IO::Socket::SSL
SSL(3) User Contributed Perl Documentation SSL(3)
NAME
IO::Socket::SSL -- Nearly transparent SSL encapsulation for
IO::Socket::INET.
SYNOPSIS
use strict;
use IO::Socket::SSL;
my $client = IO::Socket::SSL->new("www.example.com:https")
││ warn "I encountered a problem: ".IO::Socket::SSL::errstr();
$client->verify_hostname( ’www.example.com’,’http’ )
││ die "hostname verification failed";
print $client "GET / HTTP/1.0\r\n\r\n";
print <$client>;
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: mul-
tiple 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.
If you are trying to use it with threads see the BUGS section.
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) 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 dynam-
ically or get it from a string (see openssl PEM_read_bio_Pri-
vateKey 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 loca-
tion 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 unau-
thenticated 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 certifi-
cate 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 com-
mand 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 certifi-
cate exists; ignored for clients), and 0x04 (verify client once)
to change the default. See OpenSSL man page for
SSL_CTX_set_verify for more information.
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 indi-
cates what OpenSSL thinks of the certificate, 2) a C-style mem-
ory 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_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 automati-
cally verify the 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 caan be determined.
Using PeerHost or PeerAddr works only if you create the connec-
tion 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.
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() func-
tion.
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 num-
ber 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_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_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 mes-
sage.
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 rea-
son, 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. $socket->close(SSL_no_shutdown => 1)) and one or more of the
following parameters:
SSL_no_shutdown
If set to a true value, this option will make close() not use
the SSL_shutdown() call on the socket in question so that the
close operation can complete without problems if you have used
shutdown() or are working on a copy of a socket.
SSL_fast_shutdown
If set to true only a unidirectional shutdown will be done, e.g.
only the close_notify (see SSL_shutdown(3)) will be called. Oth-
erwise a bidrectional shutdown will be done. If used within
close() it defaults to true, if used within stop_SSL() it
defaults to false.
SSL_ctx_free
If you want to make sure that the SSL context of the socket is
destroyed when you close it, set this option to a true value.
peek(...)
This function has exactly the same syntax as sysread(), and per-
forms nearly the same task (reading data from the socket) but will
not advance the read position so that successive calls to peek()
with the same arguments will return the same results. This func-
tion requires OpenSSL 0.9.6a or later to work.
pending()
This function will let you know how many bytes of data are immedi-
ately ready for reading from the socket. This is especially handy
if you are doing reads on a blocking socket or just want to know
if new data has been sent over the socket.
get_cipher()
Returns the string form of the cipher that the IO::Socket::SSL
object is using.
dump_peer_certificate()
Returns a parsable string with select fields from the peer SSL
certificate. This method directly returns the result of the
dump_peer_certificate() method of Net::SSLeay.
peer_certificate($field)
If a peer certificate exists, this function can retrieve values
from it. If no field is given the internal representation of cer-
tificate 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 certifi-
cates.
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_subjectAlt-
Names.
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 dif-
ferent 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 are possible, e.g.
*.example.org or even www*.example.org. Wildcards in the
common name are not allowed. 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.
The scheme can be given either by specifying the name for one of
the above predefined schemes, by using a callback (see below) 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 subjectAlt-
Names (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*.exam-
ple.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.
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’).
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 sock-
ets 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 suceeded 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_ctx_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 call-
back 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.
IPv6
Support for IPv6 with IO::Socket::SSL is expected to work and basic
testing is done. If IO::Socket::INET6 is available it will
automatically use it instead of IO::Socket::INET4.
Please be aware of the associated problems: If you give a name as a
host and the host resolves to both IPv6 and IPv4 it will try IPv6
first and if there is no IPv6 connectivity it will fail.
To avoid these problems you can either force IPv4 by specifying and
AF_INET as the Domain (this is per socket) or load IO::Socket::SSL
with the option ’inet4’ (This is a global setting, e.g. affects all
IO::Socket::SSL objects in the program).
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 sub-
routines (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 mean-
ingful.
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 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. It might probably
work if you don’t use SSL_verify_callback and SSL_password_cb.
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.
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 com-
patibility:
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()
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 depreceated).
issuer_name()
use peer_certificate( ’issuer’ ) instead
subject_name()
use peer_certificate( ’subject’ ) 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-Certifi-
cates-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 trans-
ported 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. Theoreti-
cally, 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 iden-
tifying information. In this case, because of some clever number
theory, it is extremely difficult to falsify the stamping pro-
cess. Another useful consequence of number theory is that the cer-
tificate 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 2009-04-01 SSL(3)
