IPCop firewall 1.x uses dnsmasq to proxy client DNS requests. From the IPCop command line, issue the following command to cause dnsmasq to clear the currently cached DNS entries and start building new entries:
kill -s SIGHUP processid
should be replaced with the process id number for dnsmasq.
... will return the process id number. Putting it all together, the full command line to flush the cache would be:
kill -s SIGHUP $(pidof dnsmasq)