Using the CORE GUI

The following image shows the CORE GUI:


CORE can be used via the GUI or Python_Scripting. Often the GUI is used to draw nodes and network devices on the canvas. A Python script could also be written, that imports the CORE Python module, to configure and instantiate nodes and networks. This chapter primarily covers usage of the CORE GUI.

The following image shows the various phases of a CORE session:

After pressing the start button, CORE will proceed through these phases, staying in the runtime phase. After the session is stopped, CORE will proceed to the data collection phase before tearing down the emulated state.

CORE can be customized to perform any action at each phase in the workflow above. See the Hooks… entry on the Session Menu for details about when these session states are reached.

Note: The CORE GUI is currently in a state of transition. The replacement candidate is currently in an alpha version, and can be found here.


Beyond installing CORE, you must have the CORE daemon running. This is done on the command line with either Systemd or SysV

# systed
sudo systemctl daemon-reload
sudo systemctl start core-daemon

# sysv
sudo service core-daemon start

You can also invoke the daemon directly from the command line, which can be useful if you’d like to see the logging output directly.

# direct invocation
sudo core-daemon

Modes of Operation

The CORE GUI has two primary modes of operation, Edit and Execute modes. Running the GUI, by typing core-gui with no options, starts in Edit mode. Nodes are drawn on a blank canvas using the toolbar on the left and configured from right-click menus or by double-clicking them. The GUI does not need to be run as root.

Once editing is complete, pressing the green Start button (or choosing Execute from the Session menu) instantiates the topology within the Linux kernel and enters Execute mode. In execute mode, the user can interact with the running emulated machines by double-clicking or right-clicking on them. The editing toolbar disappears and is replaced by an execute toolbar, which provides tools while running the emulation. Pressing the red Stop button (or choosing Terminate from the Session menu) will destroy the running emulation and return CORE to Edit mode.

CORE can be started directly in Execute mode by specifying –start and a topology file on the command line:

core-gui --start ~/.core/configs/myfile.imn

Once the emulation is running, the GUI can be closed, and a prompt will appear asking if the emulation should be terminated. The emulation may be left running and the GUI can reconnect to an existing session at a later time.

The GUI can be run as a normal user on Linux.

The GUI can be connected to a different address or TCP port using the –address and/or –port options. The defaults are shown below.

core-gui --address --port 4038


The toolbar is a row of buttons that runs vertically along the left side of the CORE GUI window. The toolbar changes depending on the mode of operation.

Editing Toolbar

When CORE is in Edit mode (the default), the vertical Editing Toolbar exists on the left side of the CORE window. Below are brief descriptions for each toolbar item, starting from the top. Most of the tools are grouped into related sub-menus, which appear when you click on their group icon.

Execution Toolbar

When the Start button is pressed, CORE switches to Execute mode, and the Edit toolbar on the left of the CORE window is replaced with the Execution toolbar Below are the items on this toolbar, starting from the top.

The menubar runs along the top of the CORE GUI window and provides access to a variety of features. Some of the menus are detachable, such as the Widgets menu, by clicking the dashed line at the top.

File Menu

The File menu contains options for manipulating the .imn Configuration Files. Generally, these menu items should not be used in Execute mode.

Edit Menu

Canvas Menu

The canvas menu provides commands for adding, removing, changing, and switching to different editing canvases.

View Menu

The View menu features items for controlling what is displayed on the drawing canvas.

Tools Menu

The tools menu lists different utility functions.

Widgets Menu

Widgets are GUI elements that allow interaction with a running emulation. Widgets typically automate the running of commands on emulated nodes to report status information of some type and display this on screen.

Periodic Widgets

These Widgets are those available from the main Widgets menu. More than one of these Widgets may be run concurrently. An event loop fires once every second that the emulation is running. If one of these Widgets is enabled, its periodic routine will be invoked at this time. Each Widget may have a configuration dialog box which is also accessible from the Widgets menu.

