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Generate /etc/hosts with Ansible
# Idempotent way to build a /etc/hosts file with Ansible using your Ansible hosts inventory for a source.
# Will include all hosts the playbook is run on.
# Inspired from http://xmeblog.blogspot.com/2013/06/ansible-dynamicaly-update-etchosts.html
- name: "Build hosts file"
lineinfile: dest=/etc/hosts regexp='.*{{ item }}$' line="{{ hostvars[item].ansible_default_ipv4.address }} {{item}}" state=present
when: hostvars[item].ansible_default_ipv4.address is defined
with_items: groups['all']
@tuxillo
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tuxillo commented Jan 27, 2018

Hi,

I've kind of mixed all info shared here plus some bits from a bunch of Google searches.

First you need a playbook that includes all hosts because the 'ansible_play_batch' variablle has the list of hostnames that apply to the current play. I have a 'common' role like in the 'Best practices' document from Ansible.

---
# Main site file - allhosts.yml

- hosts: all
  roles:
    - common

The hosts file task:

- name: update /etc/hosts file
  blockinfile:
    dest: /etc/hosts
    content: "{{ lookup('template', 'templates/etc/hosts.j2') }}"
    state: present

And the Jinja2 template used in the task:

{% for item in ansible_play_batch %}
{% set short_name = item.split('.') %}
{{ hostvars[item].ansible_default_ipv4.address }}       {{ item }}      {{ short_name[0] }}
{% endfor %}

By using 'blockinfile' module you don't override the already existing entries in your hosts file. The calculated entries are added in a block at the end.

@rhinoceros
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rhinoceros commented Feb 11, 2018

@tuxillo
+1
I used your way to configure /etc/hosts
thanks!

inventory file

[servers]
master ansible_ssh_host=172.18.23.69
node1 ansible_ssh_host=172.18.23.70
node2 ansible_ssh_host=172.18.23.71
node3 ansible_ssh_host=172.18.23.72

templates/etc/hosts.j2

{% for item in ansible_play_batch %}
{{ hostvars[item].ansible_ssh_host }}       {{ item }}.example.com 
{% endfor %}

playbook task

- hosts: servers
  gather_facts: False
  tasks:
  - name: update /etc/hosts file
    become: true
    blockinfile:
      dest: /etc/hosts
      content: "{{ lookup('template', 'templates/etc/hosts.j2') }}"
      state: present

@chuckwm
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chuckwm commented Feb 22, 2018

Thanks for the templates/etc/hosts.j2 stuff, works nicely. What I can't get it to work is setting string padding using format. So basically 12 chars for IP, 10 chars for shortname, etc. Supposedly possible with:

{{ "%-12s%10" | format("192.168.244.244", "my-short-name" ) }}

But not:

{{ "%-12s%10" | format({{ hostvars[item]['ansible_host'] }}, {{ item }} ) }}

do I need something like str() in front or something? Sorry for the silly question. I just like things to line up.

@zswanson
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zswanson commented Mar 2, 2018

@rhinoceros note that as of ansible 2.0 the ansible_ssh_user, ansible_ssh_host, ansible_ssh_port variables are deprecated and you should just use 'ansible_host' etc.

@tiendungitd
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tiendungitd commented Mar 28, 2018

I got issue when using this way, try with ansible version 2.4 and 2.5, any idea?

fatal: [db-master]: FAILED! => {
    "msg": "The task includes an option with an undefined variable. The error was: 'dict object' has no attribute 'ansible_default_ipv4'\n\nThe error appears to have been in '/home/ubuntu/postgres_ha/tasks/install.yml': line 2, column 5, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n---\n  - name: Build hosts file\n    ^ here\n"
}

try to debug

`[root@origin-deployer postgres_ha]# ansible pgmaster -i inventory -m debug -a "var=ansible_default_ipv4" -b

db-master | SUCCESS => {
    "ansible_default_ipv4": "VARIABLE IS NOT DEFINED!",
    "changed": false
}
`

@haridsv
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haridsv commented Apr 8, 2018

@tiendungitd I got the same error using a 2.5 version of Ansible, not sure what happened to these variables. I just got around by using hostvars[item].ip instead.

@dpneumo
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dpneumo commented Apr 25, 2018

@rhinoceros, @tiendungitd, @haridsv
Ansible requires that facts have been gathered before 'hostvars' can be used to obtain the desired data. So be careful with 'gather_facts: False'.

Ansible 2.5, at least, seems to require that the 'hostvars' values be referenced like this:

{{ hostvars[item].ansible_facts.default_ipv4.address }}

Notice the 'ansible_facts' reference. It was a bit painful, but I started with only 'hostvars[item]' in the template. Then in the generated 'hosts' file I 'walked' into the resultant text to find the appropriate nested fields. There are other reference paths to the desired ip address that could be followed.

etc_hosts.yml:

---
- hosts: all
  become: yes
  become_user: root
  tasks:
    - name: update /etc/hosts file
      blockinfile:
        dest: /etc/hosts
        content: "{{ lookup('template', 'hosts.j2') }}"
        state: present

template/hosts.j2:

{% for item in ansible_play_batch %}
{%- if item != 'localhost' %}
{{ hostvars[item].ansible_facts.default_ipv4.address }}       {{ item }}
{% endif %}
{% endfor %}

To solve this I initially tried:

---
- hosts: localhost
  become: yes
  become_user: root
  tasks:
    - debug: var=inventory_hostname
    - debug: var=hostvars[inventory_hostname]

to decipher the reference path to the desired ip address, but that does not show that the 'ansible_facts' reference is required in the template.

There may be someplace that documents current references to the values available in 'hostvars' and current appropriate methods to access them. But I haven't found it yet.

