1. DNS – Domain Name Service WeeSan Lee http://www.cs.ucr.edu/~weesan/cs183/

2. Roadmap Introduction The DNS Namespace Top-level Domains Second-level Domains Domain Names How to Register a Domain Name? How DNS Works? BIND Tools Q&A

3. Introduction A service that maps between hostnames and IP addresses A hierarchical distributed caching database with delegated authority. Uses port 53  UDP for the queries and responses  TCP for the zone transfer

4. Introduction (cont) momo.cs.ucr.edu root name server (.) edu berkeley.edu cs.berkeley.edu Q Q R Q R Q R Q A A Recursive serversNon-recursive servers eon http://www.cs.berkeley.edu/

5. The DNS Namespace A tree structure that starts with the root (.) Each node represents a domain name 2 branches  Forward mapping hostnames → IP addresses  Reverse mapping IP addresses → hostnames

6. Top-level Domains gTLDs (generic TLDs)  com, edu, net, org, gov, mil, int, arpa  aero, biz, coop, info, jobs, museum, name, pro ccTLDs (country code TLDs)  au, ca, br, de, fi, fr, jp, se, hk, cn, tw, my, … Profitable domain names  CreditCards.com - $2.75M  Loans.com – $3M  Business.com - $7.5M

7. Second-level Domain Name Examples  ucr.edu  sony.co.jp Must apply to a registrar for the appropriate TLD Network Solutions, Inc used to monopolize the name registration Now, ~500 registrars

8. Domain Names Valid domain names  Each component: [a-zA-Z0-9\-]{1,63}  Each name < 256 chars Case insensitive  www.cs.ucr.edu == WWW.CS.UCR.EDU www.cs.ucr.eduWWW.CS.UCR.EDU FQDN  Fully Qualified Domain Name  eon.cs.ucr.edu eon – hostname cs.ucr.edu – domain name

9. How To Register A Domain Name? Pick a domain name of interest Dedicate 2 NS servers  RFC1219 stated that each domains should be served by at least 2 servers: a master & a slave One technical contact person One administrative contact person Then, register the name to a registrar of your choice Used to be done via email or fax, now all web-based

10. How DNS Works? Delegation  All name servers read all the 13 root servers from a local configuration file [a-m].root-servers.net $ dig  Those servers in turn knows all the TLDs .edu knows.ucr.edu .com knows.google.com  etc

11. DNS Caching DNS servers cache results they receive from other servers Each result is saved based on its TTL Negative caching  For nonexistent hostname (for 10 mins)  Also for unreachable/unresponsive servers

12. Authoritative vs. Non-authoritative An authoritative answer from a name server (such as reading the data from the disk) is “guaranteed” to be accurate A non-authoritative answer (such as an answer from the cache) may not Primary and secondary servers are authoritative for their own domains

13. Recursive vs. Non-recursive Recursive  Queries on a client behalf until it returns either an answer or an error Non-recursive  Refers the client to another server if it can’t answer a query

14. DNS Database A set of text files, called zone files, maintained by the system admin. on the master NS 2 types of entries  Parser commands, eg. $ORIGIN and $TTL  Resource Records (RR) [name] [tt] [class] type data eon76127 IN A 138.23.169.9 orpheus.cs.ucr.edu.76879 IN A 138.23.169.17 A very important. there!

15. DNS Database (cont) Resource Record Types  SOAStart Of Authority  NSName Server  AIPv4 name-to-address translation  AAAAIPv6 name-to-address translation  PTRAddress-to-name translation  MXMail eXchanger  CNAMECanonical NAME  TXTText  …

16. BIND The Berkeley Internet Name Domain system Current maintainer: Paul Vixie @ ISC BIND 9 Use RTT to pick the best root servers and use them in round-robin fashion named

17. /etc/named.conf options { directory "/var/named"; // query-source address * port 53; forwarders { 138.23.169.10; }; }; zone "." IN { type hint; file "named.ca"; // Read from /var/named/named.ca };

