This document provides an overview of the Domain Name System (DNS) including: the hierarchical namespace structure with top-level and second-level domains; how domain names are registered; how DNS works through name servers and caching; the BIND software; DNS database and zone files; and common DNS tools like dig and host.

1. DNS – Domain Name
Service
WeeSan Lee <weesan@cs.ucr.edu>
http://www.cs.ucr.edu/~weesan/cs183/

2. Roadmap
 Introduction
 The DNS Namespace
 Top-level Domains
 Second-level Domains
 Domain Names
 How to Register a Domain Name?
 How DNS Works?
 BIND
 Tools
 Q&A

3. Introduction
 A service that maps between hostnames and
IP addresses
 A hierarchical distributed caching database
with delegated authority.
 Uses port 53
 UDP for the queries and responses
 TCP for the zone transfer

4. Introduction (cont)
momo.cs.ucr.edu
root name server (.)
edu
berkeley.edu
cs.berkeley.edu
Q
Q
R
Q
R
Q
R
Q
A
A
Recursive servers Non-recursive servers
eon
http://www.cs.berkeley.edu/

5. The DNS Namespace
 A tree structure that starts with the root (.)
 Each node represents a domain name
 2 branches
 Forward mapping
 hostnames → IP addresses
 Reverse mapping
 IP addresses → hostnames

6. Top-level Domains
 gTLDs (generic TLDs)
 com, edu, net, org, gov, mil, int, arpa
 aero, biz, coop, info, jobs, museum, name, pro
 ccTLDs (country code TLDs)
 au, ca, br, de, fi, fr, jp, se, hk, cn, tw, my, …
 Profitable domain names
 CreditCards.com - $2.75M
 Loans.com – $3M
 Business.com - $7.5M

7. Second-level Domain Name
 Examples
 ucr.edu
 sony.co.jp
 Must apply to a registrar for the appropriate
TLD
 Network Solutions, Inc used to monopolize
the name registration
 Now, ~500 registrars

8. Domain Names
 Valid domain names
 Each component: [a-zA-Z0-9-]{1,63}
 Each name < 256 chars
 Case insensitive
 www.cs.ucr.edu == WWW.CS.UCR.EDU
 FQDN
 Fully Qualified Domain Name
 eon.cs.ucr.edu
 eon – hostname
 cs.ucr.edu – domain name

9. How To Register A Domain Name?
 Pick a domain name of interest
 Dedicate 2 NS servers
 RFC1219 stated that each domains should be served by at
least 2 servers: a master & a slave
 One technical contact person
 One administrative contact person
 Then, register the name to a registrar of your choice
 Used to be done via email or fax, now all web-based

10. How DNS Works?
 Delegation
 All name servers read all the 13 root servers from
a local configuration file
 [a-m].root-servers.net
 $ dig
 Those servers in turn knows all the TLDs
 .edu knows .ucr.edu
 .com knows .google.com
 etc

11. DNS Caching
 DNS servers cache results they receive from
other servers
 Each result is saved based on its TTL
 Negative caching
 For nonexistent hostname (for 10 mins)
 Also for unreachable/unresponsive servers

12. Authoritative vs. Non-authoritative
 An authoritative answer from a name server
(such as reading the data from the disk) is
“guaranteed” to be accurate
 A non-authoritative answer (such as an
answer from the cache) may not
 Primary and secondary servers are
authoritative for their own domains

13. Recursive vs. Non-recursive
 Recursive
 Queries on a client behalf until it returns either an
answer or an error
 Non-recursive
 Refers the client to another server if it can’t
answer a query

14. DNS Database
 A set of text files, called zone files,
maintained by the system admin. on the
master NS
 2 types of entries
 Parser commands, eg.
 $ORIGIN and $TTL
 Resource Records (RR)
 [name] [tt] [class] type data
 eon 76127 IN A 138.23.169.9
 orpheus.cs.ucr.edu. 76879 IN A 138.23.169.17
A very important . there!

