Add config for ipip interface

Supplying an empty .network file is weird but without it the tunl0
interface is left in a DOWN state even with Independent=true.

Maybe this is related to "tunl0" being automatically created when the
"ipip" kernel module is loaded.

global/overlay/etc/puppet/modules/cdn/manifests/cache.pp

@@ -60,6 +60,27 @@ class cdn::cache(
     refreshonly => true,
   }
 
+  file { '/etc/systemd/network/10-cdn-ipip.netdev':
+    ensure  => file,
+    owner   => 'root',
+    group   => 'root',
+    mode    => '0644',
+    content => template('cdn/cache/10-cdn-ipip.netdev.erb'),
+  }
+
+  file { '/etc/systemd/network/10-cdn-ipip.network':
+    ensure  => file,
+    owner   => 'root',
+    group   => 'root',
+    mode    => '0644',
+    content => template('cdn/cache/10-cdn-ipip.network.erb'),
+  }
+  # Reload the network config if it has changed
+  exec { 'networkctl reload':
+    subscribe   => File['/etc/systemd/network/10-cdn-ipip.network'],
+    refreshonly => true,
+  }
+
   $sysctl_file = '/etc/sysctl.d/99-cdn-cache.conf'
   file { $sysctl_file:
     ensure  => file,

global/overlay/etc/puppet/modules/cdn/templates/cache/10-cdn-ipip.netdev.erb

@@ -0,0 +1,8 @@
+[NetDev]
+Name=tunl0
+Kind=ipip
+
+[Tunnel]
+Independent=true
+Local=0.0.0.0
+Remote=0.0.0.0

global/overlay/etc/puppet/modules/cdn/templates/cache/10-cdn-ipip.network.erb

@@ -0,0 +1,2 @@
+[Match]
+Name=tunl0