@rwngallego
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rwngallego commented Jan 26, 2019

It works in Ansible 2.7.6:

  - name: Add the inventory into /etc/hosts
    lineinfile:
      dest: /etc/hosts
      regexp: '.*{{ item }}$'
      line: "{{ hostvars[item]['ansible_default_ipv4']['address'] }} {{item}}"
      state: present
    when: hostvars[item]['ansible_facts']['default_ipv4'] is defined
    with_items:
      - "{{ groups['all'] }}"

@zx1986
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zx1986 commented Aug 26, 2019

I got:

"msg": "The conditional check 'hostvars[item].ansible_default_ipv4.address is defined' failed. 
The error was: 
error while evaluating conditional (hostvars[item].ansible_default_ipv4.address is defined): 
u\"hostvars['groups['all']']\" is undefined\n\n

The error appears to be in '/var/lib/awx/projects/_16__openpoint_ansible/playbooks/hostname.yml': line 12, column 7, 
but may\n
be elsewhere in the file depending on the exact syntax problem.
\n\n

The offending line appears to be:
\n\n  
post_tasks:
\n    - name: Build hosts file
\n      ^ here
\n",

And I fix it with @rwngallego 's codes.


https://serverfault.com/questions/832799/ansible-add-ip-of-all-hosts-to-etc-hosts-of-all-other-hosts

@ilyesAj
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ilyesAj commented Jan 9, 2020

this works for ansible 2.2.1

  tasks:
          - name: "generate hosts file from inventory "
            copy:
              src: "{{inventory_for_bastion}}"
              dest: /etc/ansible/hosts
              mode: 766
          - name: "Build hosts file"
            lineinfile: dest=/etc/hosts regexp='.*{{ item }}$' line="{{ hostvars[item].ansible_host}} {{item}}" state=present
            with_items: "{{ groups['internal'] }}"

make sur that the group internal is defined in your inventory located in /etc/ansible/hosts and looks like that :

[internal]
terraform-controller-0   ansible_host=10.240.0.4

@smareti
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smareti commented Jan 24, 2020

It works in ansible 2.9.2
Screen Shot 2020-01-23 at 7 38 14 PM

@soliverprofesional
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soliverprofesional commented Mar 2, 2020

Hi there, sorry for refloating. I'm sharing my update just in the case helps someone else.

Based on the solutions already posted, I've got the issue where I had hosts with multiple NICs/IPs that I wanted to add to the /etc/hosts.
e.g.:

192.168.10.11 10.0.2.15 10.10.10.11       ha-1      ha-1
192.168.10.12 10.0.2.15 10.10.10.12       ha-2      ha-2

So I changed the variable {{ hostvars[item].ansible_facts.default_ipv4.address }} for {{ hostvars[item].ansible_facts.all_ipv4_addresses | join(" ") }} that is basically the list of IP address in the host gather from the ansible facts, converted to a list of strings separated by space.

Thank you guys for adding your solutions. Were extremely helpful for me.

@dywanik
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dywanik commented Mar 30, 2020

Hi, I was wondering if anybody was able to test this solution using Molecule?

@enr0s
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enr0s commented Jun 26, 2020

@dywanik After some research, I could confirm there's no way to manipulate /etc/hosts with Molecule using docker platform. The /etc/hosts file is crucial for Docker's linking system and it should only be manipulated manually at the image level, rather than the container level.
See:
https://docs.docker.com/network/links/#updating-the-etchosts-file

William-Yeh/docker-ansible#4 (comment)

@enr0s
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enr0s commented Jun 26, 2020

Anyway the solution proposed here works fine.
I have simply added this code in my molecule.yml

...
provisioner:
  name: ansible
  inventory:
    group_vars:
      all:
        run_not_in_container: False
...

and then modified my tasks/main.yml

- name: Generate /etc/hosts file
  template:
    src: etc/hosts.j2
    dest: /etc/hosts
    owner: root
    group: root
    mode: 0644
  when: run_not_in_container

when you test your role remember to set the variable

[all:vars]
run_not_in_container=True

@gardar
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gardar commented Aug 5, 2021

@enr0s, you can also use the tag/variable molecule-notest that's already set by molecule, so you don't have to define your own.

tasks/main.yml

---
- name: Generate /etc/hosts file
  template:
    src: etc/hosts.j2
    dest: /etc/hosts
    owner: root
    group: root
    mode: 0644
  tags:
    - molecule-notest

or
tasks/main.yml

---
- name: Generate /etc/hosts file
  template:
    src: etc/hosts.j2
    dest: /etc/hosts
    owner: root
    group: root
    mode: 0644
  when: molecule-notest not in ansible_skip_tags

Another solution to this issue would be to use a "dummy" hosts file with molecule, which is not a perfect solution but is better than skipping the task in my opinion because you can then confirm that the hosts file is generated correctly.
The way to achieve that would be something like:

defaults/main.yml

---
etc_hosts_file: /etc/hosts

molecule.yml

provisioner:
  name: ansible
  inventory:
    group_vars:
      all:
        etc_hosts_file: /tmp/molecule_etc_hosts

tasks/main.yml

---
- name: Generate /etc/hosts file
  template:
    src: etc/hosts.j2
    dest: "{{ etc_hosts_file }}"
    owner: root
    group: root
    mode: 0644

@irigon
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irigon commented Dec 9, 2021

@dywanik , I am not sure if that is what you are searching for... in my case, I wanted simply to add an entry to /etc/hosts.
The following worked for me:

molecule.yml:

---
...
platforms:
  - name: "myplatformname"
    etc_hosts:
      "repo": "8.8.8.8"

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