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<a name="Environment-for-local-transports"></a>
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<h1 class="chapter"> 23. Environment for running local transports </h1>
<p>Local transports handle deliveries to files and pipes. (The <code>autoreply</code>
transport can be thought of as similar to a pipe.) Exim always runs transports
in subprocesses, under specified uids and gids. Typical deliveries to local
mailboxes run under the uid and gid of the local user.
</p>
<p>Exim also sets a specific current directory while running the transport; for
some transports a home directory setting is also relevant. The <code>pipe</code>
transport is the only one that sets up environment variables; see section
<a href="spec_29.html#SEC239">Environment variables</a> for details.
</p>
<p>The values used for the uid, gid, and the directories may come from several
different places. In many cases, the router that handles the address associates
settings with that address as a result of its <code>check_local_user</code>, <code>group</code>,
or <code>user</code> options. However, values may also be given in the transport's own
configuration, and these override anything that comes from the router.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#SEC216">23.1 Concurrent deliveries</a></td><td> </td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top"><a href="#SEC217">23.2 Uids and gids</a></td><td> </td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top"><a href="#SEC218">23.3 Current and home directories</a></td><td> </td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top"><a href="#SEC219">23.4 Expansion variables derived from the address</a></td><td> </td><td align="left" valign="top">
</td></tr>
</table>
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<h2 class="section"> 23.1 Concurrent deliveries </h2>
<p>If two different messages for the same local recipient arrive more or less
simultaneously, the two delivery processes are likely to run concurrently. When
the <code>appendfile</code> transport is used to write to a file, Exim applies locking
rules to stop concurrent processes from writing to the same file at the same
time.
</p>
<p>However, when you use a <code>pipe</code> transport, it is up to you to arrange any
locking that is needed. Here is a silly example:
</p>
<table><tr><td> </td><td><pre class="example">my_transport:
driver = pipe
command = /bin/sh -c 'cat >>/some/file'
</pre></td></tr></table>
<p>This is supposed to write the message at the end of the file. However, if two
messages arrive at the same time, the file will be scrambled. You can use the
<code>exim_lock</code> utility program (see section <a href="spec_50.html#SEC467">Mailbox maintenance (exim_lock)</a>) to lock a
file using the same algorithm that Exim itself uses.
</p>
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<h2 class="section"> 23.2 Uids and gids </h2>
<p>All transports have the options <code>group</code> and <code>user</code>. If <code>group</code> is set, it
overrides any group that the router set in the address, even if <code>user</code> is not
set for the transport. This makes it possible, for example, to run local mail
delivery under the uid of the recipient (set by the router), but in a special
group (set by the transport). For example:
</p>
<table><tr><td> </td><td><pre class="example"># Routers ...
# User/group are set by check_local_user in this router
local_users:
driver = accept
check_local_user
transport = group_delivery
# Transports ...
# This transport overrides the group
group_delivery:
driver = appendfile
file = /var/spool/mail/$local_part
group = mail
</pre></td></tr></table>
<p>If <code>user</code> is set for a transport, its value overrides what is set in the
address by the router. If <code>user</code> is non-numeric and <code>group</code> is not set, the
gid associated with the user is used. If <code>user</code> is numeric, <code>group</code> must be
set.
</p>
<a name="IDX1966"></a>
<p>When the uid is taken from the transport's configuration, the <code>initgroups()</code>
function is called for the groups associated with that uid if the
<code>initgroups</code> option is set for the transport. When the uid is not specified
by the transport, but is associated with the address by a router, the option
for calling <code>initgroups()</code> is taken from the router configuration.
</p>
<a name="IDX1967"></a>
<p>The <code>pipe</code> transport contains the special option <code>pipe_as_creator</code>. If this
is set and <code>user</code> is not set, the uid of the process that called Exim to
receive the message is used, and if <code>group</code> is not set, the corresponding
original gid is also used.
</p>
<p>This is the detailed preference order for obtaining a gid; the first of the
following that is set is used:
</p>
<ul class="toc">
<li>
A <code>group</code> setting of the transport;
</li><li>
A <code>group</code> setting of the router;
</li><li>
A gid associated with a user setting of the router, either as a result of
<code>check_local_user</code> or an explicit non-numeric <code>user</code> setting;
</li><li>
The group associated with a non-numeric <code>user</code> setting of the transport;
</li><li>
In a <code>pipe</code> transport, the creator's gid if <code>deliver_as_creator</code> is set and
the uid is the creator's uid;
</li><li>
The Exim gid if the Exim uid is being used as a default.
</li></ul>
<p>If, for example, the user is specified numerically on the router and there are
no group settings, no gid is available. In this situation, an error occurs.
This is different for the uid, for which there always is an ultimate default.
The first of the following that is set is used:
</p>
<ul class="toc">
<li>
A <code>user</code> setting of the transport;
</li><li>
In a <code>pipe</code> transport, the creator's uid if <code>deliver_as_creator</code> is set;
</li><li>
A <code>user</code> setting of the router;
</li><li>
A <code>check_local_user</code> setting of the router;
</li><li>
The Exim uid.
</li></ul>
<p>Of course, an error will still occur if the uid that is chosen is on the
<code>never_users</code> list.
</p>
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<h2 class="section"> 23.3 Current and home directories </h2>
<p>Routers may set current and home directories for local transports by means of
the <code>transport_current_directory</code> and <code>transport_home_directory</code> options.
However, if the transport's <code>current_directory</code> or <code>home_directory</code> options
are set, they override the router's values. In detail, the home directory
for a local transport is taken from the first of these values that is set:
</p>
<ul class="toc">
<li>
The <code>home_directory</code> option on the transport;
</li><li>
The <code>transport_home_directory</code> option on the router;
</li><li>
The password data if <code>check_local_user</code> is set on the router;
</li><li>
The <code>router_home_directory</code> option on the router.
</li></ul>
<p>The current directory is taken from the first of these values that is set:
</p>
<ul class="toc">
<li>
The <code>current_directory</code> option on the transport;
</li><li>
The <code>transport_current_directory</code> option on the router.
</li></ul>
<p>If neither the router nor the transport sets a current directory, Exim uses the
value of the home directory, if it is set. Otherwise it sets the current
directory to ‘<tt>/</tt>’ before running a local transport.
</p>
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<h2 class="section"> 23.4 Expansion variables derived from the address </h2>
<p>Normally a local delivery is handling a single address, and in that case the
variables such as <code>$domain</code> and <code>$local_part</code> are set during local
deliveries. However, in some circumstances more than one address may be handled
at once (for example, while writing batch SMTP for onward transmission by some
other means). In this case, the variables associated with the local part are
never set, <code>$domain</code> is set only if all the addresses have the same domain,
and <code>$original_domain</code> is never set.
<a name="IDX1968"></a>
<a name="IDX1969"></a>
<a name="IDX1970"></a>
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