Provided by: tcllib_1.19-dfsg-2_all 
NAME
transfer::connect - Connection setup
SYNOPSIS
package require Tcl 8.4
package require snit ?1.0?
package require transfer::connect ?0.2?
transfer::connect objectName ?options...?
objectName method ?arg arg ...?
objectName destroy
objectName connect command
_________________________________________________________________________________________________
DESCRIPTION
This package provides objects holding enough information to enable them to either actively
connect to a counterpart, or to passively wait for a connection from said counterpart.
I.e. any object created by this packages is always in one of two complementary modes,
called active (the object initiates the connection) and passive (the object receives the
connection).
Of the two objects in a connecting pair one has to be configured for active mode, and the
other then has to be configured for passive mode. This establishes which of the two
partners connects to whom (the active to the other), or, who is waiting on whom (the
passive on the other). Note that this is completely independent of the direction of any
data transmission using the connection after it has been established. An active object
can, after establishing the connection, either transmit or receive data. Equivalently the
passive object can do the same after the waiting for its partner has ended.
API
PACKAGE COMMANDS
transfer::connect objectName ?options...?
This command creates a new connection object with an associated Tcl command whose
name is objectName. This object command is explained in full detail in the
sections Object command and Object methods. The set of supported options is
explained in section Options.
The object command will be created under the current namespace if the objectName is
not fully qualified, and in the specified namespace otherwise. The fully qualified
name of the object command is returned as the result of the command.
OBJECT COMMAND
All objects created by the ::transfer::connect command have the following general form:
objectName method ?arg arg ...?
The method method and its arg'uments determine the exact behavior of the command.
See section Object methods for the detailed specifications.
OBJECT METHODS
objectName destroy
This method destroys the object. This is safe to do for an active object when a
connection has been started, as the completion callback is synchronous. For a
passive object currently waiting for its partner to establish the connection
however this is not safe and will cause errors later on, when the connection setup
completes and tries to access the now missing data structures of the destroyed
object.
objectName connect command
This method starts the connection setup per the configuration of the object. When
the connection is established the callback command will be invoked with one
additional argument, the channel handle of the socket over which data can be
transfered.
The detailed behaviour of the method depends on the configured mode.
active The connection setup is done synchronously. The object waits until the
connection is established. The method returns the empty string as its
result.
passive
The connection setup is done asynchronously. The method returns immediately
after a listening socket has been set up. The connection will be established
in the background. The method returns the port number of the listening
socket, for use by the caller. One important use is the transfer of this
information to the counterpart so that it knows where it has to connect to.
This is necessary as the object might have been configured for port 0,
allowing the operating system to choose the actual port it will listen on.
The listening port is closed immediately when the connection was established
by the partner, to keep the time interval small within which a third party
can connect to the port too. Even so it is recommended to use additional
measures in the protocol outside of the connect and transfer object to
ensure that a connection is not used with an unidentified/unauthorized
partner One possibility for this is the use of SSL/TLS. See the option
-socketcmd and section Secure connections for information on how to do this.
OPTIONS
Connection objects support the set of options listed below.
-mode mode
This option specifies the mode the object is in. It is optional and defaults to
active mode. The two possible modes are:
active In this mode the two options -host and -port are relevant and specify the
host and TCP port the object has to connect to. The host is given by either
name or IP address.
passive
In this mode the option -host has no relevance and is ignored should it be
configured. The only option the object needs is -port, and it specifies the
TCP port on which the listening socket is opened to await the connection
from the partner.
-host hostname-or-ipaddr
This option specifies the host to connect to in active mode, either by name or ip-
address. An object configured for passive mode ignores this option.
-port int
For active mode this option specifies the port the object is expected to connect
to. For passive mode however it is the port where the object creates the listening
socket waiting for a connection. It defaults to 0, which allows the OS to choose
the actual port to listen on.
-socketcmd command
This option allows the user to specify which command to use to open a socket. The
default is to use the builtin ::socket. Any compatible with that command is
allowed.
The envisioned main use is the specfication of tls::socket. I.e. this option
allows the creation of secure transfer channels, without making this package
explicitly dependent on the tls package.
See also section Secure connections.
-encoding encodingname
-eofchar eofspec
-translation transspec
These options are the same as are recognized by the builtin command fconfigure.
They provide the configuration to be set for the channel between the two partners
after it has been established, but before the callback is invoked (See method
connect).
SECURE CONNECTIONS
One way to secure connections made by objects of this package is to require the package
tls and then configure the option -socketcmd to force the use of command tls::socket to
open the socket.
# Load and initialize tls
package require tls
tls::init -cafile /path/to/ca/cert -keyfile ...
# Create a connector with secure socket setup,
transfer::connect C -socketcmd tls::socket ...
...
TLS SECURITY CONSIDERATIONS
This package uses the TLS package to handle the security for https urls and other socket
connections.
Policy decisions like the set of protocols to support and what ciphers to use are not the
responsibility of TLS, nor of this package itself however. Such decisions are the
responsibility of whichever application is using the package, and are likely influenced by
the set of servers the application will talk to as well.
For example, in light of the recent POODLE attack
[http://googleonlinesecurity.blogspot.co.uk/2014/10/this-poodle-bites-exploiting-
ssl-30.html] discovered by Google many servers will disable support for the SSLv3
protocol. To handle this change the applications using TLS must be patched, and not this
package, nor TLS itself. Such a patch may be as simple as generally activating tls1
support, as shown in the example below.
package require tls
tls::init -tls1 1 ;# forcibly activate support for the TLS1 protocol
... your own application code ...
BUGS, IDEAS, FEEDBACK
This document, and the package it describes, will undoubtedly contain bugs and other
problems. Please report such in the category transfer of the Tcllib Trackers
[http://core.tcl.tk/tcllib/reportlist]. Please also report any ideas for enhancements you
may have for either package and/or documentation.
When proposing code changes, please provide unified diffs, i.e the output of diff -u.
Note further that attachments are strongly preferred over inlined patches. Attachments can
be made by going to the Edit form of the ticket immediately after its creation, and then
using the left-most button in the secondary navigation bar.
KEYWORDS
active, channel, connection, passive, secure, ssl, tls, transfer
CATEGORY
Transfer module
COPYRIGHT
Copyright (c) 2006-2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>