I am new to Ansible, but I have to maintain a set of playbooks, which correspond to services to be setup in a given environment. They need to be assigned a port, certificates, etc. This results in many files with lists of essentially always the same names and an assignment to them.

In many cases I think I can easily reuse the service_name as variable, but when mapping to IPs, ports or other numeric identifiers I have not yet figured out a way to deterministically assign them different numbers in a way that is reproducible, and preferably remains the same even when new services are added. I have considered using a SQLite database to store the services from and to generate the values from their ids, but I have no idea how to integrate that with Ansible.

I assume that assigning increasing port numbers is not something entirely new; it is something a lot of sysadmins to on a day-to-day basis so there has to be some way to do that.

Edit: We directly add the port numbers etc. in group_vars/all.yml like this:


The inventory is generated automatically, since we create additional jails (BSD) and depends on the roles that will be executed.

  • 2
    Since you probably have to add the service to the inventory anyway, what's stopping you to add a specific port number there manually? Or if your inventory is automatically generated, then you should probably solve the problem at the inventory generation level
    – SztupY
    Mar 2, 2017 at 11:49
  • Would you please include some Ansible snippets? It's not clear if you are using group_vars or adding variables right into inventory files. It's also not clear how your inventory is being generated.
    – Woodland
    Mar 2, 2017 at 23:25
  • Have you considered using a service discovery tool like HashiCorp's Consul? Mar 4, 2017 at 3:18

1 Answer 1


Disclaimer: I'm not using Ansible.

What I would do is use a random "predictable" number. According to Ansible doc you can seed the random number generator:

As of Ansible version 2.3, it’s also possible to initialize the random number generator from a seed. This way, you can create random-but-idempotent numbers:

"{{ 59 |random(seed=inventory_hostname) }} * * * * root /script/from/cron"

So in your case for a port number (I assume unpriviledged) I would go for a variable with something like:

port="{{ 32767 |random(start=1024,seed=service_name) }}"

Max at 32767 to avoid clashing with any client initiated port (See Ephemeral port for the reason).

  • That is a good idea, which I considered, but it makes the increasing part impossible imo. Having certain services which are logically related in a certain port range is probably very desirable.
    – midor
    Mar 2, 2017 at 13:13

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