|
|
Library
> Articles >
The Internet
> 018
|
|
|
|
The
Domain Name System
Contributor: Pay As You
Host
The
Domain Name System is at the heart of the Internet,
and understanding
how it functions is especially crucial if you manage
a website. Below you'll find basic and detailed explanations
about the world of DNS.
Basic Explanation
Overview
DNS is a system which maps your domain name to the
actual computer it's hosted on. DNS converts your
domain name into an address, which can be used to
locate your site somewhere on the Internet, allowing
you to move your website from one provider to another
without changing your domain name, and allowing the
host to move your site around within a particular
host's network (i.e. from one server to another).
In
order for other people to know where your site is
located, hosts run a system which, on request, will
return the address of a server (called an IP address)
depending on the domain name supplied. This means
that if the host wants to move your site from one
server to another on the host's network, the host
would first move the files from one computer to another,
and then adjust the settings to point your domain
name to a new address (the address for the new server).
The service, run by nearly every ISP or hosting company,
which does the above (i.e. converts a domain name
to a computer address) is called a DNS server or Name
server.
When you move your website from one hosting company
to another, the DNS server for your domain changes.
This means that if, for example, you moved a domain
away from an existing host, people would stop requesting
DNS information from that DNS server, and begin requesting
it from the DNS server of your new hosting company.
DNS
Recursing
When someone wants to find out the address of your
website, they would not normally connect directly
to the host's DNS server to find the information,
as this would require them to be able to find out
which DNS server a domain name is hosted on. When
someone wants to find a website, they will normally
make a request to the DNS server of their ISP or connection
provider. When they make a request from their ISP's
DNS server, this server then in turn requests the
information from the 'authoritative nameserver' for
the domain. This is known as DNS recursing, and a
single request of this sort is known as a recursive
DNS lookup (recursive simply implies that the request
was passed to more than one server).
In this way, someone accessing the Internet only need
know about one DNS server - their ISP's server.
DNS Caching
In the above example (where someone requests details
of a site from their DNS server, which in turn requests
the details from the DNS server hosting the site),
an ISP's DNS server will not pass on a request every
time one of their customers wants to view a site,
as this would cause massive amounts of extra Internet
traffic, and would also mean that many DNS servers
(especially ones hosting popular sites) would be inundated
with requests.
Instead of passing on a request every time a DNS server
receives one, it will remember the last address it
was given for the domain name for a fixed period of
time, and simply pass this response back to any customers
who access the same site. This is known as caching.
Normally, a DNS server will not cache the address
of a domain name for more than 24 hours. However,
some may be less (say, once per hour or few hours),
and, in some rare cases, slightly more. The period
of time a DNS server will cache details is up to the
administrator of that server.
Problems caused by DNS
caching
An unfortunate effect of DNS caching is that if a
site needs to be moved, it can take up to 24 hours
before all other DNS servers re-check with the DNS
server hosting the site. This means that there may
be a period of time when some DNS servers have the
new address, and some have the old address. As time
passes, more people will be able to see the new site.
Detailed
Explanation
Overview
Every domain name on the Internet has an IP address
associated with it. This IP address in turn is associated
with a computer. The process of converting from a
domain name to an IP address is called a DNS lookup.
There are two types of DNS server. A forwarding DNS
server and a full DNS server. A forwarding DNS server
will take a request for a domain name, and simply
pass it on to another DNS server (normally always
the same server). A full DNS server can do a number
of things, including recursive lookups, DNS hosting
of local sites, and caching (see below for details
of these).
Locally hosted sites
If a domain is hosted locally, normally it will be
listed on the local DNS server. When someone makes
a request for this domain to the local DNS server,
it will simply respond with the associated IP address.
Recursive
Lookups
If a request is made to a DNS server, and the domain
requested is not hosted on that server, the DNS server
will lookup the domain name by traversing a number
of other servers.
Initially, the DNS server will connect to a root name
server (of which there are approximately 10). The
root name server will then examine the domain requested,
and inform the DNS server of where it may be hosted.
For example, with a .co.uk domain, the root name server
would suggest ns1.nic.uk, and possibly some alternative
servers.
The DNS server will then move onto one of the returned
servers, and request the same domain from the new
server. This server will in turn suggest another DNS
server. This continues until the DNS server on which
the site is hosted is found (known as an authoritative
nameserver for the domain), and the IP address is
returned. At this point, the IP address is passed
back to the original client, and they then access
the site.
Caching
When a recursive lookup is done, the DNS server will
cache the supplied IP address for a fixed period of
time (usually 24 hours). If any clients request that
same domain name again in this time, rather than doing
another lookup, the DNS server will report with the
cached IP address. This prevents excessive load on
DNS servers and bandwidth.
Caching
problems
The downside to caching is that if a change needs
to be made to a domain name, it can take up to 24
hours before all DNS servers' caches have cleared,
and they are supplying the new IP address to their
clients.
It is possible for a DNS server to request a timeout
period other than 24 hours for specific domain names.
This means that when a DNS server caches the details
it should read this timeout and instead of caching
the details for the full 24 hours, is will only wait
the requested period. This can be useful if the details
for a domain name are changed regularly, or if it
is expected to be changed in the near future.
Avoid the Endless
Wait
One piece of advice about changing Name Servers. Although
your existing DNS company (Domain Name reseller) may
'partially transfer' your Domain Name, they may not
configure the actual Name Server to point towards
your new website host's Name Sever. You may otherwise
have an endless wait as your Domain Name continues
to point towards your existing server. Make doubly
sure you do not need to configure the DNS yourself
by discussing this issue with your existing Domain
Name provider.
|
|
|
|
| |
|
|
|
 |
| |
| Contributor
Information |
|
Pay As You Host (PAYH) offers great value,
high quality web hosting that features flexibility
and control including: no enforced annual
hosting contract; their own infrastructure;
hardware and software customised in-house;
automated account and hosting set-up; exclusive
hosting control panel; and full support services
(phone and email).
As part of the Metronet group, PAYH assures
you're not dealing with a new company that
might disappear overnight, but rather one
that has been providing a strong portfolio
of Internet Services to thousands of people
since 1996.
To find out more about Pay As You Host visit
their website at: http://www.payasyouhost.com/,
or email: info@pay-as-you-host.com
If you observe inaccuracies in our articles
or wish to contribute an article or review
to be included at AbleStable® visit Feedback.
Copyright
Notice
Although our contents are free to browse,
copyright resides with the originators of
all works accessed at AbleStable®, and
unauthorised copying or publication of our
site contents is strictly prohibited. To use
our specially selected premium content go
to Content
Syndication and Licensing.
AbleStable © 2002-2008
|
| |
|
|
|
|
|
| |
|
|
|
|
 |
|
| All
Material: AbleStable © 2002-2008 |
|
|
|
