Using host resources

It's not always practical or convenient to use DNS to map your machine names to IP addresses, especially in cloud infrastructures, where those addresses may change all the time. However, if you use entries in the /etc/hosts file instead, you then have the problem of how to distribute these entries to all machines and keep them up to date.

Here's a better way to do it; Puppet's host resource type controls a single /etc/hosts entry, and you can use this to map a hostname to an IP address easily across your whole network. For example, if all your machines need to know the address of the main database server, you can manage it with a host resource.

How to do it...

Follow these steps to create an example host resource:

1. Modify your site.pp file as follows:

   node 'cookbook' {
     host { 'packtpub.com':
       ensure => present,
       ip     => '83.166.169.231',
     }
   }

2. Run Puppet:

   [root@cookbook ~]# puppet agent -t
   Info: Caching catalog for cookbook.example.com
   Info: Applying configuration version '1413781153'
   Notice: /Stage[main]/Main/Node[cookbook]/Host[packtpub.com]/ensure: created
   Info: Computing checksum on file /etc/hosts
   Notice: Finished catalog run in 0.12 seconds
   

How it works...

Puppet will check the target file (usually /etc/hosts) to see whether the host entry already exists, and if not, add it. If an entry for that hostname already exists with a different address, Puppet will change the address to match the manifest.

There's more...

Organizing your host resources into classes can be helpful. For example, you could put the host resources for all your DB servers into one class called admin::dbhosts, which is included by all web servers.

Where machines may need to be defined in multiple classes (for example, a database server might also be a repository server), virtual resources can solve this problem. For example, you could define all your hosts as virtual in a single class:

class admin::allhosts {
  @host { 'db1.packtpub.com':
    tag => 'database'
    ...
  }
}

You could then realize the hosts you need in the various classes:

class admin::dbhosts {
  Host <| tag=='database' |>
}

class admin::webhosts {
  Host <| tag=='web' |>
}