How to build your inventory

Ansible automates tasks on managed nodes or “hosts” in your infrastructure, using a list or group of lists known as inventory. You can pass host names at the command line, but most Ansible users create inventory files. Your inventory defines the managed nodes you automate, with groups so you can run automation tasks on multiple hosts at the same time. Once your inventory is defined, you use patterns to select the hosts or groups you want Ansible to run against.

The simplest inventory is a single file with a list of hosts and groups. The default location for this file is /etc/ansible/hosts. You can specify a different inventory file at the command line using the -i <path> option or in configuration using inventory.

Ansible Inventory plugins supports a range of formats and sources to make your inventory flexible and customizable. As your inventory expands, you may need more than a single file to organize your hosts and groups. Here are three options beyond the /etc/ansible/hosts file:

You can create a directory with multiple inventory files. See Organizing inventory in a directory. These can use different formats (YAML, ini, and so on).
You can pull inventory dynamically. For example, you can use a dynamic inventory plugin to list resources in one or more cloud providers. See Working with dynamic inventory.
You can use multiple sources for inventory, including both dynamic inventory and static files. See Passing multiple inventory sources.

Note

The following YAML snippets include an ellipsis to indicate they are part of a larger YAML file. You can find out more about YAML syntax at YAML Basics.

Inventory basics: formats, hosts, and groups 

You can create your inventory file in one of many formats, depending on the inventory plugins you have. The most common formats are INI and YAML. A basic INI /etc/ansible/hosts might look like this:

mail.example.com

[webservers]
foo.example.com
bar.example.com

[dbservers]
one.example.com
two.example.com
three.example.com

The headings in brackets are group names, which are used in classifying hosts and deciding what hosts you are controlling at what times and for what purpose. Group names should follow the same guidelines as Creating valid variable names.

Here’s that same basic inventory file in YAML format:

ungrouped:
  hosts:
    mail.example.com:
webservers:
  hosts:
    foo.example.com:
    bar.example.com:
dbservers:
  hosts:
    one.example.com:
    two.example.com:
    three.example.com:

Default groups 

Even if you do not define any groups in your inventory file, Ansible creates two default groups: all and ungrouped. The all group contains every host. The ungrouped group contains all hosts that don’t have another group aside from all. Every host will always belong to at least 2 groups (all and ungrouped or all and some other group). For example, in the basic inventory above, the host mail.example.com belongs to the all group and the ungrouped group; the host two.example.com belongs to the all group and the dbservers group. Though all and ungrouped are always present, they can be implicit and not appear in group listings like group_names.

Hosts in multiple groups 

You can put each host in more than one group. For example, a production web server in a data center in Atlanta might be included in groups called [prod] and [atlanta] and [webservers]. You can create groups that track:

What - An application, stack or microservice (for example, database servers, web servers, and so on).
Where - A datacenter or region, to talk to local DNS, storage, and so on (for example, east, west).
When - The development stage, to avoid testing on production resources (for example, prod, test).

Extending the previous YAML inventory to include what, when, and where would look like this:

ungrouped:
  hosts:
    mail.example.com:
webservers:
  hosts:
    foo.example.com:
    bar.example.com:
dbservers:
  hosts:
    one.example.com:
    two.example.com:
    three.example.com:
east:
  hosts:
    foo.example.com:
    one.example.com:
    two.example.com:
west:
  hosts:
    bar.example.com:
    three.example.com:
prod:
  hosts:
    foo.example.com:
    one.example.com:
    two.example.com:
test:
  hosts:
    bar.example.com:
    three.example.com:

You can see that one.example.com exists in the dbservers, east, and prod groups.

Grouping groups: parent/child group relationships 

You can create parent/child relationships among groups. Parent groups are also known as nested groups or groups of groups. For example, if all your production hosts are already in groups such as atlanta_prod and denver_prod, you can create a production group that includes those smaller groups. This approach reduces maintenance because you can add or remove hosts from the parent group by editing the child groups.

To create parent/child relationships for groups:

in INI format, use the :children suffix
in YAML format, use the children: entry

Here is the same inventory as shown above, simplified with parent groups for the prod and test groups. The two inventory files give you the same results:

ungrouped:
  hosts:
    mail.example.com:
webservers:
  hosts:
    foo.example.com:
    bar.example.com:
dbservers:
  hosts:
    one.example.com:
    two.example.com:
    three.example.com:
east:
  hosts:
    foo.example.com:
    one.example.com:
    two.example.com:
west:
  hosts:
    bar.example.com:
    three.example.com:
prod:
  children:
    east:
test:
  children:
    west:

Child groups have a couple of properties to note:

Any host that is a member of a child group is automatically a member of the parent group.
Groups can have multiple parents and children, but not circular relationships.
Hosts can also be in multiple groups, but there will only be one instance of a host at runtime. Ansible merges the data from multiple groups.

