About Ohai

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Ohai is a tool that is used to collect system configuration data, which is provided to the chef-client for use within cookbooks. Ohai is run by the chef-client at the beginning of every Chef run to determine system state. Ohai includes many built-in plugins to detect common configuration details as well as a plugin model for writing custom plugins.

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

Attributes that are collected by Ohai are automatic level attributes, in that these attributes are used by the chef-client to ensure that these attributes remain unchanged after the chef-client is done configuring the node.

Ohai collects data for many platforms, including AIX, Darwin, Linux, FreeBSD, OpenBSD, NetBSD, Solaris, and any Microsoft Windows operating systems.

View the 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 the chef-client to ensure that they are handled properly during every chef-client run. The most commonly accessed automatic attributes are:

Attribute Description
node['platform'] The platform on which a node is running. This attribute helps determine which providers will be used.
node['platform_version'] The version of the platform. This attribute helps determine which providers will be used.
node['ipaddress'] The IP address for a node. If the node has a default route, this is the IPV4 address for the interface. If the node does not have a default route, the value for this attribute should be nil. The IP address for default route is the recommended default value.
node['macaddress'] The MAC address for a node, determined by the same interface that detects the node['ipaddress'].
node['fqdn'] The fully qualified domain name for a node. This is used as the name of a node unless otherwise set.
node['hostname'] The host name for the node.
node['domain'] The domain for the node.
node['recipes'] A list of recipes associated with a node (and part of that node’s run-list).
node['roles'] A list of roles associated with a node (and part of that node’s run-list).
node['ohai_time'] The time at which Ohai was last run. This attribute is not commonly used in recipes, but it is saved to the Chef server and can be accessed using the knife status subcommand.

Get a list of automatic attributes for a node

The list of automatic attributes that are collected by Ohai at the start of each chef-client run vary from organization to organization, and will often vary between the various server types being configured and the platforms on which those servers are run. All attributes collected by Ohai are unmodifiable by the chef-client. To see which automatic attributes are collected by Ohai for a particular node, run the following command:

find  /opt/chefdk/embedded/lib/ruby/gems/*/gems/ohai-*/lib -name "*.rb" -print | xargs grep -R "provides" -h |sed 's/^\s*//g'|sed "s/\\\"/\'/g"|sort|uniq|grep "\sprovides"

Note

Attributes can be configured in cookbooks (attribute files and recipes), roles, and environments. In addition, Ohai collects attribute data about each node at the start of the chef-client run. See https://docs.chef.io/attributes.html for more information about how all of these attributes fit together.

Blacklist Attributes

Warning

When attribute blacklist settings are used, any attribute defined in a blacklist will not be saved and any attribute that is not defined in a blacklist will be saved. Each attribute type is blacklisted independently of the other attribute types. For example, if automatic_attribute_blacklist defines attributes that will not be saved, but normal_attribute_blacklist, default_attribute_blacklist, and override_attribute_blacklist are not defined, then all normal attributes, default attributes, and override attributes will be saved, as well as the automatic attributes that were not specifically excluded through blacklisting.

Attributes that should not be saved by a node may be blacklisted in the client.rb file. The blacklist is a Hash of keys that specify each attribute to be filtered out.

Attributes are blacklisted by attribute type, with each attribute type being blacklisted independently. Each attribute type—automatic, default, normal, and override—may define blacklists by using the following settings in the client.rb file:

Setting Description
automatic_attribute_blacklist A hash that blacklists automatic attributes, preventing blacklisted attributes from being saved. For example: ['network/interfaces/eth0']. Default value: nil, all attributes are saved. If the array is empty, all attributes are saved.
default_attribute_blacklist A hash that blacklists default attributes, preventing blacklisted 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.
normal_attribute_blacklist A hash that blacklists normal attributes, preventing blacklisted 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.
override_attribute_blacklist A hash that blacklists override attributes, preventing blacklisted 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.

