Domain Name System r IP assigns 32-bit addresses to hosts (interfaces) m Binary addresses easy for computers to manage m All applications use IP addresses.

Domain Name System

IP assigns 32-bit addresses to hosts (interfaces) Binary addresses easy for computers to manage All applications use IP addresses through the TCP/IP

protocol software Difficult for humans to remember:

% telnet 134.82.11.70

The Domain Name System (DNS) provides translation between symbolic names and IP addresses

DNS serves listen on UDP port 53 and TCP port 53. For short requests and replies UDP is used. TCP is used for ZONE transfer.

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., neon.ece.utoronto.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 Services

Hostname to IP Address mapping Inverse Mapping: IP Address to Hostname mapping Mail Domain to Mail Server mapping; needed for mail

delivery by mail servers Host aliasing; Mail Domain aliasing

Canonical (real) and alias names

Load distribution Replicate a heavily accessed web site (e.g. “

www.cnn.com”) to multiple web servers (i.e. different hosts with different IP addresses)

Associate “www.cnn.com” with the IP addresses of web servers

Structure of DNS names

Each name consists of a sequence of alphanumeric components separated by periods

Examples: www.eg.bucknell.edu

www.netbook.cs.purdue.edu

charcoal.eg.bucknell.edu

Names are hierarchical, with most-significant component on the right

Left-most component is computer name Local name server, root name server, authoritative

name server

Domain Name Tree

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

e NASA Mt View, CAf Internet Software C. Palo Alto, CA

i NORDUnet Stockholm

k RIPE London

m WIDE Tokyo

a NSI Herndon, VAc PSInet Herndon, VAd U Maryland College Park, MDg DISA Vienna, VAh ARL Aberdeen, MDj NSI (TBD) Herndon, VA

13 root name servers worldwide, according to www.dnso.org 2002

Root Server

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

Domain Names are assigned independent of geographic locations or IP addresses. However, in practice, allocation of domain names generally follows the allocation of IP address, e.g., All hosts with network prefix 128.143/16 have domain name suffix

virginia.edu All hosts on network 128.143.136/24 are in the Computer Science

Department of the University of Virginia

Top-level domains

Three 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

Reverse domains: A special domain (in-addr.arpa) used for IP address-to-name mapping

gTLDS (generic TLDs) were originally meant to be US-centric. Nowadays, only mil and gov TLDs are for US.

ccTLDs (countrycode TLDs) used for organizations in other countries:

Countries define their own internal hierarchy: ac.uk and .edu.au are used for academic organizations in the United Kingdom and Australia. In SA, it is edu.sa.

gTLDs versus ccTLDs

DNS naming structure

Top level domains (rightmost components; also known as TLDs) defined by global authority

com Commercial organization

edu Educational institution gov Government organization mil Military organization Organizations apply for names under a top-level domain:

kfupm.edu

macdonalds.com Organizations determine their own internal structure

ccse.kfupm.edu

cs.purdue.edu

Domain names within an organization Organizations can create any internal DNS hierarchy Uniqueness of TLD and organization name guarantee

uniqueness of any internal name (much like file names in your directories)

All but the left-most component of a domain name is called the domain for that name:

• Authority for creating new subdomains is delegated to each domain

• Administrator of kfupm.edu has authority to create ri.kfupm.edu and need not contact any central naming authority

• DNS domains are logical concepts and need not correspond to physical location of organizations

• DNS domain for an organization can span multiple networks

DNS and client-server computing

DNS names are managed by a hierarchy of DNS servers There could be multiple ways of dividing up DNS name

space Root server at top of tree knows about next level servers. Next level servers, in turn, know about lower level servers Each DNS server is the authoritative server for the names it

manages What a server is responsible for or has authority over is

called a zone. A domain can span multiple servers. Primary server is also called authoritative server Second server has a copy

Distributed, Hierarchical Database

Root servers and TLD servers typically do not contain hostname to IP mappings; they contain mappings for locating authoritative servers.

Root DNS Servers

com DNS servers ca DNS servers edu DNS servers

poly.eduDNS servers

umass.eduDNS servers

yahoo.comDNS servers

amazon.comDNS servers

ucalgary.caDNS servers

TLDServers

usask.caDNS servers

Root name servers

The root name servers know how to find the authoritative name servers for all top-level zones.