Adding ranges of hosts 

If you have a lot of hosts with a similar pattern, you can add them as a range rather than listing each hostname separately:

In INI:

[webservers]
www[01:50].example.com

In YAML:

# ...
  webservers:
    hosts:
      www[01:50].example.com:

You can specify a stride (increments between sequence numbers) when defining a numeric range of hosts:

In INI:

[webservers]
www[01:50:2].example.com

In YAML:

# ...
  webservers:
    hosts:
      www[01:50:2].example.com:

The example above would make the subdomains www01, www03, www05, …, www49 match, but not www00, www02, www50 and so on, because the stride (increment) is 2 units each step.

For numeric patterns, leading zeros can be included or removed, as desired. Ranges are inclusive. You can also define alphabetic ranges:

[databases]
db-[a:f].example.com

Passing multiple inventory sources 

You can target multiple inventory sources (directories, dynamic inventory scripts or files supported by inventory plugins) at the same time by giving multiple inventory parameters from the command line or by configuring ANSIBLE_INVENTORY. This can be useful when you want to target normally separate environments, like staging and production, at the same time for a specific action.

To target two inventory sources from the command line:

ansible-playbook get_logs.yml -i staging -i production

Organizing inventory in a directory 

You can consolidate multiple inventory sources in a single directory. The simplest version of this is a directory with multiple files instead of a single inventory file. A single file gets difficult to maintain when it gets too long. If you have multiple teams and multiple automation projects, having one inventory file per team or project lets everyone easily find the hosts and groups that matter to them.

You can also combine multiple inventory source types in an inventory directory. This can be useful for combining static and dynamic hosts and managing them as one inventory. The following inventory directory combines an inventory plugin source, a dynamic inventory script, and a file with static hosts:

inventory/
  openstack.yml          # configure inventory plugin to get hosts from OpenStack cloud
  dynamic-inventory.py   # add additional hosts with dynamic inventory script
  on-prem                # add static hosts and groups
  parent-groups          # add static hosts and groups

You can target this inventory directory as follows:

ansible-playbook example.yml -i inventory

You can also configure the inventory directory in your ansible.cfg file. See Configuring Ansible for more details.

Managing inventory load order 

Ansible loads inventory sources in ASCII order according to the filenames. If you define parent groups in one file or directory and child groups in other files or directories, the files that define the child groups must be loaded first. If the parent groups are loaded first, you will see the error Unable to parse /path/to/source_of_parent_groups as an inventory source.

For example, if you have a file called groups-of-groups that defines a production group with child groups defined in a file called on-prem, Ansible cannot parse the production group. To avoid this problem, you can control the load order by adding prefixes to the files:

inventory/
  01-openstack.yml          # configure inventory plugin to get hosts from OpenStack cloud
  02-dynamic-inventory.py   # add additional hosts with dynamic inventory script
  03-on-prem                # add static hosts and groups
  04-groups-of-groups       # add parent groups

You can find examples of how to organize your inventories and group your hosts in Inventory setup examples.

Adding variables to inventory 

You can store variable values that relate to a specific host or group in inventory. To start with, you may add variables directly to the hosts and groups in your main inventory file.

We document adding variables in the main inventory file for simplicity. However, storing variables in separate host and group variable files is a more robust approach to describing your system policy. Setting variables in the main inventory file is only a shorthand. See Organizing host and group variables for guidelines on storing variable values in individual files in the ‘host_vars’ directory. See Organizing host and group variables for details.

Assigning a variable to one machine: host variables 

You can easily assign a variable to a single host and then use it later in playbooks. You can do this directly in your inventory file.

In INI:

[atlanta]
host1 http_port=80 maxRequestsPerChild=808
host2 http_port=303 maxRequestsPerChild=909

In YAML:

atlanta:
  hosts:
    host1:
      http_port: 80
      maxRequestsPerChild: 808
    host2:
      http_port: 303
      maxRequestsPerChild: 909

Unique values like non-standard SSH ports work well as host variables. You can add them to your Ansible inventory by adding the port number after the hostname with a colon:

badwolf.example.com:5309

Connection variables also work well as host variables:

[targets]

localhost              ansible_connection=local
other1.example.com     ansible_connection=ssh        ansible_user=myuser
other2.example.com     ansible_connection=ssh        ansible_user=myotheruser

Note

If you list non-standard SSH ports in your SSH config file, the openssh connection will find and use them, but the paramiko connection will not.

