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1 Kyung Hee University Chapter 19 DNS (Domain Name System)

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1 1 Kyung Hee University Chapter 19 DNS (Domain Name System)

2 2 Kyung Hee University 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 Server DOWN !!!): year 2003

3 3 Kyung Hee University Resolver and Name Server  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 1. Host name(khu.ac.kr)2. Host name(khu.ac.kr) 3. IP(163.180.96.52) 4. IP(163.180.96.52) Client ResolverDNS Server

4 4 Kyung Hee University 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 5 Kyung Hee University 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 6 Kyung Hee University Distributed, Hierarchical Database Root DNS Servers com DNS servers org DNS serversedu DNS servers poly.edu DNS servers umass.edu DNS servers yahoo.com DNS servers amazon.com DNS servers pbs.org DNS servers  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 7 Kyung Hee University Resolution Process  Let’s look at the resolution process step by step Client Local DNS What is the IP address of khu.ac.kr?

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

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

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

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

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

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

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

15 15 Kyung Hee University Resolution Process (Caching)  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 Client

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

17 17 Kyung Hee University Resolution Process (Caching)  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 Client Local DNS m.root-server f.kr-server g.ac.kr-server What is the IP address of networking.khu.ac.kr?

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

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

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

21 21 Kyung Hee University DNS: Root name servers  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 13 root name servers worldwide b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA e NASA, Mt View, CA f Internet Software C. Palo Alto, CA (and 17 other locations) i Autonomica, Stockholm (plus 3 other locations) k RIPE, London (also Amsterdam, Frankfurt) m WIDE Tokyo 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)

22 22 Kyung Hee University 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

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