The domain name system, commonly abbreviated as DNS, is a hierarchical, distributed database used to resolve, or translate, domain and host names to IP addresses. Using DNS, users, computers, and applications that query DNS can specify remote systems by fully qualified domain names (FQDNs).
When a DNS client needs to contact a host system, it first attempts to do so by using local resources. The client first checks its local cache, which is created by saving the results of previous queries. Items in the local cache remain until one of three things occurs:
1. The Time-to-Live (TTL) period, which is set on each item, expires.
2. The client runs the ipconfig /flushdnscommand.
3. The DNS client is shut down.
Next, the client attempts to resolve the query using the local HOSTS file, which, on Windows systems, is located in the %systemroot%\system32\drivers\etcdirectory. This file is used to manually map host names to IP addresses, and remains in place even if the system is rebooted.
If the DNS server is unable to resolve the query, it forwards the request to other DNS servers, sending what is known as a recursive query. The server forwards to other servers that are listed as “forwarders,” or to a set of servers configured in the DNS server’s “Root Hints” file.
If DNS is not configured and working properly, Exchange Server 2010 will not work at all.
Even Lightweight Directory Access Protocol (LDAP) queries for local mailbox users require the DNS client to be properly configured and functioning on your Exchange Server 2010 servers.
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