Chapter 19. Domain Name System (DNS)
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1 Chapter 19 Domain Name System (DNS) TCP/IP Protocol Suite 1 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
2 OBJECTIVES: To describe the purpose of DNS. To define the concept of domains and domain name space. To describe the distribution of name spaces and define zones. To discuss the use of DNS in the Internet and describe three categories of domains: generic, country, and reverse. To discuss name-address resolution and show the two resolution methods: recursive and iterative. To show the format of DNS message and how they can be compressed. To discuss DDNS and DNSSEC.. TCP/IP Protocol Suite 2
3 Chapter Outline TCP/IP Protocol Suite 3
4 19-1 NEED FOR DNS To identify an entity, TCP/IP protocols use the IP address, which uniquely identifies the connection of a host to the Internet. However, people prefer to use names instead of numeric addresses. Therefore, we need a system that can map a name to an address or an address to a name. TCP/IP Protocol Suite 4
5 Figure 19.1 Purpose of DNS User 1 Host name 2 Host name 6 IP address Transport layer 5 IP address 3 Query Response 4 TCP/IP Protocol Suite 5
6 19-2 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. In other words, the names must be unique because the addresses are unique. A name space that maps each address to a unique name can be organized in two ways: flat or hierarchical. TCP/IP Protocol Suite 6
7 Topics Discussed in the Section Flat Name Space Hierarchical Name Space Domain Name Space Domain Distribution of Name Space TCP/IP Protocol Suite 7
8 Figure 19.2 Domain name space TCP/IP Protocol Suite 8
9 Figure 19.3 Domain names and labels TCP/IP Protocol Suite 9
10 Figure 19.4 FQDN and PQDN TCP/IP Protocol Suite 10
11 Figure 19.5 Domains TCP/IP Protocol Suite 11
12 Figure 19.6 Hierarchy of name servers TCP/IP Protocol Suite 12
13 Figure 19.7 Zones and domains TCP/IP Protocol Suite 13
14 Note A primary server loads all information from the disk file; the secondary server loads all information from the primary server. When the secondary downloads information from the primary, it is called zone transfer. TCP/IP Protocol Suite 14
15 19-3 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 (see Figure 19.8). TCP/IP Protocol Suite 15
16 Topics Discussed in the Section Generic Domains Country Domains Inverse Domain Registrar TCP/IP Protocol Suite 16
17 Figure 19.8 DNS used in the Internet TCP/IP Protocol Suite 17
18 Figure 19.9 Generic domains TCP/IP Protocol Suite 18
19 TCP/IP Protocol Suite 19
20 Figure Country domains TCP/IP Protocol Suite 20
21 Figure Inverse domain TCP/IP Protocol Suite 21
22 19-4 RESOLUTION Mapping a name to an address or an address to a name is called name-address resolution. TCP/IP Protocol Suite 22
23 Topics Discussed in the Section Resolver Mapping Names to Addresses Mapping Addresses to Names Recursive Resolution Iterative Resolution Caching TCP/IP Protocol Suite 23
24 Figure Recursive resolution TCP/IP Protocol Suite 24
25 Figure Iterative resolution TCP/IP Protocol Suite 25
26 19-5 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 (see Figure 19.14). TCP/IP Protocol Suite 26
27 Topics Discussed in the Section Header TCP/IP Protocol Suite 27
28 Figure Query and response messages TCP/IP Protocol Suite 28
29 Figure Header format TCP/IP Protocol Suite 29
30 Figure Flags field TCP/IP Protocol Suite 30
31 TCP/IP Protocol Suite 31
32 19-6 TYPES OF RECORDS As we saw in the previous section, 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. TCP/IP Protocol Suite 32
33 Topics Discussed in the Section Question Record Resource Record TCP/IP Protocol Suite 33
34 Figure Question record format TCP/IP Protocol Suite 34
35 Figure Query name format TCP/IP Protocol Suite 35
36 TCP/IP Protocol Suite 36
37 TCP/IP Protocol Suite 37
38 Figure Resource record format TCP/IP Protocol Suite 38
39 19-7 COMPRESSION DNS requires that a domain name be replaced by an offset pointer if it is repeated. For example, in a resource record the domain name is usually a repetition of the domain name in the question record. For efficiency, DNS defines a 2-byte offset pointer that points to a previous occurrence of the domain or part of it. The format of the field is shown in Figure TCP/IP Protocol Suite 39
