Guide to TCP/IP, Third Edition

1. Guide to TCP/IP, Third Edition Chapter 7: Domain Name System

2. Objectives • Understand the history of Domain Name System (DNS), types of services that DNS provides, types of roles that DNS servers can play on a network, and DNS database structure • Understand how DNS servers handle name resolution requests, including the role of nearby and root servers in the resolution process, and the difference between recursive and iterative name resolution requests • Explain the significance of DNS caching and the value of data in DNS caches IP Addressing and Related Topics

3. Objectives (continued) • Understand DNS configuration files and resource record formats, and describe the most common DNS RR types • Understand how to handle the loopback address and root server data • Use the NSLOOKUP command to gather name server information, test DNS server configurations, and inspect the contents of key DNS records on such servers IP Addressing and Related Topics

4. Objectives (continued) • Explain how DNS queries and responses work, and how they handle name resolution, DNS record lookups, zone data transfers, and reverse DNS queries • Understand the purposes of, and drawbacks to, implementing DNS in the real world IP Addressing and Related Topics

5. DNS History and Background • RFCs 882 and 883 • Original RFCs for DNS • Created by Paul Mockapetris (also created JEEVES) • BIND (Berkeley Internet Name Domain) • Written by Kevin Dunlap in 1988 • Database segments • Include only a portion of the overall namespace that DNS can access for its clients IP Addressing and Related Topics

6. DNS History and Background (continued) • DNS combines the following virtues • Allows local control over domain name database segments • Data from all database segments is available everywhere • Database information is robust and highly available • DNS • One of the most effective uses of distributed database technology in the world today IP Addressing and Related Topics

7. DNS Database Structure • Mirrors structure of the domain namespace itself • Top-level domains in the U.S. • .com • .edu • .gov • .mil • .net • .org IP Addressing and Related Topics

8. IP Addressing and Related Topics

9. The DNS Namespace • DNS • Arbitrarily partitions tree and creates subtrees for database information • Domains (such as ibm.com) • Can be broken into subdomains(such as clearlake.ibm.com) • Any valid domain name • Ultimately resides within some specific DNS database IP Addressing and Related Topics

10. DNS Database Records • Resource records • Stores data associated with domain names, address records, and other specific data • Most commonly used types • Address (A) record • Canonical name (CNAME) record • Host information (HINFO) record • Mail exchange (MX) record • Name server (NS) record IP Addressing and Related Topics

11. Delegating DNS Authority • DNS • Permits database record for primary DNS serverto delegate authority to DNS servers lower in domain namespace • Once authority is delegated • Database for name server includes NS records that point to name servers • Organization of global DNS database • Designed to make it quick and easy for name servers to point to other name servers IP Addressing and Related Topics

12. Types of DNS Servers • Primary master name DNS server • Where the primary DNS database files for the domain(s) or subdomain(s) reside • Primary master • Distinguished from other name servers for a domain • For any DNS zone • There can be only one primary master name server IP Addressing and Related Topics

13. Secondary DNS Server • Gets its data for the zone from the master server for that zone • Zone data on a secondary server • Always originates from a primary server • Zone transfer • Secondary DNS server gets data for the zone from the master server for that zone • Secondary, or slave, DNS servers • Provide a back-up copy of the domain database for a specific zone IP Addressing and Related Topics

14. Caching Servers • Store recently accessed DNS records from other domains • Caching-only server • Speeds access to specific domain names by storing a copy of the lookup data locally • Size and Internet access volume • Factors that determine if an organization implements separate caching-only servers IP Addressing and Related Topics

15. The Client Side of DNS • Resolvers • Issue requests for service, called name queriesor address requests, to domain name servers • An address request • Seeks to resolve a domain name to a corresponding numeric IP address • Name query (inverse DNS query) • Seeks to resolve an address to a domain name IP Addressing and Related Topics

16. How Domain Name Servers Work • A TCP/IP client • Usually some application or service that encounters a domain name for which it needs an IP address • Servers • Queried in the order in which they appear in TCP/IP configuration files (from top down) • DNS servers • Handle real name resolution IP Addressing and Related Topics

17. Recursive Query • Used by DNS resolvers to • Delegate the first DNS server that they contact to go out and find the necessary address translation • In the grand DNS server hierarchy • Any DNS server can issue iterative queries • Only a DNS client or a root server can issue recursive queries IP Addressing and Related Topics

18. Iterative or Non-Recursive Queries • Issued when one DNS server receives a recursive request • Do not cause other queries to be issued • Reason some recursive name queries involve a root server • Root server always knows how to find whatever DNS server is authoritative for the domain IP Addressing and Related Topics

19. The Importance of DNS Caching • All data in a DNS cache • Has an expiration value • DNS servers • Cache name and address pairs for addresses they resolved • Keep information about name requests that result in error messages IP Addressing and Related Topics