Warning

The recommended practice is to use only automatic_attribute_blacklist for blacklisting attributes. This is primarily because automatic attributes generate the most data, but also that normal, default, and override attributes are typically much more important attributes and are more likely to cause issues if they are blacklisted incorrectly.

For example, normal attribute data similar to:

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

To blacklist the filesystem attributes and allow the other attributes to be saved, update the client.rb file:

normal_attribute_blacklist ['filesystem']

When a blacklist is defined, any attribute of that type that is not specified in that attribute blacklist will be saved. So based on the previous blacklist for normal attributes, the filesystem and map - autohome attributes will not be saved, but the network attributes will.

For attributes that contain slashes (/) within the attribute value, such as the filesystem attribute '/dev/diskos2', use an array. For example:

automatic_attribute_blacklist [['filesystem','/dev/diskos2']]

Whitelist Attributes

Warning

When attribute whitelist settings are used, only the attributes defined in a whitelist will be saved and any attribute that is not defined in a whitelist will not be saved. Each attribute type is whitelisted independently of the other attribute types. For example, if automatic_attribute_whitelist defines attributes to be saved, but normal_attribute_whitelist, default_attribute_whitelist, and override_attribute_whitelist are not defined, then all normal attributes, default attributes, and override attributes are saved, as well as the automatic attributes that were specifically included through whitelisting.

Attributes that should be saved by a node may be whitelisted in the client.rb file. The whitelist is a hash of keys that specifies each attribute to be saved.

Attributes are whitelisted by attribute type, with each attribute type being whitelisted independently. Each attribute type—automatic, default, normal, and override—may define whitelists by using the following settings in the client.rb file:

Setting Description
automatic_attribute_whitelist A hash that whitelists automatic attributes, preventing non-whitelisted attributes from being saved. For example: ['network/interfaces/eth0']. Default value: nil, all attributes are saved. If the hash is empty, no attributes are saved.
default_attribute_whitelist A hash that whitelists default attributes, preventing non-whitelisted attributes from being saved. For example: ['filesystem/dev/disk0s2/size']. Default value: nil, all attributes are saved. If the hash is empty, no attributes are saved.
normal_attribute_whitelist A hash that whitelists normal attributes, preventing non-whitelisted attributes from being saved. For example: ['filesystem/dev/disk0s2/size']. Default value: nil, all attributes are saved. If the hash is empty, no attributes are saved.
override_attribute_whitelist A hash that whitelists override attributes, preventing non-whitelisted attributes from being saved. For example: ['map - autohome/size']. Default value: nil, all attributes are saved. If the hash is empty, no attributes are saved.

Warning

The recommended practice is to only use automatic_attribute_whitelist to whitelist attributes. This is primarily because automatic attributes generate the most data, but also that normal, default, and override attributes are typically much more important attributes and are more likely to cause issues if they are whitelisted incorrectly.

For example, normal attribute data similar to:

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

To whitelist the network attributes and prevent the other attributes from being saved, update the client.rb file:

normal_attribute_whitelist ['network/interfaces/']

When a whitelist is defined, any attribute of that type that is not specified in that attribute whitelist will not be saved. So based on the previous whitelist for normal attributes, the filesystem and map - autohome attributes will not be saved, but the network attributes will.

Leave the value empty to prevent all attributes of that attribute type from being saved:

normal_attribute_whitelist []

For attributes that contain slashes (/) within the attribute value, such as the filesystem attribute '/dev/diskos2', use an array. For example:

automatic_attribute_whitelist [['filesystem','/dev/diskos2']]

