DNS – Domain Name Service

1. DNS – Domain Name Service WeeSan Lee <weesan@cs.ucr.edu> 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) Recursive servers Non-recursive servers root name server (.) Q R http://www.cs.berkeley.edu/ Q Q momo.cs.ucr.edu edu A R A Q Q eon R berkeley.edu 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 • 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 • eon 76127 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 • SOA Start Of Authority • NS Name Server • A IPv4 name-to-address translation • AAAA IPv6 name-to-address translation • PTR Address-to-name translation • MX Mail eXchanger • CNAME Canonical NAME • TXT Text • …

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 Email address: weesan@voicense.com Remember to increment the serial # after each editing • $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 NS ns.voicense.com. • IN MX 10 mail.voicense.com. • IN MX 20 mail.myisp.com. • IN A 10.0.0.1 • mail IN CNAME voicense.com. • www IN CNAME voicense.com. • ns IN CNAME voicense.com. • lee IN A 10.0.0.31 • wee IN A 10.0.0.32

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 NS ns.voicense.com. • 1 IN PTR fw.voicense.com. • 31 IN PTR lee.voicense.com. • 32 IN PTR wee.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