> You're not getting responses back from __any__ of those NTP servers. If
> you have a firewall *in front* of your BSD box (meaning a separate box,
> not ipfw/ipfilter/pf on the same BSD box!), then this is likely the
> cause of the problem.

The question is that two weeks ago, with same machine, same gateway, same NAT and same firewall config, when I was on FreeBSD 6.2-RELEASE and behind NAT, I could sync with ALL IPv6 servers (IPv4 is not functioning there) I said that in my first post.
I'm pretty sure that if I went back to 6.2 even behind NAT, I could get sync with IPv6. Long writing since my first post I shall summarize my events here for better understanding, and sorry for redundancy.
1. FreeBSD 5.4-RELEASE(dial up) - can sync all servers
2. FreeBSD 6.2-RELEASE(dial up) - can sync all servers
3. FreeBSD 6.2-RELEASE(NAT) - can sync IPv6 servers
4. FreeBSD 6.3-STABLE (NAT) - not sync at all
The issue is the different result between 3. and 4.
It seems something different between 6.2-RELEASE and 6.3-STABLE.
But today I hear good news for issuance of 7.0-RELEASE.
I shall go on with the new RELEASE.
My bad news is I've just updated to 6.3 for 2 weeks. :-(

> If not, there's two explanations:
> * Your uplink provider is filtering incoming packets destined to your
> network on port 123.

It works with IPv6 on 6.2 behind NAT. So this reason might not be the case. IPv4 or IPv6 bind() the same port (123). In addition, I look in ntpdate/ntpd source code. They both use datagram socket (UDP). But only ntpd support AF_INET6 address family. So in my case ntpdate, which supports only IPv4, will never work on my system even on 6.2 behind NAT.

> * If you're using NAT on this BSD box, somehow your NAT rules are
> broken, or you're doing something bizarre with network interfaces.

I'm not using BSD box as a router.

> You also confirm this by stating that you're able to talk to NTP servers
> if you use a dial-up connection on the same box, so it really sounds
> like you have a NAT problem and not an NTP problem.

Probably, but I'm not quite sure as my reason above.

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