iptables rules cannot be loaded at boot time when domain names are used in iptables rules

Hi, I have the following rule in my iptables, and all chains are empty when the server is booted. -A INPUT -s yahoo.com -j DROP Or -A INPUT -s yahoo.com -p tcp --dport 21 -j DROP if I manually run /etc/init.d/iptables start, all iptables rules can be loaded. Also if an IP address is used as follows, iptables can be loaded at boot time. -A INPUT -s 66.33.210.0/24 -j DROP I have checked a few books and websites, they all show that domain names can be used in iptables rules. Thanks.