Ansible Mastery Part 5 :Dynamic Hostname Configuration with AnsibleNew Post

Welcome back to our Ansible Mastery Journey. If you are new to here , don’t miss to check previous posts before diving into this post.

Ever stare at a bunch of IPs and think, "This would be so much easier if these routers had names?" Same. Instead of setting hostnames manually on each Cisco box, here’s a simple way to get Ansible to do it for you. No copy-pasting the same command again and again. Just clean automation.

Let’s get straight into it.

Start With Your Inventory

Open up your hosts.ini file. Here's how I set mine up:

[anurudh@localhost ansible_start]$ cat hosts.ini 

[routers]
1.1.1.1
2.2.2.2

[ios_devices:children]
routers

[ios_devices:vars]
ansible_user=cisco
ansible_password=cisco
ansible_connection=network_cli
ansible_network_os=ios[anurudh@localhost ansible_start]$ 

This tells Ansible: “Hey, we’re working with Cisco IOS gear, here are the devices, and here’s how to log in.”

Make Sure Ansible Knows What’s Up

Next, check your ansible.cfg. You don’t need anything fancy — just make sure it points to your inventory file and uses the right connection method:

[anurudh@localhost ansible_start]$ cat ansible.cfg 

[defaults]
gathering = explicit
inventory = hosts.ini
transport = network_cli
#host_key_checking = False  # Disables SSH host key checking
[anurudh@localhost ansible_start]$ 

That commented-out line disables SSH key checking — helpful during testing, but use it wisely.

Now the Fun Part: Auto-Generate Hostnames

You don’t want to manually set hostnames like R1111 and R2222, right? Let Ansible do that. Here's a tiny playbook that makes it happen:

[anurudh@localhost ansible_start]$ cat 04_config_hostname.yml 

---
 - name: Configure Hostane on Cisco routers
   hosts: ios_devices
   gather_facts: no 
   
   tasks:
    - name: set hostname based on IP
      cisco.ios.ios_config :
         lines:
            - hostname R{{ inventory_hostname | regex_replace('\\.', '') }}
            
[anurudh@localhost ansible_start]$ 

What’s happening here?

• inventory_hostname is the IP of each device.

• The regex_replace('\\.', '') bit strips out the dots.

• So 1.1.1.1 becomes R1111. Neat, right?

Let’s See It in Action

Run the playbook like this:

[anurudh@localhost ansible_start]$ ansible-playbook  03_ping_router 

PLAY [check connectivity to cisco routers] *************************************************

TASK [ping the router] *********************************************************************
ok: [1.1.1.1]
ok: [2.2.2.2]

PLAY RECAP *********************************************************************************
1.1.1.1                    : ok=1    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0   
2.2.2.2                    : ok=1    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0   

[anurudh@localhost ansible_start]$ 

Once it runs, SSH into your devices to confirm the hostname stuck:

[anurudh@localhost ansible_start]$ ssh [email protected]
([email protected]) Password: 

R1.1.1.1#exit
Connection to 1.1.1.1 closed.
[anurudh@localhost ansible_start]$ ssh [email protected]
([email protected]) Password: 

R2.2.2.2#

If you see that hostanme R111 for router 1 and R2222 for router 2, congrats — it worked.

Recap

• You learned how to build hostnames from router IPs

• You used Jinja2 to clean them up

• You pushed config with Ansible

• You did it without repeating yourself 50 times

If you’ve got more routers to name, this setup will save you hours. Add this to your playbook toolbox — you’ll use it again.

Action Step for You Today

Just one thing: make sure to go through the concepts and follow along with the tasks.

I’ll guide you, one simple post at a time.

We will deep dive more into Ansible in upcoming posts. Don’t miss to DM or ping me with your queries and comments.

Smiles :)

Anurudh