TCP IP Illustrated

14 even so a name server still should not ask a root

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Unformatted text preview: et). A name server normally cycles through the various servers for a zone until round-trip estimates are accumulated. The server with the smallest round-trip time is then used. Since our server is contacting a root server, the recursion-desired flag is not set. This root server does not clear the recursion-available flag, as we saw in line 2 in Figure 14.14. (Even so, a name server still should not ask a root server for a recursive query.) In line 2 the response comes back with no answers, but four authority RRs and four additional information RRs. As we can guess, the four authority RRs are the names of the name servers for, and the four other RRs contain the IP addresses of these four servers. Line 3 is the query of the name server (the first of the four name servers returned in line 2). The recursion-desired flag is set. The response in line 4 is different from previous responses. Two answer RRs are returned and tcpdump says that the first one is a CNAME RR. The canonical name of is This is a common usage of CNAME records. The FTP site for LBL always has a name beginning with ftp, but it may move from one host to another over time. Users need only know the name and the DNS will replace this with its canonical name when referenced. Remember that when we ran host, it printed both the CNAME and the IP address of the canonical name. This is because the response (line 4 in Figure 14.15) contained two answer RRs. The first one is the CNAME and the second is the A record. If the A record had not been returned with the CNAME, our server would have issued another query, asking for the IP address of This is another implementation optimization-both the CNAME and the A record of the canonical name are returned in one response. 14.8 UDP or TCP We've mentioned that the well-known port numbers for DNS name servers are UDP port 53 and TCP port 53. This implies that the DNS supports both UDP and TCP. But all the...
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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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