The Domain Name System

1
The Domain Name System

• URL
• HTTP
• Domain name system

2
Web servers and web browsers
3
Several technological components

• HTML (HyperText Markup Language) a language for
  specifying the contents and layout of pages as
  they are displayed by web browsers.
• URLs (Uniform Resource Locators), which identify
  documents and other resources stored as part of
  the Web.
• A client-server system architecture, with
  standard rules for interaction (the HyperText
  Transfer Protocol--HTTP) by which browsers and
  other clients fetch documents and other resources
  from web servers.

4
URLs

• Every HTTP URL is of the following form
  http//host_nameport/pathparameters?query
  
• -- where items in square brackets are optional.
• Examples
• http//www.ulster.ac.uk/
• http//www.ulster.ac.uk88/
• http//www.w3.org/Protocols/Activity.html
• http//www.google.com/search?qurl

5
HTTP

• A protocol that specifies the messages involved
  in a request-reply exchange, the methods,
  arguments and results and the rules for
  representing them in the messages.
• A set of methods include GET, PUT, POST, TRACE,
  etc.

6
HTTP Get request

• It requests the resource whose URL is given as
  argument.
• If the URL is a static web page, then the web
  server replies by returning that page.
• If the URL refers to a program, then the web
  server runs the program and returns its output to
  the client.

7
HTTPS

• The secure hypertext transfer protocol (HTTPS) is
  a communications protocol designed to transfer
  encrypted information between computers over the
  World Wide Web. HTTPS is http using a Secure
  Socket Layer (SSL). A secure socket layer is an
  encryption protocol invoked on a Web server that
  uses HTTPS.
• Most implementations of the HTTPS protocol
  involve online purchasing or the exchange of
  private information. Accessing a secure server
  often requires some sort of registration, login,
  or purchase.
• The successful use of the HTTPS protocol requires
  a secure server to handle the request.
• An example https//uumail.ulster.ac.uk/

8
Some characteristics of HTTP (1)

• Application level HTTP operates at the
  application level. It assumes a reliable,
  connection-oriented transport protocol such as
  TCP, but does not provide reliability or
  retransmission itself.
• Request/response once a transport session has
  been established, one side (usually a browser)
  must send an HTTP request to which the other side
  responds.
• Stateless each HTTP request is self-contained
  the server does not keep a history of previous
  requests or previous sessions.
• Bi-directional transfer in most cases, a browser
  requests a web page, and the server transfers a
  copy to the browser. HTTP also allows transfer
  from a browser to a server (e.g., when a user
  submits a so-called form).

9
Some characteristics of HTTP (2)

• Capability Negotiation HTTP allows browsers and
  servers negotiate details such as the character
  set to be used during transfer. A sender can
  specify the capabilities it offers and a receiver
  can specify the capabilities it accepts.
• Support for caching to improve respond time, a
  browser caches a copy of each web page it
  retrieves. If a user requests a page again, HTTP
  allows the browser to interrogate the server to
  determine whether the contents of the page have
  been changed since the copy was cached.
• Support from intermediaries HTTP allows a
  machine along the path between a browser and a
  server to act as a proxy server that caches web
  pages and answer a browsers request from its
  cache.

10
Name and name space

• Any process that require access to a specific
  resource must possess a name or an identifier for
  it.
• A name space is the collection of all valid names
  recognized by a particular service.
• For example, we can define a name space which is
  composed of decimal numbers, then 555 and
  1234567 are valid names, but 3r and _33 are
  not.

11
The role of names and name services (1)

• Resources are accessed using identifier or name
• An identifier can be stored in variables and
  retrieved from tables quickly
• An identifier includes or can be transformed to
  an address for an object.
• A name is human-readable value (usually a string)
  that can be resolved to an identifier or address
• Internet domain name, file pathname, process
  number
• E.g ./etc/passwd, http//www.w3.org/

12
The role of names and name services (2)

• For many purposes, names are preferable to
  identifiers
• because the binding of the named resource to a
  physical location is deferred and can be changed
• because they are more meaningful to users
• Resource names are resolved by name services
• to give identifiers and other useful attributes

13
Requirements for name spaces

• Allow simple but meaningful names to be used
• Potentially infinite number of names
• Structured
• to allow similar subnames without clashes
• to group related names
• Allow re-structuring of name trees
• for some types of change, old programs should
  continue to work
• Management of trust

