DNS及在Exchange Server 中的做用

For computer systems to communicate with each other,  whether you are talking about a local area network (LAN), a wide area  network (WAN), or the Internet, they must have the ability to identify  one another using some type of name resolution. Several strategies have  been developed over the years, but the most reliable one to date (and  the current industry standard) is the use of a DNS.

  Accurate name resolution is critical in a mail environment as well. For a  message to reach its destination, it might pass through several systems  that need to know where it came from and where it is going.

In the past, Microsoft has continued to support the Windows Internet Naming Service, commonly  known as WINS, as an alternative way of performing name resolution  within an environment. WINS provided a distributed database for  registering and querying dynamic mappings of NetBIOS names for computers and groups. WINS mapped these NetBIOS names to IP addresses, and was  originally designed to resolve problems that surrounded NetBIOS name  resolution in routed networks.

However, in Microsoft  Exchange Server 2010, support for WINS/NetBIOS broadcasts has been done  away with. This makes the importance of DNS in Exchange Server 2010  greater than ever because 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.

Domain Name System Defined

The Internet, as well as most home and business networks, relies on Internet Protocol (IP)  addresses to allow computers to connect to one another. If we had to  remember the IP addresses of every website, server, workstation, and  printer that we connect to on a daily basis, it would be very difficult  to accomplish anything!

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).

DNS is the primary method  for name resolution for the Microsoft Windows Server platforms. DNS is  also a requirement for deploying Active Directory (AD), though Active  Directory is not a requirement for deploying DNS. That being said, in a  Microsoft Windows environment, integrating DNS and Active Directory  enables DNS servers to take advantage of the security, performance, and  fault-tolerance capabilities designed into Active Directory.

Using DNS

DNS is composed of two components: clients and servers. Servers store information about specific components.

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 /flushdns command.

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\etc directory. This file is used to manually map host names to IP addresses, and remains in place even if the system is rebooted.

Finally, if the client  is unable to resolve the query locally, it forwards the request to a DNS server for resolution. The DNS server attempts to resolve the client’s  query as detailed next:

• If the query  result is found in any of the zones for which the DNS server is  authoritative, the server responds to the host with an authoritative  answer.

• If the result is in the zone entries of the DNS server, the server checks its own local cache for the information.

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.

The DNS query is  forwarded through communications channels on the Internet until it  reaches a DNS server that is listed as being authoritative for the zone  listed in the query. That DNS server then sends back a reply—either an  「affirmative,」 with the IP address requested, or a 「negative」 stating  that the host in question could not be resolved.

Understanding Who Needs DNS

Not all situations  require the use of DNS. There are other name resolution mechanisms that  exist besides DNS, some of which come standard with the operating system (OS) that companies deploy. Although not all scenarios have the  requirement of a complex name resolution structure, DNS makes life  easier by managing name servers in a domain, sometimes with little  overhead.

In the past, an  organization with a standalone, non-interconnected network could get  away with using only host files or WINS to provide NetBIOS-to-IP address name translation. Some very small environments could also use broadcast protocols such as NetBEUI to provide name resolution. In modern  networks, however, DNS becomes a necessity, especially in Active  Directory environments.

As stated before, WINS is no longer used by Exchange Server with the release of Exchange Server 2010. The proper installation and  configuration of DNS is critical to the successful deployment of  Exchange Server 2010.

http://allcomputers.us/windows_server/understanding-dns-requirements-for-exchange-server-2010.aspx