At times you need to gather information from a client machine before you can generate its configuration. For example, if some of your machines have both a local scratch disk and a system disk while others only have the system disk, you would want to know this information to correctly generate an /etc/auto.master autofs config file for each type. Here we will look at how to do this.

First you will need to set up the TCheetah plugin, as described on the TCheetah page.

Next, we need to create a Probes directory in our toplevel repository location:

mkdir /var/lib/bcfg2/Probes

This directory will hold any small scripts we want to use to grab information from client machines. These scripts can be in any scripting language; the shebang line (the #!/usr/bin/env some_interpreter_binary line at the very top of the script) is used to determine the script’s interpreter.


Bcfg2 uses python mkstemp to create the Probe scripts on the client. If your /tmp directory is mounted noexec, you will likely need to modify the TMPDIR environment variable so that the bcfg2 client creates the temporary files in a directory from which it can execute.

Now we need to figure out what exactly we want to do. In this case, we want to hand out an /etc/auto.master file that looks like:

/software  /etc/ --timeout 3600
/home      /etc/auto.home --timeout 3600
/hometest  /etc/auto.hometest --timeout 3600
/nfs       /etc/auto.nfs --timeout 3600
/scratch   /etc/auto.scratch --timeout 3600

for machines that have a scratch disk. For machines without an extra disk, we want to get rid of that last line:

/software  /etc/ --timeout 3600
/home      /etc/auto.home --timeout 3600
/hometest  /etc/auto.hometest --timeout 3600
/nfs       /etc/auto.nfs --timeout 3600

So, from the Probes standpoint we want to create a script that counts the number of SCSI disks in a client machine. To do this, we create a very simple Probes/scratchlocal script:

cat /proc/scsi/scsi | grep Vendor | wc -l

Running this on a node with n disks will return the number n+1, as it also counts the controller as a device. To differentiate between the two classes of machines we care about, we just need to check the output of this script for numbers greater than 2. We do this in the template.

The TCheetah/ directory is laid out much like the Cfg/ directory. For this example we will want to create a TCheetah/etc/auto.master directory to hold the template of the file in question. Inside of this template we will need to check the result of the Probe script that got run and act accordingly. The TCheetah/etc/auto.master/template file looks like:

/software  /etc/ --timeout 3600
/home      /etc/auto.home --timeout 3600
/hometest  /etc/auto.hometest --timeout 3600
/nfs       /etc/auto.nfs --timeout 3600
#if int($self.metadata.Probes["scratchlocal"]) > 2
/scratch   /etc/auto.scratch --timeout 3600
#end if

Any Probe script you run will store its output in $self.metadata.Probes["scriptname"], so we get to our scratchlocal script’s output as seen above. Note that we had to wrap the output in an int() call; the script output is treated as a string, so it needs to be converted before it can be tested numerically.

With all of these pieces in place, the following series of events will happen when the client is run:

  1. Client runs
  2. Server hands down our scratchlocal probe script
  3. Client runs the scratchlocal probe script and hands its output back up to the server
  4. Server generates /etc/auto.master from its template, performing any templating substitutions/actions needed in the process.
  5. Server hands /etc/auto.master down to the client
  6. Client puts file contents in place.

Now we have a nicely dynamic /etc/auto.master that can gracefully handle machines with different numbers of disks. All that’s left to do is to add the /etc/auto.master to a Bundle:

<Path name='/etc/auto.master'/>

Host and Group Specific probes

Bcfg2 has the ability to alter probes based on client hostname and group membership. These files work similarly to files in Cfg.

If multiple files with the same basename apply to a client, the most specific one is used. Only one instance of a probe is served to a given client, so if a host-specific version and generic version apply, only the client-specific one will be used.

Ohai probes

The Ohai plugin is used to detect information about the client operating system. The data is reported back to the server using JSON.

Client prerequisites

On the client, you need to install Ohai. See Ohai-Install for more information.

Server prerequisites

If you have python 2.6 or later installed, you can continue on to Setup. Otherwise, you will need to install the python-simplejson module found packaged in most distributions.


To enable the Ohai plugin, you need to first create an Ohai directory in your Bcfg2 repository (e.g. /var/lib/bcfg2/Ohai). You then need to add Ohai to the plugins line in bcfg2.conf. Once this is done, restart the server and start a client run. You will have the JSON output from the client in the Ohai directory you created previously.

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