Search in iptables.log file

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http://stackoverflow.com – I have used iptables to save information about connections. So i have modified /etc/syslog.conf file in order to save information in /var/log/iptables.log. So, every X hour, i will extract information. But the problem is the file will be bigger and the search will be slower. So i used sed -i '/string/d' iptables.log In order to delete what i don't need more. But when i use this command, iptables stop saving data in iptables.log file. So , what's wrong ? How can i solve this problem ? (HowTos)