TCP IP Illustrated

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Unformatted text preview: ry name server, however, knows how to contact every other name server. Instead every name server must know how to contact the root name servers. As of April 1993 there were eight root servers and all the primary servers must know the IP address of each root server. (These IP addresses are contained file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/dns_the.htm (3 of 18) [12/09/2001 14.47.06] Chapter 14. DNS: The Domain Name System in the primary's configuration files. The primary servers must know the IP addresses of the root servers, not their DNS names.) The root servers then know the name and location (i.e., the IP address) of each authoritative name server for all the second-level domains. This implies an iterative process: the requesting name server must contact a root server. The root server tells the requesting server to contact another server, and so on. We'll look into this procedure with some examples later in this chapter. You can fetch the current list of root servers using anonymous FTP. Obtain the file netinfo/root-servers.txt from either ftp.rs.internic.net or nic.ddn.mil. A fundamental property of the DNS is caching. That is, when a name server receives information about a mapping (say, the IP address of a hostname) it caches that information so that a later query for the same mapping can use the cached result and not result in additional queries to other servers. Section 14.7 shows an example of caching. 14.3 DNS Message Format There is one DNS message defined for both queries and responses. Figure 14.3 shows the overall format of the message. Figure 14.3 General format of DNS queries and responses. The message has a fixed 12-byte header followed by four variable-length fields. The identification is set by the client and returned by the server. It lets the client match responses to requests. The 16-bit flags field is divided into numerous pieces, as shown in Figure 14.4. file:///D|/Documents%20and%20Settings/bigini/Docu.../ho...
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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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