Inventory aliases 

You can also define aliases in your inventory using host variables:

In INI:

jumper ansible_port=5555 ansible_host=192.0.2.50

In YAML:

# ...
  hosts:
    jumper:
      ansible_port: 5555
      ansible_host: 192.0.2.50

In this example, running Ansible against the host alias “jumper” will connect to 192.0.2.50 on port 5555. See behavioral inventory parameters to further customize the connection to hosts.

Defining variables in INI format 

Values passed in the INI format using the key=value syntax are interpreted differently depending on where they are declared:

When declared inline with the host, INI values are interpreted as Python literal structures (strings, numbers, tuples, lists, dicts, booleans, None). Host lines accept multiple key=value parameters per line. Therefore they need a way to indicate that a space is part of a value rather than a separator. Values that contain whitespace can be quoted (single or double). See the Python shlex parsing rules for details.
When declared in a :vars section, INI values are interpreted as strings. For example var=FALSE would create a string equal to ‘FALSE’. Unlike host lines, :vars sections accept only a single entry per line, so everything after the = must be the value for the entry.

If a variable value set in an INI inventory must be a certain type (for example, a string or a boolean value), always specify the type with a filter in your task. Do not rely on types set in INI inventories when consuming variables.

Consider using YAML format for inventory sources to avoid confusion on the actual type of a variable. The YAML inventory plugin processes variable values consistently and correctly.

Assigning a variable to many machines: group variables 

If all hosts in a group share a variable value, you can apply that variable to an entire group at once.

In INI:

[atlanta]
host1
host2

[atlanta:vars]
ntp_server=ntp.atlanta.example.com
proxy=proxy.atlanta.example.com

In YAML:

atlanta:
  hosts:
    host1:
    host2:
  vars:
    ntp_server: ntp.atlanta.example.com
    proxy: proxy.atlanta.example.com

Group variables are a convenient way to apply variables to multiple hosts at once. Before executing, however, Ansible always flattens variables, including inventory variables, to the host level. If a host is a member of multiple groups, Ansible reads variable values from all of those groups. If you assign different values to the same variable in different groups, Ansible chooses which value to use based on internal rules for merging.

Inheriting variable values: group variables for groups of groups 

You can apply variables to parent groups (nested groups or groups of groups) as well as to child groups. The syntax is the same: :vars for INI format and vars: for YAML format:

In INI:

[atlanta]
host1
host2

[raleigh]
host2
host3

[southeast:children]
atlanta
raleigh

[southeast:vars]
some_server=foo.southeast.example.com
halon_system_timeout=30
self_destruct_countdown=60
escape_pods=2

[usa:children]
southeast
northeast
southwest
northwest

In YAML:

usa:
  children:
    southeast:
      children:
        atlanta:
          hosts:
            host1:
            host2:
        raleigh:
          hosts:
            host2:
            host3:
      vars:
        some_server: foo.southeast.example.com
        halon_system_timeout: 30
        self_destruct_countdown: 60
        escape_pods: 2
    northeast:
    northwest:
    southwest:

A child group’s variables will have higher precedence (override) than a parent group’s variables.

Organizing host and group variables 

Although you can store variables in the main inventory file, storing separate host and group variables files may help you organize your variable values more easily. You can also use lists and hash data in host and group variables files, which you cannot do in your main inventory file.

Host and group variable files must use YAML syntax. Valid file extensions include ‘.yml’, ‘.yaml’, ‘.json’, or no file extension. See YAML Syntax if you are new to YAML.

Ansible loads host and group variable files by searching paths relative to the inventory file or the playbook file. If your inventory file at /etc/ansible/hosts contains a host named ‘foosball’ that belongs to two groups, ‘raleigh’ and ‘webservers’, that host will use variables in YAML files at the following locations:

/etc/ansible/group_vars/raleigh # can optionally end in '.yml', '.yaml', or '.json'
/etc/ansible/group_vars/webservers
/etc/ansible/host_vars/foosball

For example, if you group hosts in your inventory by datacenter, and each datacenter uses its own NTP server and database server, you can create a file called /etc/ansible/group_vars/raleigh to store the variables for the raleigh group:

---
ntp_server: acme.example.org
database_server: storage.example.org

You can also create directories named after your groups or hosts. Ansible will read all the files in these directories in lexicographical order. An example with the ‘raleigh’ group:

/etc/ansible/group_vars/raleigh/db_settings
/etc/ansible/group_vars/raleigh/cluster_settings

All hosts in the ‘raleigh’ group will have the variables defined in these files available to them. This can be very useful to keep your variables organized when a single file gets too big, or when you want to use Ansible Vault on some group variables.

