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,
        # 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
        # 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:

• 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 '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:

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:

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:

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:

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.

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.