SSL(3) User Contributed Perl Documentation SSL(3)
NAME
IO::Socket::SSL -- Nearly transparent SSL encapsulation for IO::Socket::INET.
SYNOPSIS
use IO::Socket::SSL;
my $client = IO::Socket::SSL->new("www.example.com:https");
if ($client) {
print $client "GET / HTTP/1.0\r\n\r\n";
print <$client>;
close $client;
} else {
warn "I encountered a problem: ",
IO::Socket::SSL::errstr();
}
DESCRIPTION
This module is a true drop-in replacement for IO::Socket::INET that uses SSL to encrypt data before
it is transferred to a remote server or client. 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, and SSL version selection. As an extra bonus, it works
perfectly with mod_perl.
If you have never used SSL before, you should read the appendix labelled 'Using SSL' before
attempting to use this module.
If you have used this module before, read on, as versions 0.93 and above have several changes from
the previous IO::Socket::SSL versions (especially see the note about return values).
If you are using non-blocking sockets read on, as version 0.98 added better support for non-blocking.
METHODS
IO::Socket::SSL inherits its methods from IO::Socket::INET, overriding them as necessary. If there
is an SSL error, the method or operation will return an empty list (false in all contexts). The
methods that have changed from the perspective of the user are re-documented here:
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_version
Sets the version of the SSL protocol used to transmit data. The default is SSLv2/3, which
auto-negotiates between SSLv2 and SSLv3. You may specify 'SSLv2', 'SSLv3', or 'TLSv1' (case-insensitive) (caseinsensitive)
insensitive) if you do not want this behavior.
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 used the openssl builtin default is used which is suitable for most cases.
SSL_use_cert
If this is set, 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_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).
SSL_key
This is an EVP_PKEY* and can be used instead of SSL_key_file. 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).
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. Note that a key and certificate are only required for an SSL server, so you
do not need to bother with these trifling options should you be setting up an unauthenticated
client.
SSL_cert
This is an X509* or an array of X509*. The first X509* is the internal representation of the
certificate while the following ones are extra certificates. 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).
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.
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.
SSL_verify_mode
This option sets the verification mode for the peer certificate. The default (0x00) does no
authentication. You may combine 0x01 (verify peer), 0x02 (fail verification if no peer
certificate exists; ignored for clients), and 0x04 (verify client once) to change the default.
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). 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.
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. 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_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_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.
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 and the full text of the error message.
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. $socket->close(SSL_no_shutdown => 1)) and one or more of the following parameters:
SSL_no_shutdown
If set to a true value, this option will make close() not use the SSL_shutdown() call on the
socket in question so that the close operation can complete without problems if you have used
shutdown() or are working on a copy of a socket.
SSL_ctx_free
If you want to make sure that the SSL context of the socket is destroyed when you close it, set
this option to a true value.
peek(...)
This function has exactly the same syntax as sysread(), and performs nearly the same task
(reading data from the socket) but will not advance the read position so that successive calls to
peek() with the same arguments will return the same results. This function requires OpenSSL
0.9.6a or later to work.
pnig)
This function will let you know how many bytes of data are immediately ready for reading from the
socket. This is especially handy if you are doing reads on a blocking socket or just want to
know if new data has been sent over the socket.
gt_cpe(
Returns the string form of the cipher that the IO::Socket::SSL object is using.
dm_pe_criiae)
Returns a parsable string with select fields from the peer SSL certificate. This method directly
returns the result of the dump_peer_certificate() method of Net::SSLeay.
peer_certificate($field)
If a peer certificate exists, this function can retrieve values from it. Right now, the only
fields it can return are "authority" and "owner" (or "issuer" and "subject" if you want to use
OpenSSL names), corresponding to the certificate authority that signed the peer certificate and
the owner of the peer certificate. This function returns a string with all the information about
the particular field in one parsable line. If no field is given it returns the full certificate
(x509).
ert(
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 $SSL_ERROR against the
exported symbols SSL_WANT_READ and SSL_WANT_WRITE.
IO::Socket::SSL->start_SSL($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, $socket will remain blessed in its original
class. For non-blocking sockets you better just upgrade the socket to IO::Socket::SSL and call
accept_SSL or connect_SSL and the upgraded object. To just upgrade the socket set
SSL_startHandshake explicitly to 0. If you call start_SSL w/o this parameter it will revert to
blocking behavior for accept_SSL and connect_SSL.
IO::Socket::SSL->new_from_fd($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).
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.
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.
IPv6
Support for IPv6 with IO::Socket::SSL is expected to work, but is experimental, as none of the
author's machines use IPv6 and hence he cannot test IO::Socket::SSL with them. However, a few brave
people have used it without incident, so if you wish to make IO::Socket::SSL IPv6 aware, pass the
'inet6' option to IO::Socket::SSL when calling it (i.e. "use IO::Socket::SSL qw(inet6);"). You will
need IO::Socket::INET6 and Socket6 to use this option, and you will also need to write "use Socket6;"
before using IO::Socket::SSL. If you absolutely do not want to use this (or want a quick change back
to IPv4), pass the 'inet4' option instead.
Currently, there is no support for using IPv4 and IPv6 simultaneously in a single program, but it is
planned for a future release.
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 4) to IO::Socket::SSL when calling it:
use IO::Socket::SSL qw(debug0);
#No debugging (default).
use IO::Socket::SSL qw(debug1);
#Only print out errors.
use IO::Socket::SSL qw(debug2);
#Print out errors and cipher negotiation.
use IO::Socket::SSL qw(debug3);
#Print out progress, ciphers, and errors.
use IO::Socket::SSL qw(debug4);
#Print out everything, including data.
You can also set $IO::Socket::SSL::DEBUG to 0-4, but that's a bit of a mouthful, isn't it?
EXAMPLES
See the 'example' directory.
BUGS
IO::Socket::SSL is not threadsafe. This is because IO::Socket::SSL is based on Net::SSLeay which
uses a global object to access some of the API of openssl and is therefore not threadsafe.
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)
get_peer_certificate() and friends
(use the peer_certificate() function instead)
The following classes have been removed:
SSL_SSL
(not that you should have been directly accessing this anyway):
X509_Certificate
(but get_peer_certificate() will still Do The Right Thing)
SEE ALSO
IO::Socket::INET, IO::Socket::INET6, 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
Working support for non-blocking was added by Steffen Ullrich.
The rewrite of this module is Copyright (C) 2002-2005 Peter Behroozi.
The original versions of this module are Copyright (C) 1999-2002 Marko Asplund.
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 v5.8.8 2006-08-15 SSL(3)
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