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:

aix
  cpu.rb
  filesystem.rb
  kernel.rb
  memory.rb
  network.rb
  os.rb
  platform.rb
  uptime.rb
  virtualization.rb
azure.rb
bsd
 filesystem.rb
 virtualization.rb
c.rb
chef.rb
cloud.rb
command.rb
darwin
  cpu.rb
  filesystem.rb
  hardware.rb
  memory.rb
  network.rb
  platform.rb
  system_profiler.rb
  virtualization.rb
digital_ocean.rb
dmi.rb
dragonflybsd
 cpu.rb
 memory.rb
 network.rb
 os.rb
 platform.rb
ec2.rb
elixir.rb
erlang.rb
eucalyptus.rb
freebsd
  cpu.rb
  memory.rb
  network.rb
  os.rb
  platform.rb
gce.rb
go.rb
groovy.rb
haskell.rb
hostname.rb
init_package.rb
ip_scopes.rb
java.rb
joyent.rb
kernel.rb
keys.rb
languages.rb
linode.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
  sessons.rb
  virtualization.rb
lua.rb
mono.rb
netbsd
  cpu.rb
  memory.rb
  network.rb
  platform.rb
network.rb
nodejs.rb
ohai.rb
ohai_time.rb
openbsd
  cpu.rb
  memory.rb
  network.rb
  platform.rb
openstack.rb
os.rb
packages.rb
passwd.rb
perl.rb
php.rb
platform.rb
powershell.rb
ps.rb
python.rb
rackspace.rb
root_group.rb
ruby.rb
rust.rb
scala.rb
shard.rb
shells.rb
softlayer.rb
solaris2
  cpu.rb
  dmi.rb
  filesystem.rb
  memory.rb
  network.rb
  platform.rb
  virtualization.rb
  zpools.rb
ssh_host_key.rb
sysconf.rb
timezone.rb
uptime.rb
virtualbox.rb
virtualization.rb
vmware.rb
windows
  cpu.rb
  drivers.rb
  filesystem.rb
  fips.rb
  memory.rb
  network.rb
  platform.rb
  virtualization.rb

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

The default directory in which hint files are located is /etc/chef/ohai/hints/. Use the Ohai::Config[:hints_path] setting in the client.rb file to customize this location.

ohai Resource

A resource defines the desired state for a single configuration item present on a node that is under management by Chef. A resource collection—one (or more) individual resources—defines the desired state for the entire node. During a chef-client run, the current state of each resource is tested, after which the chef-client will take any steps that are necessary to repair the node and bring it back into the desired state.

Use the ohai resource to reload the Ohai configuration on a node. This allows recipes that change system attributes (like a recipe that adds a user) to refer to those attributes later on during the chef-client run.

Syntax

A ohai resource block reloads the Ohai configuration on a node:

ohai 'reload' do
  action :reload
end

The full syntax for all of the properties that are available to the ohai resource is:

ohai 'name' do
  name                       String
  notifies                   # see description
  plugin                     String
  provider                   Chef::Provider::Ohai
  subscribes                 # see description
  action                     Symbol # defaults to :reload if not specified
end

where

  • ohai is the resource
  • name is the name of the resource block
  • action identifies the steps the chef-client will take to bring the node into the desired state
  • name, plugin, and provider are properties of this resource, with the Ruby type shown. See “Properties” section below for more information about all of the properties that may be used with this resource.

Actions

This resource has the following actions:

:nothing
Define this resource block to do nothing until notified by another resource to take action. When this resource is notified, this resource block is either run immediately or it is queued up to be run at the end of the chef-client run.
:reload
Default. Reload the Ohai configuration on a node.

Attributes

This resource has the following properties:

ignore_failure

Ruby Types: TrueClass, FalseClass

Continue running a recipe if a resource fails for any reason. Default value: false.

name

Ruby Type: String

Always the same value as the name of the resource block. See “Syntax” section above for more information.

notifies

Ruby Type: Symbol, ‘Chef::Resource[String]’

A resource may notify another resource to take action when its state changes. Specify a 'resource[name]', the :action that resource should take, and then the :timer for that action. A resource may notifiy more than one resource; use a notifies statement for each resource to be notified.

A timer specifies the point during the chef-client run at which a notification is run. The following timers are available:

:before
Specifies that the action on a notified resource should be run before processing the resource block in which the notification is located.
:delayed
Default. Specifies that a notification should be queued up, and then executed at the very end of the chef-client run.
:immediate, :immediately
Specifies that a notification should be run immediately, per resource notified.

