linux - How to allow a domain name in iptables? -

i have linux server gets time offset strange reason

i set cron job run , update time using following command

/usr/sbin/ntpdate pool.ntp.org 

the problem command not run because have firewall (iptables)

i have use ip allow traffic in network:

iptables -a input -p tcp -m tcp -i eth0 -s 11.11.11.11 --dport 5060 -j accept 

i know how using domain name in case pool.ntp.org

or maybe tell me better way keep clocks in sync

please advice

typically, iptables setup restrict incoming tcp , udp connections initiated remote hosts server except needed. but, outgoing tcp , udp connections initiated server remote hosts allowed, , state kept replies allowed in, so:

# allow tcp/udp connections out. keep state conns out allowed in. iptables -a input  -p tcp -m state --state established     -j accept iptables -a output -p tcp -m state --state new,established -j accept iptables -a input  -p udp -m state --state established     -j accept iptables -a output -p udp -m state --state new,established -j accept 

if iptables setup so, allow ntpdate make outgoing connection pool.ntp.org, , allow reply in. and, can still block down incoming connections server initiated other hosts.