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DNS – Domain Name Service

WeeSan Lee <weesan@cs.ucr.edu> http://www.cs.ucr.edu/~weesan/cs183/. DNS – Domain Name Service. 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. Introduction.

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DNS – Domain Name Service

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

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

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

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