The syntax for notifies is:

notifies :action, 'resource[name]', :timer
plugin

Ruby Type: String

Optional. The name of an Ohai plugin to be reloaded. If this property is not specified, the chef-client will reload all plugins.

provider

Ruby Type: Chef Class

Optional. Explicitly specifies a provider.

retries

Ruby Type: Integer

The number of times to catch exceptions and retry the resource. Default value: 0.

retry_delay

Ruby Type: Integer

The retry delay (in seconds). Default value: 2.

subscribes

Ruby Type: Symbol, ‘Chef::Resource[String]’

A resource may listen to another resource, and then take action if the state of the resource being listened to changes. Specify a 'resource[name]', the :action to be taken, and then the :timer for that action.

Note that subscribes does not apply the specified action to the resource that it listens to - for example:

file '/etc/nginx/ssl/example.crt' do
   mode '0600'
   owner 'root'
end

service 'nginx' do
   subscribes :reload, 'file[/etc/nginx/ssl/example.crt', :immediately
end

In this case the subscribes property reloads the nginx service whenever its certificate file, located under /etc/nginx/ssl/example.crt, is updated. subscribes does not make any changes to the certificate file itself, it merely listens for a change to the file, and executes the :reload action for its resource (in this example nginx) when a change is detected.

A timer specifies the point during the chef-client run at which a notification is run. The following timers are available:

:before
Specifies that the action on a notified resource should be run before processing the resource block in which the notification is located.
:delayed
Default. Specifies that a notification should be queued up, and then executed at the very end of the chef-client run.
:immediate, :immediately
Specifies that a notification should be run immediately, per resource notified.

The syntax for subscribes is:

subscribes :action, 'resource[name]', :timer

Providers

This resource has the following providers:

Chef::Provider::Ohai, ohai
The default provider for all platforms.

Examples

The following examples demonstrate various approaches for using resources in recipes. If you want to see examples of how Chef uses resources in recipes, take a closer look at the cookbooks that Chef authors and maintains: https://github.com/chef-cookbooks.

Reload Ohai

ohai 'reload' do
  action :reload
end

Reload Ohai after a new user is created

ohai 'reload_passwd' do
  action :nothing
  plugin 'etc'
end

user 'daemonuser' do
  home '/dev/null'
  shell '/sbin/nologin'
  system true
  notifies :reload, 'ohai[reload_passwd]', :immediately
end

ruby_block 'just an example' do
  block do
    # These variables will now have the new values
    puts node['etc']['passwd']['daemonuser']['uid']
    puts node['etc']['passwd']['daemonuser']['gid']
  end
end

ohai Command Line Tool

ohai is the command-line interface for Ohai, a tool that is used to detect attributes on a node, and then provide these attributes to the chef-client at the start of every chef-client run.

Options

This command has the following syntax:

$ ohai OPTION

This tool has the following options:

ATTRIBUTE_NAME ATTRIBUTE NAME ...
Use to have Ohai show only output for named attributes.
-d PATH, --directory PATH
The directory in which Ohai plugins are located. For example: /etc/ohai/plugins.
-h, --help
Show help for the command.
-l LEVEL, --log_level LEVEL
The level of logging to be stored in a log file.
-L LOGLOCATION, --logfile c
The location of the log file.
-v, --version
The version of Ohai.

Examples

None.

Ohai Settings in client.rb

Ohai configuration settings can be added to the client.rb file.

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 directory. For example, disabling a single plugin:

  ohai.disabled_plugins = [
    :MyPlugin
  ]

or disabling multiple plugins:
ohai.disabled_plugins = [
  :MyPlugin,
  :MyPlugin,
  :MyPlugin
]

and to disable multiple plugins, including Ohai 6 plugins:

ohai.disabled_plugins = [
          :MyPlugin,
  :MyPlugin,
  'my_ohai_6_plugin'
]

When a plugin is disabled, the chef-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'
  ]
ohai.version
The version of Ohai.

Note

The Ohai executable ignores settings in the client.rb file when Ohai is run independently of the chef-client.