How Domain Name Servers Work
If
you've ever used the Internet, it's a good bet that you've used the Domain Name System, or DNS, even without
realizing it. DNS is a protocol within the set of standards for how computers
exchange data on the Internet and on many private networks, known as the TCP/IP
protocol suite. Its basic job is to turn a user-friendly domain name like "howstuffworks.com"
into an Internet Protocol (IP) address like 70.42.251.42 that computers use
to identify each other on the network. It's like your computer's GPS for the
Internet.
Computers and other network devices on the Internet use an IP
address to route your request to the site you're trying to reach. This is
similar to dialing a phone number to connect to the person you're trying to
call. Thanks to DNS, though, you don't have to keep your own address book of IP
addresses. Instead, you just connect through a domain name server,
also called a DNS
server or name server, which
manages a massive database that maps domain names to IP addresses.
Whether you're accessing a Web site or sending e-mail, your computer uses a DNS server to
look up the domain name you're trying to access. The proper term for this
process is DNS
name resolution, and you would say that the DNS server resolves
the domain name to the IP address. For example, when you enter
"http://www.howstuffworks.com" in your browser, part of the network
connection includes resolving the domain name "howstuffworks.com"
into an IP address, like 70.42.251.42, for HowStuffWorks' Web servers.
You can always bypass a DNS lookup by entering 70.42.251.42
directly in your browser (give it a try). However, you're probably more likely
to remember "howstuffworks.com" when you want to return later. In
addition, a Web site's IP address can change over time, and some sites
associate multiple IP addresses with a single domain name.
Without DNS servers, the Internet would shut down very quickly.
But how does your computer know what DNS server to use? Typically, when you
connect to your home network, Internet service provider (ISP)
or WiFi network, the modem or router that assigns your computer's network
address also sends some important network configuration information to your
computer or mobile device. That configuration includes one or more DNS servers
that the device should use when translating DNS names to IP address.
So far, you've read about some important DNS basics. The rest of
this article dives deeper into domain name servers and name resolution. It even
includes an introduction to managing your own DNS server. Let's start by
looking at how IP addresses are structured and how that's important to the name
resolution process.
DNS
Servers and IP Addresses
You just
learned that the primary job of a domain name server, or DNS server, is to
resolve (translate) a domain name into an IP address. That sounds like a simple
task, and it would be, except for the following points:
·
There are billions of IP addresses currently in use, and most machines
have a human-readable name as well.
·
DNS servers (cumulatively) are processing billions of requests
across the Internet at any given time.
·
Millions of people are adding and changing domain names and IP
addresses each day.
With so much to handle, DNS servers rely on network efficiency
and Internet protocols. Part of the IP's effectiveness is that each machine on
a network has a unique IP address in both the IPV4 and IPV6 standards managed
by the Internet Assigned Numbers Authority (IANA). Here are some ways to
recognize an IP address:
·
An IP address in the IPV4 standard has four numbers separated by
three decimals, as in: 70.74.251.42
·
An IP address in the IPV6 standard has eight hexadecimal numbers
(base-16) separated by colons, as in 2001:0cb8:85a3:0000:0000:8a2e:0370:7334.
Because IPV6 is still a very new standard, we'll concentrate on the more common
IPV4 for this article.
·
Each number in an IPV4 number is called an "octet"
because it's a base-10 equivalent of an 8-digit base-2 (binary) number used in
routing network traffic. For example, the octet written as 42 stands for
00101010. Each digit in the binary number is the placeholder for a certain
power of two from 20 to 27,
reading from right to left. That means that in 00101010, you have one each of 21, 23 and 25. So,
to get the base-10 equivalent, just add 21 + 23 + 25 = 2 + 8 + 32 = 42. For more about how IP
addresses are constructed, see our article "What is an
IP address?"
·
There are only 256 possibilities for the value of each octect:
the numbers 0 through 255.
·
Certain addresses and ranges are designated by the IANA as reserved a IP address, which means
they have a specific job in IP. For example, the IP address 127.0.0.1 is
reserved to identify the computer you're currently using. So, talking to 127.0.0.1
is just talking to yourself.
Where does your computer's IP address come from? If we're
talking about your desktop or laptop computer, it probably comes from a
Dynamic Host Configuration Protocol (DHCP) server on your network. The job of a
DHCP server is to make sure your computer has the IP address and other network
configuration it needs whenever you're online. Because this is
"dynamic," the IP address for your computer will probably change from
time to time, such as when you shut down your computer for a few days. As the
user, you'll probably never notice all this taking place. See the sidebar on
this page for hints on where to find the IP address assigned to your computer
or mobile device.
Web servers and other computers that need a
consistent point of contact use static
IP addresses. This means that the same IP address is always
assigned to that system's network interface when it's online. To make sure that
interface always gets the same IP address, IP associates the address with the
Media Access Control (MAC) address for that network interface. Every network
interface, both wired and wireless, has a unique MAC address embedded in it by
the manufacturer.