There are only 13 root name servers

Root servers are critical for the proper functioning of name resolution

Addresses of root servers

A.ROOT-SERVERS.EDU. (formerly NS.INTERNIC.NET) 10.0.2.32A.ROOT-SERVERS.NET. (formerly NS1.ISI.EDU) 198.41.0.4B.ROOT-SERVERS.NET. (formerly C.PSI.NET) 128.9.0.107C.ROOT-SERVERS.NET. (TERP.UMD.EDU) 192.33.4.12D.ROOT-SERVERS.NET. (NS.NASA.GOV) 128.8.10.90E.ROOT-SERVERS.NET. (NS.ISC.ORG) 192.203.23F.ROOT-SERVERS.NET. (NS.NIC.DDN.MIL) 192.5.5.241G.ROOT-SERVERS.NET. (AOS.ARL.ARMY.MIL) 192.112.36.4H.ROOT-SERVERS.NET. (NIC.NORDU.NET) 128.63.2.53I.ROOT-SERVERS.NET. (at NSI (InterNIC)) 192.36.148.17J.ROOT-SERVERS.NET. (operated by RIPE NCC) 198.41.0.10K.ROOT-SERVERS.NET. (at ISI (IANA)) 193.0.14.129 L.ROOT-SERVERS.NET. (operated by WIDE, Japan) 198.32.64M.ROOT-SERVERS.NET. 202.12.27.33

TLD and Authoritative Servers

Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains like: uk, fr, ca, jp. Network solutions maintains servers for COM TLD Educause for EDU TLD

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

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. Reduces lookup latency for commonly searched

hostnames

Resolver and name server

1. An application program on a host accesses the domain system through a DNS client, called the resolver

2. Resolver contacts DNS server, called name server

3. DNS server returns IP address to resolver which passes the IP address to application

Reverse lookups are also possible, i.e., find the hostname given an IP address

HTTP Resolver

Hostname (neon.tcpip-lab.edu)

IP address (128.143.71.21)

Name server

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Managed by UofT

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

Managed by ECE Dept.

. (root)

com

toronto.edu

goveduorg

uci.edu

ece.toronto.edumath.toronto.edu

neon.ece.toronto.edu

Top-level Domains

Domain name system

Each node in the DNS tree represents a domain (or a domain name)

Each branch below a node is a DNS domain. DNS domain can contain

hosts or other domains (subdomains)

Example: DNS domains are . (root domain), edu, virginia.edu, cs.virginia.edu

virginia.edu

cs.virginia.eduwww.virginia.edu

neon.cs.virginia.edu

edu

.

Hosts and DNS domains are named based on their position in the domain tree

Every node in the DNS domain tree can be identified by a unique Fully Qualified Domain Name (FQDN). The FQDN gives the position in the DNS tree.

A FQDN consists of labels (“cs”,“virginia”,”edu”) separated by a period (“.”)

There can be a period (“.”) at the end. Each label can be up to 63 characters long FQDN may contain letters, numerals, and dash character (“-”) FQDNs are not case-sensitive

Domain names

cs.virginia.edu cs.virginia.edu.or

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

Authority and delegation

Authority for the root domain is with the Internet Corporation for Assigned Numbers and Names (ICANN)

ICANN delegates to accredited registrars (for gTLDs) and countries for country code top level domains (ccTLDs)

Authority can be delegated further Chain of delegation can be obtained by reading domain

name from right to left. Unit of delegation is a “zone”.

DNS domain and zones

Each zone is anchored at a specific domain node, but zones are not domains.

A DNS domain is a branch of the namespace

A zone is a portion of the DNS namespace generally stored in a file (It could consists of multiple nodes)

A server can divide part of its zone and delegate it to other servers

. (root)

.virginia.edu

.edu

.uci.edu

cs.virginia.edumath.virginia.edu

DomainZone

anddomain

Zone

Primary and secondary name servers

For each zone, there must be a primary name server and a secondary name server The primary server (master server) maintains a zone file which has

information about the zone. Updates are made to the primary server The secondary server copies data stored at the primary server.

Adding a host: When a new host is added (“gold.cs.virginia.edu”) to a zone, the

administrator adds the IP information on the host (IP address and name) to a configuration file on the primary server

Domain name resolution

1. User program issues a request for the IP address of a hostname

2. Local resolver formulates a DNS query to the name server of the host

3. Name server checks if it is authorized to answer the query. a) If yes, it responds.

b) Otherwise, it will query other name servers, starting at the root tree

4. When the name server has the answer it sends it to the resolver.

HTTP Resolver

Hostname (neon.tcpip-lab.edu)

IP address (128.143.71.21)

Nameserver

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DNS server architecture

All DNS servers are linked together to form a unified system. Each server keeps a set of NS (Name Server) Resource Records that identify root servers and servers for child domains.

