-negotiates
	     between SSLv2 and SSLv3, while 'SSLv2', 'SSLv3' or 'TLSv1' restrict the protocol to
	     the specified version. All values are case-insensitive.

	     You can limit to set of supported protocols by adding !version separated by ':'.

	     The default SSL_version is 'SSLv23:!SSLv2' which means, that SSLv2, SSLv3 and TLSv1
	     are supported for initial protocol handshakes, but SSLv2 will not be accepted,
	     leaving only SSLv3 and TLSv1. You can also use !TLSv11 and !TLSv12 to disable TLS
	     versions 1.1 and 1.2 while allowing TLS version 1.0.

	     Setting the version instead to 'TLSv1' will probably break interaction with lots of
	     clients which start with SSLv2 and then upgrade to TLSv1. 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:!TLSv11:!TLSv12' 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:!ADH'. Look into the OpenSSL documentation
	     (<http://www.openssl.org/docs/apps/ciphers.html#CIPHER_STRINGS>) for more details.

	     If this option is not set 'ALL:!LOW' will be used.  To use OpenSSL builtin default
	     (whatever this is) set it to ''.

	   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. To mitigate BEAST attack you might use something like

	       SSL_honor_cipher_order => 1,
	       SSL_cipher_list => 'RC4-SHA:ALL:!ADH:!LOW',

	   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_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_cert_file
	     If your SSL certificate is not in the default place (certs/server-cert.pem for
	     servers, certs/client-cert.pem for clients), then you should use this option to
	     specify the location of your certificate.	A certificate is usually needed for an
	     SSL server, but might also be needed, if the client should authorize itself with a
	     certificate.

	     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>>.

	     Examples:

	      SSL_cert_file => 'mycert.pem'

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

	   SSL_cert
	     This option can be used instead of "SSL_cert_file" to specify the certificate.

	     Instead with a file the certificate is given as an X509* object or array of X509*
	     objects, where the first X509* is the internal representation of the certificate
	     while the following ones are extra certificates.  The option is useful if you create
	     your certificate dynamically (like in a SSL intercepting proxy) or get it from a
	     string (see openssl PEM_read_bio_X509 etc for getting a X509* from a string).

	     For SNI support a hash reference can be given, similar to the "SSL_cert_file"
	     option.

	   SSL_key_file
	     If your RSA private key is not in default place (certs/server-key.pem for servers,
	     certs/client-key.pem for clients), then this is the option that you would use to
	     specify a different location.  Keys should be PEM formatted, and if they are
	     encrypted, you will be prompted to enter a password before the socket is formed
	     (unless you specified the SSL_passwd_cb option).

	     For SNI support a hash reference can be given, similar to the "SSL_cert_file"
	     option.

	   SSL_key
	     This option can be used instead of "SSL_key" to specify the certificate.  Instead of
	     a file an EVP_PKEY* should be given.  This option is useful if you don't have your
	     key in a file but create it dynamically or get it from a string (see openssl
	     PEM_read_bio_PrivateKey etc for getting a EVP_PKEY* from a string).

	     For SNI support a hash reference can be given, similar to the "SSL_key" option.

	   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.

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

	   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_ca_file
	     If you want to verify that the peer certificate has been signed by a reputable
	     certificate authority, then you should use this option to locate the file containing
	     the certificate(s) of the reputable certificate authorities if it is not already in
	     the file certs/my-ca.pem.	If you definitely want no SSL_ca_file used you should set
	     it to undef.

	   SSL_ca_path
	     If you are unusually friendly with the OpenSSL documentation, you might have set
	     yourself up a directory containing several trusted certificates as separate files as
	     well as an index of the certificates.  If you want to use that directory for
	     validation purposes, and that directory is not ca/, then use this option to point
	     IO::Socket::SSL to the right place to look.  If you definitely want no SSL_ca_path
	     used you should set it to undef.

	   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).
	     For historical reasons the default for client is currently also SSL_VERIFY_NONE, but
	     this will change to SSL_VERIFY_PEER in the near future. To aid transition a warning
	     is issued if the client is used with the default SSL_VERIFY_NONE, unless
	     SSL_verify_mode was explicitly set by the application.

