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 文章标题 : DNS Servers
帖子发表于 : 2006-08-25 13:01 

注册: 2006-08-25 13:00
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Good day Everyone,

Im working in the one of the biggest data centers in US
as a system administrator and always have people asking me about Internet
in a real live. Their big misunderstanding place is exactly about the hostname
resoving, they cannot understand how IP could be attached to a hostname
and thatswhy most people associate Internet with webpages, what is
definately not right. I will try to explain here in short what is DNS. This
will allow people to have more deep understanding about Interner.

Litte bit of history:
The practice of using a name as a more human-legible abstraction of a
machine's numerical address on the network predates even TCP/IP, and
goes all the way back to the ARPAnet era. Originally, each computer on the
network retrieved a file called HOSTS.TXT from SRI (now SRI International)
which mapped an address (such as 192.0.34.166) to a name
(such as www.example.net.) The Hosts file still exists on most modern
operating systems, either by default or through configuration, and allows
users to specify an IP address to use for a hostname without checking the DNS server.
This file now serves primarily for troubleshooting DNS errors or for mapping
local addresses to more organic names. (The Hosts file can also help in
ad-blocking, and spyware may utilize it to hijack a computer.) But a system
based on a HOSTS.TXT file had inherent limitations, because of the obvious
requirement that every time a given computer's address changed, every
computer that wanted to communicate with it would need an update to its
Hosts file.

DNS in the real world
Users generally do not communicate directly with a DNS resolver. Instead
DNS resolution takes place transparently in client applications such as web
browsers (like Internet Explorer, Opera, Mozilla Firefox, Safari, Netscape Navigator,
etc), mail clients (Outlook Express, Mozilla Thunderbird, etc), and other Internet
applications. When a request is made which necessitates a DNS lookup, such
programs send a resolution request to the local DNS resolver in the operating
system which in turn handles the communications required.

The DNS resolver will almost invariably have a cache containing recent
lookups. If the cache can provide the answer to the request, the resolver
will return the value in the cache to the program that made the request.
If the cache does not contain the answer, the resolver will send the request
to a designated DNS server or servers. In the case of most home users,
the Internet service provider to which the machine connects will usually
supply this DNS server: such a user will either configure that server's
address manually or allow DHCP to set it; however, where systems
administrators have configured systems to use their own DNS servers,
their DNS resolvers will generally point to their own nameservers. This
name server will then follow the process outlined above in DNS in theory,
until it either successfully finds a result, or does not. It then returns its
results to the DNS resolver; assuming it has found a result, the resolver
duly caches that result for future use, and hands the result back to the
software which initiated the request.

Types of DNS records
Important categories of data stored in the DNS include the following:
*An A record or address record maps a hostname to a 32-bit
IPv4 address.
* An AAAA record or IPv6 address record maps a hostname
to a 128-bit IPv6 address.
* A CNAME record or canonical name record makes one domain
name an alias of another. The aliased domain gets all the subdomains
and DNS records of the original.
* An MX record or mail exchange record maps a domain name to
a list of mail exchange servers for that domain.
* A PTR record or pointer record maps an IPv4 address to the
canonical name for that host. Setting up a PTR record for a hostname
in the in-addr.arpa domain that corresponds to an IP address implements
reverse DNS lookup for that address. For example (at the time of writing),
www.icann.net has the IP address 192.0.34.164, but a PTR record maps
164.34.0.192.in-addr.arpa to its canonical name, referrals.icann.org.
* An NS record or name server record maps a domain name to a list
of DNS servers authoritative for that domain. Delegations depend on
NS records.
* An SOA record or start of authority record specifies the DNS server
providing authoritative information about an Internet domain, the email of
the domain administrator, the domain serial number, and several timers
relating to refreshing the zone.
* An SRV record is a generalized service location record.
* A TXT record allows an administrator to insert arbitrary text into a
DNS record. For example, this record is used to implement the Sender
Policy Framework specification.
* NAPTR records (NAPTR stands for "Naming Authority Pointer") are
a newer type of DNS record that support regular expression based rewriting.

Conclusion
Many investigators have voiced criticism of the methods currently used to control
ownership of domains. Critics commonly claim abuse by monopolies or
near-monopolies, such as VeriSign, Inc. Particularly noteworthy was the
VeriSign Site Finder system which redirected all unregistered .com and .net
domains to a VeriSign webpage, this was rapidly removed after widespread critism.

There is also significant disquiet regarding United States political influence over
the Internet Corporation for Assigned Names and Numbers (ICANN). This was
a significant issue in the attempt to create a .xxx Top-level domain and sparked
greater interest in Alternative DNS roots that would be beyond the control of any
single country.

Information was combined by using different websites inluding Wikipedia.

I hope this article help to understand DNS as well and assume internet is
a NETWORK not a WEBPAGE.

----
Thank you for the patience.
Servo.


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