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Unformatted text preview: Chapter 17 Domain Name System: DNS
Upon completion you will be able to: • Understand how the DNS is organized • Know the domains in the DNS • Know how a name or address is resolved • Be familiar with the query and response formats • Understand the need for DDNS
TCP/IP Protocol Suite 1 17.1 NAME SPACE
The names assigned to machines must be unique because the addresses The are unique. A name space that maps each address to a unique name can be organized in two ways: flat or hierarchical. be The topics discussed in this section include: Flat Name Space Flat Hierarchical Name Space TCP/IP Protocol Suite 2 17.2 DOMAIN NAME SPACE
The domain name space is hierarchical in design. The names are The defined in an inverted-tree structure with the root at the top. The tree can have 128 levels: level 0 (root) to level 127. have The topics discussed in this section include: Label Label Domain Name Domain TCP/IP Protocol Suite 3 Figure 17.1 Domain name space TCP/IP Protocol Suite 4 Figure 17.2 Domain names and labels TCP/IP Protocol Suite 5 Figure 17.3 FQDN and PQDN TCP/IP Protocol Suite 6 Figure 17.4 Domains TCP/IP Protocol Suite 7 17.3 DISTRIBUTION OF NAME SPACE
The information contained in the domain name space is distributed The among many computers called DNS servers. The topics discussed in this section include: The Hierarchy of Name Servers Hierarchy Zone Root Server Primary and Secondary Servers TCP/IP Protocol Suite 8 Figure 17.5 Hierarchy of name servers TCP/IP Protocol Suite 9 Figure 17.6 Zones and domains TCP/IP Protocol Suite 10 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 11 17.4 DNS IN THE INTERNET
The domain name space (tree) is divided into three different sections: The generic domains, country domains, and the inverse domain. generic The topics discussed in this section include: Generic Domains Generic Country Domains Inverse Domain Registrar TCP/IP Protocol Suite 12 Figure 17.7 DNS used in the Internet TCP/IP Protocol Suite 13 Figure 17.8 Generic domains TCP/IP Protocol Suite 14 Table 17.1 Generic domain labels Table Generic TCP/IP Protocol Suite 15 Table 17.1 Generic domain labels (Continued) Table Generic TCP/IP Protocol Suite 16 Figure 17.9 Country domains TCP/IP Protocol Suite 17 Figure 17.10 Inverse domain TCP/IP Protocol Suite 18 17.5 RESOLUTION
Mapping a name to an address or an address to a name is called nameaddress resolution. The topics discussed in this section include: Resolver Resolver Mapping Names to Addresses Mapping Addresses to Names Recursive Resolution Iterative Resolution Caching TCP/IP Protocol Suite 19 Figure 17.11 Recursive resolution TCP/IP Protocol Suite 20 Figure 17.12 Iterative resolution TCP/IP Protocol Suite 21 17.6 DNS MESSAGES
The DNS query message consists of a header and question records; the The DNS response message consists of a header, question records, answer records, authoritative records, and additional records. records, The topics discussed in this section include: Header Header TCP/IP Protocol Suite 22 Figure 17.13 DNS messages TCP/IP Protocol Suite 23 Figure 17.14 Query and response messages TCP/IP Protocol Suite 24 Figure 17.15 Header format TCP/IP Protocol Suite 25 Figure 17.16 Flags field TCP/IP Protocol Suite 26 Table 17.2 Values of rCode Table Values TCP/IP Protocol Suite 27 17.7 TYPES OF RECORDS
Two types of records are used in DNS. The question records are used in Two 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. sections The topics discussed in this section include: Question Record Question Resource Record TCP/IP Protocol Suite 28 Figure 17.17 Question record format TCP/IP Protocol Suite 29 Figure 17.18 Query name format TCP/IP Protocol Suite 30 Table 17.3 Types Table Types TCP/IP Protocol Suite 31 Table 17.4 Classes Table Classes TCP/IP Protocol Suite 32 Figure 17.19 Resource record format TCP/IP Protocol Suite 33 17.8 COMPRESSION
