4. Python ScriptingΒΆ

CORE can be used via the GUI or Python scripting. Writing your own Python scripts offers a rich programming environment with complete control over all aspects of the emulation. This chapter provides a brief introduction to scripting. Most of the documentation is available from sample scripts, or online via interactive Python.

The best starting point is the sample scripts that are included with CORE. If you have a CORE source tree, the example script files can be found under core/daemon/examples/netns/. When CORE is installed from packages, the example script files will be in /usr/share/core/examples/netns/ (or the /usr/local/... prefix when installed from source.) For the most part, the example scripts are self-documenting; see the comments contained within the Python code.

The scripts should be run with root privileges because they create new network namespaces. In general, a CORE Python script does not connect to the CORE daemon, core-daemon; in fact, core-daemon is just another Python script that uses the CORE Python modules and exchanges messages with the GUI. To connect the GUI to your scripts, see the included sample scripts that allow for GUI connections.

Here are the basic elements of a CORE Python script:

#!/usr/bin/python

from core import pycore

session = pycore.Session(persistent=True)
node1 = session.addobj(cls=pycore.nodes.CoreNode, name="n1")
node2 = session.addobj(cls=pycore.nodes.CoreNode, name="n2")
hub1 = session.addobj(cls=pycore.nodes.HubNode, name="hub1")
node1.newnetif(hub1, ["10.0.0.1/24"])
node2.newnetif(hub1, ["10.0.0.2/24"])

node1.icmd(["ping", "-c", "5", "10.0.0.2"])
session.shutdown()

The above script creates a CORE session having two nodes connected with a hub. The first node pings the second node with 5 ping packets; the result is displayed on screen.

A good way to learn about the CORE Python modules is via interactive Python. Scripts can be run using python -i. Cut and paste the simple script above and you will have two nodes connected by a hub, with one node running a test ping to the other.

The CORE Python modules are documented with comments in the code. From an interactive Python shell, you can retrieve online help about the various classes and methods; for example help(pycore.nodes.CoreNode) or help(pycore.Session).

An interactive development environment (IDE) is available for browsing the CORE source, the Eric Python IDE. CORE has a project file that can be opened by Eric, in the source under core/daemon/CORE.e4p. This IDE has a class browser for viewing a tree of classes and methods. It features syntax highlighting, auto-completion, indenting, and more. One feature that is helpful with learning the CORE Python modules is the ability to generate class diagrams; right-click on a class, choose Diagrams, and Class Diagram.

Note

The CORE daemon core-daemon manages a list of sessions and allows the GUI to connect and control sessions. Your Python script uses the same CORE modules but runs independently of the daemon. The daemon does not need to be running for your script to work.

The session created by a Python script may be viewed in the GUI if certain steps are followed. The GUI has a File Menu, Execute Python script... option for running a script and automatically connecting to it. Once connected, normal GUI interaction is possible, such as moving and double-clicking nodes, activating Widgets, etc.

The script should have a line such as the following for running it from the GUI.

if __name__ == "__main__" or __name__ == "__builtin__":
    main()

Also, the script should add its session to the session list after creating it. A global server variable is exposed to the script pointing to the CoreServer object in the core-daemon.

def add_to_server(session):
    ''' Add this session to the server's list if this script is executed from
    the core-daemon server.
    '''
    global server
    try:
        server.addsession(session)
        return True
    except NameError:
        return False
session = pycore.Session(persistent=True)
add_to_server(session)

Finally, nodes and networks need to have their coordinates set to something, otherwise they will be grouped at the coordinates <0, 0>. First sketching the topology in the GUI and then using the Export Python script option may help here.

switch.setposition(x=80,y=50)

A fully-worked example script that you can launch from the GUI is available in the file switch.py in the examples directory.

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