All DNS servers perform caching ; whenever they receive information from other servers, they store the information for a certain amount of time. This speeds the performance of DNS resolution, reduces DNS-related query traffic, and improves reliability. For more information, see "Caching and Time to Live" later in this chapter.
Certain DNS servers, known as caching-only servers, simply perform queries, cache the answers, and return the results. They are not authoritative for any DNS domains and do not host any zones. They only store data that they have cached while resolving queries.
The benefit provided by caching-only servers is that they do not generate zone transfer network traffic because they do not contain any zones. However, there is one disadvantage: when the server is initially started, it has no cached information and must build up this information over time as it services requests.