TCP IP Illustrated

A plus sign would have been printed after the

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Unformatted text preview: e cache to be empty. When the name server starts up, it initializes the identifier to 1. When we type our query, looking for the IP address of the host ftp.uu.net, the name server contacts one of the eight root servers, ns.nic.ddn.mil (line 1). This is the normal A type query that we've seen before, but notice that the recursion-desired flag is not specified. (A plus sign would have been printed after the identifier 2 if the flag was set.) In our earlier examples we always saw the resolver set the recursion-desired flag, but here we see that our name server doesn't set the flag when it's contacting one of the root servers. This is because the root servers shouldn't be asked to recursively answer queries-they should be used only to find the addresses of other, authoritative servers. Line 2 shows that the response comes back with no answer RRs, five authority RRs, and five additional information file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/dns_the.htm (14 of 18) [12/09/2001 14.47.06] Chapter 14. DNS: The Domain Name System RRs. The minus sign following the identifier 2 means the recursion-available (RA) flag was not set-this root server wouldn't answer a recursive query even if we asked it to. Although tcpdump doesn't print the 10 RRs that are returned, we can execute the host command to see what's in the cache: sun % host -v ftp.uu.net Query about ftp.uu.net for record types A Trying ftp.uu.net ... Query done, 1 answer, status: no error The following answer is not authoritative: 19109 A ftp.uu.net IN 192.48.96.9 Authoritative nameservers: 170308 IN NS UU.NET NS.UU.NET 170308 IN NS UU.NET UUNET.UU.NET 170308 IN UU.NET NS UUCP-GW-1.PA.DEC.COM 170308 IN NS UU.NET UUCP-GW-2.PA.DEC.COM 170308 IN NS UU.NET NS.EU.NET Additional information: 170347 IN A NS.UU.NET 137.39.1.3 170347 IN A UUNET.UU.NET 192.48.96.2 A UUCP-GW-1.PA.DEC.COM 170347 IN 16.1.0.18 A UUCP-GW-2.PA.DEC.COM 170347 IN 16.1.0.19 170347 IN A NS.EU.NET 192.16.202.11 This time we specified the -v op...
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