Exercise: Selecting hosts with host patterns

Selecting hosts with host patterns

• Use names, groups, wildcards, lists, exclusions and intersections to select hosts from an inventory.

Prerequisites: Complete control-node setup and the preceding lessons in this chapter. Run commands as student on workstation unless a step names another machine.

From the project directory, run the relevant home-lab check. For an exercise with several checkpoints, grade each state before changing or removing it:

lab grade projects-host

The grader reads project files and host state. A passing report covers the listed checks; also perform the task’s independent connection, repeat-run, and reboot checks where requested.

Find hosts in London that are not databases. Predict their names before testing your pattern.

Record the published result first. The grader checks the original requirements, so a changed name or value may intentionally fail those checks.

Predict and verify host selection without altering any systems. Use the two supplied inventories and playbook.yml.

• In inventory1, select a single named host, a listed IP, all hosts, names ending in example.com, and that domain excluding lab.example.com.
• Select exactly lb1.lab.example.com, s1.lab.example.com, and db1.example.com; then select addresses beginning 172.25..
• Compare a wildcard that matches stage with one that matches individual host names. Select the union of production, addresses beginning 172, and names containing lab; then the intersection of database and London hosts.
• In inventory2, verify London, Europe, ungrouped, and the missing Australia group. Explain why a missing group can produce a successful run that performs no work.

Look at the project

[student@workstation ~]$ cd ~/projects-host
[student@workstation projects-host]$ ls
ansible.cfg  inventory1  inventory2  playbook.yml

Read inventory1. It has four hosts outside any group, groups by function (web, db, lb), by city (boston, london) and by environment (dev, stage, prod, new), and three layers of parent groups. Some hosts are in lab.example.com, some directly in example.com, and new lists three IP addresses.

[web]
jupiter.lab.example.com
saturn.example.com

[db]
db1.example.com
db2.example.com
db3.example.com

[lb]
lb1.lab.example.com
lb2.lab.example.com

[boston]
db1.example.com
jupiter.lab.example.com
lb2.lab.example.com

[london]
db2.example.com
db3.example.com
file1.lab.example.com
lb1.lab.example.com

[dev]
web1.lab.example.com
db3.example.com

[stage]
file2.example.com
db2.example.com

[prod]
lb2.lab.example.com
db1.example.com
jupiter.lab.example.com

[function:children]
web
db
lb
city

[city:children]
boston
london
environments

[environments:children]
dev
stage
prod
new

[new]
172.25.252.23
172.25.252.44
172.25.252.32

inventory2 is much smaller: workstation.lab.example.com outside any group, one server in each of london, berlin, tokyo and atlanta, and a parent group europe made of london and berlin.

The playbook prints the name of each host it matches:

---
- name: Resolve host patterns
  hosts: db1.example.com
  gather_facts: false
  tasks:
    - name: Display managed hosts matching the host pattern
      ansible.builtin.debug:
        msg: "{{ inventory_hostname }}"

One host by name

Run the playbook against inventory1 as it is:

[student@workstation projects-host]$ ansible-navigator run \
> -m stdout playbook.yml -i inventory1

PLAY [Resolve host patterns] ***************************************************

TASK [Display managed hosts matching the host pattern] *************************
ok: [db1.example.com] => {
    "msg": "db1.example.com"
}

PLAY RECAP *********************************************************************
db1.example.com            : ok=1    changed=0    unreachable=0    failed=0  ...

For the rest of the exercise, only change the hosts: line and run the same command. The outputs below show just the ok: line of each matched host.

One host by IP address

Set hosts: 172.25.252.44.

ok: [172.25.252.44]

This works only because the inventory lists that address. An address that merely resolves to an inventory host, or a DNS name that the inventory does not contain, matches nothing.

Every host

Set hosts: all. All seventeen hosts are listed: the four without a group, the ten named hosts in groups, and the three addresses.

A domain, with a wildcard

Set hosts: '*example.com', with the quotes.

ok: [srv1.example.com]
ok: [srv2.example.com]
ok: [s1.lab.example.com]
ok: [s2.lab.example.com]
ok: [jupiter.lab.example.com]
ok: [saturn.example.com]
ok: [db1.example.com]
ok: [db2.example.com]
ok: [db3.example.com]
ok: [lb1.lab.example.com]
ok: [lb2.lab.example.com]
ok: [file1.lab.example.com]
ok: [web1.lab.example.com]
ok: [file2.example.com]

Fourteen hosts: every name that ends in example.com, including those in lab.example.com. Only the three IP addresses are left out.

Leave out a subdomain

Keep the hosts in example.com but drop everything in lab.example.com:

  hosts: '*.example.com, !*.lab.example.com'

ok: [srv1.example.com]
ok: [srv2.example.com]
ok: [saturn.example.com]
ok: [db1.example.com]
ok: [db2.example.com]
ok: [db3.example.com]
ok: [file2.example.com]

A list of hosts

Without using any group, select exactly lb1.lab.example.com, s1.lab.example.com and db1.example.com.

  hosts: lb1.lab.example.com,s1.lab.example.com,db1.example.com

ok: [lb1.lab.example.com]
ok: [s1.lab.example.com]
ok: [db1.example.com]

Addresses by prefix

Select every host whose address starts with 172.25.

  hosts: '172.25.*'

ok: [172.25.252.23]
ok: [172.25.252.44]
ok: [172.25.252.32]

A wildcard that matches a group

Set hosts: 's*' and predict the result before you run it.

ok: [file2.example.com]
ok: [db2.example.com]
ok: [srv1.example.com]
ok: [srv2.example.com]
ok: [s1.lab.example.com]
ok: [s2.lab.example.com]
ok: [saturn.example.com]

file2 and db2 do not start with s. They are here because they are in the group stage, which does. A wildcard is matched against host names and group names, and it cannot tell them apart.

Mix groups and wildcards

In one pattern, select the hosts in prod, every address that starts with 172, and every host with lab in its name.

  hosts: 'prod,172*,*lab*'

ok: [lb2.lab.example.com]
ok: [db1.example.com]
ok: [jupiter.lab.example.com]
ok: [172.25.252.23]
ok: [172.25.252.44]
ok: [172.25.252.32]
ok: [s1.lab.example.com]
ok: [s2.lab.example.com]
ok: [lb1.lab.example.com]
ok: [file1.lab.example.com]
ok: [web1.lab.example.com]

Each host appears once, even when several terms match it.

Hosts in two groups at once

Select the hosts that are in both db and london.

  hosts: db,&london

ok: [db2.example.com]
ok: [db3.example.com]

db1 is in db but in boston, so the intersection drops it.

Groups and nested groups in inventory2

Switch to the second inventory and try three patterns, running with -i inventory2:

[student@workstation projects-host]$ ansible-navigator run \
> -m stdout playbook.yml -i inventory2

europe has no hosts of its own; it contains the hosts of its child groups. ungrouped is a built-in group of the hosts that are in no other group.

A group that does not exist

Set hosts: australia and run against inventory2:

[student@workstation projects-host]$ ansible-navigator run \
> -m stdout playbook.yml -i inventory2
[WARNING]: Could not match supplied host pattern, ignoring: australia

PLAY [Resolve host patterns] ***************************************************
skipping: no hosts matched

PLAY RECAP *********************************************************************

Not an error: a warning, and a play that does nothing. A misspelled group name behaves exactly like this, which is why the warning is worth reading.

Finish

Check your work with lab grade projects-host, then put the exercise away with lab finish projects-host.