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359 lines
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
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<HEAD>
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<title>TitleOfArticle LG #43</title>
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<BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#0000AF"
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<H4>
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"Linux Gazette...<I>making Linux just a little more fun!</I>"
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</H4>
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<P> <HR> <P>
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<center>
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<H1><font color="maroon">Setting Up Mail for a Home Network Using Exim</font></H1>
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<H4>By <a href="mailto:JW.Stumpel@inter.NL.net">Jan Stumpel</a></H4>
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[Revised at author's request. Originally published in issue #42.]
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</center>
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<P> <HR> <P>
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<h4><a NAME="exim1"></a>1 Introduction</h4>
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Setting up a home network with Linux and Win95, using Samba, IP
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Masquerading, and diald has been described many times, also in the Linux
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Gazette, but so far I have not found a recipe for setting up <i>mail</i> on
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a small network with only one dial-up e-mail account. In this article I want
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to explain how I did it. With this system:
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<p><ul><li> users on the network can send local mail to each other, and
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reply to it, also locally.</li>
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<li> outgoing mail has a proper From: address, so the outside world can
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reply to it.</li>
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<li> the e-mail account is shared by the users, but each only receives
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his/her personal mail.</li>
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<li> users on the network receive a notification (a pop-up window) when
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personal mail for them arrives.</li></ul>
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<p>This is realized on my system (running Debian Linux 2.1) using the
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following programs:
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<p><ul><li> exim as the mail transfer agent (it is
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<i>much</i> easier to configure than sendmail).</li>
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<li> fetchmail for collecting the mail from the ISP.</li>
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<li> pine as the mail client on the Linux side (but other clients can be
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used as well, including <tt>mail</tt>).</li>
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<li> Microsoft Internet Mail on the Windows side (but other clients can be
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used as well).</li>
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<li> qpopper as the POP3 server, for moving mail from the Linux system to
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the Win95 machine.</li>
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<li> smbclient and Winpopup for mail notification.</li></ul>
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<p>I have this set up for two machines (1 Linux + 1 Win95) but it will
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probably also work for a somewhat larger network, and may be sufficient for
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a small office. Note: this article is Debian-oriented. If you use another
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distribution, change where appropriate!
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<h4> <a NAME="exim2"></a>2 The network and the names</h4>
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For this article I assume the following <i>names</i> (change these to
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correspond with your own situation):
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<p><ul><li> the owner / system administrator is called Joe Bloggs.</li>
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<li> the Linux machine is called <b>heaven</b>.</li>
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<li> the Win95 machine is called <b>earth</b>. It is mostly used by Emily
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Bloggs.</li>
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<li> Joe's user name on heaven is <tt>joe</tt>.</li>
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<li> Emily's user name on heaven is <tt>emi</tt>.</li>
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<li> Emily's user name on earth is also <tt>emi</tt>; her Linux password on
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heaven and her 'password for Microsoft networking' on earth are the
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same.</li>
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<li> Joe has a dialup account (dynamic IP address) with an ISP called
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<tt>isp.com</tt>. Mail from the ISP can be collected using POP3.</li>
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<li> Joe's account name at the ISP is <tt>jbloggs</tt>.</li>
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<li> Joe's e-mail address (also used by Emily) is
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<tt>joe.bloggs@isp.com</tt>.</li>
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<li> Joe's password for collecting POP3 mail is <tt>zaphod</tt>.</li>
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<li> The ISP's mail server (for sending mail) is <tt>smtp.isp.com</tt>.</li>
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<li> The ISP's POP3 server (for collecting mail) is <tt>pop3.isp.com</tt>.
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</li>
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<li> heaven and earth belong to a domain called <tt>home</tt>. This domain
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name is meant for use only inside the home network; Joe has not registered
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his domain name and it cannot be recognized by the outside world.</li> </ul>
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I also assume that the local networking works, and that there is on-demand
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dialup access using diald. There is no name server on heaven.
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/etc/resolv.conf contains the addresses of two name servers supplied by the
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ISP. These same addresses are entered into the TCP/IP configuration on
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earth.
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<p><tt>/etc/hostname</tt> on heaven is
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<p><tt>heaven</tt>
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<p><tt>/etc/hosts</tt> on heaven is
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<p><tt>127.0.0.1 localhost</tt>
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<br><tt>192.168.1.1 heaven.home heaven</tt>
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<br><tt>192.168.1.2 earth.home earth</tt>
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<p>On earth there is a file <tt>c:\windows\hosts</tt> with the same contents
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as <tt>/etc/hosts</tt>.
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<h4><a NAME="exim3"></a>3 Mail addresses</h4>
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Mail messages can have more than just the address in the 'To:' and 'From:'
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lines, for instance :
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<p><tt>To: Emily Bloggs <joe.bloggs@isp.com></tt>
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<p>'Emily Bloggs' in the above example is the 'real-name part'. It is set in
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the e-mail program which composes the message. This 'real-name part' can be
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used for delivering Emily's mail to her. Note: if the 'real-name part' has
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dots in it, it must be quoted using " characters ("Joe C. Bloggs"). See also
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<tt>man mailaddr</tt>.