DNS requires that a domain name be replaced by an offset pointer if it is DNS repeated. DNS defines a 2-byte offset pointer that points to a previous occurrence of the domain name or part of it. occurrence TCP/IP Protocol Suite 34 Figure 17.20 Format of an offset pointer TCP/IP Protocol Suite 35 Example 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 17.21 shows the query message sent by the resolver. The first 2 bytes show the identifier (1333). It is used as a sequence number and relates a response to a query. Because a resolver may even send many queries to the same server, the identifier helps to sort responses that arrive out of order. The next bytes contain the flags with the value of 0x0100 in hexadecimal. In binary it is 0000000100000000, but it is more meaningful to divide it into the fields as shown below: QR 0 OpCode 0000 AA 0 TC 0 RD 1 RA 0 Reserved 000 rCode 0000
36 TCP/IP Protocol Suite Figure 17.21 Example 1: Query message TCP/IP Protocol Suite 37 Example 1 (Continued)
The QR bit defines the message as a query. The OpCode is 0000, which defines a standard query. The recursion desired (RD) bit is set. (Refer back to Figure 17.16 for the flags field descriptions.) The message contains only one question record. The domain name is 4chal4fhda3edu0. The next 2 bytes define the query type as an IP address; the last 2 bytes define the class as the Internet. Figure 17.22 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 1000000110000000, but again we divide it into fields as shown below: QR 1 OpCode 0000 AA 0 TC 0 RD 1 RA 1 Reserved 000 rCode 0000
38 TCP/IP Protocol Suite Example 1 (Continued)
The QR bit defines the message as a response. The OpCode is 0000, which defines a standard response. The recursion available (RA) and RD bits are set. The message contains one question record and one answer record. The question record is repeated from the query message. The answer record has a value of 0xC00C (split in two lines), which points to the question record instead of repeating the domain name. The next field defines the domain type (address). The field after that defines the class (Internet). The field with the value 12,000 is the TTL (12,000 s). The next field is the length of the resource data, which is an IP address (184.108.40.206). TCP/IP Protocol Suite 39 Figure 17.22 Example 1: Response message TCP/IP Protocol Suite 40 Example 2 An FTP server has received a packet from an FTP client with IP address 220.127.116.11. 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. TCP/IP Protocol Suite 41 Example 2 (Continued)
Figure 17.23 shows the query message sent from the resolver to the server. The first 2 bytes show the identifier (0x1200). The flags value is 0x0900 in hexadecimal. In binary it is 0000100100000000, and we divide it into fields as shown below: QR 0 OpCode 0001 AA 0 TC 0 RD 1 RA 0 Reserved 000 rCode 0000 The OpCode is 0001, which defines an inverse query. The message contains only one question record. The domain name is 19171231537in-addr4arpa. The next 2 bytes define the query type as PTR, and the last 2 bytes define the class as the Internet.
42 TCP/IP Protocol Suite Figure 17.23 Example 2: Inverse query message TCP/IP Protocol Suite 43 Example 2 (Continued)
Figure 17.24 shows the response. The flags value is 0x8D80 in hexadecimal. In binary it is 1000110110000000, and we divide it into fields as shown below: QR 1 OpCode 0001 AA 1 TC 0 RD 1 RA 1 Reserved 000 rCode 0000 TCP/IP Protocol Suite 44 Figure 17.24 Example 2: Inverse response message TCP/IP Protocol Suite 45 Example 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. $ nslookup fhda.edu
Name: fhda.edu Address: 18.104.22.168 The nslookup utility can also be used to retrieve the domain name when the address is given as shown below: $ nslookup 22.214.171.124
126.96.36.199.in-addr.arpa name = tiptoe.fhda.edu. TCP/IP Protocol Suite 46 17.9 DDNS
The Dynamic Domain Name System (DDNS) updates the DNS master The file dynamically. TCP/IP Protocol Suite 47 17.10 ENCAPSULATION
DNS uses UDP as the transport protocol when the size of the response DNS message is less than 512 bytes. If the size of the response message is more than 512 bytes, a TCP connection is used. TCP/IP Protocol Suite 48 Note: DNS can use the services of UDP or TCP using the well-known port 53. TCP/IP Protocol Suite 49 ...
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This note was uploaded on 04/26/2010 for the course CSE CS501 taught by Professor Dmathur during the Winter '10 term at National Institute of Technology, Calicut.
- Winter '10