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<title>s6-linux-init: the s6-linux-init-logouthookd program</title>
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<p>
<a href="index.html">s6-linux-init</a><br />
<a href="//skarnet.org/software/">Software</a><br />
<a href="//skarnet.org/">skarnet.org</a>
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<h1> The <tt>s6-linux-init-logouthookd</tt> program </h1>
<p>
s6-linux-init-logouthookd cleans up its client's utmp record when it dies.
</p>
<h2> Interface </h2>
<pre>
s6-ipcserver <em>socket</em> s6-linux-init-logouthookd
</pre>
<p>
s6-linux-init-logouthookd implements a
<a href="//skarnet.org/software/s6/localservice.html">local service</a>
for getty programs that add a utmp record when a user logs in.
</p>
<p>
In the sysvinit model, <tt>getty</tt>/<tt>login</tt> and similar programs add a utmp
record for every user that logs in, then exec into the user's shell.
At logout time, the shell dies; sysvinit is supervising the <tt>getty</tt>
program, so it's watching the pid, and respawns the <tt>getty</tt> when the
shell dies. But before respawning the <tt>getty</tt>, it cleans up the
utmp record, to correctly report that the user isn't logged in on this
terminal anymore.
</p>
<p>
utmp is an old, clunky, insecure system (unless you're using
<a href="//skarnet.org/software/utmps/">utmps</a>) and it is definitely
not pid 1's job to have any knowledge of utmp and play janitor after
<tt>getty</tt>. <a href="//skarnet.org/software/s6/s6-svscan.html">s6-svscan</a>
will definitely not do it.
</p>
<p>
Some distributions use versions of <tt>login</tt> that fork the user's
shell instead of execing it. When the user logs out, the <tt>login</tt>
program cleans up after itself. This is a better model, but it's not
always easy to patch <tt>login</tt> to go from an "exec the shell" model to a
"fork the shell as a child" model.
</p>
<p>
<tt>s6-linux-init</tt> comes with a small library which makes it easy
for a distribution to fully support utmp cleanup with an s6 init system
if they so choose. Before execing into the user's shell, the <tt>login</tt>
program should just make a call to <tt>s6_linux_init_logouthook()</tt>,
and that's it. That function will call the <tt>s6-linux-init-logouthookd</tt>
local service, which will do nothing but wait until the user's shell dies;
and when it happens, the user's utmp record will automatically be cleaned up.
</p>
<h2> Exit codes </h2>
<p>
<tt>s6-linux-init-logouthookd</tt>'s exit code does not matter, because
no program uses it. However, here's the list for completeness:
</p>
<ul>
<li> 0: success, whether or not there was a utmp record to clean up. </li>
<li> 1: connection attempt from a non-root user. </li>
<li> 2: write attempt from a (misprogrammed) client. </li>
<li> 111: system call failed. </li>
</ul>
<h2> Notes </h2>
<ul>
<li> The <a href="//skarnet.org/software/s6/localservice.html">local service</a>
implementing logouthook support is automatically created at boot time when the
<tt>-L</tt> option has been given to
<a href="s6-linux-init-maker.html">s6-linux-init-maker</a>. Client-side, though,
the various login programs must be patched at the source level. </li>
</ul>
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