fqdncache_size
Squid’s FQDN cache holds recent DNS address-to-name lookups. However, Squid makes these reverse DNS lookups only when you enable the log_fqdn directive or use a dstdomain ACL. This directive limits the number of names in the cache. Each FQDN cache entry uses a relatively small amount of memory, so you can safely increase this limit to 10,000 or more.