TCP IP Illustrated

Applications contact resolvers to convert a hostname

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Unformatted text preview: em compares the A records with the IP address from. the client's TCP connection request. Caching can reduce the number of packets exchanged in this figure. 14.10 Summary The DNS is an essential part of any host connected to the Internet, and widely used in private internets also. The basic organization is a hierarchical tree that forms the DNS name space. Applications contact resolvers to convert a hostname to an IP address, and vice versa. Resolvers then contact a local name server, and this server may contact one of the root servers or other servers to fulfill the request. All DNS queries and responses have the same message format. This message contains questions and possibly answer resource records (RRs), authority RRs, and additional RRs. We saw numerous examples, showing the resolver configuration file and some of the DNS optimizations: pointers to domain names (to reduce the size of messages), caching, the domain (to look up a name given an IP address), and returning additional RRs (to save the requestor from issuing another query). Exercises 14.1 Classify a DNS resolver and a DNS name server as either client, server, or both. 14.2 Account for all 75 bytes in the response in Figure 14.12. 14.3 In Section 12.3 we said that an application that accepts either a dotted-decimal IP address or a hostname should assume the former, and if that fails, then assume a hostname. What happens if the order of the tests is reversed? 14.4 Every UDP datagram has an associated length. A process that receives a UDP datagram is told what its length is. When a resolver issues a query using TCP instead of UDP, since TCP is a stream of bytes without any record markers, how does the application know how much data is returned? Notice that there is no length field in the DNS header (Figure 14.3). (Hint: Look at RFC 1035.) 14.5 We said that a name server must know the IP addresses of the root servers and that this information is available via anonymous FTP. Unfortunately not all system admin...
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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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