Você precisa configurar seu firewall para permitir respostas UDP na porta 123.
Eu configurei o ntpd em uma instalação do CentOS com um simples /etc/ntp.conf com apenas 3 linhas:
server 0.pool.ntp.org
server 1.pool.ntp.org
server 2.pool.ntp.org
server 192.168.1.1
Infelizmente, não importa qual host eu execute, não consigo obter o ntpdate para sincronizar corretamente com um servidor externo. Aqui está a saída de depuração que recebo quando a executo:
[root@myhost superuser]# ntpdate -d pool.ntp.org
3 May 12:23:31 ntpdate[3912]: ntpdate [email protected] Fri Feb 22 11:23:28 UTC 2013 (1)
Looking for host pool.ntp.org and service ntp
host found : 173.44.32.10
transmit(173.44.32.10)
transmit(69.41.163.31)
transmit(18.85.44.118)
transmit(65.182.144.36)
transmit(173.44.32.10)
transmit(69.41.163.31)
transmit(18.85.44.118)
transmit(65.182.144.36)
transmit(173.44.32.10)
transmit(69.41.163.31)
transmit(18.85.44.118)
transmit(65.182.144.36)
transmit(173.44.32.10)
transmit(69.41.163.31)
transmit(18.85.44.118)
transmit(65.182.144.36)
transmit(173.44.32.10)
transmit(69.41.163.31)
transmit(18.85.44.118)
transmit(65.182.144.36)
173.44.32.10: Server dropped: no data
69.41.163.31: Server dropped: no data
18.85.44.118: Server dropped: no data
65.182.144.36: Server dropped: no data
server 173.44.32.10, port 123
stratum 0, precision 0, leap 00, trust 000
refid [173.44.32.10], delay 0.00000, dispersion 64.00000
transmitted 4, in filter 4
reference time: 00000000.00000000 Thu, Feb 7 2036 1:28:16.000
originate timestamp: 00000000.00000000 Thu, Feb 7 2036 1:28:16.000
transmit timestamp: d52e6306.7c9cb981 Fri, May 3 2013 12:23:34.486
filter delay: 0.00000 0.00000 0.00000 0.00000
0.00000 0.00000 0.00000 0.00000
filter offset: 0.000000 0.000000 0.000000 0.000000
0.000000 0.000000 0.000000 0.000000
delay 0.00000, dispersion 64.00000
offset 0.000000
server 69.41.163.31, port 123
stratum 0, precision 0, leap 00, trust 000
refid [69.41.163.31], delay 0.00000, dispersion 64.00000
transmitted 4, in filter 4
reference time: 00000000.00000000 Thu, Feb 7 2036 1:28:16.000
originate timestamp: 00000000.00000000 Thu, Feb 7 2036 1:28:16.000
transmit timestamp: d52e6306.afcfee21 Fri, May 3 2013 12:23:34.686
filter delay: 0.00000 0.00000 0.00000 0.00000
0.00000 0.00000 0.00000 0.00000
filter offset: 0.000000 0.000000 0.000000 0.000000
0.000000 0.000000 0.000000 0.000000
delay 0.00000, dispersion 64.00000
offset 0.000000
server 18.85.44.118, port 123
stratum 0, precision 0, leap 00, trust 000
refid [18.85.44.118], delay 0.00000, dispersion 64.00000
transmitted 4, in filter 4
reference time: 00000000.00000000 Thu, Feb 7 2036 1:28:16.000
originate timestamp: 00000000.00000000 Thu, Feb 7 2036 1:28:16.000
transmit timestamp: d52e6306.e3031a51 Fri, May 3 2013 12:23:34.886
filter delay: 0.00000 0.00000 0.00000 0.00000
0.00000 0.00000 0.00000 0.00000
filter offset: 0.000000 0.000000 0.000000 0.000000
0.000000 0.000000 0.000000 0.000000
delay 0.00000, dispersion 64.00000
offset 0.000000
server 65.182.144.36, port 123
stratum 0, precision 0, leap 00, trust 000
refid [65.182.144.36], delay 0.00000, dispersion 64.00000
transmitted 4, in filter 4
reference time: 00000000.00000000 Thu, Feb 7 2036 1:28:16.000
originate timestamp: 00000000.00000000 Thu, Feb 7 2036 1:28:16.000
transmit timestamp: d52e6307.16364db7 Fri, May 3 2013 12:23:35.086
filter delay: 0.00000 0.00000 0.00000 0.00000
0.00000 0.00000 0.00000 0.00000
filter offset: 0.000000 0.000000 0.000000 0.000000
0.000000 0.000000 0.000000 0.000000
delay 0.00000, dispersion 64.00000
offset 0.000000
3 May 12:23:36 ntpdate[3912]: no server suitable for synchronization found