The Domain Name System

1. The Domain Name System

2. Hostnames • IP Addresses are great for computers • IP address includes information used for routing. • IP addresses are tough for humans to remember. • IP addresses are impossible to guess. • ever guessed at the name of a WWW site? CeylonLinux DNS concepts using BIND

3. The Domain Name System • The domain name system is usually used to translate a host name into an IP address . • Domain names comprise a hierarchy so that names are unique, yet easy to remember. CeylonLinux DNS concepts using BIND

4. DNS Hierarchy edu com org jp rpi albany CeylonLinux DNS concepts using BIND

5. Host name structure • Each host name is made up of a sequence of labels separated by periods. • Each label can be up to 63 characters • The total name can be at most 255 characters. • Examples: • whitehouse.gov • barney.the.purple.dinosaur.com • monica.cs.rpi.edu CeylonLinux DNS concepts using BIND

6. Domain Name • The domain name for a host is the sequence of labels that lead from the host (leaf node in the naming tree) to the top of the worldwide naming tree. • A domain is a subtree of the worldwide naming tree. CeylonLinux DNS concepts using BIND

7. Top level domains • edu, gov, com, net, org, mil, … • Countries each have a top level domain (2 letter domain name). • New top level domains include: .aero .biz .coop .info .name .pro CeylonLinux DNS concepts using BIND

8. DNS Organization • Distributed Database • The organization that owns a domain name is responsible for running a DNS server that can provide the mapping between hostnames within the domain to IP addresses. • So - some machine run by RPI is responsible for everything within the rpi.edu domain. CeylonLinux DNS concepts using BIND

9. DNS Distributed Database • There is one primary server for a domain, and typically a number of secondary servers containing replicated databases. rpi.edu DNS server rpi.edu DNS DB rpi.edu DNS DB rpi.edu DNS DB rpi.edu DNS DB Authoritative Replicas CeylonLinux DNS concepts using BIND

10. DNS Clients • A DNS client is called a resolver. • A call to gethostbyname()is handled by a resolver (typically part of the client). • Most Unix workstations have the file /etc/resolv.conf that contains the local domain and the addresses of DNS servers for that domain. CeylonLinux DNS concepts using BIND

11. /etc/resolv.conf domain rpi.edu 128.113.1.5 128.113.1.3 CeylonLinux DNS concepts using BIND

12. nslookup • nslookup is an interactive resolver that allows the user to communicate directly with a DNS server. • nslookup is usually available on Unix workstations. CeylonLinux DNS concepts using BIND

13. DNS Servers • Servers handle requests for their domain directly. • Servers handle requests for other domains by contacting remote DNS server(s). • Servers cache external mappings. CeylonLinux DNS concepts using BIND

14. Server - Server Communication • If a server is asked to provide the mapping for a host outside it’s domain (and the mapping is not in the server cache): • The server finds a nameserver for the target domain. • The server asks the nameserver to provide the host name to IP translation. • To find the right nameserver, use DNS! CeylonLinux DNS concepts using BIND

15. DNS Data • DNS databases contain more than just hostname-to-address records: • Name server records NS • Hostname aliases CNAME • Mail Exchangers MX • Host Information HINFO CeylonLinux DNS concepts using BIND

16. edu com org jp albany The Root DNS Server • The root server needs to know the address of 1st (and many 2nd) level domain nameservers. rpi CeylonLinux DNS concepts using BIND

17. Server Operation • If a server has no clue about where to find the address for a hostname, ask the root server. • The root server will tell you what nameserver to contact. • A request may get forwarded a few times. CeylonLinux DNS concepts using BIND

18. DNS Message Format HEADER QUERIES Response RESOURCE RECORDS Response AUTHORITY RECORDS Response ADDITIONAL INFORMATION CeylonLinux DNS concepts using BIND

19. Question Format • Name: domain name (or IP address) • Query type (A, NS, MX, …) CeylonLinux DNS concepts using BIND

20. Response Resource Record • Domain Name • Response type • Class (IP) • Time to live (in seconds) • Length of resource data • Resource data CeylonLinux DNS concepts using BIND

21. UDP & TCP • Both UDP and TCP are used: • TCP for transfers of entire database to secondary servers (replication). • UDP for lookups • If more than 512 bytes in response - requestor resubmits request using TCP. CeylonLinux DNS concepts using BIND

22. Name to Address Conversion • There is a library of functions that act as DNS client (resolver). CeylonLinux DNS concepts using BIND

23. DNS Data Flow CeylonLinux DNS concepts using BIND

24. Example suranga.com This is named.conf in /etc/ folder According to the following configuration I have created a zone called suranga.com and in that zone im3 is a host. Therefore the full name of that host would be im3.suranga.com. like that you can have any amount of PCs in your domain. This will definitely works if you type following according to your network. CeylonLinux DNS concepts using BIND