Here are some standard widgets:

Observer Widgets

These Widgets are available from the Observer Widgets submenu of the Widgets menu, and from the Widgets Tool on the toolbar. Only one Observer Widget may be used at a time. Mouse over a node while the session is running to pop up an informational display about that node.

Available Observer Widgets include IPv4 and IPv6 routing tables, socket information, list of running processes, and OSPFv2/v3 neighbor information.

Observer Widgets may be edited by the user and rearranged. Choosing Edit… from the Observer Widget menu will invoke the Observer Widgets dialog. A list of Observer Widgets is displayed along with up and down arrows for rearranging the list. Controls are available for renaming each widget, for changing the command that is run during mouse over, and for adding and deleting items from the list. Note that specified commands should return immediately to avoid delays in the GUI display. Changes are saved to a widgets.conf file in the CORE configuration directory.

Session Menu

The Session Menu has entries for starting, stopping, and managing sessions, in addition to global options such as node types, comments, hooks, servers, and options.

Help Menu

Connecting with Physical Networks

CORE’s emulated networks run in real time, so they can be connected to live physical networks. The RJ45 tool and the Tunnel tool help with connecting to the real world. These tools are available from the Link-layer nodes menu.

When connecting two or more CORE emulations together, MAC address collisions should be avoided. CORE automatically assigns MAC addresses to interfaces when the emulation is started, starting with 00:00:00:aa:00:00 and incrementing the bottom byte. The starting byte should be changed on the second CORE machine using the MAC addresses… option from the Tools menu.

RJ45 Tool

The RJ45 node in CORE represents a physical interface on the real CORE machine. Any real-world network device can be connected to the interface and communicate with the CORE nodes in real time.

The main drawback is that one physical interface is required for each connection. When the physical interface is assigned to CORE, it may not be used for anything else. Another consideration is that the computer or network that you are connecting to must be co-located with the CORE machine.

To place an RJ45 connection, click on the Link-layer nodes toolbar and select the RJ45 Tool from the submenu. Click on the canvas near the node you want to connect to. This could be a router, hub, switch, or WLAN, for example. Now click on the Link Tool and draw a link between the RJ45 and the other node. The RJ45 node will display “UNASSIGNED”. Double-click the RJ45 node to assign a physical interface. A list of available interfaces will be shown, and one may be selected by double-clicking its name in the list, or an interface name may be entered into the text box.

NOTE: When you press the Start button to instantiate your topology, the interface assigned to the RJ45 will be connected to the CORE topology. The interface can no longer be used by the system. For example, if there was an IP address assigned to the physical interface before execution, the address will be removed and control given over to CORE. No IP address is needed; the interface is put into promiscuous mode so it will receive all packets and send them into the emulated world.

Multiple RJ45 nodes can be used within CORE and assigned to the same physical interface if 802.1x VLANs are used. This allows for more RJ45 nodes than physical ports are available, but the (e.g. switching) hardware connected to the physical port must support the VLAN tagging, and the available bandwidth will be shared.

You need to create separate VLAN virtual devices on the Linux host, and then assign these devices to RJ45 nodes inside of CORE. The VLANning is actually performed outside of CORE, so when the CORE emulated node receives a packet, the VLAN tag will already be removed.

Here are example commands for creating VLAN devices under Linux:

ip link add link eth0 name eth0.1 type vlan id 1
ip link add link eth0 name eth0.2 type vlan id 2
ip link add link eth0 name eth0.3 type vlan id 3

Tunnel Tool

The tunnel tool builds GRE tunnels between CORE emulations or other hosts. Tunneling can be helpful when the number of physical interfaces is limited or when the peer is located on a different network. Also a physical interface does not need to be dedicated to CORE as with the RJ45 tool.

The peer GRE tunnel endpoint may be another CORE machine or another host that supports GRE tunneling. When placing a Tunnel node, initially the node will display “UNASSIGNED”. This text should be replaced with the IP address of the tunnel peer. This is the IP address of the other CORE machine or physical machine, not an IP address of another virtual node.

