Discover linux interface name based on IP address - linux

Question

I have code which search specified IP in linux system:
- name: find IP
set_fact:
ip: "{{ item }}"
with_items: "{{ansible_all_ipv4_addresses}}"
when: "item.startswith('10.')"
And works ok, but I can't find out how to discover interface name based on IP from fact "ip".
Does anybody could give some advice or maybe have some example how to do it?

Answer 1

Ansible provides a list of interfaces in the ansible_interfaces fact. You can use this to iterate over available interfaces, checking each one for a given ip address.
That ends up being trickier than it sounds because you'll need to construct fact names, which means rather than something simple like:
ansible_eth0
You instead need:
hostvars[inventory_hostname]["ansible_%s" % item]
An additional complication is that Ansible divides ip addresses into "primary" (which is ansible_eth0.ipv4.address) and "secondaries" (ansible_eth0.ipv4_secondaries), where the latter is a list of dictionaries with address keys. Assuming that we are iterating with item set to an interface name, we can get the primary address like this:
hostvars[inventory_hostname]["ansible_%s" % item].ipv4.address
But! That will fail for interfaces that don't have an ipv4 address assigned, or that don't have a corresponding ansible_<interface> fact for some reason. So we need to deal gracefully with that situation:
(hostvars[inventory_hostname]["ansible_%s" % item]|default({}).get('ipv4', {}).get('address')
This uses the default filter to ensure that we start with a dictionary, and then we use a few levels of Python's .get(key, default) method.
Checking against the secondary addresses is similar but requires that we use the map filter because ipv4_secondaries gives us a list of dictionaries and what we really want is a list of addresses (so that we can check if our target address is in that list):
((hostvars[inventory_hostname]['ansible_%s' % item]|default({}))
.get('ipv4_secondaries'))|map(attribute='address')|list
Putting it all together:
- hosts: localhost
vars:
target_address: 192.168.122.1
tasks:
- set_fact:
target_interface: "{{ item }}"
when: >
(hostvars[inventory_hostname]['ansible_%s' % item]|default({}))
.get('ipv4', {}).get('address') == target_address
or
target_address in ((hostvars[inventory_hostname]['ansible_%s' % item]|default({}))
.get('ipv4_secondaries'))|map(attribute='address')|list
with_items:
- "{{ ansible_interfaces }}"
- debug:
msg: >-
found interface {{ target_interface }}
with address {{ target_address }}
If I run this on my system, the playbook run concludes with:
TASK [debug] *******************************************************************
ok: [localhost] => {
"msg": "found interface virbr0 with address 192.168.24.1"
}
If I run:
ansible-playbook playbook.yml -e target_address=192.168.1.75
I get:
TASK [debug] *******************************************************************
ok: [localhost] => {
"msg": "found interface eth0 with address 192.168.1.75"
}
As you can see from the above, this isn't exactly the sort of task that Ansible is meant for. You would probably be better off stuffing all this logic into an Ansible module so that you could use Python (or some other language) to perform the lookup in a more graceful fashion.
Update
Here is a module-based solution to the same problem.