1. 25.1 Chapter 25 Domain Name System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. 25.2 Figure 25.1 Example of using the DNS service server Typo in textbook

3. 25.3 25-1 NAME SPACE To be unambiguous, the names assigned to machines must be carefully selected from a name space with complete control over the binding between the names and IP addresses. Flat Name Space: hard to manage for large-scale system Hierarchical Name Space: name has several parts Topics discussed in this section:

4. 25.4 25-2 DOMAIN NAME SPACE To have a hierarchical name space, a domain name space was designed. In this design the names are defined in an inverted-tree structure with the root at the top. The tree can have only 128 levels: level 0 (root) to level 127. Label Domain Name Domain Topics discussed in this section:

5. 25.5 Figure 25.2 Domain name space root

6. 25.6 Figure 25.3 Domain names and labels

7. 25.7 Figure 25.5 Domains: subtree of the domain name space

8. 25.8 25-3 DISTRIBUTION OF NAME SPACE The information contained in the domain name space must be stored. However, it is very inefficient and also unreliable to have just one computer store such a huge amount of information. In this section, we discuss the distribution of the domain name space. Hierarchy of Name Servers Zone Root Server Primary and Secondary Servers Topics discussed in this section:

9. 25.9 Figure 25.6 Hierarchy of name servers

10. DNS: Root name servers b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 17 other locations) i Autonomica, Stockholm (plus 3 other locations) k RIPE London (also Amsterdam, Frankfurt) m WIDE Tokyo a Verisign, Dulles, VA c Cogent, Herndon, VA (also Los Angeles) d U Maryland College Park, MD g US DoD Vienna, VA h ARL Aberdeen, MD j Verisign, ( 11 locations) 13 root name servers worldwide

11. TLD and Authoritative Servers Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp. Network solutions maintains servers for com TLD Educause for edu TLD Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web and mail). Can be maintained by organization or service provider (paid by the organization)

12. 25.12 Figure 25.7 Zones and domains

13. 25.13 Two types of DNS server: A primary server loads all information from the disk file; the secondary server loads all information from the primary server. Reason: redundancy When the secondary downloads information from the primary, it is called zone transfer. Note

14. 25.14 25-4 DNS IN THE INTERNET DNS is a protocol that can be used in different platforms. In the Internet, the domain name space (tree) is divided into three different sections: generic domains, country domains, and the inverse domain. Generic Domains Country Domains Inverse Domain Topics discussed in this section:

15. 25.15 Unix: nslookup, dig Windows: nslookup DNS Query Commands

16. 25.16 Figure 25.8 DNS IN THE INTERNET

17. 25.17 Figure 25.9 Generic domains

18. 25.18 Table 25.1 Generic domain labels

19. 25.19 Figure 25.10 Country domains

20. 25.20 25-5 RESOLUTION Mapping a name to an address or an address to a name is called name-address resolution. Resolver Mapping Names to Addresses Mapping Addresses to Names Recursive Resolution Caching Topics discussed in this section:

21. 25.21 Figure 25.12 Recursive resolution

22. 25.22 Figure 25.13 Iterative resolution

23. 25.23 Caching: Main Reason for the Efficiency of DNS All DNS servers cache prior query results Normal DNS query will not go through the full steps of recursive/iterative resolution

24. 25.24 25-6 DNS MESSAGES DNS has two types of messages: query and response. Both types have the same format. The query message consists of a header and question records; the response message consists of a header, question records, answer records, authoritative records, and additional records. Header Topics discussed in this section:

25. 25.25 Figure 25.14 Query and response messages

26. 25.26 Figure 25.15 Header format

27. 25.27 25-7 TYPES OF RECORDS As we saw in Section 25.6, two types of records are used in DNS. The question records are used in the question section of the query and response messages. The resource records are used in the answer, authoritative, and additional information sections of the response message. Question Record Resource Record Topics discussed in this section:

28. DNS records DNS: distributed db storing Resource Records (RR) Type=NS name is domain (e.g. foo.com) value is name of authoritative DNS server for this domain RR format: (name, value, type, ttl) rType=A  name is hostname  value is IP address rType=CNAME  name is alias name for some “canonical” (the real) name www.ibm.com is really servereast.backup2.ibm.com  value is canonical name rType=MX  value is name of mailserver associated with name 25.28

29. DNS protocol, messages DNS protocol : query and reply messages, both with same message format msg header ridentification: 16 bit # for query, reply to query uses same # rflags:  query or reply  recursion desired  recursion available  reply is authoritative 25.29

30. DNS protocol, messages (UDP 53) Name, type fields for a query RRs in response to query records for authoritative servers additional “helpful” info that may be used Let’s check a web example using Wireshark! (MX record: nslookup –type=MX cs.ucf.edu or dig mx cs.ucf.edu) 25.30

31. Inserting records into DNS Example: just created startup “netwar” Register name netwar.com at a registrar (e.g., Network Solutions) Need to provide registrar with names and IP addresses of your authoritative name server (primary and secondary) Registrar inserts two RRs into the com TLD server: (netwar.com, dns1.netwar.com, NS) (dns1.netwar.com, 212.212.212.1, A) Put in authoritative server dns1.netwar.com Type A record for www.netwar.com Type CName for netwar.com (alias) Type MX record for netwar.com (email) Type A record for the email server How do people get the IP address of your Web site? 25.31

32. 25.32 25-8 REGISTRARS How are new domains added to DNS? This is done through a registrar, a commercial entity accredited by ICANN. A registrar first verifies that the requested domain name is unique and then enters it into the DNS database. A fee is charged.

33. 25.33 25-9 DYNAMIC DOMAIN NAME SYSTEM (DDNS) Used when a server has a dynamic-changeable IP The DNS master file must be updated dynamically. The Dynamic Domain Name System (DDNS) therefore was devised to respond to this need. In DDNS, when a binding between a name and an address is determined, the information is sent, usually by DHCP to a primary DNS server. The primary server updates the zone. The secondary servers are notified either actively or passively.

34. 25.34 25-10 ENCAPSULATION DNS can use either UDP or TCP. In both cases the well-known port used by the server is port 53. UDP is used when the size of the response message is less than 512 bytes because most UDP packages have a 512-byte packet size limit. If the size of the response message is more than 512 bytes, a TCP connection is used.

35. 25.35 DNS can use the services of UDP or TCP using the well-known port 53. Note