If you spend
any time on the Internet sending
e-mail or browsing the Web, then you use domain
name servers without even realizing it. Domain
name servers, or DNS, are an incredibly important but
completely hidden part of the Internet, and they are
fascinating! The DNS system forms one of the largest
and most active distributed databases on the planet.
Without DNS, the Internet would shut down very
quickly.
The Basics
When you use the Web or send an e-mail message, you
use a domain name to do it. For example, the
URL "http://www.howstuffworks.com" contains the domain
name howstuffworks.com. So does the e-mail
address "iknow@howstuffworks.com."
Human-readable names like "howstuffworks.com" are
easy for people to remember, but they don't do
machines any good. All of the machines use names
called IP addresses to refer to one another.
For example, the machine that humans refer to as
"www.howstuffworks.com" has the IP address
216.183.103.150. Every time you use a domain name,
you use the Internet's domain name servers (DNS) to
translate the human-readable domain name into the
machine-readable IP address. During a day of browsing
and e-mailing, you might access the domain name
servers hundreds of times!
Domain name servers translate domain names to IP
addresses. That sounds like a simple task, and it
would be -- except for five things:
- There are billions of IP addresses currently in
use, and most machines have a human-readable name as
well.
- There are many billions of DNS requests made
every day. A single person can easily make a hundred
or more DNS requests a day, and there are hundreds
of millions of people and machines using the
Internet daily.
- Domain names and IP addresses change daily.
- New domain names get created daily.
- Millions of people do the work to change and add
domain names and IP addresses every day.
The DNS system is a database, and no other
database on the planet gets this many requests. No
other database on the planet has millions of people
changing it every day, either. That is what makes the
DNS system so unique!
IP
Addresses
To keep all of the machines on the Internet straight,
each machine is assigned a unique address called an
IP address. IP stands for Internet protocol,
and these addresses are
32-bit numbers normally expressed as four "octets"
in a "dotted decimal number." A typical IP address
looks like this:
216.183.103.150
The four numbers in an IP address are called
octets because they can have values between 0 and
255 (28
possibilities per octet).
Every machine on the Internet has its own IP
address. A
server has a static IP address that does not
change very often. A home machine that is dialing up
through a
modem often has an IP address that is assigned by
the
ISP when you dial in. That IP address is unique
for your session and may be different the next time
you dial in. In this way, an ISP only needs one IP
address for each modem it supports, rather than for
every customer.
If you are working on a Windows machine, you can
view your current IP address with the command
WINIPCFG.EXE (IPCONFIG.EXE for Windows
2000/XP). On a UNIX machine, type nslookup
along with a machine name (such as "nslookup
www.howstuffworks.com") to display the IP address of
the machine (use the command hostname to learn
the name of your machine).
For more information on IP addresses, see
IANA.
As far as the Internet's machines are concerned, an
IP address is all that you need to talk to a server.
For example, you can type in your browser the URL
http://216.183.103.150 and you will arrive at the
machine that contains the Web server for
HowStuffWorks. Domain names are strictly a human
convenience.
Domain Names
If we had to remember the IP addresses of all of the
Web sites we visit every day, we would all go nuts.
Human beings just are not that good at remembering
strings of numbers. We are good at remembering words,
however, and that is where domain names come in. You
probably have hundreds of domain names stored in your
head. For example:
- www.howstuffworks.com - a typical name
- www.yahoo.com - the world's best-known name
- www.mit.edu - a popular EDU name
- encarta.msn.com - a Web server that does not
start with www
- www.bbc.co.uk - a name using four parts rather
than three
- ftp.microsoft.com - an
FTP server rather than a Web server
The COM, EDU and UK portions of these domain names
are called the top-level domain or
first-level domain. There are several hundred
top-level domain names, including COM, EDU, GOV, MIL,
NET, ORG and INT, as well as unique
two-letter combinations for every country.
Within every top-level domain there is a huge list
of second-level domains. For example, in the
COM first-level domain, you've got:
- howstuffworks
- yahoo
- msn
- microsoft
- plus millions of others...
Every name in the COM top-level domain must be
unique, but there can be duplication across
domains. For example, howstuffworks.com and
howstuffworks.org are completely different
machines.
In the case of bbc.co.uk, it is a third-level
domain. Up to 127 levels are possible, although
more than four is rare.
