Caching in DNS
Every server has a cache for recently used names as well as a record of where the mapping information for that name was obtained. When a client asks the server to resolve certain name the server does as follows:
- Check if it has authority for the name. If yes, the server doesn't need caching information.
- If it hasn't, the server checks its cache whether the name has been resolved recently. If yes, the server reports the caching information to its clients.
We can examine the cache when the server cashed the information once, but didn't change it. Since information about a particular name can change, the server may have incorrect information in its caching table. The Time to Live (TTL) value is used to decide when to age information. Whenever an authority responds to a request, it includes a TTL value in the response, which specifies how long it guarantees the binding to remain.