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<h4> <a NAME="exim4"></a>4 Configuring exim</h4>
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On a Debian system this is done by running <tt>eximconfig. </tt>It asks a
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number of questions which you can answer as follows:
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<p><ul><li>your system is an Internet site using smarthost.</li>
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<li>the 'visible mail domain' is <tt>home</tt></li>
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<li>other names apart from <tt>home</tt> and <tt>heaven.home</tt>: answer
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<tt>heaven:localhost</tt></li>
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<li>you don't want to relay for any non-local domains.</li>
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<li>you want to relay for the local network <tt>192.168.1.0/16</tt></li>
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<li>RBL (spam filter database): whatever you like. I said <tt>n</tt></li>
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<li>The smarthost, handling outgoing mail, is <tt>smtp.isp.com</tt></li>
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<li>System administrator mail should go to <tt>joe</tt> (not to
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<tt>root</tt>!)</li> </ul>
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In MS Internet Mail (or whatever mail client you use on Win95) <tt>heaven</tt>
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must be entered both as the SMTP server and as the POP3 server. Under 'pop3
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account' and 'pop3 password', enter the username <tt>emi</tt> and her Linux
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password. Enter the the name, Emily Bloggs, and the e-mail address, <tt>emi@home</tt>,
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in the appropriate place. Note that the e-mail address must be in the <i>local
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domain</i>!
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<p>On the Linux side, nothing special has to be set. /etc/pine/conf and
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the users' ~/.pinerc can be used 'out of the box'. The mail client (pine)
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constructs local addresses using the hostname together with user information
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from /etc/passwd.
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<p>With the above setup, local users can happily send mail to each other
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and reply to it. For instance, in pine at heaven, user <tt>joe</tt> sends
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mail to user <tt>emi</tt>. Automatically, pine changes this to:
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<p><tt>To: Emily Bloggs <emi@heaven.home></tt>
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<p>The message is delivered immediately (as you can see if you run eximon,
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the exim monitoring utility). <tt>emi</tt> (should she log in to heaven)
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would see the message as coming from
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<p><tt>From: Joe Bloggs <joe@home></tt>
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<p>So <tt>home</tt> really functions like a local domain within which messages
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can be exchanged. The problem is sending messages to the outside world.
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A From: address like <tt><joe@home></tt> is no good because nobody on
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the outside could reply to an address in the non-existent domain <tt>home</tt>.
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<h4><a NAME="exim5"></a>5 Fixing the From: address</h4>
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We must change the local From: address into a valid e-mail address (the
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e-mail account at the ISP), but <i>only</i> in the case of outgoing messages.
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With exim, we can do this by means of a 'transport filter'. The outgoing
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mail passes through this filter, and the From: address is changed. Local
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mail will not be affected.
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<p>The following filter will do the trick, provided we are <i>sure</i> that
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the address that we want to change is always between < and > signs. This
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is not guaranteed, but <i>very</i> common: <tt>pine</tt>, <tt>mutt</tt>, and
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<tt>mail</tt>, as well as MS Internet Mail all generate such addresses.
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<p><pre>
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#!/usr/bin/perl
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$address = 'joe.bloggs@isp.com';
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while (<>) {
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if (/^From: /) { s/<.*>/<$address>/; print; last; }
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print; }
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while (<>) { print; }
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</pre>
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Don't forget to change the e-mail address to yours! Call this program
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<tt>outfilt</tt>, do <tt>chmod +x outfilt</tt> and put it in
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<tt>/usr/local/bin</tt>. Now we must add a line to <tt>/etc/exim.conf</tt>, so the
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last lines of the <tt>TRANSPORTS CONFIGURATION</tt> section read:
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<p><pre>remote_smtp:
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driver = smtp
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headers_remove = "sender"
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transport_filter = "/usr/local/bin/outfilt"
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end</pre>
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Actually, we added <i>two</i> lines. The <tt>headers_remove</tt> line is
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also new. This prevents exim from adding a Sender: header to the message (as
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it would do with this setup, if you use pine). The Sender: line can cause
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trouble with some (badly configured) mail destinations.
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<p>With these changes to /etc/exim.conf, whenever anyone sends an e-mail
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message to the outside world it is now delivered properly by exim. Exim
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(through diald) opens the outside line at once. In a home situation this is
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probably what you want. In a small office, with a lot of e-mail traffic, you
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may want to defer messages and send them as a bunch at certain times, to
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save phone costs. This is possible, but I don't need it myself and have not
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looked into it. You could look at the 'Linux Mail-Queue mini-HOWTO'.