The left-most word, such as www or
encarta, is the host name. It specifies the
name of a specific machine (with a specific IP
address) in a domain. A given domain can potentially
contain millions of host names as long as they are all
unique within that domain.
Distributing Domain Names
Because all of the names in a given domain need to be
unique, there has to be a single entity that controls
the list and makes sure no duplicates arise. For
example, the COM domain cannot contain any duplicate
names, and a company called
Network Solutions is in charge of maintaining
this list. When you register a domain name, it goes
through one of several dozen
registrars who work with Network Solutions to add
names to the list. Network Solutions, in turn, keeps a
central database known as the
whois database that contains information about the
owner and name servers for each domain. If you go to
the
whois form, you can find information about any
domain currently in existence.
While it is important to have a central authority
keeping track of the database of names in the COM (and
other) top-level domain, you would not want to
centralize the database of all of the information in
the COM domain. For example, Microsoft has hundreds of
thousands of IP addresses and host names. Microsoft
wants to maintain its own domain name server for the
microsoft.com domain. Similarly, Great Britain
probably wants to administrate the uk top-level
domain, and Australia probably wants to administrate
the au domain, and so on. For this reason, the
DNS system is a distributed database. Microsoft
is completely responsible for dealing with the name
server for microsoft.com -- it maintains the machines
that implement its part of the DNS system, and
Microsoft can change the database for its domain
whenever it wants to because it owns its domain name
servers.
Every domain has a domain name server somewhere
that handles its requests, and there is a person
maintaining the records in that DNS. This is one of
the most amazing parts of the DNS system -- it is
completely distributed throughout the world on
millions of machines administered by millions of
people, yet it behaves like a single, integrated
database!
The Distributed System
Name servers do two things all day long:
- They accept requests from programs to convert
domain names into IP addresses.
- They accept requests from other name servers to
convert domain names into IP addresses.
When a request comes in, the name server can do one
of four things with it:
- It can answer the request with an IP address
because it already knows the IP address for the
domain.
- It can contact another name server and try to
find the IP address for the name requested. It may
have to do this multiple times.
- It can say, "I don't know the IP address for the
domain you requested, but here's the IP address for
a name server that knows more than I do."
- It can return an error message because the
requested domain name is invalid or does not exist.
When you type a URL into your browser, the
browser's first step is to convert the domain name and
host name into an IP address so that the browser can
go request a
Web page from the machine at that IP address (see
How Web Servers Work for details on the whole
process). To do this conversion, the browser has a
conversation with a name server.
When you set up your machine on the Internet, you
(or the software that you installed to connect to your
ISP) had to tell your machine what name server it
should use for converting domain names to IP
addresses. On some systems, the DNS is dynamically fed
to the machine when you connect to the ISP, and on
other machines it is hard-wired. If you are working on
a Windows 95/98/ME machine, you can view your current
name server with the command WINIPCFG.EXE
(IPCONFIG for Windows 2000/XP). On a UNIX machine,
type nslookup along with your machine name. Any
program on your machine that needs to talk to a name
server to resolve a domain name knows what name server
to talk to because it can get the IP address of your
machine's name server from the
operating system.
The browser therefore contacts its name server and
says, "I need for you to convert a domain name to an
IP address for me." For example, if you type
"www.howstuffworks.com" into your browser, the browser
needs to convert that URL into an IP address. The
browser will hand "www.howstuffworks.com" to its
default name server and ask it to convert it.
The name server may already know the IP address for
www.howstuffworks.com. That would be the case if
another request to resolve www.howstuffworks.com came
in recently (name servers
cache IP addresses to speed things up). In that
case, the name server can return the IP address
immediately. Let's assume, however, that the name
server has to start from scratch.
A name server would start its search for an IP
address by contacting one of the root name servers.
The root servers know the IP address for all of the
name servers that handle the top-level domains. Your
name server would ask the root for
www.howstuffworks.com, and the root would say
(assuming no caching), "I don't know the IP address
for www.howstuffworks.com, but here's the IP address
for the COM name server." Obviously, these root
servers are vital to this whole process, so:
- There are many of them scattered all over the
planet.
- Every name server has a list of all of the known
root servers. It contacts the first root server in
the list, and if that doesn't work it contacts the
next one in the list, and so on.
Here is a typical list of root servers held by a
typical name server:
; This file holds the information on root name servers
; needed to initialize cache of Internet domain name
; servers (e.g. reference this file in the
; "cache . <file>" configuration file of BIND domain
: name servers).
