Enable user-id based packet routing on Mac OS

If you would like to route all socket (TCP/UDP) traffic from processes running by a particular user on a Mac OS to be routed differently, you can do that.

1. Add the user to your Mac OS if not already done. In this example, I will add an user named "test1"
2. run the command:
        sudo vi /private/etc/pf.conf
    and add the following line before ' anchor "com.apple/*"
         pass out quick on en0 route-to { utun4 192.168.15.2 } user test1

   Note:
   a) change en0 to your default network interface name on Mac
   b) change utun4 to the network interface you would these packets to be routed to

3. restart pf by doing:
    sudo pfctl -d; sudo pfctl -e -f /etc/pf.conf

Now all processes running by user test1 should be routed to the new interface as specified.