About Ohai

Ohai is a tool for collecting system configuration data, which it then provides to Chef Infra Client to use in cookbooks. Chef Infra Client runs Ohai at the start of every Chef Infra run to determine system state. The attributes that Ohai collects are called automatic attributes. Chef Infra Client uses these attributes to ensure that nodes are in the desired state after each configuration run.

The types of attributes Ohai collects include but are not limited to:

• Operating System
• Network
• Memory
• Disk
• CPU
• Kernel
• Host names
• Fully qualified domain names
• Virtualization
• Cloud provider metadata

Ohai includes required and optional plugins to detect common configuration information. Ohai has a plugin model and language to write custom plugins to collect additional system state information.

Ohai collects data for many platforms, including AIX, macOS, Linux, FreeBSD, Solaris, and any Windows operating systems.

See the Chef Infra Client release notes for the latest information on Ohai.

Automatic Attributes

An automatic attribute is a specific detail about a node, such as an IP address, a host name, a list of loaded kernel modules, and so on. Automatic attributes are detected by Ohai and are then used by Chef Infra Client to ensure that they are handled properly during every Chef Infra Client run. The most commonly accessed automatic attributes are:

Get a list of automatic attributes for a node

Attributes Blocklist

Attributes are blocklisted by attribute type, with each attribute type being blocklisted independently in the client.rb file.

The four attribute types are:

• automatic
• default
• normal
• override

The blocklist settings are:

blocked_automatic_attributes

An array that blocklists automatic attributes, preventing blocklisted attributes from being saved. For example: ['packages'].

Default value: nil, all attributes are saved.

If the array is empty, all attributes are saved.

blocked_default_attributes

An array that blocklists default attributes, preventing blocklisted attributes from being saved. For example: ['filesystem/dev/disk0s2/size'].

Default value: nil, all attributes are saved.

If the array is empty, all attributes are saved.

blocked_normal_attributes

An array that blocklists normal attributes, preventing blocklisted attributes from being saved. For example: ['filesystem/dev/disk0s2/size'].

Default value: nil, all attributes are saved.

If the array is empty, all attributes are saved.

blocked_override_attributes

An array that blocklists override attributes, preventing blocklisted attributes from being saved. For example: ['map - autohome/size'].

Default value: nil, all attributes are saved.

If the array is empty, all attributes are saved.

Blocklisting Ohai (automatic) attributes

Use blocked_automatic_attributes to block attributes populated by Ohai’s system information gathering.

Ohai gathers a large number of attributes that can consume a significant amount of storage space on the Chef Infra Server. Many of these attributes may be considered highly valuable, while others could be blocklisted without any impact to data available in search. Normal, default, and override attributes are typically much more important attributes used within cookbooks and are more likely to cause issues if they’re blocklisted incorrectly.

Example

The following shows an example of automatic attribute data.

{
  "filesystem" => {
    "/dev/disk0s2" => {
      "size" => "10mb"
    },
    "map - autohome" => {
      "size" => "10mb"
    }
  },
  "network" => {
    "interfaces" => {
      "eth0" => {...},
      "eth1" => {...},
    }
  }
}

To blocklist the filesystem attributes and allow Infra Client to save the other attributes, update the client.rb.

blocked_automatic_attributes ['filesystem']

This blocklist blocks Chef Infra Client from saving the filesystem and map - autohome attributes, but saves the network attributes.

Attribute Allowlist

Default Plugins

The following list shows the type of plugins that are included with Ohai. See the ohai/lib/ohai/plugins directory in the version of Ohai installed on your system for the full list:

General Purpose Plugins

azure.rb
c.rb
chef.rb
cloud.rb
command.rb
cpu.rb
digital_ocean.rb
dmi.rb
docker.rb
ec2.rb
elixir.rb
erlang.rb
eucalyptus.rb
filesystem.rb
freebsd
gce.rb
go.rb
groovy.rb
haskell.rb
hostname.rb
init_package.rb
java.rb
joyent.rb
kernel.rb
keys.rb
languages.rb
libvirt.rb
linode.rb
lua.rb
mono.rb
network.rb
nodejs.rb
ohai_time.rb
ohai.rb
memory.rb
network.rb
platform.rb
openstack.rb
os.rb
packages.rb
perl.rb
php.rb
platform.rb
powershell.rb
ps.rb
python.rb
rackspace.rb
root_group.rb
ruby.rb
rust.rb
scala.rb
scaleway.rb
shard.rb
shells.rb
softlayer.rb
ssh_host_key.rb
timezone.rb
uptime.rb
virtualbox.rb
vmware.rb
zpools.rb

