69 lines
3.3 KiB
HTML
69 lines
3.3 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>Mail Routing on the Internet</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="node192.html">Mail Routing in the </A>
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<B>Up:</B> <A HREF="node190.html">How does Mail Routing </A>
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<B> Previous:</B> <A HREF="node190.html">How does Mail Routing </A>
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<BR> <P>
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<H2><A NAME="SECTION0015410000">Mail Routing on the Internet</A></H2>
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<a name="mail.routing.internet">Mail Routing on the Internet</a>
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On the Internet, it depends entirely on the destination host whether any
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specific mail routing is performed at all. The default is to deliver the
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message to the destination host directly by looking up its IP address,
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and leave the actual routing of the data to the IP transport layer.
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<P>
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<A NAME="6948"></A>
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Most sites will usually want to direct all in-bound mail to a highly
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available mail server that is capable of handling all this traffic, and
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have it distribute this mail locally. To announce this service, the site
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publishes a so-called MX record for their local domain in the DNS
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database. MX stands for <em>Mail Exchanger</em> and basically states
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that the server host is willing to act as a mail forwarder for all
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machines in this domain. MX records may also be used to handle traffic
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for hosts that are not connected to the Internet themselves, like UUCP
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networks, or company networks with hosts carrying confidential
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information.
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<P>
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MX records also have a <em>preference</em> associated with them. This is
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a positive integer. If several mail exchangers exist for one host, the
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mail transport agent will try to transfer the message to the exchanger
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with the lowest preference value, and only if this fails will it try a
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host with a higher value. If the local host is itself a mail exchanger
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for the destination address, it must not forward messages to any MX
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hosts with a higher preference than its own; this is a safe way of
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avoiding mail loops.
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<P>
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Suppose that an organization, say Foobar Inc., want all their mail
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handled by their machine called mailhub. They will then have an
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MX record like this in the DNS database:
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<PRE>
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foobar.com IN MX 5 mailhub.foobar.com
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</PRE>
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This announces mailhub.foobar.com as a mail exchanger for
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foobar.com with a preference value of 5. A host that wishes to
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deliver a message to joe@greenhouse.foobar.com will check DNS for
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foobar.com, and finds the MX record pointing at mailhub.
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If there's no MX with a preference smaller than 5, the message will be
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delivered to mailhub, which then dispatches it to
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greenhouse.
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<P>
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The above is really only a sketch of how MX records work. For more
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information on the mail routing on the Internet, please refer to
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RFC-974.
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<P>
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<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="node192.html">Mail Routing in the </A>
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<B>Up:</B> <A HREF="node190.html">How does Mail Routing </A>
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<B> Previous:</B> <A HREF="node190.html">How does Mail Routing </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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