Internet Engineering Course

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Internet Engineering Course

• DHCP, DNS

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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
• 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

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DHCP

• Provides configuration parameters specific to the
  DHCP client host to participate on an IP network
• Methods of IP allocation
• Manual
• Only requesting clients with a MAC address listed
  in the table (MAC-IP pairs) get the IP address
  according to the table
• Automatic
• DHCP server permanently assigns to a requesting
  client a free IP-address from a range given by
  the administrator
• Dynamic
• The only method which provides dynamic re-use of
  IP addresses
• The request-and-grant process uses a lease
  concept with a controllable time period.

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DHCP cont.

• DHCP server can provide optional configuration
• e.g. Subnet Mask, 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
• Cable Internet providers
• DSL providers
• broadband ISP networks
• Office networks
• public internet access
• Places where there are mobile nodes that want to
  access the net

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DHCP Implementations

• Microsoft introduced DHCP on their servers 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
• Cisco since Cisco IOS 12.0 in February 1999
• Sun added DHCP support in Solaris 8, July 2001

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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

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BOOTP cont.

• Recently is used for booting a Windows OS in
  diskless standalone media center PCs
• DHCP is a more advanced protocol base on BOOTP
• Far more complex to implement than BOOTP
• Most DHCP servers also offer BOOTP support
• Duration based leases is the fundamental addition
  in DHCP
• Dynamic in DHCP is for this

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DHCP Anatomy

• Uses the same IANA assigned ports as BOOTP
• 67/udp for the server, 68/udp for the client
• DHCP Messages
• Discover
• Client broadcasts on the local physical subnet to
  find servers
• UDP packet (broadcast dest. 255.255.255.255)
• Also request last-known IP address (optional
  parameter)
• Offer
• Server determines the configuration based on the
  clients MAC addr.
• Server specifies the IP address and put optional
  parameters
• Request
• Client selects a configuration out the DHCP Offer
  packet and broadcasts it again
• Acknowledge
• Server acknowledges the request and sends the ack
  to the client

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DHCP Anatomy
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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
  )

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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
• Obsolete by BOOTP and the more modern DHCP
• Try rarp command in Linux (if supported by
  Kernel), and RARP daemon - RARPd

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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 are
  provided by DNS
• Most important, IP address associated with domain
  name
• Mainly UDP
• TCP only when response data size exceeds 512
  bytes or for things like zone transfer

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DNS is Decentralized

• No single point of failure
• Less traffic volume
• Easier maintenance
• Scalable
• Less distant (delay) issues
• Delegation

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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 overhead of finding an answer to the
  query is placed on the name server
• The DNS specs call this kind of resolver a stub
  resolver

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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 zones 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
• Both
• An authoritative ns may be master for some of its
  zones and be slave for some others
• Its imprecise to call an ns, master or slave!

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DNS Applications

• Attach IP addresses to domain names (ease of use)
• Many to many mapping
• Virtual Hosting
• Sender Policy Framework
• Makes it possible for people to assign
  authoritative names, without needing to
  communicate with a central registrar
• Load balancing between hosts

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DNS History

• Idea in ARPAnet
• Originally, each computer retrieved a file called
  HOSTS.TXT 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 computers address changed, all
  computers should update their Hosts file
• DNS invented by Paul Mockapetris in 1983
• First implementation was called JEEVES by himself

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Parts of a Domain Name

• Domain name consists of two or more parts
  separated by dots (eng.ui.ac.ir for example)
• Rightmost label Top-level domain (ir)
• Each label to the left specifies a subdomain of
  the domain above it.
• ac is a subdomain of the ir domain
• ui is a subdomain of the ac.ir domain
• Theoretical limits 127 level, each level 63
  chars, total domain name 255 chars
• A domain name with one or more IP addresses is
  called a hostname (eng.ui.ac.ir, ui.ac.ir but not
  ac.ir)

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A Distributed Hierarchical Database

• Root Servers (13 root servers worldwide)
• TLD Servers (.com, .org, .net, .uk, .ir, )
• Authoritative DNS Servers (organizations DNS
  server)

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Distributed, Hierarchical Database

• Root servers and TLD servers typically do not
  contain hostname to IP mappings they contain
  mappings for locating authoritative servers.

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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

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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
• Glue record

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DNS Queries

• Recursive query example

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DNS Queries
root DNS server

• Iterative query example

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TLD DNS server
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5
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1
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authoritative DNS server dns.cs.umass.edu
requesting host
gaia.cs.umass.edu
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DNS Caching and Updating Records

• Once a name server learns mapping, it caches it
• Itll expire (TTL defined by the authoritative
  server)
• TLD servers typically cached in local name server
• Root name servers not often visited
• Update/Notify Mechanisms
• RFC 2136

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DNS records

• DNS distributed db storing resource records (RR)

• TypeA
• name is hostname
• value is IP address

• TypeCNAME
• name is alias name for some canonical (the
  real) name
• www.ibm.com is really
• servereast.backup2.ibm.com
• value is canonical name

• TypeNS
• name is domain (e.g. foo.com)
• value is IP address of authoritative name server
  for this domain

• TypeMX
• value is name of mail server associated with name

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Legal Users of Domains

• Registrant
• Administrative contact
• Technical contact
• Billing contact
• Name servers
• Try whois in Linux and see these information for
  different hosts

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DNS Software

• BIND (Berkeley Internet Name Daemon) full
  featured, most popular, de facto Internet
  standard
• Djbdns (Daniel J. Bernstein's DNS) composed of
  several small-footprint components
• MaraDNS UDP only
• VitalQIP (Lucent Technologies)
• Adonis DNS Management Appliance (BlueCat Networks
  Inc)
• NSD (Name Server Daemon) small footprint, UDP
  only, authoritative only
• PowerDNS
• Microsoft DNS (in the server editions of Windows
  2000 and Windows 2003)

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References

• Wikipedia, the free encyclopedia
• http//en.wikipedia.org/wiki/Domain_Name_System
• Computer Networking A Top Down Approach
  Featuring the Internet, 3rd edition, Jim Kurose,
  Keith Ross, Addison-Wesley, July 2004
• DNS and BIND, 3rd edition, Cricket Liu, Paul
  Albitz, OReilly, September 1998
• BIND9 Administrator Reference Manual