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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
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
Introduction (cont)
momo.cs.ucr.edu
root name server (.)
edu
berkeley.educs.berkeley.edu
Q
Q
R
Q
R
Q
R
QA
A
Recursive servers Non-recursive servers
eon
http://www.cs.berkeley.edu/
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
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
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
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
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
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
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
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
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
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!
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 …
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
/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 };
/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; }; };
/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; }; };
/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: [email protected]
Remember to increment the serial #
after each editing
/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
/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.
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
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
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
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
Tools (cont)
nslookup $ nslookup eon.cs.ucr.edu $ nslookup eon.cs.ucr.edu momo.cs.ucr.edu
whois $ whois google.com $ whois ucr.edu
/etc/resolv.conf
Resolver $ cat /etc/resolv.conf
search cs.ucr.edu weesan.com nameserver 138.23.169.10 nameserver 138.23.178.2
/etc/nsswitch.conf
Used by C library gethostbyname()
$ cat /etc/nsswitch.conf hosts: file nis dns
Reference
LAH Ch 15: DNS – The Domain Name System
