Internetworking II

1. Internetworking II Organizational Communications and Technologies Prithvi Rao H. John Heinz III School of Public Policy and Management Carnegie Mellon University

2. Objectives • Understand how DNS works • Present a DNS scenario

3. Naming Hosts • Nameserver is vehicle for mapping a name to a network • telnet akasha.tic.com vs telnet 192.135.128.129 • Network object is passed to transport protocol interface • Naming evolved with other protocols

4. History of Naming • Predecessor of Internet was ARPANET • Most important resource was IP address • Used naming authority to assign IP addresses • Most hosts had single network interfaces; hostname were synonymous to interface • Central registry to maintained names and corresponding IP addresses • Administrator received a host and IP address for each new machine to be added to network • IP address known when network was established • Name collision avoided by searching a host file

5. History of Naming • Host files were copied to each machine • Unix systems consisted of /etc/hosts file • Operating systems supported lookup using library functions • gethostbyname() and gethostbyaddr() • Worked well for small number of hosts (100s) • Other operating systems used similar mechanisms but basically the same • Worked well because relatively few requests and table size relatively small

6. History of Naming • Exponential growth of the internet made static host table impractical • Load on servers hosting registry introduced delays in access • Names had to be unique to avoid name clashes • Solution to support growing internet was Domain Name System (DNS)

7. Domain Name System • Internet’s official naming system • Distributed naming system • Database is scattered across many hosts • Maintained by many organizations (each has a small part) • Defines resource named and protocols used to communicate between nameservers that maintain the database

8. Domain Name System • Delegation • Naming is delegated leaving central registry to register only naming authorities • Every host is not named by central authority • Dynamic Distribution • Name lookup is dynamically distributed • Site administrators did not have to copy host files • Redundancy • Lookup algorithms were redundant; no single server • Reliability was improved

9. Domain Name System • Extensibility • Not necessarily restricted to IP addresses

10. Delegation • Defines a name space that is a tree structure • Each node owned by single authority • Child nodes can be created • Each child node must have a unique name • Domain is any node and its descendant nodes • Domain name uniquely indentifies single node within domain • Node names are written with separated period

11. Delegation root com org nz ……. edu cmu ac kiwilabs co tic akasha andrew unix5

12. Delegation • Children of root are “top-level domains” • Domain name that traverses from node to root is called a Fully Qualified Domain Name (FQDN) • Always ends with a period cs.edu(.) • Practically the period is dropped cs.edu • Some applications (mail) do not permit the appending of a period • Domain name traversing part of node is called a Relative Domain Name

13. Dynamic Distribution • Descendants of a domain called subdomains • kiwilabs.com has authority for all names under kiwilabs.com • Grant of authority is given when new subdomain is registered • Naming authority can assign subdomain names arbitrarily • Child node must be unique • ux4.sp.cs.cmu.edu? • Hierarchy is broader than deeper

14. Extensibility • Name gives resource a convenient reference; name is mapped to resource • Can map DNS name to other resources • DNS uses a typed resource record to identify resource being named<domain-name ttl IN resource_type resource_value) • domain_name is the FQDN for the resource that is key to identifying resource

15. Extensibility • ttl is the time to live value • Time that the resource record can be cached before being discarded • Field is decremented every second and resource is discarded when ttl reaches zero • IN identifies resource as belonging to TCP/IP or INternet protocol

16. Extensibility • resource_type is a unique identifier for type of resource named • During lookup resource_type is used to distinguish between resource records mqpped to the domain name • resource_value is value of resource. Can be single value (IP address) or record with multiple values • DNS has standard set of resource record types

17. Resource Types • IP addresses domain_name A ip_addresses Example ticmac.tic.com A 192.135.128.131 and A is the record type corresponding to IP addresses maps domain name ticmac.tic.com to 192.135.128.131

18. IP Address • Multi-homed host or router has an A record for each network interfacerouter.tic.com A192.135.128.1router.tic.com A193.1.1.1This illustrates mapping of name router.tic.com totwo IP addresses.Machine has two interface cards

19. Host Information • HINFO record indentifies and operating system of host with given domain namedomain_name HINFO hardware osExampleakasha.tic.com HINFO Sun SunOs

20. Alias • Alias is CNAME record associating domain name with another domain namedomain_name CNAME canonical_nameExamplemac.tic.com CNAME ticmac.tic.com says that namemac.tic.com is alias for ticmac.tic.com

21. DNS Operational Architecture Query or reply Server To/from another server Query or reply reply query Resolver library function return function call Application

22. DNS Query Format header question answer authority additional

23. DNS Operational Architecture • question contains the target domain name and the type and classof query • Can match resource record type or be wildcarded to ask for any resource • answer is completed by nameserver that replies to query • authority can name other authority that can answer query

24. DNS Operational Architecture • additional completed by nameserver and assists client with needed information

25. DNS Operational Steps • Application sends DNS query to nameserver and waits for response from resolver • Resolver generates query and and transmits it to nameserver and handles response and retransmits a query request Examples of API for DNSgethostbyname() and gethostbyservice()

26. DNS Zones root com org nz ……. edu cmu ac kiwilabs co tic akasha andrew unix5

27. DNS Zones • Each DNS zone has its own zone database • Primary name-server exists for each zone and maintains an up-to-date copy of zone database • Copies maintained in secondary nameservers (reliability)

28. DNS Scenario • Query from machine able.widget.com is sent to nameserveron ns.widget.com for the IP address for the domain namebaker.austin.tic.com: step 1 • ns.widget.com has no cached resource records forbaker.austin.tic.com so the nameserver tries to find anNS record for the parent domain austin.tic.com • Finding no cached records for that domain it attempts tofind an NS record for the tic.com domain. It looks for thecom domain without success. It forwards original query toa root nameserver: step 2

29. DNS Scenario 4) Root nameserver repeats step 3 and finds an NS record for the com server and passes the query to that server 5) Nameserver for com domain once again repeats above algorithm and finds NS record and associated A record for the domain tic.com and returns information to nameserver on ns.widget.com: step 4 6) Information is cached on ns.widget.com (NS and A records) and sends original query to server for tic.com. Second server for that domain is contacted if timeout occurs: step 5

30. DNS Scenario 7) Server for tic.com receiving query forwards it to server for austin.tic.com domain: step 6 8) Destination server has answer desired by original node (baker.austin.tic.com) and returns answer to tic.com (7) which then sends answer to ns.widget.com (8) which in turn returns answer to able.widget.com (9) and this machine caches answer for later use

31. Query Example 1 ns.widget.com rootserver 2 able.widget.com 9 4 3 8 5 6 ns.austin.tic.com akasha.tic.com comserver 7

32. Summary • Presented a brief history of domains and host naming • Examined the use of resource records • Presented DNS query example