Ansible: Find (first) free IP-address and set it - ansible

Question

I have a couple of dozen Fedora-clients with DHCP-IP-addresses and I need to set them to static IPs. Usually there is (in this case) only one client per VLAN, but there can be more. The idea is to check if a IP-address (in the same range) is pingable and if not I set the clients IP to this address. This would be sufficient for my case but I'm happy to hear better suggestions.
Setting a given IP-address works fine, but how can I find the first "free" address and use it later in the playbook? So i guess: How can I register the first "failed" IP-address?
My ping-test-command
- name: "Test if the ip address is already online."
command: "ping -q -c 1 -W 1 192.168.178.{{ item }}"
register: res
loop: "{{ range(1,5) | list }}"
#failed_when: res.rc == 0
Output:
[user#host ansible]$ ansible-playbook change_network.yml
PLAY [all] **************************************************************************************************************************************************************************************************
TASK [Gathering Facts] **************************************************************************************************************************************************************************************
ok: [192.168.178.40]
TASK [change_network : Test if the ip address is already online.] *******************************************************************************************************************************************
changed: [192.168.178.40] => (item=1)
changed: [192.168.178.40] => (item=2)
failed: [192.168.178.40] (item=3) => {"ansible_loop_var": "item", "changed": true, "cmd": ["ping", "-q", "-c", "1", "-W", "1", "192.168.178.3"], "delta": "0:00:01.003836", "end": "2020-12-30 12:04:18.700349", "item": 3, "msg": "non-zero return code", "rc": 1, "start": "2020-12-30 12:04:17.696513", "stderr": "", "stderr_lines": [], "stdout": "PING 192.168.178.3 (192.168.178.3) 56(84) bytes of data.\n\n--- 192.168.178.3 ping statistics ---\n1 packets transmitted, 0 received, 100% packet loss, time 0ms", "stdout_lines": ["PING 192.168.178.3 (192.168.178.3) 56(84) bytes of data.", "", "--- 192.168.178.3 ping statistics ---", "1 packets transmitted, 0 received, 100% packet loss, time 0ms"]}
failed: [192.168.178.40] (item=4) => {"ansible_loop_var": "item", "changed": true, "cmd": ["ping", "-q", "-c", "1", "-W", "1", "192.168.178.4"], "delta": "0:00:01.004021", "end": "2020-12-30 12:04:20.741041", "item": 4, "msg": "non-zero return code", "rc": 1, "start": "2020-12-30 12:04:19.737020", "stderr": "", "stderr_lines": [], "stdout": "PING 192.168.178.4 (192.168.178.4) 56(84) bytes of data.\n\n--- 192.168.178.4 ping statistics ---\n1 packets transmitted, 0 received, 100% packet loss, time 0ms", "stdout_lines": ["PING 192.168.178.4 (192.168.178.4) 56(84) bytes of data.", "", "--- 192.168.178.4 ping statistics ---", "1 packets transmitted, 0 received, 100% packet loss, time 0ms"]}
So 192.168.178.3 is the first free address and could be used in this example.
The second problem is, that I need to search in the correct range, i.e. replace the hardcoded "192.168.178" with an ansible variable (the first three octets of the DHCP-address stay the same). I tried something like from this question: Ansible increment IP address
- name: Append the last octet to the first 3 octets
set_fact: new_ip_address="{{ ansible_default_ipv4.address | regex_replace('(^.*\.).*$', '\\1') }}{{ item }}"
loop: "{{ range(1,5) | list }}"
- debug: var=new_ip_address
It works in this form, but I couldn't get this to work in my ping-command. Also maybe there is a prettier solution with a jinja-filter instead of the regex?

Answer 1

as serverfault question you can use nmap to issue a subnet scan.
Pay attentions: unless you know the networking infrastructure, there is still the possibility that picked IP address is used, firewall can block scanning.
To increase the probability that the found IP is really free, you can also use a DNS reverse query, to check if that IP has a PTR record:
- name: Scanning for free ip address
command:>
nmap -v -sn -n "{{ subnet }}" -oG - \
| awk '/Status: Down/{print $2}' \
| awk 'NR>2 {print last} {last=$0}'
register: ipscan
- name: Get fist free ip address
set_fact:
ipv4_address: "{{ ipscan.stdout_lines | first }}"
We need the second awk to remove the network address and broadcast address (first and last) from the output.
Checking also for reverse DNS on picked ip address:
- name: Retrieving host DNS Data.
set_fact:
vm_ipv4_dns_names: "{{ lookup('dig', ipv4_address ~ '/PTR').split(',') | map('regex_replace', '\\.?$', '' ) }}"
- name: Check reverse DNS returns data
fail:
msg: Picked ip address is on DNS
when: ('NXDOMAIN' not in vm_ipv4_dns_names)
I hope you find this useful.