NOTE: Be aware of possible MTU (Maximum Transmission Unit) issues with GRE devices. The gretap device has an interface MTU of 1,458 bytes; when joined to a Linux bridge, the bridge’s MTU becomes 1,458 bytes. The Linux bridge will not perform fragmentation for large packets if other bridge ports have a higher MTU such as 1,500 bytes.

The GRE key is used to identify flows with GRE tunneling. This allows multiple GRE tunnels to exist between that same pair of tunnel peers. A unique number should be used when multiple tunnels are used with the same peer. When configuring the peer side of the tunnel, ensure that the matching keys are used.

Here are example commands for building the other end of a tunnel on a Linux machine. In this example, a router in CORE has the virtual address and the CORE host machine has the (real) address The Linux box that will connect with the CORE machine is reachable over the (real) network at The emulated router is linked with the Tunnel Node. In the Tunnel Node configuration dialog, the address is entered, with the key set to 1. The gretap interface on the Linux box will be assigned an address from the subnet of the virtual router node,

# these commands are run on the tunnel peer
sudo ip link add gt0 type gretap remote local key 1
sudo ip addr add dev gt0
sudo ip link set dev gt0 up

Now the virtual router should be able to ping the Linux machine:

# from the CORE router node

And the Linux machine should be able to ping inside the CORE emulation:

# from the tunnel peer

To debug this configuration, tcpdump can be run on the gretap devices, or on the physical interfaces on the CORE or Linux machines. Make sure that a firewall is not blocking the GRE traffic.

Communicating with the Host Machine

The host machine that runs the CORE GUI and/or daemon is not necessarily accessible from a node. Running an X11 application on a node, for example, requires some channel of communication for the application to connect with the X server for graphical display. There are several different ways to connect from the node to the host and vice versa.

Control Network

The quickest way to connect with the host machine through the primary control network.

With a control network, the host can launch an X11 application on a node. To run an X11 application on the node, the SSH service can be enabled on the node, and SSH with X11 forwarding can be used from the host to the node:

# SSH from host to node n5 to run an X11 app
ssh -X xclock

Note that the coresendmsg utility can be used for a node to send messages to the CORE daemon running on the host (if the listenaddr = is set in the /etc/core/core.conf file) to interact with the running emulation. For example, a node may move itself or other nodes, or change its icon based on some node state.

Other Methods

There are still other ways to connect a host with a node. The RJ45 Tool can be used in conjunction with a dummy interface to access a node:

sudo modprobe dummy numdummies=1

A dummy0 interface should appear on the host. Use the RJ45 tool assigned to dummy0, and link this to a node in your scenario. After starting the session, configure an address on the host.

sudo brctl show
# determine bridge name from the above command
# assign an IP address on the same network as the linked node
sudo ip addr add dev b.48304.34658

In the example shown above, the host will have the address and the node linked to the RJ45 may have the address

Building Sample Networks

Wired Networks

Wired networks are created using the Link Tool to draw a link between two nodes. This automatically draws a red line representing an Ethernet link and creates new interfaces on network-layer nodes.

Double-click on the link to invoke the link configuration dialog box. Here you can change the Bandwidth, Delay, Loss, and Duplicate rate parameters for that link. You can also modify the color and width of the link, affecting its display.

Link-layer nodes are provided for modeling wired networks. These do not create a separate network stack when instantiated, but are implemented using Linux bridging. These are the hub, switch, and wireless LAN nodes. The hub copies each packet from the incoming link to every connected link, while the switch behaves more like an Ethernet switch and keeps track of the Ethernet address of the connected peer, forwarding unicast traffic only to the appropriate ports.

The wireless LAN (WLAN) is covered in the next section.

