Test fix for rapid7#4306

Author: jhart-r7

lib/msf/core/exploit/capture.rb

@@ -39,7 +39,6 @@ def initialize(info = {})
           [
             OptInt.new('UDP_SECRET', [true, 'The 32-bit cookie for UDP probe requests.', 1297303091]),
             OptAddress.new('GATEWAY', [false, 'The gateway IP address. This will be used rather than a random remote address for the UDP probe, if set.']),
-            OptInt.new('NETMASK', [false, 'The local network mask. This is used to decide if an address is in the local network.', 24]),
           ], Msf::Exploit::Capture
         )
 
@@ -402,9 +401,11 @@ def check_pcaprub_loaded
       def lookupnet
         check_pcaprub_loaded
         dev  = datastore['INTERFACE'] || ::Pcap.lookupdev
-        mask = datastore['NETMASK'] || 24
         begin
-          my_net = IPAddr.new("#{Pcap.lookupnet(dev).first}/#{mask}")
+          my_ip, my_mask = Pcap.lookupnet(dev)
+          # convert the netmask obtained from the relevant interface to CIDR
+          cidr_mask = my_mask.to_s(2).count('1')
+          my_net = IPAddr.new("#{my_ip}/#{cidr_mask}")
         rescue RuntimeError => e
           @pcaprub_error = e
           print_status("Cannot stat device: #{@pcaprub_error}")
@@ -414,10 +415,7 @@ def lookupnet
       end
 
       def should_arp?(ip)
-        @mydev  ||= datastore['INTERFACE'] || ::Pcap.lookupdev
-        @mymask ||= datastore['NETMASK'] || 24
-        @mynet  ||= lookupnet
-        @mynet.include?(IPAddr.new(ip))
+        lookupnet.include?(IPAddr.new(ip))
       end
 
       attr_accessor :capture, :arp_cache, :arp_capture, :dst_cache