crawler.cache.DNSCache_max_size (collection.cfg setting)
This is the maximum size of the DNS cache used during the crawl. When this limit is reached the least recently used elements in the cache will be dropped. The DNS cache is used to store DNS (domain name system) information on hostnames encountered during the crawl e.g. hostname and IP address.
Unless you are doing very large crawls you will not normally need to increase this value.