	   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()).

	     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
	     Set the scheme used to automatically verify the hostname of the peer.  See the
	     information about the verification schemes in verify_hostname.

	     The default is undef, e.g. to not automatically verify the hostname.  If no
	     verification is done the other SSL_verifycn_* options have no effect, but you might
	     still do manual verification by calling verify_hostname.

	   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 the PeerHost and PeerAddr
	     settings and fail if no name can be determined.

	     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.

	   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_reuse_ctx
	     If you have already set the above options (SSL_version through SSL_check_crl; this
	     does not include SSL_cipher_list yet) 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_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 Negotioation (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.

       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.

	   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 called. Otherwise a bidirectional
	     shutdown will be done. If used within close() it defaults to true, if used within
	     stop_SSL() it defaults to false.

	   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.

       peek(...)
	   This function has exactly the same syntax as sysread(), and performs nearly the same
	   task (reading data from the socket) 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 will let you know how many bytes of data are immediately ready for
	   reading from the socket.  This is especially handy if you are doing reads on a
	   blocking socket or just want to know if new data has been sent over the socket.

       get_cipher()
	   Returns the string form of the cipher that the IO::Socket::SSL object is using.

       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)
	   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.  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.

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

       verify_hostname($hostname,$scheme)
	   This verifies the given hostname against the peer certificate using the given scheme.
	   Hostname is usually what you specify within the PeerAddr.

	   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.

	   To ease the verification the following schemes are predefined:

	   ldap (rfc4513), pop3,imap,acap (rfc2995), nntp (rfc4642)
		   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, but there are no wildcards allowed.

	   http (rfc2818), alias is www
		   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 names are given in subjectAltNames.

	   smtp (rfc3207)
		   This RFC doesn't say much useful about the verification so it just assumes
		   that subjectAltNames are possible, but no wildcards are possible anywhere.

	   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|'leftmost'|'anywhere'
		   Determines if and where wildcards in subjectAltNames are possible. If
		   'leftmost' 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.

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

	   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()>.

       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(), 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 EAGAIN 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 is until it succeeds.

       IO::Socket::SSL->new_from_fd($fd, ...)
	   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.

       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 and add_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.

	   mode - set default SSL_verify_mode
	   callback - set default SSL_verify_callback
	   scheme - set default SSL_verifycn_scheme
	   name - set default SSL_verifycn_name
		   If not given and scheme is hash reference with key callback it will be set to
		   'unknown'

       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.

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 "errstr" method give a dualvar similar to $!,
       e.g.  providing an error number in numeric context or an error description in string
       context.

NON-BLOCKING I/O
       If you have a non-blocking socket, the expected behavior on read, write, accept or connect
       is to set $! to EAGAIN if the operation can not be completed immediately.

       With SSL there are cases, like with SSL handshakes, where the write operation can not be
       completed until it can read from the socket or vice versa.  In these cases $! is set to
       EGAIN like expected, and additionally $SSL_ERROR is set to either SSL_WANT_READ or
       SSL_WANT_WRITE.	Thus if you get EAGAIN 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.

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
       only 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". 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".

RETURN VALUES
       A few changes have gone into IO::Socket::SSL v0.93 and later with respect to return
       values. The behavior on success remains unchanged, but for all functions, the return value
       on error is now an empty list.	  Therefore, the return value will be false in all
       contexts, but those who have been using the return values as arguments to subroutines
       (like "mysub(IO::Socket::SSL(...)-"new, ...)>) may run into problems.  The moral of the
       story: always check the return values of these functions before using them in any way that
       you consider meaningful.

DEBUGGING
       If you are having problems using IO::Socket::SSL despite the fact that can recite
       backwards the section of this documentation labelled 'Using SSL', you should try enabling
       debugging.     To specify the debug level, pass 'debug#' (where # is a number from 0 to 3)
       to IO::Socket::SSL when calling it.  The debug level will also be propagated to
       Net::SSLeay::trace, see also Net::SSLeay:

       use IO::Socket::SSL qw(debug0);
	   No debugging (default).

       use IO::Socket::SSL qw(debug1);
	   Print out errors from IO::Socket::SSL and ciphers from Net::SSLeay.

       use IO::Socket::SSL qw(debug2);
	   Print also information about call flow from IO::Socket::SSL and progress information
	   from Net::SSLeay.

       use IO::Socket::SSL qw(debug3);
	   Print also some data dumps from IO::Socket::SSL and from Net::SSLeay.