18. /etc/named.conf zone "localhost" IN { type master; file "localhost.zone"; // Read from /var/named/localhost.zone allow-update { none; }; }; zone "0.0.127.in-addr.arpa" IN { type master; file "named.local"; // Read from /var/named/named.local allow-update { none; }; };

19. /etc/named.conf zone "voicense.com" IN { type master; file "voicense.com.zone"; }; zone "0.0.10.in-addr.arpa" IN { type master; file "voicense.com.rev"; }; zone "macrohard.com IN { type slave; file "macrohard.com.zone.bak"; masters { 10.0.0.1; }; };

20. /var/named/voicense.com.zone $TTL 86400 $ORIGIN voicense.com. @IN SOA voicense.com. weesan.voicense.com. ( 20040304 ; serial # 7200 ; refresh (2 hrs) 1800 ; retry (30 mins) 604800 ; expire (1 week) 7200 ) ; mininum (2 hrs) IN NSns.voicense.com. IN MX10 mail.voicense.com. IN MX20 mail.myisp.com. IN A10.0.0.1 mailIN CNAMEvoicense.com. wwwIN CNAMEvoicense.com. nsIN CNAMEvoicense.com. leeIN A10.0.0.31 weeIN A10.0.0.32 Email address: weesan@voicense.com Remember to increment the serial # after each editing

21. /var/named/voicense.com.zone Serial #  An increasing integer number (for sync’ing) Refresh  How often the slave servers should sync. with the master Retry  How long the slave servers should retry before giving up Expire  How long should the slave servers continue to serve the domains in the absent of the master Mininum  TTL for negative answers that are cached

22. /var/named/voicense.com.rev $TTL 86400 @IN SOA voicense.com. weesan.voicense.com. ( 20040304 ; serial # 7200 ; refresh (2 hrs) 1800 ; retry (30 mins) 604800 ; expire (1 week) 7200 ) ; mininum (2 hrs) IN NSns.voicense.com. 1 IN PTRfw.voicense.com. 31IN PTRlee.voicense.com. 32IN PTRwee.voicense.com.

23. How To Load Balance A Web Server? www IN A 10.0.0.1 www IN A 10.0.0.2 www IN A 10.0.0.3

24. How To Load Balance A Web Server? $ host www.google.com  www.google.com is an alias for www.l.google.com.  www.l.google.com has address 74.125.19.104  www.l.google.com has address 74.125.19.103  www.l.google.com has address 74.125.19.147  www.l.google.com has address 74.125.19.99 $ host www.google.com  www.google.com is an alias for www.l.google.com.  www.l.google.com has address 74.125.19.99  www.l.google.com has address 74.125.19.104  www.l.google.com has address 74.125.19.103  www.l.google.com has address 74.125.19.147

25. Zone Transfer DNS servers sync with each other via zone transfer All-at-once and incremental updates A slave server compares the serial number on the master’s and save backup zone files on disk. Uses TCP on port 53

26. Tools dig  $ dig eon.cs.ucr.edu  $ dig eon.cs.ucr.edu ns  $ dig @momo.cs.ucr.edu eon.cs.ucr.edu mx  $ man dig host  $ host eon.cs.ucr.edu  $ host -t ns cs.ucr.edu  $ host -t mx eon.cs.ucr.edu momo.cs.ucr.edu  $ man host

27. Tools (cont) nslookup  $ nslookup eon.cs.ucr.edu  $ nslookup eon.cs.ucr.edu momo.cs.ucr.edu whois  $ whois google.com  $ whois ucr.edu

28. /etc/resolv.conf Resolver $ cat /etc/resolv.conf  search cs.ucr.edu weesan.com  nameserver 138.23.169.10  nameserver 138.23.178.2

29. /etc/nsswitch.conf Used by C library  gethostbyname() $ cat /etc/nsswitch.conf  hosts: file nis dns

30. Reference LAH  Ch 15: DNS – The Domain Name System