DNS – Domain Name
Service
WeeSan Lee <weesan@cs.ucr.edu>
http://www.cs.ucr.edu/~weesan/cs183/
Roadmap
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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
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A service that maps between hostnames and
IP addresses
A hierarchical distributed caching database
with delegated authority.
Uses port 53
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UDP for the queries and responses
TCP for the zone transfer
Introduction (cont)
Recursive servers
Non-recursive servers
root name server (.)
Q
R
http://www.cs.berkeley.edu/
Q
Q
momo.cs.ucr.edu
edu
A
R
A
eon
Q
Q
R
cs.berkeley.edu
berkeley.edu
The DNS Namespace
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A tree structure that starts with the root (.)
Each node represents a domain name
2 branches
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Forward mapping
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hostnames → IP addresses
Reverse mapping
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IP addresses → hostnames
Top-level Domains
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gTLDs (generic TLDs)
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ccTLDs (country code TLDs)
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com, edu, net, org, gov, mil, int, arpa
aero, biz, coop, info, jobs, museum, name, pro
au, ca, br, de, fi, fr, jp, se, hk, cn, tw, my, …
Profitable domain names
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CreditCards.com - $2.75M
Loans.com – $3M
Business.com - $7.5M
Second-level Domain Name
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Examples
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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
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Valid domain names
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Case insensitive
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Each component: [a-zA-Z0-9\-]{1,63}
Each name < 256 chars
www.cs.ucr.edu == WWW.CS.UCR.EDU
FQDN
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Fully Qualified Domain Name
eon.cs.ucr.edu
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eon – hostname
cs.ucr.edu – domain name
How To Register A Domain Name?
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Pick a domain name of interest
Dedicate 2 NS servers
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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?
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Delegation
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All name servers read all the 13 root servers from
a local configuration file
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[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
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DNS servers cache results they receive from
other servers
Each result is saved based on its TTL
Negative caching
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For nonexistent hostname (for 10 mins)
Also for unreachable/unresponsive servers
Authoritative vs. Non-authoritative
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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
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Recursive
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Queries on a client behalf until it returns either an
answer or an error
Non-recursive
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Refers the client to another server if it can’t
answer a query
DNS Database
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A set of text files, called zone files,
maintained by the system admin. on the
master NS
2 types of entries
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Parser commands, eg.
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$ORIGIN and $TTL
Resource Records (RR)
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[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)
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Resource Record Types
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SOA
NS
A
AAAA
PTR
MX
CNAME
TXT
…
Start Of Authority
Name Server
IPv4 name-to-address translation
IPv6 name-to-address translation
Address-to-name translation
Mail eXchanger
Canonical NAME
Text
BIND
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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
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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
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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
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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
Email address:
weesan@voicense.com
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Remember to
increment the serial #
after each editing
$TTL 86400
$ORIGIN voicense.com.
@
IN SOA voicense.com. weesan.voicense.com. (
20040304 ; serial #
7200
; refresh (2 hrs)
1800
; retry
(30 mins)
604800 ; expire (1 week)
7200 )
; mininum (2 hrs)
IN NS
ns.voicense.com.
IN MX
10 mail.voicense.com.
IN MX
20 mail.myisp.com.
IN A
10.0.0.1
mail
IN CNAME
voicense.com.
www
IN CNAME
voicense.com.
ns
IN CNAME
voicense.com.
lee
IN A
10.0.0.31
wee
IN A
10.0.0.32
/var/named/voicense.com.zone
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Serial #
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Refresh
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How long the slave servers should retry before giving up
Expire
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How often the slave servers should sync. with the master
Retry
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An increasing integer number (for sync’ing)
How long should the slave servers continue to serve the
domains in the absent of the master
Mininum
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TTL for negative answers that are cached
/var/named/voicense.com.rev
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$TTL
@
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1
31
32
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.
IN PTR
fw.voicense.com.
IN PTR
lee.voicense.com.
IN PTR
wee.voicense.com.
How To Load Balance A Web Server?
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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?
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$ host www.google.com
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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
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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
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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
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dig
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$ dig eon.cs.ucr.edu
$ dig eon.cs.ucr.edu ns
$ dig @momo.cs.ucr.edu eon.cs.ucr.edu mx
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$ man dig
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host
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$ 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)
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nslookup
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$ nslookup eon.cs.ucr.edu
$ nslookup eon.cs.ucr.edu momo.cs.ucr.edu
whois
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$ whois google.com
$ whois ucr.edu
/etc/resolv.conf
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Resolver
$ cat /etc/resolv.conf
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search cs.ucr.edu weesan.com
nameserver 138.23.169.10
nameserver 138.23.178.2
/etc/nsswitch.conf
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Used by C library
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gethostbyname()
$ cat /etc/nsswitch.conf
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hosts: file nis dns
Reference
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LAH
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Ch 15: DNS – The Domain Name System