For ansible-playbook you can also add group_vars/ and host_vars/ directories to your playbook directory. Other Ansible commands (for example, ansible, ansible-console, and so on) will only look for group_vars/ and host_vars/ in the inventory directory. If you want other commands to load group and host variables from a playbook directory, you must provide the --playbook-dir option on the command line. If you load inventory files from both the playbook directory and the inventory directory, variables in the playbook directory will override variables set in the inventory directory.

Keeping your inventory file and variables in a Git repo (or other version control) is an excellent way to track changes to your inventory and host variables.

How variables are merged 

By default, variables are merged/flattened to the specific host before a play is run. This keeps Ansible focused on the Host and Task, so groups do not survive outside of inventory and host matching. By default, Ansible overwrites variables including the ones defined for a group and/or host (see DEFAULT_HASH_BEHAVIOUR). The order/precedence is (from lowest to highest):

all group (because it is the ‘parent’ of all other groups)
parent group
child group
host

By default, Ansible merges groups at the same parent/child level in ASCII order, and variables from the last group loaded overwrite variables from the previous groups. For example, an a_group will be merged with b_group and b_group vars that match will overwrite the ones in a_group.

Note

Ansible merges variables from different sources and applies precedence to some variables over others according to a set of rules. For example, variables that occur higher in an inventory can override variables that occur lower in the inventory. See Variable precedence: Where should I put a variable? for more information.

You can change this behavior by setting the group variable ansible_group_priority to change the merge order for groups of the same level (after the parent/child order is resolved). The larger the number, the later it will be merged, giving it higher priority. This variable defaults to 1 if not set. For example:

a_group:
  vars:
    testvar: a
    ansible_group_priority: 10
b_group:
  vars:
    testvar: b

In this example, if both groups have the same priority, the result would normally have been testvar == b, but since we are giving the a_group a higher priority the result will be testvar == a.

Note

ansible_group_priority can only be set in the inventory source and not in group_vars/, as the variable is used in the loading of group_vars.

Managing inventory variable load order 

When using multiple inventory sources, keep in mind that any variable conflicts are resolved according to the rules described in How variables are merged and Variable precedence: Where should I put a variable?. You can control the merging order of variables in inventory sources to get the variable value you need.

When you pass multiple inventory sources at the command line, Ansible merges variables in the order you pass those parameters. If [all:vars] in staging inventory defines myvar = 1 and production inventory defines myvar = 2, then:

Pass -i staging -i production to run the playbook with myvar = 2.
Pass -i production -i staging to run the playbook with myvar = 1.

When you put multiple inventory sources in a directory, Ansible merges them in ASCII order according to the filenames. You can control the load order by adding prefixes to the files:

inventory/
  01-openstack.yml          # configure inventory plugin to get hosts from Openstack cloud
  02-dynamic-inventory.py   # add additional hosts with dynamic inventory script
  03-static-inventory       # add static hosts
  group_vars/
    all.yml                 # assign variables to all hosts

If 01-openstack.yml defines myvar = 1 for the group all, 02-dynamic-inventory.py defines myvar = 2, and 03-static-inventory defines myvar = 3, the playbook will be run with myvar = 3.

For more details on inventory plugins and dynamic inventory scripts see Inventory plugins and Working with dynamic inventory.

Connecting to hosts: behavioral inventory parameters 

As described above, setting the following variables controls how Ansible interacts with remote hosts.

Host connection:

Note

Ansible does not expose a channel to allow communication between the user and the ssh process to accept a password manually to decrypt an ssh key when using the ssh connection plugin (which is the default). The use of ssh-agent is highly recommended.

ansible_connection: Connection type to the host. This can be the name of any Ansible connection plugin. SSH protocol types are ssh or paramiko. The default is ssh.

General for all connections:

ansible_host: The name of the host to connect to, if different from the alias you wish to give to it. Never set it to depend on inventory_hostname if you use delegation.
ansible_port: The connection port number, if not the default (22 for ssh)
ansible_user: The username to use when connecting to the host
ansible_password: The password to use to authenticate to the host (never store this variable in plain text; always use a vault. See Keep vaulted variables safely visible)

Specific to the SSH connection:

ansible_ssh_private_key_file: Private key file used by SSH. Useful if using multiple keys and you do not want to use SSH agent.
ansible_ssh_common_args: This setting is always appended to the default command line for sftp, scp, and ssh. Useful to configure a ProxyCommand for a certain host (or group).
ansible_sftp_extra_args: This setting is always appended to the default sftp command line.
ansible_scp_extra_args: This setting is always appended to the default scp command line.
ansible_ssh_extra_args: This setting is always appended to the default ssh command line.
ansible_ssh_pipelining: Determines whether or not to use SSH pipelining. This can override the pipelining setting in ansible.cfg.
ansible_ssh_executable (added in version 2.2): This setting overrides the default behavior to use the system ssh. This can override the ssh_executable setting in ansible.cfg under ssh_connection.

