I just spent far too long troubleshooting why an old RedHat 9 box couldn't do any NTP. I was sniffing packets, running debug mode, nothing made sense. I hit the NTP faq specifically the Linux section and found this:

According to Hal Murray, at least RedHat Linux 7.1 was shipped with this line in /etc/ntp.conf:

restrict default ignore Thus any NTP network messages are ignored. Either remove that offending line, or selectively allow individual addresses. Usually one would use the IP address, but see how the restrict directive really works. Removing that line and my server starts receiving NTP packets and syncronizing itself! Woohoo! Update: Found the same problem on a FC1 box I have. Why did they ship with that in the config?