25. Example Network CeylonLinux DNS concepts using BIND

26. This is named.conf in /etc/ folder • options { • directory "/var/named"; • // query-source address * port 53; • }; • zone "." IN { • type hint; • file "named.ca"; • }; • zone "localhost" IN { • type master; • file "localhost.zone"; • allow-update { none; }; • }; • zone "0.0.127.in-addr.arpa" IN { • type master; • file "named.local"; • allow-update { none; }; • }; • zone "suranga.com" IN { • type master; • file "suranga.com.zone"; • allow-update { none; }; • }; • zone "200.168.192.in-addr.arpa" IN { • type master; • allow-update { none; }; • }; CeylonLinux DNS concepts using BIND

27. BIND Files • Following files should be created in /var/named/ folder we do not need to customize the default file named.ca in the /var/named/ folder. • 1. 0.0.127.in-addr.arpa.zone • 2. 200.168.192.in-addr.arpa.zone • 3. suranga.com.zone • 4. localhost.zone • 5. named.local CeylonLinux DNS concepts using BIND

28. The contents of 0.0.127.in-addr.arpa.zone as follows • $TTL 86400 • @ IN SOA localhost. root.localhost ( • 1 ; serial • 28800 ; refresh • 7200 ; retry • 604800 ; expire • 86400 ; ttk • ) • @ IN NS localhost. • 1 IN PTR localhost. CeylonLinux DNS concepts using BIND

29. 200.168.192.in-addr.arpa.zone file • $TTL 86400 • @ IN SOA 192.168.200.8. root.localhost ( • 1 ; serial • 28800 ; refresh • 7200 ; retry • 604800 ; expire • 86400 ; ttk • ) • @ IN NS localhost. • 1 IN PTR localhost. • 2 IN PTR im3. CeylonLinux DNS concepts using BIND

30. suranga.com.zone file • $TTL 86400 • @ IN SOA @ root.suranga.com ( • 1 ; serial • 28800 ; refresh • 7200 ; retry • 604800 ; expire • 86400 ; ttl • ) • IN NS suranga.com. • @ IN A 192.168.200.8 • im3 IN A 192.168.200.250 CeylonLinux DNS concepts using BIND

31. localhost.zone file • $TTL 86400 • @ IN SOA @ root.localhost ( • 1 ; serial • 28800 ; refresh • 7200 ; retry • 604800 ; expire • 86400 ; ttl • ) • IN NS localhost. • @ IN A 127.0.0.1 CeylonLinux DNS concepts using BIND

32. named.localhost file • $TTL 86400 • @ IN SOA localhost. root.localhost. ( • 1997022700 ; Serial • 28800 ; Refresh • 14400 ; Retry • 3600000 ; Expire • 86400 ) ; Minimum • IN NS localhost. • 1 IN PTR localhost. CeylonLinux DNS concepts using BIND

33. Results • Now go to the prompt and type service named start and type nslookup commands to verify your domain as follows. • #nslookup >suranga.com • Server: 192.168.200.8Address: 192.168.200.8#53Name: suranga.comAddress: 192.168.200.8 • >im3.suranga.com • Server: 192.168.200.8Address: 192.168.200.8#53Name: im3.suranga.comAddress: 192.168.200.250 CeylonLinux DNS concepts using BIND

34. Configuration Files • named configuration file (/etc/named.conf): • It basically defines the parameters that point to the sources of domain database information, which can be local files or on remote servers. • Hint file (cache file)(/var/named/named.ca): • It actually provides the name of root server which gets activated in case the machine name, which is to be searched, is not there in user defined zone. • localhost file (/var/named.local): • All configuration have a local domain Database for resolving address to the host name localhost. • Zone: • Basically a zone that keeps the information about the domain database. CeylonLinux DNS concepts using BIND

35. More on zone files • @: It means from the origin to the lastname object that is suranga.com. • IN: This stands for Internet servers • SOA: This stands for `Start Of Authority’. It marks the beginning of a zone’s data and defines the parameter that affects the entire zone. Followed by the current machine name where the DNS server is maintained. • 20000011301;serial: This is the serial number--a numeric value that tells or notifies the slave server, that the database has been updated. So slave server should also update it. • 3600;refresh: This is the refresh cycle in seconds. In every refresh cycle the slave server comes to master server and checks for the updated database. CeylonLinux DNS concepts using BIND

36. More on zone files • 1800;retry: This particular line refers to the retry cycle which in turn means that the slave server should wait before asking the master server again in case master server doesn’t respond. • 1209600;expire: This is the time for slave server to respond to queries of client for the expiration time if master server fails and has to be up and not getting up. After this period slave server also fails to solve the queries of clients and sits idle. • 432100;default_ttl: This refers to the default time to leave, for this domain to work for, when named is once started. Remember the user doesn’t have to play with this unless he wants that the query time from the slave server should be somewhat less or more. In case we want to change, we should change only the refresh time in both master and slave. The best way is to make it 2, which means after each 2 seconds slave server will query to master server. CeylonLinux DNS concepts using BIND