Find name of default network interface to set MTU.

On Ubuntu 16.04, the default networks interface is not always called
eth0, depending on the hardware. The name of the default network
interface on some VM types on AWS is ens3, for example.

Determine the interface name using ip route command instead of
hardcoding to eth0.

Also, /etc/network/interfaces.d/<interface-name>.cfg files no longer
exist in Ubuntu 16.04. Use a script in /etc/network/if-up.d to
adjust the MTU value at startup instead.

playbooks/roles/aws/tasks/main.yml

@@ -30,24 +30,23 @@
   tags:
     - deploy
 
-- name: Set the MTU to 1500 temporarily
-  shell: /sbin/ifconfig eth0 mtu 1500 up
+- name: Get name of default network interface
+  shell: "ip route show 0/0 | awk '{print $NF}'"
+  register: get_interface_name
   when: ansible_distribution in common_debian_variants
 
-- name: Check for eth0.cfg
-  stat:
-    path: /etc/network/interfaces.d/eth0.cfg
-  register: eth0_cfg
+- name: Set the MTU to 1500 temporarily
+  shell: "/sbin/ifconfig {{ get_interface_name.stdout }} mtu 1500 up"
   when: ansible_distribution in common_debian_variants
 
-- name: Set the MTU to 1500 inside eth0.cfg file
-  lineinfile:
-    dest: /etc/network/interfaces.d/eth0.cfg
-    regexp: "^post-up /sbin/ifconfig eth0 mtu 1500"
-    line: "post-up /sbin/ifconfig eth0 mtu 1500"
-    insertafter: "^iface"
-  when: ansible_distribution in common_debian_variants and eth0_cfg.stat.exists
-
+- name: Set the MTU to 1500 permanently
+  template:
+    dest: /etc/network/if-up.d/mtu
+    src: mtu.j2
+    mode: 0755
+    owner: root
+    group: root
+  when: ansible_distribution in common_debian_variants
 #
 # End dealing with Jumbo frames issue in mixed MTU deployements in AWS
 #

playbooks/roles/aws/templates/mtu.j2

+#!/bin/sh
+ifconfig {{ get_interface_name.stdout }} mtu 1500