Domain Name System (DNS)

Need of DNS

Tree structure of Domain Name

Distributed hierarchy of DNS

DNS Header

Features of DNS

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Purpose of naming (domain naming)

• Addresses are used to locate objects like websites, Mail servers etc.

• Names are easier to remember than numbers/ IP addresses.

• It is user friendly to get IP address of Websites, Mail server etc. using

a name.

• DNS provides a mapping from names to IP addresses and vice

versa.

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Names and addresses in general

• An address is how you get to an endpoint

Typically, hierarchical (for scaling):

MANIT Bhopal, M.P., India, 462051

+91-755-405-1500

204.152.187.11

www.manit.ac.in, www.upsc.gov.in, www.google.com

Subdomain for emails:- hod@ece.manit.ac.in

DNS was developed in 1983 by Paul Mockapetris (RFCs 1034 and

1035), modified, updated, and enhanced by subsequent RFCs.

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4

DNS Terms

• Domain name: any name represented in the DNS format – mail.yahoo.com

– www.manit.ac.in

– opm@manit.ac.in (to target mail server @ symbol is used)

• DNS label: each string between two “.” – ac.in – 2 labels

• DNS zone: a set of names that are under the same authority – cs.manit.ac.in, ee.manit.ac.in and www.manit.ac.in

• Delegation: transfer of authority for a domain – gmail.com is a delegation from com.

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DNS name server

• DNS name server is also called as DNS server.

• DNS server stores the DNS records for a domain name.

• DNS server maintains database of domain names and

corresponding IP addresses.

• DNS name server responds with answers to queries against its

database.

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DNS name Servers (cont.)

• Name servers answer ‘DNS’ questions.

• Several types of name servers

– Authoritative servers

• master (primary)

• slave (secondary)

– (Caching) recursive servers

• also caching forwarders

– Mixture of functionality

• NMS, DHCP and DNS server

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Domain Name System - DNS

• Provides mapping from ASCII domain names to IP addresses

– Hierarchical naming system.

– The set of names are divided into mutually exclusive parts.

• Supplies mechanism for global data storage and information

retrieval.

• Most important principles:

– Distributed system – set of servers sharing information.

– Efficiency - most of the requests resolved by local servers.

• Use of Caching.

– Reliability – works even if some of the servers fail.

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DNS Servers

authoritative name server

• Give authoritative answers for one or more zones based on domains

and country.

• The master server normally loads the data from a zone file

• A slave server normally replicates the data from the master via a

zone transfer

master

slave

slave 8 OPM

Domain name syntax

• consists of one or more parts (label ) concatenated, and delimited by dots, such as www.gmail.com

• The right-most label conveys the top-level domain; for example, the domain name www.gmail.com belongs to the top-level domain com.

• The hierarchy of domains descends from right to left; each label to the left specifies a subdivision, or subdomain of the domain to the right. For example: the label gmail specifies a subdomain of the com domain, and www is a sub domain of gmail.com. This tree of subdomain may have up to 127 levels.

• Each label may contain up to 63 characters. The full domain name may not exceed a total length of 253 characters in its external dotted-label specification.

• The characters allowed in a label are a subset of the ASCII character set, and includes the characters a through z, A through Z, digits 0 through 9, and the hyphen (-). This rule is known as the LDH rule(Letters, Digits, Hyphen). There are demands for domain name in regional languages.

• Domain names are interpreted in case-independent manner. Labels may not start or end with a hyphen. 9 OPM

10

DNS tree

root

org com gov net in

cnn ibm gov co ac

iitk manit

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11

Reliability over UDP

• DNS is an application which runs over UDP

– Advantage: not as complex as TCP.

– Disadvantage: requires reliability implementation at application level .

• Reliability scheme:

– Time out.

– Retransmissions - resends timed out query to a different server.

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DNS Elements

• Resolver:

– only asks questions;

– recursive: takes simple query and makes all necessary steps to get the full answer.

• Server (some perform both roles of resolver & server at the same time):

– authoritative: the servers that contain the zone file for a zone, one Primary, one or more Secondary;

– caching: a recursive resolver that stores prior results and reuses them (local network servers and web browsers like google Crome etc.).

