TCP IP Illustrated

The name is terminated with a byte of 0 which is a

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Unformatted text preview: t 1, and the remaining two counts can be 0 or nonzero. Question Portion of DNS Query Message The format of each question in the question section is shown in Figure 14.5. There is normally just one question. The query name is the name being looked up. It is a sequence of one or more labels. Each label begins with a 1-byte count that specifies the number of bytes that follow. The name is terminated with a byte of 0, which is a label with a length of 0, which is the label of the root. Each count byte must be in the range of 0 to 63, since labels are limited. Figure 14.5 Format of question portion of DNS query message. to 63 bytes. (We'll see later in this section that a count byte with the two high-order bits turned on, values 192 to 255, is used with a compression scheme.) Unlike many other message formats that we've encountered, this field is allowed to end on a boundary other than a 32-bit boundary. No padding is used. Figure 14.6 shows how the domain name is stored. file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/dns_the.htm (5 of 18) [12/09/2001 14.47.06] Chapter 14. DNS: The Domain Name System Figure 14.6 Representation of the domain name Each question has a query type and each response (called a resource record, which we talk about below) has a type. There are about 20 different values, some of which are now obsolete. Figure 14.7 shows some of these values. The query type is a superset of the type: two of the values we show can be used only in questions. Name Numeric value Description A NS CNAME PTR HINFO MX 1 2 5 12 13 15 IP address name server canonical name pointer record host info mail exchange record AXFR * or ANY 252 255 type? query type? * * * * * * * * * * * * request for zone transfer request for all records * * Figure 14.7 type and query type values for DNS questions and responses. The most common query type is an A type, which means an IP address is desired for the query name. A PTR query requests the names corresponding to an IP address. This is a pointer query that we describe in Section 14.5. We describe the oth...
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