Wireless Networks

The wireless LAN node allows you to build wireless networks where moving nodes around affects the connectivity between them. The wireless LAN, or WLAN, node appears as a small cloud. The WLAN offers several levels of wireless emulation fidelity, depending on your modeling needs.

The WLAN tool can be extended with plug-ins for different levels of wireless fidelity. The basic on/off range is the default setting available on all platforms. Other plug-ins offer higher fidelity at the expense of greater complexity and CPU usage. The availability of certain plug-ins varies depending on platform. See the table below for a brief overview of wireless model types.

Model Type Supported Platform(s) Fidelity Description  
  Basic on/off Linux Low Ethernet bridging with ebtables
  EMANE Plug-in Linux High TAP device connected to EMANE emulator with pluggable MAC and PHY radio types

To quickly build a wireless network, you can first place several router nodes onto the canvas. If you have the Quagga MDR software installed, it is recommended that you use the mdr node type for reduced routing overhead. Next choose the wireless LAN from the Link-layer nodes submenu. First set the desired WLAN parameters by double-clicking the cloud icon. Then you can link all of the routers by right-clicking on the WLAN and choosing Link to all routers.

Linking a router to the WLAN causes a small antenna to appear, but no red link line is drawn. Routers can have multiple wireless links and both wireless and wired links (however, you will need to manually configure route redistribution.) The mdr node type will generate a routing configuration that enables OSPFv3 with MANET extensions. This is a Boeing-developed extension to Quagga’s OSPFv3 that reduces flooding overhead and optimizes the flooding procedure for mobile ad-hoc (MANET) networks.

The default configuration of the WLAN is set to use the basic range model, using the Basic tab in the WLAN configuration dialog. Having this model selected causes core-daemon to calculate the distance between nodes based on screen pixels. A numeric range in screen pixels is set for the wireless network using the Range slider. When two wireless nodes are within range of each other, a green line is drawn between them and they are linked. Two wireless nodes that are farther than the range pixels apart are not linked. During Execute mode, users may move wireless nodes around by clicking and dragging them, and wireless links will be dynamically made or broken.

The EMANE tab lists available EMANE models to use for wireless networking. See the EMANE chapter for details on using EMANE.

Mobility Scripting

CORE has a few ways to script mobility.

For the first method, you can create a mobility script using a text editor, or using a tool such as BonnMotion, and associate the script with one of the wireless using the WLAN configuration dialog box. Click the ns-2 mobility script… button, and set the mobility script file field in the resulting ns2script configuration dialog.

Here is an example for creating a BonnMotion script for 10 nodes:

bm -f sample RandomWaypoint -n 10 -d 60 -x 1000 -y 750
bm NSFile -f sample
# use the resulting 'sample.ns_movements' file in CORE

When the Execute mode is started and one of the WLAN nodes has a mobility script, a mobility script window will appear. This window contains controls for starting, stopping, and resetting the running time for the mobility script. The loop checkbox causes the script to play continuously. The resolution text box contains the number of milliseconds between each timer event; lower values cause the mobility to appear smoother but consumes greater CPU time.

The format of an ns-2 mobility script looks like:

# nodes: 3, max time: 35.000000, max x: 600.00, max y: 600.00
$node_(2) set X_ 144.0
$node_(2) set Y_ 240.0
$node_(2) set Z_ 0.00
$ns_ at 1.00 "$node_(2) setdest 130.0 280.0 15.0"

The first three lines set an initial position for node 2. The last line in the above example causes node 2 to move towards the destination (130, 280) at speed 15. All units are screen coordinates, with speed in units per second. The total script time is learned after all nodes have reached their waypoints. Initially, the time slider in the mobility script dialog will not be accurate.

Examples mobility scripts (and their associated topology files) can be found in the configs/ directory.

Multiple Canvases

CORE supports multiple canvases for organizing emulated nodes. Nodes running on different canvases may be linked together.

