Manual Reference Pages  - SSL (3)


IO::Socket::SSL -- Nearly transparent SSL encapsulation for IO::Socket::INET.



    use IO::Socket::SSL;

    my $client = IO::Socket::SSL->new("");

    if ($client) {
        print $client "GET / HTTP/1.0\r\n\r\n";
        print <$client>;
        close $client;
    } else {
        warn "I encountered a problem: ",


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.


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 (<>) 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. $socket->close(SSL_no_shutdown => 1)) and one or more of the following parameters:
SSL_no_shutdown If set to a true value, this option will make close() not use the SSL_shutdown() call on the socket in question so that the close operation can complete without problems if you have used shutdown() or are working on a copy of a socket.
SSL_ctx_free If you want to make sure that the SSL context of the socket is destroyed when you close it, set this option to a true value.
peek(...) This function has exactly the same syntax as sysread(), and performs nearly the same task (reading data from the socket) but will not advance the read position so that successive calls to peek() with the same arguments will return the same results. This function requires OpenSSL 0.9.6a or later to work.
pending() This function will let you know how many bytes of data are immediately ready for reading from the socket. This is especially handy if you are doing reads on a blocking socket or just want to know if new data has been sent over the socket.
get_cipher() Returns the string form of the cipher that the IO::Socket::SSL object is using.
dump_peer_certificate() Returns a parsable string with select fields from the peer SSL certificate. This method directly returns the result of the dump_peer_certificate() method of Net::SSLeay.
peer_certificate($field) If a peer certificate exists, this function can retrieve values from it. 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).
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.

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:
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.


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.


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.


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?


See the ’example’ directory.


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.


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 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() 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)


IO::Socket::INET, IO::Socket::INET6, 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.


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 <>. 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 <>.

perl v5.8.8 SSL (3) 2007-01-21
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