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2. Internet hosts:  IP address (32 bit) - used for addressing datagrams  “name”, e.g., ww.yahoo.com - used by humans DNS: provides translation between host name and IP address  distributed database implemented in hierarchy of many name servers  Distributed for scalability & reliability 2

3.  Hostname to IP address translation  Host aliasing  Canonical and alias names  Mail server aliasing  Load distribution  Replicated Web servers: set of IP addresses for one canonical name 3

4.  Host at cis.poly.edu wants IP address for gaia.cs.umass.edu  Infrastructure:  Client resolver  Local DNS server  Authoritative DNS Server  Root DNS Server  Top-Level Domain DNS Server 4 requesting host cis.poly.edu gaia.cs.umass.edu root DNS server local DNS server dns.poly.edu 1 2 3 4 5 6 authoritative DNS server dns.cs.umass.edu 7 8 TLD DNS server

5.  Root servers and TLD servers typically do not contain hostname to IP mappings; they contain mappings for locating authoritative servers. 5 Root DNS Servers com DNS servers ca DNS serversedu DNS servers poly.edu DNS servers umass.edu DNS servers yahoo.com DNS servers amazon.com DNS servers ucalgary.ca DNS servers TLD Servers usask.ca DNS servers

6.  contacted by local name server that can not resolve name  root name server:  contacts authoritative name server if name mapping not known  gets mapping  returns mapping to local name server 6

7.  Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp.  Network solutions maintains servers for com TLD  Education for edu TLD  Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web and mail).  Can be maintained by organization or service provider 7

8.  Each ISP (residential ISP, company, university) has one.  Also called “default name server”  When a host makes a DNS query, query is sent to its local DNS server  Acts as a proxy, forwards query into hierarchy.  Reduces lookup latency for commonly searched hostnames 8

9. 9 requesting host cis.poly.edu gaia.cs.umass.edu root DNS server local DNS server dns.poly.edu 1 2 4 5 6 authoritative DNS server dns.cs.umass.edu 7 8 TLD DNS server 3 recursive query: rputs burden of name resolution on contacted name server rheavy load? iterated query: rcontacted server replies with name of server to contact r“I don’t know this name, but ask this server”

10. 10 “root” zone TLDs & ccTLDs 2lds 3lds 4lds

11. 11 DNS vsFile System

12. 12 Naming a Domain Naming a Directory yahoo.com.au. C:\windows\system32\drivers\ Start Here A “.” is used as separator A “\” is used as separator

13. Accessing a web page You type http://www.google.com into your web browser and hit enter. What happens now? 13

14. Step 1: Your PC sends a resolution request to its configured DNS Server, typically at your ISP. 14 Tell me the Address of “www.google.com”

15. 15 Step 2: Your ISPs recursive name server starts by asking one of the root servers predefined in its “hints” file. Tell me the Address of “www.google.com” I don’t know the address but I know who’s authoritative for the ”com” domain ask them

16. 16 Step 3: Your ISPs recursive name server then asks one of the “com” name servers as directed. Tell me the Address of “www.google.com” I don’t know the address but I know who’s authoritative for the ”google.com” domain ask them

17. 17 Step 4: Your ISPs recursive name server then asks one of the “google.com” name servers as directed. Tell me the Address of “www.google.com” The Address of www.google.com is 216.239.53.99

18. Step 5: ISP DNS server then send the answer back to your PC. The DNS server will “remember” the answer for a period of time. 18 The Address of www.google.com is 216.239.53.99

19. Step 6: Your PC can then make the actual HTTP request to the web server. 19 Here it is! Send me the www.google.com.au web page

20. Summary 20 The actual web request DNS