- Filter out empty strings when constructing the list of network interfaces

- Sort interfaces properly whose name does not contain a numeric suffix (e.g. lo)
- Filter out loopback interfaces

plugins/guests/linux/cap/network_interfaces.rb

@@ -19,6 +19,7 @@ module VagrantPlugins
           machine.communicate.sudo("#{path} -o -0 addr | grep -v LOOPBACK | awk '{print $2}' | sed 's/://'") do |type, data|
             s << data if type == :stdout
           end
+
           # In some cases net devices may be added to the guest and will not
           # properly show up when using `ip`. This pulls any device information
           # that can be found in /proc and adds it to the list of interfaces
@@ -26,7 +27,17 @@ module VagrantPlugins
           machine.communicate.sudo("cat /proc/net/dev | grep -E '^[a-z0-9 ]+:' | awk '{print $1}' | sed 's/://'", error_check: false) do |type, data|
             s << data if type == :stdout
           end
-          ifaces = s.split("\n")
+
+          # Collect all loopback interfaces
+          los = ""
+          machine.communicate.sudo("#{path} -o -0 addr | grep LOOPBACK | awk '{print $2}' | sed 's/://'") do |type, data|
+            los << data if type == :stdout
+          end
+          loifaces = los.split("\n")
+          @@logger.debug("loopback interfaces: #{loifaces.inspect}")
+
+          ifaces = s.split("\n").reject { |x| x.empty? or loifaces.include?(x) }
+
           @@logger.debug("Unsorted list: #{ifaces.inspect}")
           # Break out integers from strings and sort the arrays to provide
           # a natural sort for the interface names
@@ -34,7 +45,7 @@ module VagrantPlugins
           #  as expected. This is generally seen with veth* devices, and proper ordering
           #  is currently not required
           ifaces = ifaces.map do |iface|
-            iface.scan(/(.+?)(\d+)/).flatten.map do |iface_part|
+            iface.scan(/(.+?)(\d+)?/).flatten.map do |iface_part|
               if iface_part.to_i.to_s == iface_part
                 iface_part.to_i
               else

test/unit/plugins/guests/linux/cap/network_interfaces_test.rb

@@ -23,60 +23,84 @@ describe "VagrantPlugins::GuestLinux::Cap::NetworkInterfaces" do
 
     it "sorts discovered classic interfaces" do
       expect(comm).to receive(:sudo).twice.and_yield(:stdout, "eth1\neth2\neth0")
+      expect(comm).to receive(:sudo).and_yield(:stdout, "lo")
       result = cap.network_interfaces(machine)
       expect(result).to eq(["eth0", "eth1", "eth2"])
     end
 
+    it "sorts discovered classic interfaces and handles trailing newline" do
+      expect(comm).to receive(:sudo).twice.and_yield(:stdout, "eth1\neth2\neth0\n")
+      expect(comm).to receive(:sudo).and_yield(:stdout, "lo")
+      result = cap.network_interfaces(machine)
+      expect(result).to eq(["eth0", "eth1", "eth2"])
+    end
+
+    it "filters out lo interface" do
+      expect(comm).to receive(:sudo).twice.and_yield(:stdout, "lo\neth0")
+      expect(comm).to receive(:sudo).and_yield(:stdout, "lo")
+      result = cap.network_interfaces(machine)
+      expect(result).to eq(["eth0"])
+    end
+
     it "sorts discovered predictable network interfaces" do
       expect(comm).to receive(:sudo).twice.and_yield(:stdout, "enp0s8\nenp0s3\nenp0s5")
+      expect(comm).to receive(:sudo).and_yield(:stdout, "lo")
       result = cap.network_interfaces(machine)
       expect(result).to eq(["enp0s3", "enp0s5", "enp0s8"])
     end
 
     it "sorts discovered classic interfaces naturally" do
       expect(comm).to receive(:sudo).twice.and_yield(:stdout, "eth1\neth2\neth12\neth0\neth10")
+      expect(comm).to receive(:sudo).and_yield(:stdout, "lo")
       result = cap.network_interfaces(machine)
       expect(result).to eq(["eth0", "eth1", "eth2", "eth10", "eth12"])
     end
 
     it "sorts discovered predictable network interfaces naturally" do
       expect(comm).to receive(:sudo).twice.and_yield(:stdout, "enp0s8\nenp0s3\nenp0s5\nenp0s10\nenp1s3")
+      expect(comm).to receive(:sudo).and_yield(:stdout, "lo")
       result = cap.network_interfaces(machine)
       expect(result).to eq(["enp0s3", "enp0s5", "enp0s8", "enp0s10", "enp1s3"])
     end
 
     it "sorts ethernet devices discovered with classic naming first in list" do
       expect(comm).to receive(:sudo).twice.and_yield(:stdout, "eth1\neth2\ndocker0\nbridge0\neth0")
+      expect(comm).to receive(:sudo).and_yield(:stdout, "lo")
       result = cap.network_interfaces(machine)
       expect(result).to eq(["eth0", "eth1", "eth2", "bridge0", "docker0"])
     end
 
     it "sorts ethernet devices discovered with predictable network interfaces naming first in list" do
       expect(comm).to receive(:sudo).twice.and_yield(:stdout, "enp0s8\ndocker0\nenp0s3\nbridge0\nenp0s5")
+      expect(comm).to receive(:sudo).and_yield(:stdout, "lo")
       result = cap.network_interfaces(machine)
       expect(result).to eq(["enp0s3", "enp0s5", "enp0s8", "bridge0", "docker0"])
     end
 
     it "sorts ethernet devices discovered with predictable network interfaces naming first in list with less" do
       expect(comm).to receive(:sudo).twice.and_yield(:stdout, "enp0s3\nenp0s8\ndocker0")
+      expect(comm).to receive(:sudo).and_yield(:stdout, "lo")
       result = cap.network_interfaces(machine)
       expect(result).to eq(["enp0s3", "enp0s8", "docker0"])
     end
 
     it "does not include ethernet devices aliases within prefix device listing" do
       expect(comm).to receive(:sudo).twice.and_yield(:stdout, "eth1\neth2\ndocker0\nbridge0\neth0\ndocker1\neth0:0")
+      expect(comm).to receive(:sudo).and_yield(:stdout, "lo")
       result = cap.network_interfaces(machine)
       expect(result).to eq(["eth0", "eth1", "eth2", "bridge0", "docker0", "docker1", "eth0:0"])
     end
 
     it "does not include ethernet devices aliases within prefix device listing with dot separators" do
       expect(comm).to receive(:sudo).twice.and_yield(:stdout, "eth1\neth2\ndocker0\nbridge0\neth0\ndocker1\neth0.1@eth0")
+      expect(comm).to receive(:sudo).and_yield(:stdout, "lo")
       result = cap.network_interfaces(machine)
       expect(result).to eq(["eth0", "eth1", "eth2", "bridge0", "docker0", "docker1", "eth0.1@eth0"])
     end
 
     it "properly sorts non-consistent device name formats" do
       expect(comm).to receive(:sudo).twice.and_yield(:stdout, "eth0\neth1\ndocker0\nveth437f7f9\nveth06b3e44\nveth8bb7081")
+      expect(comm).to receive(:sudo).and_yield(:stdout, "lo")
       result = cap.network_interfaces(machine)
       expect(result).to eq(["eth0", "eth1", "docker0", "veth8bb7081", "veth437f7f9", "veth06b3e44"])
     end