DHCP - Internet Engineering - Lecture Slides, Slides of Internet and Information Access

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Internet Engineering

DHCP, DNS

Introduction

• Client administration:

• IP address management:
  • They need to ease the process of joining the network and

they do not want users to do any special configuration

(DHCP)

• They want to network boot their workstations
  • i.e. Diskless workstations or remote OS installation (acquiring the network setting during boot process)
• Solution: Deploy a DHCP server
• Machine names management:
• They need to be able to name machines and access them by

names instead of IP addresses

• Solution: Deploy A DNS server

• So we discuss on DHCP and DNS in this session

DHCP cont.

• DHCP server can provide optional configuration
  • e.g. Subnet Mask, Router, Name Server, …
  • RFC 2132 defines DHCP options Usage
• DHCP relay agent (mostly in network routers/high-end switches)
  • Relays DHCP Discover broadcasts from a LAN without DHCP to a network which has one
• Usage
  • US
    • Cable Internet providers use DHCP
    • DSL providers prefer PPPoE
  • UK
    • Many broadband ISP networks use DHCP
    • XDSL providers use infinite lease  Semi-static IPs
  • Office networks, public internet access
  • Places where there are mobile nodes that want to access the net

DHCP Implementations

• Microsoft introduced DHCP on their NT server with Windows NT 3.5 in late

1994

• DHCP did not originate from Microsoft
• Internet Software Consortium published DHCP for Unix variants
• Version 1.0.0 released on December 6, 1997
• Version 2.0 on June, 1999 – A more RFC-compliant one
• Novell included a DHCP server in NetWare OS since v. 5, 1999
• It integrates with Novell eDirectory
• Weird solutions introducing a variety of multiplatform DHCP

implementations since 1997

• Cisco since Cisco IOS 12.0 in February 1999
• Sun added DHCP support in Solaris 8, July 2001

DHCP Anatomy cont.

– Inform

• Client requests more information than the server sent with the DHCPACK, or to repeat data for a particular application (e.g. to obtain web proxy settings by a browser)

– Release

• Client requests the server to release the DHCP and the client unconfigures its IP address
• Sending this message is not mandatory (unplug or …)

BOOTP

• BOOTstrap Protocol (RFC 951)

• UDP

• Used to obtain IP address automatically

• Usually in booting process of computers or OSs
  • Diskless workstations

• Historically used for UNIX-like diskless workstations

• Also obtains the locations of the boot image

• Also can be used for installing a pre-configured OS

• Protocol became embedded in the BIOS of some NICs

• Allowing direct network booting without need for a floppy

RARP

• ARP

• Address Resolution Protocol
• Resolve a hardware address from a given IP address
• Try arp command in both Windows and Linux
• RARP
• Reverse Address Resolution Protocol (RFC 903)
• Complement of ARP
• Resolve an IP address from a given hardware address
• Needs manual configuration on a central server
• Not scalable
• Obsoleted by BOOTP and the more modern DHCP
• Try rarp command in Linux (if supported by Kernel), and RARP daemon
• RARPd

DNS

• Domain Name System (RFC 1034, 1035)

• RFC 1034 and 1035 made RFC 882, 883 obsolete

• A system that stores info associated with domain

names in a distributed database on networks (such as

Internet)

• Many types of information for the domain provided by

DNS

• Most important, IP address associated with domain name
• Mail eXchange servers accepting e-mail for each domain

• Mainly UDP

• TCP only when response data size exceeds 512 bytes or for things like zone transfer

Resolvers

• Clients that access name servers

– Querying a name server

– Interpreting responses

– Returning the information to the programs that

requested it

• In BIND, the resolver is just a set of library

routines that is linked into programs

– Not even a separate process

– Most of the burden of finding an answer to the

query is placed on the name server

Types of DNS Servers

• Primary master
  • Reads the data for the zone from a file on its host
• Secondary master (Slave)
  • Gets the zone data from another ns that is authoritative for the zone (master server)
  • Often, master server is the zone’s primary master
    • Not always the case
    • Secondary master may get the info from another secondary server
  • Zone transfer
    • Contacting master ns and if necessary pulling the zone data
  • Redundancy
• An authoritative ns may be master for some of its zones and be slave for some others - It’s imprecise to call an ns, master or slave!

DNS History

• Idea in ARPAnet

• Originally, each computer retrieved a file

called HOSTS.TXT from SRI which contained

the mappings

– Hosts file exists today (Looked up before querying

DNS)

• /etc/hosts, C:\WINDOWS\system32\drivers\etc\hosts

– Limitations

• Not scalable
• Each time a given computer’s address changed, all computers should update their Hosts file Docsity.com

Parts of a Domain Name

• Domain name consists of two or more parts

separated by dots (here ce.sharif.edu for

example)

– Rightmost label: Top-level domain (edu)

– Each label to the left specifies a subdomain of the

domain above it.

• Relative dependence, not absolute dependence
• sharif is a subdomain of the edu domain
• ce is a subdomain of the sharif.edu domain
• Theoretical limits: 127 level, each level 63 chars, total domain name 255 chars Docsity.com

Local DNS Server

• Does not belong to hierarchy

• Also called default name server

• Acts as a proxy (forwarder), forwards query

into hierarchy

– Caches the results if of interest

DNS Queries

• Recursive

– Contacted name server should recurs and find the

mapping for the requesting host

– Heavy load on the servers

• Iterative

– Contacted server replies with the name of the

server to contact

– An ns provides the name of the next ns

• Bootstrapping problem (another query is required and …)
• So the IP of the next ns is provided Docsity.com