Name Resolution and DNS

Domain names and IP addresses

People prefer to use easy-to-remember names instead of IP addresses

Domain names are alphanumeric names for IP addresses e.g., syslab.csd.uwo.ca, www.google.com, ietf.org

The domain name system (DNS) is an Internet-wide distributed database that translates between domain names and IP addresses

How important is DNS? Imagine what happens when the local DNS server is down.

Before there was DNS ….…. there was the HOSTS.TXT file

Before DNS (until 1985), the name-to-IP address was done by downloading a single file (hosts.txt) from a central server with FTP. Names in hosts.txt are not structured. The hosts.txt file still works on most

operating systems. It can be used to define local names.

DNS: Domain Name System

Distributed database implemented in hierarchy of many name servers

Application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation)note: core Internet function,

implemented as application-layer protocol

complexity at network’s “edge”

Hierarchy of Name Servers The resolution of the

hierarchical name space is done by a hierarchy of name servers

Each server is responsible (authoritative) for a contiguous portion of the DNS namespace, called a zone.

Zone is a part of the subtree

DNS server answers queries about hosts in its zone

root server

com servergov serveredu serverorg server

uci.eduserver

.virginia.edu server

cs.virginia.edu server

Hierarchical Names

Internet hosts and other resources need globally unique names

Difficult to keep unstructured names unique would require a single list of all names in use

Hierarchical names are much easier to make unique

Why Not Centralize DNS?

Single point of failure Traffic volume Distant centralized database Maintenance

doesn’t scale!

Design principle of DNS The naming system on which DNS is based is a

hierarchical and logical tree structure called the domain namespace.

An organization obtains authority for parts of the name space, and can add additional layers of the hierarchy

Names of hosts can be assigned without regard of location on a link layer network, IP network or autonomous system

Let’s discuss the different levels

Domain Name Hierarchy

.

edu

Root domain

com gov mil net org ro fr at . . . . . . jp

ici rnc ase pub utt vsat

roearn ns std cs lmn dsp

ulise paul

ac co gv or . . .

uni-linz tuwien . . . . . . .

eunet

cc

univie

mat exp itc . . . . . .

phytia alpha chris

Top-Level-Domains

Second Level Domains

DNS Name hierarchy DNS hierarchy can be represented by a tree Root and top-level domains are

administered by an Internet central name registration authority (ICANN)

Below top-level domain, administration of name space is delegated to organizations

Each organization can delegate further

DNS: Root Name Servers Contacted by local name server that can not resolve name Root name server:

Contacts authoritative name server if name mapping not known

Gets mapping Returns mapping to local name server Does not have name

13 root name servers worldwide

b USC-ISI Marina del Rey, CAl ICANN Los Angeles, CA

e NASA Mt View, CAf Internet Software C. Palo Alto, CA (and 17 other locations)

i Autonomica, Stockholm (plus 3 other locations)

k RIPE London (also Amsterdam, Frankfurt)

m WIDE Tokyo

a Verisign, Dulles, VAc Cogent, Herndon, VA (also Los Angeles)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 11 locations)

Top-level domains Types of top-level domains:

Organizational: 3-character code indicates the function of the organization

• Used primarily within the US • Examples: gov, mil, edu, org, com, net

Geographical: 2-character country or region code

• Examples: us, va, jp, de

There are more than 200 top-level domains.

Organizational top-level domains (TLD)

com Commercial organizations

edu Educational institutions

gov Government institutions

int International organizations

mil U.S. military institutions

net Networking organizations

org Non-profit organizations

TLD and Authoritative Servers

Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp.

Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web and mail). Can be maintained by organization or service

provider

Local Name Server

Does not strictly belong to hierarchy Each ISP (residential ISP, company,

university) has one. Also called “default name server”

When a host makes a DNS query, query is sent to its local DNS server Acts as a proxy, forwards query into

hierarchy.

Root DNS Servers

com DNS servers org DNS servers edu DNS servers

poly.eduDNS servers

umass.eduDNS servers

yahoo.comDNS servers

amazon.comDNS servers

pbs.orgDNS servers

Distributed, Hierarchical Database

Client wants IP for www.amazon.com; 1st approximation: 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

requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

1

23

4

5

6

authoritative DNS serverdns.cs.umass.edu

78

TLD DNS server

Example

Host at cis.poly.edu wants IP address for gaia.cs.umass.edu.

First, check locally. Then, check root. Root

notes .edu suffix and suggests TLD servers for .edu.

Check TLD server. This provides the authoritative server.

Check authoritative server, and get the IP address!

This uses both iterative and recursive queries. (Usually the way its done.)

requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

1

2

45

6

authoritative DNS serverdns.cs.umass.edu

7

8

TLD DNS server

3

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”

DNS: Caching and Updating Records

Once (any) name server learns mapping, it caches mapping Cache entries timeout (disappear) after

some time TLD servers typically cached in local name

servers• Thus root name servers not often visited

Update/notify mechanisms under design by IETF RFC 2136 http://www.ietf.org/html.charters/dnsind-charter.html

Summary

We have examined how DNS works