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<h4><a NAME="exim6"></a>6 Fetchmail configuration</h4>
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At the command <tt>fetchmail</tt> diald opens the line and the mail from the
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ISP is collected (and passed to exim for local delivery). Only users who
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have a <tt>.fetchmailrc</tt>, owned by themselves, in their home directory
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can run fetchmail. This file can be created using the configuration tool
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fetchmailconf. You get something like:
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<p><pre># Configuration created Sun Mar 28 03:15:20 1999 by fetchmailconf
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set postmaster "postmaster"
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poll pop3.isp.com with proto POP3
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user "jbloggs" there with password "zaphod" is joe here options fetchall warnings 3600
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</pre>
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The <tt>.fetchmailrc</tt> files belonging to the various users could all be
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copies of each other, but with the ownership set to the user concerned. It
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is not so nice that every user has the password in plain view. Maybe there
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is a better way, but in a home situation it does not matter.
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<p>The main point is that whoever runs fetchmail, the mail must always
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be delivered to the <i>same</i> user mailbox (<tt>joe</tt>'s mailbox in
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this case).
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<h4><a NAME="exim7"></a>7 Removing exim's delivery limit</h4>
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Exim by default does not deliver more than 10 messages at a time. I am sure
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there are circumstances where this makes perfect sense, but having a dialup
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account is not one of them. To get rid of this restriction, you must put
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into the <tt>MAIN CONFIGURATION</tt> section of <tt>/etc/exim.conf</tt>,
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before the <tt>end</tt> statement, a line
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<p><tt>smtp_accept_queue_per_connection = 0</tt>
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<h4><a NAME="exim8"></a>8 Delivering personal mail</h4>
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Through <tt>fetchmail</tt> and <tt>exim</tt>, all mail from the outside
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is by default delivered to Joe's mailbox (<tt>var/spool/mail/joe</tt>)
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at heaven. In Joe's home directory he puts a file called <tt>.forward</tt>,
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containing the following text:
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<p><tt># Exim filter</tt>
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<br><tt>if $header_to: contains Emily then deliver emi endif</tt>
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<p>The <tt>.forward</tt> file must have permissions <tt>-rw-r--r--</tt>. If
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you're not sure, give the command <tt>chmod 644 .forward</tt>.
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<p>If mail contains 'Emily' in (the 'real name part' of) the To: address
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(and this will almost always be the case when her friends send her mail) it
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will now go into <i>her</i> mail account on heaven, not into Joe's. She can
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move the mail to her own machine using POP3 (see below).
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<p>Delivery to other users than Emily can be arranged with <tt>elif ...
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then</tt> clauses in the <tt>.forward</tt> file. Actually, exim's
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<tt>.forward</tt> files can perform a lot of complicated functions. See the
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text "Exim's user interface to mail filtering" which comes with the exim
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doc's.
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<h4><a NAME="exim9"></a>9 Transferring mail with qpopper</h4>
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To let heaven act as POP3 server for earth, qpopper can be installed. I
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installed the Debian package <tt>qpopper_2.3-4.deb</tt>. Installation is
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automatic; no configuration is necessary. If Emily presses 'get/send
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messages' in MS Internet Mail, the contents of her mailbox on heaven get
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transferred to earth (and all mail, local or outside, which she has written
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gets delivered).
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<h4><a NAME="exim10"></a>10 Automatic mail notification</h4>
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Emily likes to be <i>notified</i> if mail arrives for her at heaven. If Samba
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is installed on heaven and Winpopup on earth, this is easy. Mail
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notification on earth can be done using smbclient (a program which comes
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with Samba).
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<p>Joe (as root) has put a Perl program called <tt>mailwarn</tt> into
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usr/local/bin:
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<p><pre>#!/usr/bin/perl
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open POPUP, "| smbclient -M $ARGV[0] >/dev/null 2>&1";
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print POPUP "YOU'VE GOT MAIL! GO AND CHECK IT!\n";
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print POPUP "It's from $ARGV[1]";
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close POPUP; </pre>
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<p>This file was, of course, first made executable using <tt>chmod +x
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mailwarn</tt>.
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<p>In Emily's home directory at heaven there is now also also a
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<tt>.forward</tt> file (<tt>emi</tt> is the owner; permissions are 644):
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<p><pre># Exim filter
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unseen pipe "/usr/local/bin/mailwarn earth $header_from:"</pre>
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<p>If mail (local or from the outside) for Emily arrives at heaven, a window
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will now pop up on earth telling her this.
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<h4><a NAME="exim11"></a>11 Manually collecting the outside mail</h4>
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Thanks to a 'shortcut' on earth's Win95 'desktop', which does a telnet to
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heaven, Emily can log into heaven and start <tt>fetchmail</tt> by hand. That
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is, if she does not want to wait for the scheduled <tt>cron</tt> times when
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fetchmail runs. After the mail has been transferred from the ISP, she can
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press 'get/send messages' to move any mail from her heaven mailbox into the
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earth one.
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<!--===================================================================-->
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<P> <hr> <P>
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<center><H5>Copyright © 1999, Jan Stumpel<BR>
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Published in Issue 43 of <i>Linux Gazette</i>, July 1999</H5></center>
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