20. DNS Configuration Files and Resource Record Formats • domain.dns • The files that map host names to addresses • addr.in-addr.arpa.dns • Files that map addresses to domain names for reverse lookups • Every DNS zone file must contain • SOA and NS records • Records about host names or addresses in that zone IP Addressing and Related Topics

21. Start of Authority Record • Identifies the current name server as the best source of information for data in its zone • Both secondary and primary name servers can • Designate themselves as authoritative in their own SOA records IP Addressing and Related Topics

22. Address and Canonical Name Records • DNS, by default • Accesses only the first IP address for a host when multiple entries for a single domain name are defined • DNS round robinload balancing • Permits a DNS server to keep track of which IP addresses it has provided for a specific translation • Rotates the IP addresses within the list of addresses available IP Addressing and Related Topics

23. Mapping Addresses to Names • Records in the db.addr file • Provided to support reverse DNS lookups • Reverse address lookups • Used primarily to determine if IP address that user presents matches originating domain name • Classful • File structure of reverse DNS lookups IP Addressing and Related Topics

24. Handling the Loopback Address • To ensure proper handling of the reserved loopback address • The db.127.0.0 file must be createdon each individual machine • db.127.0.0 file • Begins with the required SOA record • Then provides NS records • Concludes with a PTR record IP Addressing and Related Topics

25. Obtaining and Storing Root Server Data • On a Windows server running DNS • Copy file to directory named %SystemRoot%\System32\DNS directory • Rename the file to cache.dns IP Addressing and Related Topics

26. The NSLOOKUP Command • Supported by Windows and UNIX • Provides access to all kinds of DNS information • Essential tool for testing, when configuring or troubleshooting a DNS server • Syntax • nslookup domain-name [name-server] IP Addressing and Related Topics

27. IP Addressing and Related Topics

28. IP Addressing and Related Topics

29. Using NSLOOKUP • set OPTION command • Used to examine specific types of resource records • ls –a or ls –d • Used toextract information from certain well-known name servers IP Addressing and Related Topics

30. IP Addressing and Related Topics

31. IP Addressing and Related Topics

32. DNS Query/Response Packet Formats • Four sections in the DNS response packets • Question section • Answer section • Authority section • Additional section IP Addressing and Related Topics

33. IP Addressing and Related Topics

34. DNS Query/Response Packet Formats (continued) • DNS query packet fields • ID Number Field • QR (Query/Response) Field • Opcode (Operation Code) Field • AA (Authoritative Answer) Field • TC (Truncation) Field • RD (Recursion Desired) Field • RA (Recursion Available) Field • Z (Reserved) Field IP Addressing and Related Topics

35. IP Addressing and Related Topics

36. IP Addressing and Related Topics

37. IP Addressing and Related Topics

38. DNS Query/Response Packet Formats (continued) • DNS query packet fields • Rcode (Response Code) Field • Question Count Field • Answer Count Field • Name Server Count Field • Additional Records Count Field • Question Name Field • Question Type Field • Question Class Field IP Addressing and Related Topics

39. DNS Query/Response Packet Formats (continued) • DNS query packet fields • Name Field • Type Field • Class Field • Time to Live Field • Resource Data Length Field • Resource Data Field IP Addressing and Related Topics

40. DNS Implementation • Handles two major address related-activities • Resolving symbolic domain names into numeric IP addresses • Providing authoritative hostname-to-IP mapping • DNS servers for internal use • Usually focus on alleviating administration hassles by leveraging DHCP, WINS, Active Directory IP Addressing and Related Topics

41. The Trouble with DNS • DNS database updates • Require that a qualified administrator operates directly on the DNS database files, or • Uses special-purpose tools to make changes • “Propagation delay” • Time it takes for cached values to catch up with changes to authoritative databases IP Addressing and Related Topics

42. Additional Readings on DNS • Cricket Liu, et al.: DNS on Windows Server 2003, O’Reilly & Associates • William Wong: Windows 2000 DNS Server, Osborne/McGraw-Hill • Paul Albitz and Cricket Liu: DNS and BIND, 4th Edition, O’Reilly & Associates • Craig Hunt: Linux DNS Server Administration, Sybex Books, Alameda, CA IP Addressing and Related Topics

43. Summary • The Domain Name System • Provides key address resolution service that makes today’s Internet possible • Impetus for DNS • Arose from difficulty of maintaining static HOSTS files for computers on the ARPANET • DNS name servers • Come in multiple varieties IP Addressing and Related Topics

44. Summary (continued) • DNS • Maintains its data on a large collection of name servers around the Internet • DNS databases • Consist of a collection of resource records (RRs) • DNS clients • Rely on a software component called a resolver to interact with an available IP Addressing and Related Topics

45. Summary (continued) • DNS clients • Rely on resolver to interact with available DNS server for name resolution services • DNS packet structures • Incorporate type information that identifies the kind of RR being carried IP Addressing and Related Topics