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

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

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

	   # simple HTTP client -----------------------------------------------
	   my $client = IO::Socket::SSL->new(
	       # where to connect
	       PeerHost => "",
	       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 => '', # defaults to PeerHost
	       SSL_verifycn_scheme => 'http',

	       # SNI support
	       SSL_hostname => '', # 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 => '',
	       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('');
	   # ... receive greeting, send STARTTLS, receive ok ...
	       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' )
	       or die "hostname verification failed";

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

	   # use non-blocking socket (BEWARE OF SELECT!) -------------------
	   my $cl = IO::Socket::SSL->new($dst);
	   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

	       # new data on socket or eof
	       # 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 ${EAGAIN};
		   next if $SSL_ERROR == SSL_WANT_READ;
		   if ( $SSL_ERROR == SSL_WANT_WRITE ) {
		       # need to write data on renegotiation
		   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 $self->pending;	 # goto sysread
		   next;			 # goto $sel->can_read

       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 section 'Using
       SSL' before attempting to use this module.

       If you used IO::Socket before you should read the following section
       'Differences to IO::Socket'.

       If you want to use SSL with non-blocking sockets and/or within an event
       loop please read very carefully the sections about non-blocking I/O and
       polling of SSL sockets.

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

   Differences to IO::Socket
       Although IO::Socket::SSL tries to behave similar to IO::Socket there
       are some important differences due to the way SSL works:

       ·   buffered input

	   Data are transmitted inside the SSL protocol using encrypted
	   frames, which can only be decrypted once the full frame is
	   received. So if you use "read" or "sysread" to receive less data
	   than the SSL frame contains, it will read the whole frame, return
	   part of it and buffer the rest for later reads.  This does not make
	   a difference for simple programs, but if you use select-loops or
	   polling or non-blocking I/O please read the related sections.

       ·   SSL handshakes

	   Before any encryption can be done the peers have to agree to common
	   algorithms, verify certificates etc. So a handshake needs to be
	   done before any payload is send or received and might additionally
	   happen later in the connection again.

	   This has important implications when doing non-blocking or event-
	   based I/O (please read the related sections), but means also, that
	   connect and accept calls include the SSL handshake and thus might
	   block or fail, if the peer does not behave like expected. For
	   instance accept will wait infinitly if a TCP client connects to the
	   socket but does not initiate an SSL handshake.

       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
       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

       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.

	   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 it's best in using secure defaults, e.g. chosing good
	   ciphers, enabling proper verification etc.

	     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.

	     Sets the version of the SSL protocol used to transmit data.
	     'SSLv23' auto-negotiates between SSLv2 and SSLv3, while 'SSLv2',
	     'SSLv3', 'TLSv1', 'TLSv1_1' or 'TLSv1_2' restrict the 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.

	     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 !TLSv1_1 and !TLSv1_2 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:!TLSv1_1:!TLSv1_2'
	     might help.

	     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
	     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 ''.

	     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.

	     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).

	     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 | 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 in PEM format with
	     SSL_cert_file or as an internal representation of a X509* object
	     with SSL_cert.

	     For each certificate a key is need, which can either be given as
	     a file in PEM format with SSL_key_file or as an internal
	     representation of a EVP_PKEY* object with SSL_key.

	     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 ="

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


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

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

	     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.

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

	     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.

	     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 | 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.  If both SSL_ca_file and
	     SSL_ca_path are undefined and not builtin defaults (see "Defaults
	     for Cert, Key and CA".) can be used, it will try to use the
	     system defaults used built into the OpenSSL library.  If you
	     really don't want to set a CA set this key to ''.

	     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

	     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
	     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

	     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

	     See the OpenSSL documentation for SSL_CTX_set_verify for more

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

	     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.

	     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.

	     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.

	     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.

	     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()

	     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;

	     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

	     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

	     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

	     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.

	     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).

	     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.

	     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.

	     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.

	   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.

	   This behaves similar to the connnect 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

	   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:

	     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.

	     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.