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Registries, Registrars, and Registrants

• The Domain Wars resulted in a codification of roles in the operation of a domain name space

• Registry – the name space’s database

– the organization which has edit control of that database

• Including dispute resolution, policy control, etc.

– The organization which runs the authoritative name servers for that name space

• Registrar – the agent which submits change requests to the registry on behalf of the

registrant

• Registrant – The entity (person or organization) which makes use of the domain name

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Obtaining Authority for a subname

• Before an Institution is granted authority for an official second-

level domain, it must agree to operate a domain name server that

meets internet standards.

• In case of main Name server and backup name server, there should

not be a common point of failure like common power backup.

• Examples of Donain and Subdomains

– www.manit.ac.in

– www.ece.manit.ac.in, www.cse.manit.ac.in, etc.

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Registries, Registrars, and Registrants

Registry Zone DB

Registrants

End user requests add/modify/delete

Registrar submits add/modify/delete to registry

Registrar Registrar Registrar

Master server updated Registry updates

zone

Slaves server updated

NIR: National Internet Registry to

maintain records of IP allocation and

domain names in India, has been set

up in March 2013 under ministry of

Information and Communication,

govt. of India. 15 OPM

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How Does DNS Work?

• The resolver sends a DNS request message over UDP to a local

domain name server.

• DNS caches stored data for a short time defined by TTL on the

Record.

• When a name server does not have the requested information,

it starts at longest match on query name it has when looking for data.

• The root server knows the IP address of each second-level domain

name server.

• Every server knows how to reach servers that are authorities for

names further down the hierarchy.

• The resolver follows delegations until it receives an answer.

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Name Server Architecture

Master

server Zone transfer

Zone

data

file

From

disk Authoritative Data

(primary master and

slave zones)

Agent

(looks up queries

on behalf of resolvers)

Cache Data

(responses from

other name servers)

Name Server Process

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Authoritative Data

Resolver Query

Response

Authoritative Data

(primary master and

slave zones)

Agent

(looks up queries

on behalf of resolvers)

Cache Data

(responses from

other name servers)

Name Server Process

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Using Other Name Servers

Arbitrary

name

server

Response

Resolver Query

Query

Authoritative Data

(primary master and

slave zones)

Agent

(looks up queries

on behalf of resolvers)

Cache Data

(responses from

other name servers)

Name Server Process

Response

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Cached Data

Query

Response

Authoritative Data

(primary master and

slave zones)

Agent

(looks up queries

on behalf of resolvers)

Cache Data

(responses from

other name servers)

Name Server Process

Resolver

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21

DNS query example (from IETF slides)

Root Server

ietf.org Server

Org Server

Ask org NS

Ask ietf.org NS

www.ietf.org A 65.256.255.51

www.ietf.org A 65.256.255.51

Recursive Resolver

www.ietf.org

Stub resolver

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DNS recursive server

• Recursive servers do the actual lookups; they ask questions to the

DNS on behalf of the clients.

• Answers are obtained from authoritative servers but the answers

forwarded to the clients are marked as not authoritative

• Answers are stored for future reference in the cache

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slideset 1 -23

Places where DNS data lives

Registry DB

Master

Slave server

Slave

Cache server

Changes in DNS do not propagate instantly!

Not going to net if TTL>0

Might take up to refresh to get data from master

Upload of zone data is local policy

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Host device

DNS message

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DNS Header (cont.)

• Identification:- A 16 bit identifier assigned by the program that generates any kind of query. This identifier is copied into the corresponding reply and can be used by the requester to match up replies to outstanding queries.

• Parameter:- 16 bit flag field classified as below

– QR:-A one bit field that specifies whether this message is a query (0), or a

response (1).

– Opcode:- A four bit field that specifies type of query in this message. Standard (0), Inverse (1), completion (2 & 3 now obsolete).

– AA(Authoritative Answer) :- this bit is only meaningful in responses, and specifies that the responding name server is an authority for the domain name in question section.

– TC (TrunCation):- specifies that this message was truncated.

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DNS Header (cont.)

• RD (Recursion Desired) :- this bit directs the name server to pursue the query recursively.

