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The Plug-in Skeleton

The federation distribution includes a functional skeleton that illustrates handling the interface calls though it does not manipulate any external testbed. It does illustrate most of the support for creating plug-ins, including reading configuration files, creating access control databases that map to local permission structures, storing persistent state and allocating local resources. The skeleton merely assigns an non-negative integer to each request. The range of integers is set from a configuration file.

This page describes how to set-up and run the skeleton plug-in, as well as what it does. First, install the fedd software and its dependencies. Then configure it as below.


These are the steps to configure the skeleton:

  • Create a configuration directory that's readable by the user that the plugin will run as (probably your uid)
  • make subdirectories in it called certs and userconf. The certs directory must be writeable.
  • Copy skel.conf into thie configuration directory.
  • Copy skel_access into the configuration directory
  • Create a certificate representing a fedid in this directory.
  • Copy guest.pem to your local machine. It need not be in the configuration directory.
  • Follow the steps below to edit skel.conf for your installation.

If you put the certificate you created above into fedd.pem all you need to do is change the first line to include the full pathname to your configuration directory. Something like:

base: /my/config/directory

If you named the certificate something other than fedd.pem, you need to change the line that starts with cert_file: to point to your file. This means changing the pathname. For example if the certificate was called mycert.pem the line would change to:

cert_file: %(base)s/mycert.pem

If the certificate you created has a password, that password must be included as the cert_pwd parameter. For example:

cert_pwd: mypwd

In a real installation, including such passwords in the configuration file implies that the configuration file must be secured.

The next sections explains the contents of the configuration files in more detail, but you can skip ahead to running the plug-in if you perfer.

Configuration File Contents

The skel.conf file contains three sections, each set off by a section name in braces, e.g., [access]. We will explain this file section by section.


The first section, [DEFAULT] contains parameters that will be expanded in later sections. The base parameter will be used in other pathnames, to avoid having to hard code each full pathname. In later sections,the %(base)s construction will expand to the contents of the base} parameter. This is a standard construction in a configuration file parsed by python's ConfigParser module. The value inside the %() is the variable to expand, which is followed by a single character for formatting. More information about these conventions is available.


The [access] section contains most of the plug-in configuration information. Field by field:

project_priority: false
This means that if two three level names match for the current request, give priority to the user match rather than the project match. Practically this has little effect on the skeleton.
log_level: debug
Log messages at or above log.DEBUG level will be printed to stderr or a log file.
access_state: %(base)s/skel_access.state
The persistent state of the plug-in will be stored in this file (skel_access.state in the base directory). The plug-in will create this file if it is not there and state needs to be stored.
accessdb: %(base)s/skel_access
The access database is in skel_access in the base directory. We will discuss the contents of that file below.
certdir: %(base)s/certs
This directory is used to store transient certificates. It should be readable only by the federation user.
userconfdir: %(base)s/userconf
Used to store information about exported user configurations. Again, it should be readable only by the federation user.
maxint: 3
The maximum integer that the skeleton will allocate is 3. This is not inclusive, so the skeleton will actually allocate from {0,1,2,}.

A line of the form create_debug: true is commented out. When true, that parameter generally means that the plug-in will not make allocations when requests come in. Because the skeleton only manipulates its internal state, the parameter is not used.

There are other common attributes in this section that are left as defaults in the example file, and defined elsewhere.


The two or three parameters in this section control overall operation of the plug-in, specifically what prinicpal ID (that is which fedid) it uses and what TCP port it provides services on. cert_file and cert_pwd select the X.509 certificate and encyrption password, if any, used to identify the plug-in. The services parameter controls which ports and transports are used. The sample file serves SOAP traffic on port 13230. Other choices are possible.

The access_type parameter chooses the type of access controller to run, in this case skel for the skeleton controller.

The Access Database

The access database has only one entry in it, which allows access from a user guest in the testing project project when asserted by fedid 0a778934b93d546406d195d18e4f208af11ea84a . That fedid is the one asserted by the fedd experiment controller running at DETER.

For testing plug-ins the DETER experiment controller will permit anyone connecting with the fedid encoded in the guest.pem file to request operations with that three-level name. Once your plug-in does something interesting, you should reduce or eliminate the access rights of that user.

The rest of the line gives that user access permissions and the local permission state Local_attr. This is more to show how to assign such allocations, which will be meaningful in real installations. For example, see the configurations of other plug-ins.

Running The Skeleton

To start the plug-in, assuming that the configuration files are in /usr/local/etc/skel run the command:

$ --config=/usr/local/etc/skel.conf --debug

This assumes you have properly installed fedd and that is in your path. You will see a few log messages indicating that the access and state databases have been read or are not yet created.

The easiest way to see the skeleton operate is to access it through the DETER experiment controller. To do that we need two things, permission to use an experiment controller and a way to tell that experiment controller where our skeleton is. Permission is handled by using the guest fedid certificate on the DETER experiment controller. In order to tell that experiment controller where to find our skeleton, we use the --map parameter to

Running a command like the following should cause DETER's experiment controller to start an experiment on the running skeleton:

$ ./ --cert=./guest.pem --url= --experiment_name=faber-test --file=./skel-only.tcl --map=skeleton:

The --cert parameter must point to your local copy of guest.pem, you can pick any --experiment_name you like, though names prefixed by you suername are less likely to collide with the names chosen by others, --file must point to an experiment description containing nodes destined for testbed "skeleton", again a local copy of skel-only.tcl is a good choice. Finally the --map parameter needs to map the "skeleton" testbed into your running skeleton's URI. The hostname part should be a globally reachable IP address or DNS name on which your controller is running and the port part should be the value in the services parameter in the [globals] section of the configuration file. If you are using the example file, the 13230 port is correct.

