TCP IP Illustrated

As weve mentioned before the resolver is part of the

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Unformatted text preview: un) to use the name server on the host noao.edu (140.252.1.54). Figure 14.9 shows the arrangement of the three systems. file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/dns_the.htm (7 of 18) [12/09/2001 14.47.06] Chapter 14. DNS: The Domain Name System Figure 14.9 Systems being used for simple DNS example. As we've mentioned before, the resolver is part of the client, and the resolver contacts a name server to obtain the IP address before the TCP connection can be established between Telnet and the daytime server. In this figure we've omitted the detail that the connection between sun and the 140.252.1 Ethernet is really a SLIP link (see the figure on the inside front cover) because that doesn't affect the discussion. We will, however, run tcpdump on the SLIP link to see the packets exchanged between the resolver and name server. The file /etc/resolv.conf on the host sun tells the resolver what to do: sun % cat /etc/resolv.conf nameserver 140.252.1.54 domain tuc.noao.edu The first line gives the IP address of the name server - the host noao.edu. Up to three nameserver lines can be specified, to provide backup in case one is down or unreachable. The domain line specifies the default domain. If the name being looked up is not a fully qualified domain name (it doesn't end with a period) then the default domain .tuc.noao.edu is appended to the name. This is why we can type telnet gemini instead of telnet gemini.tuc.noao.edu. Figure 14.10 shows the packet exchange between the resolver and name server. 1 0.0 2 0.290820 (0.2908) 140.252.1.29.1447 > 140.252.1.54.53: 1+ A? gemini.tuc.noao.edu. (37) 140.252.1.54.53 > 140.252.1.29.1447: 1* 2/0/0 A 140.252.1.11 (69) Figure 14.10 tcpdump output for name server query of the hostname gemini.tuc.noao.edu. We've instructed tcpdump not to print domain names for the source and destination IP addresses of each IP datagram. Instead it prints 140.252.1.29 for the client (the resolver) and 140.252.1.54 for the name server. Port 1447 is the ephemeral port used by the client and 53 is the we...
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