Creating /etc/hosts from ansible hosts

[budhac]

I am trying to figure out if there is a way to use Ansible to automate the creation of /etc/hosts files depending on which group the Ansible host belongs to. I am creating multiple test Kubernetes clusters that will use hosts files for DNS resolution.

[Group 1]

kube-g1-master1 ansible_host=10.90.1.11

kube-g1-master2 ansible_host=10.90.1.12

kube-g1-master3  ansible_host=10.90.1.13

kube-g1-node1      ansible_host=10.90.1.14

kube-g1-node2      ansible_host=10.90.1.15

kube-g1-lb      ansible_host=10.90.1.16

[Group 2]

kube-g2-master1 ansible_host=10.90.1.21

kube-g2-master2 ansible_host=10.90.1.22

kube-g2-master3 ansible_host=10.90.1.23

kube-g2-node1 ansible_host=10.90.1.24

kube-g2-node2 ansible_host=10.90.1.25

kube-g2-lb ansible_host=10.90.1.26

[Group 3]

kube-g3-master1 ansible_host=10.90.1.31

kube-g3-master2 ansible_host=10.90.1.32

kube-g3-master3 ansible_host=10.90.1.33

kube-g3-node1 ansible_host=10.90.1.34

kube-g3-node2 ansible_host=10.90.1.35

kube-g3-lb ansible_host=10.90.1.36
etc...

When I use [all], as Ansible iterates through the hosts file can it search which Group the host belongs to and create an /etc/hosts from the group hostnames and IPs.

For example:

1. Ansible interacts with: kube-g2-master3

inventory_hostname = kube-g2-master3

2. Finds kube-g2-master3 belongs [Group 2]

3. Creates /etc/hosts on kube-g2-master3 using the inventory_hostname and ansible_hosts from [Group 2]

/etc/hosts

kube-g2-master1  10.90.1.21

kube-g2-master2  10.90.1.22

kube-g2-master3  10.90.1.23

kube-g2-node1  10.90.1.24

kube-g2-node2  10.90.1.25

kube-g2-lb  10.90.1.26

Thanks for any help.

[Kai Stian Olstad]

> When I use* [all]*, as Ansible iterates through the hosts file can it
> search which Group the host belongs to and create an* /etc/hosts* from

> the
> group hostnames and IPs.

Space in group names is not allowed.


> For example:
>
> 1. Ansible interacts with: *kube-g2-master3*
>
> inventory_hostname = kube-g2-master3
>
> 2. Finds *kube-g2-master3* belongs* [Group 2]*

Groups in Ansible is just for grouping variables, and the grouping is
lost when Ansible is running.
What you can do is check if a host is in a group, but you would need to
check every group.
when: "'kube-g2-master3' in groups['group2']"

And the list of all group is in the special variable "group_names".


> 3. Creates */etc/hosts* on *kube-g2-master3* using the
> *inventory_hostname*
> and *ansible_hosts* from* [Group 2]*

>
> /etc/hosts
>
> kube-g2-master1 10.90.1.21
> kube-g2-master2 10.90.1.22
> kube-g2-master3 10.90.1.23
> kube-g2-node1 10.90.1.24
> kube-g2-node2 10.90.1.25
> kube-g2-lb 10.90.1.26

Since you are building a host list, all you need in the variable
groups['group2'] this is a list of all members in that group.


--
Kai Stian Olstad

[Vladimir Botka]

> When I use* [all]*, as Ansible iterates through the hosts file can it
> search which Group the host belongs to and create an* /etc/hosts* from the

> group hostnames and IPs.
>
> For example:
>

> 1. Ansible interacts with: *kube-g2-master3*
>
> inventory_hostname = kube-g2-master3
>

> 2. Finds *kube-g2-master3* belongs* [Group 2]*
>

> 3. Creates */etc/hosts* on *kube-g2-master3* using the *inventory_hostname*
> and *ansible_hosts* from* [Group 2]*
>

> /etc/hosts
>
> kube-g2-master1 10.90.1.21
> kube-g2-master2 10.90.1.22
> kube-g2-master3 10.90.1.23
> kube-g2-node1 10.90.1.24
> kube-g2-node2 10.90.1.25
> kube-g2-lb 10.90.1.26
>
> Thanks for any help.
>
>

Use "group_names: List of groups the current host is part of"
https://docs.ansible.com/ansible/latest/reference_appendices/special_variables.html

It's possible to create lists of [host, ip]. For example (given a host may be
a member of multiple groups) the tasks below

- set_fact:
my_hosts: "{{ my_hosts|default([]) +
groups[item]|
zip(groups[item]|
map('extract', hostvars, 'ansible_host')|list)| list }}"
loop: "{{ group_names }}"

- set_fact:
my_hosts: "{{ my_hosts|unique }}"

give

"my_hosts": [
[
"test_01",
"10.1.0.51"
],
[
"test_02",
"10.1.0.52"
],
[
"test_03",
"10.1.0.53"
]
]

Then create /etc/hosts. For example

- template:
src: hosts.j2
dest: /tmp/hosts

$ cat hosts.j2
{% for item in my_hosts %}
{{ item.0 }} {{ item.1 }}
{% endfor %}

Cheers,

-vlado