iptables -A INPUT -p udp --dport 123 -j ACCEPT iptables -A OUTPUT -p udp --sport 123 -j ACCEPT
Also, from the NTP website:
... ntpd requires full bidirectional access to the privileged UDP port 123. ...
My question is, why? To someone not familiar with NTP, this seems like a potential security hole, especially when I'm asking a client of mine to open up that port in their firewall so that my servers can keep their time synchronised. Does anyone have a decent justification I can give to my client to convince them that I need this access in the firewall? Help is appreciated! :)