	     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 futher 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.	 syswrite will
	   write all the data within a single SSL frame, which means, that no
	   more than 16.384 bytes, which is the maximum size of an SSL frame,
	   can 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

	   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.

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

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

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

	   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.

	   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

	   subjectAltNames - only for Net::SSLeay version >=1.33
		   Alternative names for the subject, usually different names
		   for the same server, like,,

		   It returns a list of (typ,value) with typ GEN_DNS,
		   GEN_IPADD etc (these constants are exported from
		   IO::Socket::SSL).  See

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

	   This verifies the given hostname against the peer certificate using
	   the given scheme. Hostname is usually what you specify within the

	   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.
		   * matches but not 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.  * or even www* The
		   common name will be only checked if no names are given in

	   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 * will
		   be possible (like in ldap), for 'anywhere' www*
		   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.

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

	   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()>.

	   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

	   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

       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.

	   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.

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

       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

	   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.

	   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).

	   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.

	   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'

	   Similar to "set_defaults", but only sets the defaults for client

	   Similar to "set_defaults", but only sets the defaults for server

       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:

	   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.

   Defaults for Cert, Key and CA
       Only if no SSL_key*, no SSL_cert* and no SSL_ca* options are given it
       will fall back to the following builtin defaults:

	   Depending on the SSL_server setting it will be either
	   "certs/server-cert.pem" or "certs/client-cert.pem".

	   Depending on the SSL_server setting it will be either
	   "certs/server-key.pem" or "certs/client-key.pem".

       SSL_ca_file | SSL_ca_path
	   It will set SSL_ca_file to "certs/my-ca.pem" if it exist.
	   Otherwise it will set SSL_ca_path to "ca/" if it exist.

       Please note, that these defaults are depreciated and will be removed in
       the near future, e.g. you should specify all the certificates and keys
       you use.	 If you don't specify a CA file or path it will fall back to
       the system default built into OpenSSL.

       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.

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 packet 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

       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 16k all the time. 16k is the maximum
       size of an SSL frame and because sysread returns data from only a
       single SSL frame you guarantee this way, that there are no pending
       data.  Please see the example on top of this documentation on how to
       use SSL within a select loop.

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 handshakes might occure at any time, even within an
       established connections. In this 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 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.

       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 it you should better do an TCP accept and later upgrade the TCP
       socket in a non-blocking way with "start_SSL" and "accept_SSL".

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

       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.

       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

       See the 'example' directory.

       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

       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

       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.

       The following functions are deprecated and are only retained for

	 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

	 use close() instead

	 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).

	 use peer_certificate( 'issuer' ) instead

	 use peer_certificate( 'subject' ) instead

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

       Steffen Ullrich, <steffen at> is the current maintainer.

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

       Marko Asplund, <marko.asplund at>, was the original author
       of IO::Socket::SSL.

       Patches incorporated from various people, see file Changes.

       The original versions of this module are Copyright (C) 1999-2002 Marko

       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
       <>.  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, e.g. you need to authenticate the person.
       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" (signed) by someone
       (a CA, e.g. 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 signing
       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?  So most common case is that at least the
       server has a certificate which the client can verify, but the server
       may also ask back for a certificate to authenticate the client.	To
       verify that a certificate is trusted, one checks if the certificate is
       signed by the expected CA (Certificate Authority), which often means
       any CA installed on the system (IO::Socket::SSL tries to use the CAs
       installed on the system by default). So if you trust the CA, trust the
       number theory and trust the used algorithms you can be confident, that
       no-one is reading your data.

       Beside the authentication using certificates there is also anonymous
       authentication, which effectivly means no authentication. In this case
       it is easy for somebody in between to intercept the connection, e.g.
       playing man in the middle and nobody notices.  By default
       IO::Socket::SSL uses only ciphers which require certificates and which
       are safe enough, but if you want to set your own cipher_list make sure,
       that you explicitly exclude anonymous authentication. E.g. setting the
       cipher list to HIGH is not enough, you should use at least HIGH:!aNULL.

   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 <>.

perl v5.18.2			  2014-01-16		  IO::Socket::SSL(3pm)

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