To create a new canvas, choose New from the Canvas menu. A new canvas tab appears in the bottom left corner. Clicking on a canvas tab switches to that canvas. Double-click on one of the tabs to invoke the Manage Canvases dialog box. Here, canvases may be renamed and reordered, and you can easily switch to one of the canvases by selecting it.

Each canvas maintains its own set of nodes and annotations. To link between canvases, select a node and right-click on it, choose Create link to, choose the target canvas from the list, and from that submenu the desired node. A pseudo-link will be drawn, representing the link between the two nodes on different canvases. Double-clicking on the label at the end of the arrow will jump to the canvas that it links.

Check Emulation Light

The |cel| Check Emulation Light, or CEL, is located in the bottom right-hand corner of the status bar in the CORE GUI. This is a yellow icon that indicates one or more problems with the running emulation. Clicking on the CEL will invoke the CEL dialog.

The Check Emulation Light dialog contains a list of exceptions received from the CORE daemon. An exception has a time, severity level, optional node number, and source. When the CEL is blinking, this indicates one or more fatal exceptions. An exception with a fatal severity level indicates that one or more of the basic pieces of emulation could not be created, such as failure to create a bridge or namespace, or the failure to launch EMANE processes for an EMANE-based network.

Clicking on an exception displays details for that exception. If a node number is specified, that node is highlighted on the canvas when the exception is selected. The exception source is a text string to help trace where the exception occurred; “service:UserDefined” for example, would appear for a failed validation command with the UserDefined service.

Buttons are available at the bottom of the dialog for clearing the exception list and for viewing the CORE daemon and node log files.

NOTE: In batch mode, exceptions received from the CORE daemon are displayed on the console.

Configuration Files

Configurations are saved to xml or .imn topology files using the File menu. You can easily edit these files with a text editor. Any time you edit the topology file, you will need to stop the emulation if it were running and reload the file.

The .xml file schema is specified by NRL and there are two versions to date: version 0.0 and version 1.0, with 1.0 as the current default. CORE can open either XML version. However, the xmlfilever line in /etc/core/core.conf controls the version of the XML file that CORE will create.

In version 1.0, the XML file is also referred to as the Scenario Plan. The Scenario Plan will be logically made up of the following:

The .imn file format comes from IMUNES, and is basically Tcl lists of nodes, links, etc. Tabs and spacing in the topology files are important. The file starts by listing every node, then links, annotations, canvases, and options. Each entity has a block contained in braces. The first block is indented by four spaces. Within the network-config block (and any custom--config* block), the indentation is one tab character.

TIP: There are several topology examples included with CORE in the configs/ directory. This directory can be found in ~/.core/configs, or installed to the filesystem under /usr[/local]/share/examples/configs.

TIP: When using the .imn file format, file paths for things like custom icons may contain the special variables $CORE_DATA_DIR or $CONFDIR which will be substituted with /usr/share/core or ~/.core/configs.

TIP: Feel free to edit the files directly using your favorite text editor.

Customizing your Topology’s Look

Several annotation tools are provided for changing the way your topology is presented. Captions may be added with the Text tool. Ovals and rectangles may be drawn in the background, helpful for visually grouping nodes together.

During live demonstrations the marker tool may be helpful for drawing temporary annotations on the canvas that may be quickly erased. A size and color palette appears at the bottom of the toolbar when the marker tool is selected. Markings are only temporary and are not saved in the topology file.

The basic node icons can be replaced with a custom image of your choice. Icons appear best when they use the GIF or PNG format with a transparent background. To change a node’s icon, double-click the node to invoke its configuration dialog and click on the button to the right of the node name that shows the node’s current icon.

A background image for the canvas may be set using the Wallpaper… option from the Canvas menu. The image may be centered, tiled, or scaled to fit the canvas size. An existing terrain, map, or network diagram could be used as a background, for example, with CORE nodes drawn on top.


The Preferences Dialog can be accessed from the Edit_Menu. There are numerous defaults that can be set with this dialog, which are stored in the ~/.core/prefs.conf preferences file.