For more information on IP addresses, see the IANA,
operated by the Internet Corporation for Assigned Names and Numbers (ICANN).
Now, though, let's look at the other side of the DNS equation: domain names.
Domain
Names
If we
had to remember the IP addresses of all our favorite Web sites, we'd
probably go nuts! Human beings are just 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,
such as:
·
howstuffworks.com -- our favorite domain name
·
google.com -- one of the most used domain names in the world
·
mit.edu -- a popular EDU name
·
bbc.co.uk -- a three-part domain name using the country code UK
You'll recognize domain names as having strings of characters
separated by dots (periods). The last word in a domain name represents a top-level domain.
Thesetop-level
domains are controlled
by the IANA in what's called the Root Zone Database, which
we'll examine more closely later. The following are some common top-level
domains:
·
COM -- commercial Web sites, though open to everyone
·
NET -- network Web sites, though open to everyone
·
ORG -- non-profit organization Web sites, though open to
everyone
·
EDU -- restricted to schools and educational organizations
·
MIL -- restricted to the U.S. military
·
GOV -- restricted to the U.S. government
·
US, UK, RU and other two-letter country codes -- each is
assigned to a domain name authority in the respective country
In a domain name, each word and dot combination you add before a
top-level domain indicates a level in the domain structure. Each level refers
to a server or a group of servers that manage that domain level. For example,
"howstuffworks" in our domain name is a second-level domain off the
COM top-level domain. An organization may have a hierarchy of sub-domains further organizing its Internet
presence, like "bbc.co.uk" which is the BBC's domain under CO, an
additional level created by the domain name authority responsible for the UK
country code.
The left-most word in the domain name, such as www or mail, is a host name. It
specifies the name of a specific machine (with a specific IP address) in a
domain, typically dedicated to a specific purpose. A given domain can
potentially contain millions of host names as long as they're all unique to
that domain.
Because all of the names in a given domain need to be unique,
there has to be some way to control the list and makes sure no duplicates
arise. That's where registrars come in. A registrar is an authority that can
assign domain names directly under one or more top-level domains and register
them with InterNIC, a service of ICANN, which enforces uniqueness of domain
names across the Internet. Each domain registration becomes part of a central
domain registration database known as the whois database. Network Solutions,
Inc. (NSI) was one of the first registrars, and today companies like
GoDaddy.com offer domain registration in addition to many other Web site and
domain management services. [source: InterNIC]
Later, when we look at how to create a domain name, we'll see
that part of registering a domain requires identifying one or more name servers
(DNS servers) that have the authority to resolve the host names and sub-domains
in that domain. Typically, you would do this through a hosting service, which
has its own DNS servers. Next, we'll look at how these DNS servers manage your
domain, and how DNS servers across the Internet work together to ensure traffic
is routed properly between IP addresses.
The
Distributed System
Every
domain has a domain name server handling its requests, and there is a person or
IT team maintaining the records in that DNS server's database. No other
database on the planet gets as many requests as DNS servers, and they handle
all those queries while also processing data updates from millions of people
every day. That's one of the most amazing parts of DNS -- it is completely
distributed throughout the world on millions of machines, managed by millions
of people, and yet it behaves like a single, integrated database!
Because managing DNS seems like such a big job, most people tend
to leave it to the IT professionals. However, by learning a little bit about
how DNS works and how DNS servers are distributed across the Internet, you can manage DNS with confidence.
The first thing to know is what the purpose of a DNS server is on the network
where it resides. A DNS server will have one of the following as its primary
task:
·
Maintain a small database of domain names and IP addresses most
often used on its own network, and delegate name resolution for all other names
to other DNS servers on the Internet.
·
Pair IP addresses with all hosts and sub-domains for which that
DNS server has authority.
DNS servers that perform the first task are normally managed by
your Internet service provider (ISP). As mentioned earlier, the ISP's DNS
server is part of the network configuration you get from DHCP as soon as you go
online. These servers reside in your ISP's data centers, and they handle
requests as follows:
·
If it has the domain name and IP address in its database, it
resolves the name itself.
·
If it doesn't have the domain name and IP address in its
database, it contacts another DNS server on the Internet. It may have to do
this multiple times.
·
If it has to contact another DNS server, it caches the lookup
results for a limited time so it can quickly resolve subsequent requests to the
same domain name.
·
If it has no luck finding the domain name after a reasonable
search, it returns an error indicating that the name is invalid or doesn't
exist.
