PerlStalker’s SysAdmin Notes

Notes from the life of a systems administrator

Managing /etc/hosts With Puppet

So, here’s the situation. I have a stack of VM servers running KVM and libvirt. The hosts need to connect to a SAN for ISO storage and, potentially, VM disks. The problem is that the VM running DNS may not be up yet when the host starts. That’s a problem since I’m referencing the san by it’s host name rather than the IP address. Yes, I could change all of my configs to use the IP instead but host names are a lot easier to deal with, most of the time.

Well, I could work around the lack of DNS by putting an entry for the server in /etc/hosts but then I’d have to update it on every server if I ever change the IP address. Fortunately, puppet makes it easy. Sort of.

Puppet is a wonderful tool for managing Linux (and other *nix) servers. It’s a little weak, though, when all you want to do is add a line to file. That’s exactly what I wanted to do with /etc/hosts.

The good news is that puppet has a hook into the tool augeus. That makes editing the config relatively easy but plugging that into puppet is still a little messy. To make it easier, I created a class and define to tidy that up a bit.

Update [2012-09-03 Mon 07:15]: Puppet actually makes this easier than I thought. There already exists a host data type that I totally missed before. Dominic Cleal also pointed me to a module that he wrote that adds augeus providers to some of the default data types, including host.

class hosts {
  define entry(
    $ipaddr,
    $canonical,
    $aliases = 'UNSET' # I want to make this an array
    )
    {
      augeas { "create_$title":
        context => '/files/etc/hosts',
        changes => [
                    "ins 01 after 1",
                    "set 01/ipaddr $ipaddr",
                    "set 01/canonical $canonical"
                    ],
        onlyif  => "match *[ipaddr = '$ipaddr'] size == 0"
      }

      augeas { "update_$title":
        context => '/files/etc/hosts',
        changes => [
                    "set *[ipaddr = '$ipaddr']/canonical $canonical"
                    ],
      }

      Augeas["create_$title"] -> Augeas["update_$title"]

      # It would be great if I could loop this
      if ($aliases == 'UNSET') {
        augeas { "alias_$title":
          context => '/files/etc/hosts',
          changes => [
                      "rm *[ipaddr = '$ipaddr']/alias[1]"
                      ],
        }
      }
      else {
        augeas { "alias_$title":
          context => '/files/etc/hosts',
          changes => [
                      "set *[ipaddr = '$ipaddr']/alias[1] '$alias'"
                      ],
        }
      }
      Augeas["update_$title"] -> Augeas["alias_$title"]
    }
}

There are a couple of known issues. First, you can’t unset an entry. If you add a host with this and then decide that you no longer want it there, you can’t take it out. I don’t think it would hard to add that feature but I haven’t.

Second, the define only lets you set the first alias. Augeas allows for multiple aliases and I could pass an array to the define but I don’t know how to loop through that list.

Anyway, here’s how you use the define within a node or class definition.

include hosts
hosts::entry { 'san':
  ipaddr    => '192.168.1.5',
  canonical => 'san.example',
  aliases   => 'san'
}

You can use as many hosts::entry blocks as you want.

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