Understanding DNS: From Domain Names to IP Addresses, Slides of Computer Networks

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Domain Name System (DNS) Ch 9.1 in text Book
Problem statement:

Average brain can easily remember 10 digits

On average, IP addresses have 10.28 digits

We need an easier way to remember IP addresses

Solution:

Solution:

Use alphanumeric names to refer to hosts

Add a distributed, hierarchical protocol (called DNS) tomap between alphanumeric host names and IP addresses

We call this Address Resolution

⥠ʵ DNS Root name servers
Responsible for root zone
About 12 root name servers worldwide
Loal name servers are confgured to contact well

known root servers well

known root servers

Domain Name Service
The domain name service consists of

Domain name space

Name servers

In each zone, there is a primary name server and one or more secondary name servers Name servers contain two kinds of address mappings:

Name servers contain two kinds of address mappings:

Authoritative mappings

: For hosts within the zone

Cached mappings

: For previously requested mappings to hosts

not in the zone

Resolvers

Programs that extract information from name servers inresponse to client requests

DNS Protocol
When client wants to know an IP address for a host name

Client sends a DNS query to the local name server in its zone

If name server contains the mapping, it returns the IP address tothe client

Otherwise, the name server forwards the request to the root

Otherwise, the name server forwards the request to the root name server

The request works its way down the tree toward the host until itreaches a name server with the correct mapping

DNS Protocol Example

remus.rutgers.edu

ns-lcsr.rutgers.edu 1 2

Scenario:recursive processing remus.rutgers.edu tries to resolve an IP address for venus.cs.yale.edu a.root-servers.net

yale.edu cs.yale.edu

venus.cs.yale.edu using a recursive queryNs-lcsr.rutgers.edu obtains theIP address for cs.yale.edu

DNS Caching
Going to the root server and then down the tree every time weneed to resolve an address is inefficient

Introduce address caching at name servers

Store host-to-IP-address mappings from recently requested host names at name servernames at name server

When the same address is requested later, use the cachedversion at the local name server instead of recursively queryingother name servers again

DNS Caching Example remus.rutgers.edu ns-lcsr.rutgers.edu 1

remus.rutgers.edu

First time: remus.rutgers.edu

tries

to resolve an IP address for

Later: venus.cs.yale.edu

has

been cached at ns-lcsr.

a.root-servers.net yale.edu cs.yale.edu 2 3 4

ns-lcsr.rutgers.edu 1

address for venus.cs.yale.edu using a recursive query

remus.rutgers.edu

(and

any other host thatuses ns-lcsr) willreceive the cached IPaddress for venus.cs.yale.edu

DNS records
All information stored in resource records (RR): a four tuple

<name, value, type, TTL>

If type = A then name=hostname, value = IP address<rags.rutgers.edu,168.14.2.4, A>

If type= NS then name=domain, value = name of DNS server <ns.lcsr.rutgers.edu,128.64.13.2, NS><ns.lcsr.rutgers.edu,128.64.13.2, NS>

If type=CNAME then name = generic host name, value is theactual hostname <cs.rutgers.edu, aramis.rutgers.edu, CNAME>

If type = MX, then value is the mail server for the host name inthe type field <cs.rutgers.edu,dragon.rutgers.edu,MX>

DNS message In DNS, all communications use a single format called a message. The top level format of message is divided into 5 sections(some of which are empty in certain cases) RR( Resource record) Header Question the question for the name server Answer RRs answering the question Authority RRs pointing toward an authority Additional RRs holding additional information

The answer section contains RRs that answer the question;the authority section contains RRs that point toward an authoritative name server;the additional records section contains RRs which relate to the query,but are not strictly answers for the question.

DNS Message Fields
Transaction Identification : Random number used to match client queries with name server responses

Flags :

QR

opcode

AA

TC

RD

RA

(unused) rcode 1 4 1 1 1 1 3 4

QR

: 0=Query, 1=Response

opcode

: 0=standard query, 1=inverse query, 2=status request

AA

: Authoritative answer

TC

: Truncated DNS packet

RD

: Recursion desired

RA

: Recursion available

rcode

: Return code. 0=no error, 3=name error

QR

opcode

AA

TC

RD

RA

(unused) rcode

DNS Message Fields (cont’d)

Number of Questions : Number of DNS queries in the packet

Number of Answer RRs : Number of non-authoritative DNS responses in the packet

Number of Authoritative RRs : Number of authoritative DNS responses in the packetresponses in the packet

Number of Additional RRs : Number of other DNS responses in the packet (usually contains other DNS servers in domain)

Questions & Answers : Variable length fields to store DNS queries and DNS server responses

CDN terms
Origin server

Server that holds the authoritative copy of thecontent
CDN server
CDN server

A replica server owned by the CDN provider
CDN name server

A DNS like name server used for redirection
Client

Types of CDN
DNS redirection

Full site Content delivery

All requests are redirected by DNS to CDN server

Partial site content delivery

URL are modified to be resolved by CDN name server

URL rewriting

Hybrid scheme

CDN used by ISPs to improve end-user latency

Caches used to reduce bandwidth required