Domain Name System (DNS)

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Presentation transcript:

Domain Name System (DNS)

DNS The “Domain Name System” What Internet users use to reference anything by name on the Internet It translates easily memorized domain names to the numerical IP addresses needed for the purpose of locating computer services and devices worldwide

DNS A lookup mechanism for translating names to addresses and vice versa Users generally prefer names to numbers Computers prefer numbers to names DNS provides the mapping between the two I have “x”, give me “y” Copyright © 2001, Nominum, Inc.

DNS Components There are 3 components: Name Space: Specifications for a structured name space and data associated with the names Resolvers: Client programs that extract information from Name Servers. Name Servers: Server programs which hold information about the structure and the names.

Overview Introduction to the DNS DNS Components The name space The servers The resolvers Copyright © 2001, Nominum, Inc.

The Name Space The name space is the structure of the DNS database An inverted tree with the root node at the top Each node has a label The root node has a null label, written as “” Copyright © 2001, Nominum, Inc.

Copyright © 2001, Nominum, Inc. Labels Each node in the tree must have a label A-Z, 0-9, and “-” only with a-z and A-Z treated as the same Sibling nodes must have unique labels The null label is reserved for the root node Copyright © 2001, Nominum, Inc.

Domain Name Each node in the DNS tree represents a domain name. Some examples of DNS names are DNS domains computers and services (host)

Copyright © 2001, Nominum, Inc. Domain Names A domain name is the sequence of labels from a node to the root, separated by dots (“.”s), read left to right The name space has a maximum depth of 127 levels Domain names are limited to 255 characters in length A node’s domain name identifies its position in the name space Copyright © 2001, Nominum, Inc.

DNS domain A DNS domain is a branch under the node. DNS domains can contain both hosts (computers or services) and other domains (referred to as subdomains ).

Subdomains One domain is a subdomain of another if its apex node is a descendant of the other’s apex node More simply, one domain is a subdomain of another if its domain name ends in the other’s domain name So sales.nominum.com is a subdomain of nominum.com com nominum.com is a subdomain of com Copyright © 2001, Nominum, Inc.

The parts of a domain name mail.ksu.edu The particular host is called main The organization that controls it is called ksu This host is at an educational organization A domain is a group of related hosts.

How are these hosts related? bpastudio.csudh.edu www.ucla.edu sws.csudh.edu chemistry.stanford.edu They all belong to educational institutions. They are members of the edu top-level domain (TLD).

How are these hosts related? www.ksu.edu sms.ksu.edu chemistry.ksu.edu They all belong to the same organization, ksu. They are members of the ksu.edu second-level domain.

A unique host mail.ksu.edu This is the domain name of a unique host. It is a member of the ksu.edu domain, but it is the only one with the name mail.

There are a limited number of top-level domains (TLDs), including: .edu, educational .com, commercial .gov, government .org, non profit .net, networking organizations These are called “generic” TLDs.

There are also country code top-level domain names for every nation, like: .us, United States .sa, Saudi Arabia .cl, Chile .uk, United Kingdom These are called “country code” top-level domains (ccTLDs).

“root” zone TLDs & ccTLDs 2lds 3lds 4lds

Overview Introduction to the DNS DNS Components The name space The servers The resolvers Copyright © 2001, Nominum, Inc.

Zones In addition to dividing DNS namespace into domains, DNS namespace are divided into zones that store name information about one or more DNS domains. A zone is the authoritative source for information about each DNS domain name that is included in the zone.

Name Servers Name servers store information about the name space in units called “zones” The name servers that load a complete zone are said to “have authority for” or “be authoritative for” the zone Usually, more than one name server are authoritative for the same zone This ensures redundancy and spreads the load Also, a single name server may be authoritative for many zones Server responsible for answering DNS queries Exists at all levels of hierarchy Authoritative name servers hold part of the DNS database One name server can serve more then one zone Many name servers “should” serve the same zone Some name servers are authoritative for certain zones Copyright © 2001, Nominum, Inc.

Name Servers and Zones Name Servers Zones nominum.com isc.org 128.8.10.5 serves data for both nominum.com and isc.org zones Name Servers Zones 128.8.10.5 nominum.com 202.12.28.129 serves data for nominum.com zone only 202.12.28.129 isc.org 204.152.187.11 serves data for isc.org zone only 204.152.187.11 Copyright © 2001, Nominum, Inc.

Types of Name Servers Two main types of servers Other types exist… Authoritative – maintains the data Master – where the data is edited Slave – where data is replicated to Caching – stores data obtained from an authoritative server The most common name server implementation (BIND) combines these two into a single process Other types exist… No special hardware necessary Copyright © 2001, Nominum, Inc.

Name Server Architecture You can think of a name server as part: database server, answering queries about the parts of the name space it knows about (i.e., is authoritative for), cache, temporarily storing data it learns from other name servers, and agent, helping resolvers and other name servers find data that other name servers know about Also, the caching resolver can insert data into the cache, and can "query" the cache and the database server. Copyright © 2001, Nominum, Inc.

Name Server Architecture 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 Master server Zone transfer Copyright © 2001, Nominum, Inc.

on behalf of resolvers) Authoritative Data 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 Resolver Query Copyright © 2001, Nominum, Inc.

Using Other Name Servers 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 Response Arbitrary name server Resolver Query Query Copyright © 2001, Nominum, Inc.

on behalf of resolvers) Cached Data 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 Resolver Query Copyright © 2001, Nominum, Inc.

Overview Introduction to the DNS DNS Components The name space The servers The resolvers Copyright © 2001, Nominum, Inc.