15. DNS Database (cont)
 Resource Record Types
 SOA Start Of Authority
 NS Name Server
 A IPv4 name-to-address translation
 AAAA IPv6 name-to-address translation
 PTR Address-to-name translation
 MX Mail eXchanger
 CNAME Canonical NAME
 TXT Text
 …

16. BIND
 The Berkeley Internet Name Domain system
 Current maintainer: Paul Vixie @ ISC
 BIND 9
 Use RTT to pick the best root servers and
use them in round-robin fashion
 named

17. /etc/named.conf
 options {
 directory "/var/named";
 // query-source address * port 53;
 forwarders { 138.23.169.10; };
 };
 zone "." IN {
 type hint;
 file "named.ca"; // Read from /var/named/named.ca
 };

18. /etc/named.conf
 zone "localhost" IN {
 type master;
 file "localhost.zone"; // Read from /var/named/localhost.zone
 allow-update { none; };
 };
 zone "0.0.127.in-addr.arpa" IN {
 type master;
 file "named.local"; // Read from /var/named/named.local
 allow-update { none; };
 };

19. /etc/named.conf
 zone "voicense.com" IN {
 type master;
 file "voicense.com.zone";
 };
 zone "0.0.10.in-addr.arpa" IN {
 type master;
 file "voicense.com.rev";
 };
 zone "macrohard.com IN {
 type slave;
 file "macrohard.com.zone.bak";
 masters { 10.0.0.1; };
 };

20. /var/named/voicense.com.zone
 $TTL 86400
 $ORIGIN voicense.com.
 @ IN SOA voicense.com. weesan.voicense.com. (
 20040304 ; serial #
 7200 ; refresh (2 hrs)
 1800 ; retry (30 mins)
 604800 ; expire (1 week)
 7200 ) ; mininum (2 hrs)
 IN NS ns.voicense.com.
 IN MX 10 mail.voicense.com.
 IN MX 20 mail.myisp.com.
 IN A 10.0.0.1
 mail IN CNAME voicense.com.
 www IN CNAME voicense.com.
 ns IN CNAME voicense.com.
 lee IN A 10.0.0.31
 wee IN A 10.0.0.32
Email address:
weesan@voicense.com
Remember to
increment the serial #
after each editing

21. /var/named/voicense.com.zone
 Serial #
 An increasing integer number (for sync’ing)
 Refresh
 How often the slave servers should sync. with the master
 Retry
 How long the slave servers should retry before giving up
 Expire
 How long should the slave servers continue to serve the
domains in the absent of the master
 Mininum
 TTL for negative answers that are cached

22. /var/named/voicense.com.rev
 $TTL 86400
 @ IN SOA voicense.com. weesan.voicense.com. (
 20040304 ; serial #
 7200 ; refresh (2 hrs)
 1800 ; retry (30 mins)
 604800 ; expire (1 week)
 7200 ) ; mininum (2 hrs)
 IN NS ns.voicense.com.
 1 IN PTR fw.voicense.com.
 31 IN PTR lee.voicense.com.
 32 IN PTR wee.voicense.com.

23. How To Load Balance A Web Server?
 www IN A 10.0.0.1
 www IN A 10.0.0.2
 www IN A 10.0.0.3

24. How To Load Balance A Web Server?
 $ host www.google.com
 www.google.com is an alias for www.l.google.com.
 www.l.google.com has address 74.125.19.104
 www.l.google.com has address 74.125.19.103
 www.l.google.com has address 74.125.19.147
 www.l.google.com has address 74.125.19.99
 $ host www.google.com
 www.google.com is an alias for www.l.google.com.
 www.l.google.com has address 74.125.19.99
 www.l.google.com has address 74.125.19.104
 www.l.google.com has address 74.125.19.103
 www.l.google.com has address 74.125.19.147

25. Zone Transfer
 DNS servers sync with each other via zone
transfer
 All-at-once and incremental updates
 A slave server compares the serial number
on the master’s and save backup zone files
on disk.
 Uses TCP on port 53

26. Tools
 dig
 $ dig eon.cs.ucr.edu
 $ dig eon.cs.ucr.edu ns
 $ dig @momo.cs.ucr.edu eon.cs.ucr.edu mx
 $ man dig
 host
 $ host eon.cs.ucr.edu
 $ host -t ns cs.ucr.edu
 $ host -t mx eon.cs.ucr.edu momo.cs.ucr.edu
 $ man host

27. Tools (cont)
 nslookup
 $ nslookup eon.cs.ucr.edu
 $ nslookup eon.cs.ucr.edu momo.cs.ucr.edu
 whois
 $ whois google.com
 $ whois ucr.edu

28. /etc/resolv.conf
 Resolver
 $ cat /etc/resolv.conf
 search cs.ucr.edu weesan.com
 nameserver 138.23.169.10
 nameserver 138.23.178.2

29. /etc/nsswitch.conf
 Used by C library
 gethostbyname()
 $ cat /etc/nsswitch.conf
 hosts: file nis dns

30. Reference
 LAH
 Ch 15: DNS – The Domain Name System