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 > 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 >
About the Author:
Robert Evans writes on topics such as DNS , TCP and UDP for The Tech FAQ.
Tue, 29 Jul 2008 13:11:03 - 100%
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