Privilege escalation (see Ansible Privilege Escalation for further details):

ansible_become: Equivalent to ansible_sudo or ansible_su, allows to force privilege escalation
ansible_become_method: Allows to set privilege escalation method
ansible_become_user: Equivalent to ansible_sudo_user or ansible_su_user, allows you to set the user you become through privilege escalation
ansible_become_password: Equivalent to ansible_sudo_password or ansible_su_password, allows you to set the privilege escalation password (never store this variable in plain text; always use a vault. See Keep vaulted variables safely visible)
ansible_become_exe: Equivalent to ansible_sudo_exe or ansible_su_exe, allows you to set the executable for the escalation method selected
ansible_become_flags: Equivalent to ansible_sudo_flags or ansible_su_flags, allows you to set the flags passed to the selected escalation method. This can be also set globally in ansible.cfg in the become_flags option under privilege_escalation.

Remote host environment parameters:

ansible_shell_type: The shell type of the target system. You should not use this setting unless you have set the ansible_shell_executable to a non-Bourne (sh) compatible shell. By default, commands are formatted using sh-style syntax. Setting this to csh or fish will cause commands executed on target systems to follow those shell’s syntax instead.

ansible_python_interpreter: The target host Python path. This is useful for systems with more than one Python or not located at /usr/bin/python such as *BSD, or where /usr/bin/python is not a 2.X series Python. We do not use the /usr/bin/env mechanism as that requires the remote user’s path to be set right and also assumes the python executable is named python, where the executable might be named something like python2.6.
ansible_*_interpreter: Works for anything such as ruby or perl and works just like ansible_python_interpreter. This replaces shebang of modules that will run on that host.

New in version 2.1.

ansible_shell_executable: This sets the shell the ansible control node will use on the target machine, overrides executable in ansible.cfg which defaults to /bin/sh. You should only change this value if it is not possible to use /bin/sh (in other words, if /bin/sh is not installed on the target machine or cannot be run from sudo.).

Examples from an Ansible-INI host file:

some_host         ansible_port=2222     ansible_user=manager
aws_host          ansible_ssh_private_key_file=/home/example/.ssh/aws.pem
freebsd_host      ansible_python_interpreter=/usr/local/bin/python
ruby_module_host  ansible_ruby_interpreter=/usr/bin/ruby.1.9.3

Non-SSH connection types 

As stated in the previous section, Ansible executes playbooks over SSH but it is not limited to this connection type. With the host-specific parameter ansible_connection=<connector>, the connection type can be changed. For a full list with available plugins and examples, see Plugin list.

Inventory setup examples 

See also Sample Ansible setup, which shows inventory along with playbooks and other Ansible artifacts.

Example: One inventory per environment 

If you need to manage multiple environments it is sometimes prudent to have only hosts of a single environment defined per inventory. This way, it is harder to, for example, accidentally change the state of nodes inside the “test” environment when you wanted to update some “staging” servers.

For the example mentioned above you could have an inventory_test file:

[dbservers]
db01.test.example.com
db02.test.example.com

[appservers]
app01.test.example.com
app02.test.example.com
app03.test.example.com

That file only includes hosts that are part of the “test” environment. Define the “staging” machines in another file called inventory_staging:

[dbservers]
db01.staging.example.com
db02.staging.example.com

[appservers]
app01.staging.example.com
app02.staging.example.com
app03.staging.example.com

To apply a playbook called site.yml to all the app servers in the test environment, use the following command:

ansible-playbook -i inventory_test -l appservers site.yml

Example: Group by function 

In the previous section, you already saw an example of using groups in order to cluster hosts that have the same function. This allows you, for example, to define firewall rules inside a playbook or role affecting only database servers:

- hosts: dbservers
  tasks:
  - name: Allow access from 10.0.0.1
    ansible.builtin.iptables:
      chain: INPUT
      jump: ACCEPT
      source: 10.0.0.1

Example: Group by location 

Other tasks might be focused on where a certain host is located. Let’s say that db01.test.example.com and app01.test.example.com are located in DC1 while db02.test.example.com is in DC2:

[dc1]
db01.test.example.com
app01.test.example.com

[dc2]
db02.test.example.com

In practice, you might even end up mixing all these setups as you might need to, on one day, update all nodes in a specific data center while, on another day, update all the application servers no matter their location.