Platform Specific Plugins

aix
  kernel.rb
  memory.rb
  network.rb
  platform.rb
  uptime.rb
  virtualization.rb
bsd
 virtualization.rb
darwin
  cpu.rb
  filesystem.rb
  hardware.rb
  memory.rb
  network.rb
  platform.rb
  system_profiler.rb
  virtualization.rb
dragonflybsd
 cpu.rb
 memory.rb
 network.rb
 os.rb
 platform.rb
freebsd
  cpu.rb
  memory.rb
  network.rb
  os.rb
  platform.rb
linux
  block_device.rb
  cpu.rb
  filesystem.rb
  fips.rb
  hostnamectl.rb
  lsb.rb
  machineid.rb
  mdadm.rb
  memory.rb
  network.rb
  platform.rb
  sessions.rb
  virtualization.rb
netbsd
  cpu.rb
  memory.rb
  network.rb
  platform.rb
openbsd
  cpu.rb
  memory.rb
  network.rb
  platform.rb
solaris2
  cpu.rb
  dmi.rb
  filesystem.rb
  memory.rb
  network.rb
  platform.rb
  virtualization.rb
windows
  cpu.rb
  drivers.rb
  filesystem.rb
  fips.rb
  memory.rb
  network.rb
  platform.rb
  system_enclosure.rb
  virtualization.rb

Optional Plugins

Ohai ships several optional plugins that you can enable in the client.rb configuration file.

:Grub2

Information from the Linux Grub2 bootloader

:IPC

SysV IPC shmem information (New in Chef Infra Client 16)

:Interupts

Data from /proc/interrupts and /proc/irq (New in Chef Infra Client 16)

:Lspci

PCI device information on Linux hosts.

:Lsscsi

SCSI device information on Linux hosts.

:Passwd

User and Group information. This plugin can result in large node sizes if a system connects to Active Directory or LDAP.

:Sessions

Sessions data from loginctl on Linux hosts.

:Sysctl

All sysctl values on Linux hosts.

Enabling Optional Plugins

Optional plugins can be enabled in the client.rb configuration file:

ohai.optional_plugins = [
  :Sessions,
  :Lspci,
]

Ohai Settings in client.rb

ohai.directory

The directory in which Ohai plugins are located.

ohai.disabled_plugins

An array of Ohai plugins to be disabled on a node. The list of plugins included in Ohai can be found in the ohai/lib/ohai/plugins source. For example, disabling a single plugin:

ohai.disabled_plugins = [
  :MyPlugin
]

or disabling multiple plugins:

ohai.disabled_plugins = [
  :MyPlugin,
  :MyPlugin2,
  :MyPlugin3
]

When a plugin is disabled, the Chef Infra Client log file will contain entries similar to:

[2014-06-13T23:49:12+00:00] DEBUG: Skipping disabled plugin MyPlugin

ohai.hints_path

The path to the file that contains hints for Ohai.

ohai.log_level

The level of logging to be stored in a log file.

ohai.log_location

The location of the log file.

ohai.plugin_path

An array of paths at which Ohai plugins are located. Default value: [<CHEF_GEM_PATH>/ohai-9.9.9/lib/ohai/plugins]. When custom Ohai plugins are added, the paths must be added to the array. For example, a single plugin:

ohai.plugin_path << '/etc/chef/ohai_plugins'

and for multiple plugins:

ohai.plugin_path += [
  '/etc/chef/ohai_plugins',
  '/path/to/other/plugins'
  ]

Custom Plugins

Custom Ohai plugins can be written to collect additional information from systems as necessary. See the Ohai Custom Plugins docs for more information.

Hints

Ohai hints are used to tell Ohai something about the system that it is running on that it would not be able to discover itself. An Ohai hint exists if a JSON file exists in the hint directory with the same name as the hint. For example, calling hint?('antarctica') in an Ohai plugin would return an empty hash if the file antarctica.json existed in the hints directory, and return nil if the file does not exist.

If the hint file contains JSON content, it will be returned as a hash from the call to hint?.

{
  "snow": true,
  "penguins": "many"
}

antarctica_hint = hint?('antarctica')
if antarctica_hint['snow']
  "There are #{antarctica_hint['penguins']} penguins here."
else
  'There is no snow here, and penguins like snow.'
end

Hint files are located in the /etc/chef/ohai/hints/ directory by default. Use the Ohai.config[:hints_path] setting in the client.rb configuration file to customize this location.

ohai Resource

Chef Infra Client includes an ohai resource that allows you to reload the Ohai data on a node. This allows recipes or resources that change system attributes (like a recipe that adds a user) to refer to those attributes later on during a Chef Infra Client run. See the ohai resource for complete usage information.

ohai Command Line Tool

Ohai can be run on the command line outside of the Chef Infra Client run. See Ohai (executable) for more information.