Chapter 17 DNS (Domain Name System)

1. Chapter 17DNS (Domain Name System)

2. Introduction • People prefer to use easy to remember names instead of IP addresses • Domain names are alphanumeric names for IP addresses • e.g., kr.yahoo.com, khu.ac.kr, ietf.org • The domain name system (DNS) is an Internet-wide distributed database that translates between domain names and IP addresses • How important is DNS? • Imagine what happens when the local DNS server is down. • KT 1.25 Big Accident(DNS ServerDOWN !!!): year 2003

3. Resolver and Name Server Resolver DNS Server Client 1. Host name(khu.ac.kr) 2. Host name(khu.ac.kr) 4. IP(163.180.96.52) 3. IP(163.180.96.52) An application program on a host accesses the domain system through a DNS client, called the Resolver Resolver contacts DNS Server, called name server DNS server returns IP address to resolver which passes the IP address to application

4. Design principle of DNS The naming system on which DNS is based is a hierarchical and logical tree structure called the domain namespace. An organization obtains authority for parts of the name space, and can add additional layers of the hierarchy Names of hosts can be assigned without regard of location on a link layer network, IP network or autonomous system

5. Design principle of DNS (Cont’d) • In practice, allocation of the domain names generally follows the allocation of IP address, e.g., • All hosts with network prefix 163.180/16 have domain name suffix khu.ac.kr

6. Root DNS Servers org DNS servers edu DNS servers com DNS servers poly.edu DNS servers umass.edu DNS servers pbs.org DNS servers yahoo.com DNS servers amazon.com DNS servers Distributed, Hierarchical Database • Client wants IP for www.amazon.com; At first, client queries a root server to find com DNS server • Then, client queries com DNS server to get amazon.com DNS server • Thirdly, Client queries amazon.com DNS server to get IP address for www.amazon.com

7. Resolution Process Local DNS What is the IP address of khu.ac.kr? Client Let’s look at the resolution process step by step

8. Resolution Process (cont’d) m.root-server Local DNS What is the IP address of khu.ac.kr? Client The Local DNS asksa root name server, m, for khu.ac.kr’s address

9. Resolution Process (cont’d) m.root-server Local DNS Here’s a list of the kr name servers. Ask one of them. Client The root server m refers local DNS to the kr name servers This type of response is called a “referral”

10. Resolution Process (cont’d) m.root-server Local DNS f.kr-server What is the IP address of khu.ac.kr? Client The local DNS asks a kr name server, f, for khu.ac.kr

11. Resolution Process (cont’d) m.root-server Local DNS f.kr-server Here’s a list of the ac.kr name servers. Ask one of them. Client The kr name sever f refers Local DNS to the ac.kr name servers

12. Resolution Process (cont’d) m.root-server Local DNS What is the IP address of khu.ac.kr? f.kr-server g.ac.kr-server Client The local DNS asks an ac.kr name sever, g.ac-kr for khu.ac.kr’s address

13. Resolution Process (cont’d) m.root-server Local DNS Here’s the IP address for khu. ac.kr (163.180.96.52) f.kr-server g.ac.kr-server Client The g.ac.kr-server responds with khu.ac.kr’s address

14. Resolution Process (cont’d) m.root-server Local DNS Here’s the IP address for khu. ac.kr (163.180.96.52) f.kr-server g.ac.kr-server Client The local DNS responds to client with khu.ac.kr’s address

15. Resolution Process (Caching) Client • After the previous query, the Local DNS now knows: • The names and IP addresses of the kr name servers • The names and IP addresses of the ac.kr name servers • The IP address of khu.ac.kr • Let’s look at the resolution process again

16. Resolution Process (Caching) m.root-server Local DNS What is the IP address of networking.khu.ac.kr? f.kr-server g.ac.kr-server h.khu.ac.kr-server Client The client asks its Local DNS for networking.khu.ac.kr’s address

17. Resolution Process (Caching) m.root-server Local DNS f.kr-server What is the IP address of networking.khu.ac.kr? g.ac.kr-server Client Local DNS has cached an NS record indicating g.ac.kr-server is an ac.kr name server, so it asks it for networking.khu.ac.kr’s address

18. Resolution Process (Caching) m.root-server Local DNS Here’s a list of the khu.ac.kr name servers. Ask one of them. f.kr-server g.ac.kr-server h.khu.ac.kr-server Client g.ackr-server responds with networking.khu.ac.kr’s address

19. Resolution Process (Caching) m.root-server Local DNS f.kr-server What is the IP address of networking.khu.ac.kr? g.ac.kr-server h.khu.ac.kr-server Client g.ackr-server responds with networking.khu.ac.kr’s address

20. Resolution Process (Caching) m.root-server Local DNS Here’s the IP address for networking.khu.ac.kr (163.180.116.228) f.kr-server g.ackr-server h.khu.ac.kr-server Client Finally, the Local DNS respondsto client with networking.khu.ac.kr’s address

21. DNS: Root name servers a Verisign, Dulles, VA c Cogent, Herndon, VA (also Los Angeles) d U Maryland, College Park, MD g US, DoD, Vienna, VA h ARL, Aberdeen, MD j Verisign, (11 locations) k RIPE, London (also Amsterdam, Frankfurt) i Autonomica, Stockholm (plus 3 other locations) m WIDE Tokyo e NASA, Mt View, CA f Internet Software C. Palo Alto, CA (and 17 other locations) 13 root name servers worldwide b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA • contacted by local name server that can not resolve name • root name server: • contacts authoritative name server if name mapping is not known • gets mapping • returns mapping to local name server

22. Summary • The domain name system (DNS) is an Internet-wide distributed database that translates between domain names and IP addresses • An application program on a host accesses the domain system through a DNS client, called the Resolver • Resolver contacts DNS Server, called name server • DNS server returns IP address to resolver which passes the IP address to application • The naming system on which DNS is based is a hierarchical and logical tree structure called the domain namespace. • There are 13 root name servers worldwide