14
Composed naming domains used to access a resource
from a URL
15
Names and resources

• Currently, different name systems are used for
  each type of resource
• resource name identifies
• file pathname file within a
  given file system
• Process process id process on a given
  computer
• port port number IP port on a given computer
• Uniform Resource Identifiers (URI) offer a
  general solution for any type of resource. There
  are two main classes
• URL Uniform Resource Locator
• typed by the protocol field (http, ftp, nfs,
  etc.)
• part of the name is service-specific
• resources cannot be moved between domains
• URN Uniform Resource Name
• requires a universal resource name lookup service
  - a DNS-like system for all resources

16
The Domain Naming System

• IP addresses are difficult to remember. Instead
  the Internet maintains a set of high-level
  addresses which map to IP addresses. These
  high-level addresses are easier to remember than
  IP addresses.
• The addresses are organised hierarchically, as
  domain names.
• A domain name consists of a sequence of labels
  separated by dots. Each label is a sequence of
  alphabetic characters.
• Anything to the right of a label in a domain name
  is called a domain.

17
The Internet Domain Name System (2)

• A distributed naming database
• Name structure reflects administrative structure
  of the Internet
• Rapidly resolves domain names to IP addresses
• exploits caching heavily
• typical query time 100 milliseconds
• Scales to millions of computers
• partitioned database
• caching
• Resilient to failure of a server
• replication

18
An example of domain name

• isun1.infj.ulst.ac.uk
• In this example there are five labels.
• infj.ulst.ac.uk
• ulst.ac.uk
• ac.uk
• uk are all domains.

19
Name resolution process

• Basic DNS algorithm for name resolution (domain
  name -gt IP number)
• Look for the name in the local cache
• Try a superior DNS server, which responds with
• another recommended DNS server
• the IP address (which may not be entirely up to
  date)

20
DNS name servers
Note Name server names are in italics, and the
corresponding domains are in parentheses.Arrows
denote name server entries
authoritative path to lookup jeans-pc.dcs.qmw.ac.
uk


21
DNS in typical operation
a.root-servers.net
(root)
Without caching
uk
purdue.edu
ns1.nic.uk
yahoo.com ....
(uk)
ns.purdue.edu
(purdue.edu)
co.uk
ac.uk...
.purdue.edu
ns0.ja.net
(ac.uk)
ic.ac.uk
alpha.qmw.ac.uk
IP alpha.qmw.ac.uk
qmw.ac.uk...
(qmw.ac.uk)
client.ic.ac.uk
dns0-doc.ic.ac.uk
(ic.ac.uk)
dcs.qmw.ac.uk
.qmw.ac.uk
.ic.ac.uk
IPjeans-pc.dcs.qmw.ac.uk
dns0.dcs.qmw.ac.uk
4
(dcs.qmw.ac.uk)

.dcs.qmw.ac.uk
22
DNS server functions and configuration

• Main function is to resolve domain names for
  computers, i.e. to get their IP addresses
• caches the results of previous searches until
  they pass their 'time to live'
• Other functions
• get mail host for a domain
• reverse resolution - get domain name from IP
  address
• Host information - type of hardware and OS
• Well-known services - a list of well-known
  services offered by a host
• Other attributes can be included (optional)

23
DNS resource records
Record type
Meaning
Main contents
A
A computer address
IP number
NS
An authoritative name server
Domain name for server
CNAME
The canonical name for an alias
Domain name for alias
SOA
Marks the start of data for a zone


Parameters governing the zone
WKS
A well-known service description
List of service names and protocols
PTR
Domain name pointer (reverse
Domain name
lookups)
HINFO
Host information
Machine architecture and operating
system
preference, host
gt pairs
MX
Mail exchange
List of lt
TXT
Text string
Arbitrary text

24
DNS issues

• Name tables change infrequently, but when they
  do, caching can result in the delivery of stale
  data.
• Clients are responsible for detecting this and
  recovering
• Its design makes changes to the structure of the
  name space difficult. For example
• merging previously separate domain trees under a
  new root
• moving subtrees to a different part of the
  structure (e.g. if Scotland became a separate
  country, its domains should all be moved to a new
  country-level domain.