Small organizations can use a single “primary” server Easy to administer Inexpensive

Large organizations often use multiple servers Reliability through redundancy Improved response time through load-sharing Delegation of naming authority

Locality of reference applies - users will most often look up names of computers within same organization

Recursive and Iterative Queries

There are two types of queries: Recursive queries Iterative (non-recursive) queries

The type of query is determined by a bit in the DNS query

Recursive query: When the contacted Name Server cannot resolve a query, the server becomes a DNS client, issues a query, wait for reply and pass reply back to requestor

Iterative queries: When the contacted Name Server cannot resolve a query, it replies to the requestor with the IP address/name of another Name Server

Name Resolution

requesting hostsurf.eurecom.fr

www.umass.edu

root name serverA.ROOT-SERVERS.NET.

local name serverdns.eurecom.fr

1

23

4 5

6

authoritative name serverdns.umass.edu

edu name serverA.GTLD-SERVERS.NET.

7

8

host surf.eurecom.fr wants IP address of www.umass.edu

1. contacts its local DNS server, dns.eurecom.fr

2. dns.eurecom.fr contacts root name server, if necessary

3. root name server eventually contacts authoritative name server, dns.cs.umass.edu, if necessary

This is called “Recursive Resolution”

Types of Queries

recursive query:puts burden of name resolution on contacted name serverheavy load?

iterated query:contacted server replies with name of server to contact“I don’t know this name, but ask the following server(s)”Gives more control to client requesting host

surf.eurecom.fr

gaia.cs.umass.edu

root name server

local name serverdns.eurecom.fr

1

23

4

5 6

authoritative name serverdns.cs.umass.edu

7

8

iterated query

edu name serverA.GTLD-SERVERS.NET.

DNS: Root Name Servers

contacted by local name servers that can not resolve query (mapping)

Root Name Server: contacts authoritative name server if mapping not known gets mapping returns mapping to local name server

DNS Resource Records (RRs)A DNS server stores DNS info. in a DNS database (act. a simple txt file):

A set of records known as Resource Records (RRs)

Type=NS* name is some domain

name (e.g. foo.com) value is hostname of

authoritative name serverfor the domain

RR format: (class, name, type, value, TTL); class value set to “IN” for Internet

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* name is a mail domain value is hostname of mail

server responsible for mail domain

*Note: IP addresses associated with the relevant hostnames are entered as A records

DNS Resource Records (RRs): continued

Type= PTR used to resolve IP-address to hostname queries name is IP address (e.g. 1.2.3.196.in-addr.arpa.) value is hostname

Type=SOA Start of Authority Record. The SOA is the first record in

entered in the zone’s file name is domain name for this zone value (MNAME, RNAME, SERIAL, REFRESH , RETRY, Min.

TTL); the Serial field acts as a time stamp that is cahnged whenever the zone’s file is modified. This filed is regularly checked by secondary to tell if a zone transfer is needed.

DNS protocol, messages

DNS protocol : query and reply messages, both with same message format

msg header identification: 16 bit #

for query, reply to query uses same #

flags: query or reply recursion desired recursion available reply is authoritative

DNS protocol, messages

Name, type fields for a query

RRs in responseto query

NS records forauthoritative servers

additional “helpful”info that may be used

Becoming Part of DNS

Example: just created startup “Network Utopia” Register name networkutopia.com at a Registrar (e.g., Network

Solutions) Need to provide Registrar with names and IP addresses of your

authoritative name servers (primary and secondary) Registrar inserts two RRs into the COM TLD server:

(networkutopia.com, NS, dns1.networkutopia.com)(dns1.networkutopia.com, A, 200.200.200.1)

Network Utopia adds it own RRs as part of the configuration of its DNS server (dns1.networkutopia.com), for example: Type A record for www.networkuptopia.com Type MX record for networkutopia.com

Measures to Improve Performance of DNS•DNS resolution can be very inefficient

• Every host referenced by name triggers a DNS request

• Every DNS request for the address of a host in a different organization goes through the root server

•Caching: Name Servers and hosts use caching of DNS replies to reduce the need to issue further DNS requests

• Cache is a list of recently resolved names and IP addresses

• A DNS reply (i.e. RR) is kept in Cache for no more than the time set by the TTL (time-to-live) field

• Replication of Name Servers

• There are 13 root name servers

• Every zone maintains at least two name servers (primary and secondary)

(DDNS) Dynamic Domain name System

• When a new node is added/deleted authoritative DNS server is updated accordingly.

• When a new domain is added, update message is sent to upper servers

• Servers query other servers periodically for latest changes update/notify mechanisms under design by IETF

RFC 2136 http://www.ietf.org/html.charters/dnsind-charter.html