NET0183 Networks and Communications

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NET0183 Networks and Communications

Lecture 25

DNS Domain Name System

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8/25/2009 NET0183 Networks and Communications by Dr

Andy Brooks 2

DNS is a distributed database implemented

in a hierarchy of many servers.

DNS is an application layer protocol that

runs over UDP and uses port 53.

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DNS @ Webopedia 17/3/10

Andy Brooks 3

Short for Domain Name System (or Service or Server), an Internet service that translates domain names into IP addresses. Because domain names are alphabetic, they're easier to remember. The Internet however, is really based on IP addresses. Every time you use a domain name, therefore, a DNS service must translate the name into the corresponding IP address. For example, the domain name www.example.com might translate to 198.105.232.4.

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4.20 Name Resolution

• The translation of a domain name into an address is called

name resolution

and the name is said to be

resolved

to an

address.

• Software to perform translation is known as a

name resolver

(or simply

resolver

).

• In the socket API, for example, the resolver is invoked by

calling function gethostbyname.

• Each resolver is configured with the address of one or more

local DNS servers.

• The resolver forms a

DNS request

message and sends the

message to the local DNS server.

– The resolver the waits for the local DNS server to send a DNS reply

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Root server system @ Webopedia 17/3/10

Andy Brooks 5

A system of 13 file servers that are distributed around the globe and contain authoritative databases that form a master list of all top-level domain names (TLDs). There is one central, or "A",

server that replicates changes to the other servers on a daily basis. Different organizations maintain the servers on the root server

system. The U.S. government plays a role in maintaining about half of the servers.

“While only 13 names are used for the root nameservers, there are many more physical servers; C, F, I, J, K, L and M servers now exist in multiple locations on

different continents, using anycast address announcements to provide decentralized service. As a result most of the physical root servers are now outside the United States, allowing for high performance worldwide.”

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TLD @ Webopedia 17/3/10

Andy Brooks 6

Short for top-level domain, and refers to the suffix attached to

Internet domain names. There are a limited number of predefined suffixes, and each one represent a top-level domain. Current top-level domains include:

• com - commercial businesses; this is the most common TLD • gov - U.S. government agencies

• edu - Educational institutions such as universities • org - Organizations (mostly nonprofit)

• mil - Military

• net - Network organizations • ca - Canada

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Distributed, Hierarchical Database

Client wants IP for www.amazon.com; 1

st

_approx:

• Client queries a root server to find com DNS server

• Client queries com DNS server to get amazon.com DNS

server

• Client queries amazon.com DNS server to get IP address

for www.amazon.com

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Example



Host at cis.poly.edu

wants IP address for

gaia.cs.umass.edu

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Recursive queries

recursive query:

 puts burden of name

resolution on contacted name server  heavy load?

iterated query:

 contacted server

replies with name of server to contact

 “I don’t know this

name, but ask this server”

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DNS: caching



once (any) name server learns a mapping, it

caches

the mapping



cache entries timeout (disappear) after some

time



Top-level domain servers are typically cached

in local name servers.



Thus root name servers are not often

visited.

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Domain Name System @ Wikipedia 17/3/10

Andy Brooks 12

“In principle, authoritative name servers are sufficient for the

operation of the Internet. However, with only authoritative name servers operating, every DNS query must start with recursive queries at the root zone of the Domain Name System and each user system must implement resolver software capable of recursive operation.” “To improve efficiency, reduce DNS traffic across the Internet, and increase performance in end-user applications, the Domain Name System supports DNS cache servers which store DNS query results for a period of time determined in the configuration (time-to-live) of the domain name record in question.”

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DNS caching in Web browsers

Andy Brooks 13

http://developer.yahoo.net/blog/archives/2007/07/high_performanc_7.html

“DNS has a cost. It typically takes 20-120 milliseconds for DNS to lookup the IP address for a given hostname. The browser can’t download anything from this hostname until the DNS lookup is completed.”

“DNS lookups are cached for better performance. This caching can occur on a special caching server, maintained by the user's ISP or local area network, but there is also caching that occurs on the individual user's computer. The DNS information remains in the operating system's DNS cache (the "DNS Client service" on Microsoft

Windows).”

“Most browsers have their own caches, separate from the operating system's cache. As long as the browser keeps a DNS record in its own cache, it doesn't bother the operating system with a request for the record.”

“Internet Explorer caches DNS lookups for 30 minutes by default, as specified by the DnsCacheTimeout registry setting. Firefox caches DNS lookups for 1 minute,

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DNS records

DNS:

a distributed database storing resource records

(RR)



Type=NS

 name is domain (e.g.

foo.com)

 value is hostname of

authoritative name server for this domain

RR format:

(name, value, type, ttl) 

Type=A

 name is hostname

 value is IP address



Type=CNAME

 name is alias name for some

“canonical” (the real) name

www.ibm.com is really

servereast.backup2.ibm.com

 value is canonical name



Type=MX

 value is name of mailserver

associated with name

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DNS protocol, messages

query

&

reply

messages both have the same

message format

msg header

 Identification: 16 bit # for query, reply to query uses same #  Flags, e.g.

 query or reply

 Bit 16 - QR bit. The message is a query if the value is 0.

The message is a response if the value is 1.

 reply is authoritative

 Bit 21 - AA bit. - Authoritative answer is set if the

responding name server is an aurhority for the domain name in question.

 recursion desired

 Bit 23 – RD bit. Recursion Desired is set in a query and

indicates the query should be persued recursively.

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DNS protocol, messages

slide from Kurose & Ross