TCP IP Illustrated

But the resolver function automatically sends an ip

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Unformatted text preview: 1.252.140.in-addr.arpa. (43) noao.edu.domain > sun.1812: 1* 1/0/0 PTR sun.tuc.noao.edu. (73) sun. 1813 > noao.edu.domain: 2+ A? sun.tuc.noao.edu. (33) noao.edu.domain > sun.1813: 2* 2/0/0 A 140.252.1.29 (69) Figure 14.13 Calling resolver function to perform pointer query Line 1 is the expected pointer query, and line 2 is the expected response. But the resolver function automatically sends an IP address query in line 3 for the name returned in line 2. The response in line 4 contains two answer records, since the host sun has two IP addresses. If one of the addresses does not match the argument to gethostbyaddr, a message is sent to the system logging facility, and the function returns an error to the application. 14.6 Resource Records We've seen a few different types of resource records (RRs) so far: an IP address has a type of A, and PTR means a pointer query. We've also seen that RRs are what a name server returns: answer RRs, authority RRs, and additional information RRs. There are about 20 different types of resource records, some of which we'll now describe. Also, more RR types are being added over time. A An A record defines an IP address. It is stored as a 32-bit binary value. PTR This is the pointer record used for pointer queries. The IP address is represented as a domain name (a sequence of labels) in the in-addr.arpa domain. This stands for "canonical name." It is represented as a domain name (a sequence of labels). The domain name that has a canonical name is often called an alias. These are used by some FTP sites to provide an easy to remember alias for some other system. For example, the gated server (mentioned in Section 10.3) is available through anonymous FTP from the server gated.cornell.edu. But there is no system named gated, this is an alias for some other CNAME system. That other system is the canonical name for gated.cornell.edu: sun % host -t cname gated.cornell.edu gated.cornell.edu CNAME COMET.CIT.CORNELL.EDO Here we use the -t option to specify one particular query type. file:///D|/Document...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.

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