EXAMPLES
       See the 'example' directory.

BUGS
       IO::Socket::SSL depends on Net::SSLeay.	Up to version 1.43 of Net::SSLeay it was not
       thread safe, although it did probably work if you did not use SSL_verify_callback and
       SSL_password_cb.

       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.

       The default for SSL_verify_mode on the client is currently SSL_VERIFY_NONE, which is a
       very bad idea, thus the default will change in the near future.	See documentation for
       SSL_verify_mode for more information.

LIMITATIONS
       IO::Socket::SSL uses Net::SSLeay as the shiny interface to OpenSSL, which is the shiny
       interface to the ugliness of SSL.    As a result, you will need both Net::SSLeay and
       OpenSSL on your computer before using this module.

       If you have Scalar::Util (standard with Perl 5.8.0 and above) or WeakRef, IO::Socket::SSL
       sockets will auto-close when they go out of scope, just like IO::Socket::INET
       sockets.     If you do not have one of these modules, then IO::Socket::SSL sockets will
       stay open until the program ends or you explicitly close them.	This is due to the fact
       that a circular reference is required to make IO::Socket::SSL sockets act simultaneously
       like objects and glob references.

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

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

AUTHORS
       Steffen Ullrich, <steffen at genua.de> 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) 2006-2013 Steffen Ullrich.

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

Appendix: Using SSL
       If you are unfamiliar with the way OpenSSL works, good references may be found in both the
       book "Network Security with OpenSSL" (Oreilly & Assoc.) and the web site
       <http://www.tldp.org/HOWTO/SSL-Certificates-HOWTO/>.  Read on for a quick overview.

   The Long of It (Detail)
       The usual reason for using SSL is to keep your data safe.  This means that not only do you
       have to encrypt the data while it is being transported over a network, but you also have
       to make sure that the right person gets the data.    To accomplish this with SSL, you have
       to use certificates.   A certificate closely resembles a Government-issued ID (at least in
       places where you can trust them).     The ID contains some sort of identifying information
       such as a name and address, and is usually stamped with a seal of Government Approval.
       Theoretically, this means that you may trust the information on the card and do business
       with the owner of the card.  The same ideas apply to SSL certificates, which have some
       identifying information and are "stamped" [most people refer to this as signing instead]
       by someone (a Certificate Authority) who you trust will adequately verify the identifying
       information.	 In this case, because of some clever number theory, it is extremely
       difficult to falsify the stamping process.   Another useful consequence of number theory
       is that the certificate is linked to the encryption process, so you may encrypt data
       (using information on the certificate) that only the certificate owner can decrypt.

       What does this mean for you?  It means that at least one person in the party has to have
       an ID to get drinks :-).  Seriously, it means that one of the people communicating has to
       have a certificate to ensure that your data is safe.   For client/server interactions, the
       server must always have a certificate.	   If the server wants to verify that the client
       is safe, then the client must also have a personal certificate.	To verify that a
       certificate is safe, one compares the stamped "seal" [commonly called an encrypted
       digest/hash/signature] on the certificate with the official "seal" of the Certificate
       Authority to make sure that they are the same.	 To do this, you will need the
       [unfortunately named] certificate of the Certificate Authority.	With all these in hand,
       you can set up a SSL connection and be reasonably confident that no-one is reading your
       data.

   The Short of It (Summary)
       For servers, you will need to generate a cryptographic private key and a certificate
       request.  You will need to send the certificate request to a Certificate Authority to get
       a real certificate back, after which you can start serving people. For clients, you will
       not need anything unless the server wants validation, in which case you will also need a
       private key and a real certificate.     For more information about how to get these, see
       <http://www.modssl.org/docs/2.8/ssl_faq.html#ToC24>.

perl v5.16.3				    2013-06-01			       IO::Socket::SSL(3)
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