Name Resolution Name resolution is the process by which resolvers and name servers cooperate to find data in the name space To find information anywhere in the name space, a name server only needs the names and IP addresses of the name servers for the root zone (the “root name servers”) The root name servers know about the top-level zones and can tell name servers whom to contact for all TLDs Copyright © 2001, Nominum, Inc.

Name Resolution A name server receiving a query from a resolver looks for the answer in its authoritative data and its cache If the answer isn’t in the cache and the server isn’t authoritative for the answer, the answer must be looked up Types of resolution: Recursive Resolution or Iterative Resolution Copyright © 2001, Nominum, Inc.

The Resolution Process Let’s look at the resolution process step-by-step: annie.west.sprockets.com ping www.nominum.com. Copyright © 2001, Nominum, Inc.

The Resolution Process The workstation annie asks its configured name server, dakota, for www.nominum.com’s address dakota.west.sprockets.com What’s the IP address of www.nominum.com? annie.west.sprockets.com ping www.nominum.com. Copyright © 2001, Nominum, Inc.

The Resolution Process The name server dakota asks a root name server, m, for www.nominum.com’s address m.root-servers.net dakota.west.sprockets.com What’s the IP address of www.nominum.com? annie.west.sprockets.com ping www.nominum.com. Copyright © 2001, Nominum, Inc.

The Resolution Process The root server m refers dakota to the com name servers This type of response is called a “referral” m.root-servers.net Here’s a list of the com name servers. Ask one of them. dakota.west.sprockets.com annie.west.sprockets.com ping www.nominum.com. Copyright © 2001, Nominum, Inc.

The Resolution Process The name server dakota asks a com name server, f, for www.nominum.com’s address What’s the IP address of www.nominum.com? m.root-servers.net dakota.west.sprockets.com f.gtld-servers.net annie.west.sprockets.com ping www.nominum.com. Copyright © 2001, Nominum, Inc.

The Resolution Process The com name server f refers dakota to the nominum.com name servers Here’s a list of the nominum.com name servers. Ask one of them. m.root-servers.net dakota.west.sprockets.com f.gtld-servers.net annie.west.sprockets.com ping www.nominum.com. Copyright © 2001, Nominum, Inc.

The Resolution Process The name server dakota asks an nominum.com name server, ns1.sanjose, for www.nominum.com’s address What’s the IP address of www.nominum.com? m.root-servers.net dakota.west.sprockets.com ns1.sanjose.nominum.net f.gtld-servers.net annie.west.sprockets.com ping www.nominum.com. Copyright © 2001, Nominum, Inc.

The Resolution Process The nominum.com name server ns1.sanjose responds with www.nominum.com’s address m.root-servers.net dakota.west.sprockets.com Here’s the IP address for www.nominum.com ns1.sanjose.nominum.net f.gtld-servers.net annie.west.sprockets.com ping www.nominum.com. Copyright © 2001, Nominum, Inc.

The Resolution Process The name server dakota responds to annie with www.nominum.com’s address Here’s the IP address for www.nominum.com m.root-servers.net dakota.west.sprockets.com ns1.sanjose.nominum.net f.gtld-servers.net annie.west.sprockets.com ping www.nominum.com. Copyright © 2001, Nominum, Inc.

Resolution Process (Caching) After the previous query, the name server dakota now knows: The names and IP addresses of the com name servers The names and IP addresses of the nominum.com name servers The IP address of www.nominum.com Let’s look at the resolution process again annie.west.sprockets.com ping ftp.nominum.com. Copyright © 2001, Nominum, Inc.

Resolution Process (Caching) The workstation annie asks its configured name server, dakota, for ftp.nominum.com’s address m.root-servers.net dakota.west.sprockets.com What’s the IP address of ftp.nominum.com? ns1.sanjose.nominum.net f.gtld-servers.net annie.west.sprockets.com ping ftp.nominum.com. Copyright © 2001, Nominum, Inc.

Resolution Process (Caching) dakota has cached an NS record indicating ns1.sanjose is an nominum.com name server, so it asks it for ftp.nominum.com’s address What’s the IP address of ftp.nominum.com? m.root-servers.net dakota.west.sprockets.com ns1.sanjose.nominum.net f.gtld-servers.net annie.west.sprockets.com ping ftp.nominum.com. Copyright © 2001, Nominum, Inc.

Resolution Process (Caching) The nominum.com name server ns1.sanjose responds with ftp.nominum.com’s address m.root-servers.net dakota.west.sprockets.com Here’s the IP address for ftp.nominum.com ns1.sanjose.nominum.net f.gtld-servers.net annie.west.sprockets.com ping ftp.nominum.com. Copyright © 2001, Nominum, Inc.

Resolution Process (Caching) The name server dakota responds to annie with ftp.nominum.com’s address Here’s the IP address for ftp.nominum.com m.root-servers.net dakota.west.sprockets.com ns1.sanjose.nominum.net f.gtld-servers.net annie.west.sprockets.com ping ftp.nominum.com. Copyright © 2001, Nominum, Inc.

Recursive Resolution root server 3 edu server 2 com server 4 8 7 recursive request “What is the IP address of www.google.com?” udel server 1 google server 5 9 recursive response “The IP address of www.google.com is 216.239.37.99.” 6 10 client

Iterative Resolution 5 7 3 iterative response (referral) a.root server 5 a3.nstld.com 3 a.gtld- server 7 iterative response (referral) “I don't know. Try a.gtld-servers.net.” 6 iterative request “What is the IP address of www.google.com?” udel server 1 ns1.google.com 9 iterative response (referral) “I don't know. Try a.root-servers.net.” 4 iterative response (referral) “I don't know. Try ns1.google.com.” 8 iterative response (referral) “I don't know. Try a3.nstld.com.” 2 iterative response “The IP address of www.google.com is 216.239.37.99.” 10 client