Templates From Hell

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Templates in cfengine 2

Here is what I've cf-written to handle templates with cfengine version 2. This configuration has three goals:

• Centralize templates in a configuration repository,
• Only fetch a template if it's modified on the repository,
• Keep the final configuration file (made from the template) from direct editing.

A template, in this article, is a configuration file in which we replaced keywords, that are host-specific, with tags. Cfagent will replace automatically tags with the right keyword.

We'll use the following cfagent actions: copy and editfiles. No external script is needed.

The idea

The main idea is to use a "temporary" directory, used by cfagent to store the edited template file, and maintain the destination file. cfagent will follow this procedure:

1. Copy the template file from the repository to a "templates" directory by checking the modification time difference,
2. If a copy occured, edit the template file in the "templates" directory,
3. Locally copy the edited template to the final destination, with checksum check.

The code

First, the control section.

control: 

  # Copy from repository, edit templates, copy edited templates
  actionsequence = ( copy editfiles copy.templates )

  # Configuration files may be big.
  EditfileSize = ( 40000 )

  # This class will be defined by the first copy
  AddInstallable = ( copiedTemplates ) 

  # The repository's address
  reposerver = ( 192.168.1.1 )

The copy action might look like this, path are examples:

copy: 

  
  # First copy pass - don't do anything if we locally copy templates
  !templates::

  /repository/postfix.templates
  dest=/var/cfengine/templates/postfix
  server=${reposerver}
  recurse=2
  action=fix
  type=mtime
  define=copiedTemplates

  # Second copy pass - check if the destination conf file has been altered
  templates::

  /var/cfengine/templates/postfix
  dest=/etc/postfix
  recurse=2
  action=fix
  type=checksum

The editfiles action:

editfiles: 

  
  # Edit only if a new template arrived
  copiedTemplates::
  { /var/cfengine/templates

    Recurse "inf"
    ReplaceAll "_TAG_HOSTNAME_" With "${host}"
  }

If you have written the tag _TAG_HOSTNAME_ in /repository/postfix.templates/main.cf, cfengine will replace it with the machine's hostname and copy main.cf on the right place, /etc/postfix.