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DNS Fundamentals: Name to Address Mapping, Client-Server Model, and More

Explore the importance of name-to-address mappings, the evolution from HOSTS.TXT to DNS, hierarchical structure of DNS, resolver setup on UNIX, and resource records. Learn about different types of nameservers, client resolver lookups, and utilities for testing DNS.

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DNS Fundamentals: Name to Address Mapping, Client-Server Model, and More

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  1. Domain Name System (DNS) Ayitey Bulley abulley@ghana.com AfNOG-2003

  2. Objectives • Describe why we need name to address mappings and what was wrong with HOSTS.TXT • Describe the client-server model of DNS • Setup and test a resolver on a UNIX machine • List and describe resource records (RR) a client would be interested in • Use dig tool to resolve names with more detail AfNOG-2003

  3. Why Names? • The Internet infrastructure depends on IP addresses • Machines communicate with each other via IP addresses • However human beings can remember names better than numbers • A single file mapping name to IP address was created (HOSTS.TXT) AfNOG-2003

  4. HOSTS.TXT • Maintained by SRI’s NIC and distributed from a single host • Administrators emailed changes to the NIC and periodically downloaded the current HOSTS.TXT • Changes were compiled into a new HOSTS.TXT once or twice a week • As the network grew this scheme became impracticable. AfNOG-2003

  5. What was wrong with HOSTS.TXT • Traffic and load • Name collisions • Consistency • Single point of editing and maintenance • Did not scale well • The need for a more scalable system/scheme, hence DNS AfNOG-2003

  6. What is DNS? • DNS is a distributed database • Allows local control of segments of the overall database • Employs a client-server architecture • Robustness and performance achieved through replication and caching • Name servers constitute the server half of the client-server mechanism • Resolvers constitute the client half of the client-server mechanism • Structure of the DNS database is hierarchical AfNOG-2003

  7. Hierarchical Structure of DNS • Very similar to the structure of the UNIX file system • Pictured as an inverted tree with root node at the top • Each node in the tree has a text label • The null label “” is reserved for the root node • Root node is written as a single dot (.) AfNOG-2003

  8. Hierarchical Structure of DNS (contd.) / “” ug opt edu biz bin usr com etc DNS Database Unix Filesystem AfNOG-2003

  9. ws1 ws2 ftp • www www disi • sun • ripe isi tislabs moon • • • google net edu com • Hierarchical Structure of DNS (contd.) • Domain names can be mapped to a tree. • New branches at the ‘dots’ • No restriction to the amount of branches. AfNOG-2003

  10. Hierarchical Structure of DNS (contd.) • Hostnames are globally unique • E.g. pc1.t1.ws.afnog.org and pc1.t2.ws.afnog.org • Name space is administered in zones • E.g. afnog.org and ws.afnog.org can be administered by different organizations AfNOG-2003

  11. afnog.org domain org domain Domains • Domains are “namespaces” • Everything below .com is in the com domain. • Everything below afnog.org is in the afnog.org domain and in the .org domain. • com org edu • • • google sun afnog tislabs isi • moon • ws www www ftp • www noc AfNOG-2003

  12. Client-Server Model Question: www.afnog.org A root-server www.afnog.org A ? Ask org server @A7.NSTLD.COM. (+ glue) www.afnog.org A ? Caching forwarder (recursive) Resolver 216.129.132.164 www.afnog.org A ? nstld-server Ask afnog server @ austin.gh.com (+ glue) Add to cache www.afnog.org A ? 216.129.132.164 afnog-server AfNOG-2003

  13. Client-Server Model Authoritative Server First query is forwarded, and reply is cached Resolver Client1 Caching/recursive server Authoritative Server aa Authoritative Server non-aa Resolver Client2 Next query is answered from cache AfNOG-2003

  14. Types of Nameservers • Caching-Only Server • Non-authoritative for any zone except 0.0.127.in-addr.arpa • Resolves recursively by querying authoritative nameservers • Authoritative Servers • Master (Primary) • Slave (Secondary) • Can be authoritative for one or more domains AfNOG-2003

