What Is Dns

By Author: Robert Evans
Total Articles: 3698
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DNS (Domain Name System), is the service which translates between Internet names and Internet addresses.

Internet names are the names which we use to refer to hosts on the Internet, such as www.tech-faq.com and www.freebsd.org.

Internet addresses are the numbers which routers use to move traffic across the Internet, such as 216.17.138.115 and 216.136.204.117.
nslookup

One of the best ways to understand DNS is to look at the DNS testing tool, `nslookup`.

Let's start by doing a forward DNS lookup on www.freebsd.org:

bash-2.05a$ nslookup www.freebsd.org
Server: localhost.net
Address: 127.0.0.1

Name: www.freebsd.org
Address: 216.136.204.117

Next, let's do a reverse DNS lookup on 216.136.204.117:

bash-2.05a$ nslookup 216.136.204.117
Server: localhost.net
Address: 127.0.0.1

Name: www.freebsd.org
Address: 216.136.204.117
DNS Record Types

DNS records which map names to numbers are known as "A Records" or "Address Records".

DNS records which map numbers to names are known as ...
... "PTR Records" or "Pointer Records".

Two other types of useful records which are stored in DNS are MX (Mail eXchange) Records and NS (Name Server) Records.

NS records show where the DNS servers are for a domain. Let's find IBM's name servers using `nslookup`:

bash-2.05a$ nslookup
Default Server: localhost.net
Address: 127.0.0.1

> set type=ns
> ibm.com
Server: localhost.net
Address: 127.0.0.1

Non-authoritative answer:
ibm.com nameserver = ns.watson.ibm.com
ibm.com nameserver = internet-server.zurich.ibm.com
ibm.com nameserver = ns.almaden.ibm.com
ibm.com nameserver = ns.austin.ibm.com

Authoritative answers can be found from:
ns.watson.ibm.com internet address = 129.34.20.80
internet-server.zurich.ibm.com internet address = 195.176.20.204
ns.almaden.ibm.com internet address = 198.4.83.35
ns.austin.ibm.com internet address = 192.35.232.34
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MX Records tell us where the e-mail servers are for a domain. This tells our mail systems where to send mail to. Let's use `nslookup` to find the mail systems for IBM:

bash-2.05a$ nslookup
Default Server: localhost.net
Address: 127.0.0.1

> set type=mx
> ibm.com
Server: localhost.net
Address: 127.0.0.1

ibm.com preference = 10, mail exchanger = e35.co.us.ibm.com
ibm.com preference = 10, mail exchanger = e1.ny.us.ibm.com
ibm.com preference = 10, mail exchanger = e2.ny.us.ibm.com
ibm.com preference = 10, mail exchanger = e3.ny.us.ibm.com
ibm.com preference = 10, mail exchanger = e4.ny.us.ibm.com
ibm.com preference = 10, mail exchanger = e5.ny.us.ibm.com
ibm.com preference = 10, mail exchanger = e6.ny.us.ibm.com
ibm.com preference = 10, mail exchanger = e31.co.us.ibm.com
ibm.com preference = 10, mail exchanger = e32.co.us.ibm.com
ibm.com preference = 10, mail exchanger = e33.co.us.ibm.com
ibm.com preference = 10, mail exchanger = e34.co.us.ibm.com
ibm.com nameserver = internet-server.zurich.ibm.com
ibm.com nameserver = ns.almaden.ibm.com
ibm.com nameserver = ns.austin.ibm.com
ibm.com nameserver = ns.watson.ibm.com
e35.co.us.ibm.com internet address = 32.97.110.133
e1.ny.us.ibm.com internet address = 32.97.182.101
e2.ny.us.ibm.com internet address = 32.97.182.102
e3.ny.us.ibm.com internet address = 32.97.182.103
e4.ny.us.ibm.com internet address = 32.97.182.104
e5.ny.us.ibm.com internet address = 32.97.182.105
e6.ny.us.ibm.com internet address = 32.97.182.106
e31.co.us.ibm.com internet address = 32.97.110.129
e32.co.us.ibm.com internet address = 32.97.110.130
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