• RA (Recursion Available) - this be is set or cleared in a response, and denotes whether recursive query support is available in the name server. Recursive query support is optional.

• Z Reserved for future use.

• AD (Authentic Data)- to show data authenticity and integrity of record and errors.

• CD (Checking Disabled)- to disable signature validation in a security-aware name server's processing of a particular query.

• RCODE (Response code) - this 4 bit field is set as part of responses. The values have the following interpretation: – 0 No error condition

– 1 Format error - The name server was unable to interpret the query.

– 2 Server failure - The name server was unable to process this query due to a problem with3 the name server.

– 3 Name Error - Meaningful only for responses from an authoritative name server, this code signifies that the domain name referenced in the query does not exist.

– 4 Not Implemented - The name server does not support the requested kind of query.

– 5 Refused - The name server refuses to perform the specified operation for policy reasons.

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DNS Header (cont.)

• Number of Questions:- an unsigned 16 bit integer specifying the number of

entries in the question section.

• Number of Answers:- an unsigned 16 bit integer specifying the number of

resource records in the answer section.

• Number of Authority:- an unsigned 16 bit integer specifying the number of

name server resource records in the authority records section.

• Number of Additional :- ARCOUNT an unsigned 16 bit integer specifying

the number of resource records in the additional records section.

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Question section

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• QNAME (Query Domain Name) :-A domain name represented as a sequence of

labels, where each label consists of a length octet followed by that number of octets.

The domain name terminates with the zero length octet for the null label of the root.

•QTYPE (Query Type):- A two octet code which specifies the type of the query.

0x0001 for representing A records (host addresses/web), 0x000f for mail server

(MX) records and 0x0002 for name servers (NS) records.

•QCLASS (Query Class):- A two octet code that specifies the class of the query.

0x0001 for representing Internet addresses.

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Answer Section

30

NAME (Resource Domain Name) The domain name that was queried, in the same

format as the QNAME in the questions.

TYPE Two octets containing one of the type codes. This field specifies the meaning

of the data in the RDATA field. type 0x0001 (A record), type 0x0002 (name servers)

, type 0x0005 (CNAME/ Alias) and 0x000f (mail servers).

CLASS Two octets which specify the class of the data in the RDATA field.

0x0001 for Internet addresses etc.

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Answer Section (cont.)

• TTL (Time To Live)The number of seconds the results can be

cached.

• RDLENGTH (Resource data length) The length of the RDATA

field.

• RDATA (Resource data) The data of the response. The format is

dependent on the TYPE field:

– if the TYPE is 0x0001 for A records, then this is the IP address (4 octets).

– If the type is 0x0005 for CNAMEs, then this is the name of the alias.

– If the type is 0x0002 for name servers, then this is the name of the server.

– Finally if the type is 0x000f for mail servers,

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Authority & Additional sections

• Authority Section

– Resource Records (RRs) pointing toward an authority

• Additional Section

– Resource Records (RRs) holding additional information

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DNS Features: Dynamicity

• Database can be updated dynamically

– Add/delete/modify of any record

• Modification of the master database triggers replication

– Only master can be dynamically updated

• Creates a single point of failure

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DNS Features: Global Distribution

• Data is maintained locally, but retrievable globally

– No single computer has all DNS data

• DNS lookups can be performed by any device

• Remote DNS data is locally cachable to improve performance

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DNS Features: Loose Coherency

• The database is always internally consistent

– Each version of a subset of the database (a zone) has a serial number

• The serial number is incremented on each database change

• Changes to the master copy of the database are replicated according to timing set by the zone administrator

• Cached data expires according to timeout set by zone administrator

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DNS Features: Scalability

• No limit to the size of the database

– One server has over 20,000,000 names

• No limit to the number of queries

– 24,000 queries per second handled easily

• Queries distributed among masters, slaves, and caches

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DNS Features: Reliability

• Data is replicated

– Data from master is copied to multiple slaves

• Clients can query

– Master server

– Any of the copies at slave servers

• Clients typically query local caches

• DNS protocols can use either UDP or TCP

– If UDP, DNS protocol handles retransmission, sequencing, etc.

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