How to add resolvers via ansible on Ubuntu 16.04?

Question

I'm starting on some Ubuntu 14.04 upgrades and creating some Ansible playbooks along the way to be used on several other hosts later on. First off is setting up /etc/resolv.conf to point to local DNS resources.

The old method of provisioning /etc/resolv.conf was a shell script with awk/sed/grep which ran over ssh which edited the file directly. This seems to be a no-no in 16.04 which warns against this with:

# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8) # DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN.

The man page for resolvconf mentions adding dns-nameservers into /etc/network/interfaces and also a note discouraging the use of resolvconf to add nameserver info by hand:

The administrator can run resolvconf from the command line...but this is not normally necessary or advisable.

It seems the only option left is to edit /etc/network/interfaces but how do I specify the network interface in the playbook? It's not the standard eth0..1..2 but some mix of alphabet soup like enp6s0 or ens18 which I don't quite get. On an different upgrade, I was able to override this odd naming schemed by adding biosdevname=0 to grub, but this does not seem to have any effect on this new host even after running update-grub and rebooting.

Is there some way to tell ansible to find the primary nic and add a nameserver line in /etc/network/interfaces for these hosts? My current non-working playbook is below:

(updated working example from accepted answer)

- hosts: all
  tasks:
    - name: setup resolv.conf in DMZ9 for Ubuntu 16.04 hosts
      when: ansible_default_ipv4.address is match("192.1.9")
      when: ansible_distribution_release is match ("xenial")
      interfaces_file:
        iface: "{{ ansible_default_ipv4['interface'] }}" 
        option: dns-nameservers
        value: 192.1.9.4 192.1.9.10

You probably want to look at this question about how to disable interfaces naming scheme https://askubuntu.com/questions/767786/changing-network-interfaces-name-ubuntu-16-04 — AlexD, May 11 '18 at 16:17
The network interface names are available in the `ansible_interfaces` fact. Keep in mind that `lo` is also listed in that fact. — Michael Hampton, May 11 '18 at 16:26
@AlexD Thanks. I added `net.ifnames=0` to grub and now I have `eth0` back. — Server Fault, May 11 '18 at 17:46

score 1 · Accepted Answer · answered May 11 '18 at 19:36

1

ansible_default_ipv4['interface'] contains the name of the interface. I find running

ansible all -i localhost, -m setup -c local

helpful to determine what variables ansible defines.

answered May 11 '18 at 19:36

Mark Wagner

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So like this?: `iface: "{{ ansible_default_ipv4['interface'] }}"`. I've updated the playbook in my question with this. – Server Fault May 11 '18 at 20:16

score 0 · Answer 2 · answered May 11 '18 at 18:27

Just to answer the playbook part of the question for anyone else stumbling through this -- I ended up removing the resolvconf package by hand and was able to forego any twiddling with udev rules. The added net.ifnames=0 boot arg is puzzling, but apparently necessary, in some contexts, for ethX nic naming. I have other 16.04 hosts which were setup months back which do not require this boot arg so something else is apparently involved.

How to add resolvers via ansible on Ubuntu 16.04?

2 Answers2