how to configuring pf to NAT traffic ?

I use SimepleTunnel project , iPhone app PacketTunel extion work with tunnel_server , but tunnel_server writev packet to kerel , how config pf to NAT traffic and the packets can route to Internet?

pf.conf

vpn_net = "192.168.3.0/24"
ext_if = "en0"
nat on en0  from $vpn_net to any ->  (en0)

check pf state

rMBP:etc root# pfctl -sa
No ALTQ support in kernel
ALTQ related functions disabled
TRANSLATION RULES:
nat on en0 inet from 192.168.3.0/24 to any -> (en0) round-robin

I use tcpdump check write status

tcpdump -i utun0 and found 192.168.3.0/16 to my dnsserver 192.168.0.24, but no dnserver replay to 192.168.3.0/16

14:02:46.467038 IP 192.168.3.3.57257 > 192.168.0.245.domain: 38998+ A? guzzoni.apple.com. (35)
14:02:46.467877 IP 192.168.3.3.49218 > 192.168.0.245.domain: 59621+ A? www.apple.com. (31)
14:02:53.068894 IP 192.168.3.3.60834 > 192.168.0.245.domain: 52069+ A? apple.com. (27)
14:02:53.069427 IP 192.168.3.3.57257 > 192.168.0.245.domain: 38998+ A? guzzoni.apple.com. (35)
14:02:53.070072 IP 192.168.3.3.49218 > 192.168.0.245.domain: 59621+ A? www.apple.com. (31)
14:03:05.456122 IP 192.168.3.3.60834 > 192.168.0.245.domain: 52069+ A? apple.com. (27)
14:03:05.456653 IP 192.168.3.3.57257 > 192.168.0.245.domain: 38998+ A? guzzoni.apple.com. (35)
14:03:05.457140 IP 192.168.3.3.49218 > 192.168.0.245.domain: 59621+ A? www.apple.com. (31)

how to configuring pf let utun packet to NAT traffic ?