The second category of DNS servers mentioned above is typically associated
with Web, mail and other Internet domain hosting services. Though some hardcore
IT gurus set up and manage their own DNS servers, hosting services have made
DNS management much easier for the less technical audience. A DNS server that
manages a specific domain is called the start
of authority (SOA)
for that domain. Over time, the results from looking up hosts at the SOA will
propagate to other DNS servers, which in turn propagate to other DNS servers,
and so on across the Internet.
This propagation is a result of each DNS server caching the
lookup result for a limited time, known as its Time To Live (TTL), ranging from
a few minutes to a few days. People managing a DNS server can configure its
TTL, so TTL values will vary across the Internet. So, each time you look up
"www.howstuffworks.com," it's possible that the DNS server for your
ISP will find the lookup results "70.42.251.42" in its own cache if you or someone else using that
server looked for it before within the server's TTL.
This great web of DNS servers includes the root name servers,
which start at the top of the domain hierarchy for a given top-level domain.
There are hundreds of root name servers to choose from for each top-level
domain. Though DNS lookups don't have to start at a root name server, they can
contact a root name server as a last resort to help track down the SOA for a
domain.
Now that you know how DNS servers are interconnected to improve
the name resolution process, let's look at how you can configure a DNS server
to be the authority for your domain.
Creating
a New Domain Name
When
you want to create a new domain name, you need to do the following:
·
Use the Whois database to find a unique domain name that isn't
yet registered. There are several sites that offer free Whois database
searches, such as Network Solutions. If the
search comes up empty, you know the domain name is available.
·
Register the domain name with a registrar. There are a lot of
registrars to choose from, and some offer special prices for registering the
COM, NET, and ORG versions of a domain at the same time, for registering for
two or more years, or for hosting the domain with the same company.
·
If you're hosting the domain at a different company than your
registrar, configure the registrar to point your domain name to the correct
host name or IP address for your hosting company (see
information below about A records).
Using the DNS servers from your registrar or hosting company
means that you have a parked
domain. This means that someone else owns the computer hardware
for the DNS servers, and your domain is just part of that company's larger DNS
configuration. Alternatively, if you're passionate about hosting your own DNS,
you can set up your own server, either as a physical or virtual machine.
Whichever DNS setup you decide on, that DNS server (or group of servers)
becomes the SOA for your domain, as described earlier.
Whether your SOA is somewhere else or on your own system, you
can extend and modify your DNS settings to add sub-domains, redirect e-mail and
control other services. This information is kept in a zone file on the DNS server [source: GoDaddy.com]. If you're
running your own server, you'll probably need to manually edit the zone file in
a text editor. Many registrars today have a Web interface you can use to manage
DNS for your domain. Each new configuration you add is called a record, and the
following are the most common types of records you can configure for your DNS
server:
·
Host (A) -- This is the basic mapping of IP address to host
name, the essential component for any domain name.
·
Canonical Name (CNAME) -- This is an alias for your domain.
Anyone accessing that alias will be automatically directed to the server
indicated in the A record.
·
Mail Exchanger (MX) -- This maps e-mail traffic to a specific
server. It could indicate another host name or an IP address. For example,
people who use Google for the e-mail for their domain will create an MX
record that points to ghs.google.com.
·
Name Server (NS) -- This contains the name server information
for the zone. If you configure this, your server will let other DNS servers
know that yours is the ultimate authority (SOA) for your domain when caching
lookup information on your domain from other DNS servers around the world.
·
Start of Authority (SOA) -- This is one larger record at the
beginning of every zone file with the primary name server for the zone and some
other information. If your registrar or hosting company is running your DNS
server, you won't need to manage this. If you're managing your own DNS,
Microsoft's support information has a helpful article on the structure of a DNS SOA Record.
The following is an example of what a zone file might look like
for those who are editing it directly in a text editor. Note that the center
column (second item on each line) includes a record type from those listed
above. When you see an "@" in the left column, it means that the
record applies in all cases not otherwise specified:
@ NS auth-ns1.howstuffworks.com
@ NS auth-ns2.howstuffworks.com
@ MX 10 mail
mail A 209.170.137.42
vip1 A 216.183.103.150
www CNAME vip1
Typical users will probably get the most use out of MX and CNAME
records. The MX records allows you to point your mail services somewhere other
than your hosting company if you choose to use something like Google Apps for
your domain. The CNAME records let you point host names for your domain to
various other locations. This could include setting google.example.com to
redirect to google.com, or setting up a dedicated game server with its own IP
address and pointing it to something like gameserver.example.com.
HowStuffWorks' parent company, Discovery, does this: dsc.discovery.com is the
main Web site, science.discovery.com is The Science Channel Web site, and so
on.
Throughout this article, you've read about the role of domain
name servers, how DNS maps domain names to IP addresses and how to choose your
domain name and configure it to work within the distributed system of DNS
servers around the world. Now that you're in the zone with zone files and
registered for success with domain name servers, look up lots more information
about DNS on the next page.
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