When you download the experiment file and the certificate, be sure to use the "Original format" link on the bottom of the page.

The command has run, you should see output similar to:

Warning:Neither master/project nor services requested
localname: faber-test
fedid: b3af52e09614b7f4731361ae82c8d66dcf50ef12
status: starting

You needn't worry about the warning. The localname is the string by which you can access the experiment. If there was another experiment running on the DETER controller with the same name as you requested in the --experiment_name parameter, you would need to use the one returned. If you miss the name here, you can always retrieve it using the command.

In the window running the skeleton, you should see output similar to:

28 Jun 10 09:32:50 fedd.access [read_state]: No saved state: Can't open /users/faber/fedd-config/skel/skel_access.state: [Errno 2] No such file or directory: '/users/faber/fedd-config/skel/skel_access.state'
28 Jun 10 09:33:02 fedd.access [lookup_access] Checking access for (fedid(hexstr='b55205ac843c40ce9c9feb3b358bff782ed337fd'), u'testing', u'guest')
28 Jun 10 09:33:02 fedd.access [lookup_access] Access granted
28 Jun 10 09:33:02 fedd.access [RequestAccess] Access granted to (fedid(hexstr='b55205ac843c40ce9c9feb3b358bff782ed337fd'), u'testing', u'guest') with local creds Local_attr
28 Jun 10 09:33:02 fedd.access [generate_fedid] /usr/bin/openssl req -text -newkey rsa:2048 -keyout /tmp/key-gY474.pem -nodes -subj /CN=alloc -x509 -days 30 -out /tmp/certAJ3SiY.pem
28 Jun 10 09:33:02 fedd.access rv = 0
28 Jun 10 09:33:02 fedd.access Wrote state to /users/faber/fedd-config/skel/skel_access.state
28 Jun 10 09:33:02 fedd.access [RequestAccess] Returning allocation ID: 32b21906165aa76bbc629511b81b7504d01a0f20
28 Jun 10 09:33:02 fedd Successful SOAP request code 200
28 Jun 10 09:33:03 fedd.access Wrote state to /users/faber/fedd-config/skel/skel_access.state
28 Jun 10 09:33:03 fedd.access [StartSegment] Allocated 0 to 32b21906165aa76bbc629511b81b7504d01a0f20
28 Jun 10 09:33:03 fedd.access Wrote state to /users/faber/fedd-config/skel/skel_access.state
28 Jun 10 09:33:03 fedd Successful SOAP request code 200

That's the output from a successful experiment creation on the skeleton. Notice the request for access from (b55205ac843c40ce9c9feb3b358bff782ed337fd, 'testing', 'guest') that is approved, the creation of the access principal and the saving of state all before the first Successful SOAP request code 200 line. That is all output from a successful RequestAccess call, and you can trace the RequestAccess method in federation/

You'll also notice that the plug-in state file has been created, it is /users/faber/fedd-config/skel/skel_access.state in the example.

The rest of the output is the successful StartSegment call.

To see the state of your experiment do the following. Make sure to give the --cert option. Without it the commands will either fail, or if you have a certificate in a standard emulab set-up, return confusing results.

$ ./ --cert=./guest.pem --url=

Which outputs:


The first two fields are the readable name and the fedid of the experiment, which will match the output from not the ones in this document. The command only queries the experiment controller, so no changes to the skeleton log will be happen.

Removing the experiment will exercise the StopSegment and ReleaseAccess methods of the skeleton. You can do this by running the command (again, use --cert and the experiment name returned by and above):

$ ./ --cert=./guest.pem --url= --experiment_name=faber-test

This will not produce any output (on success) though the log on your skeleton controller should now include lines like:

28 Jun 10 09:39:42 fedd.access Terminate request for 32b21906165aa76bbc629511b81b7504d01a0f20
28 Jun 10 09:39:42 fedd.access Wrote state to /users/faber/fedd-config/skel/skel_access.state
28 Jun 10 09:39:42 fedd Successful SOAP request code 200
28 Jun 10 09:39:42 fedd.access [ReleaseAccess] deallocation requested for 32b21906165aa76bbc629511b81b7504d01a0f20
28 Jun 10 09:39:42 fedd.access [ReleaseAccess] Found allocation for 32b21906165aa76bbc629511b81b7504d01a0f20
28 Jun 10 09:39:42 fedd.access Wrote state to /users/faber/fedd-config/skel/skel_access.state
28 Jun 10 09:39:42 fedd.access [ReleaseAccess] Removing /users/faber/fedd-config/skel/certs/32b21906165aa76bbc629511b81b7504d01a0f20.pem
28 Jun 10 09:39:42 fedd Successful SOAP request code 200

That shows how to run the skeleton and exercise its interfaces with well formed commands from the experiment controller. You can now begin customizing your plug-in. The first step in that process is making a dynamically loadable plug-in.

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