Chapter 9
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The main problem with moving the host-level entries, such as for www.cisco, to a single root nameserver entry such as cisco, is that this either limits organizations to a single externally visible host, or else (if the change is interpreted slightly differently) significantly increases root nameserver traffic as it returns some kind of block of multiple host addresses. In effect this takes DNS back to a single central server. Perhaps just as importantly, the updating of the IP addresses corresponding to host names is now out of the hands of the organizations owning the host names, leading to a considerable administrative bottleneck. However, if we’re just browsing the web and need only one address for each organization, the traffic would be roughly equivalent to the way DNS works now. (We are assuming that local resolvers still exist and still maintain request caches; the loss of local caches would put an intolerable burden on the root nameservers.) 11. DNS records contain a TTL value, specified by the DNS server, representing how long a DNS record may be kept in the client cache. RFC 1034 puts it this way: If a change can be anticipated, the TTL can be reduced prior to the change to minimize inconsistency during the change, and then increased back to its former value following the change. 12. Here is a series of dig queries and edited responses. First we try to find top level servers for the edu. domain:
% dig +norecurse edu. ;; AUTHORITY SECTION: edu. 143993 IN edu. 143993 IN edu. 143993 IN edu. 143993 IN ...
NS NS NS NS
C3.NSTLD.COM. D3.NSTLD.COM. E3.NSTLD.COM. G3.NSTLD.COM.
Next, picking one of the top level servers:
% dig @c3.nstld.com princeton.edu. ;; AUTHORITY SECTION: princeton.edu. princeton.edu. princeton.edu. princeton.edu. ;; ADDITIONAL SECTION:
172800 172800 172800 172800
IN IN IN IN
NS NS NS NS
NS1.UCSC.edu. NS2.FAST.NET. NS3.NIC.FR. DNS.princeton.edu.