  15. Exercise 1 • In this exercise we will be testing name resolution using: • ping and a browser • and by configuring the local resolver on your PCs (/etc/resolv.conf) • Please refer to Exercise 1 in the handouts given to you. AfNOG-2003

  16. Client Resolver Lookups • Possible queries from a client to a nameserver are: • Name to IP Address (A) • browser • Name to Mail exchanger (MX) • Mail Server (MTA) e.g. Exim • IP Address to Name (PTR) - [Reverse DNS] • Logging of incoming connections (apache logs) • Alias to Name (CNAME) • Other resource record (RR) types • SOA, NS (Mainly Server to Server) AfNOG-2003

  17. Client Resolver Lookups (contd.) • Possible responses from a nameserver to a client are: • Positive answer • Negative answer (Name does not exist) • Server Fail (Could not find any answer) AfNOG-2003

  18. Client Utilities for Testing DNS • BIND comes with utilities for testing and troubleshooting nameserver issues. Some of these tools are: • nslookup • dig • Most client programs use the local resolver • E.g. ping, browsers etc. • In this workshop we will focus on the dig and ping utilities. AfNOG-2003

  19. The BIND dig utility • Syntax dig [@server] domain [q-type] [other options] • Server – The server you want to use to resolve the query (defaults to servers listed in /etc/resolv.conf) • Domain - a name in the Domain Name System • q-type - is one of (a,any,mx,ns,soa,hinfo,axfr,txt,...) [default: a] • Examples # dig @81.199.109.1 ws.afnog.org a # dig @ns.t1.ws.afnog.org ws.afnog.org a # dig @noc.ws.afnog.org –x 81.199.110.100 # man dig AfNOG-2003

  20. Question • From the output from the last example, what is the default query type? AfNOG-2003

  21. Understanding output from dig • Queries using the dig utility outputs a lot of information, however the most important for us are • Status • Flags • Answer Section • Authority Section • Additional Section • TTL • Total query time • “From …. To …. Server” …. Section AfNOG-2003

  22. ns# dig @81.199.110.100 www.gouv.bj a ; <<>> DiG 8.3 <<>> @81.199.110.100 www.gouv.bj a ; (1 server found) ;; res options: init recurs defnam dnsrch ;; got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 4 ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 4, ADDITIONAL: 3 ;; QUERY SECTION: ;; www.gouv.bj, type = A, class = IN ;; ANSWER SECTION: www.gouv.bj. 1D IN CNAME waib.gouv.bj. waib.gouv.bj. 1D IN A 208.164.179.196 ;; AUTHORITY SECTION: gouv.bj. 1D IN NS rip.psg.com. gouv.bj. 1D IN NS ben02.gouv.bj. gouv.bj. 1D IN NS nakayo.leland.bj. gouv.bj. 1D IN NS ns1.intnet.bj. ;; ADDITIONAL SECTION: ben02.gouv.bj. 1D IN A 208.164.179.193 nakayo.leland.bj. 1d23h59m59s IN A 208.164.176.1 ns1.intnet.bj. 1d23h59m59s IN A 81.91.225.18 ;; Total query time: 2084 msec ;; FROM: ns.t1.ws.afnog.org to SERVER: 81.199.110.100 81.199.110.100 ;; WHEN: Sun Jun 8 21:18:18 2003 ;; MSG SIZE sent: 29 rcvd: 221 AfNOG-2003

  23. Exercise 2 • In this exercise we will be using the dig utility to resolve domain names • dig using your local resolver • dig using another caching server • dig for reverse lookups • dig for a non-existent domain AfNOG-2003

  24. Best Practices • Choose caching nameservers close by you for your resolver • Select at least two (2) caching nameservers for your resolver (redundancy) • Use search lists in the resolver for non-FQDN AfNOG-2003

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