Microsoft 70-411 Exam – Q88

Your network contains a DNS server named Server1 that runs Windows Server 2012 R2. Server1 has a zone named contoso.com. The network contains a server named Server2 that runs Windows Server 2008 R2. Server1 and Server2 are members of an Active Directory domain named contoso.com.

You change the IP address of Server2. Several hours later, some users report that they cannot connect to Server2. On the affected users’ client computers, you flush the DNS client resolver cache, and the users successfully connect to Server2. You need to reduce the amount of time that the client computers cache DNS records from contoso.com.

Which value should you modify in the Start of Authority (SOA) record? To answer, select the appropriate setting in the answer area.
Hot Area:
88a

Correct Answer:
88b

Explanation:

Time-to-Live for resource records
The Time-to-Live (TTL) value in a resource record indicates a length of time used by other DNS servers to determine how long to cache information for a record before expiring and discarding it. For example, most resource records created by the DNS Server service inherit the minimum (default) TTL of one hour from the start of authority (SOA) resource record, which prevents extended caching by other DNS servers.

A DNS client resolver caches the responses it receives when it resolves DNS queries. These cached responses can then be used to answer late queries for the same information. The cached data, however, has a limited lifetime specified in the TTL parameter returned with the response data. TTL ensures that the DNS server does not keep information for so long that it becomes out of date. TTL for the cache can be set on the DNS database (for each individual resource record, by specifying the TTL field of the record and per zone through the minimum TTL field of the SOA record) as well as on the DNS client resolver side by specifying the maximum TTL the resolver allows to cache the resource records.

There are two competing factors to consider when setting the TTL. The first is the accuracy of the cached information, and the second is the utilization of the DNS servers and the amount of network traffic. If the TTL is short, then the likelihood of having old information is reduced considerably, but it increases utilization of DNS servers and network traffic, because the DNS client must query DNS servers for the expired data the next time it is requested. If the TTL is long, the cached responses could become outdated, meaning the resolver could give false answers to queries. At the same time, a long TTL decreases utilization of DNS servers and reduces network traffic because the DNS client answers queries using its cached data.

If a query is answered with an entry from cache, the TTL of the entry is also passed with the response. This way the resolvers that receive the response know how long the entry is valid. The resolvers honor the TTL from the responding server; they do not reset it based on their own TTL. Consequently, entries truly expire rather than live in perpetuity as they move from DNS server to DNS server with an updated TTL.

https://technet.microsoft.com/en-us/library/dd197427