Fetch first network device and pass to template for rendering

plugins/guests/debian/cap/configure_networks.rb

@@ -15,11 +15,12 @@ module VagrantPlugins
           entries    = []
           interfaces = machine.guest.capability(:network_interfaces)
 
+          root_device = interfaces.first
           networks.each do |network|
             network[:device] = interfaces[network[:interface]]
 
             entry = TemplateRenderer.render("guests/debian/network_#{network[:type]}",
-              options: network,
+              options: network.merge(:root_device => root_device),
             )
             entries << entry
           end

templates/guests/debian/network_dhcp.erb

@@ -6,8 +6,8 @@ iface <%= options[:device] %> inet dhcp
     post-up route del default dev $IFACE || true
 <% else %>
     # We need to disable eth0, see GH-2648
-    post-up route del default dev $IFACE || true
+    post-up route del default dev <%= options[:root_device] %> || true
     post-up dhclient $IFACE
-    pre-down route add default dev $IFACE
+    pre-down route add default dev <%= options[:root_device] %>
 <% end %>
 #VAGRANT-END

test/unit/templates/guests/debian/network_dhcp_test.rb

@@ -23,6 +23,7 @@ describe "templates/guests/debian/network_dhcp" do
     it "renders the template" do
       result = Vagrant::Util::TemplateRenderer.render(template, options: {
         device: "eth1",
+        root_device: "eth0",
         use_dhcp_assigned_default_route: true,
       })
       expect(result).to eq <<-EOH.gsub(/^ {8}/, "")
@@ -31,9 +32,9 @@ describe "templates/guests/debian/network_dhcp" do
         auto eth1
         iface eth1 inet dhcp
             # We need to disable eth0, see GH-2648
-            post-up route del default dev $IFACE || true
+            post-up route del default dev eth0 || true
             post-up dhclient $IFACE
-            pre-down route add default dev $IFACE
+            pre-down route add default dev eth0
         #VAGRANT-END
       EOH
     end