;
; This file is made available by InterNIC registration
; services under anonymous FTP as
; file /domain/named.root
; on server FTP.RS.INTERNIC.NET
; -OR- under Gopher at RS.INTERNIC.NET
; under menu InterNIC Registration Services (NSI)
; submenu InterNIC Registration Archives
; file named.root
;
; last update: Aug 22, 1997
; related version of root zone: 1997082200
;
;
; formerly NS.INTERNIC.NET
;
. 3600000 IN NS A.ROOT-SERVERS.NET.
A.ROOT-SERVERS.NET. 3600000 A 198.41.0.4
;
; formerly NS1.ISI.EDU
;
. 3600000 NS B.ROOT-SERVERS.NET.
B.ROOT-SERVERS.NET. 3600000 A 128.9.0.107
;
; formerly C.PSI.NET
;
. 3600000 NS C.ROOT-SERVERS.NET.
C.ROOT-SERVERS.NET. 3600000 A 192.33.4.12
;
; formerly TERP.UMD.EDU
;
. 3600000 NS D.ROOT-SERVERS.NET.
D.ROOT-SERVERS.NET. 3600000 A 128.8.10.90
;
; formerly NS.NASA.GOV
;
. 3600000 NS E.ROOT-SERVERS.NET.
E.ROOT-SERVERS.NET. 3600000 A 192.203.230.10
;
; formerly NS.ISC.ORG
;
. 3600000 NS F.ROOT-SERVERS.NET.
F.ROOT-SERVERS.NET. 3600000 A 192.5.5.241
;
; formerly NS.NIC.DDN.MIL
;
. 3600000 NS G.ROOT-SERVERS.NET.
G.ROOT-SERVERS.NET. 3600000 A 192.112.36.4
;
; formerly AOS.ARL.ARMY.MIL
;
. 3600000 NS H.ROOT-SERVERS.NET.
H.ROOT-SERVERS.NET. 3600000 A 128.63.2.53
;
; formerly NIC.NORDU.NET
;
. 3600000 NS I.ROOT-SERVERS.NET.
I.ROOT-SERVERS.NET. 3600000 A 192.36.148.17
;
; temporarily housed at NSI (InterNIC)
;
. 3600000 NS J.ROOT-SERVERS.NET.
J.ROOT-SERVERS.NET. 3600000 A 198.41.0.10
;
; housed in LINX, operated by RIPE NCC
;
. 3600000 NS K.ROOT-SERVERS.NET.
K.ROOT-SERVERS.NET. 3600000 A 193.0.14.129
;
; temporarily housed at ISI (IANA)
;
. 3600000 NS L.ROOT-SERVERS.NET.
L.ROOT-SERVERS.NET. 3600000 A 198.32.64.12
;
; housed in Japan, operated by WIDE
;
. 3600000 NS M.ROOT-SERVERS.NET.
M.ROOT-SERVERS.NET. 3600000 A 202.12.27.33
; End of File
The formatting is a little odd, but basically it
shows you that the list contains the actual IP
addresses of 13 different root servers.
The root server knows the IP addresses of the name
servers handling the several hundred top-level
domains. It returns to your name server the IP address
for a name server for the COM domain. Your name server
then sends a query to the COM name server asking it if
it knows the IP address for www.howstuffworks.com. The
name server for the COM domain knows the IP addresses
for the name servers handling the HOWSTUFFWORKS.COM
domain, so it returns those. Your name server then
contacts the name server for HOWSTUFFWORKS.COM and
asks if it knows the IP address for
www.howstuffworks.com. It does, so it returns the IP
address to your name server, which returns it to the
browser, which can then contact the server for
www.howstuffworks.com to get a Web page.
One of the keys to making this work is
redundancy. There are multiple name servers at
every level, so if one fails, there are others to
handle the requests. There are, for example, three
different machines running name servers for
HOWSTUFFWORKS.COM requests. All three would have to
fail for there to be a problem.
The other key is caching. Once a name server
resolves a request, it
caches all of the IP addresses it receives. Once
it has made a request to a root server for any COM
domain, it knows the IP address for a name server
handling the COM domain, so it doesn't have to bug the
root servers again for that information. Name servers
can do this for every request, and this caching helps
to keep things from bogging down.
