Internet Engineering Course

1. Internet Engineering Course 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 • 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 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.

4. 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

5. 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

6. 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

7. 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

8. 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

9. DHCP Anatomy

10. 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 …)

11. 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

12. 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

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

14. 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

15. 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 • Both • 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!

16. 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

17. 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 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

18. Parts of a Domain Name • Domain name consists of two or more parts separated by dots (eng.ui.ac.irfor example) • Rightmost label: Top-level domain (ir) • Each label to the left specifies a subdomain of the domain above it. • acis a subdomain of the irdomain • uiis a subdomain of the ac.irdomain • 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)

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

20. Root DNS Servers TLD Servers ir DNS servers edu DNS servers com DNS servers co.ir DNS servers poly.edu DNS servers ac.ir DNS servers amazon.com DNS servers umass.edu DNS servers yahoo.com DNS servers Distributed, Hierarchical Database • Root servers and TLD servers typically do not contain hostname to IP mappings; they contain mappings for locating authoritative servers.

21. 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

22. 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

23. root DNS server 2 3 6 7 TLD DNS server 4 local DNS server Ns.ui.ac.ir 5 1 8 authoritative DNS server dns.cs.umass.edu requesting host gaia.cs.umass.edu DNS Queries • Recursive query example

24. local DNS server ns.ui.ac.ir DNS Queries root DNS server • Iterative query example 2 3 TLD DNS server 4 5 6 7 1 8 authoritative DNS server dns.cs.umass.edu requesting host gaia.cs.umass.edu

25. 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

26. DNS: distributed db storing resource records (RR) Type=NS name is domain (e.g. foo.com) value is IP address of authoritative name server for this domain RR format: (name, value, type, ttl) DNS records • Type=A • name is hostname • value is IP address • Type=CNAME • name is alias name for some “canonical” (the real) name • www.ibm.com is really • servereast.backup2.ibm.com • value is canonical name • Type=MX • value is name of mail server associated with name

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

28. 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)

29. 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