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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

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introduction
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
slide3
DHCP
  • Provides configuration parameters specific to the DHCP client host requesting, information required by the client host to participate on an IP network
  • Method 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.
dhcp cont
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
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
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 client’s 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
dhcp anatomy cont
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
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
bootp cont
BOOTP cont.
  • Recently 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
slide10
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
slide11
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
dns is decentralized
DNS is Decentralized
  • No single point of failure
  • Less traffic volume
  • Easier maintenance
  • Scalable
  • Less distant (delay) issues
  • Delegation
resolvers
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
    • The DNS specs call this kind of resolver a stub resolver
types of dns servers
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 applications
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
dns history
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
  • DNS invented by Paul Mockapetris in 1983
    • First implementation was called JEEVES by himself
parts of a domain name
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
    • A domain name with one or more IP addresses is called a hostname (sharif.edu, ce.sharif.edu but not edu)
a distributed hierarchical database
A Distributed Hierarchical Database
  • Root Servers (13 root servers worldwide)
  • TLD Servers (.com, .org, .net, .uk, .ir, …)
  • Authoritative DNS Servers (organization’s DNS server)
local dns server
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
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
dns queries1
DNS Queries
  • Recursive query example
dns queries2
DNS Queries
  • DNS in the real world
dns caching and updating records
DNS Caching and Updating Records
  • Once a name server learns mapping, it caches it
    • It’ll 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
  • TTL is specified in the Start Of Authority (SOA) record
    • Serial – Incremented when the zone file modified, others know when the zone has been changed and should be reloaded
    • Refresh – Number of seconds between update requests
    • Retry – Number of seconds between retries (if a request failed)
    • Expire – Number of seconds before considering the data stale
    • Minimum – Used for minimum TTL, used for negative caching
dns records
DNS Records
  • Resource Records
    • Tuples which are stored in the distributed database
    • (name, value, type, ttl)
  • Types
    • There are many types, most famous ones (IPv4 mostly)
      • A: Maps a hostname to an IPv4 address
      • NS: Maps a domain name to a list of authoritative DNS servers
      • CNAME: Makes one domain name an alias of another
      • MX: Maps a domain name to a list of mail exchange servers
      • PTR: Maps an IPv4 address to canonical name for that host
      • SOA: Specifies the authoritative DNS server
        • Info like email of the domain administrator, serial number, …
advanced features of dns servers
Advanced Features of DNS Servers
  • Address Match Lists and Address Control Lists
    • i.e. defining a network and referring to it with the name we defined. e.g.
      • acl “ce” { { 81.31.164.0/24; 81.13.179.0/24; };};
  • DNS Notify
    • Notify the listed servers on zone change
  • DNS Dynamic Update
    • This permits authorized updaters to add and delete resource records from a zone for which the server is authoritative
    • Used in DNS, DHCP servers integration
legal users of domains
Legal Users of Domains
  • Registrant
  • Administrative contact
  • Technical contact
  • Billing contact
  • Name servers
  • Try whois in Linux and see these information for different hosts
dns bind
DNS - BIND
  • BIND (Berkeley Internet Name Domain) written for Berkeley’s 4.3BSD UNIX OS by Kevin Dunlap
    • It is not maintained by Internet Software Consortium
  • The most popular implementation of DNS today
  • Ported to many flavors of UNIX
    • Shipped as a standard part of most vendors’ UNIX offerings
    • Has even been ported to Microsoft Windows
references
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, O’Reilly, September 1998
  • BIND9 Administrator Reference Manual