Vagrant tips’n’tricks: changing /etc/hosts automatically for Oracle Universal Installer

Oracle Universal Installer, or OUI for short, doesn’t at all like it if the hostname resolves to an IP address in the 127.0.0.0/0 range. At best it complains, at worst it starts installing and configuring software only to abort and bury the real cause deep in the logs.

I am a great fan of HashiCorp’s Vagrant as you might have guessed reading some of the previous articles, and as such wanted a scripted solution to changing the hostname to something more sensible before I begin provisioning software. I should probably add that I’m using my own base boxes; the techniques in this post should equally apply to other boxes as well.

Each of the Vagrant VMs I’m creating is given a private network for communication with its peers. This is mainly done to prevent me from having to deal with port forwarding on the NAT device. If you haven’t used Vagrant before you might not know that by default, each Vagrant VM will come up with a single NIC that has to use NAT. The end goal for this post is to ensure that my VM’s hostname maps to the private network’s IP address, not 127.0.0.1 as it would normally do.

Setting the scene

By default, Vagrant doesn’t seem to mess with the hostname of the VM. This can be changed by using a configuration variable. Let’s start with the Vagrantfile for my Oracle Linux 7 box:

# -*- mode: ruby -*-
# vi: set ft=ruby :

Vagrant.configure("2") do |config|
  config.vm.define "ol7guest" do |u|
    # this is a base box I created and stored locally
    u.vm.box = "oracleLinux7Base"

    u.ssh.private_key_path = "/path/to/key"

    u.vm.hostname = "ol7guest"
    u.vm.network "private_network", ip: "192.168.56.204"

    u.vm.provider "virtualbox" do |v|
      v.memory = 2048
      v.name = "ol7guest"
      v.cpus = 1
    end
  end
end 

Please ignore the fact that my Vagrantfile is slightly more complex than it needs to be. I do like having speaking names for my VMs, rather than “default” showing up in vagrant status. Using this terminology in the Vagrantfile also makes it easier to add more VMs to the configuration should I so need.

Apart from you just read the only remarkable thing to mention about this file is this line:

    u.vm.hostname = "ol7guest"

As per the Vagrant documentation, I can use this directive to set the hostname of the VM. And indeed, it does:

$ vagrant ssh ol7guest
Last login: Thu Jan 09 21:14:59 2020 from 10.0.2.2
[vagrant@ol7guest ~]$  

The hostname is set, however it resolves to 127.0.0.1 as per /etc/hosts:

[vagrant@ol7guest ~]$ cat /etc/hosts
127.0.0.1    ol7guest    ol7guest
127.0.0.1   localhost localhost.localdomain localhost4 localhost4.localdomain4
::1         localhost localhost.localdomain localhost6 localhost6.localdomain6 

Not quite what I had in mind, but apparently expected behaviour. So the next step is to change the first line in /etc/hosts to match the private IP address I assigned to the second NIC. As an Ansible fan I am naturally leaning towards using a playbook, but I also understand that not everyone has Ansible installed on the host and using the ansible_local provisioner might take longer than necessary unless your box has Ansible pre-installed.

The remainder of this post deals with an Ansible solution and the least common denominator, the shell provisioner.

Using an Ansible playbook

Many times I’m using Ansible playbooks to deploy software to Vagrant VMs anyway, so embedding a little piece of code into my playbooks to change /etc/hosts isn’t a lot of work. The first step is to amend the Vagrantfile to reference the Ansible provisioner. One possible way to do this in the context of my example is this:

# -*- mode: ruby -*-
# vi: set ft=ruby :

Vagrant.configure("2") do |config|
  config.vm.define "ol7guest" do |u|
    # this is a base box I created and stored locally
    u.vm.box = "oracleLinux7Base"

    u.ssh.private_key_path = "/path/to/key"

    u.vm.hostname = "ol7guest"
    u.vm.network "private_network", ip: "192.168.56.204"

    u.vm.provision "ansible" do |ansible|
      ansible.playbook = "change_etc_hosts.yml"
      ansible.verbose = "v"
    end

    u.vm.provider "virtualbox" do |v|
      v.memory = 2048
      v.name = "ol7guest"
      v.cpus = 1
    end
  end
end  

It is mostly the same file with the addition of the call to Ansible. As you can imagine the playbook is rather simple:

---
- hosts: ol7guest
  become: yes
  tasks:
  - name: change /etc/hosts
    lineinfile:
      path: '/etc/hosts'
      regexp: '.*ol7guest.*' 
      line: '192.168.56.204   ol7guest.example.com   ol7guest' 
      backup: yes

It uses the lineinfile module to find lines containing ol7guest and replaces that line with the “correct” IP address. The resulting hosts file is exactly what I need:

[vagrant@ol7guest ~]$ cat /etc/hosts
192.168.56.204   ol7guest.example.com   ol7guest
127.0.0.1   localhost localhost.localdomain localhost4 localhost4.localdomain4
::1         localhost localhost.localdomain localhost6 localhost6.localdomain6
[vagrant@ol7guest ~]$ 

The first line of the original file has been replaced with the private IP which should enable OUI to progress past this potential stumbling block.

Using the shell provisioner

The second solution involves the shell provisioner, which – unlike Ansible – isn’t distribution agnostic and needs to be tailored to the target platform. On Oracle Linux, the following worked for me:

# -*- mode: ruby -*-
# vi: set ft=ruby :

$script = <<-SCRIPT
/usr/bin/cp /etc/hosts /root && \
/usr/bin/sed -ie '/ol7guest/d' /etc/hosts && \
/usr/bin/echo '192.168.56.204 ol7guest.example.com ol7guest' >> /etc/hosts
SCRIPT

Vagrant.configure("2") do |config|
  config.vm.define "ol7guest" do |u|
    # this is a base box I created and stored locally
    u.vm.box = "oracleLinux7Base"

    u.ssh.private_key_path = "/path/to/key"

    u.vm.hostname = "ol7guest"
    u.vm.network "private_network", ip: "192.168.56.204"

    u.vm.provision "shell", inline: $script

    u.vm.provider "virtualbox" do |v|
      v.memory = 2048
      v.name = "ol7guest"
      v.cpus = 1
    end
  end
end 

The script copies /etc/hosts to root’s home directory and then changes it to match my needs. At the end, the file is in exactly the shape I need it to be in.

Summary

Whether you go with the shell provisioner or embed the change to the hostname in an (existing) Ansible playbook doesn’t matter much. I would definitely argue in support of having the code embedded in a playbook if that’s what will provision additional software anyways. If installing Ansible on the host isn’t an option, using the shell as a fallback mechanism is perfectly fine, too. Happy hacking!