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Internet Engineering

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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Internet Engineering

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  1. Internet Engineering DHCP, DNS

  2. 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

  3. 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.

  4. 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

  5. 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

  6. 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

  7. 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 …)

  8. 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

  9. 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

  10. 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

  11. 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

  12. DNS is Decentralized • No single point of failure • Less traffic volume • Easier maintenance • Scalable • Less distant (delay) issues • Delegation

  13. 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

  14. 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!

  15. 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

  16. 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

  17. 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)

  18. A Distributed Hierarchical Database • Root Servers (13 root servers worldwide) • TLD Servers (.com, .org, .net, .uk, .ir, …) • Authoritative DNS Servers (organization’s DNS server)

  19. 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

  20. 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

  21. DNS Queries • Recursive query example

  22. DNS Queries • DNS in the real world

  23. 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

  24. 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, …

  25. 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

  26. Legal Users of Domains • Registrant • Administrative contact • Technical contact • Billing contact • Name servers • Try whois in Linux and see these information for different hosts

  27. 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

  28. 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

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