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
Note
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:
Attribute | Description |
---|---|
node['platform'] | The platform on which a node is running. This attribute helps determine which providers will be used. |
node['platform_family'] | The platform family is a Chef Infra specific grouping of similar platforms where cookbook code can often be shared. For example, `rhel` includes Red Hat Linux, Oracle Linux, CentOS, and several other platforms that are almost identical to Red Hat Linux. |
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 Infra Server and can be accessed using the knife status subcommand. |
Get a list of automatic attributes for a node
Ohai collects a list of automatic attributes at the start of each Chef Infra Client run. This list will vary from organization to organization, by server type, and by the platform that runs those servers. All the attributes collected by Ohai are unmodifiable by Chef Infra Client. Run theohai
command on a system to see which automatic attributes Ohai
has collected for a particular node.Attributes Blocklist
Warning
blocked_automatic_attributes
defines attributes that will not be saved, but blocked_normal_attributes
, blocked_default_attributes
, and blocked_override_attributes
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 blocklisting.Attributes that should not be saved by a node may be blocklisted in the client.rb file. The blocklist is a Hash of keys that specify each attribute to be filtered out.
Attributes are blocklisted by attribute type, with each attribute type being blocklisted independently. Each attribute type—automatic
, default
, normal
, and override
—may define blocklists by using the following settings in the client.rb file:
blocked_automatic_attributes
A hash that blocklists
automatic
attributes, preventing blocklisted 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.blocked_default_attributes
A hash 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
A hash 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
A hash 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
The recommended practice is to 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 are blocklisted incorrectly.
For example, automatic attribute data similar to:
{
"filesystem" => {
"/dev/disk0s2" => {
"size" => "10mb"
},
"map - autohome" => {
"size" => "10mb"
}
},
"network" => {
"interfaces" => {
"eth0" => {...},
"eth1" => {...},
}
}
}
To blocklist the filesystem
attributes and allow the other attributes to be saved, update the client.rb file:
blocked_automatic_attributes ['filesystem']
When a blocklist is defined, any attribute of that type that is not specified in that attribute blocklist will be saved. So based on the previous blocklist for automatic 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:
blocked_automatic_attributes [['filesystem', '/dev/diskos2']]
Attribute Allowlist
Warning
automatic_attribute_allowlist
defines attributes to be saved, but normal_attribute_allowlist
, default_attribute_allowlist
, and
override_attribute_allowlist
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 allowlisting.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,
]
Note
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 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' ]
Note
The Ohai executable ignores settings in the client.rb file when Ohai is run independently of Chef Infra Client.
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.