This blog is part of my series on Devops for Networking. Ansible is a very popular Devops tool and serves similar purposes as Puppet, Chef etc. Ansible has the unique feature that there is no need to install agent on the device side and this makes it very popular for Network device configuration since Network devices are still predominantly a closed system which does not allow agent installation in the device. In this blog, I will cover how to get started with Ansible and in the next blog, I will cover a sample application that I have written.
Ansible basics:
Ansible modules can be run locally or remotely. With the local approach, the module runs locally using apis to talk to remote devices. In remote scenario ,modules are pushed to remote devices, executed as python script and results are returned. Even though there is no need to install remote agent, remote device should allow execution of Python script. Ansible can either be run in command-line for simple tasks or can be executed using a playbook.
Basic modules can be executed from command-line. Following example shows usage of Ansible ping module.
# ansible -m ping 192.168.56.104
192.168.56.104 | success >> {
"changed": false,
"ping": "pong"
}
The above result shows ping was successful.
Following example shows an example of Ansible shell module. “remote” is an Ansible variable that contains host list and will be explained later.
ansible -m shell -a 'df -k' remote 192.168.56.104 | success | rc=0 >> Filesystem 1K-blocks Used Available Use% Mounted on /dev/sda1 6192704 4564984 1313148 78% / udev 2011508 4 2011504 1% /dev tmpfs 404924 880 404044 1% /run none 5120 0 5120 0% /run/lock none 2024616 124 2024492 1% /run/shm cgroup 2024616 0 2024616 0% /sys/fs/cgroup /dev/sr0 63132 63132 0 100% /media/VBOXADDITIONS_4.3.10_93012
In the above example, we get disk usage in the remote system using Ansible shell module.
Following are some terminologies used:
Variables:
- Used both in commandline and in playbooks
- Used to specify inventory(group vars, host vars)
- Variables can also be discovered like environment variables in remote system.
Following are some important variables:
# cat /etc/ansible/hosts: [remote] 192.168.56.104
Above, remote has 1 host defined. Multiple hosts can be part of remote group.
# cat /etc/ansible/group_vars/remote --- ansible_ssh_user: root
Above, remote group is defined to use username “root”.
Playbook:
- Contains plays which is a list of tasks
- Tasks call modules
- Modules are Ansible libraries for performing common tasks.
- Plays contain handlers that execute at the end
- Playbook use YAML syntax.
Following is a simple playbook:
---
- hosts: remote
user: root
sudo: no
tasks:
- name: enables apache module
apache2_module: state=present name=wsgi
- name: installs latest version of perl
apt: name=perl state=latest
- name: installs apache2
apt: name=apache2 state=present
- name: removes wireshark if present
apt: name=wireshark state=absent
Above playbook is defined to run on “remote” hosts group with user “root”. The first task is to enable apache module. The module used is “apache2_module”. The last task is to remove wireshark package if installed. Ansible executes playbooks in an idempotent manner. No operation is done if the task is already executed before.
Modules:
Modules are pre-defined Ansible libraries. Most of the common tasks already have libraries and the library list keeps growing. For a complete list of Ansible modules, refer here.
Important Ansible directories:
/usr/share/ansible - Modules location /etc/ansible/group_vars - Group variables /etc/ansible/hosts - Host details
Getting started:
Ansible needs to be installed first. I used the instructions in this link to install Ansible in my Ubuntu 12.04 system. Password less access needs to be enabled to access remote hosts using ssh. I used this link to setup passwordless ssh access. (I had to run ‘ssh-add’ in localagent, otherwise, I was getting this error “Agent admitted failure to sign using the key”)
I used the following steps to install ansible on Ubuntu 14.04.
$ sudo apt-get install software-properties-common $ sudo apt-add-repository ppa:ansible/ansible $ sudo apt-get update $ sudo apt-get install ansible
Sreenivas Makam's Blog 
Thanks for providing the samples. Now got an idea about how can we use Ansible to control network devices and servers.