Name servers do not cache forever, though. The
caching has a component, called the Time To Live
(TTL), that controls how long a server will cache a
piece of information. When the server receives an IP
address, it receives the TTL with it. The name server
will cache the IP address for that period of time
(ranging from minutes to days) and then discard it.
The TTL allows changes in name servers to propagate.
Not all name servers respect the TTL they receive,
however. When HowStuffWorks moved its machines over to
new servers, it took three weeks for the transition to
propagate throughout the Web. We put a little tag that
said "new server" in the upper left corner of the home
page so people could tell whether they were seeing the
new or the old server during the transition.
Creating a New Domain Name
When someone wants to create a new domain, he or she
has to do two things:
- Find a name server for the domain name to
live on.
- Register the domain name.
Technically, there does not need to be a machine in
the domain -- there just needs to be a name server
that can handle the requests for the domain name.
There are two ways to get a name server for a
domain:
- You can create and administer it yourself.
- You can pay an ISP or hosting company to handle
it for you.
Most larger companies have their own domain name
servers. Most smaller companies pay someone.
The history of HowStuffWorks is typical. When
howstuffworks.com was first created, it began as a
parked domain. This domain lived with a company
called
http://computer.howstuffworks.com/framed.htm?parent=dns.htm&url=http://www.webhosting.com.
Webhosting.com maintained the name server and also
maintained a machine that created the single "under
construction" page for the domain.
To create a domain, you fill out a form with a
company that does domain name registration (examples:
register.com,
verio.com,
networksolutions.com). They create an "under
construction page," create an entry in their name
server, and submit the form's data into the
whois database. Twice a day, the COM, ORG, NET,
etc. name servers get updates with the newest IP
address information. At that point, a domain exists
and people can go see the "under construction" page.
HowStuffWorks then started publishing content under
the domain www.howstuffworks.com. We set up a hosting
account with Tabnet (now part of Verio, Inc.), and
Tabnet ran the DNS for HowStuffWorks as well as the
machine that hosted the HowStuffWorks Web pages. This
type of machine is called a virtual Web hosting
machine and is capable of hosting multiple domains
simultaneously. Five-hundred or so different domains
all shared the same processor.
As HowStuffWorks became more popular, it outgrew
the virtual hosting machine and needed its own server.
At that point, we started maintaining our own machines
dedicated to HowStuffWorks, and began administering
our own DNS. We have a primary server and a secondary:
- AUTH-NS1.HOWSTUFFWORKS.COM 209.116.69.78
- AUTH-NS2.HOWSTUFFWORKS.COM 209.116.69.79
Our primary DNS is auth-ns1.howstuffworks.com.
Any changes we make to it propagate automatically to
the secondary, which is also maintained by our ISP.
All of these machines run name server software
called
BIND. BIND knows about all of the machines in our
domain through a text file on the main server that
looks like this:
Decoding this file from the top, you can see that:
- The first two lines point to the primary and
secondary name servers.
- The next line is called the MX record.
When you send e-mail to anyone at howstuffworks.com,
the piece of software sending the e-mail contacts
the name server to get the MX record so it knows
where the SMTP server for HowStuffWorks is (see
How E-mail Works for details). Many larger
systems have multiple machines handling incoming
e-mail, and therefore multiple MX records.
- The next line points to the machine that will
handle a request to mail.howstuffworks.com.
- The next line points to the IP address that will
handle a request to oak.howstuffworks.com.
- The next line points to the IP address that will
handle a request to howstuffworks.com (no
host name).
You can see from this file that there are several
physical machines at separate IP addresses that make
up the HowStuffWorks server infrastructure. There are
aliases for hosts like mail and www. There can be
aliases for anything. For example, there could be an
entry in this file for scoobydoo.howstuffworks.com,
and it could point to the physical machine called
walnut. There could be an alias for
yahoo.howstuffworks.com, and it could point to
yahoo. There really is no limit to it. We could also
create multiple name servers and segment our domain.
The Beauty of DNS
As you can see from this description, DNS is a rather
amazing distributed database. It handles billions of
requests for billions of names every day through a
network of millions of name servers administered by
millions of people. Every time you send an e-mail
message or view a URL, you are making requests to
multiple name servers scattered all over the globe.
What's amazing is that the process is usually
completely invisible and extremely reliable!
For more information on domain name servers and
related topics, check out the links on the next page.
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