transfer::connect - Connection setup
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.
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.
All objects created by the ::transfer::connect command have the following general form:
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.
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.
The connection setup is done synchronously. The object waits until the connection is established. The method returns the empty string as its result.
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.
Connection objects support the set of options listed below.
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 ... ...
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 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 ...
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. Please also report any ideas for enhancements you may have for either package and/or documentation.