73 lines
3.7 KiB
HTML
73 lines
3.7 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
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<!--Converted with LaTeX2HTML 96.1-c (Feb 29, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds -->
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<HTML>
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<HEAD>
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<TITLE>The DNS Database</TITLE>
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</HEAD>
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<BODY LANG="EN">
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<A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node40.html">Reverse Lookups</A>
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<B>Up:</B> <A HREF="node34.html">The Domain Name System</A>
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<B> Previous:</B> <A HREF="node38.html">Domain Name Servers</A>
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<BR> <P>
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<H2><A NAME="SECTION004650000">The DNS Database</A></H2>
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<A NAME="tcpipdnsrecords"></A>
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We have seen above that DNS does not only deal with IP-addresses of
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hosts, but also exchanges information on name servers. There are in
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fact a whole bunch of different types of entries the DNS database may
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have.
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<P>
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A single piece of information from the DNS database is called a
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<em>resource record</em>, or RR for short. Each record has a type
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associated with it, describing the sort of data it represents, and a
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class specifying the type of network it applies to. The latter
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accommodates the needs of different addressing schemes, like
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IP-addresses (the IN class), or addresses of Hesiod networks (used at
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MIT), and a few more. The prototypical resource record type is the A
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record which associates a fully qualified domain name with an
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IP-address.
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<P>
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Of course, a host may have more than one name. However, one of these
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names must be identified as the official, or <em>canonical host name</em>,
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while the others are simply aliases referring to the former. The
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difference is that the canonical host name is the one with an A record
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associated, while the others only have a record of type CNAME which
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points to the canonical host name.
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<P>
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We will not go through all record types here, but save them for a later
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chapter, but rather give you a brief example here.
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Figure-<A HREF="node40.html#introfighosts"><IMG ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif"></A> shows a part of the domain database that is
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loaded into the name servers for the physics.groucho.edu zone.
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<P>
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Apart from A and CNAME records, you can see a special record at the top
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of the file, stretching several lines. This is the SOA resource record,
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signaling the <em>Start of Authority</em>, which holds general information
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on the zone the server is authoritative for. This comprises, for
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instance, the default time-to-live for all records.
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<P>
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Note that all names in the sample file that do not end with a dot should
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be interpreted relative to the groucho.edu domain. The special
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name ``@'' used in the SOA record refers to the domain name by
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itself.
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<P>
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We have seen above that the name servers for the groucho.edu
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domain somehow have to know about the physics zone so that they
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can point queries to their name servers. This is usually achieved by a
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pair of records: the NS record that gives the server's FQDN, and
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an A record associating an address with that name. Since these records
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are what holds the name space together, they are frequently called the
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<em>glue records</em>. They are the only instances of records where a
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parent zone actually holds information on hosts in the subordinate zone.
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The glue records pointing to the name servers for
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physics.groucho.edu are shown in figure-<A HREF="node40.html#introfignsptr"><IMG ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif"></A>.
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<P><HR><A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node40.html">Reverse Lookups</A>
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<B>Up:</B> <A HREF="node34.html">The Domain Name System</A>
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<B> Previous:</B> <A HREF="node38.html">Domain Name Servers</A>
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<P><ADDRESS>
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<I>Andrew Anderson <BR>
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Thu Mar 7 23:22:06 EST 1996</I>
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</ADDRESS>
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</BODY>
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</HTML>
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