pdnsd is a proxy DNS server with permanent caching (the cache contents are
written to hard disk on exit) that is designed to cope with unreachable or 
down DNS servers (e.g., in dial-in networking).

Add this to /etc/rc.d/rc.local:
if [ -x /etc/rc.d/rc.pdnsd ]; then
   /etc/rc.d/rc.pdnsd start
fi

Add this to /etc/rc.d/rc.local_shutdown
if [ -x /etc/rc.d/rc.pdnsd ]; then
   /etc/rc.d/rc.pdnsd stop
fi
