use xor for generate_mac

- add entropy / reduce probability of mac collisions
- makes macs more opaque / non-sequential
- SQUASH THIS

lib/puppet/functions/generate_mac.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require 'digest'
+require "digest"
 
 # generates a deterministic pseudorandom mac address, using hostname as seed
 #
@@ -12,27 +12,27 @@
 #
 # generated address contains
 #   - 3 byte fixed OUI (prefix)
-#   - 21 "random" bits (seeded from hostname)
-#   - 3 bit sequential number (index)
+#   - 3 "random" byte (seeded from hostname)
+#   - XOR with up to 3 bits (index) for additonal interfaces
 Puppet::Functions.create_function(:generate_mac) do
   dispatch :generate_mac do
     param "String", :prefix
     param "String", :hostname
-    param "Integer", :index
+    optional_param "Integer", :index
   end
 
-  def generate_mac(prefix, hostname, index)
-    unless(index.between?(0,7))
+  def generate_mac(prefix, hostname, index = 0)
+    unless index.between?(0, 7)
       raise(ArgumentError, "#{index} must be between 0 and 7, I can only generate 8 mac addresses per host!")
     end
 
-    oui = prefix.downcase.gsub(/[^0-9a-z]/,'')
-    unless(oui =~ /^[a-f0-9][26ae][a-f0-9]{4}$/)
+    oui = prefix.downcase.gsub(/[^0-9a-z]/, "")
+    unless /^[a-f0-9][26ae][a-f0-9]{4}$/.match?(oui)
       raise(ArgumentError, "invalid mac prefix!")
     end
 
-    integer_id = (Digest::SHA256.hexdigest(hostname)[0,6].to_i(16) & 0xFF_FF_F8) + index
-    hex_id = sprintf('%06x',integer_id)
-    "#{oui}#{hex_id}".each_char.each_slice(2).map{|x| x.join}.join(':')
+    integer_id = Digest::SHA256.hexdigest(hostname)[0, 6].to_i(16) ^ index
+    hex_id = sprintf("%06x", integer_id)
+    "#{oui}#{hex_id}".each_char.each_slice(2).map { |x| x.join }.join(":")
   end
 end

spec/functions/generate_mac_spec.rb

@@ -7,23 +7,22 @@
 
 describe "generate_mac" do
   it "generates simple addresses" do
-    is_expected.to run.with_params('02:00:00', 'example.com', 0).and_return('02:00:00:a3:79:a0')
-    is_expected.to run.with_params('12-0f-00', 'example.com', 2).and_return('12:0f:00:a3:79:a2')
+    is_expected.to run.with_params("02:00:00", "example.com").and_return("02:00:00:a3:79:a6")
+    is_expected.to run.with_params("12-0f-00", "example.com", 2).and_return("12:0f:00:a3:79:a4")
   end
   it "fails on invalid chars" do
-    is_expected.to run.with_params('g2:00:00', 'example.com', 0).and_raise_error(ArgumentError)
+    is_expected.to run.with_params("g2:00:00", "example.com", 0).and_raise_error(ArgumentError)
   end
   it "correctly masks final 3 bits" do
-    is_expected.to run.with_params('06 12 34', 'my.host.name', 0).and_return('06:12:34:3b:d9:98')
-    is_expected.to run.with_params('5a.67.89', 'my.host.name', 3).and_return('5a:67:89:3b:d9:9b')
-    is_expected.to run.with_params('AE:BC:DE', 'my.host.name', 7).and_return('ae:bc:de:3b:d9:9f')
+    is_expected.to run.with_params("06 12 34", "my.host.name", 0).and_return("06:12:34:3b:d9:9a")
+    is_expected.to run.with_params("5a.67.89", "my.host.name", 3).and_return("5a:67:89:3b:d9:99")
+    is_expected.to run.with_params("AE:BC:DE", "my.host.name", 7).and_return("ae:bc:de:3b:d9:9d")
   end
   it "fails on non-private oui" do
-    is_expected.to run.with_params('03:00:00', 'example.com', 0).and_raise_error(ArgumentError)
+    is_expected.to run.with_params("03:00:00", "example.com", 0).and_raise_error(ArgumentError)
   end
   it "fails on out of range index" do
-    is_expected.to run.with_params('02:00:00', 'example.com', 11).and_raise_error(ArgumentError)
-    is_expected.to run.with_params('02:00:00', 'example.com', -1).and_raise_error(ArgumentError)
+    is_expected.to run.with_params("02:00:00", "example.com", 11).and_raise_error(ArgumentError)
+    is_expected.to run.with_params("02:00:00", "example.com", -1).and_raise_error(ArgumentError)
   end
 end
-