TCP IP Illustrated

This is another implementation optimization in the

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Unformatted text preview: tion to see more than just the A record. This shows that there are five authoritative name servers for the domain uu.net. The five RRs with additional information that are returned by the root server contain the IP addresses of these five name servers. This saves us from having to contact the root server again, to look up the address of one of the servers. This is another implementation optimization in the DNS. The host command states that the answer is not authoritative. This is because the answer was obtained from our name server's cache, not by contacting an authoritative server. Returning to line 3 of Figure 14.14, our name server contacts the first of the authoritative servers (ns.uu.net) with the same question: What is the IP address of ftp.uu.net? This time our server sets the recursion-desired flag. The answer is returned on line 4 as a response with one answer RR. We then execute the host command again, asking for the same name: sun % host ftp.uu.net ftp.uu.net A 192.48.96.9 This time there is no tcpdump output. This is what we expect, since the answer output by host is returned from the server's cache. We execute the host command again, looking for the address of ftp.ee.lbl.gov: sun%hostftp.ee.lbl.gov CNAME ftp.ee.lbl.gov A ee.lbl.gov ee.lbl.gov 128.3.112.20 file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/dns_the.htm (15 of 18) [12/09/2001 14.47.06] Chapter 14. DNS: The Domain Name System Figure 14.15 shows the tcpdump output. 1 18.664971 (17.6555) 2 19.429412 ( 0.7644) 3 19.432271 ( 0.0029) 4 19.909242 ( 0.4770) sun.tuc.noao.edu.domain > c.nyser.net.domain: 4 A? ftp.ee.lbl.gov. (32) c.nyser.net.domain > sun.tuc.noao.edu.domain: 4 0/4/4 (188) sun.tuc.noao.edu.domain > nsl.lbl.gov.domain: 5+ A? ftp.ee.lbl.gov. (32) nsl.lbl.gov.domain > sun.tuc.noao.edu.domain: 5* 2/0/0 CNAME ee.lbl.gov. (72) Figure 14.15 tcpdump output for: host ftp.ee.lbl.gov. Line 1 shows that this time our server contacts another of the root servers (c.nyser.n...
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