В ipfw можно выдернуть счетчики трафика, содержащие информацию о UID-е пользователя:
# /sbin/ipfw show
20000 7087 669681 count ip from me to any uid 20088
20001 35 3171 count ip from me to any uid 20089
...
...
pfctl -vsn
No ALTQ support in kernel
ALTQ related functions disabled
nat on 192.168.101.255 inet from 192.168.101.0/24 to any -> 192.168.101.255
[ Evaluations: 3072607 Packets: 0 Bytes: 0 States: 0 ]
[ Inserted: uid 0 pid 99399 ]