DNS

DNS records: Type = cname

name is alias name for some "canonical" (the real) name ex. IBM.com is servereast.backup2.ibm.com

DNS records: Type = ns

name is domain (ex. foo.com) value is hostname of authoritative name server for this domain (networkutopia.com, dns1.networkutopia.com, NS)

DNS records: Type = A

name is hostname value is IP address (dns1.networkutopia.com, 212.212.212.., A)

DNS caching

once any name server learns mapping, it caches the mapping Cache entries disappear after some time TLD server typically cached in local name servers

Authoritative DNS servers

organization's DNS servers, providing authoritative hostname to IP mappings for organization's servers (e.g. Web, mail) can be maintained by organization or service provider

People have many identifiers..

SSN, name, passport #

recursive query

puts task of name resolution on contacted server "I don't know but, I will ask this server for you" - subsequent servers react in same way once server is found mapping is backtracked to client

DNS protocol

query and reply messages, both with same message format

Inserting records into DNS

register name at DNS registrar (GoDaddy.com) provide names, IP of authoritative name server registrar inserts 2 RRs into com TLD server

Top-level domain(TLD) servers

responsible for .com, .org, .net, .edu, etc... Network solutions maintains servers for .com TLD Educause for .edu TLD

load distribution

set of IP addresses for one canonical name

Why not centralize DNS?

single point of failure, traffic volume, distant centralized database, maintenance

Client wants amazon.com

(1) client queries a root server to find .com DNS server (2) client queries .com DNS server to get amazon.com DNS server (3) client queries amazon.com DNS server to get IP address for amazon.com

Internet hosts, routers have many identifiers...

IP address(32-bit): user for addressing datagrams "name": for readability by humans

Root name servers

contacted by local name server that cannot resolve a name root name server then contacts authoritative name server gets mapping(address) returns mapping to local name server

Iterated query

contacted server replies with name of server to contact "I don't know this name, but ask this server"

DNS records

distributed db storing resource records (name, value, type)

Local Name Server

does not strictly belong to hierarchy each ISP (residential ISP, company, university) has one when host makes DNS query, query is sent to its local DNS server (acts as proxy to hierarchy)

DNS - application layer protocol

host, routers, name servers to communicate to resolve names(address/name translation)

DNS services

hostname to IP translation host aliasing(creating name) mail server aliasing

Domain Name System (DNS) - distributed database

implemented in hierarchy of many name servers

DNS updating

update/ notify mechanisms under design by IETF

DNS Records: Type = MX

value is name of mailserver associated with name