1. how to compile
   cp if_utun.c /sys/net/.
   mkdir /sys/modules/if_utun
   cp Makefile /sys/modules/if_utun/.
   cd /sys/modules/if_utun
   make

2. how to load
   kldload ./if_utun.ko
   ifconfig utun create a.b.c.d netmask A.B.C.D

3. config
   sysctl net.link.utun.0.sport=40001
   sysctl net.link.utun.0.dport=40002
   sysctl net.link.utun.0.destaddr="127.0.0.1"

   receive the udp packets from 127.0.0.1:40002
   send the packets to 127.0.0.1:40002

   sysctl net.link.utun.0.sport=40001
   sysctl net.link.utun.0.dport=40002
   sysctl net.link.utun.0.destaddr="192.168.1.1"

   receive the udp packets from 192.168.1.1:40002
   send the packets to 192.168.1.1:40002

   Plesae note, port 40001 should not be used by local application.
