the way FlashFXP detects inet ip is perfectly fine.
i know for a fact that traceroute is not a reliable method to detect internet ip.
for one, some dsl modems, when in router mode do not support traceroute from the inside.
2nd, some isp use nice tricks to protect their servers by making them reply back with ips from nonroutable range, making them impossible to access unless you are on the same subnet. check out my tracerote.
Code:
Tracing route to dslreports.com [209.123.109.175]
over a maximum of 30 hops:
1 23 ms 23 ms 27 ms 172.31.255.251
2 22 ms 23 ms 22 ms 192.168.13.49
3 22 ms 21 ms 20 ms gigabitethernet5-0-146.ipcolo2.NewYork1.Level3.net [166.90.137.213]
4 20 ms 23 ms 23 ms gigabitethernet2-1.core1.NewYork1.Level3.net [64.159.17.69]
5 22 ms 24 ms 21 ms unknown.wcg.net [209.244.160.138]
6 25 ms 23 ms 30 ms nycmny2wcx3-pos9-0.wcg.net [64.200.87.226]
7 23 ms 22 ms 26 ms nycmnyhlce1-oc48.wcg.net [64.200.87.110]
8 20 ms 20 ms 26 ms nycmny2lce1-netaccess-atm.wcg.net [64.200.86.150]
9 24 ms 25 ms 25 ms 1187.at-0-1-0.gbr1.oct.nac.net [209.123.11.150]
10 22 ms 22 ms 23 ms 0095.gi-1-1.msfc1.oct.nac.net [64.21.102.2]
11 24 ms 22 ms 24 ms www.dslreports.com [209.123.109.175]
Trace complete.
as you can see 1st hop is a nonroutable ip, which is actually my machine w/o any firewalls or routers. 2nd hop is my dsl provider's redback, and 3rd is internet.
so as you can see it's impossible to determine my ip from outgoing traceroute. incoming traceroute looks similar only 1st hop(last hop for incoming) will have my real inet ip.