40 Figure Format of an offset pointer TCP/IP Protocol Suite 40
41 Example 19.1 A resolver sends a query message to a local server to find the IP address for the host chal.fhda.edu.. We discuss the query and response messages separately. Figure shows the query message sent by the resolver. The first 2 bytes show the identifier (1333) 16. It is used as a sequence number and relates a response to a query. The next bytes contain the flags with the value of 0x0100 in hexadecimal. In binary it is , but it is more meaningful to divide it into the fields as shown below: TCP/IP Protocol Suite 41
42 Figure Example 19.1: Query message TCP/IP Protocol Suite 42
43 Example 19.1 Continued Figure shows the response of the server. The response is similar to the query except that the flags are different and the number of answer records is one. The flags value is 0x8180 in hexadecimal. In binary it is , but again we divide it into fields as shown below: TCP/IP Protocol Suite 43
44 Figure Example 19.1: Response message TCP/IP Protocol Suite 44
45 Example 19.2 An FTP server has received a packet from an FTP client with IP address The FTP server wants to verify that the FTP client is an authorized client. The FTP server can consult a file containing the list of authorized clients. However, the file consists only of domain names. The FTP server has only the IP address of the requesting client, which was the source IP address in the received IP datagram. The FTP server asks the resolver (DNS client) to send an inverse query to a DNS server to ask for the name of the FTP client. We discuss the query and response messages separately. Figure shows the query message sent from the resolver to the server. TCP/IP Protocol Suite 45
46 Figure Example 19.2: Inverse query message TCP/IP Protocol Suite 46
47 Example 19.2 Continued The first 2 bytes show the identifier (0x1200). The flags value is 0x0900 in hexadecimal. In binary it is , and we divide it into fields as shown below: The OpCode is 0001, which defines an inverse query. The message contains only one question record. The domain name is in-addr4arpa. The next 2 bytes define the query type as PTR, and the last 2 bytes define the class as the Internet. Figure shows the response. The flags value is 0x8D80 in hexadecimal. In binary it is , and we divide it into fields as shown below: TCP/IP Protocol Suite 47
48 Figure Example 19.2: Inverse response message TCP/IP Protocol Suite 48
49 Example 19.3 In UNIX and Windows, the nslookup utility can be used to retrieve address/name mapping. The following shows how we can retrieve an address when the domain name is given. The nslookup utility can also be used to retrieve the domain name when the address is given as shown below: TCP/IP Protocol Suite 49
50 19-8 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. In that case, one of two scenarios can occur: TCP/IP Protocol Suite 50
51 19-9 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. TCP/IP Protocol Suite 51
52 19-10 DDNS When the DNS was designed, no one predicted that there would be so many address changes. In DNS, when there is a change, such as adding a new host, removing a host, or changing an IP address, the change must be made to the DNS master file. The DNS master file must be updated dynamically. The Dynamic Domain Name System (DDNS) therefore was devised to respond to this need. TCP/IP Protocol Suite 52
53 19-11 SECURITY OF DNS DNS is one of the most important systems in the Internet infrastructure; it provides crucial services to the Internet users. Applications such as Web access or e- mail are heavily dependent on the proper operation of DNS. DNS can be attacked in several Ways. To protect DNS, IETF has devised a technology named DNS Security (DNSSEC) that provides the message origin authentication and message integrity using a security service called digital signature (See Chapter 29). TCP/IP Protocol Suite 53
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